MESOSCALE PRECIPITATION DISCUSSION 0185
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1125 PM EDT TUE MAY 22 2018

AREAS AFFECTED...SOUTH-CENTRAL MT...NORTHERN WY

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

VALID 230325Z - 230925Z

SUMMARY...SLOW-MOVING SHOWERS AND THUNDERSTORMS WITH HEAVY
RAINFALL RATES WILL CONTINUE. RUNOFF AND FLASH FLOODING WILL
REMAIN POSSIBLE.

DISCUSSION...GOES-16 1-MINUTE CLEAN IR SATELLITE IMAGERY HAS BEEN
SHOWING A NOTABLE UPTICK IN CONVECTIVE CLOUD TOP COOLING OVER
CARBON AND STILLWATER COUNTIES OF SOUTH-CENTRAL MT...AND DOWN
ACROSS SHERIDAN AND JOHNSON COUNTIES OF NORTHERN WY AS HEAVY
SHOWERS AND THUNDERSTORMS EVOLVE OVER THIS REGION.

THE ACTIVITY IS BEING FOSTERED BY A RELATIVELY UNSTABLE BOUNDARY
LAYER CHARACTERIZED BY MUCAPE VALUES OF 500 TO 1000 J/KG AND A
SOMEWHAT FOCUSED AREA OF SURFACE MOISTURE CONVERGENCE.
ADDITIONALLY...THERE IS INCREASINGLY DIVERGENT FLOW ALOFT AROUND
THE NORTHEAST FLANK OF AN UPPER LOW OVER THE GREAT BASIN. IN
FACT...THE LATEST GOES-16 WV AND AIRMASS-RGB IMAGERY SUITE ARE
SUGGESTING A PV ANOMALY ANGLING NORTHEAST ACROSS NORTHEAST UT AND
NORTHWEST CO.

LARGER SCALE FORCING IS EXPECTED TO INCREASE FURTHER OVERNIGHT
ACROSS AREAS OF NORTHERN WY AND SOUTH-CENTRAL MT AS THE UPPER LOW
AND EJECTING PV ANOMALY APPROACHES FROM THE SOUTHWEST AND THE
REGION BEGINS TO GET INTO A MORE FAVORABLE MID LEVEL DEFORMATION
AXIS. SO WHILE THE INSTABILITY PARAMETERS WILL TEND TO WEAKEN WITH
THE LOSE OF DIURNAL HEATING...THE DYNAMICAL SET-UP IS EXPECTED TO
FAVOR AT LEAST SOME DEGREE OF ORGANIZED CONVECTION FOR SEVERAL
MORE HOURS ALONG WITH HEAVY RAINFALL POTENTIAL.

IN THE SHORT-TERM...RAINFALL RATES ARE EXPECTED TO BE AS MUCH AS
1.5 INCHES/HR GIVEN SUCH COLD CONVECTIVE TOPS AND PWATS OF LOCALLY
NEAR 1 INCH WHICH IS ABOUT 1.5 STANDARD DEVIATIONS ABOVE NORMAL.
AFTER ABOUT 06Z...THE RAINFALL RATES SHOULD TEND TO COME DOWN TO
UNDER 1 INCH/HR...BUT THERE WILL REMAIN A FAIR DEGREE OF
CONCENTRATED AND LOCALLY HEAVY RAINFALL GIVEN THE LARGER SCALE
ASCENT.

EXPECT ADDITIONAL RAINFALL TOTALS OF AS MUCH AS 2 TO 4 INCHES
GOING THROUGH 09Z...WITH LOCALLY HEAVIER AMOUNTS NOT OUT OF THE
QUESTION GIVEN SOME OF SLOW CONVECTIVE CELL MOTIONS. SOME RUNOFF
PROBLEMS AND FLASH FLOODING CONCERNS WILL REMAIN THROUGH 09Z. WILL
CONTINUE TO MONITOR.

ORRISON

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

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

LAT...LON   46510799 45670682 44420630 43630658 43370730
            43550842 44060922 45020972 45890994 46490953