Quantitative Precipitation Forecast
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FOUS30 KWBC 211339
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
938 AM EDT WED SEP 21 2016

...VALID 15Z WED SEP 21 2016 - 12Z THU SEP 22 2016...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
OTM 15 E ICL 45 S HSI 20 SSE ODX 30 NNW YKN 15 NE VVV
15 SSE AIT 10 WNW LNL 20 ENE SUE 30 WSW LDM ENW 30 E MLI OTM.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
45 S BPI 50 W VEL 10 WNW U28 40 SE CNY 35 NNW CPW 20 SE CPW
30 W SKX 15 NW SAF 15 ENE 4SL 10 ESE FMN 15 SSW CEZ 20 S 4BL
40 WSW 4BL 10 ENE PGA 45 N GCN 35 ESE SGU 30 WSW CDC 75 WNW CDC
65 ENE TPH 15 ESE TPH 45 NNW TPH 45 NW P68 45 NE 9BB 15 N PIH
25 NW IDA 50 WNW IDA 45 E SUN 20 ESE SUN 20 NW SUN 30 ESE SNT
30 SE SMN 20 S BTM 25 SSW HLN 35 NE 3DU 30 SE GPI 20 NNE GPI
20 SSW CWFJ 30 SSE CTB 35 ESE GFA LWT 45 WNW JDN 40 NE OLF
35 W ISN 35 SSE OLF 50 S JDN 35 S BIL 55 NW RIW 45 S BPI.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
95 SSE HSE 25 SSW MRH 15 ENE ILM 20 WNW MYR 30 SSW FLO UDG
20 ESE HNZ 10 S WAL 80 ESE OXB.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
10 S GHW 20 NE RZN 10 SE T08 10 SW ATW 10 WNW UNU 15 N DBQ
30 W DSM 10 W LNK 20 WSW OLU 30 ENE YKN 10 S GHW.

MODERATE RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
20 SW MDZ 20 S ISW 15 NNW LNR 15 NNW ALO 20 W BNW DNS 10 S EST
HCD 20 WSW RPD 20 SW MDZ.


...CENTRAL PLAINS AND MIDWEST / PARTS OF NE/SD/IA/MN/WI/IL...

HEAVY RAIN WITH OBSERVED TOTALS AROUND 2.5 INCHES HAD OCCURRED
EARLY THIS MORNING IN SE MN / SW WI...ALONG THE EASTERN EDGE OF
THE GREAT PLAINS CAP AND COINCIDENT WITH GRADIENTS OF MOISTURE AND
INSTABILITY. A LOW AMPLITUDE SHORTWAVE TROUGH WAS PROGRESSING
DOWNSTREAM...SUGGESTING DEEP LAYER ASCENT WILL WEAKEN...ALTHOUGH
WARM ADVECTION MAY CONTINUE TO SUPPORT SCATTERED HEAVY DOWNPOURS
OVER THE SAME AREAS NEAR THE MISSISSIPPI RIVER THROUGH LATE
MORNING. CONVECTION IS THEN EXPECTED TO REORGANIZE AND REFORM THIS
AFTERNOON AND OVERNIGHT. STRONG INSTABILITY WITHIN THE LOW LEVEL
SOURCE REGION OVER IOWA AND ADJACENT STATES...ALONG WITH
MODERATELY STRONG FORCING AND LOW LEVEL MOISTURE
TRANSPORT...SHOULD RESULT IN LOCALLY INTENSE RAINFALL. A MODERATE
RISK OF EXCESSIVE RAINFALL IS MAINTAINED AND EXPANDED JUST
SLIGHTLY EASTWARD TO ACCOUNT FOR NOW WET ANTECEDENT CONDITIONS.
THE MODERATE RISK IS ALSO EXPANDED NORTHWARD WHERE THE NAM AND RAP
INDICATE THE SURFACE FRONTAL BOUNDARY SHARPENING UP ACROSS THE
SOUTHERN SIDE OF MINNEAPOLIS THIS AFTERNOON. THIS REGION WILL
EXPERIENCE LESS CAPPING THAN AREAS FARTHER SOUTH...AND SHOULD
CONVECT VIGOROUSLY...WHEREAS AFTERNOON / EVENING THUNDERSTORMS
DOWN ACROSS SD/NE/IA MAY BE MORE SCATTERED...AND THOSE ACROSS
WISCONSIN WILL BE MORE ELEVATED...ROOTED CLOSER TO 700 MB AS THE
UPPER PATTERN BEGINS ONCE AGAIN TO SPUR STRONG DIVERGENCE ALOFT.
THE EARLY MORNING OUTFLOW CAUSES US TO EXPAND A MARGINAL RISK
CONTOUR INTO MORE OF NORTHWEST ILLINOIS...BUT WE DID NOT GO AS FAR
AS THE NSSL WRF WOULD SUGGEST...AS LOW LEVEL INFLOW REALLY DROPS
OFF WITH SOUTHEASTWARD EXTENT.

