Quantitative Precipitation Forecast
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621
FOUS30 KWBC 290040
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
839 PM EDT WED SEP 28 2016

...VALID 01Z THU SEP 29 2016 - 12Z THU SEP 29 2016...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
15 ENE IDI 30 SE ELZ 25 SW BGM 30 NNE AVP 15 W 12N 15 NNE MIV
10 NW OXB 20 S ASJ EYF 25 SSW FLO 15 NNW SSC 15 NW VUJ 20 E HLX
20 NE HSP 15 ENE IDI.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
HBI 20 S LYH 15 ENE SHD 20 NNE CBE 10 SSE UNV 10 ENE SEG
10 NNE RDG 10 ENE ILG 15 W SBY 10 WNW FKN 20 S JNX HFF HBI.

MODERATE RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
25 WNW CJR 20 WNW HGR 25 ENE HGR 25 NW APG 10 NNE W29
20 SSW NHK 10 SSW FYJ 15 N EMV 25 SE FVX FVX 25 WNW CJR.


01Z UPDATE...
HAVE EXPANDED THE MDT RISK AREA TO THE SOUTH AND WEST WITH THE
SOUTHERN EXPANSION DUE PRIMARILY FROM ONGOING CONVECTIVE
TRENDS/TRAINING ALONG STATIONARY BNDRY ACRS CNTL/SRN VA WHICH HAS
LEAD TO LOCAL RAINFALL REPORTS IN EXCESS OF 6 INCHES IN SEVERAL
HOURS.  THE STRONGER CONVECTION CONTINUES TO EXTEND NWD ALONG AXIS
OF OF HIER MLCAPES FROM CNTL NC NWD WHERE STORMS HAVE BECOME A BIT
MORE FOCUSED ALONG THE STALLED FRONTAL BNDRY ACRS SRN VA. FEEL THE
HRRR MODELS HAVE HAD A BETTER HANDLE IN KEEPING HEAVIER RAINS TO
THE EAST TOWARD THE BETTER INSTABILITY/MUCAPES SO FAR AND SHOW A
GRADUAL SHIFT OF THE HEAVIER RAINS NWD ALONG THE I-95 CORRIDOR
TOWARD DCA/BWI DURING THE EARLY OVERNIGHT WITH STRENGTHENING SELY
LOW LEVEL JET.  A SEPARATE AREA OF HEAVIER RAINS IS LIKELY TO
EXPAND ALONG THE UPSLOPE AREAS OF THE BLUE RIDGE/ERN SLOPES OF THE
APLCNS AS WELL BASED ON VAD WIND PROFILES OUT OF LWX WHICH
CONTINUE TO SHOW SLOW STRENGTHENING OF LOW LEVEL E TO SELY FLOW IN
THE SFC-85H LAYER.  THIS INCREASE IN UPSLOPE FLOW/MSTR FLUX IN
CONCERT WITH INCREASING DIFFLUENCE ALOFT WITH APPROACH OF DEEP UPR
LOW OVER THE LOWER OH VALLEY SHOULD LEAD TO AN INCREASING HEAVY
RAIN THREAT WITH POTNL TRAINING ACRS THAT REGION DURING THE
REMAINDER OF THE NIGHT AND ESPECIALLY INTO THE DAY ON THURSDAY.
DESPITE THE PREFERRED ERN FOCUS BY THE HRRR AND HRRR EXPERIMENTAL
RUNS EARLY IN THE PD..SPREAD REMAINS QUITE HIGH BETWEEN THE HRRR
SOLNS WITH THE PLACEMENT OF HEAVIEST RAINS.  THAT SAID..TIME
LAGGED ENSEMBLE RUNS OF THE HRRR SHOW 6 HR PROBABILITIES IN EXCESS
OF 6 INCHES OF BETWEEN 25 TO OVER 40 PCT FOR PARTS OF CNTL MD SWD
INTO CNTL/ERN VA..SO THE THREAT OF SIGNFICANT HEAVY RAIN AND PSBL
FLASH FLOODING CERTAINLY HAS SUFFICIENT MODEL SUPPORT FOR A MDT
RISK.  SULLIVAN

PREVIOUS DISCUSSION

...MID ATLANTIC INTO THE NORTHEAST...

