Small Craft Advisory 859, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
130 PM AKST FRI NOV 25 2011

.ANALYSIS AND UPPER LEVELS...
A LARGE COMPLEX UPPER LOW REMAINS OVER CENTRAL ALASKA WITH THE JET
STREAM WELL SOUTH OF OUR DOMAIN. MULTIPLE VORTICES ROTATING AROUND
THE BROAD CYCLONIC CIRCULATION ALOFT AND AT THE SURFACE. THE TWO MOST
CRITICAL LOWS ARE LOCATED NEAR MIDDLETON ISLAND AND ONE OVER THE
ALASKA RANGE NEAR RAINY PASS. THE DEFORMATION ZONE BETWEEN THESE TWO
FEATURES IS WHAT LED TO A DECENT SNOW BAND OVER ANCHORAGE EARLY
FRIDAY MORNING.

FURTHER WEST...HIGH PRESSURE QUICKLY TRANSITIONING FROM THE CENTRAL
TO THE EASTERN BERING SEA WHILE A KAMCHATKA PENINSULA LOW AND
ASSOCIATED FRONT APPROACH FROM THE WEST.

.MODEL DISCUSSION...MAIN CHALLENGES TODAY INVOLVE THE VERY SUBTLE
MESO SCALE FEATURES OVER THE EASTERN DOMAIN. THE VARIOUS MODEL
SOLUTIONS DEPICT A SIMILAR OVERALL PICTURE...BUT DIFFER ON THE
DETAILS. MODELS DID NOT INITIALIZE WELL ON THE DEFORMATION ZONE NEAR
ANCHORAGE. ALL MODELS KEPT THE ZONE OVER ANCHORAGE DURING THE DAY
WHICH WOULD HAVE KEPT SIGNIFICANT SNOW FALL ACCUMULATING. THE PROBLEM
WAS THE INITIALIZATION OF THE UPPER LOW NEAR RAINY PASS. MODELS HAD
IT JUST 50 MILES TOO FAR TO THE SOUTH AND WEST. THAT 50 MILES WAS
ENOUGH OF A DIFFERENCE FOR ANCHORAGE TO CLEAR OUT THIS MORNING WHILE
PALMER CONTINUES TO GET THE SNOW.

THE OVERALL PATTERN IS THAT THE DOUBLE BARRELED LOW IN THE NORTHERN
GULF WILL CONSOLIDATE AND DRIFT TO THE NORTHWEST TOWARD THE KENAI
PENINSULA. AN UPPER LEVEL WAVE WILL MOVE ACROSS THE CHUGACH RANGE AND
HELP BACK THE DEFORMATION ZONE TOWARD ANCHORAGE THIS AFTERNOON.
MODELS ARE ALL IN GOOD AGREEMENT WITH THIS SOLUTION SO FEEL CONFIDENT
TO CONTINUE ADVERTISING SNOW IN ANCHORAGE DESPITE THE CLEAR SKIES
EARLY THIS AFTERNOON.

OTHER SIGNIFICANT DIFFERENCES IN THE MODELS ARE OUT WEST AS A TRIPLE
POINT LOW DEVELOPS NEAR THE EASTERN ALEUTIANS. AT THIS
POINT...BELIEVE THAT A COMBINATION OF THE MODELS IS PROBABLY THE BEST
SOLUTION. INSTEAD OF ONE CONCENTRIC LOW DEVELOPING ALONG THE
BOUNDARY...IT WILL MOST LIKELY BE TWO OR EVEN THREE CIRCULATIONS THAT
EVENTUALLY CONSOLIDATE INTO ONE LOW IN THE WESTERN GULF LATER IN THE
WEEK. WITH THAT IN MIND...WE HAVE NUDGED TOWARD A BLEND OF THE GFS
WITH THE INHERITED FORECAST TO TRY AND CAPTURE THE MULTI-CENTERED LOW
DEVELOPING ALONG THE ALASKA PENINSULA THIS WEEKEND.

