26th Conference on Hurricanes and Tropical Meteorology


Forecasting the impact of a statically stable layer in the lower atmosphere on surface wind conditions during Hurricane Isabel

Patrick F. Maloit, NOAA/NWSFO, Wakefield, VA; and J. A. Billet

Hurricane Isabel was a category 2 hurricane on September 18, 2003 when landfall occurred near Ocracoke Inlet, NC. Strong high pressure over New England and Southeast Canada prevented Isabel from taking the typical north to northeast track that parallels the east coast of the United States. Instead, the high-pressure system deflected Isabel to the northwest, through Eastern North Carolina and Central Virginia during the afternoon and evening of the 18th. By midday September 19, Isabel had tracked to near Erie, PA. The high pressure to the north provided a statically stable environment as Hurricane Isabel moved across the area. This storm downed numerous trees and caused major disruption of the power grids across the Wakefield Weather Forecast Office (WFO) county warning area, which took several weeks to repair and restore. The estimated cost from this storm will approach one billion dollars in Virginia alone.

Hurricane Isabel provided the first opportunity to view a hurricane in the Mid-Atlantic Region using WSR-88D radar 8-bit Reflectivity and Velocity data. This data revealed detailed banded structures, and discerned gravity waves that were not possible using non 8-bit WSR-88D data. This detailed data, coupled with the use of LAPS soundings, enabled forecasters to anticipate where and when peak wind gusts would affect land areas. Isabel was unusual for a land falling hurricane in the Mid-Atlantic Region, in that winds at 5 to 10 thousand feet above the surface were not mixing down to the surface due to an statically stable layer in the lowest levels of the atmosphere. This layer is indicated in the LAPS soundings, and by the presence of gravity waves in the radar 0.5 volume scans (indicating a ducting layer). It was noted that the winds from 2 to 5 thousand feet only mixed down when convective bands of approximately 35 dBz or greater were over an area.

This paper will examine the statically stable layer and how it changed as Isabel progressed through the area. This stable layer will be analyzed through LAPS soundings and cross-sections. With the higher resolution WSR-88D 8-bit radar data, smaller features could be resolved. These smaller features when combined with LAPS data provide details needed to anticipate when higher wind gusts would arrive over an area. A technique will be shown to enable the anticipation of higher wind gusts at the surface in advance of the event (increased lead time). This technique allowed special weather statements to be provided to heightened awareness to customers in local areas.

extended abstract  Extended Abstract (2.6M)

Poster Session 1, Posters
Wednesday, 5 May 2004, 1:30 PM-1:30 PM, Richelieu Room

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