Quantitative Precipitation Forecast
Issued by NWS

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659
FOUS30 KWBC 230657
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
256 AM EDT SAT SEP 23 2017

...VALID 12Z SAT SEP 23 2017 - 12Z SUN SEP 24 2017...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
35 WNW CWCH 20 SE GPZ 20 S RWF OFK 10 SW BVN 25 SW BBW
10 NNW LBF 20 SE TIF 30 NNW ONL 25 SW MHE 20 WNW ATY 10 SW FFM
DTL 15 E KGAF 25 SSE CXWN 45 SE CYQK 35 WNW CWCH.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
70 S MMCS 25 SSW HMN 15 ESE CQC 30 S RTN 25 NNE LAA 50 N GCK
10 ESE GUY 35 WNW PVW 45 NNE HOB 10 NNE MRF 30 ENE MMCU.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
70 S MMCS 10 ESE SRR 50 WSW CAO 25 SSE SPD 30 NE DHT 50 NNE CVN
30 WNW HOB 85 W MRF.


...HIGH PLAINS / PLAINS / CENTRAL U.S...

A MULTI-DAY HEAVY RAIN EVENT COVERING PARTS OF MANY CENTRAL U.S.
STATES FROM TEXAS TO MINNESOTA...IS SET TO BEGIN TODAY. THE
PRECIPITATION...OCCURRING ALONG AND STRADDLING BOTH A DRYLINE AND
COLD FRONT...WILL SLIDE SLOWLY EASTWARD...WHILE CELL MERGERS AND
SSW TO NNE TRAINING ACT TO PRODUCE LOCALLY EXCESSIVE RAINFALL. WE
EXPECT POCKETS OF VERY HEAVY LOCAL RAINFALL PERHAPS EXCEEDING 5
INCHES OVER A MATTER OF 2 TO 4 HOURS IN SOME INSTANCES...WHICH
WILL CARRY A RISK OF FLASH FLOODING.

THE DRIVER OF THIS EVENT IS A STOUT UPPER LEVEL TROUGH/RIDGE
PATTERN WITH NEGATIVE 3.5 DEVIATION HEIGHT ANOMALIES OVER THE
SOUTHWEST STATES COUNTERED BY PLUS-TWO DEVIATION ANOMALIES OVER
THE GREAT LAKES. THE RESULTING MERIDIONAL FLOW WILL TRANSPORT AIR
MARKED BY PRECIPITABLE WATER VALUES 1.25 TO ABOUT 1.60
INCHES...PLENTY ANOMALOUSLY HIGH FOR LATE SEPTEMBER...ESPECIALLY
AT HIGHER LATITUDES. A NARROW STREAM OF HIGH LEVEL MOISTURE
EMANATING FROM A TROPICAL WAVE OVER SOUTHWEST MEXICO...WILL ALSO
CONTRIBUTE TO DEEPER COLUMN MOISTURE AND MORE EFFICIENT RAINFALL
PROCESSES OVER AT LEAST THE SOUTHERN PLAINS/ROCKIES STATES THROUGH
SATURDAY NIGHT.

A SHORTWAVE TROUGH ROUNDING THROUGH THE EAST SIDE OF THE WESTERN
U.S. LONGWAVE WILL ENHANCE THE PATTERN INFLECTION AND ASSOCIATED
JET STREAK SUCH THAT SYNOPTIC DEEP LAYER FORCING WILL BECOME
EXTREMELY FAVORABLE FOR PRODUCING A RAINFALL MAXIMUM FROM EASTERN
COLORADO THROUGH WESTERN NEBRASKA INTO SOUTH DAKOTA. ON THE
SOUTHERN EDGE OF THE SHORTWAVE...CLUSTERS OF EQUALLY PRODUCTIVE
THUNDERSTORMS SHOULD OCCUR OVER NM/TX/OK...THOUGH MESOSCALE
INFLUENCES / OUTFLOWS MAY PLAY A BIGGER ROLE HERE AT DETERMINING
WHERE THE RAINFALL MAXIMA OCCUR.

ON DAY 1 A LENGTHY SLIGHT RISK OF EXCESSIVE RAINFALL WAS DRAWN.
THE RISK MAY BECOME MORE FOCUSED AND BETTER PRONOUNCED OVER PARTS
OF EASTERN NEW MEXICO AND CENTRAL/WESTERN NEBRASKA...BUT WITH
THESE AREAS HAVING BEEN DRY RECENTLY...WITH HIGH FFG VALUES...AND
SOME OF THE HEAVIEST RAIN PROGGED OVER THE SAND HILLS IN
NEBRASKA...THE PROBABILITY OF FLASH FLOODING AT A GIVEN POINT IS
BEST DESCRIBED BY THE SLIGHT RISK CATEGORY AT LEAST FOR THE
INITIAL ISSUANCE TODAY.


...FLORIDA PENINSULA...

MOIST 20 KNOT EASTERLY FLOW AT LOW LEVELS...DIURNAL HEATING...AND
A SLIGHTLY DIFLUENT UPPER PATTERN...SHOULD FAVOR AN ACTIVE DAY
WITH NUMEROUS SHOWERS / THUNDERSTORMS. A SCENARIO EXISTS FOR
EXCESSIVE RAINFALL TO OCCUR...ESPECIALLY ALONG THE EAST COAST OF
THE PENINSULA. WHILE MEAN STEERING FLOW WOULD CARRY CELLS WESTWARD
AT 10 KNOTS...CORFIDI VECTORS ARE DIRECTED BACK EAST INTO THE
INFLOW AT 15 KNOTS...WHICH COULD YIELD A BALANCE RESULTING IN SLOW
NET MOVEMENT. THIS IS SEEMINGLY SIGNALED IN SOME OF THE 00Z HI-RES
GUIDANCE WHICH PRODUCES HEAVY RAIN ALONG THE EAST COAST OF
FLORIDA. THE WRF-ARW AND NSSL WRF INTRODUCED SPOT AMOUNTS GREATER
THAN SIX INCHES...WHICH IS PLAUSIBLE GIVEN THE CELL MOTION BALANCE
AND PRECIPITABLE WATER VALUES AROUND 2.00 INCHES OR GREATER.

BURKE
$$





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