Quantitative Precipitation Forecast
Issued by NWS

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000
FOUS30 KWBC 130047
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
846 PM EDT TUE SEP 12 2017

...VALID 01Z WED SEP 13 2017 - 12Z WED SEP 13 2017...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
90 SE SUT 45 SE SUT 20 SE EYF 10 S TTA EXX 15 SSW MRN
15 WNW CEU 30 NNE GVL 15 SE RHP 20 W AVL 20 WNW AVL 20 W TNB
15 NNE MKJ HSP 25 E HSP 25 SSW SHD 20 N FVX 15 ESE FVX 20 NW AVC
25 SSW ASJ 25 NE MRH 95 SE MRH.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
45 W MMH 15 SSE BLU 45 E RDD 25 N RDD 35 S O54 35 WSW O54
30 SE CEC 20 W SIY 15 W LMT 65 NNE LMT 50 N LKV 20 N LKV
25 SW LKV 35 SSW AAT 25 NW RNO 55 S NFL 30 SSE BIH 60 NE PTV
40 ENE VIS 45 W MMH.


...NORTHERN CA AND THE SIERRA NEVADA...
...ADJACENT PORTION OF INTERIOR NEVADA...

MODEL DIFFERENCES REMAIN MODEST DURING THE 12/12Z AND 12/18Z
CYCLES ACROSS MUCH OF THE AREA WITH RESPECT TO THE CLOSED MID
LEVEL LOW OFF THE CENTRAL CA COAST.  THIS SYSTEM IS STILL EXPECTED
TO DRIFT CLOSER TO THE COAST TOWARD THE END OF THE PERIOD LATER
TONIGHT AND EARLY WEDNESDAY MORNING.  BROADLY DIFFLUENT UPPER FLOW
IN AN AXIS OF ABOVE-AVERAGE PWAT VALUES HAS ALREADY BEEN A STRONG
ENOUGH COMBINATION TO TRIGGER SCATTERED SHOWERS AND THUNDERSTORMS
LATE THIS AFTERNOON/EARLY EVENING OVER NORTHERN CA INTO THE
INTERIOR PORTION OF NEVADA WITH GREATEST INITIALLY NEAR THE WEST
ASPECT OF THE SIERRA NEVADA RANGE.

A QUICK LOOK AT THE 00Z SOUNDINGS SHOWED MORE MOISTURE IN THE
LOW/MID LEVELS THAN LAST NIGHT...BUT THE PROFILES STILL SUGGEST
THAT ENOUGH DRY AIR IN THE SUBCLOUD LAYER WILL KEEP THE BEST
RAINFALLS OVER THE HIGHER TERRAIN WHERE THERE WILL BE LESS TIME
FOR EVAPORATION OF RAIN FALLING OUT OF THE CLOUD.  THE ALSO
PROFILES AND HODOGRAPHS SUGGEST THAT CELLS SHOULD GENERALLY BE
PROGRESSIVE...ESPECIALLY IN AREAS CLOSER TO THE MID LEVEL
CIRCULATION CENTER ALONG THE COAST.

A COUPLE OF THE HIGH RESOLUTION MODELS HAD A SIGNAL FOR VERY HEAVY
RAINFALL ALONG THE COAST S OF SFO OVERNIGHT.  THINKING IS THAT THE
AMOUNTS SHOWN BY THOSE MODELS WERE TOO AGGRESSIVE...THE ABOVE
AVERAGE MOISTURE AND INTERACTION WITH COMPLEX TERRAIN COULD STILL
RESULT IN PERIODS OF HEAVY RAINFALL BUT THAT AMOUNTS SHOULD BE
HELD IN CHECK BY STEADY FORWARD PROPAGATION.  NOTE THAT WE
EXPANDED THE MARGINAL RISK AREA FARTHER NORTH WHERE THE UPPER
LEVEL DIFLUENCE WAS GREATEST AND LOW LEVEL INSTABILITY WAS A BIT
BETTER THAN TO THE SOUTH.

BANN

$$





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