Quantitative Precipitation Forecast
Issued by NWS

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000
FOUS30 KWBC 210953
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
553 AM EDT MON AUG 21 2017

...VALID 12Z MON AUG 21 2017 - 12Z TUE AUG 22 2017...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
10 NNW HYS 25 S JYR 25 SSE BVN 20 NW BVN 25 NNW YKN 15 N MKT
10 WNW HYR 40 ENE IWD 25 NNW SAW 15 ENE ESC 20 NNE MTW 10 NE MKE
15 NNW GYY PRG MDH FAM 25 WNW SUS 15 W AIZ 20 NNW JLN 35 SSE DDC
20 WNW GCK 10 NNW HYS.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
30 NNW 4CR 25 NW SAF 25 S TEX 40 NNW CPW 15 SSW VTP 20 SW CAO
45 WSW AMA 35 SSE CVN 30 E ROW 25 SW ATS 25 ESE ALM 30 NNW 4CR.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
25 ESE BIE 15 S OFF 15 ENE OLU 25 NNW OFK 20 E SPW 20 WSW PDC
FEP 25 SE PNT 20 S DEC 15 NNE PPQ 50 N JLN 20 SE HUT 25 E GBD
25 NW SLN 25 ESE BIE.


...LOWER MISSOURI VALLEY INTO THE MID AND UPPER MISSISSIPPI
VALLEY...

THE MODELS...INCLUDING THE HIGH-RES CAMS... CONTINUE TO STRUGGLE
WITH THE QPF DETAILS ON THE SOUTHERN PERIPHERY OF THE
WESTERLIES...DUE IN LARGE PART TO THE CONVECTIVE CONTRIBUTION WITH
LOCATION/TIMING DIFFERENCES WITH MCS GENESIS AND TRACK.
DIFFERENCES BEGIN TO SUBSIDE LATER IN THE PERIOD (MON NIGHT-EARLY
TUE) PER A NOTICEABLE UPTICK IN BROAD-SCALE FORCING...AS AN
AMPLIFYING UPPER SHORTWAVE TROUGH ACROSS THE UPPER MS VALLEY AND
ATTENDANT HEIGHT FALLS ALLOWS FOR STRENGTHENING LOWER TROPOSPHERIC
FRONTOGENESIS AND A MORE PROGRESSIVE COLD FRONT/PRE-FRONTAL
CONVECTION.

WPC QPF LEANED HEAVILY TOWARD A BLEND OF CONTINUITY (PREVIOUS
FCST) WITH HRRR/NBM TRENDS EARLY (THROUGH 18Z MON)...THEREAFTER A
COMBINATION OF THE 00Z WPC GUIDANCE WITH THE LATEST NBM AND
HREF/HREF VERSION 2 ENSEMBLE MEANS. THE NSSL-WRF OF LATE HAS BEEN
STRUGGLING WITH THE 6 HOURLY DETAILS...PARTICULARLY WITH THE
FORMATION OF NOCTURNAL/ELEVATED CONVECTION (TOO FAR SOUTH OF THE
PRIMARY LOW-LEVEL BOUNDARY WHERE LIMITED SUPPORT FOR DEEP-LAYER
ASCENT RESULTS IN A STRONGER MID LEVEL CAP). THE AFOREMENTIONED
MEANS OFFER A VIABLE COMPROMISE BETWEEN THE LATITUDINAL SPREAD IN
THE HIGH-RES GUIDANCE OVERNIGHT MON-EARLY TUE...WHICH WOULD PUT
EASTERN KS AND NORTHWEST- WEST-CENTRAL MO IN THE CROSS HAIRS FOR
HEAVIEST RAINFALL DURING THE DAY 1 PERIOD (12Z MON-12Z TUE). THE
RESULT WAS A SLIGHT SOUTHWARD SHIFT OF THE RAINFALL MAX
(AREAL-AVERAGE TOTALS OF 2-3+ INCHES) COMPARED TO THE PREVIOUS
(00Z) WPC QPF. STRENGTHENING PRE-FRONTAL MOISTURE POOLING
OVERNIGHT MON-EARLY TUE (PWAT AND 850 MB MOISTURE FLUX ANOMALIES ~
2 STANDARD DEVIATIONS ABOVE NORMAL)...ALONG WITH AN ELONGATION OF
MODEST ELEVATED INSTABILITY AHEAD OF THE FRONT...WILL RESULT IN A
FAIRLY BROAD SLIGHT RISK AREA OVERNIGHT. THE RISK WILL BE
HEIGHTENED ACROSS THE AFOREMENTIONED AREAS IN EASTERN KS-WESTERN
MO...WHERE THE STRENGTHENING LOW-LEVEL INFLOW IS EXPECTED TO
REMAIN NEARLY PARALLEL TO THE MEAN 850-300 MB FLOW FOR A LONGER
PERIOD (THUS RESULTING IN WEAK MBEs/CORFIDI VECTORS...INCREASED
UPWIND PROPAGATION...AND HIGHER PROBABILITY OF CELL TRAINING).


...SOUTHERN ROCKIES AND FOOTHILLS...

SHORTWAVE UPPER TROUGH ACROSS CA WILL EVENTUALLY CLOSE OFF AND
MIGRATE OFF THE SOUTHERN-CENTRAL CA COAST LATER IN THE PERIOD
(LATE MON- MON NIGHT). THIS WILL ALLOW FOR STRENGTHENING
DEEP-LAYER RIDGING ACROSS THE SOUTHERN ROCKIES-HIGH PLAINS THIS
PERIOD...ALONG WITH A BREAKDOWN IN WHAT UP TO THIS POINT HAS BEEN
A PERSISTENT (ALBEIT COMPACT) MOISTURE CONVEYOR BELTS PER THE WV
SATELLITE LOOPS. THE MESOSCALE GUIDANCE FOLLOWS SUIT...NOTING A
MORE ISOLATED (MARGINAL) RISK FOR ROBUST CONVECTION AND ASSOCIATED
HEAVY/POTENTIALLY EXCESSIVE RAINFALL.

HURLEY
$$





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