MESOSCALE PRECIPITATION DISCUSSION 0160
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
559 PM EDT THU MAY 17 2018

AREAS AFFECTED...SRN ND INTO SD

CONCERNING...HEAVY RAINFALL...FLASH FLOODING LIKELY

VALID 172155Z - 180355Z

SUMMARY...FLASH FLOODING APPEARS LIKELY OVER SOUTHERN NORTH DAKOTA
INTO CENTRAL/NORTHERN NORTH DAKOTA OVER THE NEXT FEW HOURS AS
ONGOING CONVECTION CONTINUES TO EVOLVE. 2-4 ADDITIONAL INCHES OF
RAIN IS EXPECTED THROUGH 04Z.

DISCUSSION...CONVECTION CONTINUES TO EVOLVE NEAR A STATIONARY
FRONT WHICH EXTENDED EAST TO WEST NEAR THE NORTH DAKOTA/SOUTH
DAKOTA BORDER AT 21Z. TRENDS IN RADAR REFLECTIVITY SHOWED CELL
COVERAGE INCREASING NEAR THE FRONT AND ALONG AN OUTFLOW BOUNDARY
LOCATED WEST-EAST THROUGH NORTHERN SOUTH DAKOTA...JUST SOUTH OF
THE FRONT...WHILE A SOUTH-NORTH LINE OF SHOWERS/THUNDERSTORMS IN
WESTERN SOUTH DAKOTA WAS BEGINNING TO SHOW SIGNS OF EASTWARD
MOVEMENT. A LARGE POOL OF WEAK TO MODERATE INSTABILITY HAS
DEVELOPED SOUTH OF THE STATIONARY FRONT IN THE PLAINS WITH A
MIXTURE OF STORM MODES AND STORM MOTIONS OBSERVED.

FLOW ALOFT IS ALREADY BROADLY DIFFLUENT OUT AHEAD OF A LARGE UPPER
TROUGH IN THE WESTERN U.S. BUT INCREASED FORCING FOR ASCENT WILL
DEVELOP WITH SHORT TERM MODEL FORECASTS INDICATING INCREASING
UPPER LEVEL DIVERGENCE OVER NORTHERN SOUTH DAKOTA OVER THE NEXT
FEW HOURS WITH THE DEVELOPMENT OF A 50-70 KT UPPER LEVEL JET
STREAK OVER NORTH DAKOTA. THIS COMBINED WITH AN INCREASING
SOUTHERLY LOW LEVEL JET INTO THE 30-50 KT RANGE WILL ALLOW FOR
OVERRUNNING OF RAIN COOLED AIR AS CONVECTION EXPANDS ALONG THE
WEST-EAST ORIENTED OUTFLOW BOUNDARY...AND AS CONVECTION PROPAGATES
EASTWARD FROM WESTERN SOUTH DAKOTA ALLOWING FOR CELL MERGERS.

FLASH FLOOD GUIDANCE VARIES ACROSS THE DAKOTAS...BUT IS AS LOW AS
1.5 INCHES IN THREE HOURS OVER MUCH OF WESTERN SOUTH DAKOTA. GIVEN
THE POTENTIAL FOR TRAINING AND REPEATING CONVECTION...2-4 INCHES
OF ADDITIONAL RAINFALL IS EXPECTED...WHICH IS LIKELY TO EXCEED
FLASH FLOOD GUIDANCE IN A FEW LOCATIONS.

OTTO

ATTN...WFO...ABR...BIS...CYS...FGF...UNR...

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

LAT...LON   46739917 46689763 46479705 45389729 44479935
            43730160 43290347 43240407 43460431 44080426
            44930370 45750237 46450101