EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1023 AM EDT SAT SEP 23 2017

...VALID 15Z 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
70 S MMCS 20 NW HMN 10 S SKX GXY 10 ENE BRX 15 NW DGW 35 N GCC
HEI 40 WNW FGN 20 NNW CWCH 20 SSW ELO 15 E 2P3 35 SSW YKN
40 NNE HLC LBL 30 N PVW 30 NW BPG 45 SW 6R6 135 NW MMMV.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
75 S MYGF 70 SE HST 40 S HST 30 WSW HST 45 E APF LAL 10 WSW GNV
CRG 60 ENE SGJ 170 ENE TTS.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
105 SSE MMCS 25 SE ALM 10 NNW LVS 20 E MNH 20 NNE BFF MBG
15 NE KGWR ATY 30 SW ONL 25 E GLD 45 SW DUX 45 WSW LBB 20 NW FST
25 SSE E38 95 S MRF.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
55 NW MYGF 35 ENE PBI 15 N PBI 30 WSW FPR 15 S LEE 35 ENE OCF
25 NNE DAB 60 NE TTS 65 ENE TTS.


1500 UTC UPDATE

NO CHANGES MADE TO THE PREVIOUS EXCESSIVE RAINFALL POTENTIAL
OUTLOOK

ORAVEC

INITIAL DISCUSSION


...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 DRY LINE 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