Alex slowly organizing; a Texas or Mexico landfall most likely

Tropical Storm Alex is slowly growing more organized as it steams away from Mexico's Yucatan Peninsula. Satellite loops show that Alex's heavy thunderstorms have increased in areal extent over the past few hours, and low level spirals bands are beginning to form to the south and north. The clockwise flow around an upper-level high pressure system a few hundred miles west of Alex is bringing about 15 knots of wind shear to the storm, which is slowing intensification, and limiting heavy thunderstorm activity on the storm's northwest side. Sea Surface Temperatures are very warm, 29°C, and there is some dry air to the northwest of Alex that may be inhibiting development. The latest Hurricane Hunter center fixes, at 6:32 am CDT and 7:16 am CDT, both had central pressures of 989 mb, with top surface winds in the 50 - 55 mph range.


Figure 1. Alex over the Yucatan Peninsula on Sunday, June 27, as seen by NASA's Aqua satellite. Image credit: NASA.

Track forecast for Alex: which model should you trust?
Our most reliable computer models have come into much better agreement this morning. A consensus forecast arrived at by averaging together most or all of the tracks of our top models--the GFS, ECMWF, GFDL, NOGAPS, HWRF, UKMET, and Canadian--is pretty much what NHC always uses as the basis of their forecast. This consensus forecast has narrowed in on the region near the Texas/Mexico border as being the most likely landfall location, with the usual cone of uncertainty surrounding it. The northernmost landfall location is Port O'Connor, as predicted by the Canadian model. The southernmost landfall location is near Tampico, Mexico, as predicted by the ECMWF model. Alex's landfall time varies from Wednesday evening to Thursday morning. Which model should you trust? Last year, the best performing models at the 3 day forecast period were the GFS, Canadian, ECMWF, and GFDL. Three out of four of those models are predicting a landfall between Brownsville and Corpus Christi, with only the ECMWF model predicting a landfall well south of the Texas border. With steering currents relatively weak, the uncertainty in landfall location is high. The average error in an NHC 72-hour track forecast last year was 230 miles, which is about the distance from Brownsville to Port O'Connor. Consider also that the NHC cone of uncertainty is the region where 2/3 of the time (using the last 5 years of statistics) the center of a storm will go. That means that 1/3 of the time a storm will not be in the cone of uncertainty. Given the slow motion of Alex and the recent uncertainty of the computer models, people living just beyond the edge of the cone of uncertainty should not be confident yet that Alex will miss them.

To get the probability of receiving tropical storm force winds or hurricane force winds for your location, I recommend the NHC wind probability forecasts. The 5am EDT (09 UTC) wind probability product predicted that Brownsville, Texas had the highest odds of getting a direct hit from Alex:

Brownsville, TX: 64% chance of tropical storm conditions (winds 39+ mph), 14% chance of hurricane force winds (74+ mph). This is the cumulative probability through Saturday morning. The wind probability forecasts also include separate probabilities for each 12-hour period between now and three days from now, and each 24 hours for the period 4 - 5 days from now.

La Pesco, MX: 59% tropical storm, 9% hurricane.

Tampico, MX: 42% tropical storm, 6% hurricane.

Corpus Christi, TX: 38% tropical storm, 5% hurricane.

Freeport, TX: 23% tropical storm, 2% hurricane.

Galveston, TX: 18% tropical storm, 1% hurricane.


Figure 2. Skill of computer model forecasts of Atlantic named storms during 2009. OFCL=Official NHC forecast; GFS=Global Forecast System model; GFDL=Geophysical Fluid Dynamic Laboratory model; HWRF=Hurricane Weather Research Forecasting model; NOGAPS=Navy Operational Global Prediction System model; UKMET=United Kingdom Met Office model; ECMWF=European Center for Medium Range Weather Forecasting model; CMC=Canadian GEM model; TVCN=one of the consensus models that lends together all (or most) of the above models; BAMM=Beta and Advection Model (Medium Layer.) Image credit: National Hurricane Center 2009 verification report.

Intensity forecast for Alex
Alex is currently over a region of ocean with relatively low total ocean heat content (about 10 - 30 kJ/cm^2). By Tuesday and Wednesday, the heat content will increase to 40 - 70 kJ/cm^2, which is high enough to allow Alex to rapidly intensify. Wind shear is currently a moderate 15 knots, and is projected by the SHIPS model to decrease to the low range, below 10 knots, on Tuesday and Wednesday. The combination of low wind shear and high ocean heat content should allow Alex to intensify into a hurricane. NHC is giving Alex an 81% chance of being a hurricane on Wednesday morning, and a 17% chance it will be a major hurricane at that time. Water vapor satellite images, though, show plenty of dry air over Texas and the adjoining waters, and this dry air may turn out to be a significant detriment to Alex. Another factor limiting Alex's intensification may be that the atmosphere is more stable than usual right now--temperatures at 200 mb are a rather warm -50°C, and are expected to warm an additional 1 - 2 degrees by Wednesday. Another factor limiting Alex's intensification may be its slow forward speed. Alex is moving at just 6 mph, and it would not take much of a slackening of the steering currents to stall out the storm. A slow-moving storm tends to pull up cold water from the depths, limiting intensification. In fact, the ECMWF model predicts that Alex could stall out right at landfall on Thursday. In short, Alex has the potential to intensify into a major hurricane, but there are plenty of roadblocks that make this only a 20% probability in my estimation.

Elsewhere in the tropics
None of the reliable computers models is calling for tropical storm formation over the the next seven days in the Atlantic.

Wind and ocean current forecast for the BP oil disaster
It currently appears that Alex will not directly affect the oil slick location, other than to bring 2 - 4 foot swells to the region on Wednesday. However, because Alex is such a deep low pressure region, strong east to southeast winds of 20 - 25 knots will develop over the oil slick region today through Thursday, according to the latest marine forecast from NOAA. The resulting currents should act to push oil to the west and northwest onto portions of the Louisiana, Mississippi, and Alabama coasts, according to the latest trajectory forecasts from NOAA and the State of Louisiana. Oil will also move westward along the central Louisiana coast towards the Texas border.

Resources for the BP oil disaster
Map of oil spill location from the NOAA Satellite Services Division
My post, What a hurricane would do the Deepwater Horizon oil spill
My post on the Southwest Florida "Forbidden Zone" where surface oil will rarely go
My post on what oil might do to a hurricane
NOAA's interactive mapping tool to overlay wind and ocean current forecasts, oil locations, etc.
Gulf Oil Blog from the UGA Department of Marine Sciences
Oil Spill Academic Task Force
University of South Florida Ocean Circulation Group oil spill forecasts
ROFFS Deepwater Horizon page
Synthetic Aperture Radar (SAR) imagery from the University of Miami

Portlight continues its Haiti response
Hurricane season is here, and Haiti is not ready. Over 1.5 million Haitians are living outside in tents or under tarps, and are highly vulnerable to a hurricane. Portlight is working on constructing steel shelters out of shipping containers for homeless Haitians, as detailed in the Haitian Relief Recap blog post. Please visit the Portlight.org web site or the Portlight blog to learn more and to donate to Portlight's efforts in Haiti.


Figure 3. Still frame from the remarkable video taken inside the Haitian Presidential Palace during the 2010 earthquake.

To remind people of just how devastating the earthquake was, the Haitian government released a a href= video earlier this month showing the inside of the Haitian Presidential Palace during the mighty Haitian earthquake.

Next post
I'll have an update between 2 - 3pm CDT today, when the next set of model runs will be available.

Jeff Masters