Quantitative Precipitation Forecast
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FOUS30 KWBC 210830
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
429 AM EDT TUE MAR 21 2017

...VALID 12Z TUE MAR 21 2017 - 12Z WED MAR 22 2017...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
10 N PTV 25 NE MOD 15 SSW RBL 10 ESE O54 25 WSW MHS 10 NNW MHS
20 E MHS 30 E RDD 30 NE CIC 10 SSE BLU 35 SSW TVL 50 NE MCE
35 SSW MMH 30 NE PTV 10 N PTV.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
10 WNW CIR CSV 10 SSW RHP 15 NW GVL 15 E RMG 30 N MSL 15 W JBR
15 E FYV 20 SE JLN 10 SSW TBN 10 WNW CIR.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
50 SSW SBA 30 SSW LPC XVW 30 N SBA 25 W SDB 20 NE CMA
10 ESE CMA 20 S NTD.


...CALIFORNIA....

THE FULL LATITUDE TROUGH OFF THE WEST COAST WILL AGAIN BECOME
REINFORCED FROM BOTH NORTHERN AND SOUTHERN STREAM COMPONENTS
UPSTREAM -- THIS AS THE INITIAL CYCLONE OFF THE PAC NW BECOMES
ABSORBED INTO THE DEEPENING LOW OFF THE B.C. COAST WEDNESDAY
MORNING. MEANWHILE...THE APPROACHING UPPER LEVEL JET STREAK
ROUNDING THE TROUGH BASE WILL FURTHER AMPLIFY THE SOUTHERN STREAM
WAVE AS IT APPROACHES CA LATE IN THE PERIOD. PRE-FRONTAL
STRENGTHENING ONSHORE SOUTH-SOUTHWESTERLY FLOW IN AN AXIS OF 3-3.5
STANDARD DEVIATIONS ABOVE THE MEAN PW ANOMALIES WILL SUPPORT
WIDESPREAD MODERATE TO HEAVY PRECIP TOTALS FROM THE CENTRAL COAST
RANGE INTO THE SOUTHERN-CENTRAL SIERRA...WHILE THE POST-FRONTAL
WESTERLY FLOW (STILL MOIST YET NOT NEARLY AS ANOMALOUS) WILL
RESULT IN ADDITIONAL MOD-HEAVY PCPN AMOUNTS ACROSS FROM THE BAY
AREA-NORTH COAST AND INTO THE SHASTA/SISKIYOUS AND NORTHERN
SIERRA. IN TERMS OF THE QPF...WPC AGAIN UTILIZED A MULTI-MODEL
BLEND OF (MAINLY) THE HIGHER RESOLUTION GUIDANCE...GIVEN THE
BETTER SENSITIVITY OVER THE TERRAIN.

A MARGINAL RISK OF EXCESSIVE RAINFALL WILL REMAIN ACROSS EASTERN
SACRAMENTO/SAN JOAQUIN VALLEYS INTO THE WESTERN SLOPES OF THE
SIERRA DURING THE DAY 1 PERIOD...AIDED BY THE WET ANTECEDENT
CONDITIONS AND (OVER THE HIGHER TERRAIN) ADDED CONTRIBUTION FROM
SNOWMELT.


...LOWER MISSISSIPPI VALLEY / TENNESSEE VALLEY / SOUTHEAST...

ANOTHER ROUND OF ELEVATED CONVECTION...BUOYED TO A LARGE DEGREE BY
STEEP MID LEVEL LAPSE RATES (7.5-8.0+ C/KM BETWEEN 700-500 MB)
WILL ALIGN ALONG AND BEHIND THE QUASI WEST-EAST SURFACE FRONT
DROPPING INTO THE LOWER MS VALLEY-SOUTHEAST REGION DURING THE
PERIOD. THE MODEL QPF SPREAD IS NOT AS SIGNIFICANT TODAY AS IT WAS
YESTERDAY WHEN THE CONVECTION WAS FARTHER NORTH (MIDWEST-OH
VALLEY). WPC THEREFORE WAS ABLE TO INCORPORATE A MULTI-MODEL BLEND
(GFS/ECMWF/RGEM/PARALLEL NAM)...WHICH MATCHED UP FAIRLY WELL WITH
THE HIGH-RES MEANS.

GIVEN THE AVAILABLE INSTABILITY (MODEL MUCAPES AVERAGING 1000-2000
J/KG)...ALONG WITH THE POTENTIAL FOR TRAINING THIS AFTERNOON AND
EVENING AS THE WESTERLY LOW-LEVEL FLOW ALIGNS NEARLY PARALLEL TO
THE MEAN 850-300 MB FLOW... LOCALIZED SHORT-TERM RUNOFF ISSUES
WILL BE POSSIBLE (MARGINAL RISK OF EXCESSIVE
RAINFALL)...PARTICULARLY FROM SOUTHERN MO-NORTHERN AR ESE THROUGH
MUCH OF THE TN VALLEY WHERE THE ANTECEDENT CONDITIONS (LOWER 1-3
HOURLY FFG VALUES) WOULD BE MORE CONDUCIVE.

HURLEY
$$





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