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BMS Tropical Update 10/10/2016 12 PM CDT

Now that Matthew’s story is complete, immediate attention will turn to Nicole, a tropical storm currently 450 miles south of Bermuda. The models generally agree that Nicole will slowly strengthen back into a hurricane and that there is a good chance that Matthew will become a strong category 1 or weak category 2 hurricane as it tracks close to Bermuda this Thursday.
So with no tropical troubles threatening the U.S. coastline in the immediate future, this is the time of year that the insurance industry often wonders if there is any other tropical trouble forecasted for the remainder of the year.

Climatologically over the next two weeks we tend to see stable named storm activity, but after October 18th, the activity in the Atlantic Basin drastically trails off.

Season

This is also roughly the time of year that the West African – Cape Verde type hurricane season trails off due to the equator-ward shift of the African Monsoon. This past weekend the overall tropical rainfall seemed to shift below 10 degrees north latitude, which is the benchmark for tropical waves coming off Africa to obtain enough spin to become named storms.

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Here is a look at the tropical rainfall totals since mid July. It shows the height of the African Waves during middle of August with a trailing off over since this period. The axis to the left is latitude showing a trend towards the equator.

This is also the time of year that colder Canadian troughs of low pressure air start to invade the U.S. Often these weather systems leave trailing cold fronts that sometimes extend into the southern Gulf of Mexico or Caribbean. Low pressure can form at the tail end of these fronts, and if the water is warm enough in the Gulf or Caribbean, it can encourage tropical convection which could then become organized. These typical developments are consistent with some of the models’ long-range weather forecasts for later next week.

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Forecasts for Thursday morning show an area of low pressure moving across the upper Great Lakes, which could provide the first freeze of the year for the upper Midwest. This forecast also shows Nicole moving toward Bermuda.

This low pressure moves across the north Atlantic late next week, and it leaves a trailing cold front with an area of low pressure off the east coast of Mexico in the western Caribbean. This is the area that needs to be watched for tropical trouble later next week as the water in this area is plenty warm enough to support tropical convection.

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The long range forecast from Oct 17  – Oct 24th  total precipitation plot below show a decent amount of rain which could be  from  convection thunderstorms.

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The graphic below illustrates the climatological pattern that is typical of tropical storms that originate in October so the Western Caribbean is the place to watch over the next three week.

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Approaching the Peak of the Hurricane Season

Historically, September 10 is the peak of the North Atlantic hurricane season, which typically sees 10 or 11 named tropical storms. This climatology number  usually represents the last 30 or 50 years, but the average since 1995 is higher – at 15 named storms, 8 hurricanes and 4 major hurricanes – and corresponds to the so-called active era in the North Atlantic, caused by warm Atlantic Multidecadal Oscillation. With the storm Erin just being named in the eastern Atlantic Main Development Region (MDR), the storm total for this year (through to the second week of August) sits at 5. This is about two weeks ahead of climatology, which suggests the fifth named storm is often observed around August 31. The first hurricane is climatology-observed on August 10, so in terms of hurricane activity and the Accumulated Cyclone Energy will continue to fall behind the climatological norm.

In August, Sea Surface Temperatures (SST) in the MDR extending from the Lesser Antilles to the Cape Verde Islands warm significantly, which is one reason why three storms have formed in the MDR so far this year. It is also part of the reason seasonal forecasts are calling for a more active than normal hurricane season.

Figure 1 shows the SST in the MDR. Image Credit: www.weatherbell.com

SSTs are solidly 81°F off the African coast and rise slowly to 83°-84°F as tropical waves approach the Caribbean islands. However, lately the main issue is the presence of Saharan dust in the MDR.