THE SOUTHWARD MOVING CLOSED LOW OVER THE OH VALLEY WILL BE BACKING
THE LOW TO MID LEVEL FLOW DOWNSTREAM OF IT ACROSS THE MID
ATLANTIC---BEGINNING A MULTI DAY PRECIPITATION EVENT THAT WILL
BRING QUITE A BIT OF RAIN OVER NORTHERN PORTIONS OF THE MID
ATLANTIC REGION. AS THE DEEP-LAYER FLOW BACKS S-SE...THE INJECTION
OF MOISTURE OFF THE ATLANTIC GULF STREAM WILL BRING HIGHER PWS
W-NW INTO THE MID ATLANTIC REGION. TAKING A LOOK AT THE LATEST 00Z
GFS-BASED ESRL FORECAST OF INTEGRATED WATER VAPOR TRANSPORT (IVT)
SHOWS A WELL-DEFINED ATMOSPHERIC RIVER SET UP FROM OFF THE
ATLANTIC COAST NORTHWEST THROUGH THE NORTHERN MID ATLANTIC
REGION...THEN WESTWARD INTO THE OH VALLEY AND SOUTHERN LOWER MI.
MEANWHILE...THE NEGATIVE U (EASTERLY) WIND MOISTURE FLUX ANOMALIES
CLIMB TO AROUND 3 STANDARD DEVIATIONS ABOVE NORMAL ALONG THE
MASON-DIXON LINE PER THE GEFS/ECENS. ADDING TO WHAT WILL BECOME A
VERY FAVORABLE THERMODYNAMIC ENVIRONMENT IS SOME ELEVATED
INSTABILITY TO WORK WITH -- WITH MUCAPES AVERAGING 400-800 J/KG
PER THE MODELS.

IN TERMS OF THE DAY 1 QPF...WPC FOLLOWED A BLEND OF CONTINUITY
(PREVIOUS FORECAST) ALONG WITH THE 00Z GFS...UKMET...AND RGEM. THE
00Z ECMWF IS SOMEWHAT OF A WET OUTLIER WITH THE HEAVY RAIN AXIS
REACHING FARTHER NORTHEAST INTO SOUTHEAST PA...NORTHERN DE...AND
SOUTHERN NJ...THUS FOR NOW HAVE NOT INCORPORATED THE ECMWF IN THE
QPF BLEND OVER THIS AREA. MEANWHILE...THE 00Z NAM IS A NOTABLE
WESTERN OUTLIER WITH THE HEAVIEST RAINFALL (2-3+ INCHES) ACROSS
THE BLUE RIDGE AND EASTERN SLOPES OF THE APPALACHIANS. GIVEN THAT
THE MEAN LOW-LEVEL INFLOW WILL HAVE MORE OF A SOUTHERLY VS.
EASTERLY COMPONENT (I.E. MINIMAL UPSLOPE) THROUGH MUCH OF THE
PERIOD...ALONG WITH THE INSTABILITY GRADIENT EAST OF THE BLUE
RIDGE...HAVE DISMISSED THE 00Z NAM QPF.

HEAVIEST QPF -- 2 TO 3+ INCHES -- WAS NOTED ALONG AND WEST OF THE
I-95 CORRIDOR TO INCLUDE RIC...DCA...BWI...AND MDT. DESPITE THE
DRY ANTECEDENT CONDITIONS (RELATIVELY HIGH FFG)...WILL MAINTAIN
THE LARGE SLIGHT AND SMALL MODERATE RISK AREAS (WITH SOME MINOR
TWEAKS) IN THE NEW DAY 1 EXCESSIVE RAINFALL OUTLOOK THAT WERE
INHERITED FROM YESTERDAY`S DAY 2 ERO. PER THE HIGH RESOLUTION CAM
GUIDANCE... INCLUDING THE WRF-ARW...NMMB...AND NAM
CONEST...LOCALIZED TOTALS OF 4-7" WERE NOTED.

HURLEY
$$





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