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BMS Tropical Update 10/07/2017 11 AM CDT

Last night at 11:30 PM EDT, the NHC upgraded Nate to hurricane status about 18 hours ahead of when it was originally predicted to become a Category 1 storm. This is now the first time since 2012 that more than nine hurricanes have developed in one season. This is also the first time since 1893 that nine Atlantic named storms have consecutively reached hurricane status (Franklin-Nate). But, it should be noted that, obviously, vastly different systems are now in place than in the 19th century, which means weaker systems likely were missed before satellite and aircraft observation.

Since Nate is now a hurricane, there is no doubt it will continue to strengthen and be a Category 2 storm as it makes landfall around Gulfport or Biloxi, MS.  There is even a possibility that Nate could be a major hurricane at landfall if it undergoes rapid intensification over the next 12 hours. However, this is more difficult to predict with a fast-moving storm like Nate. Regardless, a stronger storm at landfall will increase the insured losses expected from this event, as yesterday’s estimates were based on a weak hurricane or tropical storm at landfall.

Nate continues to improve its satellite presentation as it cruises north/northwest this morning. With winds at 90 mph, Nate has maintained its gradual strengthening overnight. Nate is located 180 miles south/southeast from the mouth of the Mississippi River. Intense thunderstorms have wrapped all the way around the center, and distinct spiral bands have developed both northwest and southeast of the center. Strong outflow is noted in all directions, which indicates low wind shear. It will be a race against time to see how much Nate can intensify before it moves onshore and weakens.

Nate’s Landfall Impacts
The general landfall area continues to narrow with an expected location between New Orleans, LA and Mobile, AL with the consensus of a landfall location near Gulfport or Biloxi, MS. The main question continues to be: how much strengthening can happen in the next 12-15 hours before landfall?  Nate is already close to a Category 2 hurricane.

Wind Impacts
What we do know is based on observations from the hurricane hunter – the overall wind structure is quite lopsided. This is likely due to Nate’s fast forward motion of 26 mph. This will cause Nate to have a small core of hurricane-force winds only to the northeast of its center.

Cross section of Nate showing pressure and wind. The wind profile clearly shows that the stronger winds are to the right side of the storm with weaker winds on the left side of the storm center. This is likely due to Nate’s rapid forward motion.

There will be little, if any, hurricane force winds to the west of the storm which would be good for New Orleans and cities to the west side of the landfall location. Tropical storm-force winds will extend well east into Alabama, and possibly the Florida panhandle, as Nate makes landfall around 7 pm CDT.  This is also much sooner than expected based on Nate’s rapid forward motion.

Because of Nate’s swift forward motion east of its center, locations further inland could experience higher winds, versus a typically slower-moving storm, as Nate will be further inland before it weakens over land.

As with all landfilling hurricanes, weak tornadoes are the primary threat away from the center of the storm. However, due to the incredibly warm, moist nature of hurricanes, hail is not expected to be an issue.

BMS iVision allows clients to run risks to better understand various impacts from Nate. This is the 3 sec gust wind speed, which shows only coastal areas will see the strongest winds. However, tropical storm-force winds will be widespread, particularly along the eastern side of the storm. Given current fwd speed (26 mph), concerned about higher inland wind potential in southeast as well.

 

BMS iVision allows clients to run risks to better understand various impacts from Nate. This is the 1 minute wind speed, which shows only coastal areas will see the strongest winds. However, tropical storm-force winds will be widespread, particularly along the eastern side of the storm. Given current fwd speed (26 mph), concerned about higher inland wind potential in southeast as well.

