July Tropical Update

With the formation of Tropical Depression Four (TD4) on July 5th, I wanted to remind the insurance industry of what to expect this hurricane season. TD4 is now located 690 miles to the east of the Lesser Antilles and is not currently a threat to the insurance industry due to expected weakening in the coming 24 hours. However, keep an eye on the location of it’s left over moisture plume later next week as it could be a sign of future storm tracks towards the heart of the 2017 hurricane season.
In my May 15th tropical update, I went into detail discussing how many early Atlantic Basin seasonal hurricane forecasts were putting a lot of weight into the formation of an El Niño event during the 2017 Atlantic hurricane season, but spring El Niño forecasts can be notoriously misleading. In fact, the seasonal climate models continue to back off the idea that an El Niño will develop, and currently there is only a 40% chance of an El Niño with neutral condition developing during the heart of the hurricane season.

I also mentioned in the May 15th update that many of the early season forecasts would likely be adjusted upward to a “more active or normal season,” and this is what has happened. In fact, on July 5th, Phil Klotzbach and the team of seasonal forecasters at Colorado State University revised their forecast and now call for an above average hurricane season with 15 named storms, including 8 hurricanes and 3 major hurricanes during the 2017 Atlantic hurricane season.

A great collection of 18 different seasonal hurricane forecasts can be found here:

http://www.bsc.es/ESS/seasonalhurricanepredictions/
As indicated in my May 15th update, although sea surface temperatures are conducive for tropical development in the Main Development Region (MDR), a large negative influence on named storm activity over the last few years has been the Saharan Air Layer (SAL). This is a factor this year and is likely the reason why TD4 has struggled as it tracks across the MDR.

Dusty dry air to its east of TD4

The SAL will likely continue to be an influence on named storm development in the MDR for the remainder of the season, but there will be windows of less dust which will provide opportunities for named storm development.

12z NASA-GEOS5 dust forecast for next 5-days shows series of typical Saharan air layers heading west across tropical Atlantic from Africa.

These opportunities for storm development could arise with the passing of the lesser known phenomenon of the Madden Julian Oscillation (MJO). The MJO is long known to influence tropical convection as it traverses the planet’s tropic regions every 30 to 60 days. During times when El Niño neutral, like it is forecasted to be this year, this oscillation brings more positive atmospheric upward motion and can help us determine when the Atlantic Ocean might have more named storm activity. During the first passage of the MJO this year, the Atlantic saw the formation of tropical storms Cindy and Bret. Although TD4 is likely struggling because of the SAL, the Atlantic basin is also now in the suppressive phase of the MJO which limits large scale tropical development due to a large scale atmospheric sinking motion. The next phase of the MJO which could possibly enhance Atlantic named storm activity would occur near the end of July or first part of the August, followed by a suppressive phase in the middle to end of August. If the pattern holds, it could be followed by an active phase of the MJO again in mid-September which could enhance named storm activity at that time.

With the understanding that there could be pulses of activity in the Atlantic Basin associated with the MJO, where might these storms track? As highlighted in the May 15th update, the more important concept to grasp is where named storms may make landfall and not necessarily the total number of named storms that will develop. The climate forces this season are much more conducive to named storm development closer to the U.S. coastline. This, combined with the current placement of the Bermuda – Azores high pressure and the high pressure that has been dominating the central high plains states, would allow for storms to track toward the east coast of the U.S., putting the coastline at higher risk of landfall if storms were to develop there. Add in the fact that the sea surface temperatures off the east coast are warmer then average, and this has the potential to create interesting conditions for named storm activity later this season.

 

Long range models continue to develop tropical waves over western Africa into named storm. Could these thunderstorm over central Africa be the next system of interest?

With that being said, the long range forecast modeling always seems to try to spin up a named storm in the long range, with most of these scenarios often failing to come to fruition. In looking at the extreme long range forecast (next 46 days), the Atlantic basin looks to remain quiet with only a few hints at Cape Verde storm development through mid August.

Not much activity for the next 45 days as indicated by the ECMWF MEPS Cyclone Tracks by WeatherBell. However, a few Cape Verde storm development so need to keep as we get into the heart of the season.

This is interesting because globally named storm tropical activity in terms of Accumulated Cyclone Energy (ACE) is at record low levels. In fact, the North Pacific has had 0 typhoons so far in 2017, and this is only the seventh time since 1950 that this has happened through July 7th of a season. This also follows the quietest season on record for the southern hemisphere, which recently just ended. It should be noted that years with an ACE under 200 in the southern hemisphere have also managed to be below normal in the Atlantic, with a few exceptions like 1995. Based on the seasonal forecasts, this year looks like it could also potentially be an exception due to a higher risk of named storm activity near the east coast of the U.S., as storms struggle in the MDR.

BMS Tropical Update 6/22/2017 12 PM CDT

Cindy made landfall around 4:00 a.m. CDT this morning near the Texas and Louisiana border as a weak tropical storm with maximum sustained winds of 40 mph. I have been scouring social media and NOAA local storm reports and have not seen any significant damage. The biggest impact has been flooding, as previously forecasted, but it appears at this time that a repeat of the August 2016 Louisiana flood is unlikely. The highest rainfall amount that I have seen so far is 8.5 inches in Wiggins, Mississippi. The Gulfport-Biloxi Airport in Mississippi picked up 8.43 inches, and in Florida, the 8.25 inches in Navarre is the top total that I have seen. It should again be noted that these locations are over 300 miles from where Cindy made landfall. The BMS iVision Verisk Climate total rainfall layer suggests that some isolated coastal areas have seen upwards of 6 – 9” of rain over since Monday.