PREVIOUS DISCUSSION...A SURFACE FRONTAL BOUNDARY EXPECTED TO
REMAIN NEARLY STATIONARY DURING THE UPCOMING FORECAST PERIOD FROM
CENTRAL NEBRASKA---EAST NORTHEASTWARD INTO THE VICINITY OF THE
MN/IA BORDER---CENTRAL WI AND THE NORTHERN PORTIONS OF THE U.P.
THIS BOUNDARY SHOULD REMAIN THE FOCUS FOR ORGANIZED
CONVECTION---HEAVY TO EXCESSIVE RAINFALL AMOUNTS AS MOIST WEST
SOUTHWESTERLY LOW LEVEL FLOW PERSISTS. ADDITIONAL ORGANIZED
CONVECTION IS LIKELY TO BEGIN BY OR SHORTLY AFTER 0000 UTC THU AS
THE STRENGTHENING SOUTHWEST LOW LEVEL FLOW REFOCUSES INTO THE
FRONT.  WITH PW VALUES EXPECTED TO INCREASE TO 1.75"+...2.5 TO 3
STANDARD DEVIATIONS ABOVE THE MEAN ALONG THIS FRONT AND POTENTIAL
FOR SLOW MOVING AND TRAINING OF CELLS ALONG THE BOUNDARY---HEAVY
TO EXCESSIVE RAINFALL AMOUNTS LIKELY IN THE VICINITY OF NORTHEAST
NEBRASKA---CENTRAL TO  NORTHERN IA---SOUTHERN MN INTO SOUTHWEST
WI.  WPC QPF LEANED TOWARD THE HI RES ARW AND NSSL WRF WHICH WERE
ON THE SOUTHWEST SIDE OF THE GUIDANCE WITH RESPECT TO THE MAX AREA
OF PRECIP.  MANY OF THE OTHER NON CONVECTION ALLOWING MODELS WERE
FARTHER TO THE NORTHEAST WITH HEAVY PRECIP INTO CENTRAL WISCONSIN.
WHILE THIS PRECIPITATION WILL OCCUR WITHIN A DIVERGENCE MAXIMUM AT
250 MB...THE CONVECTION WILL BE ROOTED WELL ALOFT THERE...AND MAY
NOT PRESENT A FOCUSED FLASH FLOOD THREAT. THE NAM EXPERIMENTAL WAS
CLOSER TO THE ARW AND NSSL WRF---NOT AS FAR EAST NORTHEAST WITH
HEAVY PRECIP AS THE NAM CONEST.


...THE CAROLINAS AND SOUTHEAST VIRGINIA...