Surge Impacts
As with any northward moving hurricane, the onshore winds will cut across a wide swath of the Northern Gulf Coast from southeast Louisiana to the Florida panhandle. These onshore winds will pile water up against the coastline, resulting in storm surge flooding. As I mentioned yesterday, the slope of the Gulf Coast is quite flat, meaning that it doesn’t take much storm surge to cause issues. The factor that could limit the amount of storm surge is Nate is an extremely fast-moving system, meaning that it won’t have prolonged winds along the shore, thus limiting the storm surge. Currently, Mobile Bay seems to be the target for the highest storm surge amounts, which could be close to nine feet. One unfortunate coincidence is that the moon is full and high tide will occur around midnight locally, which coincides with landfall and peak storm surge, maximizing coastal surge.  At this time this does not look to be a big surge event for New Orleans as higher impacts should be greater in MS and AL coastal locations.

NHC Potential Storm Surge Flooding Map. These will get updated here: http://www.nhc.noaa.gov/refresh/graphics_at1+shtml/091740.shtml?wsurge#contents

Rainfall Flooding Impacts
Rainfall will be heavy as this system moves onshore, especially near the center, but could extend well east of the center to the Florida peninsula. As with the surge, the fast motion of the storm precludes any extreme impacts from rainfall, but flooding should be expected whenever 3-8″ of rain occurs. Also note that the heavy rain will continue north into the Appalachians.

Forecasted rainfall over the next 5 days. This shows up to 8″ of rain is now forecasted for parts of the eastern slopes of the Appalachian Mountains. This could create isolated flash flooding.

Insured loss
Over the last few days, I have provided a few analogs that were from tropical storms like Lee in 2012 to Cindy in 2005 and even Isidore in 2002. All of these analog events resulted in insured losses of under $300 million. However, with Nate now likely a Category 2 at landfall, a comparison to a higher analog storm such as Ethel in 1960 seems more appropriate, which today would be closer to a billion dollar insured loss event. Fortunately, it’s highly unlikely Nate will be worse than Opal was in 1995, which today could be close to a 3 billion dollar insured loss event. The intensity at which Nate makes landfall will dictate the amount of insured loss, but an insured loss closer to a billion dollars is more credible with a stronger storm at landfall.

Next Tropical Trouble

A strong suppressed Kelvin wave is forecast to push across the Atlantic this week. After Nate, there is a good chance of a lull in activity until late Oct.  There is a chance that  something manages to spin up in the sub-tropics, which are less impacted by suppressed Kelvin wave passages.  Overall Nate looks to be the last U.S. landfalling hurricane at least for the next two weeks.

 

BMS Tropical Update 10/06/2017 12 PM CDT

Nate’s overall structure is finally beginning to improve, based on satellite imagery and observation from a hurricane hunter aircraft. Strong thunderstorms have developed over/near the center of the system, and organized outflow is developing in the upper levels. Banding structures are now beginning to organize around the system. This is a marked change from late yesterday afternoon when thunderstorms were disorganized and weak. The storm is now over the warm waters of the western Caribbean and further intensification is expected.

Nate’s track forecast continues to wobble around, but the general expectation remains consistent. The storm will either make landfall on, or barely miss, the Yucatan Peninsula today before tracking north-northwest into the Gulf of Mexico. A turn north and then northeast is expected on Saturday before a landfall somewhere between Lake Charles, LA and Panama City, FL around 1 am Sunday morning. The storm’s remnants will then race northeast and bring heavy rains to the Mid-Atlantic and parts of New England.

Nate’s Forecasted Intensity
The biggest question mark continues to be just how strong Nate will be at landfall. I focused on this in yesterday’s update, but it is worth discussing again since intensity is key to the overall insured impact at landfall.

As I have been showing, warm waters and low wind shear are the only two factors that suggest strengthening may occur. Dry air and land interaction with the Yucatan Peninsula are the two opposing factors that may cause Nate to be weaker as it moves into the southern Gulf of Mexico. It appears that the dry air will be the primary inhibiting factor in terms of Nate’s intensity prior to landfall in the U.S. The last several runs of the models show dry air wrapping into the system from the west and south as it moves closer to New Orleans tomorrow. If this dry air can wrap into the inner core of the system, it will keep a lid on Nate’s intensity. However, if an inner core can organize today before the dry air really kicks in, and if interaction with the Yucatan doesn’t disrupt this inner core, the storm may be able to continue strengthening despite the dry air.

