Quantitative Precipitation Forecast
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FOUS30 KWBC 260058
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
857 PM EDT SUN SEP 25 2016

...VALID 01Z MON SEP 26 2016 - 12Z MON SEP 26 2016...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
140 WNW MMMV 50 WSW 6R6 40 SE MAF 10 NNW ABI 15 WNW 1F9
20 W AQR 15 SW SGF 10 S SUS 35 N HNB 30 E M30 10 NW HOT 10 N OCH
25 W KMZG 70 SSE MMMA.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
85 NW MMMV 35 S 6R6 35 WNW E29 25 ESE SJT 25 W 05F 15 E LNC
15 NW JSO 30 WSW UTS 50 WSW VCT 15 WNW EBG 80 NNE MMCV.


01 UTC UPDATE...

FROM THE SOUTHERN PLAINS TO THE MID MS VALLEY...SHIFTED THE
WESTERN EDGE OF THE MARGINAL AND SLIGHT RISK AREAS FURTHER TO THE
SOUTHEAST...REMOVING AREAS WHERE THE HEAVY RAINFALL THREAT HAS
DIMINISHED AS DRIER AND MORE STABLE AIR CONTINUES TO ADVECT BEHIND
THE ADVANCING COLD FRONT.  ALSO REMOVED THE MODERATE RISK IN
SOUTHWEST TEXAS THAT WAS CENTERED EAST OF THE BIG BEND.  ALTHOUGH
FLASH FLOOD GUIDANCE VALUES REMAIN VERY LOW...THE AIRMASS HAS
BECOME MORE STABLE ACROSS THE REGION...WITH HEAVY PRECIPITATION
NOW FOCUSED WELL TO THE EAST ALONG AN ADVANCING SURFACE BOUNDARY.
THE LATEST RUNS OF THE HRRR HAVE BEEN SHOWING A FAIRLY GOOD
REPRESENTATION OF THE ONGOING CONVECTION AND SHOW LITTLE TO NO
ADDITIONAL AMOUNTS FALLING DURING THE OVERNIGHT HOURS ACROSS
BASINS WHERE THE FLASH FLOOD GUIDANCE IS THE LOWEST.

ACROSS THE UPPER GREAT LAKES...GIVEN LIMITED INSTABILITY AND WITH
SHOWERS AND STORMS NOW MOVING PROGRESSIVELY EAST OF REGIONS WITH
LOWER FLASH GUIDANCE VALUES...REMOVED THE MARGINAL RISK ACROSS THE
REGION.

PREVIOUS DISCUSSION...

...TEXAS AND OKLAHOMA....

THE MODERATE RISK AREA IN SOUTHWEST TEXAS EAST OF THE BIG BEND AND
ON THE WESTERN FRINGE OF THE HILL COUNTRY IS NOW REDUCED IN SIZE A
BIT...AND REORIENTED TO ALIGN WITH THE COMBINED COLD POOLS /
EFFECTIVE FRONT IN THIS REGION. IN THE NEAR TERM SUNDAY MORNING
SEE MESOSCALE DISCUSSION 701 FOR DETAILS. LARGE SCALE FORCING WAS
INCREASING OVER THIS REGION WITH GREATER DIFFLUENCE ALOFT AND
BACKING AND ACCELERATING OF LOW LEVEL INFLOW. INDIVIDUAL CELL AND
BOUNDARY MOTIONS HAD SLOWED TO YIELD AN ENHANCED POTENTIAL FOR
HEAVY RAINFALL WITHIN AN ENVIRONMENT MARKED BY DEEP TROPICAL
MOISTURE SUPPORTIVE OF VERY HIGH RAIN RATES. WHILE THERE DOES
APPEAR TO BE A FOCUSED EVENT UNFOLDING IN THIS AREA...THE COVERAGE
OF THE EVENT IS SOMEWHAT IN QUESTION...GIVEN THAT THE UPPER LOW IS
PINCHING OFF MUCH FARTHER WEST...AND LOWER TO MIDDLE LEVEL WIND
SPEEDS ARE FORECAST TO DECREASE DURING THE DAY...FROM 20 TO 30
KNOTS EARLY ON...DOWN TO 15 TO 20 KNOTS LATER. DEFINITELY SOME
PROS AND CONS...AS THE PATTERN REMAINS DIFLUENT ALOFT WITH BACKED
INFLOW...BUT AS WIND SPEEDS FALL OFF...SO DOES THE INTENSITY OF
THE DEEP LAYER LIFT...AND DAYTIME MIXING MAY ALLOW FOR A BIT MORE
PROPAGATION OF COLD POOLS TO PUSH THE HEAVY RAINFALL OUT OF ANY
ONE GIVEN SPOT. WHEREVER LOCAL REPEATED RAINS
OCCUR...HOWEVER...MODEL GUIDANCE STILL SUPPORTS SOME HEFTY
TOTALS...ADDITIONAL RAINFALL 4 INCHES OR MORE.

