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Fig. 1-2: Tropical cyclone model tracks and intensity forecast.

Fig. 3-4: Areas of >85F SST support a category-2, possibly a category-3 hurricane. Satellite view of Isaias indicates a minimal hurricane after crossing Hispaniola which is a rare event.

Discussion: At 5AM EDT Hurricane Isaias was located at 20.9N/73.3W or about 385 miles southeast of Nassau. Maximum wind is 80 mph and Isaias is moving northwest at 17 mph. Central pressure is 990 MB. In a rare event, Isaias is able to maintain hurricane intensity after crossing Hispaniola. Tropical cyclone models are in general agreement on a northwest to north to north-northeast track keeping Isaias just off the East Coast (Fig. 1). Intensity forecasts have strengthened considerably and indicate some risk of category-2/category-3 intensity in the 60-84 hour period (Fig. 2) which coincides with passage over >85F ocean water (Fig. 3). There is potential for NOAA to strengthen their intensity forecast given the very warm water ahead. Satellite view reveals two large clusters of thunderstorms likely to re-emerge once Isaias is into the southeast Bahamas (Fig. 4).

The NOAA/NHC forecast track is agreeable (Fig. 5). The “caveat” forecast is a little closer to the coast (indicated by European model). NOAA forecasts 100 mph wind in the southeast Bahamas but lowers wind speeds to 75-85 mph range off the central Florida to Carolina Coast. The reason is interaction with upper trough to the northwest of the track. The concern is that initially while the storm is guided north and north-northeast by the upper trough the outflow across the storm increases and causes inflow at the surface to also increase therefore faster wind speeds. This is a common occurrence with tropical cyclones beginning the recurving process. Additionally, this event takes place over >85F water which gives credibility to some of the stronger tropical cyclone model intensity forecasts.

Isaias will bring a lot of rain to the entire east coast of the U.S. (Fig. 6-7). Heavy rains directly due to the storm occur along east coastal Florida while merging with an upper trough will toss flooding rains into areas as far north as New England Tuesday. The entire East Coast should plan on tropical storm force wind with hurricane force wind possible near the central east coast of Florida and the Carolina coast and possibly southeast Massachusetts (Fig. 8-9).

Fig. 5: NOAA/NHC 5-day forecast track for Hurricane Isaias.

Fig. 6-7: The NOAA/NHC rainfall profile associated with Isaias and NOAA/WPC 7-day rainfall forecast.

Fig. 8-9: Tropical storm and hurricane force wind probability forecasts by NOAA/NHC.