Weather Nerd

Target: Texas?

The computer models predicting Tropical Storm Gustav’s track have shifted noticeably to the left overnight, increasing the odds of a landfall somewhere along the Texas coast.

As Eric Berger writes, the models are reacting to “an area of high pressure that’s forecast to develop to the storm’s north. By Monday night the storm could begin bending westward if it bumps into the high.” The longer Gustav takes to approach the coast, the likelier — and potentially sharper — this “left turn” is.

Louisiana and points east are not in the clear yet. Indeed, the official NHC forecast still shows a Louisiana landfall — though the “center line” has shifted to the left, and nearly all of the Texas coast is now within the “cone.”

But the leftward trend in the models is significant, and it will be very interesting to see if it continues. As Berger wrote last night, “If tomorrow’s model runs confirm the westward shift we have seen this evening the upper Texas coast will have to look hard on Friday at evacuation contingencies. The fact that it’s Labor Day Weekend will only complicate matters, of course.”

Indeed. Unfortunately, the forecast remains strikingly uncertain. In fact, after it briefly looked like the computer models were “clustering” last night, the “spread” among the models has increased this morning, as the CSU “spaghetti” track map shows:


Computer models, 2pm EDT Thursday


Computer models, 8pm EDT Thursday


Computer models, 2am EDT Friday

The National Hurricane Center’s 5am EDT discussion explains the various forces at work — operating in a complex, multi-stage atmospheric pattern — in the track forecast:

THERE IS A COMPLICATED EVOLUTION TO THE STEERING CURRENTS AROUND GUSTAV DURING THE FORECAST PERIOD. THE MID-LEVEL RIDGE OVER FLORIDA THAT HAS BEEN STEERING THE STORM WESTWARD IS FORECAST TO WEAKEN TODAY. THE BERMUDA HIGH IS THEN EXPECTED TO BUILD IN FROM THE WESTERN ATLANTIC AND CAUSE GUSTAV TO MOVE MORE TOWARD THE NORTHWEST OVER THE WEEKEND. ONE BIG QUESTION IS THE EVOLUTION OF A MID- TO UPPER-LEVEL LOW OVER THE SOUTHEASTERN UNITED STATES. SOME OF THE MODELS…SUCH AS THE GFS/GFDL/HWRF…ARE SHOWING THIS LOW CREATING ENOUGH OF A WEAKNESS IN THE BERMUDA HIGH TO ALLOW A MOTION MORE TOWARD THE NORTH-NORTHWEST AS THE SYSTEM CROSSES CUBA. IT WOULD NOT BE SURPRISING IF THAT TRACK BRIEFLY OCCURRED…BUT IN THE LONGER-TERM…A HIGH OVER THE OHIO VALLEY IS EXPECTED TO CAUSE GUSTAV TO GRADUALLY BEND TO THE LEFT. HOWEVER…THERE IS A LARGE UNCERTAINTY IN THE TIMING AND SHARPNESS OF THE BEND. THE UKMET AND GFS HAVE HAD LARGE CHANGES TO THEIR FORECAST FROM THE PREVIOUS RUNS…AND GIVEN THE INCREASING MODEL SPREAD…WE’VE OPTED TO KEEP THE FORECAST SIMILAR TO THE PREVIOUS ONE. HOPEFULLY A G-IV MISSION LATER TODAY WILL HELP RESOLVE SOME OF THESE DIFFERENCES.

Hopefully so. For the moment, however, what we’ve been saying for several days now remains true: everyone from Texas to Florida needs to keep a close eye on Gustav.

As for the intensity forecast, the 11pm discussion called it “PROBLEMATIC” and noted that there was “EVIDENCE OF NORTHERLY VERTICAL SHEAR UNDERCUTTING THE OUTFLOW,” but that this was expected to subside within 24 hours, after which “SIGNIFICANT AND POSSIBLY RAPID INTENSIFICATION” would likely commence. However:

A COMPLICATING FACTOR IS THE UPPER-LEVEL TROUGH CURRENTLY SEEN IN WATER VAPOR IMAGERY OVER THE GULF OF MEXICO. THE LARGE-SCALE MODELS FORECAST THIS FEATURE TO RETREAT WESTWARD AS GUSTAV APPROACHES…WITH HIGH PRESSURE BUILDING OVER THE SOUTHEASTERN GULF. HOWEVER…THE MODELS DO NOT AGREE ON HOW MUCH THE TROUGH WILL MOVE…WITH THE UKMET IN PARTICULAR SHOWING THE POSSIBILITY OF SHEAR AS GUSTAV APPROACHES THE NORTHERN GULF COAST. THE SHIPS MODEL CALLS FOR A PEAK INTENSITY OF [115 MPH]…THE LGEM MODEL [110 MPH]…THE GFDL [130 MPH] KT…AND THE HWRF [160 MPH]. THE LATTER IS DEFINITELY NOT OUT OF THE QUESTION. THE INTENSITY FORECAST IS INCREASED OVER THE PREVIOUS FORECAST IN BEST AGREEMENT WITH THE GFDL. HOWEVER…IT WOULD BE NO SURPRISE IF RAPID INTENSIFICATION OCCURRED AND GUSTAV BECAME A CATEGORY 4 OR 5 HURRICANE BY 72 HR.

The 5am discussion is slightly less detailed on the question of intensity, and states simply:

AFTER THE STORM MOVES AWAY FROM JAMAICA…SIGNIFICANT INTENSIFICATION IS POSSIBLE. VERTICAL WIND SHEAR IS FORECAST TO BE VERY LIGHT DURING THE NEXT COUPLE OF DAYS…AND COMBINED WITH THE DEEP WARM CARIBBEAN WATERS…COULD PRODUCE A STRONG HURRICANE VERY QUICKLY. THE OFFICIAL FORECAST WILL SHOW RAPID INTENSIFICATION BEFORE IT REACHES WESTERN CUBA AND COULD BE CONSERVATIVE AS SOME MODELS SHOW CATEGORY FOUR STRENGTH AT THAT TIME. A LARGE TROUGH CURRENTLY SEEN IN WATER VAPOR IMAGES OVER THE CENTRAL GULF OF MEXICO IS EXPECTED TO MOVE WESTWARD SLOWLY DURING THE NEXT DAY OR TWO. THIS TROUGH MAY IMPART SOME SOUTHWESTERLY SHEAR ON GUSTAV AS IT ENTERS THE CENTRAL GULF OF MEXICO. ALTHOUGH THE FORECAST SHOWS SOME WEAKENING DUE TO THE SHEAR…GUSTAV IS EXPECTED TO BE A LARGE POWERFUL HURRICANE AS IT APPROACHES THE NORTHERN GULF COAST.

Eric Berger has more on the potential for possibly life-saving wind shear in the central Gulf.

No time for an update on Tropical Storm Hannah right now, but I’ll try to add one a bit later.