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Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1556Z Nov 20, 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
1056 AM EST THU NOV 20 2014

VALID 12Z SUN NOV 23 2014 - 12Z THU NOV 27 2014

...OVERVIEW...

AN AMPLIFYING TREND ALOFT WILL BE IN PROGRESS AT THE START OF THE
PERIOD SUN WITH A RIDGE BUILDING TOWARD THE WEST COAST AND TROUGH
BECOMING ESTABLISHED OVER THE CNTRL CONUS.  AHEAD OF THE CNTRL
CONUS RIDGE GUIDANCE IS STEADILY COMING INTO BETTER AGREEMENT
REGARDING THE STREAM INTERACTION EXPECTED TO YIELD A DEEP STORM
OVER THE UPR GRTLKS AS OF EARLY MON.  HOWEVER MODEL/ENSEMBLE
SPREAD BEGINS TO WIDEN DRAMATICALLY OVER THE ERN PAC EARLY NEXT
WEEK WITH DOWNSTREAM EFFECTS REACHING/SPREADING ACROSS THE CONUS
SOON THEREAFTER... SO FCST CONFIDENCE BY DAYS 6-7 WED-THU IS MUCH
LOWER THAN DURING DAYS 3-5 SUN-TUE.


...GUIDANCE EVALUATION/PREFERENCES...

THERE IS FAIRLY GOOD MODEL AGREEMENT WITH THE DEEPENING SFC LOW
REACHING THE UPR GRTLKS BY EARLY MON AS AMPLIFYING NRN STREAM FLOW
INTERACTS WITH A SRN STREAM SYSTEM EJECTING NEWD FROM THE SRN
PLAINS.  CONSENSUS OF OPERATIONAL MODELS WOULD SUGGEST INCREASING
CONFIDENCE IN A CNTRL PRESSURE IN THE 970'S MB.  THE 00Z-06Z
GFS/00Z ECMWF COMPARE WELL TO THE WEAKER ENSEMBLE MEANS FOR
TRACK/TIMING SO THOSE GFS/ECMWF RUNS SERVE AS THE PRIMARY BASIS
FOR THE DAYS 3-5 BLEND.  00Z/06Z GFS PARALLEL RUNS ARE A LITTLE
FASTER THAN THE MAJORITY CLUSTER.  THERE ARE STILL SOME
INDICATIONS OF WAVINESS ALONG THE TRAILING FRONT AS IT CROSSES THE
EAST COAST AROUND TUE-WED BUT MOST SOLNS ARE NOT AS WELL DEFINED
AS THE 06Z GFS WITH ANY INDIVIDUAL WAVE.  TOWARD THE END OF THE
PERIOD ERN CONUS TROUGH DETAILS BECOME INFLUENCED BY UPSTREAM
ISSUES WITH PREFERENCE DESCRIBED BELOW LEADING TO A LITTLE MORE
WEAKENING OF THE ERN TROUGH... PERHAPS JUST TEMPORARILY...
RELATIVE TO GFS/GFS PARALLEL RUNS.