BMS iVision Verisk Climate total rainfall layer for Cindy up until 1 AM EDT 6/22/2017. BMS clients can run exposure reports against these rainfall amounts.

A surprising fact for some may be the level of storm surge that has occurred along parts of the central Gulf Coast. The largest storm surge level that I have been able to find was reported in Shell Beach, AL with an observed storm surge of 6 feet above the Mean Higher High Water (MHHW). This location is roughly 250 miles from the center, suggesting that Cindy had a large circulation which allowed a lot of water to pile up along parts of the central Gulf Coast over the last several days.

Observed Water Level At Shell Beach, LA

Example of road closures and flooding in Mobile Bay, AL that is currently common along coastal areas of the central Gulf Coast.

As mentioned, the winds from Cindy have been in the 45 – 55 mph range, which is well below the international residential design minimum of 90 mph. With only a handful of damage reports in to the NOAA, all of which are reports of trees or power lines being knocked down, the wind damage should be fairly minimal.

iVision Verisk Climate maximum gust in mph. BMS cleints can run exposure reports to understand the risk to high wind speeds from Tropical Storm Cindy.

At this time PCS has not issued a catastrophe bulletin, suggesting that the insurance industry loss could remain under $25 million.

Looking ahead over the longer term, the tropics should remain quiet for the next two weeks as the active phase of the Madden Julian Oscillation moves away from the Atlantic basin and other climate forcers such as the SAL layer and high wind shear hinder tropical development.

BMS Tropical Update 6/21/2017 12 PM CDT

Just after the tropical update yesterday, the National Hurricane Center (NHC) proceeded to upgrade Potential Tropical Cyclone Three to Tropical Storm Cindy. Currently Cindy is located 170 miles SSW of Morgan City, LA and may be the ugliest organized named storm that meteorologists have seen in the Gulf of Mexico in quite some time. It’s even questionable if it meets the true definition of a named tropical storm. In their 4 a.m. tropical discussion, even the NHC suggests that the cyclone is more characteristic of a subtropical cyclone and that the current categorization is generous.

Cindy is still expected to make landfall late tonight near the Texas/Louisiana border as a tropical storm. This landfall location is no stranger to named tropical storms, with 25 other tropical storms having tracked within 55 miles of the expected landfall location, based on historical records. However, as highlighted in the past few updates, the majority of insured impacts are being felt far away from the center of Cindy across eastern Louisiana and the panhandle of Florida. Heavy rainfall and flooding still appear to be the main threats across these areas. The BMS iVision Verisk Climate total rainfall layer suggests that isolated coastal areas have seen upwards of 4 – 6” of rain over the last 24 hours.

BMS iVision Verisk Climate total rainfall layer. BMS Clients can run exposure reports against these rainfall amounts.

New Orleans recorded record rainfall yesterday of 1.62” and more is expected today as an atmospheric river of moisture is pulled north from the deep tropics over the same area, creating a storm total that may be close to 14” in some areas.


Although there have been many tornado warnings issued over the last 24 hours for the central Gulf Coast, only 2 confirmed tornados, both weak with minimal damage, have been reported. As of this morning, no wind damage reports have been logged with the National Weather Service.

Based on the iVision Verisk Climate maximum gust data layer, most coastal areas will only experience winds of 40 – 69 mph, which in most cases is below the damage threshold for many structures.

iVision Verisk Climate maximum gust in mph. BMS cleints can run exposure reports to understand the risk to high wind speeds from Tropical Storm Cindy

As history would suggest, it is difficult to estimate insured losses from weak tropical storms. Let’s look at three historical tropical storms (Chris 1982, Debra 1978, TS#2 1987) that took a similar track inland as to what Cindy is forecasted to take. The insured losses from these storms range from $2 million to $12 million. However, given the large threat of flooding from Cindy, it should be noted that the major Louisiana flood event that occurred from August 11, 2016 to August 15, 2016 reached $1 billion in insured losses.  Estimating insured losses of tropical storms can be difficult.

As expected, Tropical Storm Bret has dissipated in the southern Caribbean. Looking ahead over the longer term, the tropics should remain quiet for the next two weeks as the active phase of the Madden Julian Oscillation moves away from the Atlantic basin and other climate forcers such as the SAL layer and high wind shear hinder tropical development.

BMS Tropical Update 6/20/2017 12 PM CDT

Update:  12:39 PM CDT  the NHC will be upgrading PTC3 to Cindy.  The most recent ECMWF 12z landfalls TS Cindy into Houston.  Has some strong winds with a landfall pressure of 990 mb which is a minor hurricane pressure.

