Don't let your guard down: Harvey will still cause catastrophic flooding
HOUSTON – The news initially sounds good: Harvey weakened this afternoon to "only" a tropical storm. This is a natural process that occurs when a hurricane moves off its warm ocean water fuel source and over land. It was expected, and it happened right on schedule.
However, of greater concern is that the storm's movement has slowed to a virtual crawl, and is only moving to the north-northwest at 2 mph. As we have seen many times with these tropical systems, it doesn't have to be a hurricane to generate extraordinary rainfall rates (remember Allison?), and Harvey's slow movement will keep intense rain bands around into the middle of next week.
At the present time, the best banding extends from the the Upper Texas Coast into southwest Louisiana, with hourly rainfall rates of 1.5 inches to 2.5 inches occurring near Beaumont. Based upon satellite and radar positioning of the band and its expected movement, another four to eight inches of rain is easily attainable generally from Beaumont westward although, as of 2:30 p.m. CDT, the band is east of the immediate Houston metropolitan area...sparing us for the moment. Radar already estimates that as much as ten inches of rain has fallen in an area roughly between Angleton in Brazoria County and Sugar Land in Fort Bend County.
Keep in mind that additional banding will develop over the next few days, bringing additional heavy rain into the Upper Texas Coast. The map below is the raw output from our in-house RPM model showing expected rainfall amounts from midday Saturday through midday Monday. Notice that the model projects another sixteen-to-twenty inches of rain possible. Of course, this is just a single model...and the forecast could change...but it does show the potential we face.
One other thing to mention is that the right side of a hurricane or tropical storm is favorable for tornado development in these intense bands, and we've already seen a number of tornado warnings issued today. This afternoon, the National Weather Service said that the damage sustained in Katy and Sienna Plantation was likely the result of a tornado, but they are uncertain if some of the many other damage reports, such as in Richmond, are attributable to tornadoes, or to straight-line wind from rain bands. The environment will remain favorable for tornadoes as long as a strong circulation remains associated with Harvey, and we remain on the eastern side of that circulation.
This is going to be a long duration event, and the KPRC 2 Severe Weather Team and Click2Houston.com are working 'round the clock to keep you advised with continuously updated tornado and flood information as we receive it.
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