Quantitative Precipitation Forecast
Issued by NWS

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FOUS30 KWBC 211417
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1017 AM EDT THU AUG 21 2014

...VALID 15Z THU AUG 21 2014 - 12Z FRI AUG 22 2014...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
CWBZ 10 SSW CMSB 15 E CWFQ 30 N 1V4 15 NE 1V4 15 SSE 1V4 LEB
10 ESE VSF 10 SW EEN 20 WSW ORE 10 NNW BAF 20 SSE PSF 20 SSW PSF
10 N POU 10 W MGJ 20 SW MSV 35 NNE AVP 25 SE BGM 20 NE BGM UCA
20 NE GTB 15 ENE OGS MSS CWBZ.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
35 E HEI 15 SSE HEI 10 E 2WX 65 WSW 2WX 50 SSE MLS MLS
25 SE JDN 60 NW JDN 50 SW CWVN 15 SSW CWVN 20 SE CWAQ 10 E ISN
20 ENE KS25 15 SSE KHZE 35 E HEI.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
DUH 10 NW BJJ 20 E PHD AFJ 15 NE MGW 25 SSW 2G4 10 SSE EKN
30 NNE LWB 20 NNE BKW 20 NNW I16 25 SE HTS 35 WNW HTS 15 SSE ILN
10 NNW HAO 10 NE EYE 15 NW DNV 15 NE MQB 20 SW MPZ 10 S FFL
10 SSE PEA DSM 20 SSW BNW 20 NW BNW 10 SE FOD 15 NE CAV
20 SSE AUM 15 NE FKA 15 SSE ONA 15 NW CMY 20 NNW VOK 10 S ISW
10 WSW PCZ 10 ESE PCZ ATW 20 W MTW 30 ESE MTW 20 NNE MKG FPK
10 ESE FPK 15 NE ADG DUH.


1400 UTC UPDATE

CHANGES TO THE PREVIOUS ELONGATED SLIGHT RISK AREA FROM THE
NORTHERN PLAINS/UPPER MS VALLEY REGION INTO THE MID WEST WERE TO
CUT BACK ON THE WESTERN PORTION OF THIS RISK AREA AS PER LATEST
RADAR TRENDS SHOWING HEAVIEST PRECIPITATION MOVING EASTWARD.  THE
SLIGHT RISK AREA WAS EXTENDED SOUTHWARD INTO CENTRAL TO EASTERN IA
AND NORTHERN IL.  THIS WAS TO ACCOUNT FOR CONVECTION MOSTLY LATER
ON THIS AFTERNOON INTO THE OVERNIGHT PERIOD WHICH MAY FIRE ACROSS
IA INTO IL/IN.  WESTERLY FLOW AT THIS TIME---MAY SUPPORT TRAINING
OF CELLS ACROSS THIS AREA.  THE SOUTHWARD ADJUSTMENT WAS ALSO TO
COVER THE QPF FORECASTS FROM THE NSSL WRF AND HI RES ARW---WHICH
BOTH SHOW POTENTIAL FOR VERY HEAVY RAINFALL AMOUNTS ACROSS THESE
AREAS

OVER THE NORTHEAST---THE PREVIOUS OUTLOOK AREA WAS EXPANDED
SLIGHTLY EASTWARD INTO THE BERKSHIRES OF WESTERN MA AND THE GREEN
MOUNTAINS OF VT WHERE FFG VALUES ARE RELATIVELY LOW AND SHOWERS
HAVE ENHANCED OVER.  ISOLATED 1"+ PRECIPITATION TOTALS IN AN HOUR
POSSIBLE HERE.

A SLIGHT RISK AREA WAS ALSO ADDED OVER EASTERN MT---SOUTHWEST ND
INTO FAR NORTHWEST SD.  THIS IS TO ACCOUNT FOR CONVECTIVE
POTENTIAL LATER THIS AFTERNOON---CONTINUING INTO THE OVERNIGHT
HOURS AHEAD OF SHORTWAVE ENERGY ROTATING THROUGH THE MEAN TROF
POSITION ACROSS THESE AREAS.  WITH FFG VALUES RELATIVELY LOW
ACROSS THESE AREAS---ISOLATED RUNOFF ISSUES POSSIBLE.

