Quantitative Precipitation Forecast
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760
FOUS30 KWBC 180051
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
850 PM EDT THU AUG 17 2017

...VALID 01Z FRI AUG 18 2017 - 12Z FRI AUG 18 2017...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
35 WNW CYYU 30 SW CYYB 10 S CYTZ CWWZ 10 ENE JHW 20 ENE BTP
15 SE 3I2 35 SSE FTK 20 WNW SDF 30 NNE SGH 25 NW PHN 10 W OSC
15 NE ISQ 20 NW ESC 10 S IMT LNL 30 ENE IWD 10 NNE CKC
35 ESE CWDV 25 WNW CYGQ.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
80 ESE HXD 40 E HXD 15 SW UDG 15 W IPJ 1A5 25 NNW 1A5
25 SSW TRI 15 NNW MWK 20 E W99 10 SSE MRB NAK 10 SSW NAK
15 SE DAA 20 NNW OFP 15 WSW PTB 20 ENE LHZ 15 NE OAJ 70 S MRH
105 SE MRH.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
30 E ABI 15 SSE SLR 10 WNW GGG 15 WNW OCH 30 NE LHB 45 E 6R6
30 ESE E38 25 SE INK 30 E ABI.


0100 UTC UPDATE

OVERALL...THE EXCESSIVE RAINFALL THREAT REMAINS MARGINAL AT BEST
WITH ONGOING CONVECTION LARGELY DISORGANIZED AND/OR PROGRESSIVE
ACROSS MOST AREAS.  MADE FURTHER MODIFICATIONS TO THE PREVIOUS
OUTLOOK BASED UPON CURRENT SATELLITE/RADAR TRENDS...AS WELL AS
RECENT RUNS OF THE HRRR AND NBM.  THIS INCLUDED REMOVING THE UPPER
GREAT LAKES REGION FROM THE PREVIOUS OUTLOOK WHILE SHIFTING THE
MARGINAL RISK FURTHER EAST INTO THE NORTHEASTERN U.S.  ACROSS THE
SOUTHERN PLAINS...REMOVED PORTIONS OF THE SOUTHERN TX PANDHANDLE
REGION.

PEREIRA

1900 UTC UPDATE

CHANGES TO THE PREVIOUS OUTLOOK BASED ON THE LATEST RADAR TRENDS
AND UPDATED QPF.  TRIMMED THE MARGINAL RISK AREA SOUTHWARD OVER
THE SOUTHERN PLAINS AND EXTENDED IT SOUTHWEST IN THE VICINITY OF
THE FRONT OVER SW TX.  SLIGHT CHANGES TO THE MARGINAL RISK AREA
OVER THE UPPER LAKES FOR CURRENT RADAR TRENDS.

1500 UTC UPDATE

NO CHANGES MADE TO THE PREVIOUS EXCESSIVE RAINFALL POTENTIAL
OUTLOOK.

ORAVEC

INITIAL DISCUSSION


...GREAT LAKES/OHIO VALLEY...

AN AREA OF LOW PRESSURE WILL TRACK ACROSS THE GREAT LAKES
TODAY...WITH A COLD FRONT EXTENDING SOUTH FROM THE LOW. THE MID
AND UPPER LEVELS WILL BE QUITE FAVORABLE FOR ASCENT...WITH A
STRONG MID LEVEL SHORTWAVE MOVING EAST AND A VERY DIFFLUENT UPPER
PATTERN. ALSO WILL HAVE STRONG 850 MB MOISTURE TRANSPORT AND LOW
LEVEL CONVERGENCE...WITH PWATS ALSO ABOVE THE CLIMATOLOGICAL 90TH
PERCENTILE. THUS WE WILL CONTINUE TO SEE A BROAD AREA OF SHOWERS
AND THUNDERSTORMS MOVE EAST ACROSS THE GREAT LAKES AND OHIO VALLEY
THROUGH TODAY. THE ACTIVITY MOVING THROUGH THIS MORNING IS MORE
ASSOCIATED WITH WARM FRONTAL ADVECTION AND CONVERGENCE...WITH THE
COLD FRONT MOVING ACROSS THIS AFTERNOON/EVENING. DEPENDING ON THE
AMOUNT OF DESTABILIZATION...SHOULD AT LEAST SEE SOME CONVECTION
LATER IN THE DAY ASSOCIATED WITH THE COLD FRONT. THE PROGRESSIVE
NATURE OF THE CONVECTION SHOULD HELP KEEP RAINFALL AMOUNTS IN
CHECK. HOWEVER GIVEN THE STRONG FORCING AND ANOMALOUS PWATS IN
PLACE...WOULD STILL EXPECT TO SEE POCKETS OF LOCALIZED 1-3"
AMOUNTS ACROSS NORTHERN AND EASTERN MI. FURTHER SOUTH ACROSS
IN/OH/KY WE ARE MORE REMOVED FROM THE BETTER FORCING...WHICH MAY
TEND TO KEEP CONVECTION MORE SCATTERED IN NATURE. THIS SHOULD KEEP
AREAL AVERAGED AMOUNTS LOWER...ALTHOUGH STILL COULD SEE ISOLATED
AMOUNTS UPWARDS OF 1.5". GIVEN THE PROGRESSIVE NATURE OF BOTH THE
SYSTEM AS A WHOLE AND INDIVIDUAL CELLS...THINK FLASH FLOODING WILL
REMAIN LOCALIZED IN NATURE. THUS WILL JUST STICK WITH A MARGINAL
RISK FROM NORTHEAST MN INTO NORTHERN AND EASTERN MI AND PORTIONS
OF THE OH VALLEY.