The majority of ensembles suggest that Nate will have a pressure of 988 – 999 mb at landfall, which is what we would see in a weak hurricane or tropical storm.

Nate is expected to make landfall in 48 hours, so note the pressure forecast from the ensembles during this forecast time period.

Nate’s Forecasted Landfall

The last few model runs of the GFS suggest that land interaction with the Yucatan will be less of a factor as the track has shifted back to the east over the last 24 hours. This may cause Nate to be stronger than forecasted this afternoon, as it tracks close to the Yucatan but not over the Yucatan.

Notice over the last 12 hours a shift in Nate’s track (Green Line) versus what the NHC had forecasted (Red Line).  This may allow Nate to track between the Yucatan and Cuba and would limit land interaction.

 

Last night’s multi-model European, American, and Canadian ensemble’s strike probabilities for Nate.

The landfall location currently appears to be near Gulfport, MS, as the models shift back east away from a New Orleans landfall.  However, the range of possible landfall locations fall somewhere between Lake Charles, LA and Panama City, FL just before midnight on Sunday morning. I would not be surprised if the models continue to shift a landfall location to the Florida Panhandle.

Landfall Impacts
Nate’s impact will extend well away from the center regardless of the inner core’s organization, especially to the east side of the storm. I expect to see strong thunderstorm activity in Nate’s outer bands as it makes landfall, with the heaviest storms moving onshore along Florida’s entire west coast, hundreds of miles east of the center. Gusty winds, heavy rains, and tornadoes will be concerns in these outer bands.

This simulated satellite forecast from the ECMWF shows strong thunderstorm activity in Nate’s outer bands moving onshore in Florida.

Even the weakest of hurricanes can cause storm surge along the northern Gulf Coast. Note in the image below the wide expanse of onshore winds from New Orleans all the way east to the Big Bend of Florida. Even if these onshore winds barely reach tropical storm-force, they will pile up water along the coast. Due to the very flat slope of the Gulf Coast, even a small rise in water level is enough to cause flooding concerns.

The extent of the storm surge will depend on the storm’s intensity, but coastal locations vulnerable to storm surge flooding will likely see at least some level of storm surge. Mobile Bay, AL, Gulf Port, MS, Biloxi, MS and Pensacola, FL are all expected to see at least three feet in storm surge, with some isolated locations expected to see six feet.  Also worth noting that landfall will be close to High Tide, which means water levels will be even higher usual.

NHC Potential Storm Surge Flooding Map. These will get updated here: http://www.nhc.noaa.gov/refresh/graphics_at1+shtml/091740.shtml?wsurge#contents

Several inches of rain may also cause freshwater flash flooding, but the storm’s fast forward motion should preclude any devastating flooding like we saw with Harvey. The map below shows total rainfall for the next 5 days across the U.S. Note the swath of higher totals along Nate’s forecasted path all the way up into New England, where the storm’s remnant low will enhance rainfall associated with a cold front.

Forecasted rainfall over the next 5 days. This shows up to 8″ of rain is now forecasted for parts of the eastern slopes of the Appalachian Mountains. This could create isolated flash flooding.

 

BMS iVision allows clients to run risks to better understand various impacts from Nate. This is the 3 sec wind gust in MPH, which shows only coastal areas will see the strongest winds. However, tropical storm-force winds will be widespread, particularly along the eastern side of the storm.  Given current fwd speed (21 mph), concerned about higher inland wind potential in southeast as well.

 

Potential For Insured loss

The insured loss will depend on the intensity at which Nate makes landfall. It is likely that only minor insured losses will occur if Nate makes landfall as a Category 1 hurricane along the Gulf Coast (under $1B in insured loss). The best analog in recent memory would be Hurricane Cindy 2005 which made landfall over the Mississippi River delta as a Category 1 hurricane, tracking north-northeast with a second landfall just west of Biloxi, MS.  Cindy caused roughly $200M (in 2016 dollars) in insured loss across Louisiana, Mississippi and Alabama. Another analog might be Isidore 2002, which, in 2016 dollars, would have caused $279M in insured loss. Arlene 2005 made landfall as a tropical storm in a similar area but did not cause a PCS loss event.