 

Yesterday afternoon the National Hurricane Center (NHC) found enough evidence from the Air Force reserve hurricane hunter aircraft to upgrade Potential Tropical Cyclone Two to the second named storm of the 2017 Atlantic hurricane season. As mentioned in yesterday’s post, Bret still does not pose a significant threat to the insurance industry and will likely weaken later this week as it transverses the South American coastline in the southern Caribbean Sea.
The bigger threat to the insurance industry remains Invest 93L which, as of yesterday afternoon, was labeled Potential Tropical Cyclone Three (PTC3). Again, the idea of issuing advisories before a tropical depression or named storm forms is to highlight the threats of a developing storm earlier in its life cycle, and PTC3 is a classic example of the reason why there has been a change in policy. In fact, tropical storm warnings and watches are now in place along the Gulf Coast as PTC3 is still expected to develop into the next named storm (Cindy) before its low center makes landfall near High Island, Texas, in the late evening hours Wednesday, but tropical storm force winds could occur as early as before sunrise  on Wednesday.

This morning, it appears that PTC3 is gradually becoming better organized as it approaches the southern Gulf Coast. There is still a great deal of wind shear impacting the convection on the western side of the low center, which is likely the primary reason that the NHC has not yet upgraded PTC3 to a tropical depression or named storm.

 

This is the GFS model depiction of winds shear impacting the west side of PTC3

Forecast models still expect gradual strengthening of PTC3 until the low center moves inland later tomorrow afternoon. Historically, developing storms in the Gulf of Mexico are notorious for rapidly straightening towards the coast, but given the broad circulation along with the large radius of maximum winds, this becomes more difficult with PTC3. Regardless of how strong PTC3 becomes over the next 36 hours, given the nature of the circulation, the wind and rain hazards extend well north and east of the center which makes PTC3 a great example of the far reaching impacts a tropical storm can have away from the main track. In this case, the NHC cone is far outside of where the very heavy rains are forecasted for the Mississippi and Alabama coastlines.

NOAA QPF forecast suggesting over 10″ of rain far away from the NHC track of the low center. Heavy rainfall will be from Houston, TX to Pensacola FL

It is this rainfall which will continue to likely be the largest loss for the insurance industry, and the rainfall is already starting to reach the coastline. This rainfall will come from training of individual thunderstorms which are already creating areas of severe weather across the Gulf Coast States. Tornado warnings are being issued and individual thunderstorms are producing localized severe weather along the Gulf Coast which could also cause insured loss far outside the  forecated path. As indicated above, the NOAA Weather Prediction Center is now forecasting for as much as 10” of rain to fall over southern Mississippi and Louisiana, with as much as 7” over eastern Texas.  There could be locally even higher amounts.

Making things worse is the soil moisture is already saturated from the recent heavy rainfall that has occurred over the last 30 days. Therefore, most of the rain that falls will run off and exacerbate the flooding threat. This saturated soil could increase tree fall from higher winds as wet soil weakens the hold on a tree’s root system.

Given the broad and large circulation, storm surge risk is higher than what it might typically be with a developing named storm. Currently, it appears that inundation levels in the tropical storm warning area could be as high as 3 feet along the coastline.

BMS Tropical Update 6/19/2017 12 PM CDT

As expected from last week, two areas of tropical trouble are trying to form in the Atlantic Basin. If both of these tropical systems manage to get named this week, it would be fairly rare as only three Atlantic hurricane seasons on record have had two concurrent named storms in June. Those years were 1909, 1959 and 1968, all of which turned out to be average to slightly above average hurricane seasons with a higher than average landfall rate across the U.S. This is in line with the general thinking of what could occur this Atlantic hurricane season.

Since last week, the National Hurricane Center  (NHC) has been watching these two disturbances. One of these has been centered north of the Yucatan Peninsula and has been labeled Invest 93L. The other disturbance is labeled Invest 92L and is currently located 325 miles ESE of Trinidad, moving rapidly toward the southern Windward Islands. In fact, for the first time ever for a tropical system, the NHC has begun issuing advisories for 92L before it is a depression or named storm and has also labeled the system “Potential Tropical Cyclone Two.” I talked about this possibility in my New Hurricane Products for 2017 Season blog post. The NHC is doing this because there is an immediate threat of tropical storm force winds to land, which, in this case, would be in the southern Windward Islands where the watches and warnings are in effect.

What is the forecast and worry for insurance industry this week?
“Potential Tropical Cyclone Two”
Potential Tropical Cyclone Two has a 90% chance of tropical development over the next two days. Early visible satellite imagery suggests the system is still an open wave and does not have a closed center of circulation, which is part of the criteria for storm naming by the NHC. My guess is that the NHC will wait to name the storm (Bret is the next storm name) until this afternoon when an Air Force reserve hurricane hunter aircraft is scheduled to investigate the potential tropical cyclone. Depending on what the aircraft finds, it could get a name. Regardless of whether the potential tropical cyclone gets a name, stormy conditions will be experienced in the southern Windward Islands. The forecast models are suggesting that after today the system will struggle to maintain itself as interaction with South America and increased wind shear will likely hamper any development in the second half of this week. This means the overall impact to the U.S. insurance industry is minimal at this time, and even if named, it would be a short lived tropical system.

Last Night ECMWF Ensemble Breakdown of disturbance two tracks and intensity

Invest 93L
Invest 93L also has a high chance of tropical development over the next two days with an 80% chance of development. The reason why the NHC has not issued advisories for this system is they currently feel the threat of tropical storm conditions is less severe and it could take a few days to produce strong winds over land. So overall, the difference between these two systems in terms of advisories is the immediate threat of tropical storm conditions to land is greater for Potential Tropical Cyclone Two. However, in the longer term Invest 93L has the higher likelihood to impact the insurance industry. This morning’s satellite imagery suggests that the low level center is decoupled from the deep tropical convection on the right (east) side of the storm.

