MESOSCALE PRECIPITATION DISCUSSION 0478
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
520 PM EDT WED JUL 18 2018

AREAS AFFECTED...SOUTH CENTRAL ND...NORTH CENTRAL TO SOUTHEAST
SD...

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

VALID 182119Z - 190300Z

SUMMARY...CONFLUENT/MERGING CONVECTIVE CELLS ALONG SHEAR AXIS WITH
AMPLE INSTABILITY/MOISTURE POSE FLASH FLOODING THROUGH THIS
EVENING.

DISCUSSION...GOES-16 WV SUITE DEPICTS ELONGATED SHEAR AXIS FROM
NDAK TO LOWER MS VALLEY WITH CURRENT FULCRUM OF ROTATION/MEAN
CENTER IN SW NDAK WITH STRONG BUT SHEARING VORT CENTER OVER NE SD
LIFTING NORTH.  WEAK/LIMITED OR CONFLUENT CELL MOTION VECTORS
ALIGN NEAR THIS SHEAR AXIS BUT ALSO SLIGHTLY SOUTHWEST OF THE
DIFFERENTIAL HEATING BOUNDARY FROM MID-LEVEL CLOUD COVER THAT HAS
BEEN BREAKING THROUGH THE LAST FEW HOURS FROM SOUTH CENTRAL NDAK
TO SE SDAK.  AS SUCH NARROW AXIS OF POTENT INSTABILITY HAS BUILT
ACROSS CENTRAL SDAK WITH MLCAPES TO 2000 J/KG AND Q-AXIS JUST A
BIT EAST WITH VALUES UP TO 1.75".  DPVA FROM FULCRUM VORT CENTER
IS SPARKING CONVECTION BUT VERY STRONG DIRECTIONAL MST CONVERGENCE
IN THE LOWEST LEVELS ALONG THE MST/INSTABILITY AXIS IS SPARKING
TWO LINES OF CONVECTION...THE ONE ACROSS NW SDAK IS PROPAGATING
QUICKLY IN THE STRENGTHENING NORTHWESTERLY JET (WHICH WILL NOT
POSE MUCH OF A FLASH FLOODING RISK DUE TO THE MOTIONS) AND A NW-SE
BAND OF MORE DISCRETE FROM CORSON TO STANLEY COUNTIES...THESE
CELLS WILL INITIALLY BE SEVERE BUT IN HIGHLY DIVERGENT FLOW
THROUGH A GOOD DEPTH OF THE MID TO UPPER TROPOSPHERE WILL SUPPORT
SLOWED CELL MOTIONS WITH EVENTUAL MERGERS AS THEY CONTINUE TO
EXPAND UPSCALE.  RAIN RATES OF 1.5-2"/HR ARE LIKELY WITH THESE
CELLS BUT IT IS THE MERGERS/SLOWING OF THE CELL MOTIONS THAT WILL
ALLOW FOR 3-5" TOTALS THROUGH 03Z LEADING TO FLASH FLOODING. 
FLASH FLOODING WOULD BE MORE LIKELY IF THE CELLS WOULD START TO
STALL IN THE LOWERED FFG AXIS THAT IS GENERALLY WEST OF THE BEST
MOISTURE/INSTABILITY FROM CORSON TO GREGORY COUNTIES...BUT THIS IS
LOOKING A BIT LESS LIKELY GIVEN THE CURRENT TRENDS.  EVEN THOUGH
FFG IS HIGHER 2-3" INCHES/3HRS...THERE IS STILL A MODERATE
PROBABILITY OF EXCEEDING IT THROUGH 03Z.  THIS IS IN LINE WITH
RECENT HRRR RUNS AND THE HREF MEAN (AT LEAST AXIS/TIMING OF
CONVECTION).  

A FEW ADDITIONAL CELLS ARE DEVELOPING ALONG THE FAR NE EDGE OF THE
INSTABILITY POOL/DIFFERENTIAL HEATING GRADIENT.  THEY ARE NOT
WITHIN THE GREATEST MOISTURE POOL BUT HAVE SUFFICIENT INFLUX TO
GENERATE MODERATE TO HEAVY RAIN RATES.  CELL MOTIONS WILL BE NEAR
ZERO OR VERY SLOW MOVING TO ALLOW FOR SOME MORE ISOLATED TOTALS
WHERE THE CORES REMAIN STATIONARY.  

GALLINA

ATTN...WFO...ABR...BIS...FSD...UNR...

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

LAT...LON   46780146 45619824 44499698 43159679 43069866
            43990040 44880150 46060246