MESOSCALE PRECIPITATION DISCUSSION 0205
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
245 AM EDT SUN MAY 27 2018

AREAS AFFECTED...NORTHEAST WY...SOUTHEAST MT

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

VALID 270645Z - 271045Z

SUMMARY...STATIONARY CONVECTION EAST OF THE BIG HORNS WITH MERGERS
POSE FLASH FLOODING RISK...WITH 2-4" TOTALS POSSIBLE OVER NEXT
3-4HRS.

DISCUSSION...SURFACE ANALYSIS SHOWS SURGING COLD FRONT ACROSS NE
WY WHILE ANGLING WITH SOUTHERLY FLOW IN SD AND EASTERLY FLOW
ACROSS SW ND AHEAD OF OUTFLOW FROM LARGE MCS IN EASTERN MT MOVING
INTO W CENTRAL ND ATTM...WHILE NOT ABLE TO CONFIRM WITH
OBSERVATIONS IT IS ALLUDED A SURFACE LOW WITH AN EFFECTIVE
OCCLUDED FRONT EXISTS OVER SE MT.  GOES-16 CONUS IR AND SWIR
SUGGEST THAT STRONG INFLOW IN THE TROWAL NEAR THE SURFACE LOW LEAD
TO STRONG CONVECTIVE INITIATION THAT GIVEN THE LIMITED MID-LEVEL
FLOW BANKED UP AGAINST THE BIG HORNS HAS LEAD TO A LOCALIZED AREA
OF LIMITED FLOW AND AS SUCH CONVECTION HAS BEEN FAIRLY STATIONARY
ESPECIALLY OVER SHERIDAN TO N JOHNSON COUNTY. WHILE MID-LEVEL
SOUNDINGS SUGGEST A FAIRLY DRY ENVIRONMENT...STRONG MST
FLUX/CONVERGENCE AND STATIONARY MOTIONS LIKELY HAVE MOISTENED THE
ENVIRONMENT TO ALLOW FOR MORE EFFICIENT RAINFALL PRODUCTION.  TPWS
ALONG THE TROWAL REGION OF SE MT ARE SUGGESTED AT .9" TO 1.1" AND
LIKELY TO SUPPORT HOURLY RATES OF 1.25".  RECENT SATELLITE IMAGERY
SUPPORTED BY RADAR ALSO SHOWS UPSTREAM REDEVELOPMENT WITH SOME
POTENTIAL OF MERGING CELLS ACROSS S ROSEBUD INTO POWDER RIVER
COUNTY AND BACKBUILDING OVER SHERIDAN TO JOHNSON.  HI-RES CAMS ARE
NOT HANDLING THIS SITUATION/EVOLUTION...BEING SOLELY FOCUSED ON
THE COMPLEX/OUTFLOW TO THE NORTH OR MUCH TOO SLOW WITH THE
DPVA/LOW LEVEL MOISTURE CONVERGENCE.  STILL STRONG ASCENT IS
EXPECTED FOR AT LEAST AN ADDITIONAL HOUR OR TWO...SUGGESTING
STATIONARY CELLS MAY PRODUCE 2-4" TOTALS LOCALLY. 

ADDITIONALLY...STRONG OUTFLOW/CAA REINFORCEMENT FROM CANADA AND
THE MCS HAS LEAD TO VERY HIGH CONVERGENCE VALUES WHERE CELLS ARE
DEVELOPING SW ALONG THE FLANKING LINE WITH A GOOD LIKELIHOOD OF
CONNECTING UP TO THE AFOREMENTIONED CONVECTIVE COMPLEX OVER NE
WY/SE MT.  GIVEN THE SURFACE LOW AND WEAKER WINDS BETWEEN THE
MCS/OUTFLOW AND THE SURGING COLD FRONT SOUTH OF THE BIG HORN
RANGE...FLOW IS SLOWER/WEAKER TO ALLOW FOR CONVECTION TO PROGRESS
SLOWER AND BUILD UP RAINFALL TOTALS THAT ARE LIKELY TO NEAR 3HR
FFG IF THE 1HR LOWER FFG VALUES ARE NOT EXCEEDED. 

GALLINA

ATTN...WFO...BIS...BYZ...GGW...RIW...UNR...

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

LAT...LON   47130395 46870355 46360434 45690495 45200534
            44390550 43810574 43790681 44340712 45050736
            45500727 46000680 46550590 46820520 47030471