 

BMS Tropical Update 10/05/2017 10 AM CDT

Tropical Depression 16 has been upgraded as of this morning to Tropical Storm Nate, but it remains poorly organized as it interacts with the Central American coastline. Nate’s center is now just off the Nicaraguan coast. A well-defined core has not yet emerged at this stage, and any banding features are weak and transient. Unfortunately, environmental conditions are favorable for that to change in the coming days.

Forecasted Intensity
Water temperatures remain extremely warm in the western Caribbean Sea, as well as in the southern Gulf of Mexico. Tropical systems need ocean temperatures above 80F to develop and thrive, and the water in Nate’s path is well over that mark in the range of 84F to 87F.

Current Sea Surface Temperature analysis suggests waters warm enough to support rapid intensification. However, warm water is just one ingredient that is needed.  Temperatures above are in degrees Celsius.

Wind shear over the next few days will be light. Nate is expected to be located over the western Caribbean Sea by tomorrow, between Honduras and the Yucatan Peninsula. Wind shear values in this area, excluding those generated by the storm’s circulation, are near zero. Nate will have little to no trouble with wind shear as it moves northwest, and this combined with the warm water in its path leads me to believe that strengthening will occur.

The amount of land interaction that Nate has with the Yucatan Peninsula as it moves north will be critical in determining how strong Nate will be when it enters the southern Gulf of Mexico. Another potential inhibiter to Nate’s future intensity could be a bit of dry air it encounters as it approaches the Yucatan Peninsula and moves into the Gulf of Mexico. Notice the dry air stretching from Guatemala to the NE Gulf.

Current water vapor showing the amount of water in the air. The brown in the image above shows dry air in the path of Nate in the central Gulf of Mexico.

 

This is the 700 mb water vapor from the American Model initialization. This shows dry air is even at lower levels of the atmosphere, which is also backed up by the water vapor image above.

This is the dry air inhibiting development of the tropical wave near Florida this morning, and it may put a lid on how strong Nate can become as it enters the Gulf of Mexico.

Therefore, the intensity forecast remains even more uncertain than the track forecast at this time. If the storm can put together an inner core and keep the dry air at bay, intensification into a mid-grade hurricane is possible. If the storm’s inner core doesn’t become organized, or if dry air is able to work into the core, the system will likely remain either a strong tropical storm or a weak hurricane until landfall. However, as discussed yesterday, weaker systems tend to strengthen right up until landfall in the Gulf of Mexico, which appears to be possible with Nate.  Some model guidance is still suggesting that a Category 2 hurricane is possible at landfall.

However, given the inhibiting factors of the Yucatan Peninsula, the dry air and some increase in wind shear that is forecasted right before landfall, today there is a much lower possibility of Nate becoming a major hurricane prior to landfall, and it is more likely that it will be a weak hurricane or even a tropical storm at landfall.

Forecasted Landfall

Nate’s forecasted path has shifted slightly west over the last 24 hours, although there remains some uncertainty as to exactly where the storm will make landfall.  At this point, anyone from the Big Bend region of Florida over to western Louisiana should be watching this system closely.  The current model consensus takes the center of the storm near New Orleans, which is a perfectly reasonable scenario.

 

Current morning run of the American GFS model ensemble plot. The black line is the NHC OFCL forecast, which is east of most of the model guidance from this model.

 

Current morning run of the European ECMWF model ensemble plot. The black line is the NHC OFCL forecast, which is east of most of the model guidance from this model.  Since the GFS and ECMWF are all west of the NHC official forecast, I think the NHC will keep adjusting the landfall location west.

