ALASKA EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
225 PM EDT SAT MAY 18 2013

VALID 12Z WED MAY 22 2013 - 12Z SUN MAY 26 2013

LATEST GUIDANCE AGREES FAIRLY WELL UPON A MEAN TROUGH NEAR 180
LONGITUDE EARLY IN THE PERIOD.  WITH TIME MODEST SPREAD DEVELOPS
WITH RESPECT TO WHETHER THE MEAN TROUGH DRIFTS EWD A LITTLE... AND
MORE SIGNIFICANTLY FOR DETAILS WITHIN THE MEAN TROUGH AND THE
DEGREE OF INTERACTION BETWEEN NRN STREAM FLOW AND PACIFIC ENERGY. 
THESE ISSUES SHOULD HAVE THE GREATEST INFLUENCE ON THE FCST FROM
THE BERING SEA/ALEUTIANS/NRN PAC INTO THE WRN MAINLAND.

DURING DAYS 4-5 WED-THU THE 00Z GFS/ECMWF/CMC AND 06Z GFS RUNS
SHOWED A MORE CLOSED/SWWD MID LVL SYSTEM TRACKING FROM RUSSIA INTO
THE WRN BERING SEA VERSUS ENSEMBLE MEANS.  THIS DIFF IN THE
OPERATIONAL SOLNS YIELDS A SLIGHTLY SLOWER MEAN CENTER
INCORPORATING THIS SYSTEM AND LEADING FEATURE RELATIVE TO THE
ENSEMBLE MEANS EARLY IN THE PERIOD.  THE 12 GFS HAS ADJUSTED
FASTER/WEAKER WITH RUSSIAN ENERGY TRACKING INTO THE BERING SEA SO
AN AVG OF OPERATIONAL/ENSEMBLE GUIDANCE APPEARS BEST HERE.

AFTER DAY 5 THE 00Z NAEFS MEAN SEEMS TO PLACE MORE EMPHASIS ON THE
CMC MEAN WHICH BECOMES SOMEWHAT MORE PROGRESSIVE WITH THE OVERALL
TROUGH ALOFT VERSUS THE 00Z/06Z GEFS MEANS.  THE 00Z ECMWF MEAN IS
RELATIVELY SLOW LIKE THE GEFS MEANS BUT WITH A MORE OPEN TROUGH
OVER THE BERING SEA.  GEFS/ECMWF MEANS BOTH SHOW A SLOWER TREND
WITH THE MEAN TROUGH OVER THE PAST 1-2 DAYS... SO PREFER TIMING
THAT IS SLOWER THAN THE 00Z NAEFS/CMC MEANS.

CONFIDENCE IS LOWER WITH RESOLVING THE DISTRIBUTION OF ENERGY
WITHIN THE OVERALL MEAN TROUGH ALOFT... WHICH WILL HAVE
SIGNIFICANT IMPACT ON THE ULTIMATE EVOLUTION/TRACK OF PACIFIC LOW
PRESSURE.  THE 00Z ECMWF IS QUICKEST TO BRING A SFC LOW INTO THE
BERING SEA BY LATE FRI/SAT.  SUCH AN EVOLUTION LAST APPEARED IN
THE OPERATIONAL ECMWF THREE RUNS AGO.  THE 06Z GFS/GEFS MEAN TAKE
LONGER TO BRING THEIR SYSTEM INTO THE ERN HALF OF THE
ALEUTIANS/SERN BERING SEA WHILE THE 00Z GFS TRACKS IT FARTHER E TO
THE PENINSULA... SLIGHTLY E OF THE 00Z CANADIAN MEAN.  00Z
GEFS/ECMWF MEANS ARE RATHER WEAK WITH ANY SFC REFLECTION.  06Z
GFS/GEFS MEAN SOLNS ARE QUESTIONABLE AS THEY DISPLAY FASTER THAN
CONSENSUS HANDLING OF WRN PAC FLOW BY WED-THU.  ALTHOUGH THE 12Z
GFS HAS TRENDED BACK TO THE CONSENSUS OVER THE WRN PAC... IT STILL
CLOSES OFF A DEEP SYSTEM NEAR THE ERN ALEUTIANS BY LATE IN THE
PERIOD.  SLOWER ENS MEAN TREND WITH THE NRN STREAM TROUGH ALOFT
INCREASES POTENTIAL FOR CNTRL PAC ENERGY TO TRACK A LITTLE FARTHER
WWD THAN YDAYS CONTINUITY.  ALTHOUGH THE 00Z GFS AND 00Z CMC MEAN
ARE NOT FAVORED IN THE LARGE SCALE... THEIR IDEA OF TRACKING A SFC
LOW NEAR THE AK PENINSULA LATE IN THE PERIOD OFFERS THE MOST
REASONABLE SOLN IN LIGHT OF THE CURRENT SPREAD/UNCERTAINTY WITH
DETAILS ALOFT.

DAYS 4-5 WED-THU START WITH A COMPROMISE AMONG THE 00Z/12Z GFS AND
00Z ECMWF/EC MEAN TO REPRESENT THE DESIRED INTERMEDIATE SOLN OVER
THE BERING SEA AND CONSENSUS ELSEWHERE.  DAY 6 FRI TRANSITIONS TO
A 70/30 BLEND OF THE 00Z ECMWF MEAN/12 GFS.  DAYS 7-8 SAT-SUN
START WITH THE 00Z ECMWF MEAN DUE TO ITS PREFERRED LARGE SCALE
EVOLUTION... WITH ADJUSTMENTS TOWARD A COMPROMISE BETWEEN THE 00Z
GFS AND WEAKER CANADIAN MEAN FOR THE SYSTEM TRACKING TOWARD THE
PENINSULA.

RAUSCH