| Posted by: JeffMasters, 4:38 PM GMT on August 31, 2011 |
Tropical Storm Katia continues its long trek across the Atlantic Ocean today, and is expected to arrive at a position several hundred miles north of the Northern Lesser Antilles Islands by Monday. At this time, it appears unlikely that the islands will receive tropical storm-force winds from Katia. Satellite images show that Katia is a well-organized storm with plenty of heavy thunderstorms. The storm has good upper-level outflow channels to the north and south, is under light wind shear, and is traversing warm waters, so it should be able to overcome any dry air problems by Thursday and intensify into a hurricane. It is looking less likely that Katia will affect land. Dr. Bob Hart’s Historical Tropical Cyclone Probability web page suggests shows that tropical storms in Katia’s current position have an 11% chance of hitting North Carolina, a 12% chance of hitting Canada, a 5% chance of hitting Florida, and a 62% chance of never hitting land. It will be two more days before our computer models will be able to assess the threat to land, though, as Katia is currently still very far out at sea.
Figure 2. The morning run of the GFS Ensemble prediction. The ensemble prediction was done by taking a lower-resolution version of the GFS model and changing the initial distributions of temperature, pressure, and humidity randomly by a few percent to generate an ensemble of 20 different computer projections of where Katia might go. The operational (highest-resolution) version of the GFS model (white line) is usually more accurate, but the ensemble runs give one an idea of the uncertainty in the forecast. Very few of the ensemble members are currently showing a threat to the U.S. Canada is more at risk than the U.S., according to this model.
Gulf of Mexico disturbance a threat to develop
Surface winds over the Gulf of Mexico are rising today in advance of the approach of a tropical wave currently over the Western Caribbean, western tip of Cuba, and Mexico’s Yucatan Peninsula. This wave is headed west-northwest at 10 – 15 mph, and is under a high 20 – 30 knots of wind shear. The wave is slowly beginning to build an increased amount of heavy thunderstorms, and this process will accelerate on Thursday when the wave enters the Gulf of Mexico. By Friday, when the wave will be near the Louisiana or Texas coast, wind shear is expected to drop to low to moderate levels, and the wave may be able to organize into a tropical depression. This process will likely take several days, and formation of a tropical depression is more likely Saturday or Sunday. NHC is giving the wave just a 10% chance of developing into a tropical depression by Friday morning. Regardless, this system will spread heavy rains to portions of the Gulf Coast by Friday, with the Upper Texas coast and the coast of Louisiana the most likely recipients of heavy rain. Strong onshore winds raising tides to 1 – 2 feet above normal are likely over Louisiana beginning on Friday, and coastal flood statements have been issued for the region. Three of our four top models for predicting tropical cyclone development forecast that a tropical depression will form this weekend or early next week, and I think it is at least 50% likely we will have Tropical Depression 13 on our hands by Monday. However, steering currents will be weak in the Gulf, and it is difficult to predict where the storm might go.The GFS model has a possible tropical depression forming by Sunday off the coast of Mississippi, then moving east-northeast over the Florida Panhandle on Monday. The ECMWF model forms the storm on Monday off the coast of Texas, and leaves the storm stalled out there through Wednesday. The UKMET model forms the storm Saturday off the coast of Louisiana, and leaves it stalled out there through Monday. If the storm did remain in the Gulf of Mexico for three days as some of the recent model runs have been predicting, it would be a threat to intensify into a hurricane.