ALASKA EXTENDED FORECAST DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
159 PM EST MON JAN 19 2009
 
VALID 12Z FRI JAN 23 2009 - 12Z TUE JAN 27 2009
 
THE MEDIUM RANGE PERIOD CONTINUES TO BE ONE DOGGED BY A LACK OF
PREDICTABILITY AS PRONOUNCED UPPER CUTOFF LOWS AND CUTOFF HIGHS
BEGIN TO BREAK DOWN AND BECOME MORE PROGRESSIVE.  THERE ARE MANY
DIFFERENCES THAT DISTINGUISH THE OVERNIGHT ENSEMBLE MEANS AND LOTS
OF VARIABILITY BETWEEN INDIVIDUAL OPERATIONAL MODEL RUNS. 

DESPITE THE DIFFICULTIES IN FORECASTING SPECIFICS...THE MODELS ARE
TRENDING TOWARD A SIGNIFICANT CHANGE IN ALASKA FROM A DOMINANT
RIDGE EARLY IN THE PERIOD TO A PROGRESSIVE WEAKENING RIDGE AND A
PROGRESSIVE WEAKENING TROUGH CROSSING THE STATE BY THE MIDDLE OF
THE PERIOD...TO A REEMERGING RIDGE OVER THE BERING SEA AND
NORTHWESTERLY UPPER FLOW OVER THE STATE...AND COOLING STATEWIDE BY
THE END OF THE PERIOD.

IN THE MEAN...THE MEDIUM RANGE STARTS OFF WELL ENOUGH WITH A
PRONOUNCED UPPER RIDGE ACROSS THE STATE WITH A LARGE SURFACE HIGH
MOVING OFF EAST OF THE STATE. A DEEP UPPER LOW OVER THE
NORTHCENTRAL PACIFIC IS WEAKENING AS IT MOVES SLOWLY EASTWARD
ACROSS THE SOUTHERN BERING SEA AND ALEUTIANS. ONE RELATIVELY
PREDICTABLE FEATURE IS THE LARGE CUTOFF LOW OVER EASTERN ASIA
WHICH HAS LITTLE EFFECT ON THE REGION WITH THE EXCEPTION OF
THROWING UP AN UPPER RIDGE TO ITS EAST FROM THE NORTHWEST PACIFIC
EARLY IN THE PERIOD TO ACROSS THE BERING SEA BY LATE IN THE
PERIOD.

HOWEVER...AS THE MEDIUM RANGE PROCEEDS TO DAYS 6 THOUGH 8...THE
LARGE DIFFERENCES BECOME VERY DIFFICULT TO RESOLVE.  THE 00Z
EUROPEAN ENSEMBLE MEANS BEGIN TO DEVIATE GREATLY FROM THE
GFS/CANADIAN/NAEFS ENSEMBLE MEANS SHOWING THAT AS RIDGING
INCREASES OVER THE BERING SEA...TROUGHING AMPLIFIES ACROSS MUCH OF
THE NORTHEAST PACIFIC.  THIS CONTRASTS GREATLY WITH THE OTHER
MODELS THAT HAVE A LESS PRONOUNCED SOLUTION WITH A RIDGE REMAINING
INTACT ACROSS THE NORTHEAST PACIFIC WITH A MUCH FLATTER TROUGH
WORKING ITS WAY TOWARD THE GULF OF ALASKA BY THE END OF THE
FORECAST PERIOD. THE 12Z EUROPEAN IS NOT AS PRONOUNCED AS ITS
PREDECESSOR BUT THE TREND TOWARD COOLING IS THERE.

LARGE DIFFERENCES ALSO EXIST OVER NORTHERN ALASKA AS THE UPPER
RIDGE BEGINS TO BREAK DOWN...WITH LARGE DIFFERENCES IN WHETHER AND
HOW MUCH COLDER AIR RETURNS TO THE STATE AS INDIVIDUAL MODELS HAVE
MANY DIFFERENT SOLUTIONS ON HOW HIGH PRESSURE BUILDS TO THE NORTH
AND WEST OF THE STATE. LARGE VARIABILITY SUCH AS THIS WARRANTED
THE MORE PRONOUNCED USE OF THE MODEL ENSEMBLE MEANS IN THE
GENERATION OF THE MANUAL GRAPHICS.  THE MANUAL GRAPHICS TODAY OVER
NORTHERN ALASKA WILL DIFFER SIGNIFICANTLY THAN YESTERDAYS
GRAPHICS.

DESPITE THE PRESENCE OF A NEAR CUTOFF RIDGE OVER THE STATE...THE
OVERNIGHT AND 12Z EUROPEAN HAVE A SHORTWAVE FEATURE MOVING ACROSS
THE WEST SIDE OF ALASKA ON DAYS 4 AND 5/FRIDAY AND SATURDAY THAT
IS NOT DEPICTED IN THE GFS BUT IS PICKED UP BY THE 12Z CANADIAN
AND UKMET.

IN ADDITION...THE 12Z EUROPEAN IS SUDDENLY MORE GUNG HO ON A WAVE
DEVELOPING WELL SOUTH OF THE ALEUTIANS ON DAY 3/THURSDAY AND THEN
INTENSIFYING ON DAY 4 AS IT HEADS TOWARD THE ALASKA PENINSULA.
THIS IS SOMEWHAT A NEW FEATURE...AT LEAST IN INTENSITY.  BY THE
END OF THE FORECAST...THE 12Z EUROPEAN HAS SLIGHTLY BACKED OFF ON
THE INTENSITY OF THE COLD AIR RETURNING TO THE NORTHEAST PACIFIC.

KOCIN