Visible Satellite shows low level center is decoupled from the deep tropical convection on the right (east) side of the storm.

There is a chance it’ll never actually develop into a tropical cyclone as the wind shear over the next few days could hamper development.

ECMWF forecasted strong southwesterly wind shear associated with a trough aloft blowing t-storms away from surface center of Invest 93L inhibiting development. Also shown is the shear that could impact disturbance two.

However, later in the week this wind shear is likely to abate and formation of a named storm becomes more likely.

As always there is uncertainty in the development and final track of tropical systems. The GFS (American model) takes this system on a more northerly track towards Louisiana and even on some early weekend runs into the Florida panhandle. However, as I highlighted in my New Hurricane Products for 2017 Season blog post, the new GFS model this year is not the model to watch. The more reliable ECMWF (European model) takes 93L into southern Texas as indicated last week, so the model has been fairly consistent with the ideas Texas will see worse of the impacts in terms of tropical storm force winds if they develop.

At this time, almost all forecast models for 93L remain below hurricane strength. Regardless, deep tropical moisture will have far reaching effects along the Gulf Coast states, so heavy rainfall and flooding are currently the biggest threats to the insurance industry to areas that don’t handle a lot of rain well like New Orleans and Houston. Depending on the track,  5 inches of rain could easily fall as a sort of atmospheric river seems to be setting up along the Central Gulf Coast.

NOAA Weather Prediction Center showing much of the central gulf coast is expected to get 2 – 5″ of rain which is conservative in my opinion based on the sort of atmospheric river that seems to be setting up into the Central Gulf Coast with a possible poorly organized tropical disturbance.

 

 

New Hurricane Products for 2017 Season

Every year there are new tools and products that can help the insurance industry understand named storm risks.  In this write-up, I highlight some of these new tools and products for the 2017 hurricane season, which starts today.   For ideas on the type of activity that is expected this season please see my previous update here:  May Tropical Update issued May 15th.

Advisories Will Be Issued Before a Storm Is Named

The National Hurricane Center (NHC), which decides when a named storm gets a name, will issue advisories for tropical systems before the tropical system has a name. In these cases, these tropical systems will pose a threat of bringing tropical storm-force or hurricane-force winds to land areas within 48 hours. For decades these tropical disturbances have been called ‘Invests’, or areas of investigation, and for the last few years the NHC has been giving Invests forecasts related to the chance of formation within the next five days.

Currently some re/insurance contract language is directly related to named storm activity. However, the advisories for these tropical systems that will likely impact land could now lead to earlier activity in re/insurance contracts where coverage is triggered by storm warnings or watches, as the NHC would previously wait for a storm to be named before issuing such warnings or watches. In most cases, these types of named systems would be in a developing stage just off the U.S. coastline and would highlight not only the likelihood of genesis of a named storm, but the possible strength of winds along the coastline.

Hurricane Humberto in 2007 is a classic example of where the NHC would have likely issued watches and warnings before Humberto was named if it took place in 2017. Humberto strengthened into a hurricane in a 24 hour period. Source: AccuWeather Inc. & NOAA

Storm Surge Watches and Warnings Are Coming 

For the last several years, the NHC has made large improvements to storm surge forecasts from named storms.  In fact, the detailed Potential Storm Surge Flooding Maps now rival the high resolution flood model simulations that are becoming common in the insurance industry by various Cat Modeling vendors.  However, another tool that might help the insurance industry is that the NHC plans to release storm surge watch and warning graphics to provide further guidance on where the greatest threat to life and property from a named storm might be.   The insurance industry is all too familiar with the hazards and damaging storm surge that occur with a threatening named storm and this guidance can help pin point areas likely to be most impacted by storm surge.

New storm surge watch and warning product for 2017 hurricane season. Source: NHC

Earliest Reasonable Arrival of Tropical Storm-Force Winds Will Be Forecast

The NHC has always provided guidance as to the position and timing of the center of a named storm; however, to provide more added value, the NHC will now be directly forecasting when tropical storm-force winds will begin to affect land. This will allow the insurance industry to better understand when winds greater than 39 mph are expected, which should aid in allowing more time for an insured to apply preventative measures to mitigate risk from damaging storms. Above 39 mph, winds can make it difficult and even dangerous to be outside continuing preparations for a tropical storm or hurricane.

The IBHS has some helpful hints to reduce hurricane damage to homes and businesses.


The Cone of Uncertainty Will Be Smaller

Every year the NHC reviews the accuracy of their previous five seasons of hurricane forecasts.  This review suggests the forecasts are getting better, and with that, the average error in the NHC forecasts that make up that famous cone of uncertainty will result in a smaller cone and just maybe more certain hurricane forecasts for the 2017 season.  Track errors have gone down over the last 10 years and forecasts have gotten better.  In fact, since 2007, the size of the cone of uncertainty at 120 hours (or five days) has shrunk by more than 35%.  Since last year, the size of the cone at 120 hours has shrunk by more than 10%.

