Most people associate tropical weather with wind. This is likely because it is the scariest element of storms for us. Wind means flying debris, trees falling on homes and general mayhem beyond what we typically see from thunderstorms. In a city like Houston where heavy rainfall is often the norm, we can be lulled into a sense that anything short of a major hurricane is no big deal. Yet, the two most destructive storms in our area in the last 30 years were Hurricane Ike, a category 2 hurricane, and Tropical Storm Allison. Even a non-tropical system like the recent storms on Memorial Day can create problems we thought were only associated with hurricanes.
It's all the more reason why the area of disturbed weather in the Gulf dubbed Invest 91 by the National Hurricane Center bears watching. Though the disturbance lacks the organization necessary to even be classified a tropical depression, it is likely to create very serious rain for people along the Texas coastline including Houston and Galveston.
Uncertainty is part of all storm forecasting. In an area the size of Houston, it is not abnormal for one area to get twice as much rainfall as another. Take Memorial Day, when the Energy Corridor and Meyerland got upwards of nine inches of rain while other areas only got three or four. So, with a storm like Invest 91, we are right to be cautious.
The NHC has not upgraded the storm as of its early afternoon update, though it believes we will have Tropical Storm Bill sitting off the Texas coast by sometime tomorrow prior to landfall. There has yet to be a closed center of circulation but there is a ton of moisture being pulled into the storm from the very warm waters of the Caribbean and eastern Gulf of Mexico. It also has sustained winds up around 40 mph in isolated areas.
So, why the lack of a classification? Well, the NHC has very specific guidelines for giving storms names or even classifying them as a depression. All of us along the Gulf Coast are familiar with the Saffir-Simpson scale that ranks hurricanes on a simple number system, but that scale is not a terribly accurate measurement of the potential damage a storm can do. In the case of Invest 91, it already has some of the characteristics of a tropical storm, but not enough for classification.
Where is it going?
Because it doesn't have a closed center of circulation, forecast models have a very difficult time pinpointing its forecast track. The problem is that it is difficult at the moment to locate the exact center of the storm. A shift of only a few hundred miles in any direction for the center moves the storm inland south of Corpus Christi or across Galveston Island. Until the storm has intensified a bit more, the entire stretch from South Padre Island to Bolivar Peninsula is at risk, but Monday afternoon, storm tracks have begun to shift their focus to the Matagorda Bay area, putting Houston in the crosshairs for the worst of the rainfall.
It's why Harris County and Galveston County are already getting prepared. A voluntary evacuation of Bolivar Peninsula, the most hard hit area during Ike, has been issued. Houston Independent School District and other schools and school districts are currently on alert waiting for word as to whether they should cancel summer school classes. The Harris County Jail has even suspended prisoner visits for Tuesday and Wednesday. City officials just announce they'll activate the city's Emergency Operations Center to level 2 or "high readiness" tonight. Everybody's gotta plan ahead.
How bad will it get?
The National Weather Service has warned that our area should almost certainly see 3-5 inches of rainfall. Fortunately, the watersheds for area bayous are back to normal levels even though the ground remains rather saturated. We can certainly deal with 3-5 inches even in a compressed time frame. However, because we are on the east or "dirty" side of the storm, a landfall where some are suggesting, just to the southwest of Galveston, means we'll see the worst of rain and wind. Isolated areas could expect more than 10 inches.
We saw what happened with 8-9 inches of rain on Memorial Day. Imagine 12-15.
It is also important to note that the vast majority of the storm activity with this disturbance is well to the east of the center. So, a direct hit likely means less rainfall that a strike to the east and south, as strange as that may be. Even a strike between South Padre and Corpus Christi likely means 3-5 inches of rain for Houston and more on the western edge of the area.
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As for wind, gusts as high as 50 mph should be expected, particularly near the coast. Sustained winds of 30-40 mph are also realistic even inland, particularly in some of the large storm cells.
Is this another Allison?
The good news is that, unlike Allison, which was a multiple-day rain event, Invest 91 should move out of the area fairly quickly and skies should clear by Wednesday. In fact, there isn't any significant rain in the forecast from Wednesday evening through most of next week. That doesn't mean Invest 91 won't dump a huge amount of rainfall on us as it moves through, but it is unlikely to be long-lasting.
The first bands of rain should move in Monday night with the center of the storm making landfall Tuesday morning and the last of the rain should clear out by Wednesday evening.