By: Andrew J Siffert - August 6th, 2013
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.