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Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1529Z Aug 17, 2014)
 
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Geographic Boundaries -  Map 1: Color  Black/White       Map 2: Color  Black/White


EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1129 AM EDT SUN AUG 17 2014

VALID 12Z WED AUG 20 2014 - 12Z SUN AUG 24 2014

.SYNOPSIS AND MODEL EVALUATION...

IN THE EAST THE ECMWF- AND GFS-BASED GUIDANCE HAVE BEEN RELATIVELY
STEADY IN BRINGING SHORTWAVE TROUGHING DOWN THROUGH THE OHIO
VALLEY TO MID ATLANTIC. THIS HOLDS IN PLACE A QUASI-STATIONARY
BOUNDARY AND EVENTUALLY PUSHES THE FRONT SOUTH THROUGH THE
CAROLINAS AND NORTH GEORGIA BY THE END OF THE 7-DAY.

MODEL VARIABILITY IS MUCH GREATER FARTHER WEST. OPERATIONAL
GUIDANCE CONTINUES TO FLIP-FLOP OVER THE SOUTHWEST...WHERE IT WAS
THE GFS...THEN THE ECMWF...NOW THE 00Z GFS AGAIN...WHICH KEPT THE
STREAMS PHASED AND PUSHED TROUGHING INTO ARIZONA BY DAY 3/4. THOSE
TWO MODELS HAVE NOT BEEN IN SYNC ONCE IN THE PAST SEVERAL CYCLES.
THE VAST MAJORITY OF ENSEMBLE EVIDENCE CONTINUES TO POINT TOWARD A
SOUTHERN STREAM CLOSED LOW FORMING AND THEN RETROGRADING INTO THE
PACIFIC RATHER THAN PUSHING INTO THE SOUTHWEST U.S. THIS GIVES US
SOME CONFIDENCE IN MAINTAINING CONTINUITY AND FAVORING THE
CONSENSUS SCENARIO OF A RETROGRADING LOW...ALTHOUGH WE ACKNOWLEDGE
THIS IS AN UNUSUALLY SENSITIVE FORECAST GIVEN THE RUN TO RUN
VARIABILITY. ADDITIONALLY...THE MODELS THAT FORM A RETROGRADING
LOW PHASE THIS FEATURE TO VARYING DEGREES WITH INTENSIFYING
DISTURBANCES IN THE TROPICAL PACIFIC...WITH MULTIPLE RUNS OF THE
CANADIAN GOING SO FAR AS TO DRAW A DEFINABLE LOW LEVEL CIRCULATION
UP THROUGH BAJA AND INTO THE SOUTHWEST U.S. THESE ARE VERY-LOW
PROBABILITY SOLUTIONS...BUT INDIVIDUAL DETERMINISTIC RUNS CONTINUE
TO GO THIS ROUTE. AT THE VERY LEAST...SOME MONSOONAL MOISTURE
WOULD DRIFT WESTWARD INTO THE DESERT AREAS OF SOUTHERN CALIFORNIA
IF THE RETROGRESSION OCCURS AND SUB-TROPICAL RIDGING ACROSS THE
SOUTHERN U.S. EXPANDS WESTWARD.

LARGE SCALE VARIABILITY IS ALSO OCCURRING IN THE NORTHERN STREAM.
RUN TO RUN DIFFERENCES IN THE ECMWF WERE PARTICULARLY
EXTREME...WITH THE PREVIOUS 12Z HOLDING A MEAN TROUGH AXIS NEAR
THE WEST COAST ON DAY 6...AND THE NEW 00Z RUN PUSHING IT JUST EAST
OF THE CONTINENTAL DIVIDE BY THAT TIME. THE 00Z AND 06Z GFS
TRENDED SLIGHTLY IN THAT DIRECTION...BUT SOMEWHAT FOR THE WRONG
REASONS...BY SLING-SHOTTING THE SOUTHERN STREAM ENERGY OUT OF
CALIFORNIA... ENERGY WHICH CONSENSUS AND CONTINUITY SAYS SHOULD
NOT BE AVAILABLE TO THE NORTHERN STREAM. THE ENSEMBLES MAY HAVE
INCHED EASTWARD...BUT CERTAINLY NOWHERE NEAR THE EXTENT SEEN IN
THE ECMWF.

THE OPERATIONAL CANADIAN HAS NOT BEEN MUCH HELP...AS IT IS EVEN
MORE OUT OF PHASE WITH THE ENSEMBLES THAN IS THE ECMWF.
THEREFORE...WPC CONTINUED WITH A NEAR EVEN MIX OF ECMWF AND NAEFS
ENSEMBLE MEANS.

...CHANGES THIS CYCLE...

THE MAIN CHANGE IN THE FORECAST WAS TO GRAB ONTO THE OVERALL TREND
TOWARD MORE LEAD SHORTWAVES AND HEIGHT FALLS AFFECTING SOUTHERN
CANADA AND THE NORTHERN U.S...TO PULL THE FRONT ALONG INTO THE
PLAINS MORE QUICKLY ON DAYS 3-5. WE ALSO DIG THE EASTERN U.S.
TROUGH A LITTLE FURTHER...CAUSING THE QUASI-STATIONARY FRONT TO
PUSH SOUTH AS A COLD FRONT IN LATER PERIODS...PRODUCING A SLIGHT
COOL DOWN OVER THE CAROLINAS ON DAY 7.

...SENSIBLE WEATHER HIGHLIGHTS...

THE GREATEST COVERAGE AND INTENSITY OF RAINFALL THIS PERIOD IS
EXPECTED IN A ZONE ALONG THE QUASI-STATIONARY BOUNDARY STRETCHING
FROM THE CENTRAL/NORTHERN PLAINS TO THE MID ATLANTIC. GREATER
ORGANIZATION SHOULD OCCUR NEAR THE EJECTING LEAD SHORTWAVES OVER
THE NORTHERN PLAINS AND OVER THE APPALACHIANS TO MID ATLANTIC
AHEAD DURING SLIGHT AMPLIFICATION OF THE EASTERN U.S. TROUGH.

EXPANSION OF THE UPPER-LEVEL RIDGE ACROSS THE SOUTHEAST WILL LOCK
IN THE HEAT AND HUMIDITY ACROSS THE REGION...WHILE A BACKDOOR COLD
FRONT RETURNS THE NORTHEAST COAST TO COOLER CONDITIONS DAY 6-7.

BY DAYS 5-7 THE GRADUALLY DEEPENING TROUGH IN THE WESTERN U.S.
REACHES 1.5 STANDARD DEVIATIONS FROM CLIMATOLOGY...WITH AN
ASSOCIATED DIP IN MAX TEMPERATURES TO THE 70S IN MANY AREAS...OR
ABOUT 12F-16F DEGREES BELOW IN THE NORTHERN ROCKIES AND NORTHERN
PLAINS.

BURKE/VOJTESAK