Imelda Flooding & Why It Happened

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09/16/2019, 12:02 pm EST
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Discussion: The excessive rainfall episode of the past 2 days associated with the remnants of inland-moving Tropical Storm Imelda has finally abated in southeast Texas. However, flood warnings remain in effect from Houston and vicinity to Beaumont, TX and Lake Charles, LA. NWS indicates the flood warnings expire 9-10 AM CDT (Fig. 1). All flash flooding warnings have expired. Additional showers and thunderstorms are likely the next 2-3 days but scattered in nature and of less duration.

Fig. 1: Current watch and warning areas issued by NWS for Texas/Louisiana.

A radar rainfall estimate provided by AccuWeather, Inc. from late yesterday indicates >25 in. of rain occurred due to Imelda from Beaumont to northeast of Houston to the upper coast of Texas (Fig. 2). Nearly as much rain was observed just east of Freeport, TX where Imelda went inland. The excessive rainfall extended to all of southwest Louisiana.

The cause of the unexpected extreme rainfall was due to two factors (Fig. 3): 1.) A stalled weakening tropical storm west and northwest of Houston and 2.) Confluence of westerly flow to the south of the low pressure area into moisture-rich southerly flow off the Gulf of Mexico. The combination of long duration of vividly effective dynamic forcing to produce extreme rainfall caused yet another historic rainfall episode in southeast Texas.

The rapid formation of Imelda just-prior to moving inland over very warm 86F water also inspired the extreme rainfall especially near Freeport whereas the stalled inland system 2-day rainfall event was responsible for the Houston to Beaumont rains.

Fig. 2: Radar estimate of rainfall associated with former Tropical Storm Imelda from late yesterday provided by Accuweather, Inc.

Fig. 3: Dynamics of a perfect rain-maker after Imelda moved inland was confluence of westerly flow south of the low pressure area into moisture-rich southerlies off the Gulf of Mexico.