...VA/NC/SC...

MODELS CONTINUE TO BREAK OUT ISOLATED TO SCATTERED CONVECTION
TODAY ACROSS PORTIONS OF VA/NC/SC. ACROSS VA MODELS SHOW A SLIGHT
UPTICK IN 850MB MOISTURE TRANSPORT AS A WARM FRONT SLOWLY LIFTS TO
THE NORTH. THIS MAY BE ENOUGH TO RESULT IN ISOLATED TO SCATTERED
ACTIVITY THIS MORNING INTO THE AFTERNOON HOURS. THE APPROACH OF A
PRE FRONTAL TROUGH FROM THE WEST SHOULD ALSO PRODUCE ISOLATED TO
SCATTERED CONVECTION MOVING FROM WV INTO VA THIS
AFTERNOON/EVENING. ACROSS NC/SC A COASTAL CONVERGENCE BOUNDARY
SHOULD ACT AS A FOCUS FOR CONVECTIVE DEVELOPMENT BY EARLY
AFTERNOON WITH ADDITIONAL DIURNALLY DRIVEN CONVECTION POSSIBLE
OVER CENTRAL NC AS WELL. SC/NC WILL FEEL THE ADDED INFLUENCE OF
ENHANCED DIFFLUENCE ALOFT...IN BETWEEN THE NORTHERN STREAM JET TO
THE NORTH AND THE UPPER LEVEL HIGH CENTERED OVER THE FAR
SOUTHEAST. THIS MAY ALLOW FOR A BIT BETTER COVERAGE OF ACTIVITY
OVER THESE AREAS. PWAT VALUES WILL BE INCREASING THOUGH THE DAY AS
WELL...GENERALLY TO 2"+ BY THIS AFTERNOON. THUS WHILE THE COVERAGE
OF HIGHER TOTALS WILL NOT BE WIDESPREAD GIVEN THE LACK OF A
FORCING MECHANISM AND THE UNORGANIZED NATURE OF CONVECTION...STILL
ANTICIPATE LOCALIZED 2-4" AMOUNTS WILL BE POSSIBLE FROM VA INTO
PORTIONS OF NC/SC...WITH THE BEST CHANCE OF THESE HIGHER TOTALS
FROM NORTHEAST SC INTO SOUTHEAST NC. HAVE ISSUED A MARGINAL RISK
ACROSS PORTIONS OF VA/NC/SC WHERE WE ANTICIPATE AN ISOLATED FLASH
FLOOD RISK WILL EXIST TODAY.

...PLAINS TO THE LOWER MS VALLEY...