Figure 2 is a look at the graphics from the NASA GEOS-5 model, which shows the dust that continues to occur over the MDR. Image Credit: www.weatherbell.com

I am not quite sure what is considered to be normal in terms of dust occurrence. But as a proxy in lieu of actual dust measurements over the MDR, we can look at the 400-mb- specific humidity over the last dozen years to demonstrate how dry the MDR has been (Figure 3). This dry, dusty air is not conducive to tropical development and has been the main reason why Chantal, Dorian, and most likely Erin have stayed below hurricane status and could result in less overall named storm active compared to what has been forecast for the season.

Figure 3 is the 400 mb Specific Humidity since June 1, 2013 over the MDR. Image Credit: NOAA / Earth System Research Laboratory

Over the past week many media outlets have been hyping the upcoming few weeks of the hurricane season. This is because the strong, opposing wind shear has weakened across the MDR. Furthermore, the dry, Saharan air off the African coast has begun to dissipate, compared to earlier this season. Thus, conditions in the Atlantic seem like they are quickly becoming more favorable for hurricane development, which should come as no surprise since about 80% of the season’s hurricane activity is produced in mid-to-late August and September.

Updated Seasonal Impact Forecast

A high impact for the U.S. is still expected with fewer named storms, but the newest weather pattern forecasted for the next month suggests a shift centered at a corridor near Florida rather than in eastern Florida and up the east coast. The European Center for Medium Range Weather Forecasting (ECMWF) model September forecast appears to be much wetter in the Caribbean and eastern Gulf, which supports the idea of seeing storms track in that area at this time of year.

Florida, which just had its wettest July on record (with 12.38 inches of rainfall – 4.91 inches above average), is an example of the large amount of moisture that has been observed along the east coast this hurricane season. Because wet soil can increase basement leakage and tree fall, these wet soil conditions should lead to increased losses if the area is impacted by a named storm.

Wet East Coast Increases Loss Concern this Hurricane Season

As we approach the peak of the 2013 Atlantic hurricane season (which peaks around September 10), the forecasts (for an active season) made back in April have partially panned out – if you consider the total number of named storms. Looking at the last 50 years (1976-2012), the average formation dates of the fourth named storm and first hurricane are August 19 and August 3. So in terms named tropical storms, the season is ahead of par with climatology, but slipping behind on the occurrence of the first hurricane for 2013. In terms of Accumulated Cyclone Energy (ACE), 2013 is essentially average for this date. We’re at 6.6 compared to the 1981-2010 average of 9 for this date.

With August being the month where typically the number of tropical systems ramps up and Colorado State University’s August forecast calling for an active landfalling season, the likelihood of the season’s wet soil conditions leading to increased losses from a landfalling named storm must be considered.

The Ohio Valley and East Coast were much wetter than average. June precipitation totals for 18 states – from Georgia to Maine – ranked among their 10 wettest in the historical record. The fact that this weather continued into July undoubtedly creates concern over a named stormed impacting these rain-soaked areas.

 

 

NOAA Advanced Hydrologic Prediction Service view of the past 60-day Percent of Normal Precipitation. Over the past 60 days, much of the East Coast has seen 150% – 350% of normal precipitation.

Research shows that past hurricanes have demonstrated the combined effects of subsequent excessive rainfall and a named-storm wind speed that can have a major impact on insured losses. These losses might not otherwise be represented if the subsequent seasonal rainfall was normal or below normal. This loss increase is primarily associated with increased basement leakage and tree fall. (When soil is saturated, the connection between the root plate of a tree and the soil is lessened, which can increase tree fall.) Research also shows that the average wind speed expected to snap a hard- or softwood tree trunk is a 90 mph gust. Therefore, while more intense winds wouldn’t necessarily increase the loss, lower wind speeds might – given the weakened condition of saturated soil.

Recent examples of events that might have seen increased losses due to higher soil moisture are hurricane Isabel 2003 and Irene 2011. With New Jersey and Delaware having had their wettest June on record and 18 other eastern states having had Junes ranking in their top 10 wettest, this August has seen some of the highest soil moisture levels ever recorded. And this could increase the risk of river and basement flooding as well as tree fall if a named storm were to impact the area.