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Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1556Z May 04, 2015)
 
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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
1156 AM EDT MON MAY 04 2015

VALID 12Z THU MAY 07 2015 - 12Z MON MAY 11 2015

...OVERVIEW AND GUIDANCE PREFERENCES...

THE LARGEST INFLUENCES ON THE SYNOPTIC FLOW ACROSS THE CONUS THE
NEXT 7 DAYS INCLUDE THE DECAYING REX-BLOCK OVER THE BERING
SEA...WHICH IS PROMOTING SPLIT-FLOW DOWNSTREAM AND A MEAN TROUGH
IN THE WEST...AND THE SUBTROPICAL HIGH OVER THE ATLANTIC WHERE
ANOMALIES ARE RUNNING ABOUT 3 STANDARD DEVIATIONS ABOVE NORMAL. 
THE HIGH IS IMPORTANT BECAUSE OF ITS STEERING INFLUENCE WITH ANY
LOW OR LOWS THAT MIGHT FORM OVER OFF THE SOUTHEAST COAST THE NEXT
SEVERAL DAYS.

FOR THE MEAN TROUGH IN THE WEST...INCLUDING THE CLOSED LOW PORTION
CROSSING THE FOUR CORNERS AND THE NORTHERN STREAM PORTION RUNNING
ALONG THE CANADIAN BORDER...MODEL CLUSTERING AND RECENT TRENDS
SUPPORT A SOLUTION TOWARD THE SLOWER SIDE OF THE SOLUTION
ENVELOPE...INCLUDING THE GFS/GEFS MEAN.  THE ECMWF AND ITS OWN
ENSEMBLE MEMBERS HAVE SLOWED APPRECIABLY ACROSS CANADA THE LAST
COUPLE OF DAYS...WITH THE GFS ALSO SHOWING STRONGER RUN-TO-RUN
CONTINUITY.  THIS PREFERENCE IMPACTS THE FRONTAL TIMING ACROSS THE
NORTHERN TIER...WITH AGAIN THE SLIGHTLY SLOWER SOLUTIONS
FAVORED...WITH SMALLER IMPLICATIONS FOR EVOLUTION OF THE LOW OVER
THE FOUR CORNERS...WHICH IS MORE DEPENDENT ON THE UPSTREAM FLOW. 
FOR THE CLOSED LOW PORTION...ABOUT 70 PERCENT TOWARD THE SLOWER
06Z GEFS MEAN IS PREFERRED OVER THE 00Z ECMWF ENSEMBLE MEAN. 

THE FASTER ECMWF APPEARS TO ALSO HAVE SOME BEARING ON THE TRACK OF
ANY LOW OR LOWS THAT MAY DEVELOP OFF THE SOUTHEAST THE NEXT FEW
DAYS...BY NUDGING THE LOW MORE OUT TO SEA AFTER DAY
5/SAT...COMPARED TO THE GFS/GEFS MORE WESTWARD CAMP.  MODELS HAVE
BEEN VERY CONSISTENT THE LAST FEW DAYS WITH A LOW DEVELOPING NORTH
OF THE BAHAMAS...THEN ENTERING WEAKER FLOW AND POSSIBLY
MEANDERING...WHILE TRENDS IN THE MODEL CONSENSUS HAVE BEEN
WESTWARD.  AFTER COORDINATING WITH THE NHC AND OPC...WE ARE ALL IN
AGREEMENT TO REPOSITION ANY ANTICIPATED LOW SLIGHTLY MORE WESTWARD
AND STATIONARY FOR DAYS 5-7 IN ACCORDANCE WITH AN INTERMEDIATE
CHOICE BETWEEN THE MORE WESTWARD 06Z GEFS MEAN AND MORE OFFSHORE
00Z ECMWF ENSEMBLE MEAN.  CONFIDENCE IS LOW WITH THIS PARTICULAR
SYSTEM OWING TO MULTIPLE UNCERTAINTIES IN CORE IDENTITY AND OTHER
MESOSCALE PROCESSES...NOT THE MENTION ANY UPSTREAM INFLUENCES.

...SENSIBLE WEATHER HIGHLIGHTS...

WIDESPREAD AND IN SOME CASES HEAVY RAINFALL WILL ACCOMPANY THE
LEADING EDGES OF THE UPPER TROUGH ENTERING THE HIGH PLAINS AND
CENTRAL CONUS...WITH ADDITIONAL BUT MORE SCATTERED PRECIPITATION
FORMING BENEATH THE LOW EXTENDING FROM CALIFORNIA INTO THE
INTERMOUNTAIN WEST...WHILE A SMALLER AREA OF POSSIBLY HEAVY
RAINFALL WILL BECOME POSSIBLE WITH ANY LOW THAT DEVELOPS OFF THE
SOUTHEAST COAST.  INCREASED ONSHORE FLOW NORTH OF ANY COASTAL LOW
MAY ALSO RESULT IN ADVERSE CONDITIONS FOR OTHER COASTAL AND MARINE
INTERESTS.  FINALLY...BETWEEN THE UPPER TROUGH IN THE WEST AND THE
COMPENSATING UPPER RIDGE IN THE EAST...A DIPOLE OF BELOW NORMAL
AND ABOVE NORMAL TEMPERATURES WILL DEVELOP BENEATH THE LOW AND
HIGH RESPECTIVELY.

JAMES