The shrinking cone of uncertainty. Source Brian McNoldy Univ. of Miami

Hurricane Model Wars

As long as I have been studying meteorology there have been wars among the various forecast models as to which model is the most accurate at forecasting named storm activity.  This war was brought into the public limelight after Hurricane Sandy in 2012, when the American Model [Global Forecast System (GFS)] falsely forecasted Sandy to track offshore, and the European ECMWF model correctly predicted Sandy to make landfall in New Jersey.  Just like the various hurricane products described above, the weather models themselves are under ongoing improvements.  One item to watch this hurricane season is the upgrade to the GFS model on July 12th.  As part of these upgrades, the NOAA asks forecasting divisions like the NHC to run performance tests.   Although the tests might suggest better temperature forecasts or precipitation forecasts for different areas of the globe, one area where the upgraded GFS shows deficient skill is with hurricane forecasting.   The report I have seen from the NHC suggests for the 2014-2016 retrospective runs, in comparison to the 2016 GFS model, the 2017 GFS showed a 9-10% degradation in track forecast skill at 48-72h in the Atlantic Basin.  In terms of intensity forecasting, the 2017 GFS showed degradations in intensity forecast skill at nearly every forecast interval out to 120 hours in the Atlantic Basin.  The new GFS model also indicated less run-to-run consistency.  It likewise showed little overall improvements in TC genesis forecasts for the Atlantic Basin.

Evaluation of the proposed 2017 GFS implementation done by the NHC in February 2017

Another troubling factor is that the GFS model will likely have other fallouts with the regional models, such as the HWRF and GFDL, and the statistical models such as the GFEX.

Here is the quote from the NHC about the upgrade to the GFS model:

“The loss of short- to medium-range TC track and intensity forecast skill for the Atlantic basin in the proposed 2017 GFS is unacceptable to the National Hurricane Center.  We are also concerned about the lack of testing of the downstream impact of the 2017 GFS on the regional hurricane models.  Therefore, we oppose this implementation.”

As the insurance industry watches all of the various hurricane forecast model runs to determine where a hurricane might track, it might be good to put a bit more weight in the ECMWF model this season.  However, the ECMWF will also be upgraded on July 11th and very few people know what these upgrades will do to its hurricane forecasting.

Detail of this model upgrade can be found here:

http://www.emc.ncep.noaa.gov/gmb/noor/GFS2017/GFS2017.htm

May Tropical Update

Seasonal Hurricane Forecast Skill

Seasonal hurricane forecasts, with varying lead times, have been produced in the Atlantic basin since 1984 by the late Dr. Gray from Colorado State University (CSU).  Partly as a result of the early success of those forecasts, seasonal named storm forecasts are now a dime a dozen with many different research and operational groups making seasonal hurricane forecasts for tropical basins around the world.  Although there are some very brilliant minds making these forecasts, some might argue there has been little skill improvement over the last several years. The insurance industry is accustomed to using climatology to understand the risk of a U.S. landfalling hurricane.  This climatology also suggests, on average, the more active the overall Atlantic basin hurricane season is, the greater the probability of U.S. hurricane landfall which can be useful guidance to the insurance industry heading into the hurricane season. However, when it comes to seasonal landfall forecasts, really no skill has been demonstrated; this is ultimately what matters for the insurance industry.  In fact, there isn’t a forecast group that has skillfully predicted the current drought in major hurricane strikes over the last 11 years.

CSU Atlantic Basin wide hurricane forecasted and observed hurricanes with missed forecasted number shown.

CSU provides easy access to historical forecasts which allows for simple validation of there seasonal forecasts.  In a simple analysis, the CSU forecast has done fairly well with their April predictions when compared to using historic climatology (6.5 hurricanes per year) .  Fourteen CSU forecasts since 1995 have come within three hurricanes of the seasonal hurricane total in the Atlantic Basin.  On seven occasions, they have only missed the cumulative number by one.  They hit the mark in 2008.  Of course, CSU have posted a few wayward forecasts as well: 2005 (undershot the total by eight hurricanes) and 2012 (fell short by six hurricanes).

CSU Atlantic Basin wide major hurricane forecasted and observed hurricanes with missed forecasted number shown.

Regarding the major hurricane forecasts, CSU has made seven forecasts that failed to fall within two storms of the final tally. This should not obscure the fact that they’ve also posted four perfect forecasts of major hurricane occurrence and 13 others that approximated the seasonal total within one storm.
It is very easy for one to sit back and grade objective season hurricane forecasts like CSU, and there is no doubt a lot of work goes into the refinement of these forecasts each year. I am a firm believer that seasonal forecasts and even landfall forecasts will improve over time. Currently, many of these seasonal forecasts are statistical schemes which will inherently fail some years. There is some promise that a hybrid of statistical and dynamical forecast could be the future of seasonal hurricane forecasting.

