By: Andrew J Siffert - August 19th, 2013
Historically, September 10 is the peak of the North Atlantic hurricane season, which typically sees 10 or 11 named tropical storms. This climatology number climatology number is 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 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 unless the current activity develops into a hurricane, 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 Verdes Islands warm significantly, which is one reason why three storms have formed in the MDR so far this year. It’s also part of the reason seasonal forecasts are calling for a more active than normal hurricane season.
SSTs are solidly 27°C off the African coast and rise slowly to 28°-29° as tropical waves approach the Caribbean islands. But lately the main issue is the presence of Saharan dust in the MDR.
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 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 forecasted.
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 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, 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.