MESOSCALE PRECIPITATION DISCUSSION 0502...CORRECTED
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
656 PM EDT SAT JUL 21 2018

CORRECTED FOR SOUTHEAST MT IN AREA AFFECTED LINE

AREAS AFFECTED...WESTERN SD...NORTHEAST WY...EXT SOUTHEAST MT...

CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE

VALID 212252Z - 220300Z

SUMMARY...DEEP AVAILABLE MOISTURE/BACKED LOW LEVEL FLOW FOR SLOW
MOVING SUPERCELLS CAPABLE OF HIGH RATES (2"/HR) AND PSBL FLASH
FLOODING...

DISCUSSION...GOES-16 WV DEPICTS A FLAT SHORTWAVE FEATURE ALONG THE
SOUTHERN SIDE OF AN ANTICYCLONICALLY CURVED 3H 80KT JET CROSSING
SOUTHERN MT.  ADDITIONAL SWLY MID TO UPPER LEVEL FLOW HAS
SUPPORTED A DEVELOPING 850MB/SFC LOW NORTH OF CPR THAT IS
CONNECTED BY AN OCCLUDED FRONT TO DEEPER SFC LOW IN S CANADA WITH
A STATIONARY FRONT EXTENDING SOUTH IN THE VICINTY OF THE LARAMIE
RANGE.  WITH THESE HEIGHT FALLS...UPSLOPE COMPONENT LOW LEVEL
WINDS WILL BE BACKING ACROSS THE BLACK HILLS REGION TOWARD THE
FRONTAL BOUNDARY AND SURFACE/850MB LOW.  SFC ANALYSIS SHOWS TDS IN
THE 70S ALONG THE LOWER SLOPES OF THE BLACK HILLS AND MID 60S INTO
NE WY.  AS SUCH MOISTURE CONVERGENCE ALONG THE FRONTAL ZONE HAS
SPARKED CONVECTION IN EXT SE MT INTO NE WY.

THE VEERED SHEAR PROFILE IS SUPPORTING ROTATION FOR SUPERCELL
DEVELOPMENT AND RAP13 BUNKERS RIGHT MOVING SUPERCELL VECTORS RANGE
FROM 10-15 KTS TO THE SE FURTHER NORTH TO 0-5KTS OVER EAST CENTRAL
WY AND THE BLACK HILLS. IN FACT RECENT UDX RADAR AND VISIBLE
IMAGERY SHOW AN OVERSHOOTING TOP CELL ALONG THE NORTHERN FLANKS OF
THE BLACK HILLS WITH NO SIGNIFICANT CELL MOTIONS AT THIS TIME. 
GIVEN THE DEEP MOISTURE...RATES UP TO 2"/HR ARE POSSIBLE WHICH
EXCEEDS 1HR FFG VALUES 1-1.5" IN THE AREA AND APPROACHES 3HR
VALUES 1.5-2.5" (MINUS THE CORE OF THE BLACK HILLS WHERE IT IS
HIGHER).  FURTHER CONVECTIVE EXPANSION IS EXPECTED TOWARD THE
TRIPLE POINT LATER THIS EVENING AS WELL INCREASING FLASH FLOOD
RISK OVER EAST CENTRAL WY INTO SW SD AS WELL.

GALLINA

ATTN...WFO...BYZ...CYS...UNR...

ATTN...RFC...MBRFC...

LAT...LON   45720452 45670326 45130270 44370242 43200250
            42990451 43860596 44970515