https://www.gfdl.noaa.gov/hyhufs/

The Real Value – Landfall Forecasts

As mentioned, the real value to the insurance industry is in getting an accurate picture of potential landfalling storm activity. CSU issues probabilities for landfalling hurricanes across various regions of the U.S. This year, they are forecasting a 75% probability of a hurricane landfall. The probability of landfall for any one location along the coast is very low and reflects the fact that, in any one season, most U.S. coastal areas will not feel the effects of a hurricane no matter how active the individual season is. And, given the variation of exposure along the coast, providing accurate estimated insured loss ahead of season is very difficult if not impossible.
However, given the current state of the science of seasonal forecasting it is possible to provide guidance to which area of the coastline will see a higher than normal probability of landfall. This year, landfall forecasts may prove to be one of the more challenging years. This is due to the large amount of uncertainty of many of the climate forcers used to predict Atlantic named storm activity. One of the most common climate forcers used is El Niño Southern Oscillation (ENSO) and, this year, many forecasts are uncertain if an El Niño event will take hold during the summer, which historically suppresses hurricane activity. CSU thinks an El Niño will be weak or moderate by the peak of the Atlantic hurricane season based on their April forecast. However, as mentioned a few weeks ago, spring El Niño forecasts can be notoriously misleading, and result in difficult forecasts. (This phenomenon is so familiar that it has its own name – the “spring predictability barrier”.) Over the last few months many of the monthly climate models have backed off of the idea of a strong El Niño during the peak of the Atlantic hurricane season and now have a weak to neutral ENSO condition. If the El Niño is weak, it could result in increased landfall chances, and El Niño historically has little to do with any activity in the Western Caribbean / Gulf of Mexico and any activity in this region increases the chances of U.S. landfall.

Official Climate Prediction Center ENSO probability forecast for Mid-April (Top) and Early May (Bottom) Showing in just a half of month the probability of a El Niño during the hurricane season (Gray Box) went form 70% to below 50%. There is an increasing signal that ENSO will be neutral during the hurricane seasons.

The other challenge this year is that earlier this spring, water in the far North Atlantic and water off the coast of Africa had a relatively cold signal which is potentially indicative of a negative phase of the Atlantic Multi-Decadal Oscillation (AMO).  However, over the last month or so, the Atlantic water has warmed substantially.  This continues the debate as to whether the AMO has switched to a negative phase.  The U.S. government suggests the Atlantic is still in a positive state, whereas the CSU data suggests the AMO has trended negatively over the past few years.  If water continues to warm over the main development region, like it has over the last month, it could allow more storm formation in the heart of the hurricane season.

NOAA SST anomaly (degree C) for 4/10 (left) and 5/11 (right) showing a warming of SST off the west coast of Africa off the East Coast of the US. The cold SST in the North Atlantic have expanded and SST after months of above normal in the Gulf Of Mexico have recently turned colder than normal which should be short lived.

Another factor that has had a large influence on named storm activity over the last few years has been the Saharan Air Layer (SAL).  Named storms interact with the SAL in several ways.  Some named storms get embedded in the SAL their entire life-cycle and often struggle to intensify beyond strong tropical storm strength.  Other named storms can be overrun by the faster moving SAL and are quickly weakened.  The SAL in general can hamper convection, making it hard for named storms to develop.  This year could be similar to the last couple of years which would hamper development of named storm activity in the main development region but, as tropical waves get closer to the U.S. and Caribbean, the SAL decreases, and conditions become more favorable for development outside of the SAL.

SST anomaly for 5/11 showing warmer than normal SST around Hawaii.

Not to forget about Hawaii, which is also a target for named storm activity and, like the U.S. mainland, has gone a long time without a major hurricane impact.   This year, the insurance industry should also keep an eye on this 50th state as, just like the warm sea surface temperature (SST) off the East Coast of the U.S., the SSTs are above normal near Hawaii, and any tropical activity that might come close to the islands could be enhanced by these warmer than normal SSTs.

Summary
Forecasting of Atlantic named storm activity is not easy, and CSU has demonstrated variability in skill year over year, with no real improvement in the April seasonal forecasts over the last several years, but overall there is skill improvement over using basic climatology. Although some forecasts are currently calling for below normal activity in the Atlantic basin, climate forcers are pointing toward the ideas that these forecasts will be adjusted upward as we approach June 1 to a more active or normal season. The major climate forcers suggest there could be less development in the main development region of the Atlantic but, as tropical waves move closer to the U.S., they could have a better chance at developing, due to lack of wind shear and lower SAL environment. SSTs near the U.S. coastline are also expected to be warmer than normal which would add fuel to any disturbances that develop. Therefore, parts of the U.S. coastline have a higher probability of a landfalling named storm this season, with the Mid-Atlantic and Northeast seeing the highest chance.
Resources:
Barcelona Supercomputing Center has a nice summary of the various Atlantic Hurricane Forecasts
http://www.bsc.es/ESS/seasonalhurricanepredictions/
NOAA Climatology Products – Avg Start Dates, Return Periods, Develop Origin by Date, Strike Density
http://www.nhc.noaa.gov/climo/
CSU U.S. Landfalling Hurricane Probability Project
http://www.e-transit.org/hurricane/welcome.html

May Severe Weather Update

The Onslaught of Severe Weather
Last week I posted some general ideas of what to expect over the next few months and briefly touched on U.S. severe weather. In this update I will provide a bit more detail on 2017 severe weather and what to expect over the next few months.
As mentioned in my last blog, this year is off to a record pace in terms of severe weather reports and U.S. insured loss. In fact, if you think a major severe weather event is occurring about every week so far in 2017, you’re likely correct as there have been only four severe weather free weeks so far in 2017 according to PCS wind and thunderstorm event designations.

Highlighted dates in 2017  in which PCS has a wind  thunderstorm designations

Historically, in terms of number of PCS wind thunderstorm events, the activity has been unprecedented with 19 events so far, which is 271% above the average number of events that have occurred since 2000.

Number of PCS event designations by year as of May 1

 

January – April Wind and Thunderstorm PCS losses in billions adjusted for CPI by year.

