MESOSCALE PRECIPITATION DISCUSSION 0267
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
932 PM EDT WED JUN 06 2018

AREAS AFFECTED...NORTHERN WYOMING...SOUTHEAST MONTANA

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

VALID 070131Z - 070631Z

SUMMARY...THUNDERSTORMS MAY PERSIST INTO THE EVENING ACROSS THE
WYOMING-MONTANA BORDER REGION...AND THE SLOW STORM MOTIONS MAY
PROMOTE LOCALIZED FLASH FLOODING. RAIN RATES WILL REACH 1-2 IN/HR
WITH THE SLOWER MOVING THUNDERSTORMS.

DISCUSSION...LATEST OBJECTIVE RAP ANALYSIS SHOWS A CORRIDOR OF
HIGHER 850-750MB THETAE STRETCHING FROM NEAR KRIW... TO JUST EAST
OF KSHR... AND EXTENDING INTO SOUTHEAST MONRANA. A CORRESPONDING
MAXIMUM IN MLCAPE (2000-2500 J/KG) ALSO EXISTED IN THE SAME SWATH
ALONG A LOW-LEVEL CONVERGENCE AXIS (INDICATED BY SURFACE WIND
OBSERVATIONS). THIS LEVEL OF INSTABILITY IS CLIMATOLOGICALLY VERY
HIGH FOR INTERIOR WYOMING...AS SHOWN IN THE RIVERTON SOUNDING
CLIMATOLOGY. THE STRONG INSTABILITY WAS LIKELY CONTRIBUTING TO
STORM ORGANIZATION AND RAP FORECASTS SHOW THAT FAIRLY STRONG
INSTABILITY SHOULD BE MAINTAINED INTO SOUTHEAST MONTANA TONIGHT.

SOME CONVECTIVE CLUSTERS APPEARED TO BE ANCHORED TO TERRAIN...LIKE
THE BIGHORN MOUNTAINS...AND THAT WAS CONTRIBUTING TO THE LOCALIZED
FLASH FLOOD THREAT. THIS MAY CONTINUE...BUT THE MAINTENANCE OF THE
LOW-LEVEL CONVERGENCE AXIS AND INCREASING CONVERGENCE NEAR A
DEVELOPING SURFACE LOW IN SOUTHEAST MONTANA SHOULD ALSO BE
SUFFICIENT TO SOMEWHAT ANCHOR CONVECTION IN THE ABSENCE OF
SIGNIFICANT TERRAIN FEATURES. ADDITIONALLY...MODELS DO SHOW
INCREASING LOW-LEVEL INFLOW IN SOUTHEAST MONTANA AS THE SURFACE
LOW DEVELOPS IN THE NEXT SEVERAL HOURS...WITH A CORRESPONDING
INCREASE IN THE PRECIPITABLE WATER VALUES. ALL THIS MAY ENABLE
CONVECTIVE CLUSTERS TO BUILD OR DEVELOP INTO SOUTHEAST MONTANA
WITH A CONTINUED FLASH FLOOD THREAT BEYOND WHAT IS ALREADY BEING
OBSERVED IN NORTHERN WYOMING.

MRMS AND KRIW RADAR DUAL POL ESTIMATES ALREADY SHOW LOCALIZED
AREAS OF 1-2 IN/HR RAIN RATES...AND THE ENVIRONMENT AND STORM
MOTIONS SHOULD CONTINUE TO SUPPORT THIS. ALTHOUGH CONVECTIVE
COVERAGE MAY BEGIN TO WANE IN THE NEXT COUPLE HOURS WITH
SUNSET...THE LOW-LEVEL FORCING MECHANISMS AND AMPLE INSTABILITY
SUGGEST THAT SOME CONVECTION COULD LINGER BEYOND 03Z WITH A
CONTINUED FLASH FLOOD THREAT. PORTIONS OF THIS AREA HAVE BEEN
UPGRADED TO A SLIGHT RISK OF EXCESSIVE RAINFALL ON THE DAY 1 ERO.

LAMERS

ATTN...WFO...BYZ...RIW...UNR...

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

LAT...LON   46450475 45280415 44020557 43370682 43330833
            43760923 44880886 46340666