Quantitative Precipitation Forecast
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000
FOUS30 KWBC 181325
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
824 AM EST SAT NOV 18 2017

...VALID 15Z SAT NOV 18 2017 - 12Z SUN NOV 19 2017...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
15 W CWPS 10 NNW DKK 10 SSW DSV 40 SE ELM 30 E SEG 30 SSE UNV
20 S JST 15 SSW 2G4 15 SSE EKN LWB 20 ENE BKW 20 ESE CRW
25 NNE CRW 15 SW PKB 20 S ZZV 10 ENE LHQ 15 SW LCK ILN
15 NNW CVG 15 SE BAK RSV 10 NNW 1H2 25 S MQB GBG 30 SSW SQI RPJ
PWK 30 W BEH YIP 15 NNE CWAJ 15 W CWPS.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
15 NNW FDY 20 NE AKR 25 NNW BTP BTP 10 SE PIT 25 WNW HLG
15 NE DAY 20 SE AID 20 NW EYE GUS 15 NNW FDY.



...OH VALLEY/MID-ATLANTIC...

SEE MESOSCALE PRECIPITATION DISCUSSION 0935 FOR DETAILS ON THE
NEAR TERM FLASH FLOOD THREAT THROUGH 18Z. IN THE 15Z UPDATE OF THE
EXCESSIVE RAINFALL OUTLOOK WE EXPANDED SLIGHT RISK BACK WESTWARD
INTO ILLINOIS WHERE DEEP LAYER ASCENT WAS PHASING WITH SOME OF THE
STRONGER AVIALABLE INSTABILITY TO YIELD ROBUST CONVECTION WITH
HOURLY RAIN RATES PUSHING WELL ABOVE ONE INCH. THE RECENT HRRR
RUNS AND THE 00Z NSSL WRF WERE HANDLING THE SITUATION WELL.
OTHERWISE THE OUTLOOK AND REASONING REMAIN RELATIVELY UNCHANGED.

PREVIOUS DISCUSSION...THE MAIN FOCUS FOR HEAVY TO LOCALLY
EXCESSIVE RAIN WILL BE AHEAD OF AN EVOLVING LONG WAVE TROUGH
TRACKING FROM THE UPPER GREAT LAKES INTO THE OH/TN VALLEYS DURING
THE AFTERNOON. AHEAD OF THE TROUGH (AND ITS ASSOCIATED SURFACE
FRONT)...A 40 TO 50 KNOT LOW LEVEL SOUTHWEST FLOW PUMPS 1.25 INCH
PRECIPITABLE WATER (WHICH IS BETWEEN TWO AND THREE STANDARD
DEVIATIONS ABOVE THE MEAN) INTO TN/KY/IN/OH BETWEEN 18/12Z AND
19/00Z. SYNOPTIC SCALE ASCENT AHEAD OF THE FRONT IS AUGMENTED BY
DIVERGENCE IN THE LEFT FRONT QUADRANT OF A 130 KNOT JET MOVING
FROM THE LOWER MS VALLEY INTO THE TN VALLEY DURING THE SAME TIME
PERIOD.

MODEL SOUNDINGS ACROSS THESE AREAS GENERALLY SHOWED 250-500 J/KG
OF MUCAPE AHEAD OF THE FRONT (WITH THE 00Z NAM INDICATING THE
HIGHER END OF THE RANGE). THIS MUCH INSTABILITY SHOULD BE
SUFFICIENT TO FEED LOW TOPPED CONVECTION STRETCHING FROM CENTRAL
IL ACROSS NORTHERN IN INTO NORTHERN OH/WESTERN PA. THERE IS SOME
MODEL SPREAD CONCERNING THE PLACEMENT OF THE MAXIMUM QPF WITH THE
LOW TOPPED CONVECTION...AND THIS IS TIED AT LEAST TO SOME DEGREE
TO THE PLACEMENT OF THE AXIS OF BEST INSTABILITY. THE 00Z HREF
MEAN SHOWED THE BEST INSTABILITY GENERALLY SOUTH OF THE
GLOBAL/REGIONAL MODELS...AND THE WPC QPF AXIS OF 1.75 TO 2.25
INCHES AREAL AVERAGE PRECIPITATION WAS STRETCHED OVER THE
ABOVEMENTIONED LOCATIONS...WHERE THE BEST INSTABILITY IS EXPECTED
TO LIE.

THREE HOUR FLASH FLOOD GUIDANCE VALUES ARE AS LOW AS 1.50 TO 2.00
INCHES ACROSS PORTIONS OF OH/WESTERN PA INTO NORTHERN WV. SOME
HIGH RESOLUTIONS (MOST NOTABLY THE 00Z NAM CONEST) HAD 3.00+
INCHES OF QPF IN THESE AREAS...SO THERE IS A FLASH FLOOD THREAT.

HAYES/BURKE
$$




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