Regardless of landfall location, the model guidance on the timing of Nate’s northward track is now in better agreement with it making landfall on the Gulf Coast on Sunday morning. It should then weaken as it races northeast across the Appalachian Mountains early next week as a much weaker tropical system, bringing some much needed rain to the area. The fast movement of the system at this time is not conducive to a large inland flood threat.

Current rainfall forecast suggesting that the upward eastern slopes of the Appalachian Mountains will receive the most rain into early next week.  The amounts are all less than 5″ at this time.

Insurance Industry Impacts
Until we know Nate’s strength as it moves into the Gulf of Mexico, it is too early to determine storm surge impacts and potential insured loss. However, even a weak tropical storm such as Tropical Storm Lee that made landfall near New Orleans in 2011 caused insured loss along the Gulf Coast and inland states.

 

BMS Tropical Update 10/04/2017 9 AM CDT

Despite the lull in new activity since September 16, the hurricane season is not over yet. This is not a surprise, as even the very active year of 2005 had periods of inactivity. As I last talked about in my September 25 update, the next area to watch for tropical development is the southwest Caribbean Sea. Just as expected, the NHC is now expected to label an area of low pressure in the Southwest Caribbean Sea Tropical Depression 16 (TD 16) at 11 AM EDT.

 

The NHC is expected to send aircraft reconnaissance into this developing area of low pressure later today, and if they find strong enough winds within the embedded thunderstorm activity, it could become Tropical Storm Nate later today.

As I talked about back on September 18th, the Main Development Region (MDR) tends to shut down as October approaches and new named storm development becomes more likely to form in the western Caribbean. In fact, if you look at the landfall locations for all U.S. hurricanes during the month of October since 1851, the Gulf of Mexico coastline and Florida’s west coast have been heavily favored areas.

Landfall locations for all continental U.S. hurricanes during October since 1851. West coast of Florida gets hit fairly often. Source; CSU

Future Track of TD 16
There is growing consensus that TD 16  will track northward into the Gulf of Mexico this weekend. In fact, some of the guidance suggests a hurricane could be in the Gulf of Mexico by this weekend with a landfall as early as Sunday morning.  The model guidance is suggesting a landfall location across the eastern Gulf of Mexico, so the Texas coastline has a very low probability of landfall at this point. Florida’s panhandle has the highest probability, which goes along with the climatology shown above.

Last nights multi-model European, American, and Canadian ensemble, strike probabilities for TD 16.

It is too early to provide insured loss guidance, but we know that Hurricane Hermine caused $205 million in insured loss to the region last year.

Future Intensity of TD 16
The key to how intense TD 16  becomes will be how much land interaction it sees as it moves northward over the next few days. What’s more troublesome with the current ensemble guidance above is that many of the members are showing a track of free reign over very warm water with little land interaction.

The average sea surface temp in the area Nate is expected to develop is 84 °F a major concern for hurricane development.

The area Nate is developing is some of highest tropical cyclone heat potential in the world right now.

This would mean a stronger storm as it tracks northward. A major hurricane cannot be ruled out at this time, although most of the early guidance is keeping this area of low pressure from becoming anything more than a tropical storm or minimal hurricane as it tracks into the Gulf of Mexico this weekend.

However, the ingredients for rapid strengthening are there, with a low wind shear environment and plenty of ocean heat content and warm sea surface temperatures.

The other key to the future intensity at landfall might be just how strong the storm has become as it enters the Gulf of Mexico. Stronger hurricanes often tend to weaken just before landfall, as the shallow continental shelf allows for colder water upwelling ahead of a hurricane and deprives it of an important energy source. Lili 2002 is the most extreme example of this, where it went from Category 4 to Category 1 in only 12 hours. There are certainly exceptions to this rule though, such as Camille 1969, Eloise 1975 and Frederic 1979, which were all major hurricanes that strengthened right up until landfall.

If a weaker hurricane is in the Gulf of Mexico, however, strengthening is almost always common right up until landfall, much like we saw already this year with Harvey.