Post by SKYSUMMIT on Apr 13, 2021 11:47:18 GMT -6
Mesoscale Precipitation Discussion 0102
NWS Weather Prediction Center College Park MD
143 PM EDT Tue Apr 13 2021
Areas affected...South Louisiana into far Southern MIssissippi
Concerning...Heavy rainfall...Flash flooding possible
Valid 131738Z - 132215Z
Summary...Thunderstorms continuing over southern Louisiana will
continue to spread east. The risk of flash flooding remains
isolated with hourly rain totals to 2" and local additional
amounts to 4" through the rest of the afternoon.
Discussion...A weak impulse is moving over a surface trough along
the LA coast which is aiding lift in a rather moist environment.
Precipitable water values have increased to around 1.9" in
south-central LA (2.5 standard deviations above normal) which is
moving into a dryer airmass farther east with a sharp moisture
gradient along the MS/AL coasts per recent RAP analysis.
Convective activity is becoming more surface based and is focused
farther south than activity from this morning with SBCAPE
generally 1500 to 2000J/Kg along the southern LA coast. WSWly deep
layer flow around 25kt and light WNWly propagation vectors will
continue to allow eastward movement across southeast LA (including
NOLA) to far southern MS this afternoon.
Guidance has struggled to have activity as far east as it has
been, meaning the earlier elevated activity was feeding on
instability that had progressed more than anticipated. Now that
the activity is more rooted from the surface trough, the activity
should generally expand south as it moves east. Quite a line is
setting up along I-10 west of Lake Pontchartrain that should allow
at least some training cells over the susceptible NOLA metro area
this afternoon and cause at least some urban flash flooding.
Hourly rain totals of 1.5" to 2" with local additional afternoon
rainfall of up to 4" are expected from a combination of
backbuilding, cell training, and cell mergers. Southern LA has
been rather wet over the past week, enhancing the flash flood
threat roughly along and south of I-10 this afternoon.
Jackson