Quantitative Precipitation Forecast
Issued by NWS

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FOUS30 KWBC 241417
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1017 AM EDT SUN SEP 24 2017

...VALID 15Z SUN SEP 24 2017 - 12Z MON SEP 25 2017...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
40 E MMCU 90 WSW MRF 30 E GDP 50 NNW HOB 40 NNE TCC 35 N LAA
40 SSW ITR 25 S PIR 15 ESE KGWR 10 NNE BRD 10 NE CBG 10 WSW FRM
15 SSW OFF 25 SW SLN 10 NW CSM 30 SW SJT 100 SW DRT 90 NNW MMTC.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
90 ENE MMCU 15 NW MRF 30 NNE INK 35 SW MCK 20 N BBW 15 WSW ATY
10 SSE OVL 15 W FET 20 SSW RSL 50 SSW CDS 10 NNE 6R6
150 WNW MMMV.


1500 UTC UPDATE

NO CHANGES MADE TO THE PREVIOUS EXCESSIVE RAINFALL POTENTIAL
OUTLOOK

ORAVEC

INITIAL DISCUSSION

...GREAT PLAINS / CENTRAL U.S...

MODERATE AND EMBEDDED HEAVIER RAINFALL IS FORECAST SUNDAY FROM
TEXAS TO MINNESOTA. THIS WAS DRIVEN BY A VERY DEEP TROUGH IN THE
WEST AND EQUALLY STOUT DOWNSTREAM RIDGE OVER THE GREAT LAKES.
RAINFALL INTENSITY WAS TEMPERED...HOWEVER...BY MARGINAL LAPSE
RATES IN THE WARM SECTOR AND THE BULK OF DEEP LAYER ASCENT
REMAINING BACK IN THE COLD SECTOR. THUS...IT MAY BE THAT RAIN
RATES REMAIN SOMEWHAT MARGINAL AS HAD BEEN OBSERVED ON SATURDAY.
TWO AREAS STAND OUT AS SEEING GREATER ODDS OF ORGANIZED HEAVY
RAINFALL. SUFFICIENT
FRONTOGENESIS AND CONVERGENCE OF THE LOW LEVEL FLOW...ESPECIALLY
JUST TO THE COOL SIDE OF THE SURFACE FRONT...SHOULD ALLOW FOR NEAR
CONTINUAL RAIN/THUNDERSTORM DEVELOPMENT FROM WESTERN KANSAS
THROUGH CENTRAL NEBRASKA INTO EASTERN SOUTH DAKOTA. ADDITIONAL
CLUSTERS OF THUNDERSTORMS / SMALL MCS ACTIVITY WILL LIKELY OCCUR
DOWN THROUGH WEST TEXAS AND EASTERN NEW MEXICO...BLEEDING OVER
INTO WESTERN OKLAHOMA WITH TIME. AT THESE
MORE SOUTHERLY LATITUDES THE STEERING FLOW INCLUDES MORE WESTERLY
COMPONENT...ALLOWING CONVECTION TO PUSH OUT INTO THE WARM SECTOR.
AT THE SAME TIME HOWEVER...INSTABILITY ACTUALLY DECREASES WITH
DISTANCE INTO THE WARM SECTOR...SO THERE ARE SOME COMPETING
FACTORS AT PLAY.

ONE SIGNAL OF NOTE IS THE INCREASE OF QPF OVER THE BIG BEND AREA
IN TEXAS. THE WRF-ARW WAS PARTICULARLY AGGRESSIVE THERE...AND
OTHER GUIDANCE TRENDED UPWARDS AS WELL. THE WPC TREND WAS MORE
CONSERVATIVE...GIVEN THE POTENTIAL FOR OUTFLOW AND PROPAGATION TO
SUPPRESS ANY EXTREME VALUES...BUT THERE IS POTENTIAL FOR LOCAL
INTENSE RAIN RATES...AS THE UPPER PATTERN OPENS UP AND IS MORE
DIFLUENT ACROSS THIS AREA...WHILE ALSO TAPPING INTO MOISTURE
ASSOCIATED WITH TROPICAL STORM PILAR.

OVERALL THE SITUATION MIGHT BE BEST DESCRIBED BY A MARGINAL RISK
OF EXCESSIVE RAINFALL AT THIS TIME...BUT THERE WILL BE SOME
OVERLAP OF NEW RAINFALL ATOP AREAS THAT RECEIVED A WETTING RAIN ON
SATURDAY. THEREFORE WE MAINTAINED THE INHERITED NARROW SLIGHT RISK
AREA FROM SOUTHWEST TEXAS TO SOUTHWEST MINNESOTA.

BURKE
$$





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