Post by SKYSUMMIT on Apr 7, 2021 17:53:51 GMT -6
Mesoscale Precipitation Discussion 0090
NWS Weather Prediction Center College Park MD
745 PM EDT Wed Apr 07 2021
Areas affected...portions of Southern Louisiana
Concerning...Heavy rainfall...Flash flooding possible
Valid 072344Z - 080544Z
Summary...A cluster of thunderstorms is expected to move across
southern LA and deposit hourly rain totals to 2.5" and local
amounts to 4".
Discussion...A long-lived convective cluster which originated near
Athens, TX at 1800 UTC has been marching east-southeast and slowly
broadening its along track profile as it moves across the Piney
Woods of East TX and is about to cross the Sabine river. Broadly
convergent low-level inflow is present, keeping moisture streaming
in from the Gulf of Mexico. Divergence aloft is being sponsored
by a deep layer cyclone centered near Kansas City MO.
Precipitable water values of 1.5-1.7" lie in the vicinity of the
thunderstorm cluster per GPS data. MU/ML CAPE to its southwest of
2000+ J/kg is helping to aid in occasional backbuilding, which has
led to some slow broadening. Effective bulk shear of 40-50 kts
has been leading to the convective organization. GFS mass fields
indicate that it lies in an area of diffluent 1000-500 hPa
thickness and riding near the 5670 meter thickness contour, which
should guide it through southern LA with time.
The area has been dry over the past couple of weeks and streamflow
is below average, which lowers the overall risk of flash flooding.
However, hourly rain totals to 2.5" have been indicated per radar
estimates, and local amounts of 4" (likely over a two hour period)
would lie near the three hourly flash flood guidance values and on
the high end of the mesoscale guidance total-wise. This heavy
rain would be a combination of cell training and occasional
mesocyclone formation. As the best instability is expected to
continue to lurk to the west-southwest, some degree of
backbuilding should continue as it propagates along. The
mesoscale guidance looks too far north and an hour or so slow with
the track of this cluster, which appears to be tracking towards
I-10, if not to the south, over the next several hours. Urban
areas in southern Louisiana are close to I-10, which would be more
easily overwhelmed with such heavy rainfall, though the area rice
and sugar crops would likely benefit. Flash flooding is
considered possible.
Roth