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Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1600Z Apr 21, 2014)
 
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EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1200 PM EDT MON APR 21 2014

VALID 12Z THU APR 24 2014 - 12Z MON APR 28 2014

...OVERVIEW AND PREFERENCES...

THE PATTERN IS EXPECTED TO TRANSITION FROM MODERATE PROGRESSION
TOWARD A BLOCKIER/SLOWER MOVING REGIME BY THE WEEKEND OR EARLY
NEXT WEEK WITH HINTS OF A REX BLOCK PSBLY DEVELOPING OVER CNTRL
NOAM AND A MEAN TROUGH LINGERING OVER ERN CANADA AND NERN CONUS. 
WITHIN THIS EVOLUTION THE TWO GREATEST UNCERTAINTIES APPEAR IN THE
DAYS 5-7 SAT-MON TIME FRAME... IF/WHERE AN UPR LOW CLOSES OFF
WITHIN THE WRN-CNTRL CONUS TROUGH AND THE AMPLITUDE OF NERN CONUS
TROUGHING/LATITUDE OF THE ASSOC SFC FRONTAL BNDRY.  IN BOTH CASES
THERE IS SIGNIFICANT SPREAD AND IN SOME GUIDANCE VARIABILITY
BETWEEN 12Z/00Z CYCLES.  THIS ARGUES FOR MAINTAINING ABOUT HALF
WEIGHTING OF WPC CONTINUITY VERSUS THE NEW 00Z/06Z GUIDANCE WHICH
IS REPRESENTED BY A 30/20 PROPORTION OF THE 00Z ECMWF MEAN/06Z
GEFS MEAN.


...GUIDANCE EVALUATION...

EARLY IN THE FCST THERE IS RELATIVELY GOOD CONTINUITY AGREEMENT
WITH SYSTEMS OF INTEREST... FAIRLY STG LOW PRES DEPARTING FROM THE
CANADIAN MARITIMES... COMPLEX AREA OF LOW PRESSURE TRACKING EWD
FROM THE NRN TIER AND TRAILING FRONT... AND SYSTEM INITIALLY OFF
THE PAC NW COAST.  AN AVG OF LATEST GFS/ECMWF SOLNS IS FAIRLY
CLOSE TO CONTINUITY.

WITH THE TROUGH MOVING INTO THE WEST BY THE START OF THE WEEKEND
LATEST GFS/CMC RUNS ARE FARTHEST SWD WITH AN EMBEDDED CLOSED LOW
WHILE ECMWF RUNS HAVE TENDED TO MAINTAIN AN OPEN TROUGH THROUGH
THE WEST AND WAIT UNTIL ENERGY REACHES THE PLAINS TO CLOSE OFF A
LOW.  RESPECTIVE ENSEMBLE SYSTEMS GENERALLY REFLECT THE
OPERATIONAL IDEAS WITH GEFS/CMC MEMBERS SHOWING MORE TROUGH
AMPLITUDE AS A WHOLE RELATIVE TO ECMWF MEMBERS.  IN THE PAST
COUPLE WEEKS THERE HAVE BEEN AT TWO WRN TROUGHS THAT HAVE TURNED
OUT TO BE LESS AMPLIFIED THAN SOME GFS/GEFS SOLNS... WITH ANY
EMBEDDED UPR LOW WELL NWD.  THUS PREFER TO MINIMIZE USE OF
OPERATIONAL GFS RUNS WITH ONLY MINORITY INCLUSION OF THE 06Z GEFS
IN THE UPDATED FCST.

MEANWHILE SOLNS DISPLAY INCREASING DISPERSION WITH TIME FOR NERN
CONUS TROUGH AMPLITUDE AND RESULTING LATITUDE FOR THE SFC FRONT
OVER THE ERN HALF OF THE LOWER 48.  RELATIVE TO THE FULL RANGE OF
GUIDANCE 00Z/06Z GFS RUNS ARE RATHER AMPLIFIED WITH THE TROUGH
ALOFT/SWD WITH THE TRACK OF THE EMBEDDED UPR LOW AND SWD WITH THE
SFC FRONT.  WHILE REMAINING SOLNS ARE LESS AMPLIFIED ALOFT THERE
HAS BEEN SOME APPARENT OSCILLATION IN RECENT CYCLES WITH EVEN THE
ECMWF MEAN TENDING TO BRING THE ERN CONUS SFC FRONT FARTHER NWD
LATE IN THE PERIOD IN 00Z RUNS THAN 12Z VERSIONS.

