ALASKA EXTENDED FORECAST DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
254 PM EDT MON AUG 24 2009
 
VALID 12Z FRI AUG 28 2009 - 12Z TUE SEP 01 2009
 
TWO SEPARATE BUT ROBUST UPPER LOWS ARE POISED TO INFLUENCE MUCH OF
ALASKA DURING THE MEDIUM RANGE PERIOD. A VERY WET PATTERN IS
SHAPING UP OVER SOUTHEASTERN COASTAL ALASKA THIS WEEKEND WITH AN
EXTENDED PERIOD OF ONSHORE FLOW. SOME OPERATIONAL GUIDANCE
FORECASTS SHOW EIGHT TO THIRTEEN INCHES OF RAIN OVER THE 120 HOURS
ENDING SUNDAY.

THE FIRST UPPER LOW IS A COMBINATION OF THE REMNANTS OF TYPHOON
11W /VAMCO/ WHICH WAS ABOUT 1350 MILES SW OF THE ALEUTIANS AS OF
12Z/24 PER THE JTWC. THIS WILL CONTINUE ITS NORTH TO NORTHEAST
PATH AND BECOME EXTRATROPICAL BY THE TIME IT REACHES THE BERING
SEA ON WEDNESDAY NEAR 180. IT SHOULD THEN DIVE SOUTHEASTWARD BACK
TO ABOUT 50N/155W BY THE START OF THE MEDIUM RANGE PERIOD ON
FRI/D4. IT WILL RESUME A NORTHEASTWARD MOTION AND HEAD TOWARD THE
GULF OF ALASKA AS ITS COLD/OCCLUDED FRONT OUTRUNS ITS PARENT LOW
INTO CANADA. THE 00Z ECMWF AND 12Z GFS ARE RELATIVELY CLOSE TO
EACH OTHER AND IN BETWEEN THE FASTER 00Z UKMET AND SLOWER 00Z
CANADIAN WITH RESPECT TO THE OVERALL EVOLUTION EAST OF THE
ALEUTIANS. QUIETER WEATHER SHOULD RESUME EAST OF ABOUT 155W BY THE
START OF NEXT WEEK.

THE SECOND UPPER LOW SHOULD MATERIALIZE OVER NORTHEASTERN RUSSIA
OVER THE NEXT FEW DAYS AND ROTATE INTO THE SEA OF OKHOTSK BY
SUN/D6. THEREAFTER... THE 00Z ECMWF BECOMES MUCH FASTER TO LOWER
HEIGHTS OVER WESTERN ALASKA COMPARED TO THE SLOWER 12Z GFS AND 00Z
ECMWF ENSEMBLE MEAN /AND 06Z GEFS MEAN/. THE 00Z CANADIAN IS EVEN
SLOWER. EVEN THOUGH THE PAST TWO ECMWF RUNS ARE FASTER THAN THE
00Z/23 RUN... FELT COMPELLED TO STICK CLOSER TO THE BETTER
CONSENSUS FOR NOW. THIS IS NOT TOO FAR FROM THE HPC FORECAST
YESTERDAY. RMOP VALUES OVER ALASKA BECOME MUCH LOWER IN TIME BY
THE END OF THE PERIOD WITH THE GREATEST PREDICTABILITY FARTHER
EAST IN CANADA. 

A MOSTLY 12Z GFS AND 00Z ECMWF ENSEMBLE MEAN BLEND WAS THE BASE
FOR THE FORECAST TODAY... WHICH DID NOT CHANGE THE TIMING OF TWO
BIG SYSTEMS TOO MUCH.  

THE LATEST 12Z ECMWF IS A LITTLE LESS BULLISH WITH THE SHORTWAVE
NEAR THE BERING STRAIT AND A BIT STRONGER WITH THE UPPER LOW
BARRELING THROUGH THE SEA OF OKHOTSK BY SUN/D6. THIS IS MORE IN
LINE WITH THE 00Z ECMWF ENSEMBLE MEAN AND CURRENT 12Z GFS BUT
STILL OUTPACES THEM BY TUE/D8.

FRACASSO
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