AS OF 06Z THURSDAY A CONVECTIVE LINE WAS MOVING SOUTHEAST ACROSS
OK INTO AR AND NORTH TX. WOULD APPEAR THAT THE NSSL WRF WAS DOING
THE BEST JOB CAPTURING THE SOUTHWESTWARD EXTENT OF THIS ACTIVITY.
THUS WPC TRENDED OUR FORECAST CLOSER TO THE NSSL SOLUTION THROUGH
THE DAY TODAY FROM CENTRAL TX INTO THE LOWER MS VALLEY. THIS
CONVECTION SHOULD CONTINUE PROGRESSING SOUTHEASTWARD INTO
NORTHEAST TX AND WESTERN AR THROUGH THE MORNING HOURS. THINK THE
LINE SHOULD GENERALLY BE PROGRESSIVE...ALTHOUGH THE SOUTHWEST
FLANK MAY HANG UP FOR A BIT AND RESULT IN LOCALLY HIGHER TOTALS
AND AN ISOLATED FLASH FLOOD THREAT THROUGH THE MORNING HOURS FROM
NEAR THE RED RIVER INTO NORTHEAST TX. IN GENERAL SHOULD SEE A
WEAKENING TREND WITH THIS ACTIVITY BY MID TO LATE MORNING. HOWEVER
ANTICIPATE WE SHOULD SEE AFTERNOON REDEVELOPMENT ALONG THE
LINGERING BOUNDARY. SOME MODEL DIFFERENCES EXIST WITH EXACTLY
WHERE THIS REDEVELOPMENT OCCURS...BUT GIVEN ITS HANDLING OF THE
CURRENT ACTIVITY...AGAIN PREFER SOMETHING CLOSER TO THE NSSL WRF.
THIS POINTS TOWARDS CENTRAL TX AS THE MOST PROBABLE LOCATION. WITH
SOME LOW LEVEL SOUTHERLY FLOW INTO THE BOUNDARY THINK THIS
AFTERNOON CONVECTION MAY TEND TO BACKBUILD ALONG THE BOUNDARY FOR
A PERIOD. ALTHOUGH GIVEN THE GENERAL LACK OF SYNOPTIC SUPPORT AND
WEAK FLOW OVERALL WOULD NOT ANTICIPATE ANY ORGANIZATION TO BE LONG
LIVED...AND THUS ANY FLASH FLOOD THREAT IS EXPECTED TO REMAIN
LOCALIZED. A MARGINAL RISK WAS EXTENDED ACROSS NORTH TEXAS.

ALSO LIKELY TO HAVE AFTERNOON/OVERNIGHT CONVECTION ACROSS THE
WESTERN PLAINS WITH THE APPROACH OF A MID LEVEL SHORTWAVE TROUGH
AXIS AND INCREASING SOUTHERLY RETURN FLOW. IT REMAINS UNCLEAR HOW
ORGANIZED ANY OF THIS CONVECTION WILL BECOME. WOULD STILL APPEAR
THAT THE BEST CHANCE FOR A MORE ORGANIZED COMPLEX IS ACROSS
PORTIONS OF THE TX/OK PANHANDLES. IT IS HERE WHERE WE SHOULD END
UP WITH MODERATE INSTABILITY ALONG WITH SOME RIGHT ENTRANCE REGION
JET DYNAMICS. 850 MB MOISTURE TRANSPORT IS ALSO FORECAST TO
INCREASE OVERNIGHT...ALTHOUGH THE LATEST GUIDANCE DOES NOT SHOW
MUCH IN THE WAY OF 850 MB MOISTURE CONVERGENCE OVER THIS AREA.
THIS CONVERGENCE IS USUALLY A PRECURSOR TO ORGANIZED
CONVECTION...AND MAY BE THE REASON WHY THE LATEST GUIDANCE IS
OVERALL NOT THAT ROBUST WITH QPF AMOUNTS. ALSO ADD IN THAT PWAT
VALUES ARE ONLY NEAR AVERAGE...AND CORFIDI VECTORS SUGGEST ANY
CONVECTION SHOULD GENERALLY STAY MOVING. THUS DO HAVE MANY FACTORS
POINTING TOWARDS MORE SMALLER SCALE PROGRESSIVE DISCRETE
CONVECTION. ALTHOUGH CONFIDENCE REMAINS LOW...AND THE PERSISTENT
850MB MOISTURE TRANSPORT THAT IS FORECAST INTO THE WEAK LOW LEVEL
BOUNDARY..COMBINED WITH THE UPPER JET DYNAMICS...DOES SUGGEST THAT
ORGANIZED CONVECTION AND HEAVY RAINFALL IS STILL A POSSIBILITY.
GIVEN THE UNCERTAINTIES PRESENT AND THE LACK OF A HEAVY OF SIGNAL
IN ANY OF THE CAM GUIDANCE...THINK THE RISK IS BEST CHARACTERIZED
BY A MARGINAL RISK AT THIS POINT. HAVE THUS OPTED TO DROP THE
SLIGHT FOR NOW...BUT WILL WILL HAVE TO CONTINUE TO MONITOR TRENDS.

GRAPHICS AVAILABLE ON THE WEB AT WWW.WPC.NCEP.NOAA.GOV

CHENARD
$$





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