GUIDANCE SPREAD PROGRESSING EWD FROM THE PACIFIC ORIGINATES FROM
ONGOING UNCERTAINTY OVER HOW FLOW WITHIN A TROUGH BETWEEN 150-160W
MAY SPLIT AROUND MON-TUE.  THESE SITUATIONS TEND TO BE ONE OF THE
MORE DIFFICULT TO FCST WITH SIGNIFICANT SPREAD AND POTENTIAL FOR
LARGE FCST ERRORS.  FURTHER COMPLICATING THE FCST... THERE ARE
MEANINGFUL DIFFS IN HOW MUCH UPSTREAM NRN PAC FLOW WILL AMPLIFY
INTO THE MEAN TROUGH POSN.  FROM THE LARGE SCALE PERSPECTIVE THE
D+8 MEANS STILL SHOW THE BEST AGREEMENT/DEFINITION WITH A CORE OF
POSITIVE HGT ANOMALIES OVER THE ATLC AND NEG ANOMALIES NOT FAR
FROM HUDSON BAY WITH CORRESPONDING TELECONNECTIONS SUPPORTING THE
MAINTENANCE OF A MEAN RIDGE NEAR OR JUST OFFSHORE THE WEST COAST. 
THIS RELATIONSHIP WOULD SUGGEST A LOWER PROBABILITY FOR SOLNS SUCH
AS RECENT OPERATIONAL ECMWF RUNS OR 00Z CMC WHICH BRING
SIGNIFICANT HGT FALLS INTO THE WEST COAST/INTERIOR WEST.  AT THE
SAME TIME WHAT CONTINUITY/CONSENSUS THAT HAS EXISTED SEEMS TO
POINT TOWARD A BETTER THAN EVEN CHANCE OF MORE ERN PAC FLOW
SEPARATION THAN DEPICTED IN 00Z/06Z GFS PARALLEL RUNS AND THE 06Z
GEFS MEAN.  THE STRONG WEST COAST RIDGE OF THE 00Z/06Z OPERATIONAL
GFS RUNS COULD BE A LITTLE MORE PLAUSIBLE IN LIGHT OF THE
DOWNSTREAM PATTERN BUT THE ERN PAC TROUGH/UPR LOW MAY BE TOO
PROGRESSIVE.  GIVEN NO GREATER CONFIDENCE THAN BEFORE... PREFER TO
MAINTAIN A SOLN AS CLOSE TO CONTINUITY AS POSSIBLE WITH SOME ERN
PAC FLOW SEPARATING AND REACHING THE WRN STATES BUT WITH MUCH LESS
AMPLITUDE THAN OPERATIONAL ECMWF RUNS.  ALOFT THE 00Z ECMWF MEAN
SATISFIES THIS CRITERION SO BY DAYS 6-7 WED-THU ITS SOLN IS
BLENDED EVENLY WITH THE PREVIOUS WPC FCST TO PROVIDE COMPROMISE
TIMING FOR SFC ADJUSTMENTS.


...SENSIBLE WEATHER HIGHLIGHTS...

DEEPENING STORM EXPECTED TO TRACK THROUGH THE MIDWEST/UPR GRTLKS
SUN-MON AND LEAVING STRONG CYCLONIC FLOW IN ITS WAKE INTO TUE WILL
BE RESPONSIBLE FOR A PERIOD OF WINDY CONDS OVER MUCH OF THE
CNTRL-ERN CONUS.  AS COOLING NRN STREAM ENERGY CONSOLIDATES OVER
THE NRN TIER EXPECT MEANINGFUL SNOW ON THE COLD SIDE OF THIS
SYSTEM... AS WELL AS SOME LINGERING LAKE EFFECT ACTIVITY. 
ENHANCED RNFL IS PSBL OVER PARTS OF THE MID MS VLY/LWR OH VLY AND
SOUTHEAST EARLY IN THE PERIOD.  THE SFC LOW/TRAILING FRONT WILL
SEPARATE VERY CONTRASTING TEMPS... UP TO 10-20F BELOW NORMAL
ACROSS THE NRN TIER VERSUS PLUS 10-25F ANOMALIES IN THE WARM
SECTOR WITH HIGHEST ANOMALIES MOST LIKELY FOR MIN TEMPS.  EXPECT A
GENERAL DRYING TREND OVER THE ERN HALF OF THE CONUS BY MIDWEEK
WITH TEMPS MODERATING ASIDE FROM THE NRN PLAINS AND VICINITY WHICH
SHOULD REMAIN WELL BELOW NORMAL.

EARLY-MID PERIOD THERE IS REASONABLE AGREEMENT UPON PERIODS OF
RAIN/HIGHER ELEV SNOW OVER NRN PORTIONS OF THE WEST WITH LOCALIZED
HEAVIER AMTS OVER FAVORED TERRAIN.  WIDE SPREAD IN GUIDANCE SOLNS
BY NEXT WED-THU LEAD TO LOW CONFIDENCE IN DETERMINING
COVERAGE/AMTS OF PCPN OVER THE WEST AT THAT TIME.  PREFERRED FCST
INCORPORATING THE 00Z ECMWF MEAN AND WPC CONTINUITY WOULD YIELD A
SWD EXTENT OF MSTR BETWEEN THE MOST NRN 00Z/06Z GFS AND SWD
12Z/19-00Z/20 ECMWF RUNS.  SOME OF THIS MSTR MAY EXTEND INTO THE
PLAINS LATE IN THE PERIOD.  WRN CONUS TEMPS SHOULD BE WITHIN 10F
OF NORMAL MOST DAYS.

RAUSCH