Flash Flood Guidance
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000
AWUS01 KWNH 211454
FFGMPD
CAZ000-220000-

MESOSCALE PRECIPITATION DISCUSSION 0078
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1053 AM EDT TUE MAR 21 2017

AREAS AFFECTED...N-CNTRL...CNTRL AND SRN CA

CONCERNING...HEAVY RAINFALL

VALID 211452Z - 220000Z

SUMMARY...INCREASING INSTABILITY IS EXPECTED TO SUPPORT RAINFALL
RATES UP TO 1 IN/HR IN THE NRN SAN JOAQUIN VALLEY AND THE
FOOTHILLS OF THE SIERRAS AFTER 18Z. FARTHER SOUTH...LOCALIZED
RAINFALL RATES OF 0.5 IN/HR WILL BE POSSIBLE LATE THIS MORNING TO
THE EARLY AFTERNOON WITH THE THREAT DIMINISHING INTO THE LATE
AFTERNOON.

DISCUSSION...A 24 HR LOOP OF THE LAYERED PW PRODUCT SHOWED THE
MERGING OF TWO STREAMS OF HIGHER MOISTURE IN THE 850-700 MB LAYER
FOCUSED INTO THE CNTRL CA COAST...COINCIDENT WITH THE GREATEST
WATER VAPOR FLUX IN THE TROPOSPHERE. THE RESULT IN THE BLENDED TPW
PRODUCT WAS A LARGE AREA OF ANOMALOUS MOISTURE WITH 1+ INCH
PRECIPITABLE WATER VALUES WHICH EXTENDED FROM THE NRN SACRAMENTO
VALLEY TO THE LOS ANGELES METRO AREA AS OF 14Z. A LARGE CLOSED LOW
WAS OBSERVED ON WATER VAPOR IMAGERY OFFSHORE OF NRN CA WITH STEADY
HEIGHT FALLS SHIFTING EAST ALONG WITH A COLD FRONT AT THE SURFACE.

DESPITE EXTENSIVE CLOUD COVER SEEN ON IR SATELLITE IMAGERY ACROSS
MUCH OF CA...THE COMBINATION OF DAYTIME HEATING AND COOLER
MID-LEVEL TEMPERATURES ASSOCIATED WITH THE ADVANCING CLOSED LOW
WILL SUPPORT INCREASING INSTABILITY INTO THE CNTRL CA VALLEY WITH
A CONSENSUS OF THE LATEST RAPIDLY UPDATING MODELS INDICATING
ANYWHERE FROM 250 TO 750+ J/KG OF CAPE DEVELOPING OVER THE CNTRL
AND NRN SAN JOAQUIN VALLEY BETWEEN 18-00Z. WHILE PRECIPITABLE
WATER VALUES AND SUBSEQUENT INTEGRATED VAPOR TRANSPORT WILL BE
FALLING INTO THE AFTERNOON...UNIDIRECTIONAL FLOW FROM THE
SOUTHWEST WITH A STEADY 25-30 KTS OF 850-700 MB FLOW MAY SUPPORT A
FEW AREAS OF TRAINING HEAVY RAIN AHEAD OF THE ADVANCING COLD
FRONT. DIFFLUENT FLOW ALOFT AND THE ADDED UPSLOPE COMPONENT TO THE
FLOW INTO THE SIERRA NEVADA SHOULD HELP MAKE UP FOR THE LOSS OF
MOISTURE TO SUPPORT RAINFALL RATES OF 0.5 TO 1.0 IN AT TIMES THIS
AFTERNOON.

FARTHER SOUTH...THE BEST MOISTURE FLUX WILL BE DIRECTED INTO THE
TRANSVERSE RANGES OF SRN CA OVER THE NEXT 6 HOURS WITH A FOCUS
INTO THE SANTA YNEZ...SAN RAFAEL...AND SIERRA MADRE RANGES FOR
RAINFALL RATES POTENTIALLY REACHING 0.5 IN/HR. HOWEVER...AS THE
UPPER TROUGH CONTINUES TO ADVANCE EAST TODAY...PRECIPITABLE WATER
VALUES ARE FORECAST TO GRADUALLY DIMINISH ALONG WITH WEAKENING LOW
LEVEL FLOW WHICH SHOULD GREATLY REDUCE THE HEAVY RAINFALL THREAT
AFTER 21Z FOR SRN CA.

OTTO

...PLEASE SEE WWW.WPC.NCEP.NOAA.GOV FOR GRAPHIC PRODUCT...

ATTN...WFO...HNX...LOX...MTR...STO...

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

LAT...LON   39942137 39722099 39432066 38992037 38201994
            37661967 37401936 36981884 36461855 36131843
            35631843 35011838 34431869 33931923 33872031
            34782099 35682165 36542219 38222245 39072224
            39682184



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