Quantitative Precipitation Forecast
Issued by NWS

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FOUS30 KWBC 200814
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
412 AM EDT THU APR 20 2017

...VALID 12Z THU APR 20 2017 - 12Z FRI APR 21 2017...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
30 NE HBR 25 W BVO 15 ENE AIZ 15 NNW MDH 20 SSE PAH 10 SE DYR
15 NNE SRC 15 W RKR 10 SE LAW 30 NE HBR.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
20 SSW CWNC 20 NW SLK SCH 20 WNW FWN 10 WNW SEG 15 W JST
20 SSW YNG 10 NE HZY 20 SSW CWNC.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
30 SW SGF 20 W CGI 35 S CIR 20 N M19 25 ENE ADH 20 E CQB
30 SW SGF.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
35 SW SCH 35 ESE BGM 10 ESE ELM 25 NE PEO 15 SSE GTB 50 SSW SLK
35 SW SCH.


...CENTRAL-EASTERN OKLAHOMA EASTWARD INTO SOUTHERN MISSOURI AND
NORTHERN ARKANSAS...

THE INITIAL SHORTWAVE PROGRESSING SLOWLY ENE ACROSS THE UPPER MS
VALLEY TODAY...ALONG WITH THE PRE-FRONTAL CONVECTION AND ATTENDANT
OUTFLOW BOUNDARIES...WILL ALLOW THE SURFACE FRONT TO DROP INTO
SOUTHERN OK-CENTRAL AR BY LATE IN THE DAY. THIS BOUNDARY WILL BE
THE FOCUS FOR POTENTIALLY HEAVY RAINFALL FROM DURING DAY 1...AS A
VIGOROUS NW-SE ORIENTED UPPER LEVEL JET STREAK DROPS INTO THE 4
CORNERS REGION AND AMPLIFIES THE UPPER TROUGH OVER THE
CENTRAL-SOUTHERN ROCKIES. THE DEVELOPING LEE-SIDE LOW AND
DOWNSTREAM UPPER DIFLUENCE...ALONG WITH CONTINUED FORCING WITHIN
THE RIGHT-ENTRANCE REGION OF THE UPPER JET STREAK DRAPED OVER THE
CENTRAL PLAINS...WILL MAKE FOR A DYNAMICALLY-FAVORED ENVIRONMENT
(STRENGTHENING UPPER DIVERGENCE AND LOWER LEVEL FGEN ALONG THE
STATIONARY BOUNDARY) ACROSS CENTRAL-EASTERN OK E-ENE INTO SOUTHERN
MO AND NORTHERN AR. MEANWHILE... A POOL OF ANOMALOUS MOISTURE (PW
AND 850-700 MB MOISTURE FLUX STANDARDIZED ANOMALIES AROUND +2 PER
THE SREF AND GEFS) ADVECTING ACROSS THE STATIONARY
BOUNDARY...ALONG WITH MODEST DEEP-LAYER INSTABILITY (MUCAPES
1000-1500 J/KG)...WILL PROVIDE A FAVORABLE THERMODYNAMIC
ENVIRONMENT FOR AREAS OF HEAVY RAINFALL ACROSS THE
REGION...PARTICULARLY OVERNIGHT. FURTHERMORE... WHILE NOT OVERLY
ROBUST...THE STRENGTHENING LOW-LEVEL WSW FLOW AHEAD OF THE FRONT
WILL BECOME MORE PARALLEL AND CLOSE TO THE MAGNITUDE OF THE MEAN
850-300 MB FLOW BETWEEN 00-12Z...ENHANCING THE POTENTIAL FOR
UPWIND PROPAGATION AND THUS CELL TRAINING.

AS A RESULT...WPC WILL CONTINUE TO CARRY A "SLIGHT" RISK OF
EXCESSIVE RAINFALL INHERITED FROM THE PREVIOUS DAY 2 OUTLOOK...
THOUGH BASED ON THE LATEST GUIDANCE TRENDS (PARTICULARLY WITH THE
ENSEMBLE OF HIGH-RES WINDOWS)...HAVE SHIFTED THE OUTLOOK SLIGHTLY
ENE. IN TERMS OF THE QPF...WPC UTILIZED A MULTI-MODEL BLEND...LED
BY THE HIGH-RES MEANS ALONG WITH THE WPC BIAS-CORRECTED ENSEMBLE
MEAN...WHICH YIELDED A STRIP OF 1-1.5" AREAL-AVERAGE TOTALS ACROSS
THIS AREA. BASED ON THE AFOREMENTIONED FAVORABLE DYNAMICAL AND
THERMODYNAMICAL PARAMETERS...SOME OF THE HIGHER LOCALIZED AMOUNTS
(2-3+ INCHES) ARE REFLECTED ACCORDINGLY IN THE HIGH-RES CAMS. ALSO
HELPING TO DELINEATE THE SLIGHT RISK AREA INCLUDED THE RELATIVELY
LOWER FFG VALUES PER THE RFCS AND HIGHER SOIL SATURATION VALUES
PER THE NATIONAL WATER MODEL...BOTH OF WHICH WOULD BE MORE
CONDUCIVE TO LOCALIZED RUNOFF ISSUES.


...NEW YORK-PENNSYLVANIA...

THE AFOREMENTIONED COMPACT NORTHERN STREAM SHORTWAVE TRAVERSING
THE UPPER MS VALLEY-GREAT LAKES DURING DAY 1 WILL PROVIDE AN
EXTENSIVE SWATH OF MODERATE WAA PCPN WITH EMBEDDED HEAVIER AMOUNTS
WITHIN AREAS OF CONVECTION. PER A MULTI-MODEL BLEND...AN AREA OF
1-1.25" AMOUNTS WAS NOTED ACROSS MUCH OF WESTERN AND UPSTATE
NY...ALONG WITH NORTHWEST PA AND SOUTHEAST LOWER MI. DESPITE THE
ANOMALOUS MOISTURE AVAILABILITY AND VIGOROUS DEEP-LAYER DYNAMICAL
FORCING...INSTABILITY BECOMES MORE LIMITED N-NE OF THE OH VALLEY
(MUCAPES DROP TO UNDER 500 J/KG). THIS WILL IN-TURN MITIGATE
SHORT-TERM RAINFALL RATES...HOWEVER...GIVEN THE LOWER FFG
VALUES/ANTECEDENT WET SOILS OVER THESE AREAS...WILL MAINTAIN A
RELATIVELY SMALL "SLIGHT" RISK FOR EXCESSIVE RAINFALL OVER
PORTIONS OF NY...PARTICULARLY THE CATSKILLS WHERE THE FFG IS
LOWEST...ENCOMPASSED BY A BROADER "MARGINAL" RISK THAT COVERS THE
NORTHERN HALF OF PA AND MUCH OF NY STATE.

HURLEY
$$





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