Mesoscale Precipitation Discussion 0204
NWS Weather Prediction Center College Park MD
911 PM EDT Tue May 18 2021
Areas affected...Parts of Southern and Southeast Texas
Concerning...Heavy rainfall...Flash flooding likely
Valid 190110Z - 190700Z
Summary...The risk of flash flooding will increase during the
evening over portions of southern and southeastern Texas this
evening...with the risk persisting beyond 07Z.
Discussion...There has been an increasing signal in the high
resolution convective-allowing models for flash flooding this
evening and into the overnight hours given persistent southeast
flow interacting with a quasi-stationary northeast-southwest
oriented boundary. 00Z soundings showed southerly low-level flow
of 20 to 30 kts drawing in an airmass with precipitable water
values in excess of 1.75 inches...resulting a moisture flux
convergence max along an outflow boundary/effective boundary over
southern and southeast Texas. MLCAPE values south of the effective
boundary had reached nearly 4000 J per kg during the time of
maximum heating during the afternoon...which allowed rainfall
rates to peak at 4 inches per hour in Fayette county Texas.
Because low level flow expected to increase to 30 kts or greater
at 850 mb during the evening, isolated rainfall rates of 2 to 3
inches per hour are still expected despite the loss of solar
heating.
The HRRR began consistently showing widespread areal-average
amounts of 2 to 4 inches...with isolated maximum values of 8+
inches...as early as its 18/20Z run. The experimental version of
the HRRR looked to be aligned with the NAM solution in placement
of its 2 and 3 inch per hour values, but those ideas were
verifying too far north and west as of early this evening.
Given some overlap between areas which received heavy rainfall in
the past 24 hour and the current placement shown by the guidance,
thinking is that flash flooding will become likely during the
evening and that the risk will extend beyond 07Z.
Bann
ATTN...WFO...CRP...EWX...FWD...HGX...LCH...SHV...
ATTN...RFC...WGRFC...NWC...
