Graphic for MPD #0714

MESOSCALE PRECIPITATION DISCUSSION 0714
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
707 PM EDT TUE AUG 22 2017

AREAS AFFECTED...WESTERN NV...PORTIONS OF NORTHERN CA

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

VALID 222300Z - 230300Z

SUMMARY...CONVECTION DEVELOPING ACROSS THE HIGHER TERRAIN OF
WESTERN NV AND PORTIONS OF NORTHERN CA WILL TAP DEEP MOISTURE TO
PRODUCE SLOW MOVING STORMS...POSING A FLASH FLOOD THREAT INTO THE
EARLY EVENING HOURS.

DISCUSSION...REGIONAL RADARS SHOWED AN INCREASE IN CONVECTIVE
ACTIVITY ACROSS THE HIGHER TERRAIN OF WESTERN NV AND PORTIONS OF
NORTHEAST CA THIS AFTERNOON. THE CONVECTION IS FORMING IN AN AREA
OF WEAKLY DIFLUENT FLOW AHEAD OF A MID LEVEL TROUGH LOCATED OFF
THE CA COAST. THE CONVECTIVE DEVELOPMENT IS FOLLOWING AN AXIS OF
500-1000 J/KG OF MLCAPE...AND TRACKING SLOWLY NORTHWEST IN THE
PRESENCE OF A WEAK 850-300 MB MEAN FLOW (WHICH IS GENERALLY LESS
THAN 10 KNOTS).

THE MOST RECENT RAP ANALYSIS SHOWED 1.25 INCH PRECIPITABLE WATER
AIR OVER NORTHERN CA...AND MOST OF THE CONVECTION HAS FORMED ON
THE GRADIENT OF THIS MOISTURE AXIS. THE KRGX RADAR SHOWED LOCAL
1.50 INCH HOURLY RAINFALL RATES OVER NORTHERN ALPINE COUNTY CA.
WHILE SOME DRY AIR IN THE MID LEVELS COULD BE RESULTING IN HAIL
CONTAMINATION FOR SOME STORMS...A MESONET REPORT OF 0.55 INCHES OF
RAINFALL IN 15 MINUTES WAS RECEIVED FROM WASHOE COUNTY NV SUPPORTS
THE IDEA OF 1.50+ INCH HOURLY RAINFALL RATES.

THERE WAS A GOOD SIGNAL IN THE HIGH RESOLUTION GUIDANCE FOR
CONVECTION TO CONTINUE OVER WESTERNMOST NV AND PORTIONS OF
NORTHEAST CA THROUGH 03Z OR SO. WEAK MID LEVEL FLOW AND HIGH
MOISTURE CONTENT AIR SHOULD SUPPORT LOCAL 1.00-2.00 INCH RAINFALL
AMOUNTS...THOUGH THESE AMOUNTS SHOULD BE SCATTERED. HOWEVER...THIS
IS MORE THAN ENOUGH (SINCE ONE HOUR FLASH FLOOD GUIDANCE VALUES
ARE GENERALLY LESS THAN AN INCH) TO INITIATE FLASH FLOODING.

HAYES

ATTN...WFO...REV...STO...

ATTN...RFC...CNRFC...

LAT...LON   39352069 39342067
40902081 40631987 39951939 38371857 38061895
            38271966 38762026 38772027 39612080 40492119
           


Last Updated: 707 PM EDT TUE AUG 22 2017