OVER SOUTHEAST CA INTO THE SOUTHWEST---ADDITIONAL SCATTERED
CONVECTION LIKELY AHEAD OF THE SLOWLY MOVING CLOSED LOW INITIALLY
OFF THE SOUTHERN CALIFORNIA COAST.  PW VALUES ARE NEAR OR SLIGHTLY
ABOVE NORMAL AHEAD OF THIS CLOSED LOW FROM SOUTHERN
CALIFORNIA---EASTWARD INTO THE SOUTHWEST---BUT HAVE INCREASED FROM
YESTERDAY.  WELL DEFINED UPPER DIFFLUENCE AHEAD OF THIS CLOSED LOW
WILL SUPPORT ADDITIONAL SCATTERED CONVECTION WITH ISOLATED RUNOFF
ISSUES POSSIBLE FROM SHORT TERM PRECIPITATION TOTALS OF .50-1"+

ORAVEC


PREVIOUS DISCUSSION

A COUPLE OF STREAMS OF H5 S/WV ENERGY MOVING THRU THIS REGION IN A
GRADUALLY RISING HEIGHT REGIME..A WARM FRONT LIFTING SLOWLY
NORTHWARD WITH VERY HIGH
PWS POOLED VICINITY OF THIS FRONT..AND A BROAD AND MODEST
SOUTHWESTERLY TO WESTERLY LOW LEVEL JET WILL ALL COMBINE TO
PRODUCE AN AXIS OF ORGANIZED CONVECTION WITH MODERATE TO HEAVY AND
EVEN ISOLATED EXCESSIVE RAINFALL AMOUNTS WITHIN THIS AXIS FROM
PARTS OF THE UPPER MS VALLEY SOUTHEASTWARD INTO THE CENTRAL
APPALACHIANS THIS PERIOD.  ONGOING CONVECTION FROM PARTS OF THE
EASTERN DAKOTAS INTO WESTERN MN SHOULD BECOME BETTER ORGANIZED THE
REMAINDER OF THE NIGHT INTO EARLY THURSDAY MORNING AS THE
SOUTHWESTERLY LOW LEVEL JET STRENGTHENS A BIT..WITH THIS ACTIVITY
TENDING TO WEAKEN A LITTLE DURING THURSDAY BEFORE ENHANCING AGAIN
THURSDAY NIGHT UNDER WHAT WILL AGAIN BE AN INCREASING WESTERLY LOW
LEVEL JET THRU THE OH VALLEY AT THIS TIME.  OVERALL..THERE
CONTINUES TO BE GOOD AGREEMENT AMONGST THE MODELS FOR AN ORGANIZED
AND SIGNIFICANT CONVECTIVE/RAINFALL EVENT..BUT WITH SOME
DIFFERENCES IN THE LATITUDE OF WHERE THE HEAVIER RAINS WILL OCCUR.
 THE HIRES ARW SEEMED TO BE A BIT OF A SOUTH OUTLIER..SO STAYED
CLOSER TO SOME OF THE OTHER HIRES GUIDANCE WHICH HAD A SLIGHTLY
FARTHER NORTH AXIS..INCLUDING THE NMMB..NAM CONEST AND THE ECMWF
AND GEM REGIONAL HIRES QPFS.  EXPECT AREAL AVERAGE 1-3 INCH RAINS
ESPECIALLY ACROSS THE LOWER GREAT LAKES..AND INTO THE CENTRAL
APPALACHIANS WHERE THE WESTERLY FLOW WILL ENHANCE AMOUNTS ON
THURSDAY NIGHT..WITH ISOLATED HEAVIER TOTALS CERTAINLY
POSSIBLE..WHICH COULD CAUSE SOME ISOLATED FLOODING CONCERNS.



TEMPORARY REX BLOCK WILL REMAIN IN PLACE FROM EASTERN CANADA
SOUTHWARD INTO THE NORTHERN MID ATLANTIC STATES THIS PERIOD..WITH
THE CLOSED H5 LOW COMPRISING THE SOUTHERN PORTION OF THIS BLOCK
MOVING ONLY VERY SLOWLY FROM THE EASTERN GREAT LAKES INTO
SOUTHEASTERN NY STATES DURING THE PERIOD.  AHEAD OF THIS CLOSED H5
LOW..AN AXIS OF HIGHER PWS WILL PERSIST FROM PARTS OF PA AND NJ
NORTHWARD THRU NY STATE AND INTO WESTERN NEW ENGLAND ON A
RELATIVELY WEAK BUT PERSISTENT SOUTHERLY LOW LEVEL JET.  THIS
SHOULD SUPPORT ENHANCED BANDS OF RAINFALL DURING MUCH OF THIS
PERIOD..WITH THE DURATION OF THE EVENT SUPPORTING AT LEAST SOME
0.50-1.00+ INCH AREAL AVERAGE RAINFALL AMOUNTS..BUT WITH WITH
ISOLATED AMOUNTS OF 2+ INCHES POSSIBLE WHERE SOME BANDS MAY REPEAT.

TERRY
$$





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