Quantitative Precipitation Forecast
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FOUS30 KWBC 220909
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
509 AM EDT SAT APR 22 2017

...VALID 12Z SAT APR 22 2017 - 12Z SUN APR 23 2017...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
30 SE FTK 25 NE CRW 10 NW KW66 2W6 20 W MFV ORF 15 E ASJ
15 NE TTA 10 ESE FQD 20 ENE 47A 4A9 15 S 1M4 55 N MSL 30 SE FTK.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
25 NE NQX 10 N MTH 10 E MTH 10 SSE MTH 20 SE NQX 25 SW KEY
35 WSW KEY 25 WNW KEY 25 NE NQX.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
20 W SHD 20 ENE LYH 20 NNE INT 10 N AVL 15 W CHA 30 NNW MDQ BNA
25 NNW SME 30 SSE HTS 20 W SHD.


...TENNESSEE VALLEY / LOWER OH VALLEY / CENTRAL-SOUTHERN
APPALACHIANS / MID ATLANTIC...

THE SLOW MOVING...LARGELY W-E ORIENTED UPPER TROUGH CURRENTLY
ACROSS THE CENTRAL CONUS WILL PIVOT INTO THE LOWER MS VALLEY BY
THE END OF THE PERIOD...DEEPENING AND IN-TURN BECOMING MORE
NEUTRALLY-TILTED BY THE END OF THE PERIOD. THE MODELS ARE FAIRLY
WELL CLUSTERED MASS FIELD WISE DURING THE DAY 1 PERIOD...BOTH
ALOFT AND AT THE SURFACE...WITH THE EVOLVING PATTERN LEADING TO A
MODEL CONSENSUS OF A ~1008 MB SURFACE LOW IN NORTHWEST GA BY 12Z
SUN. THE DECELERATING SYSTEM ALONG WITH INCREASINGLY FAVORABLE
DYNAMICS (UPPER DIFFLUENCE COUPLED WITH PROLONGED RIGHT ENTRANCE
REGION (FRONTOGENETIC) FORCING VIA THE UPPER JET STREAK OVER THE
NORTHERN MID ATLANTIC-NORTHEAST) WILL FAVOR A BROAD AREA OF
OROGRAPHICALLY-ENHANCED HEAVY RAINFALL ACROSS EASTERN TN/KY
EASTWARD INTO THE CENTRAL-SOUTHERN APPALACHIANS OF SOUTHERN
WV...WESTERN VA...AND NORTHWEST NC. THE ROBUST DEEP-LAYER
DYNAMICAL FORCING WITHIN A SHARPENING LOW-MID LEVEL FRONTOGENETIC
ZONE AND MATURING WARM CONVEYOR BELT (WCB) WILL CONTINUE THE HEAVY
RAIN THREAT OVERNIGHT SAT-SUN NORTH OF THE SURFACE LOW AND
ASSOCIATED STATIONARY FRONT...EVEN AS THE ELEVATED INSTABILITY
DIMINISHES PER THE MODELS. THIS ESPECIALLY ACROSS THE
AFOREMENTIONED HIGHER TERRAIN AREAS...WHERE THE UPTICK IN
SOUTHERLY LOW-LEVEL MOIST UPSLOPE FLOW ALONG WITH MODEST PW AND
MOISTURE FLUX ANOMALIES WILL FAVOR OROGRAPHIC AUGMENTATION WITH
THE RAINFALL RATES. WPC QPF WAS COMPOSED OF A MUTLI-MODEL BLEND OF
THE ECMWF AND UKMET ALONG WITH WPC`S HIGH-RES BIAS-CORRECTED
ENSEMBLE MEAN...WHICH YIELDED AREAL-AVERAGE MAX TOTALS OF 1.5-2.0+
INCHES ACROSS EASTERN TN... SOUTHEAST KY...WESTERN VA...SOUTHERN
WV...AND NORTHWEST NC. PER THE ARRAY OF HIGH-RES CAM
GUIDANCE...LOCALIZED AMOUNTS OF 3-5" ARE ANTICIPATED.

IN TERMS OF THE EXCESSIVE RAINFALL OUTLOOK FOR DAY 1...WPC
DELINEATED A FAIRLY BROAD SLIGHT RISK AREA WHICH ENCOMPASSED NOT
ONLY THE AFOREMENTIONED MAXIMUM DETERMINISTIC QPF...BUT ALSO TOOK
INTO ACCOUNT THE LATEST NATIONAL WATER MODEL (NWM) SOIL MOISTURE
GUIDANCE...LATEST FFG VALUES...AND THE SSEO 40 KM NEIGHBORHOOD
PROBABILITIES ABOVE 30% WHERE 3 AND 6 HOURLY SSEO MEAN QPF EXCEEDS
THE CURRENT FFG.

...WESTERN KEYS...

SLOW MOVING WEAK UPPER LOW WEST OF THE FL STRAITS WILL ALLOW FOR A
PERSISTENT AREA OF UPPER LEVEL DIFFLUENCE ACROSS SOUTH FL WILL
MAKE FOR A RATHER NARROW CORRIDOR OF HEAVY RAINFALL FOCUSED OVER
THE KEYS AND THE FAR SOUTHERN PORTION OF THE FL PENINSULA...AND IN
PARTICULAR THE WESTERN KEYS CLOSER TO THE LOW-MID LEVEL
CIRCULATION AND THUS MOST CONVERGENT LOW-LEVEL MOIST INFLOW.
MUCAPES OF 1000-1500 J/KG COUPLED WITH PWATS NEAR 2 INCHES (WHICH
IS ABOUT 3 STANDARD DEVIATIONS ABOVE NORMAL FOR LATE APRIL) WILL
PROVIDE A RICH THERMODYNAMIC PROFILE THAT WILL COINCIDE WITH THE
FAVORABLE DYNAMICAL FORCING IN PRODUCING HIGHLY EFFICIENT RAINFALL
RATES. WPCQPF...USING A MULTI-MODEL BLEND WHICH INCLUDED HIGH-RES
GUIDANCE/MEANS... INDICATED A MAX OF 2-2.5" OVER THE WESTERN KEYS.
SOME OF THE INDIVIDUAL CAMS WERE DECIDEDLY MORE
BULLISH...ESPECIALLY THE 00Z NSSL WRF WHICH NOTED 4-6". THIS IS
ALSO WHERE THE LATEST SSEO 40 KM NEIGHBORHOOD PROBABILITIES SHOW A
>30% CHANCE OF 3 HOUR QPF EXCEEDING THE FFG...PARTICULARLY IN THE
AFTERNOON INTO THE EARLY EVENING.

HURLEY

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