However, in terms of insured loss it is difficult to estimate how the recent PCS events might develop. Given the wide scope of impact, it would be safe to say at least another billion dollars (or two) could still be expected to develop from existing PCS designated events that have occurred at the end of April.  If this development occurs, insured loss through the end of April would be at a historically high level. However, it would be far lower than the costly severe storm year of 2011, where the loss was driven by the deadly April tornadoes in Alabama and the Joplin, Missouri tornado in May.  This highlights the remarkable luck that has occurred with tornado related insured loss over the last several years and especially this year since reports of tornadoes are running above normal for the first time since 2012.

Since the U.S. has not experienced a major marquee tornado loss this year, most of the insured loss continues to be related to hail or localized wind damage with smaller tornado losses mixed in such as the East New Orleans tornado on February 7th of this year.   Below is a break down how insured losses have compared to 2016 by state thru May 1st, keeping in mind further development of 2017 losses is expected.  As of right now Texas, which last year saw 47% of the total reported U.S. insured loss, is reporting a lower level of loss as of May 1 compared to last year at this time.

Insured PCS Loss Difference (%) from 2016

Possible Cause of Severe Weather
The main stream media continue to put focus on El Niño–Southern Oscillation (ENSO) and its possible impacts on this year’s severe weather, but severe weather cannot be tied to just one atmosphere climate forcer. As mentioned in my previous post, several bits of research have been done around this relationship of Gulf of Mexico Sea Surface Temperatures (SST) and severe weather Convective Available Potential Energy (CAPE). Currently, the SST, are at record high levels, which appears to be helping provide extra fuel for any storm system that traverses across the U.S.

 

However, another hypothesis is the relationship the Rocky Mountains and Northern Plains snowpack has on severe weather and tornado occurrence.   There is little research around this connection, but a quick crude analysis shows there is possibly a connection here suggesting in years when May snowpack is below normal in the Colorado basin U.S. tornado count in May is also below normal.  This is not the case this year with near record snow pack across the Rocky Mountain; however, the correlation between above average snowpack years and tornadoes is not as clean.  Combining this theory with warmer than normal Gulf of Mexico SSTs creating warmer, moister air and the extensive spring snowpack in the Rocky Mountains provides an interesting hypothesis that would be a great master thesis for some young inspiring meteorologist.

Forecasted Severe Weather

As shown with the severe weather losses and number of events the first few months have been active, but the first few months of the year historically make up only 19.5% of the severe weather activity on average as recorded by NOAA Local Storm Reports (Tornado, Hail, Wind). An active January thru April can quickly be superseded by a quiet May, June, & July. In attempting to understand if early activity will lead to an overall above active year, I used a trailing 17 year average to find above and below average periods. In this sample approach, over the last 17 years, 7 years were above average in the January-April period. Of the years that had an above average January – April reports of severe weather, only one year (2016) went on to have above average numbers for the remainder of the year, the remainder of those years ended quieter then normal.

In fact other researchers have done some similar studies that suggest a fast start does not necessarily mean the reminder of the season will be active.

Source: NOAA – Michael Tippett

Annual PCS January – April loss development as a percent of yearly total loss.

In terms of insured loss as of May 1, historically the U.S. insured loss is developed at 42% and we know there will be further development of 2017 losses that have occurred at the end of April. Regionally, it would appear severe weather will continue to be a common occurrence across the Southern Gulf States into the Carolinas into June. More typical periods of warmth across the Northern Plains will trigger severe weather into the summer. However, the stormy periods are not expected to last long like the current persistence pattern the south has seen so far this year.

Note on Wildfire Risk
Florida will continue to see a higher risk for wildfire, but after June the risk could shift to Western states. Despite significant rain and mountain snow across California early this year, wildfires will still pose a threat this summer. Significant precipitation has led to abundant vegetation which can serve as fuel for fires. Early in the season, heat may be inconsistent across California, but temperatures are predicted to rise in July, which will likely dry out this new vegetation and increasing the chances for fires.

BMS Seasonal Outlook April 2017

Summary:

  • An El Niño is forecasted to emerge for late this summer, but weather patterns suggest that it has already arrived.
  • Late spring-early summer warming will occur over the central and eastern U.S.; then, temperatures will trend cooler into the summer for the northern plains.
  • Heavy spring rains across the Gulf States and into the Great Plains will accompany storm systems, but nothing like past record years (like 1993).
  • Best matching analog years: 1982, 1986, 2004, 2006 and 2014

The Pattern

There has been no shortage for the insurance industry of severe weather during the first quarter of 2017. A warmer-than-normal start to the year, aided by record Gulf of Mexico sea surface temperatures, has produced a relatively active period of severe weather. Will this weather pattern continue into the next few months, or are we on the road to a quieter-than-normal weather pattern?

Severe weather reports from Jan 1 – April 20, 2017. Bar graph shows the number of reports per day so far this year.

In reviewing the first three months of 2017, the weather patterns have been dominated by Pacific influences, with storm after storm pounding the West Coast. As these storm systems moved across the U.S. and into the central U.S., they were enhanced by tapping a large amount of warm air from the Gulf of Mexico, which also produced warmth across the eastern U.S. This weather pattern has provided plenty of opportunity for severe weather, and resulted in several significant severe weather outbreaks that impacted the insurance industry. Some of the bigger events occurred on January 2, 21-23, February 28-March 1, March 6-7, and most recently, April 2-3.