THE CLOSED LOW INITIALLY ALONG THE COASTAL NC/SC BORDER IS
EXPECTED TO BECOME INCREASINGLY SHEARED THIS PERIOD.  THIS SHOULD
RESULT IN WEAKENING OF THE ONSHORE EASTERLY FLOW TO ITS NORTH
WHICH HAS BEEN FOCUSING PERSISTENT MODERATE TO HEAVY RAINS OVER
NORTHEAST NC INTO SOUTHEAST VIRGINIA.  WHILE WEAKENING IS
EXPECTED---THIS AREA WILL STILL HAVE WELL DEFINED ONSHORE EASTERLY
FLOW---ESPECIALLY DURING THE BEGINNING OF THE FORECAST PERIOD.
THIS WILL SUPPORT ADDITIONAL MODERATE TO HEAVY RAINFALL OVER
NORTHEAST NC AND SOUTHEAST VA. WITH RECENT HEAVY RAINFALLS AMOUNTS
OF 4 TO 6 INCHES OR MORE---FFG VALUES HAVE LOWERED.
ADDITIONALLY...PARTIAL HEATING IS FORECAST TO GENERATE MIXED LAYER
CAPE VALUES EXCEEDING 500 J/KG THIS AFTERNOON ALONG THE ELONGATING
TROUGH AXIS JUST INLAND FROM VA TO SC. MEAN LAYER WINDS WILL CAUSE
UPDRAFTS TO DRIFT WEST...WHILE REDEVELOPMENT MAY OCCUR BACK TO THE
EAST INTO THE INFLOW...RESULTING IN SOME SMALL SCALE EPISODES OF
CELL TRAINING. WITH RICH TROPICAL MOISTURE IN PLACE...SOME LOCALLY
INTENSE RAIN RATES OVER 1.5 INCHES PER HOUR APPEAR POSSIBLE...AND
MAY CAUSE RAPID RUNOFF IN ANY POORLY DRAINED AREAS.


...WESTERN U.S...

MODELS CONTINUE TO SHOW GOOD CONTINUITY AND AGREEMENT WITH THE
AMPLIFICATION OF THE UPPER TROF FROM THE PACIFIC
NORTHWEST---SOUTHWARD ALONG THE WEST COAST AND INLAND INTO THE
WESTERN PORTIONS OF THE GREAT BASIN.  THE MUCH ABOVE AVERAGE PW
VALUES THAT HAVE ALREADY PUSHED NORTHWARD INTO SOUTHERN
CALIFORNIA---THE SOUTHWEST AND THE SOUTHERN GREAT BASIN AHEAD OF
POST TROPICAL CYCLONE PAINE WILL CONTINUE TO PUSH NORTHEASTWARD
AHEAD OF THIS AMPLIFYING TROF ACROSS THE GREAT BASIN AND INTO THE
NORTHERN ROCKIES. THIS WILL BE SUPPORTING INCREASING PRECIP
COVERAGE THROUGH THE GREAT BASIN INTO THE NORTHERN ROCKIES WITH
WIDESPREAD MODERATE TO LOCALLY HEAVY PRECIP TOTALS EXPECTED. IN
THE 15Z UPDATE WE EXPANDED LOW PROBABILITIES AND MARGINAL RISK TO
INCLUDE MOISTURE ADVECTING INTO THE HIGHER TERRAIN OF SOUTHWEST
COLORADO AND NORTHERN NEW MEXICO...NOTING SUBSTANTIAL PW INCREASE
BETWEEN 00Z AND 12Z AND A MARGINALLY FAVORABLE HI-RES QPF SIGNAL.
ONE OF THE MORE INTRIGUING AREAS IS OVER PARTS OF NV/UT/ID...WHERE
THERE WILL BE SOME SENSE OF JET COUPLING THIS AFTERNOON AND A BIT
MORE SUBSTANTIAL HEIGHT FALLS SWINGING EASTWARD THROUGH NEVADA.
LAPSE RATES ARE STEEP...BUT MOISTURE MAY BE SUFFICIENT ONLY TO
GENERATE CAPE VALUES TOUCHING ON 500 J/KG...AND QUICK CELL MOTIONS
ARGUE FOR MAINTAINING ONLY LOW / MARGINAL PROBABILITIES OF
RAINFALL EXCEEDING FLASH FLOOD GUIDANCE.

BURKE/ORAVEC
$$





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