EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1058 AM EST TUE DEC 10 2013

VALID 12Z FRI DEC 13 2013 - 12Z TUE DEC 17 2013

...PATTERN OVERVIEW...

THE LARGE-SCALE FLOW ACROSS NORTH AMERICA DURING THE UPCOMING
MEDIUM RANGE PERIOD...DAYS 3-7...IS EXPECTED TO REMAIN
SPLIT...WITH BROAD CYCLONIC FLOW EXTENDING FROM THE NORTHERN
PLAINS TOWARD THE EAST...WHILE A SEPARATE STREAM UNDERCUTS A
BLOCKING PATTERN OVER THE NORTHERN PACIFIC AND ENTERS THE WEST. 
THIS PATTERN FAVORS NEAR TO ABOVE NORMAL PRECIPITATION FOR THE
PACIFIC NORTHWEST...DRY WITH ABOVE NORMAL TEMPERATURES FOR THE
SOUTHWEST...BELOW NORMAL TEMPERATURES FOR THE NORTHERN PLAINS INTO
THE GREAT LAKES...AND ABOVE NORMAL PRECIPITATION WITH TRANSIENT
TEMPERATURES FOR THE EAST.

...GUIDANCE ASSESSMENT/PREFERENCES/SENSIBLE WEATHER HIGHLIGHTS...

THE SOLUTION SPREAD BY DAY 3/FRI...PARTICULARLY ACROSS THE
WEST...IS ABOVE NORMAL AND CONSISTENT WITH THE LIMITED
PREDICTABILITY THAT IS EXPECTED TO ACCOMPANY A SPLIT-FLOW REGIME
WITH NUMEROUS AND OCCASIONALLY STRONG PERTURBATIONS WHICH GUIDANCE
HAS MORE DIFFICULTY THAN USUAL INITIALIZING.  THE 00Z ECMWF LIES
NEAR THE STRONG EDGE OF THE GUIDANCE WITH THE TROUGH OVER THE
WEST...AND THUS PREFER TO TEMPER ITS STRONGER DEPICTION WITH A
DAMPED ENSEMBLE MEAN THAT IS ALSO CLOSER TO THE HIGHER RESOLVING
DETERMINISTIC CONSENSUS.  THIS TROUGH'S TIMING AND STRENGTH COULD
HAVE SIGNIFICANT IMPLICATIONS FOR THE SENSIBLE WEATHER DOWNSTREAM
FOR THIS WEEKEND AS IT ENCOUNTERS GREATER BAROCLINICITY AND
MOISTURE. 

THE SOLUTION ENVELOPE INCREASES AND BECOMES MORE COMPLICATED BY
DAYS 4-5/SAT-SUN...AS THE MODELS ALLOW THE FLOW ALONG THE CANADIAN
BORDER TO BEGIN INTERACTING WITH THE TROUGH ENTERING THE
PLAINS...WITH THE 00-06Z GFS LIE NEAR THE FAST SIDE OF THE
GUIDANCE WITH THE 00Z ECMWF NEAR THE SLOWER/STRONGER SIDE.  GIVEN
THE INCREASING UNCERTAINTY...THE PREFERENCE IS TO LEAN HEAVILY
AND/OR ENTIRELY ON A GLOBAL ENSEMBLE MEAN BY DAY 5.  REGARDING
WHICH ENSEMBLE MEAN...ITS DIFFICULT TO SAY...BUT WITH THE BLOCKING
PATTERN OVER THE NORTHERN PACIFIC...THE GFS AND ITS OWN ENSEMBLE
MEMBERS SEEM AWFULLY FAST AND AGGRESSIVE IN ALLOWING PERTURBATIONS
TO PENETRATE THE HIGHER HEIGHTS OVER BRITISH COLUMBIA...LEADING TO
A FASTER SOLUTION.  THEREFORE...PREFER TO LEAN TOWARD THE SLOWER
SIDE OF THE CAMP WHICH IS DEPICTED BY THE ECMWF ENSEMBLE
MEAN...WITH ABOUT AN 70/30 ECMWF/GEFS BLEND RESPECTIVELY TO
ACCOUNT FOR THE POSSIBILITY THAT THE FASTER GEFS MEMBERS ARE
CORRECT.  THE POTENTIAL EXISTS...PARTICULARLY IF THE
SLOWER/STRONGER ECMWF IS CORRECT...FOR ANOTHER SIGNIFICANT
PRECIPITATION EVENT...INCLUDING SNOW/ICE...TO IMPACT AREAS FROM
THE TENNESSEE VALLEY NORTHEASTWARD THIS WEEKEND...THE DETAILS OF
WHICH REMAIN UNCLEAR AT THIS TIME.

ANOTHER COLD FRONT...YET WEAKER ONE...IS EXPECTED TO REACH THE
PACIFIC NORTHWEST BY DAY 5/SUN...BEFORE MOVING INTO THE
PLAINS/MISSISSIPPI VALLEY EARLY NEXT WEEK...WITH ALL THE GLOBAL
ENSEMBLE MEANS RELATIVELY CLOSE TO EACH OTHER FOR TIMING AND
INTENSITY.

JAMES