LOOKING AT MULTI-DAY MEANS AT D+8... THE MOST COMMON FEATURES ARE
AN AREA OF POSITIVE HGT ANOMALIES OVER/NEAR HUDSON BAY AND
NEGATIVE ANOMALIES OVER THE NERN PAC.  WHILE TELECONNECTIONS
RELATIVE TO THESE TWO FEATURES YIELD DIFFERENT RESULTS OVER THE
WRN-CNTRL CONUS THERE IS A COMMON THEME OF TROUGHING CLOSER TO THE
EAST COAST... WHICH LEADS TO SOME SKEPTICISM OF THE 00Z
ECMWF/ECMWF MEAN THAT BRING THE SFC FRONT RATHER FAR NWD DUE TO
HIGHER HGTS ALOFT.  ON THE OTHER HAND RECENT HISTORY SUGGESTS GFS
RUNS COULD BE OVERDONE WITH TROUGH AMPLITUDE.  AT THIS TIME THE
BEST OPTION IS AN INTERMEDIATE SOLN THAT INCLUDES ONLY HALF
WEIGHTING OF THE 00Z ENSEMBLE MEANS ALONG WITH 50 PCT OF THE MORE
SUPPRESSED WPC CONTINUITY.  


...SENSIBLE WEATHER HIGHLIGHTS...

SYSTEM INITIALLY OVER THE NRN TIER/MS VLY ON THU WILL SPREAD PCPN
ACROSS THE ERN HALF OF THE CONUS WITH BEST ORGANIZED ACTIVITY
LIKELY NEAR THE SFC LOW TRACKING NEAR THE CANADIAN BORDER AND
ALONG THE TRAILING FRONT OVER THE NRN HALF OF THE MS VLY ON THU. 
RNFL IS FCST TO TREND LIGHTER AS THE FRONT CONTINUES EWD LATER IN
THE WEEK... THOUGH SOME ACTIVITY MAY DEVELOP ALONG THE TRAILING
PART OF THE FRONT AS IT STALLS OVER S-CNTRL PARTS OF THE CNTRL-ERN
STATES.  DEPENDING ON EXACTLY HOW THE SFC/UPR SYSTEM DEVELOPS
THERE IS POTENTIAL FOR SOME WINTRY PCPN NEAR THE CANADIAN BORDER.

EXPECT MSTR TO MOVE INTO THE NWRN STATES AS OF THU WITH AN
INCREASINGLY BROAD SHIELD OF PCPN SPREADING ACROSS THE WEST IN
ASSOC WITH THE TROUGH ALOFT CROSSING THE REGION.  SOME LOCALLY
MDT-HVY PCPN IS PSBL OVER FAVORED NRN-CNTRL TERRAIN DEPENDING ON
EXACTLY HOW THE SYSTEM EVOLVES.  AS HGT FALLS REACH THE PLAINS
EXPECT INCREASED POTENTIAL FOR SOME AREAS OF HVY RNFL OVER
PORTIONS OF THE PLAINS/MS VLY FROM SAT NIGHT INTO MON.  AT THIS
TIME THERE IS CONSIDERABLE UNCERTAINTY OVER EXACTLY HOW FAR NEWD
RNFL MAY EXTEND CORRESPONDING TO SPREAD WITH THE AMPLITUDE OF THE
DOWNSTREAM TROUGH ALOFT.

THE MOST PERSISTENT/EXTREME ANOMALIES FOR TEMPS SHOULD BE OF THE
COLD VARIETY OVER THE NRN TIER WITH DAILY HIGHS GENERALLY 5-15F
BELOW NORMAL... WITH BREEZY CONDITIONS MAKING IT FEEL COOLER THAN
THAT AT TIMES.

RAUSCH