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Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1600Z Apr 23, 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
1200 PM EDT WED APR 23 2014

VALID 12Z SAT APR 26 2014 - 12Z WED APR 30 2014

...PATTERN OVERVIEW...

THE SHORTWAVE PATTERN AFFECTING THE CONUS DURING THE MEDIUM RANGE
PERIOD/DAYS 3-7 IS EXPECTED TO SLOW AS A STRONG CLOSED LOW FORMS
OVER THE SOUTHWEST AND DRIFTS EASTWARD WHILE ENCOUNTERING A
BLOCKING PATTERN EXTENDING FROM SOUTHEAST CANADA TOWARD GREENLAND.
 THIS PATTERN IS ACCOMPANIED BY A DEEP SURFACE LOW FORMING IN THE
LEE OF THE ROCKIES WHICH PROGRESSES SLOWLY EASTWARD OVER SEVERAL
DAYS...AND HAS THE POTENTIAL TO PRODUCE EXCESSIVE RAINFALL
ANYWHERE FROM THE SOUTHERN PLAINS EASTWARD INTO THE LOWER/MIDDLE
MISSISSIPPI VALLEY AND DEEP SOUTH.

...MODEL PREFERENCES AND SENSIBLE WEATHER HIGHLIGHTS...

THE ABOVE AVERAGE AVERAGE WAVELENGTH AND STRENGTH OF THE PATTERN
ENTERING THIS WEEKEND/DAYS 3-4 ARE ASSOCIATED WITH NARROW SOLUTION
SPREAD AND THUS HIGHER THAN NORMAL CONFIDENCE IN THE DETERMINISTIC
SOLUTIONS...WITH ALMOST ANY COMBINATION OF SOLUTIONS CONSIDERED
RELIABLE.  BY DAY 5/MON HOWEVER...THE SPREAD INCREASES...WITH THE
00Z ECMWF STRAYING FROM THE CONSENSUS THE QUICKEST BY ALLOWING THE
TROUGH PORTION OF THE BLOCK ACROSS SOUTHEAST CANADA TO BECOME
REINFORCED BY A SHORTWAVE TROUGH APPROACHING FROM ITS NORTH. 
GIVEN THAT IS SOLUTION LIES NEAR THE OUTER EDGES OF THE
GUIDANCE...HAS POOR RUN-TO-RUN CONTINUITY...AND DISAGREES WITH THE
CONCEPTUAL MODEL OF THE BLOCKING PATTERN WITH AN UPSTREAM CLOSED
LOW...ITS SOLUTION IS CONSIDERED UNRELIABLE.  HOWEVER...SUFFICIENT
BUT RELATIVELY SMALL PERCENTAGE OF THE ITS OWN ENSEMBLE MEMBERS
ALONG WITH MINOR CONTRIBUTIONS FROM THE 00Z UKMET/CANADIAN
SUGGESTS THAT IT CANNOT BE COMPLETELY RULED OUT.  AMONG THE
REMAINING GUIDANCE...THE 06Z GFS CLUSTERS WELL WITH THE 06Z GEFS
MEAN AND 00Z ECMWF ENSEMBLE MEAN TO BE USEFUL IN MOST AREAS EXCEPT
FOR THE PACIFIC NORTHWEST BY DAY 7/WED...WHERE IT MAY BE TOO FAST
WITH THE APPROACH OF ANOTHER LONGWAVE TROUGH.  HOWEVER...THE 06Z
GFS IS SLOWER AND CONSIDER MUCH BETTER THAN THE 00Z GFS. 
CONFIDENCE IS CONSIDERED ABOVE AVERAGE THROUGH ABOUT DAY 5...THEN
FALLS SLIGHTLY BELOW AVERAGE THEREAFTER.  HOWEVER...AS A
WHOLE...THIS PATTERN HAS ABOVE NORMAL PREDICTABILITY.  REGARDING
SENSIBLE WEATHER...THE LARGEST AND POTENTIALLY MOST SIGNIFICANT
IMPACT WILL BE FOR THE POSSIBILITY OF EXCESSIVE RAINFALL ANYWHERE
FROM THE SOUTHERN PLAINS TOWARD THE DEEP SOUTH AS THE LOW AND COLD
FRONT MOVE SLOWLY EASTWARD.  MODEL FORECASTS OF MODERATE TO HIGH
CONVECTIVE INSTABILITY...PRECIPITABLE WATER VALUES NEAR OR IN
EXCESS OF 1.5 TO 1.75 INCHES...AND UPWIND AND DOWNWIND PROPAGATION
SPEEDS OF 15 KNOTS OR LESS...
SUPPORT 1 TO 2 DAY QPF AMOUNTS IN EXCESS OF 5 INCHES IN SOME AREAS.

JAMES