91L Enhances Great Plains Rainfall…7 in. of Rain Omaha to Des Moines This week

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Discussion: At 5 AM EDT Tropical Disturbance 91L is located about 60 miles east-southeast of Marathon, FL. TD 91L should move across the area mid-morning. Top wind is near 30 mph. NOAA/NHC expects 91L to become a tropical storm by tonight. The next storm name is Gordon.

91L is poorly organized now. USAF Hurricane Hunter investigates 91L later this morning. Upper shear is ahead for 91L (Fig. 1). However, this system will move over very warm water (Fig. 2) so intensification to a tropical storm tonight is forecast by NOAA/NHC.

If the shear is maintained a minimal to moderate tropical storm is likely and currently indicated. If the upper shear eases there is risk of a hurricane.

The preliminary tropical storm force wind profile (Fig. 3) indicates a large area affected stretching from east/southeast Louisiana to the Alabama Coast and inland to northeast Louisiana/south-central Mississippi. Coastal impacts of tropical storm force wind arrive late tomorrow afternoon.

The NOAA/WPC 72-hour rainfall forecast (Fig. 4) indicates 5-10 in. of rain along the Mississippi Coast to southeast Louisiana to northeast Louisiana. There is a 9-12 in. rainfall deficit across Louisiana now which may slow the evolution of flash flooding. The NOAA/WPC day 4-5 forecast (Fig. 5) indicates 3-5 in. of rain due to the remnants of Gordon over northeast Texas.

A stalled cold front already triggering heavy rains in the central Plains to the Corn Belt is enhanced by influence of Gordon. 7-day rainfall forecasts indicate up to 7 in. of rain from Omaha to Des Moines.

Storm surge forecasts have not yet been issued but 2-3 feet at a minimum should be expected in favored areas of the north and east Gulf of Mexico coast. Gordon is forecast to move inland near the MS/LA state line implying highest storm surge is in the upper Louisiana coast and coastal Mississippi.

Fig. 1: Upper shear axis across the North Atlantic basin. Note shear axis across the central Gulf of Mexico.

 

Fig. 2: The Gulf of Mexico SST is near or warmer than 85F which is slightly warmer than normal.

Fig. 3: NOAA/NHC projection of tropical storm force wind risk.

Fig. 4-5: NOAA/WPC 72-hour and day 4-5 rainfall amount forecast.