Harvey may rapidly develop and approach the Texas Coast

Hey everyone!

My name is Hal Needham. Some people call me Hurricane Hal.

I am a hurricane/ storm surge scientist living in Galveston, Texas. I think about hurricanes most days of the year.

I am concerned about the rapid development of what will likely be Tropical Storm or Hurricane Harvey approaching Texas this week. Here are some of my concerns:

1) This storm will likely push off the Yucatan Peninsula and rapidly develop in the Gulf of Mexico. This means the storm may be named on Wednesday and make landfall as soon as Friday. We won't have much lead time, compared to storms like Hurricane Ike, or tropical waves that form off the coast of Africa.

GEFS ensemble members forecast an open tropical wave to track NW from the Yucatan Peninsula into the Gulf of Mexico and generally approach the Central Texas Coast. This model run was made at 1:00AM CDT on Tue Aug 22.


2) The conditions in the Gulf of Mexico should be VERY favorable for rapid development. The water is warm and there should be little wind shear, which tends to impede development. Check out the Sea Surface Temperature (SST) map below. The green areas have SST values exceeding 86 degrees F (30 degrees C). That's a lot of warm water to provide fuel for Harvey.

3) Most reliable models show Harvey slowing down, or even stalling, along the Texas coast. Regardless of the storm track, this could mean DAYS of torrential rain, especially to the "right" of the track. This means the chance of street flooding is high.

Sea-Surface Temperature (SST) map of the Gulf of Mexico, on Mon Aug 21, 2017. The green area depicts SSTs greater than 86 degrees F (30 degrees C). Tropical cyclones get their fuel from warm ocean water, and this map shows that Harvey will have plenty of fuel for strengthening.

4) Most models show Harvey developing into a strong tropical storm or category-1 hurricane before it reaches the Texas Coast.

Details:
UNIQUENESS:
The uniqueness of Harvey is this idea that it could stall out, and then crawl along the Texas coast for DAYS. Long-duration wind, rain and storm surge, mean more power outages and greater wind/ flood impacts.

For example, a tree may tolerate 60 mph winds for three hours, but not 40 mph winds for 36 hours, in waterlogged soil. So we may see some surprising impacts around the Texas Coast.

Check out the forecast maps below.  These are "deterministic" maps that predict one possible scenario in the future. So don't take the exact track too literally. If possible, it is better to consider an "ensemble" of multiple models, what some people call "spaghetti models."

The take-home point of these two maps below is how slowly Harvey moves. The forecast on top is GFS model for 7:00PM on Friday night. The forecast on bottom is GFS model for 1:00AM on Monday morning. That's 54 hours of time lapsed, and Harvey has not moved very far! (Models were run on Tue Aug 22 at 1:00AM CDT).

GFS model run. Minimum central pressure (millibars) and maximum sustained winds. Each small wind barb is 10 knots, a black triangle is 50 knots. This model is a forecast scenario for Friday night at 7:00PM. The model was run at 7:00AM CDT on Tuesday. Plot courtesy: www.tropicaltidbits.com

This is GFS model run for Mon Aug 28 at 1:00AM.  The model was run at 7:00AM CDT on Tuesday. Plot courtesy: www.tropicaltidbits.com


Summarized details for Tropical Storm/ Hurricane Harvey:

TIMING:
Along the Texas Coast, rain, wind and surge should pick up during the day on Friday and possibly last through Monday, especially if the storm stalls.

IMPACTS:
Risk of street flooding from heavy rain: HIGH
Risk of storm surge flooding: MEDIUM
Risk of wind damage: MEDIUM
Risk of long-term power loss: MEDIUM

Note: Even if Harvey is "only" a strong tropical storm, it would still have the potential to generate a decent storm surge if it stalls along the coast. Storm surge takes time to build, and we should be concerned about days of strong onshore winds.

Also, Harvey could dump 10 or more inches of rain, even if it's a tropical storm and even if it makes landfall well south of Galveston. Long-duration torrential rains are most likely impact.

The Quantitative Precipitation Forecast (QPF) map above shows the potential for torrential rainfall along the Texas coast over the next week. The torrential rain should be widespread and likely impact Galveston.

TAKE ACTION:
1) Consider buying your supplies before you go to bed tonight. Harvey should hit the mainstream media Tuesday night and Wednesday. Why fight crowds?

2) Consider filling your vehicle with gas before you go to bed tonight.

3) Consider evacuation plan.
An evacuation professional once told me that most people evacuate the morning before a storm makes landfall. If Harvey makes landfall on Friday, traffic could be gridlock on Thursday. If you have any flexibility, consider leaving the island/ coastal areas by Wednesday evening, IF Harvey is forecast to approach Galveston.

You'll be glad you did.

4) If Harvey makes landfall well south of Galveston or it makes a direct hit on Galveston and you cannot evacuate for some reason, at least think about where you park your car. Avoid parking under heavy tree branches and low-lying areas, especially north of Broadway.

Anyway, let's keep an eye on this. No need to panic and hopefully Galveston will not see damage. But as the saying goes, hope for the best and prepare for the worst.

Thanks, y'all, and stay safe!
-Hurricane Hal