So far in April, it would appear this weather pattern has continued the pattern established in March, with a series of infrequent, but very energetic storm systems digging into the western U.S. before lifting up into the mid-west and northeastern U.S.  This has meant that much of the U.S. should continue to experience similar above-normal severe weather activity; but these storm systems should start traversing a more northern track across the northern-tier states.  This pattern indicates that June-like weather might appear in April and May.

Long-Range Forecasts

Many long-range climate forcer signals can provide seasonal forecasters clues about what weather to expect over the next few months.  The El Niño-Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO), and Atlantic Multidecadal Oscillation (AMO) are some of the most common signals.  Although ENSO seems the most popular climate forcer in recent seasonal forecasts, a few words of caution are required:  fickle spring weather is notoriously misleading and results in difficult forecasts (this phenomenon is so familiar that it has its own name, the “spring predictability barrier”).   Second, we are technically coming out of a weak La Niña (which has a traditionally predictable outcome), but the atmosphere isn’t following its typical pattern following a weak La Niña.  Instead, the weather pattern over the continental U.S. reflected more of an El Niño pattern, with widespread warmth across much of the country and above-normal precipitation across parts of the southern tier and West Coast states.

The PDO has been positive for a record 39 months (during a positive phase, the west Pacific becomes cooler and part of the eastern ocean warms), but the weather pattern over the western U.S. has been anything but reflective of a positive PDO pattern.  Usually a positive PDO will lead to warmer and drier conditions in the western U.S., but this has not been the case over the last few months.

Weekly U.S. Drought Monitor, which uses a composite index on the level of drought that is occurring

Seasonal forecasting can also help us understand current areas of drought.  Large areas of moderate-to-severe drought can create a positive feedback loop, meaning that already-dry regions are more susceptible to warm and dry weather than non-drought areas.  As illustrated by the above map, patches of drought are currently scattered throughout the country, but these scattered patches are too insignificant to raise red flags for long-term warm and dry conditions over next few months.

Research suggests that the continued warm water temperatures in the Gulf of Mexico will likely mean severe weather will continue to erupt over the southern and central U.S. into late spring.  Historically, when the Gulf of Mexico trends warmer than normal, there is more energy to fuel severe storms and tornadoes.

Convective energy is needed for storm development and correlates to storm activity.

The Verdict

Evaluating recent weather patterns and various climate forcers produces the following analog years: 1982, 1986, 2004, 2006, and 2014.  These years suggest an active storm pattern that may result in:

  • Much of the U.S. experiencing near-normal temperatures between April-August
  • The western U.S. and southeast warmer then normal
  • The northern states slightly colder than normal

Combining the analog years yields the following temperature and precipitation anomalies. Note the temperature scale is less than a 1 degree +/- long term average.

However, above-normal national temperatures now and into early summer should give way to more normal temperatures or cooling temperature patterns in the Great Plains later in the year.  Hit-or-miss precipitation across much of the country will be a by-product of severe weather and will provide rain in some areas but not others.  As a result, dry conditions in the southeast could progress into summer.  Severe weather should remain active until May, with activity waning to more normal levels as summer progresses; but overall, the insurance industry should expect to see much higher levels of insured loss than in the last few years.

 

BMS Tropical Update 4/20/2016 4 PM CDT

You might have noticed that the first named storm (Arlene) of the 2017 Atlantic Hurricane season has formed in the middle of the North Atlantic Ocean – 815 miles west of the Azores Islands.   The biggest impact from this storm will be the discussion in the meteorology community as to whether this system should even be monitored by the National Hurricane Center.  Currently Arlene is over relatively cold ocean water (66 F (19 C)) for a tropical system, and according to satellite data, it is questionable if the system has a warm core.

IR Satellite image show the system is over 19C water.

Typically a tropical cyclone is characterized by lack of warm/cold fronts attached, a “warm core” (air is warmer in center of the cyclone than elsewhere), and persistent deep convection wrapped close to the center; these attributes are commonly referred to as “tropical characteristics” of a cyclone.

It would appear that the decision to track Arlene is very subjective: there have likely been dozens of similar systems over the last 100 years (systems that develop in water warmer than 66 F and with a marginally warm core) that have gone unclassified, including systems that have impacted the U.S. (Sept 2008 SC coast, Sept 2009 NJ).

All this matters to the insurance industry because our hurricane catalogs in the catastrophe models are tuned to the historical data, and after decades there is still no objective guidance as to what type of system gets tracked by the NHC. In some cases like Sandy, questionably categorized storms can have large impacts on landfall definitions as well.

Regardless, Arlene is now in our record books as a storm in the North Atlantic, but it will not be around for long as a strong mid-latitude trough will merge with this system later this weekend and make it unrecognizable. These early-season tropical systems should be no surprise (this will be the sixth early-season storm in the past six years). In fact, history suggests that tropical/subtropical systems in April are uncommon, but not necessarily rare. There have been a few, but they are typically short-lived and innocuous.

Historical Storms that have occurred in April

I am currently tracking the various early season forecasts for the 2017 Atlantic basin season.  I should have my views formulated in a couple of weeks.  What I can tell you right now there is no correlation for April storms and the rest of the season, partly because of a small sample size.