Quantitative Precipitation Forecast
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FOUS30 KWBC 210034
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
833 PM EDT THU APR 20 2017

...VALID 01Z FRI APR 21 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
15 SSW DUJ 10 SW FKL 15 WSW HZY 20 WNW HZY 10 WSW ERI
25 WSW DKK 15 NNE DKK 10 S IAG CYSN 35 NE BUF 15 NE ROC 20 N FZY
10 N ART 25 NE GTB 40 SSW SLK 15 WSW GFL 15 SW SCH 25 NE MSV
15 ESE MSV 20 S MSV 35 W MSV 35 SSW BGM 35 WNW IPT FIG
15 SSW DUJ.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
40 SSE CDS 45 SW CDS 30 WSW CDS 40 WSW CSM 25 ENE CSM
15 NNW GOK 25 SE CFV 25 W SGF 25 ENE SGF 20 WSW FAM 15 SSW MDH
M30 20 S PAH 30 ENE DYR 15 SSW HKA 25 NW SRC 30 WSW RKR
15 WNW ADM 10 SSW DUC 20 SSE F05 40 SSE CDS.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
25 SW SGF 20 S SGF 20 WSW UNO 25 NW BVX 25 NW RUE ADH
20 ENE DUC 20 WNW LAW 25 ENE HBR PWA 10 NNW CQB 20 WSW RVS
10 ENE GCM 25 SW SGF.




21/01 UTC UPDATE...

SOME FURTHER WWD EXPANSION OF PREVIOUS SLIGHT RISK AREA WAS MADE
ACROSS OK OVERNIGHT WITH SEVERAL HI RES MODEL RUNS SUGGESTING
RAPID BLOSSOMING OF ORGANIZED CONVECTION NEAR AND NORTH OF
STATIONARY FRONT ACROSS SOUTHWEST TO CENTRAL OKLAHOMA BETWEEN
03-06Z WITH ARRIVAL OF STRONGER HEIGHT FALLS/MID AND UPPER FORCING
FROM APPROACHING SLIGHTLY NEGATIVELY TILTED S/WV PUSHING ACROSS
THE CNTL/SRN ROCKIES.  HRRR TIME LAGGED ENSEMBLES SUGGEST A SMALL
RISK OF 2 INCH OR GREATER RAINS ACROSS THE SLIGHT RISK AREA
INTIALLY OVER SW TO CENTRAL OKLAHOMA..BUT EXPANDING EWD INTO PARTS
OF NW ARKANSAS TOWARD 12Z.  POTNL FOR ISOLD STORM TOTALS EXCEEDING
5 INCHES FROM TRAINING/REPEAT ACTIVITY IS CLEARING SEEN IN RECENT
HRRR COMPOSITE REFLECTIVITY SIMULATIONS ACROSS THE SLIGHT RISK
AREA.

ELSEWHERE...NO CHANGES ARE PLANNED FOR MARGINAL RISK AREA ACROSS
NY/PA.  SULLIVAN

...15 UTC UPDATE...

OVER THE S-CENTRAL US...DECIDED AFTER COORDINATION TO SHRINK THE
NRN EDGE OF THE PREVIOUSLY ISSUED WPC MARGINAL AREA ACROSS THEIR
REGION BASED ON LATEST RADAR TRENDS. ALSO OPTED TO  SLIGHTLY
EXTEND THE SLIGHT AND MARGINAL AREA SWWD ACROSS S-CENTRL OK AND AS
FAR WEST AROUND CDS RESPECTIVELY CONSIDERING SOME MESO MODEL AND
THE 12 UTC NAM OFFER SOME HEAVY CONVECTIVE DEVELOPMENT SUPPORT
POTENTIAL IN LINE WITH THE APPROACHING SHORTWAVE AND SLOW MOVING
SURFACE BOUNDARY VS CAP ISSUES.

OVER NY/PA...ALSO DECIDED TO SLIGHTLY SHRINK THE MARGINAL THREAT
AREA INTO PA...BUT COMPLETELY REMOVE THE PREVIOUSLY ISSUED SLIGHT
RISK AREA FOR CENTRAL NY CONSIDERING MODEST RAINFALL
POTENTIAL/FFG. THE REST OF THE MARGINAL AREA WAS MAINTAINED.

SCHICHTEL


...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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