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Alaska Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1907Z Dec 10, 2017)
 
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ALASKA EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
207 PM EST SUN DEC 10 2017

VALID 12Z THU DEC 14 2017 - 12Z MON DEC 18 2017

MODELS/ENSEMBLES CONTINUE TO SHOW A GENERAL CONSENSUS THAT
PERSISTENT UPPER RIDGE ACROSS WESTERN CANADA AND INTO ALASKA
SHOULD BEGIN TO WEAKEN SOME BY LATE IN THE WEEK, ALTHOUGH SOME
DIFFERENCES REMAIN AS TO HOW QUICKLY AND TO WHAT DEGREE THIS
OCCURS. DURING THE EARLY PORTION OF THE MEDIUM RANGE, PREFERRED
THE ECMWF SOLUTION FOR LOW PRESSURE MOVING FROM THE GULF INTO
SOUTHERN AK THU. MODELS STILL SHOW SOME SPREAD HERE BUT THE ECMWF
HAS BEEN GENERALLY CONSISTENT FOR SEVERAL RUNS NOW AND IS ALSO
WELL CENTERED WITHIN THE ENSEMBLE CLUSTERING. FOR THE NEXT SYSTEM
CROSSING THE ALEUTIANS THROUGH FRI, TOOK A BLENDED APPROACH TO THE
FORECAST INCLUDING THE ECMWF/GFS AS WELL AS ECENS/NAEFS MEANS.
AFTER FRI SPREAD BEGINS TO INCREASE MORE SUBSTANTIALLY, AND IT IS
DIFFICULT TO JUSTIFY BUYING INTO ANY ONE DETERMINISTIC SOLUTION AT
THIS TIME. DETERMINISTIC GUIDANCE SEEMS TO HAVE SHOWN A TREND
TOWARD TRANSFERRING MOST OF THE ENERGY ASSOCIATED WITH THIS SYSTEM
TO A POTENTIAL TRIPLE POINT LOW IN THE GULF THAT EITHER MOVES EAST
TOWARD SOUTHEAST AK (AS SHOWN BY THE ECMWF) OR TURNS MORE TO THE
NORTH (PER THE GFS). AT THIS TIME, PREFERRED AN ENSEMBLE APPROACH
(INCLUDING THE ECENS/GEFS/NAEFS) FROM DAY 6 ONWARD, WHICH SHOWED A
LOW STAYING CLOSER TO THE SOUTHERN MAINLAND AK COAST. THIS KEEPS
THE FORECAST SOMEWHAT CONSISTENT WITH CONTINUITY WHILE ALSO
ADJUSTING A BIT TOWARD THE LATEST TRENDS. BY EARLY NEXT WEEK,
MODELS ALL SHOW ANOTHER SYSTEM MOVING INTO THE PICTURE, CROSSING
THE WESTERN ALEUTIANS OR THE SOUTHERN BERING SEA. DETERMINISTIC
MODELS ARE ALL OVER THE PLACE WITH THIS FEATURE, BUT ENSEMBLE
MEANS ARE MUCH CLOSER TO ONE ANOTHER, SHOWING A CYCLONE NEARING
THE WESTERN ALEUTIAN CHAIN BY 12Z NEXT MON. ADDITIONALLY,
ENSEMBLES CONTINUE TO PERSIST WITH A SLIGHTLY STRONGER RIDGE AXIS
ACROSS WESTERN NORTH AMERICA RELATIVE TO THE DETERMINISTICS, SO
THE PREFERRED ENSEMBLE APPROACH LATER IN THE PERIOD WOULD FAVOR
THIS SCENARIO OF A SLIGHTLY MORE SLOWLY WEAKENING RIDGE.

IN TERMS OF SENSIBLE WEATHER, THE RAPID SUCCESSION OF SHORTWAVES
AND SURFACE CYCLONES ACROSS THE ALEUTIANS, THE GULF, AND INTO
SOUTHERN AK WILL RESULT IN A CONTINUED STORMY PERIOD FOR SOUTHERN
AND SOUTHEASTERN AK, WITH HEAVY PRECIPITATION POSSIBLE ESPECIALLY
IN FAVORED WINDWARD TERRAIN AREAS. FARTHER NORTH, SOMEWHAT DRIER
CONDITIONS WILL PREVAIL NORTH OF THE ALASKA RANGE, BUT STILL
EXPECT SCATTERED SNOW SHOWERS THROUGH MUCH OF THE PERIOD AS ROUNDS
OF SHORTWAVE ENERGY PASS OVERHEAD. GRADUAL WEAKENING OF UPPER
RIDGING BY LATE IN THE WEEK SHOULD RESULT IN DECREASING
TEMPERATURES, CLOSER TO OR EVEN A FEW DEGREES BELOW CLIMATOLOGICAL
NORMS AS ARCTIC AIR IS ABLE TO SPREAD FARTHER SOUTH ACROSS MUCH OF
THE STATE.

RYAN