MESOSCALE PRECIPITATION DISCUSSION 0232
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
100 AM EDT TUE SEP 03 2013
 
AREAS AFFECTED...NORTHERN UT...SOUTHEAST ID 
 
CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE 
 
VALID 030500Z - 030900Z
 
SUMMARY...SCATTERED TO NUMEROUS SHOWERS AND THUNDERSTORMS ARE
EXPECTED TO MOVE ACROSS NORTHERN UT AND SOUTHEAST ID OVER THE NEXT
SEVERAL HOURS, BRINGING THE POSSIBILITY OF REGIONAL FLASH FLOODING.

DISCUSSION...DIVERGENCE ALOFT EAST OF A DEEP CYCLONE LOCATED
OFFSHORE THE PACIFIC NORTHWEST IS INTERACTING WITH PRECIPITABLE
WATER VALUES OF AROUND 1.25 INCHES, WHICH ARE ABOVE THE 95TH
PERCENTILE FOR EARLY SEPTEMBER, ALONG WITH AREA TOPOGRAPHY TO
SUPPORT SCATTERED TO NUMEROUS SHOWERS AND THUNDERSTORMS ACROSS
NORTHWEST UT AND SOUTHERN ID.  WHILE LOW-LEVEL WIND SHEAR IS WEAK,
0-8 KM SHEAR ACROSS THE REGION IS AROUND 30 KNOTS, WHICH APPEARS
TO BE HELPING OUT THUNDERSTORM DEVELOPMENT.  CAPE VALUES AROUND
2000 J/KG ARE ALSO FEEDING THE THUNDERSTORM ACTIVITY. RADAR
ESTIMATES OF RAINFALL RATES ARE 0.50-1 INCH PER HOUR IN THIS
REGION, WHICH COULD EXCEED FLASH FLOOD GUIDANCE REGIONALLY.

THUNDERSTORM ACTIVITY IS EXPECTED TO REMAIN SOMEWHAT PROGRESSIVE,
AS CORFIDI VECTORS AND THE 850-400 HPA MEAN WIND SUGGEST CELL
MOTION OF 10-15 KNOTS TO THE NORTHEAST.  LOCAL AMOUNTS OF 1-2
INCHES ARE POSSIBLE WHERE CELLS TRAIN.  AS THESE STORMS ARE MAINLY
CAPE-DRIVEN, EXPECT THE AREA OF THUNDERSTORM ACTIVITY TO SLOWLY
DECREASE WITH TIME.  BOTH THE 03Z RAP AND 00Z ARW ADVERTISE CAPES
TO REGIONALLY REMAIN ABOVE 1000 J/KG UNTIL AROUND 09Z, WHICH WAS
SELECTED AS THE VALID TIME.  CONFIDENCE IN THE ABOVE EVENTS
TRANSPIRING AS EXPECTED IS AVERAGE.

ROTH

ATTN...WFO...SLC...PIH...MSO...BOI...LKN...

ATTN...RFC...CBRFC...NWRFC...

LAT...LON   41261323 41651417 42161372 43571475 44281463 44471377
            44191302 43491209 42601142 41901144 41231307 41261323