Quantitative Precipitation Forecast
Issued by NWS

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FOUS30 KWBC 230828
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
427 AM EDT FRI SEP 23 2016

...VALID 12Z FRI SEP 23 2016 - 12Z SAT SEP 24 2016...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
10 SSE CXWN 40 WNW CWSU 30 SSE HCO 20 ESE DVL BIS 25 NNE D07
35 WSW PHP 20 WSW CDR 35 NE SIB 20 ENE DGW 35 NNE DRC 10 SSE TMH
15 W CPR 35 NW HLD 15 WSW WRL 40 WSW WRL 40 WNW RIW 10 W LND
15 SSW BVR 35 SW BVR 25 E BPI 20 SSE BPI 35 W CMS 45 WNW VEL
30 WSW PUC 15 SE U24 30 NW U24 25 NNE ENV 10 E JER 15 WSW DLN
35 SSE 3HT 65 NE LWT 25 NE CWEH 25 SSE CWLE.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
35 SSE CHS 15 N CHS 15 ESE FLO ISO 25 SSE EDE 25 NW HSE
30 SE MRH.


...WESTERN U.S. INTO THE NORTHERN PLAINS...

AN IMPRESSIVE GREAT BASIN / ROCKIES STORM WILL CONTINUE TO UNFOLD
THROUGH FRIDAY NIGHT. NEGATIVE 500 MB HEIGHT ANOMALIES OF UP TO 3
STANDARD DEVIATIONS WILL SLIDE EAST WITH THE CENTER OF THE
DEVELOPING CLOSED CIRCULATION. THE SYSTEM REMAINS
PROGRESSIVE...BUT THE PROGRESSION IS EXPECTED TO BE SO SLOW...AND
WITH A PIVOT OCCURRING OVER THE CENTRAL GREAT BASIN AND CENTRAL
ROCKIES TODAY...ADDITIONAL MOD-HVY AMOUNTS OF RAIN AND HIGH
ELEVATION SNOW ARE EXPECTED. PRECIPITATION EFFICIENCY IS ENHANCED
BY THE INVOLVEMENT OF ANOMALOUS DEEP LAYER MOISTURE HAVING BEEN
PUMPED INTO THE REGION OUT OF THE SUB-TROPICS PRIOR TO ARRIVAL OF
THE NORTHERN STREAM TROUGH...AND STEEP LAPSE RATES OWING TO THE
DEPTH OF THE TROUGH / STRENGTH OF THE DEEP LAYER LIFT. PW
ANOMALIES OF 4+ STANDARD DEVIATIONS ABOVE NORMAL WILL SHIFT INTO
THE NORTHERN HIGH PLAINS DURING THE PERIOD...ALONG WITH THE REGION
OF FAVORABLE UPPER DIFFLUENCE AHEAD OF THE HIGHLY-AMPLIFIED UPPER
TROUGH/CLOSED UPPER LOW. THE MAIN INHIBITOR FOR EXCESSIVE
SHORT-TERM (1-3 HOURLY) RAINFALL RATES...IN ADDITION TO THE UPPER
SYSTEM BECOMING MORE PROGRESSIVE WITH TIME...WILL BE THE LACK OF
DEEP LAYER INSTABILITY AS MUCAPES AVERAGE LESS THAN 1000 J/KG.
MAXIMUM AREAL-AVERAGE QPF OF 1-1.25" IS NOTED... HOWEVER PER THE
HIGH-RES CAM GUIDANCE...LOCALIZED ADDITIONAL AMOUNTS BETWEEN
1.5-2.5" WERE NOTED. THEREFORE...WILL MAINTAIN A MARGINAL RISK FOR
EXCESSIVE RAINFALL IN THE DAY 1 EXCESSIVE RAINFALL OUTLOOK (ERO)
AS EXPECT ANY SHORT-TERM RUNOFF ISSUES TO BE BE ISOLATED.


...LOWER MID ATLANTIC COAST...

BROAD LOW-MID LEVEL CYCLONIC (ONSHORE) FLOW WILL MAINTAIN DEEP
MOISTURE THROUGH THE FIRST HALF OF THE DAY 1 PERIOD (FRIDAY)...AS
PWS REMAIN BETWEEN 1.75-2.00 INCHES. THE WEAK UPPER LEVEL SUPPORT
(MID LEVEL DEFORMATION) AND INFLUX OF ADDITIONAL SUBTROPICAL BANDS
OFF THE SOUTHEAST COAST WILL BRING ADDITIONAL LOCALLY HEAVY
RAINFALL TO AREAS ALONG THE NORTHERN SC/SOUTHERN NC COASTS THAT
HAVE SEEN 3-6" OF RAIN OVER THE PAST FEW DAYS. WPC UTILIZED MORE
OF THE HIGH-RESOLUTION CAMS OVER THIS REGION EARLY IN THE PERIOD
(ESPECIALLY THROUGH 18Z FRI)...GIVEN THE BETTER HANDLE ON THE
MOIST CONVECTIVE ENVIRONMENT. WPC WILL INCLUDE A MARGINAL RISK FOR
EXCESSIVE RAINFALL IN THE DAY 1 ERO OVER THIS AREA...THOUGH FOR
NOW REFRAIN FROM A SLIGHT RISK GIVEN THE LACK OF A SIGNAL FROM THE
GUIDANCE IN TERMS OF THE COVERAGE AND MAXIMUM AMOUNTS (LOCALIZED
TOTALS UP TO 1.5-2.0").

HURLEY
$$





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