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Tropical Storm Danny not impressive yet

By: Dr. Jeff Masters, 7:19 PM GMT on August 26, 2009

Tropical Storm Danny is here, born from an African tropical wave that developed a closed circulation and some respectable heavy thunderstorm activity this morning. Data between 2 - 3 pm EDT from the Hurricane Hunters indicates the Danny has a central pressure of 1009 mb, and top surface winds of about 45 mph. Satellite imagery shows that Danny's circulation has gotten stretched and pulled away from the heaviest thunderstorm activity on the storm's east side. It is possible that Danny's center will reform at a different location, closer to the heaviest thunderstorms, so the model forecasts probably have a higher degree of uncertainty than usual at present. Wind shear has dropped to the low range, 5 - 10 knots. The upper-level low Danny is moving underneath has plenty of dry air in it, and this dry air is intruding into Danny's west side, keeping and heavy thunderstorm activity from developing on that side.

Figure 1. Oceanic heat content for August 25, 2009. Regions where the Sea Surface Temperature is below 80°F, or where the shallow waters of the Continental Shelf lie, do not have enough heat content to be plotted, and are shown in white colors. The oceanic heat content along the track of Tropical Storm Danny is below the threshold of 90 kJ/cm^2 typically seen with cases of rapid intensification. The storm position at 8am EDT today is given by the hurricane symbol with the "0" inside it. Subsequent hurricane symbols show the official NHC forecast points from the 11 am EDT forecast today, for 8am Thursday (24 hours), 8am Friday (48 hours), 8am Saturday (72 hours), and 8am Sunday (72 hours). The cold waters stirred up by Hurricane Bill last week will not impact Tropical Storm Danny. Image credit: RSMAS, University of Miami.

The forecast for Danny
As Danny continues to plow through the upper low, the low will weaken, but will dump dry air into the storm through Thursday afternoon, slowing down development. By Thursday night, when Danny should be several hundred miles off the coast of northern Florida, the upper-level low may be weak enough and far enough away that Danny will find itself in a region with light upper level anticyclonic winds, which would favor more rapid development. Ocean heat content (Figure 1) is high enough to support a hurricane, until Danny gets to about New Jersey's latitude. Most of the intensity models, including the GFDL, HWRF, and SHIPS model, forecast that Danny will become a hurricane by Friday. However, this favorable environment will not last long, since a strong trough of low pressure will be approaching the U.S. East Coast on Friday. This trough will bring high wind shear of 20 - 30 knots by Friday night. This trough should be strong enough to turn Danny to the north, then northeast on Saturday. The models have come into better agreement keeping Danny offshore as it passes North Carolina, though the storm is certainly capable of giving the Outer Banks a direct hit. As Danny passes North Carolina, it should start heading north-northeast, with a landfall likely Saturday afternoon or evening somewhere between Massachusetts and Nova Scotia. At that time, Danny is likely to be a strong tropical storm or weak Category 1 hurricane, with winds in the 55 - 80 mph range. The latest runs of the GFDL and GFS model have Danny tracking directly over Martha's Vineyard, where President Obama is on vacation. It will be interesting to see if the president stays on the island for Danny.

For those of you wondering about specific probabilities of getting tropical storm force or hurricane force winds, consult the NHC Wind Probability Product.

Figure 2. Tropical wave newly emerged from the coast of Africa, with some potential for development.

Elsewhere in the Atlantic
A well-organized tropical wave has exited the coast of Africa, and lies a few hundred miles southeast of the Cape Verdes Islands. None of the models are gung-ho about developing this wave, but shear is low, 10 knots, and waters are warm enough to support development. The dry Saharan Air Layer is relatively limited in extent and intensity, so dry air may have only a small inhibiting effect on the wave. Expect some slow development of this wave as it moves westward over the next few days. NHC is giving this system a low (less than 30% chance) of developing into a tropical depression by Friday afternoon.

I'll have an update Thursday.

Jeff Masters


The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.