Quantitative Precipitation Forecast
Issued by NWS

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930
FOUS30 KWBC 202344
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
743 PM EDT SAT AUG 20 2016

...VALID 01Z SUN AUG 21 2016 - 12Z SUN AUG 21 2016...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
45 NW CYSB 55 NNE CYZE GOV 35 ESE MTW 20 ENE CLI 25 NNW IMT
35 NNE CYXZ.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
130 ESE MMCU 125 SSW 6R6 55 SSE E38 25 SSE GDP 10 SSW CNM
50 W SJT 20 W GDJ 20 SW LBR 15 SE M19 30 S BMG BAK 35 NW DAY
15 S AOH 10 N PKB 10 WSW AGC 15 ESE 2G4 10 WNW BCB 10 NW GMU AHN
20 NW TOI AEX 30 SSE LFK VCT 70 S MMMY.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
45 NNW MMMV 15 ESE MMPG 30 S ECU 10 NW JCT 20 NW 05F 10 SSW 4F4
20 E M89 25 NE SGT 35 WSW UTA LLQ 10 SSW MLU 35 N ESF 35 WSW IER
20 NE UTS 35 N ALI 20 E MMMY.

MODERATE RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
50 SSE SSF 20 ESE COT 10 NNW COT 15 WNW ERV 20 NNW BMQ
10 ENE HLR 20 W LHB 20 N 62H 45 ESE BAZ 50 SSE SSF.

HIGH RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
25 S BMQ 10 SSE AUS 25 SE BAZ 30 S SSF 30 NNE COT 10 S ERV T82
25 S BMQ.


...SIGNIFICANT HILL COUNTRY FLASH FLOOD EVENT SLATED INTO THE
MORNING HOURS...