SURROUNDING THE MODERATE RISK IS A RATHER LARGE SLIGHT RISK AREA
WHICH WE EXPANDED ALONG A COUPLE OF AXES POINTED UP INTO THE MID
LEVEL FRONTOGENETICAL ZONE OVER WESTERN OKLAHOMA...AND ALONG THE
EFFECTIVE FRONT INTO NORTHEAST TEXAS AND SOUTHEAST OKLAHOMA.
PARTICULARLY DEEP SATURATION WAS NOTED IN THE 12Z SOUNDINGS FROM
OUN AND FWD. THE SLIGHT RISK AREA TAKES THE SHAPE OF THE 00Z
WRF-ARW QPF...WHILE TAKING INTO ACCOUNT POCKETS OF LOWER FLASH
FLOOD GUIDANCE BASED ON RECENT HEAVY RAINS.

ON THE SOUTHWEST SIDE OF THE RISK AREA...GIVEN THE STRONG EASTERLY
UPSLOPE COMPONENT TO LOW LEVEL WINDS AND ANOMALOUS PW ENVIRONMENT
(2-2.5 STANDARD DEVIATIONS ABOVE NORMAL)...DO NOT FEEL COMFORTABLE
REMOVING THE RISK AREA FROM PORTIONS OF THE PERMIAN BASIN AND
SOUTHEAST NEW MEXICO. ALTHOUGH THE HEAVY RAINFALL OVER THIS AREA
WILL MORE LIKELY FALL OVER A LONGER DURATION...FRONTOGENETICAL
FORCING WILL ACT UPON WHAT IS STILL A VERY MOIST ENVIRONMENT IN
THE POST-FRONTAL REGIME.


...THE OZARKS UP TOWARD LAKE MICHIGAN...

MARGINAL RISK SURROUNDING THE SOUTHERN PLAINS EVENT WAS STRETCHED
UP AHEAD OF THE FRONTAL BOUNDARY ACROSS MISSOURI AND ILLINOIS INTO
THE AREA TO EITHER SIDE OF LAKE MICHIGAN. FLASH FLOOD GUIDANCE IS
A BIT LOWER OVER SOUTHEAST WISCONSIN...BUT OF MORE CONCERN WAS THE
METEOROLOGICAL SETUP. A NARROW PLUME OF PRECIPITABLE WATER VALUES
NEARING 2 INCHES...WELL ABOVE CLIMATOLOGICAL NORMS...WILL BE SWEPT
UP THROUGH THIS REGION AT A FAVORABLE TIME OF DAY...21-03Z...AND
MAY CONTRIBUTE TO VERY HEAVY SHORT TERM RAIN RATES...ESPECIALLY
OVER PARTS OF WI/IL/IN...WHERE A LOCAL MAXIMUM OF CAPE SHOULD FORM
NEAR THE RETREATING WARM FRONT. ALTHOUGH THE HRRR HOURLY QPF IS
MUCH LESS...SOME OF THE NCEP HI-RES WINDOWS PRODUCE GREATER THAN
ONE INCH HOURLY RATES...AND NEIGHBORHOOD PROBABILITIES OF
FFG-EXCEEDANCE FROM THE SSEO SPIKE AT 30 PERCENT THIS AFTERNOON IN
SOUTHEAST WISCONSIN. PROGRESSIVE CELL MOTIONS OF 20 KNOTS OR
SO...SHOULD LESSEN THE OVERALL POTENTIAL...BUT ISOLATED INSTANCES
OF FLASH FLOODING ARE POSSIBLE.

PEREIRA/BURKE/HURLEY
$$





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