An examination of stability changes allowing for increased gustiness over the Mid Atlantic from tropical systems

Historically, tropical systems forced onshore while moving through the Mid-Atlantic region do so because strong high pressure becomes anchored off of southeast New England. Thus, a very stable air mass is resident ahead of the system at landfall. This increased stability may slow down the mixing of stronger winds in gusts to the surface, especially as surface wind directions are oriented from the north clockwise to an easterly direction. As the tropical air changes the stability, a noticeable increase in gustiness ensues, in particular as the winds back southerly. This decreasing stability allows more mixing of the winds aloft to penetrate to the surface, increasing surface gusts. The increased gusts can play a role in the amount of damage these storms produce. This was the case as Hurricane Isabel traversed through the Mid-Atlantic coastal region in 2003. Stronger gusts contributed to more damage once this transition occurred over much of the Chesapeake Bay region.

In this study, two tropical systems, Isabel and Hanna, were examined as each moved through the Mid Atlantic region. The observational network was used to assess the timing and location of increased gustiness. This observational data was then compared with changes in stability and to the boundary layer structure from a group of mesoscale models. Such parameters as mixing height and low level lapse rates were compared to the changes in gustiness. These were examined over both time and space for each storm. With the increasing number of mesoscale models available to the forecaster, this study helps develop methods for utilizing multiple mesoscale models for improved prediction of gusts. These mesoscale models were also combined into ensemble fields, where means and probabilistic information for elements such as boundary layer lapse rate and cape were developed. From this ensemble information, techniques were derived to aid forecasters in determining gust forecasts in both magnitude and timing. Ideally, this information will help forecasters in the future to provide better tropical weather products, services, and support to the public.