ALASKA EXTENDED FORECAST DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
252 PM EDT MON SEP 15 2008
 
VALID 12Z FRI SEP 19 2008 - 12Z TUE SEP 23 2008
 
THERE ARE SOME SIGNIFICANT DIFFERENCES SHOWING UP WITH THE 00Z AND
06Z MODEL ENSEMBLE MEANS.  WHILE THERE IS LITTLE UNCERTAINTY ABOUT
A LOW PRESSURE SYSTEM NEAR THE WESTERN GULF OF ALASKA AROUND DAY
4/FRIDAY...THE 00Z EUROPEAN ENSEMBLE MEANS ARE SIGNFICANTLY
DIFFERENT IN A COUPLE ASPECTS FROM THE 00Z GFS/CANADIAN/NAEFS
ENSEMBLE MEANS.  

FIRST...THE EUROPEAN MEANS MAINTAIN SURFACE LOW PRESSURE IN THE
GULF THROUGH DAY 6...LONGER THAN THE OTHER ENSEMBLE MEANS. 
SECOND...THE 00Z EUROPEAN ENSEMBLE MEANS ARE SLOWER AND DEEPER
WITH THE NEXT LOW PRESSURE SYSTEM MOVING ACROSS THE NORTH
PACIFIC...MOVING A DEEP LOW WELL SOUTH OF THE ALEUTIANS BETWEEN
170E AND 170 W ON DAYS 4 THROUGH 6/FRIDAY THROUGH SUNDAY. 
MEANWHILE...THE GFS/CANADIAN MEANS HAVE WEAKER MORE PROGRESSIVE
SYSTEMS MOVING INTO THE NORTHEAST PACIFIC IN THE SAME TIME FRAME.

BY DAY 8/TUESDAY...THE EUROPEAN ENSEMBLE MEANS ARE NEARLY
COMPLETELY OUT OF PHASE WITH THE 00Z GFS/CANADIAN/NAEFS ENSEMBLE
MEANS WITH A LARGE RIDGE COVERING ALASKA IN THE EUROPEAN WHILE THE
OTHERS KEEP AN UPPER TROUGH OVER THE STATE.

WITH THOSE DIFFERENCES IN MIND...THE ENSEMBLE MEANS DO ALSO HAVE
SOME AREAS OF SIMILARITIES.  A LARGE UPPER RIDGE IS FORECAST BY
ALL MODEL MEANS TO COVER THE BERING SEA AND A SERIES OF
UPPER-LEVEL TROUGHS ARE FORECAST TO MOVE NEAR KAMCHATKA FROM DAYS
5 THROUGH 7/SATURDAY THROUGH MONDAY.  

THE OPERATIONAL 00Z EUROPEAN/GFS/CANADIAN/UKMET MODELS APPEARED TO
RESEMBLE THE EUROPEAN ENSEMBLE MEANS...AT LEAST THROUGH DAY
6/SUNDAY WITH A SLOWLY WEAKENING CYCLONE OVER SW ALASKA/WESTERN
GULF OF ALASKA...A SURFACE RIDGE OVER THE BERING SEA AND A SLOW
MOVING DEEP CYCLONE WELL SOUTH OF THE ALEUTIANS BY DAY 6/SUNDAY.  

TOWARDS DAYS 7 AND 8/MONDAY AND TUESDAY...THE 00Z AND 06Z ENSEMBLE
MEANS AND OPERATIONAL MODELS APPEAR TO BE TRENDING TOWARD A
SOLUTION WHEREBY THE CYCLONE OVER THE NORTH PACIFIC MOVES AGAIN
TOWARD SW ALASKA WHILE HIGH PRESSURE BUILDS ACROSS NORTHERN
ALASKA.  THAT SEQUENCE IS MUCH CLEARER ON THE 00Z EUROPEAN
ENSEMBLE MEANS WHILE THE OTHER MODEL ENSEMBLE MEANS SHOW A MORE
STRETCHED OUT CYCLONE FROM NEAR THE EASTERN ALEUTIANS EASTWARD TO
MUCH OF SOUTHERN ALASKA WHILE HIGH PRESSURE HOLDS ACROSS NORTHERN
ALASKA.

