Quantitative Precipitation Forecast
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FOUS30 KWBC 230101
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
858 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
10 N DCU 15 WSW TUP 20 NNW GLH 20 WNW TXK 15 SSE 1F9 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 40 ESE RTN
10 SSE TCC 40 NW PVW 40 WNW CDS 35 W CSM 35 SE WWR 10 ENE WDG
10 NNW GCM 25 SE FYV 15 SSW LIT 40 ESE SGT 35 SE OLV 45 N MSL
25 NE MQY 20 W EKQ 30 SSW HTS 25 NNE CRW 10 NE CBE THV MDT
20 NNE AVP 30 SW SCH 25 SSW MPV 20 SE CYSC 30 SE CWHV 30 NW MLT
10 WSW BGR ORH HVN 15 NNW NEL PHL 10 SSW FDK W99 10 WNW LWB
10 NW BLF OQT 10 N DCU.

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 30 WNW AAT 20 S LKV 55 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
15 NNE MKO 15 E FSM 20 SSW MWT 15 SE DUA 20 W GLE 20 N 1F9
30 N ABI 20 NW BPG 25 E HOB 55 NNE HOB LBB 40 SSW CDS 10 SW LTS
10 SW JWG GOK 15 NNE MKO.


...01 UTC UPDATE...

MAIN ADJUSTMENT FOR THIS UPDATE WAS TO FURTHER SHRINK THE
EXPANSIVE MARGINAL EXCESSIVE RAINFALL THREAT AREA STRETCHING FROM
THE SRN PLAINS NEWD TO THE NORTHEAST TO TAKE INTO ACCOUNT
CONVECTIVE PROPAGATION IN LINE WITH LATEST RADAR/SATELLITE AND
HRRR GUIDANCE. ADDITIONALLY...DISCONTINUED THE EARLIER ISSUED
SLIGHT RISK AREA OVER THE NERN US DUE TO CONTINUED SPEED OF
ACTIVITY.

SCHICHTEL

...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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