Dean and Erin.

Tropical Depression 5 is now Tropical Storm Erin, as of 11:30 AM this morning. It’s expected to hit South Texas early tomorrow morning. The Houston Chronicle’s Eric Berger notes that this will be the first Texas landfall since 2003: “Rita (2005) and re-born Ivan (2004) both technically missed Texas by hitting near the Louisiana-Texas border.”

Some strengthening is expected, but Erin shouldn’t have time to get too strong. As Dr. Jeff Masters says: “Wind shear over Erin is only 5-10 knots, and an upper-level high pressure system has parked itself directly over the storm. This is an ideal situation for intensification, since the upper-level high provides very favorable outflow at the top of the storm, venting all the air forced up at the center of the storm. … Spiral banding is starting to occur, along with good upper level outflow. Erin could grow in strength rapidly. Fortunately, the storm only has about 24 hours over water, so it should not be able to become more than a 55 mph tropical storm. Heavy rains will be the main threat from [Erin].” That threat is nothing to sneeze at, though, considering that South Texas definitely doesn’t need rain. Flooding is likely.

Meanwhile, the bigger story in the grand scheme of things is Tropical Storm Dean, which has strengthened to 60 mph and, according to Bryan Woods, “is already showing the start of an eyewall structure and could become a hurricane at any time.” Luckily, a patch of Saharan dust north of storm should prevent rapid intensification in the immediate future. Says Alan Sullivan: “Some of this air will be entrained into the circulation from the north through the next day or two, and it will surely stunt Dean’s growth.” Dr. Masters “expect[s] that this dry air will impede Dean enough so that the storm passes through the Lesser Antilles as a Category 1 hurricane.” Sullivan, on the other hand, expects a burst of rapid intensification just before it reaches the islands (which is expected to happen on Friday).

Whatever happens before Dean reaches the islands, everyone seems to agree that afterward, conditions are ripe for rapid intensification. In Masters’s words, “the environment moistens, shear stays low, and the heat content of the ocean greatly increases. The 06Z run of new HWRF model is again very aggressive intensifying Dean after it crosses into the Caribbean, bringing the storm to 928 mb (Category 4) on Monday morning near Jamaica. The GFDL model is not nearly as aggressive, putting Dean at 964 mb (Category 2) Monday morning. I can’t see any reason why Dean wouldn’t become a Category 3 or 4 hurricane by the time it reaches the Cuba/Jamaica region, unless it passes very close to the mountainous island of Hispaniola.”

Berger, in Houston, is worried:

Given the warmer water and lessening wind shear, the official forecast calls for a 115-mph, category-3 hurricane [in five days], but then hastens to add that because of the favorable conditions, Dean could be “notably stronger” than this.

The bad news is that the Gulf of Mexico is still warmer (I’ll do a full analysis this afternoon), and if Dean reaches the Gulf the United States will likely face a blow from a major hurricane.

And it’s looking increasingly likely that Dean will indeed reach the Gulf. The computer models have come into general agreement on a west-northwest track, as you can see:

Again, cue Dr. Masters:

The trough of low pressure that will pass north of the islands on Saturday is now expected to be a bit weaker than earlier forecast, which should allow Dean to pass into the Caribbean on a west to west-northwest track. The trough is no longer forecast to spawn an upper-level low pressure system, which means that the danger to the U.S. East Coast north of the Carolinas is minimal. A ridge of high pressure is expected to build in after the first trough passes on Saturday, which should keep Dean on a west to west-northwest path into the middle of next week. The southernmost model solutions (GFDL, Canadian) take Dean into Honduras early next week. The more northerly solutions of the GFS and HWRF take Dean over Jamaica, then into the Gulf of Mexico. No models call for a threat to the east coast of Florida at present, but that could change once we see how strong Saturday’s trough of low pressure really will be. The NOAA jet is scheduled to make its first flight Thursday night, and by Friday morning we should have a good set of model runs that will give us a more reliable idea of Dean’s likely track. At present, it appears that Dean’s main threat to the U.S. will be to the Gulf of Mexico coast.

Berger concurs: “Although the models remain unreliable forecasting longer than five days, and Dean probably wouldn’t reach the Gulf for a week, the storm’s trend is clearly in our direction. … It remains early in Dean’s life, but it’s becoming increasingly unlikely that the high-pressure system north of the tropical storm will weaken in time for it to re-curve northward and become a ‘fish’ storm, i.e. not affect land. Indeed, if the current trend continues the East coast will be out of the woods in a day or two for a possible strike. The Gulf, however, needs to remain quite alert and closely follow Dean’s progress.”

As an aside, the Space Shuttle Endeavour is scheduled to land on August 22 — assuming it’s deemed safe despite the gash in its heat shield. That’s right around the time Dean could be reaching the Gulf or Florida.