Quantitative Precipitation Forecast
Issued by NWS

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FOUS30 KWBC 222316
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
715 PM EDT TUE AUG 22 2017

...VALID 01Z WED AUG 23 2017 - 12Z WED AUG 23 2017...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
30 NNE ELD DFW ABI 20 SW BPG 10 SSE INK 15 NNW PEQ 50 S GDP
55 ESE MMCS 35 ENE ELP 10 ESE ALM 35 WNW 4CR 25 NW SKX
40 ESE MYP PUB 30 WNW CAO 20 SW DUX 30 NNW JWG 15 SSW BVO
30 SW HRO 10 S M19 10 E MKL 25 S CKV 45 NW SME 25 E LEX
20 NE 3I2 10 NNW CKB 25 NNE 2G4 25 NNW UNV 10 SW SYR 20 SW CMCW
25 SSE CWVY 30 NW MLT 10 WSW BGR ORH TEB UKT 10 E LNS 25 WNW OKV
10 WNW LWB 10 NW BLF 40 SW CSV 30 NNE ELD.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
40 NNE MMH 20 NW MMH 40 S TVL 20 S BLU 35 NE CIC 25 SSE MHS
10 W MHS 25 NE MHS AAT 35 SE AAT 50 WNW LOL 35 W LOL 40 SSE NFL
40 NNE MMH.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
20 S PSF 10 WNW FWN CXY 25 E AOO 25 NE UNV 20 S ITH 15 N BGM
40 NW GFL 15 SSW CWQH 45 W GNR 20 NNW WVL CON 20 S PSF.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
10 N GLE 35 N DYS 30 W SNK 50 ENE HOB 50 WSW LBB 30 WSW PVW
30 ESE PVW 25 S CDS 10 NW LTS 20 ENE CSM GOK 10 N MKO 15 WNW RUE
10 NNE LRF 20 NW SGT 25 E DUA 10 N GLE.


...23 UTC SPECIAL UPDATE...

A WPC MARGINAL EXCESSIVE RAINFALL SLIGHT RISK AREA HAS BEEN ADDED
OVER PARTS OF NRN CA/WRN NV IN LINE WITH RECENTLY ISSUED WPC
MESOSCALE PRECIPITATION DISCUSSION #714 AS PER BELOW...

SUMMARY...CONVECTION DEVELOPING ACROSS THE HIGHER TERRAIN OF
WESTERN NV AND PORTIONS OF NORTHERN CA WILL TAP DEEP MOISTURE TO
PRODUCE SLOW MOVING STORMS...POSING A FLASH FLOOD THREAT INTO THE
EARLY EVENING HOURS.

DISCUSSION...REGIONAL RADARS SHOWED AN INCREASE IN CONVECTIVE
ACTIVITY ACROSS THE HIGHER TERRAIN OF WESTERN NV AND PORTIONS OF
NORTHEAST CA THIS AFTERNOON. THE CONVECTION IS FORMING IN AN AREA
OF WEAKLY DIFLUENT FLOW AHEAD OF A MID LEVEL TROUGH LOCATED OFF
THE CA COAST. THE CONVECTIVE DEVELOPMENT IS FOLLOWING AN AXIS OF
500-1000 J/KG OF MLCAPE...AND TRACKING SLOWLY NORTHWEST IN THE
PRESENCE OF A WEAK 850-300 MB MEAN FLOW (WHICH IS GENERALLY LESS
THAN 10 KNOTS).

THE MOST RECENT RAP ANALYSIS SHOWED 1.25 INCH PRECIPITABLE WATER
AIR OVER NORTHERN CA...AND MOST OF THE CONVECTION HAS FORMED ON
THE GRADIENT OF THIS MOISTURE AXIS. THE KRGX RADAR SHOWED LOCAL
1.50 INCH HOURLY RAINFALL RATES OVER NORTHERN ALPINE COUNTY CA.
WHILE SOME DRY AIR IN THE MID LEVELS COULD BE RESULTING IN HAIL
CONTAMINATION FOR SOME STORMS...A MESONET REPORT OF 0.55 INCHES OF
RAINFALL IN 15 MINUTES WAS RECEIVED FROM WASHOE COUNTY NV SUPPORTS
THE IDEA OF 1.50+ INCH HOURLY RAINFALL RATES.

THERE WAS A GOOD SIGNAL IN THE HIGH RESOLUTION GUIDANCE FOR
CONVECTION TO CONTINUE OVER WESTERNMOST NV AND PORTIONS
OFNORTHEAST CA THROUGH 03Z OR SO. WEAK MID LEVEL FLOW AND HIGH
MOISTURE CONTENT AIR SHOULD SUPPORT LOCAL 1.00-2.00 INCH RAINFALL
AMOUNTS...THOUGH THESE AMOUNTS SHOULD BE SCATTERED. HOWEVER...THIS
IS MORE THAN ENOUGH (SINCE ONE HOUR FLASH FLOOD GUIDANCE VALUES
ARE GENERALLY LESS THAN AN INCH) TO INITIATE FLASH FLOODING.


