Quantitative Precipitation Forecast
Issued by NWS

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179
FOUS30 KWBC 211500
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1100 AM EDT MON AUG 21 2017

...VALID 15Z 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 ULM
15 SSE CBG 10 NE IWD 35 NW 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...

AT 15Z WE ALTERED THE SHAPE OF THE SLIGHT RISK AREA HERE...TO
ACCOUNT FOR THE STABILIZING INFLUENCE OF A MORNING MCS OVER
EASTERN IOWA...BUT THEN ALSO TO EXTEND SLIGHT RISK UP INTO PARTS
OF MN/WI BASED ON TRENDS. IT APPEARED THAT THE HEAVY RAIN / FLASH
FLOOD RISK WILL LATER BECOME FOCUSED ALONG THE SOUTHERN AND
WESTERN PERIPHERY OF OUTFLOW FROM THE MORNING
CONVECTION...INCLUDING FROM IL/MO INTO KANSAS...AND UP ALONG THE
850-700 MB THERMAL GRADIENT / WARM ADVECTION REGIME INTO THE OMAHA
AREA. A FURTHER EXTENSION OF MOISTURE AND INSTABILITY INTO
SOUTHERN MINNESOTA AND WISCONSIN IS ALSO FORECAST...AHEAD OF A
PROGRESSIVE FRONTAL SYSTEM AND SUBSTANITAL HEIGHT FALLS. THOUGH
THE SIGNAL IN THE HRRR IS BOUNCING AROUND IN LOCATION...RECENT
RUNS GENERALLY FEATURE HEAVY RAINFALL FROM NORTHWEST IOWA INTO
WESTERN WISCONSIN LATER TODAY. WIND PROFILES ARE MILDLY FAVORABLE
FOR PERIODS OF TRAINING...AND FLASH FLOOD GUIDANCE IS IN THE MID
RANGE AT ABOUT 2 INCHES IN 3 HOURS...WHICH SHOULD BE EXCEEDED IN
SPOTS.

PREVIOUS DISCUSSION...THE HI-RES ENSEMBLE MEANS OFFER A VIABLE
COMPROMISE BETWEEN THE LATITUDINAL SPREAD IN THE DETERMINISTIC
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.


...MID ATLANTIC...

HERE WE ADDED A SMALL MARGINAL RISK AREA. THE PATTERN HAS BEEN
VERY WET FOR THE PAST 2 TO 3 WEEKS IN PARTS OF THE REGION. THE 12Z
KLWX SOUNDING SAMPLED A VERY MOIST AND POTENTIALLY UNSTABLE
ATMOSPHERE. HEATING IS VERY LIKELY TO YIELD SCATTERED TO NUMEROUS
THUNDERSTORMS THIS AFTERNOON...WITH GREATER COVERAGE BEGINNING
WEST ALONG THE SOME OF THE HIGHER TERRAIN IN VA/MD...AND EXTENDING
INTO CENTRAL PA...WITH MOVEMENT THEN SLOWLY TOWARD THE EAST. THERE
IS ENOUGH WESTERLY SHEAR IN PLACE FOR SOME QUASI-ORGANIZED STORM
MODES.


...SOUTHERN ATLANTIC COAST...

VERY HIGH PW VALUES ASSOCIATED WITH AN EASTERLY WAVE / INVERTED
TROUGH...SHOULD ENHANCE RAIN RATES WITHIN SEA BREEZE
CONVECTION...WITH MERGERS POSSIBLY LEADING TO RAPID ACCUMULATION
OF A FEW INCHES OF RAIN IN SPOTS.

HURLEY/BURKE
$$





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