Tropical Storms Nate, Maria have formed; 2011 season on the heels of 2005's numbers

Tropical Storm Nate formed in the southern Gulf of Mexico this afternoon after Hurricane Hunters found a well-defined surface circulation in Invest 96L. Nate is the 14th named storm this year, and comes three days before the climatological half-way point of the Atlantic hurricane season, September 10. A typical hurricane season has just 10 - 11 named storms, so we've already had 35% more than a whole season's worth of storms before reaching the season's half-way point. At this rate, 2011 will see 28 named storms, equalling the all-time record set in 2005. Nate's formation date of September 7 puts 2011 in 2nd place for earliest date of arrival of the season's 14th storm. Only 2005 had an earlier formation date of the season's 14th named storm (September 6, when Hurricane Nate got named.) Third place is now held jointly by 1936 and 1933, which got their 14th storm of the season on September 10.

This afternoon's Hurricane Hunter mission into Invest 96L/Nate found maximum sustained winds of at least 45 mph, and minimum central pressure of 1003 mb. Wind shear in the region is low and is expected to remain low for the next 48 hours. Sea surface temperature is toasty in the Gulf at around 30°C (86°F) and more than ample to support intensification. Nate will bring heavy rains and potential flooding and mudslides to Mexico, and according to some weather models, also has the potential to be a U.S. landfall threat. Nate's surface circulation is apparent on satellite loops, although the thunderstorm activity in the storm is displaced from the center. It appears the strongest storms are to the southwest and northeast of the center. East of the storm, thunderstorms are churning associated with the stationary front that's draped across Florida, the Gulf of Mexico, and over the Yucatan Peninsula. This stationary front is left over from the cold front that pushed south through the central and eastern U.S. earlier this week, and created a focal point in the Gulf of Mexico for Tropical Storm Nate to form. It's notoriously difficult for weather models and forecasters to predict tropical cyclones that spin up in the Gulf of Mexico, but lead-time for both Nate and our previous Gulf cyclone, Lee, was generous.


Figure 1. Infrared satellite image of Tropical Storm Nate at 6:15pm EDT.

Forecast for Tropical Storm Nate
Given the favorable environment, in addition to a very warm pocket of sea surface waters in the central Gulf of Mexico, we expect that Nate will intensify modestly over the next few days. The National Hurricane Center forecasts that Nate will become the third hurricane of the season by Friday. The HWRF and ECMWF agree with this forecast—both of these models bring Nate to a category 2 hurricane by Saturday. The IVCN/ICON consensus models that the Hurricane Center relies on are more conservative, peaking at category 1 intensity. Nate's maximum potential intensity is heavily dependent on its track, which, according to the weather models, has been up in the air for the past few of days. Until this afternoon's run, the ECMWF has held true to its forecast that Nate will track north and make landfall anywhere from Louisiana to Florida. This afternoon, it backed off of that solution and is now forecasting a northern Mexico landfall. The GFS has consistently forecasted a track that lingers in the Bay of Campeche for a few days before ultimately making dive to the west into Mexico. Over the past few days the Canadian CMC model has been reluctant to develop Nate at all, but today is forecasting the system to track north into the Southeast U.S. states. Now that there is Hurricane Hunter data to ingest (as well as confirmed 45 mph surface wind speeds), we expect the models will come into better agreement on both track and intensity for Tropical Storm Nate.

Tropical Storm Maria

Tropical Depression 14 was named Tropical Storm Maria late this morning, as well. The storm is still pretty far east in the Atlantic, 1,200 miles east of the Leeward Islands, but is moving quickly to the west at 23 mph. According to the National Hurricane Center, Maria looks well-organized on satellite, "but not really." If you look closely at satellite loops (especially the loops that you can catch before the sun sets in that area), you'll see the surface circulation is located to the west of the strongest thunderstorm activity. Well-organized tropical cyclones will be vertically stacked in the atmosphere, with the strongest thunderstorms directly on top of the surface circulation. Maria's disjointed-ness is likely due to a pretty strong clip of wind shear (30 knots worth) in the area. This would usually be deadly for a tropical cyclone of Maria's strength, but since the storm is moving so quickly to the west, the Hurricane Center is forecasting the storm to remain somewhat intact for the next 5 days, although the forecast is for no intensification. Models are coming into better agreement on the track of this system. Prior to today, the ECMWF was forecasting a track south of Puerto Rico, but has since changed its mind and is now in agreement with many of the other models on a track skirting the northern Leeward Islands and missing the Greater Antilles to the north. Beyond this, there is quite a bit of uncertainty depending on steering winds in the Atlantic. It's still too early to guess which track the models will eventually converge on. Climatology favors a track that would miss land, with Dr. Bob Hart's track history pages suggesting Maria has a 22% chance of hitting Canada, 19% chance of hitting Bermuda, and an 11% chance of hitting North Carolina.

Angela