re: SW LA getting rocked by severe storms this morning.

Mesoscale Precipitation Discussion 0219
NWS Weather Prediction Center College Park MD
147 AM EDT Fri May 21 2021

Areas affected...Southeast and South-Central Louisiana

Concerning...Heavy rainfall...Flash flooding possible

Valid 210600Z - 211200Z

Summary...Persistent onshore flow from the Gulf of Mexico will
drive rainfall into Louisiana through morning. Areas of moderate
rain with embedded convective elements could produce rain rates in
excess of 1"/hr. This rain occurring atop extremely saturated
soils could produce flash flooding.

Discussion...GOES-E IR imagery early this morning indicates a well
defined swirl associated with a mid-level wave lifting northward
across the Gulf of Mexico. East of this feature, deep and
confluent E/SE flow originating from the Atlantic and rounding the
periphery of a large high pressure is returning moisture onto the
Gulf Coast, with PWs measured by GPS of 1.8-2.0 inches, above the
90th percentile for the date. Within this tropical moisture plume,
showers and thunderstorms continue to develop over the Gulf in
response to low-level convergence near a remnant frontal boundary,
MUCape >1000 J/kg over the warm waters, and modest diffluence
within the broad RRQ of a poleward extending jet streak atop the
Plains.

Area soundings from 00Z/21 indicate deep S/SE flow with limited
directional shear, efficiently advecting better moisture into the
region. Also noted on the soundings, WBZ heights of 12,000-13,000
ft are also above the 90th percentile for the date, suggesting
efficient warm rain collision processes which will be further
enhanced as 850mb dewpoints climb towards +15C. 850mb winds of
20-30kts will maintain this warm/moist advection, and become equal
to or slightly greater than the mean 0-6km winds, enhancing lift
over Louisiana. This persistent moist inflow will also allow
Corfidi vectors to become more anti-parallel to the mean wind, and
with the greatest instability continuing just offshore, there is
an increasing likelihood for training or backbuilding of echoes
through the overnight.

Although most of the rainfall should be of moderate intensity,
embedded rates above 1"/hr are likely as shown by recent HREF
probabilities. Where these rates train, rainfall could exceed 3",
as reflected in recent runs of the HRRR, ARWs, and FV3LAM.
Rainfall in this part of Louisiana has been as much as 9" in the
last 72 hours, leading to locally reduced FFG of 2"/3hrs, and 40cm
soil moisture that is above the 95th percentile according to NASA
Sport. Despite the general moderate intensity expected overnight,
even briefly heavy rainfall could quickly transition to runoff
leading to isolated flash flooding through morning.

Heavy rainfall is likely to continue beyond this MPD through
Friday, with additional flash flooding probable.

Weiss