MESOSCALE PRECIPITATION DISCUSSION 0203
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
936 PM EDT SAT MAY 26 2018

AREAS AFFECTED...CENTRAL AND EASTERN MT...EXT NORTHWEST WY

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

VALID 270135Z - 270735Z

SUMMARY...CELL MERGERS WITHIN DEFORMATION ZONE AND POTENTIAL
TRAINING CELLS IN E MT POSE POSSIBLE FF RISK THROUGH THE EARLY
OVERNIGHT HOURS.

DISCUSSION...VERY DYNAMIC SETUP ACROSS THE HIGH PLAINS AND
NORTHERN ROCKIES AS STRONG SHORTWAVE IS LIFTING NORTH OUT OF WY
SET TO ROTATE/STALL ON THE NORTHEAST PERIPHERY OF THE LARGER SCALE
CLOSED LOW OVER THE GREAT BASIN.  STRONG SHORTWAVE CROSSING
SOUTHERN CANADA WITH THE TAIL END OF THE TROF EXTENDS SOUTHWEST
INTO N CENTRAL MT.  THE COMBINATION OF FLOW REGIMES HAS SET UP A
SOLID CONVERGENCE ZONE ALONG THE 8-7H FROM NEAR YELLOWSTONE TO NE
MT ATTM.  THIS HAS ALLOWED FOR POOLING MID-LEVEL MOISTURE TO RAISE
PRECIPITABLE WATER VALUES AT OR SLIGHTLY ABOVE 1". THOUGH LOW
LEVEL ENVIRONMENT IS A BIT DRIER PARTICULARLY ACROSS SOUTH CENTRAL
MT INTO NW WY WHERE RIW 00Z SOUNDING IS A BIT TOO DRY WITH STRONG
INVERTED V LOW LEVEL ENVIRONMENT. WHILE THIS MAY NOT BE TRULY
REPRESENTATIVE OF THE ENVIRONMENT NEAR THE SW PORTION OF THE MPD
AREA GIVEN SLIGHTLY HIGHER TDS IN THE LOW 50S PER RECENT SFC
OBS...CONVECTION HERE HAS MORE OF A CAPABILITY FOR HIGH BASES AND
CELLS SPLITS REDUCING THE THREAT FOR HIGH RAIN RATES.  GOES-16
VISIBLE DOES SHOW THAT THERE IS SUFFICIENT INSTABILITY AND
RESILIENCY FOR ADDITIONAL NEW DEVELOPMENT/OVERSHOOTING
TOPS...MARCHING TOWARD THE DEFORMATION ZONE.   CURRENT RADAR AND
SATELLITE SUGGESTS CELL MERGERS ARE LIKELY ESPECIALLY IN
WHEATLAND/GOLDEN VALLEY COUNTY.  THIS SHOULD LOCALLY INCREASE RAIN
FALL TOTALS NEAR 1-1.25" IN THE NEXT HOUR OR SO WHICH NEARS
COMPROMISED LOW FFG VALUES IN THE REGION.  

FURTHER NORTHEAST...STRONGER DAYTIME HEATING AND MID-LEVEL COOLING
AND HIGH MID-LEVEL MOISTURE NOTED IN 00Z TFX/GGW SOUNDINGS SUPPORT
STRONGER UPDRAFTS WHERE SBCAPES UP TO 2500 J/KG AND RAP ANALYSIS
SUGGESTS VALUES TO 5000 J/KG EVEN FURTHER SOUTH IN SE MT.  WHILE
FLOW IS GENERALLY WEAK IN THE MID-LEVELS BUT ALSO GENERALLY
UNIDIRECTIONAL...UPPER LEVEL FLOW DUE TO PROXIMITY TO JET FURTHER
NORTH AND BEING SLIGHTLY EAST OF THE MEAN S/W RIDGING ACROSS THE
DAKOTAS TO NE WY SUPPORTING GOOD UPPER LEVEL ASCENT FROM
DIVERGENCE.  WITH UPSTREAM CONVECTIVE DEVELOPMENT WITHIN THE
DEFORMATION ZONE/CONVERGENCE ZONE AND MEAN CELL MOTIONS TO THE
ENE/E...THERE IS A POTENTIAL FOR TRAINING OF CELLS ACROSS NE MT
INTO W NDAK INTO THE EARLY OVERNIGHT HOURS.  THE INCREASED
MOISTURE OF 1.1-1.2" TPWS AND STRONGER UPDRAFTS SHOULD SUPPORT
RATES UP TO 1.25"/HR.  COMBINING THE TRAINING AND RATES ALSO
SUGGEST POSSIBLE ECLIPSING OF LOW FFG VALUES (ABOUT 1.25 IN 3HRS).
 A LIMITING FACTOR IS GOING TO BE RELATED TO STRENGTH OF COLD
FRONT PRESSING THROUGH E MT AS SUGGESTED BY THE RAP AND OTHER
HI-RES CAMS...THIS MAY REDUCE DEW POINT DEPRESSIONS AND REDUCE
SUB-CLOUD EVAPORATION BUT ALSO SHOULD REDUCE SOME SELY SUB-CLOUD
INFLOW AT 850 BUT ALSO MOVE AWAY FROM STRONGEST
CONVERGENCE/CONFLUENCE ALONG THE DEFORMATION ZONE FROM SWLY OR
WESTERLY 7H MST FLUX.  HI-RES CAM NEIGHBORHOOD FFG EXCEEDANCE
PROBABILITIES WILL MAXIMIZE AROUND 20-30% AROUND 06Z-08Z. AS SUCH
FLASH FLOODING IS CONCERNED POSSIBLE AND LOCALIZED. 

GALLINA

ATTN...WFO...BIS...BYZ...GGW...RIW...TFX...

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

LAT...LON   48460457 48330371 47340327 46300377 46130550
            45350757 45000831 44770974 45351089 46451079
            47540898 48290668