Subj : MESO: Heavy rain - floodi
To   : All
From : Mike Powell
Date : Wed Jun 07 2023 17:11:00

AWUS01 KWNH 072036
FFGMPD
MTZ000-WYZ000-IDZ000-080120-

Mesoscale Precipitation Discussion 0441
NWS Weather Prediction Center College Park MD
435 PM EDT Wed Jun 07 2023

Areas affected...northwestern WY into west-central to east-central
MT

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

Valid 072034Z - 080120Z

SUMMARY...Scattered thunderstorms with slow movement will generate
localized areas of flash flooding across portions of MT into
northwestern WY through 01Z. Rainfall rates of 1-2 inches in 30-60
minutes are expected.

DISCUSSION...GOES East visible imagery at 20Z showed convective
initiation underway across the higher terrain of northwestern WY
into west-central and central MT. Additional convection was
located along the MT/Canada border, just north of a weak
quasi-stationary front. This region of MT and wy contained
anomalou smoisture (1.5 to 2.5 standardized PW anomalies) and
largely uninhibited MLCAPE of 500 to 2000+ J/kg (19Z SPC
mesoanalysis). Steering flow was very weak (~5 kt), with the
region caught between a closed low over CA and ridge over the
Northern Plains. The main driver for convection over MT/WY was
daytime heating along the elevated heat source of the mountains,
but a pair of subtle mid-level vorticity maxima were noted in
east-central MT on water vapor imagery.

With continued heating and reduction of remaining convective
inhibition, thunderstorms coverage will increase across
northwestern WY into MT over the next couple of hours. A lack of
better speed shear aloft will limit storm organization and
duration over any given location, but the slow moving nature of
storms and the moist environment will allow for rainfall rates of
1-2 inches in 30-60 minutes before collapsing. However, resulting
outflow boundaries are likely to initiate new cells with rapid
growth and quick hitting heavy rainfall. Outside of the weak
vorticity maxima over east-central MT, forcing for ascent will be
mainly diurnally driven through terrain influences, and a
relativae lull in coverage is anticipated into the
east-central/eastern MT Plains where some of the best instability
was located. Given the anomalous rainfall received over the past
1-2 weeks, increased soil saturation will more easily support
runoff from high rainfall rates. While coverage of flash flooding
may not be high, localized areas are considered likely through 01Z.

Otto

ATTN...WFO...BYZ...GGW...MSO...PIH...RIW...TFX...

ATTN...RFC...MBRFC...NWRFC...NWC...

LAT...LON   49081037 49050925 49030764 48970737 47730686 
            46220633 45060705 44830775 43640850 43490982 
            44131082 44941306 46061404 46921389 47561228 
            47971098 48691068 

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