THE 12Z GFS APPEARS TO DEVIATE CONSIDERABLY FROM THE 00Z
CYCLE...EVEN AS EARLY AS THE DAY 4/FRIDAY FORECAST.  THE CYCLONE
AFFECTING SW ALASKA APPEARS TO DEVELOP FASTER THAN PREVIOUS RUNS
AND IS WEAKENING SIGNIFICANTLY BY DAY 4.  THE INTENSE CYCLONE OVER
THE NORTH PACIFIC AT DAY 4 IS ALSO MUCH FLATTER AND WEAKER IN THE
12Z RUN.  WHILE THIS SOLUTION CAN NOT BE RULED OUT...THE CHANGE
FROM PREVIOUS RUNS AND OTHER OPERATIONAL MODELS MAKE IT AN OUTLIER
SOLUTION.  

ALTHOUGH THERE WAS GOOD AGREEMENT BETWEEN THE OPERATIONAL MODELS
AT 00Z...THE SIGNIFICANT DIFFERENCES BETWEEN THE ENSEMBLE MEANS
AND THE LACK OF CONTINUITY WITH THE 12Z OPERATIONAL GFS CONTINUES
TO SUGGEST THIS FORECAST HAS BELOW AVERAGE PREDICTABILITY.  FOR
THE MANUAL GRAPHICS...TO MAXIMIZE CONTINUITY...HAVE GONE WITH A
30/30/40 BLEND OF THE OPERATIONAL 00Z EUROPEAN/GFS AND THE 00Z
ENSEMBLE MEAN EUROPEAN ON DAYS 4 AND 5 AND THEN INCREASED THE
PROPORTION OF THE ENSEMBLE MEANS TOWARD THE END OF THE FORECAST
WHILE DIMINISHING THE ROLE OF THE OPERATIONAL RUNS UNTIL THERE WAS
A 20/10/70 BLEND BY DAY 8.

THE 12Z OPERATIONAL EUROPEAN MODEL HAS SOME DIFFERENCES WITH THE
PREVIOUS RUN ON THE DETAILS OF THE CYCLONE MOVING TOWARD SW ALASKA
ON DAY 4/FRIDAY...WITH THE DIFFERENCE DUE TO THE RESOLUTION OF 2
SHORTWAVE FEATURES...ONE WHICH INTENSIFIES IN THE WESTERN GULF BY
DAY 3/THURSDAY AND MOVES ASHORE AND WEAKENS BY DAY 4/FRIDAY WHILE
THE SECOND FEATURE DEVELOPS A SURFACE WAVE ALONG THE TRAILING COLD
FRONT AND INTENSIFIES INTO A SECOND LOW ALONG THE SOUTHERN ALASKA
COAST BY DAY 5/SATURDAY.  WHILE THESE FEATURES ADD ADDITIONAL
UNCERTAINTY INTO THE EVOLUTION OF FEATURES IN THE AREA...LOW
PRESSURE APPEARS TO DOMINATE THE SOUTHERN AND SW ALASKA COAST ON
DAYS 4 AND 5.

MEANWHILE...MUCH OF THE REST OF THE 12Z EUROPEAN IS CONSISTENT
WITH THE PREVIOUS FORECAST OF THE CYCLONE SOUTH OF THE ALEUTIANS
IN THE NORTH PACIFIC ON DAY 6/SUNDAY WHILE ALSO BEING RELATIVELY
CONSISTENT WITH THE SURFACE RIDGE OF HIGH PRESSURE IN THE BERING
SEA AND A SERIES OF LOW PRESSURE SYSTEMS NEAR THE COAST OF
KAMCHATKA AT THE SAME TIME.  THEREFORE...THE MANUAL GRAPHICS LOOK
PRETTY CONSISTENT WITH THE 12Z EUROPEAN ON DAY 6.  THE 12Z
EUROPEAN IS MORE AGGRESSIVE THAN EARLIER WITH A CYCLONE MOVING
EAST OF KAMCHATKA INTO THE BERING SEA ON DAYS 7 AND 8/MONDAY AND
TUESDAY.

KOCIN
Á$