Mesoscale Precipitation Discussion 0858 NWS Weather Prediction Center College Park MD 557 AM EDT Tue Sep 17 2019
Areas affected...Southeast TX & Southwest LA
Concerning...Heavy rainfall...Flash flooding possible
Valid 170957Z - 171557Z
Summary...A northward-moving tropical disturbance is expected to bring heavy rains into southeast TX and southwest LA this morning. Hourly rain totals to 3" with local amounts to 6" are possible.
Discussion...A tropical disturbance with a surface low offshore the Middle TX coast has led to an increase of shower and thunderstorm activity with cooling cloud tops over the northwest Gulf of Mexico during the past few hours, with some of the activity beginning to move inland across southeast TX. Radar indicates hourly rain totals of 0.5-1" onshore as of late with occasional 2" an hour totals well offshore. Precipitable water values of 2.0-2.4" exist per recent GPS data. Inflow at 850 hPa is southeast, up to 25 kts, northeast of Matagorda Bay, which along with a similar magnitude of effective bulk shear appears to be leading to some degree of organization on recent satellite and radar imagery. ML CAPE values of 1000-2500 J/kg exist across the region, with the highest values near and just offshore. Winds up through 500 hPa are unidirectional and cyclonic with height.
The expectation is for curved rain bands to develop over the next few hours which could yield hourly rain totals to 3" within this environment where they train, with some gradual outward expansion of the banding expected through the morning. The system as a whole should move/propagate northward around a low- to mid-level high pressure system over the central Gulf of Mexico. Rain moving ashore should cause the instability gradient to retreat back towards the coast, making it an effective front/convergence zone. The mesoscale guidance shows reasonable consistency, with a strong signal for local amounts of 5-6". Parts of these region are dry or have descended into drought, so flash flood guidance values are quite high and soils are generally desiccated. However, the high rain rates possible in this moist and unstable air mass would quickly saturate soils and cause significant problems in urban environments. Since the signal in the guidance is localized and the rain is just beginning to fall, chose the possible category.
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