Quantitative Precipitation Forecast
Issued by NWS

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FOUS30 KWBC 180825
QPFERD

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

...VALID 12Z 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
10 SSE TOL 15 SSE HZY 30 WSW BFD 15 SSE DUJ 25 NNW HLG 25 S AOH
MIE 25 ESE LAF 20 SSW ASW 10 SSE TOL.


...OH VALLEY/NORTHEAST...

THE MAIN FOCUS FOR HEAVY TO LOCALLY EXCESSIVE QPF 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 18/18Z 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 FEED LOW TOPPED CONVECTION STRETCHING FROM EAST 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 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.
THE THREAT...HOWEVER...IS HIGHLY DEPENDENT ON THE AVAILABILITY OF
INSTABILITY... WHICH LOOKS AS THOUGH IT COULD BE A LIMITING
FACTOR.

BASED PRIMARILY ON THE PREPONDERANCE OF HIGH RESOLUTION
GUIDANCE...A MARGINAL RISK WAS STRETCHED FROM EAST CENTRAL IL
ACROSS MUCH OF IN/OH INTO NORTHERN AND CENTRAL WV INTO WESTERN PA.
WITHIN IN THIS AREA...A SLIGHT RISK AREA WAS EXTENDED FROM EAST
CENTRAL IN ACROSS NORTHERN OH INTO A PORTION OF WESTERN
PAN...BASED ON THE STRENGTH OF THE HIGH RESOLUTION GUIDANCE
SIGNATURE...AND THE RELATIVELY LOW FLOOD FLOOD GUIDANCE VALUES.

HAYES
$$





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