094
AWUS01 KWNH 210133
FFGMPD
IAZ000-NEZ000-SDZ000-COZ000-210730-

Mesoscale Precipitation Discussion 0292
NWS Weather Prediction Center College Park MD
932 PM EDT Mon May 20 2024

Areas affected...far northeast CO...much of NE...far western
IA...far southeastern SD

Concerning...Heavy rainfall...Flash flooding likely

Valid 210130Z - 210730Z

Summary...Isolated to scattered instances of flash flooding are
likely overnight from repeated rounds of heavy rainfall with rates
generally 1-2"/hr. Localized totals of 4-7" are possible.

Discussion...Convection is gradually increasing in coverage across
portions of the Northern/Central Plains this evening, producing
localized rainfall rates of 1-2"/hr. A warm and unstable air mass
is in place across the region, with Tds rising through the 60s
with SB CAPE of 2000-3000 J/kg. In addition, PWs range from
0.9-1.5 inches (above the 90th percentile, per LBF/OAX sounding
climatology) and effective bulk shear of 30-60 kts. Continued
convective proliferation and organization is anticipated
overnight, likely evolving into an MCS that propagates rapidly
east to northeast near/along the surface front. While the speed of
the MCS should mitigate the extent and intensity of the flash
flooding overall, the concern is that the ongoing storms prior to
the arrival of the MCS will contribute substantially to the
overall rainfall total (particularly across portions of southeast
NE).

Hi-res CAMs have done reasonably well with the depiction of storm
mode and evolution thus far, and some of the more recent HRRR runs
are particularly concerning with localized totals through 08z of
up to 6". This signal has mainly been confined to southeast NE,
where the 12z HREF indicates the highest chance for localized 3"
exceedance (per the 40-km neighborhood probabilities of 20-40%).
This is highly supported by the observational trends, as the
surface front is located directly across this region with pre-MCS
localized rainfall totals of 1-3" already (per MRMS estimates).
Given that the MCS should track right through this area (bringing
another round of rainfall with more widespread 2-3" totals), this
should result in localized totals of 4-7". The corresponding 6-hr
Flash Flood Guidance (FFG) generally ranges from 2.0-4.0",
suggesting that the FFGs will almost certainly be eclipsed
locally, likely resulting in isolated to scattered instances of
flash flooding.


Churchill

...Please see www.wpc.ncep.noaa.gov for graphic product...

ATTN...WFO...BOU...CYS...DMX...FSD...GID...GLD...LBF...OAX...

ATTN...RFC...MBRFC...NCRFC...NWC...

LAT...LON   43389661 43119554 42079494 41169516 40629609
            40189820 40000307 41440326 41870059 42949828