Quantitative Precipitation Forecast
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000
FOUS30 KWBC 221415
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1015 AM EDT SAT APR 22 2017

...VALID 15Z 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
20 SSE JKL 20 N BKW 25 N HSP CHO 10 NNW FYJ 10 E LFI CPK
10 SSW ASJ 15 SSE LHZ EQY 15 NNW 27A 10 SE PDK 15 SW ANB
20 NNW TCL 20 NE CBM 25 WNW 1M4 20 WNW DCU 45 NE MDQ CSV
25 SE EKQ 20 SSE JKL.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
25 NNW KEY 15 NW MTH 10 NE MTH 10 SE MTH 25 SSE NQX 40 SW KEY
50 WSW KEY 40 W KEY 25 NNW KEY.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
15 NNE AVL 20 ESE 1A5 30 NNW GVL 15 NNW RMG 25 SW CHA
10 WNW CHA 25 SSE CSV 10 E 1A6 LNP 20 NNW BCB 15 W FVX
30 WNW AVC 20 S MTV 15 NNE AVL.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
KEY 10 W MTH 20 ESE NQX 20 SSW KEY 35 WSW KEY KEY.


...S-CENTRAL APPALACHIANS / MID-ATLANTIC...

...15 UTC UPDATE...

ADJUSTED THIS AREA TO ACCOUNT FOR RADAR/SATELITTE TRENDS THAT SHOW
MORE PROGRESSIVE COLD POOL/CONVECTIVE TRANSLATION.

PREVIOUS DISCUSSION...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.


...WRN KEYS...

...15 UTC UPDATE...

INTRODUCED A SLIGHT RISK AREA OVER THE WRN KEYS TO ACCOUNT FOR
ORGANIZED ONGOING CONVECTION...FUELING DEEP MOISTURE...AND
POTENTIAL FOR CELL TRAINING/LONGEVITY. THE HREFV2 SHOWS >50%
NEIGHBORHOOD PROBABILITIES FOR 24 HR RAINFALL EXCEEDING 5".

PREVIOUS DISCUSSION...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/SCHICHTEL

$$





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