A hurricane moving over the much warmer than normal waters of the Northern Gulf of Mexico is expected to rapidly strengthen to major status with 125 mph sustained winds over the next 12 hours before making landfall. Such rapid intensification brings with it the risk of severe storm surge flooding and damaging winds along the U.S. Gulf Coast. However, one of the most worrying features of this system is that it is incredibly moisture rich and new models now show a potential that the storm will dump as much as 20-35+ inches of rain across parts of Texas as the storm stalls over the region for 5-6 days.

Potential flood disaster: Like GFS, European model forecasts large area 17-32" of rain thru Wed in eastern Texas. https://t.co/H2pt4VJ0ty pic.twitter.com/nleFUh24oM — Capital Weather Gang (@capitalweather) August 24, 2017

(Present model guidance for Hurricane Harvey’s rainfall totals show a significant flooding potential for Texas. Note that maximum rainfall potential in this model is 40.6 inches.)

By late afternoon on Thursday, Harvey was a minor hurricane packing 85 mile per hour winds with a minimum central pressure of 976 mb and moving through the Central Gulf of Mexico on a path toward Texas. The storm was about 305 miles to the southeast of Corpus Christi, Texas and was moving toward the northwest at about 10 miles per hour. A curve toward the north is expected in the next 12-24 hours. Such a track would bring the storm adjacent to the Texas coast by some time late Friday or early Saturday.

As the storm moves north and west, it is expected to tap the warmer than normal waters of the Gulf of Mexico in the range of 87 to 88 degrees Fahrenheit and rapidly intensify into a major hurricane packing 125 mile per hour or higher winds.

(Rapidly intensifying Harvey approaches the Texas coast on Thursday. Image source: The National Hurricane Center.)

This is presently a very dangerous situation with the National Hurricane Center warning, as of the 4 PM CDT advisory that the storm is still expected to ‘rapidly intensify.’ And that peak intensity estimates could be conservative at this time. The storm will then bring 6-12 foot or higher surges to parts of the Texas Coast before moving slowly inland.

We should point out that some of the model guidance from earlier in the day predicted a very intense storm. This morning’s GFS run showed a 938 mb minimum central pressure just before the eye wall landfall late Friday. Such pressures are more consistent with a category four storm with maximum sustained winds in the range of 130 to 156 mph. This afternoon’s intensity guidance from GFS has backed off a little bit to 948 mb — which is more in the range of a strong Category 3 storm which jibes with the most recent NHS predicted intensity of 125 mph.

(Harvey may be stronger than even current NHC guidance indicates. GFS models from earlier this morning showed a 938 mb storm by late Friday. Intensity in the models has subsequently backed off to 948 mb — which is equivalent to the strong category 3 strong that the National Hurricane Center now predicts. Image source: Tropical Tidbits.)

As the eyewall reforms and the storm’s intensification rate varies, we’ll tend to end up with different peak intensity forecasts. In any case, we are looking at a major hurricane producing serious impacts for Texas and the U.S. Gulf Coast over multiple days.

Present model guidance further predicts that Harvey will slow down and then stall over southeastern Texas after making landfall — remaining basically stationary near the coast until Wednesday. As it hovers over this region, the storm will pull warm, moist air in over Eastern Texas while maintaining tropical storm intensity, generating a 5 to 6 day long severe flood event. In some scenarios, the storm may partially re-emerge over the Gulf and restrengthen. By Thursday, Harvey is expected to be picked up by a frontal system dipping in over the Central U.S. Exiting the state as it moves north and east.

(Harvey’s associated thunderstorms boil with hot intensity in this GOES satellite picture. Watch an animation of the massive storms swirling around Harvey here.)

Such a long-term stall is expected to bring significant torrential flooding rains over parts of Southeastern Texas. With averages of 17-32 inches over a wide swath and as much as 35+ inches in isolated locales. It’s worth noting that with Harvey’s top intensity continuing to trend toward major hurricane status, with a human-warmed atmosphere now capable of producing much more intense rainfall events, and with the storm expected to rain out over such a long period, some of the ultimate rainfall totals could be historic.

From the National Hurricane Center:

“The system is likely to slow down once it reaches the coast, increasing the threat of a prolonged period of heavy rain and flooding across portions of Texas, southwest Louisiana, and northeastern Mexico into early next week.”

(UPDATES TO FOLLOW)

Links:

The National Hurricane Center

Tropical Tidbits

Pivotal Weather

The Capital Weather Gang

Hat tip to Bostonblurp