Mesoscale Precipitation Discussion 0716
NWS Weather Prediction Center College Park MD
745 AM EDT Fri Aug 24 2018

Areas affected...Enter areas affected

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

Valid 241144Z - 241614Z

Summary...Off and on convective rainfall this morning may lead to
isolated instances of flash flooding, especially near
fast-responding creeks or in urban environments.

Discussion...Something of a low threat, but a non-zero threat of
flash flooding exists this morning in eastern MO / western IL,
near Saint Louis. RAP forecasts depicted difluent upper jet level
flow, and loops of GOES-16 water vapor imagery gave the impression
of a convective wedge feature over central Missouri - although
with broken convection then becoming more expansive downstream
near the Mississippi River.

This upper level forcing sat atop an environment marked by low
level warm advection east of a warm front, with anomalously strong
(for August) southwesterly 850-mb flow 50 knots or greater
becoming convergent over central to eastern Missouri. This 850-mb
flow was a part of deeper southwest to west flow tapping steep 
mid level lapse rates sampled by the 06Z RAOB out of Lamont,
Oklahoma, and an unstable environment with RAP MUCAPE values 1000
J/kg or greater near the western edge of the convective wedge
feature.

Ongoing convection had evolved to the point of producing what
appeared to be a developing MCV over western Illinois east of
Quincy. While deep layer synoptic lift will continue to be
maximized at that latitude, the evolution of the MCV / MCS had
suppressed stronger updrafts to the south, resulting in a
relatively narrow zone where strong convection and 1 to 2 inch per
hour rain rates will occur closer to Saint Louis and surrounding
area through 16-17Z.  The 06Z NAM CONUS Nest and 00Z NSSL WRF
solutions had a reasonable handle, but were displaced too far
north. The fractured / scattered nature of the radar echoes along
with relatively high FFG argue for lesser probabilities of flash
flood impacts, but wherever convection briefly trains, some 2 to 3
inch amounts could fall in a short time, leading to some flash
flooding.

Burke

ATTN...WFO...ILX...LSX...PAH...SGF...

ATTN...RFC...LMRFC...MBRFC...NCRFC...

LAT...LON   40179113 39888951 38528899 37719054 37729285
            38979232