Tropical Depression Harvey an Extremely Dangerous Flood Threat for Texas, Louisiana; Hurricane, Storm Surge Watches Issued
Harvey
is currently a tropical depression in the western Gulf of Mexico.
It
will track toward the Texas Gulf Coast and make landfall late Friday.
Harvey
is forecast to strengthen to a hurricane before landfall.
Harvey
may then stall or meander for a few days, leading to a dangerous
flood threat in parts of Texas and Louisiana.
Hurricane,
tropical storm and storm surge watches have been issued for parts of
Texas.
The NHC says the center of Harvey is located about 445 miles southeast of Port Mansfield, Texas, and is sluggishly moving northwest.
Current Storm Information
A hurricane watch has been issued for a portion of the Texas coast, from north of Port Mansfield to San Luis Pass, including Corpus Christi and Victoria. A hurricane watch means hurricane conditions are possible within the watch area. They are typically issued within 48 hours of the expected onset of tropical-storm-force winds.
Tropical storm watches are in effect from Boca de Catan, Mexico, to Port Mansfield, Texas, and from north of San Luis Pass, Texas, to High Island, Texas. This means tropical storm conditions are possible within 48 hours.
Tropical Alerts
The NHC has also issued its first ever public storm surge watch, which includes a swath of the Texas coast from Port Mansfield to High Island, including Corpus Christi and all of Galveston Bay. This new type of watch is issued when there is the "possibility of life-threatening inundation, from rising water moving inland from the coastline" in the next 48 hours, the NHC says.
Harvey is expected to gain strength and will draw near, if not make landfall, somewhere along the Texas coast Friday afternoon or Friday night.
Projected Path
The projected path shows the expected track of the circulation center. Note that impacts such as heavy rain and storm surge can extend beyond this forecast path.
With an otherwise favorable environment that includes deep, warm Gulf of Mexico water, Harvey is currently forecast to strengthen into a hurricane before its landfall, bringing the risk of storm-surge flooding, high surf with battering waves and strong winds.
Various NOAA aircraft reconnaissance missions will provide data that will likely help numerical forecast models determine the exact future intensity and track of Harvey.
Regardless of intensity or track, a major flooding rainfall threat looms, and it may last into next week.
Rainfall Flood Danger
Harvey is expected to be caught in a zone of light steering winds aloft this weekend that will slow or stall the circulation.
Harvey will be wedged between two areas of high pressure aloft, one over the Desert Southwest and a weaker one over the central Gulf of Mexico. Meteorologists call this zone between two areas of high pressure aloft a col.
Potential upper-level steering pattern this weekend that may stall Harvey for some time near or over the western Gulf Coast.
A tropical cyclone's rainfall potential is a function of its forward speed, not its intensity.
Therefore, if Harvey stalls for a period of a few days, it has the potential for producing prolific rainfall, capable of major flash flooding.
Rainfall amounts through next Wednesday could range from 10 to 15 inches, with locally up to 20 inches, over the Texas coast and southwest Louisiana, with heavy rainfall beginning Friday.
Harvey is also expected to produce rainfall accumulations of 3 to 9 inches in portions of south, central and eastern Texas and the lower Mississippi Valley during the same time period.
This forecast is subject to change depending on the exact path of Harvey, locations of rainbands and how long it stalls. Generally, areas along and east of Harvey's path are in the greatest threat of flooding rainfall.
For now, areas near the Texas and Louisiana Gulf coasts are in the biggest threat for torrential rainfall and major flash flooding, potentially including Houston and Corpus Christi.
Among the biggest uncertainties is the heavy rain potential in central Texas, including for the flood-prone cities of Austin andSan Antonio. That all depends on how far inland and to the west Harvey tracks.
Rainfall Outlook Through Monday
While it is too soon for specific rainfall forecasts, areas in red and purple are in the highest threat for flooding rainfall from Harvey through the weekend. Some of this rain may linger into Tuesday or beyond in some areas.
Harvey may still linger over parts of the western Gulf Coast into early next week before it finally moves east. This could spread additional heavy rainfall into parts of the lower Mississippi Valley.
There are even scenarios where Harvey either stalls just off the Gulf Coast or moves back over the Gulf of Mexico at some point after having made landfall, which could allow it to restrengthen for a brief time.
The ground is already quite saturated in many of these areas from what has been one of the wettest starts to August on record.
Long-Lived Surge, Wind Threats
Harvey's slow movement will also likely lead to additional long-lived impacts from wind.
To the east of Harvey's center, a persistent fetch of south to southeast winds will build swells over the western Gulf of Mexico. As Harvey strengthens, coastal flooding could increase along parts of the Texas and Louisiana coasts as soon as late Thursday, then should peak Friday with Harvey's intensification and landfall.
Given Harvey's expected slow crawl near the coast, this coastal flooding, along with battering waves, could persist in some form through Sunday, if not longer, to the east of the circulation.
Coastal Flood, Waves, Wind Setup
This water rise near the coast may not allow rain-swollen rivers and bayous to drain, compounding the inland flood threat.
Locations in the storm surge watch on the Texas coast (Port Mansfield to High Island) could see a water rise of 4 to 6 feet above ground level along the immediate coast if the peak surge coincides with high tide. This includes all of Galveston Bay.
Storm Surge Alerts
Furthermore, persistent winds, even if not particularly high-end, could down more trees than they otherwise would given the rain-soaked or flooded ground.
Now is a good time to make sure you have a plan in case of both a hurricane strike and flooding. The Federal Alliance for Safe Homes can help you plan for a hurricane. NOAA also has excellent resources to plan for flooding.
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