ELSEWHERE...MADE MANUAL MODIFICATIONS TO AN EXPANSIVE MARGINAL
EXCESSIVE RAINFALL THREAT AREA STRETCHING FROM THE SRN PLAINS NEWD
TO THE NORTHEAST AND EMBEDDED SLIGHT RISK AREAS TO TAKE INTO
ACCOUNT CONVECTIVE PROPAGATION/DEVELOPMENTS NOW IN BETTER FOCUS
SINCE 15 UTC AS PER LATEST RADAR/SATELLITE AND HRRR GUIDANCE.

SCHICHTEL


...EARLIER 15 UTC MESSAGE...


...EASTERN OHIO VALLEY AND LOWER GREAT LAKES INTO THE NORTHEAST...

VIGOROUS NORTHERN STREAM SHORTWAVE PIVOTING ACROSS THE UPPER MS
VALLEY AND GREAT LAKES THIS PERIOD WILL ALLOW FOR TROUGH
AMPLIFICATION/HEIGHT FALLS ACROSS THE NORTHEAST
SECTOR...ULTIMATELY LEADING TOWARD A CUTOFF MID/UPPER LOW INTO
SOUTHERN QUEBEC BY THE END OF THE PERIOD (12Z WED). GIVEN THE
DEGREE/DEPTH OF SYNOPTIC SCALE FORCING...THE MODELS ARE FAIRLY
WELL CLUSTERED WITH THE SURFACE LOW TRACK INTO SOUTHEAST
ONTARIO-SOUTHERN QUEBEC ALONG WITH THE ASSOCIATED ELONGATED COLD
FRONT...THOUGH SLIGHT TIMING DIFFERENCES REMAIN WITH THE
PROGRESSION OF PRE-FRONTAL CONVECTION. BASED ON THE LATEST MODEL
TRENDS...PARTICULARLY WITH THE ENSEMBLE OF HIGH-RES CAMS AND
PARALLEL VERSIONS...WPC INCREASED QPF TOTALS ACROSS PORTIONS OF
THE LOWER AND EASTERN OH VALLEY INTO THE NORTHEAST...ESPECIALLY
FROM EASTERN KY...WESTERN PA...AND MUCH OF NY INTO NORTHERN NEW
ENGLAND. LOW-LEVEL FRONTOGENESIS AHEAD OF THE TROUGH WILL BE MOST
FAVORABLE OVER THESE AREAS...WHILE COUPLING WITH AN IMPRESSIVE
PRE-FRONTAL THERMODYNAMIC ENVIRONMENT AS MIXED LAYER CAPES PEAK
BETWEEN 1000-2000 J/KG ALONG WITH PWs OF 1.75-2.00 INCHES. THE
HEAVY RAINFALL SIGNAL IS FURTHER SUPPORTED BY THE UPTICK IN
LOW-LEVEL MOISTURE POOLING (850 MB SOUTHWESTERLY FLOW 45-55+ KTS
PER THE MODELS) AND ATTENDANT NOTEWORTHY MOISTURE FLUX ANOMALIES
(4-5 STANDARD DEVIATIONS ABOVE NORMAL).

THE PROGRESSIVE NATURE OF THE FRONT REMAINS THE MAIN INHIBITOR TO
THE EXCESSIVE RAINFALL POTENTIAL. HOWEVER...THE AFOREMENTIONED
UPTICK IN SOUTHWESTERLY INFLOW LATER THIS AFTERNOON-EVENING
(UPWIND PROPAGATION) MAY ALLOW FOR SOME CELL
TRAINING...PARTICULARLY OVER WESTERN PA INTO WESTERN-UPSTATE NY
AND NORTHERN VT-NH. A "SLIGHT" RISK WILL BE NOTED ACROSS THESE
AREAS AND INTO EASTERN PORTIONS OF THE OH VALLEY (WITH THE HIGHER
ANTECEDENT SOIL MOISTURE/LOWER FFG) IN THE NEW DAY 1 EXCESSIVE
RAINFALL OUTLOOK (ERO).


...SOUTHERN PLAINS AND LOWER MISSISSIPPI VALLEY...

IN THE 15Z UPDATE WE EXPANDED THE SLIGHT RISK AREA TOWARD UPSLOPE
AREAS OF EASTERN NEW MEXICO WHERE SOME OF THE MORE RELIABLE HI-RES
GUIDANCE...NSSL WRF AND SSEO MEAN...HAVE A STRONG QPF SIGNAL.
ELSEWHERE...NO CHANGES.

ACROSS THE SOUTHERN PLAINS (PARTICULARLY ALONG AND JUST NORTH OF
THE RED RIVER) INTO CENTRAL AR...THE MODELS CONTINUE TO DEPICT A
SIGNAL FOR HEAVY RAINFALL ALONG AND SOUTH OF THE FRONT WHICH WILL
BECOME DRAPED MORE OR LESS IN A W-E FASHION. GIVEN THE FRONTAL
ORIENTATION AND ANTICIPATED LOW-LEVEL AND MEAN LAYER
FLOW...CHANCES FOR CELL TRAINING ARE HEIGHTENED OVER THIS REGION
AS NOTED BY THE WEAK MBEs/CORFIDI VECTORS LATE TUE INTO EARLY WED
(5 KTS OR LESS).

HURLEY/BURKE
$$





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