TEXAS
~~~~~
WE ARE VERY CONCERNED ABOUT THE INGREDIENTS IN PLACE THAT WOULD
SUPPORT EXCESSIVE RAINFALL AMOUNTS TOTALING 10-15" IN MOST OF THE
HIGH RESOLUTION GUIDANCE -- A POTENTIAL PREDECESSOR RAINFALL EVENT
(PRE) RELATED TO TROPICAL STORM KAY IN THE NORTHEAST PACIFIC.  AN
MCV THAT MOVED OUT OF NORTHWEST MEXICO ACROSS SOUTH-CENTRAL TX IS
EMBEDDED WITHIN A PLUME OF TROPICAL MOISTURE.  IT HELPED SPAWN A
LOW-LEVEL CIRCULATION NEAR THE RIO GRANDE VALLEY THAT IS UNLIKELY
TO MOVE APPRECIABLY OVERNIGHT.  THE LARGER SCALE PATTERN SUPPORTS
DIVERGENCE ALOFT ACROSS SOUTH-CENTRAL TX AND SUSTAINED LOW LEVEL
INFLOW OF 20-30 KTS FROM THE GULF OF MEXICO AROUND THE EAST SIDE
OF THE LOW-LEVEL CIRCULATION, WHICH WOULD DRAW IN MUCAPES IN THE
1000-4000 J/KG RANGE.  THIS SITUATION WOULD APPEAR TO BE RIPE FOR
A SIGNIFICANT FLASH FLOOD EVENT, PARTICULARLY DUE TO THE HISTORY
OF THESE EVENTS IN THE TEXAS HILL COUNTRY NEAR THE BALCONES
ESCARPMENT.  THE QPF SIGNAL IN THE GUIDANCE IS ORGANIZED,
SUGGESTING SOME BREADTH TO THE EXTREME RAIN TOTALS AFFECTING
MULTIPLE COUNTIES...RATHER THAN BEING VERY ISOLATED.

COORDINATION WITH TEXAS LOCAL OFFICES UPGRADED TO A SMALL HIGH
RISK AREA CENTERED AROUND SAN ANTONIO / AUSTIN. PLACEMENT OF THE
QPF MAXIMUM IS NOT OF EXTREMELY HIGH CONFIDENCE...BUT WPC
FORECASTERS USING INDEPENDENT METHODS ON THE NIGHT AND DAY SHIFT
CAME UP WITH THE SAME ANSWER FOR LOCATION...WHICH IS SUPPORTED IN
PARTICULAR BY THE WRF-ARW...WHICH TENDS TO PERFORM WELL IN SOUTH
TEXAS...AND ENDS UP BEING A COMPROMISE OF THE 12Z ECMWF/12Z GFS
GUIDANCE.  THE WRF-ARW AND WRF-NMM QPF SIGNAL INDICATING NEARLY
150 PERCENT OF THE ONE PERCENT ANNUAL EXCEEDANCE PROBABILITY
RAINFALL-WISE; ANOTHER INDICATION OF AN UNUSUALLY HEAVY RAIN
EVENT. THANKS TO WFO EWX...FWD...CRP...HGX...AND SOUTHERN REGION
FOR COORDINATION EARLY THIS AFTERNOON.


ELSEWHERE IN THE LOWER MISSISSIPPI VALLEY AND SOUTHERN PLAINS
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
MAINTAINED A MARGINAL TO SLIGHT RISK OVER THROUGH PARTS OF
ARKANSAS / LOUISIANA BASED ON OBSERVATIONAL TRENDS...THE
HRRR...AND THE OVERNIGHT WPC QPF. THOUGH CONVECTION MAY BE LOOSELY
ORGANIZED HERE...IT WILL PRODUCE STRONG SHORT TERM RAIN
RATES...AND FORCING MAY INCREASE NORTHEASTWARD OUT OF THE HILL
COUNTRY BY LATE TONIGHT AS THE MCV CONTINUES STREAKING
NORTHEASTWARD.


CENTRAL APPALACHIANS/UPPER OH VALLEY/TN VALLEY
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
THE FRONTAL BOUNDARY FROM THE AMPLIFYING TROF OVER THE UPPER MS
VALLEY/UPPER LAKES IS EXPECTED TO BE VERY PROGRESSIVE TO THE EAST
AND SOUTHEAST THROUGH THE LOWER LAKES---OH VALLEY AND TN VALLEY.
WHILE PW VALUES WILL BE ABOVE AVERAGE IN A NARROW BAND ALONG AND
AHEAD OF THE FRONT---2 TO 2.5 STANDARD DEVIATIONS ABOVE THE
MEAN---THE PROGRESSIVE NATURE OF THE BOUNDARY WILL LIMIT POTENTIAL
FOR VERY HEAVY TOTALS. STILL---WELL DEFINED FRONTAL CONVERGENCE IN
THIS HI PW AXIS WILL SUPPORT LOCALLY HEAVY PRECIP TOTALS ALONG AND
AHEAD OF THIS FRONT. A MARGINAL RISK AREA WAS DEPICTED FROM THE
CENTRAL APPALACHIANS/UPPER OH VALLEY REGION INTO THE TN VALLEY
WHERE THERE ARE POCKETS OF RELATIVELY LOWER FFG VALUES IN 1-1.5"
RANGE IN AN HOUR.  CHANGES MADE IN THIS AREA WERE DUE TO RADAR
TRENDS.


UPPER LAKES
~~~~~~~~~~~
THE DEEPENING CYCLONE ROTATING THROUGH THE UPPER MS VALLEY/UPPER
LAKES REGION WILL SUPPORT AN ENHANCING COMMA HEAD/DEFORMATION
PRECIPITATION AREA FROM PORTIONS OF THE UPPER MS
VALLEY---NORTHEASTWARD INTO THE UPPER LAKES.  WITH FAIRLY GOOD
MASS FIELD EVOLUTION AMONG THE LATEST MODELS---THERE IS SIMILAR
QPF DISTRIBUTIONS.  HEAVY RAINS LIKELY IN THIS COMMA HEAD
REGION---WITH AREAL AVERAGE 1-2"+ TOTALS FROM NORTHERN WI INTO
EASTERN PORTIONS OF THE UPPER PENINSULA OF MI AND NORTHERN LOWER
PENINSULA--- WITH ISOLATED TOTALS GREATER THAN 3"
POSSIBLE---ESPECIALLY OVER THE CENTRAL U.P. AND FAR NORTHERN WI
WHERE LAKE ENHANCEMENT TO THE PRECIPITATION IS POSSIBLE FROM
STRONG NORTHERLY TO NORTHWESTERLY FLOW OFF OF LAKE SUPERIOR.

ROTH/BURKE/ORAVEC
$$





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