Little change in strength for Nate and Maria
This afternoon's observations from an Air Force Reserve hurricane hunter aircraft show that Tropical Storm Nate in Mexico's Bay of Campeche has changed little this afternoon. The 4:10 pm EDT center fix found Nate's central pressure remaining near-steady at 998 mb, and winds were near 50 mph at the surface. Latest visible satellite loops show that Nate is having trouble with the dry air to its north, which is getting wrapped into the circulation and interfering with intensification, and the intensity of Nate's thunderstorms has decreased since early this morning. Since the storm is nearly stationary, it is upwelling cooler waters from the depths that are also slowing intensification.
Wind shear remains low, near 5 knots, so once Nate manages to wall off the dry air to its north, steady intensification should occur. The 5 pm EDT NHC wind probability forecast gives Nate a 42% chance of being a Category 1 hurricane on Sunday, and 6% chance of being Category 2 or stronger. Nate probably has time to intensify to a strong Category 1 hurricane before making landfall Sunday. The main hazard to Mexico will be very heavy rains that will cause flash flooding and mudslides. The computer models continue to agree that a ridge of high pressure will build in to the north of Nate over the next few days, forcing the storm westward or southwestward to a landfall in Mexico between Tuxpam de Rodriguez Cano and Veracruz. Nate is too far south to be turned northwards towards Louisiana, as some model runs were suggesting yesterday.
Figure 1. True-color MODIS image of Tropical Storm Nate taken at 12:45 pm EDT Friday, September 9, 2011. At the time, Nate was a tropical storm with 50 mph winds. Image credit: NASA.
Nate is a small storm, and is not likely to bring significant rains to Texas; only extreme South Texas near Brownsville could see an inch or so of rain over the weekend from an outer spiral band of Nate. Our latest wundermap wind forecast map from the European Center model, with the fire layer turned on, shows that Nate's wind field on Saturday and Sunday will not be large enough to fan the fires burning in Texas.
Figure 2. Water temperatures at buoy 42055 in the Bay of Campeche, about 150 miles northwest of the center of Nate, have declined steadily over the past few days due to the mixing action from Nate's winds. Image credit: National Data Buoy Center.
Tropical Storm Maria
Tropical Storm Maria is not changing much in intensity it bears down on the Lesser Antilles Islands. We haven't had a hurricane hunter aircraft in the storm since this morning, but satellite intensity estimates are unchanged from this morning, and support classifying Maria as a tropical storm with 45 - 50 mph winds. Satellite loops show that Maria's heavy thunderstorms decreased in areal coverage and intensity since this morning, but the cloud pattern is more organized now. Maria is slowly developing spiral bands in all quadrants, and the storm has an obvious spin to it that was not apparent earlier in its life. Wind shear has fallen to the low range, 5 - 10 knots, and Maria is taking advantage of the lower shear and getting more organized. Martinique radar shows heavy rains from Maria are now affecting the islands, and the pattern has more rotation to it than it did this morning. The thunderstorms are not yet well-organized into spiral bands, though.
The intensity forecast models predict steady strengthening for Maria, and I think likely that Maria will be a tropical storm with 45 - 55 mph winds in the Northern Lesser Antilles, Virgin Islands, and Puerto Rico, and develop into a Category 1 hurricane shortly after pulling away from Puerto Rico on Sunday morning. Assuming that the Virgin Islands and Puerto Rico stay on the weaker left side of the storm, winds in those islands should be at least 15 mph less than the peak winds of the storm--so, in the 30 - 40 mph range. Islands on the right side of the storm, such as Anguilla and Barbuda, will see the full force of the storm, with winds of 45 - 55 mph. The northeastern portion of the Dominican Republic should get heavy rains from Maria, but not tropical storm-force winds. It now appears likely that the Turks and Caicos Islands and the Bahamas will miss seeing the core of Maria, but they are still in the cone of uncertainty, and cannot count on this yet. The reliable ECMWF model has now joined the other models in predicting that Maria will get caught up by a trough of low pressure early next week and turned northwards between Bermuda and U.S. East Coast. This has been the usual pattern this hurricane season, and we can now have moderate confidence that Maria will miss hitting the U.S., given the high degree of model agreement on this. A threat to southeast Newfoundland in Canada is possible for late next week, though.
I'll have an update late Saturday morning.