.SHORT TERM FORECAST...AS PREVIOUSLY MENTIONED...VERY CHALLENGING
FORECAST TODAY WITH THE DEFORMATION ZONE JUST MOVING TOO FAR TO THE
EAST AND SHUTTING OFF THE SNOW OVER ANCHORAGE. LATEST SATELLITE
IMAGERY SHOWING AN ENHANCED AREA FINALLY BECOMING ORGANIZED AND
MOVING BACK TOWARD THE NORTHERN KENAI PENINSULA AND ANCHORAGE BOWL.
RADAR ECHOES ON THE MIDDLETON ISLAND NEXRAD ALSO SHOWING THE BAND
BECOMING MORE ORGANIZED OVER WESTERN PRINCE WILLIAM SOUND AND EASTERN
KENAI PENINSULA.

ANTICIPATING THIS SNOW BAND TO MOVE OVER ANCHORAGE THIS AFTERNOON AND
START SLIDING SOUTH AND WEST DOWN THE KENAI PENINSULA DURING THE
EVENING. THIS COULD LEAD TO SEVERAL MORE INCHES OF SNOW SO WE HAVE
DECIDED TO NOT CANCEL THE ADVISORY EVEN THOUGH WE WILL NOT LIKELY GET
THE ADVISORY THRESHOLD LEVELS THAT WE WERE ADVERTISING EARLIER.

THE COMPLEX LOW IN THE NORTHERN GULF CONSOLIDATES AND REMAINS A
POTENTIAL CHALLENGE THROUGH THE WEEKEND. ALL MODELS DRY OUT ANCHORAGE
AND THE MATSU VALLEY BUT THE PROXIMITY OF THE LOW UNDER THE BROAD
UPPER TROF COULD LEAD TO ADDITIONAL PERIODS OF LIGHT SNOWFALL.

IN ADDITION...THE CONVECTIVE NATURE IN THE EASTERLY FLOW OVER PRINCE
WILLIAM SOUND IS ALSO CONDUCIVE TO SIGNIFICANT SNOW FALL OVER THE
EASTERN KENAI PENINSULA INCLUDING GIRDWOOD. MODELS DO NOT USUALLY
HANDLE THIS SCENARIO WELL SO WE WILL BEGIN ADVERTISING HEFTIER SNOW
AMOUNTS THROUGH THE WEEKEND FOR THIS REGION WITH LOW CONFIDENCE GIVEN
THE ENORMOUS UNCERTAINTY IN THE EXACT LOCATION OF THE SURFACE
FEATURES.

THIS LOW IN THE NORTHERN GULF FINALLY DISSIPATES BY THE END OF THE
WEEKEND AS THE AFOREMENTIONED TRIPLE POINT LOW DEVELOPS SOUTH OF
KODIAK. THE POSITION OF THIS LOW AND THE FRONTAL BOUNDARY WILL PLAY A
LARGE ROLE IN DETERMINING HOW MUCH SNOW KODIAK CITY MAY RECEIVE ON
SUNDAY. THE EASTERN TRACK WILL LEAD TO MUCH MORE SNOW AND POSSIBLE
BLIZZARD CONDITIONS. WILL HAVE TO MONITOR THIS CLOSELY DURING THE
WEEKEND.

.SOUTHWEST AK AND BERING SEA...VERY BENIGN WEATHER PATTERN OVER
SOUTHWEST ALASKA THIS FRIDAY AFTERNOON WITH MOSTLY CLEAR SKIES
PREVAILING WITH JUST A FEW AREAS EXPERIENCING SOME LOCALIZED STRATUS
AND FOG. HIGH PRESSURE BUILDS ONSHORE TONIGHT SO FOG AND STRATUS MAY
BECOME A BIT MORE WIDESPREAD. THE FRONTAL BOUNDARY RACES ACROSS THE
ALEUTIAN CHAIN BY SUNDAY SPREADING SNOW AND RAIN TO THE REGION.

IT APPEARS THAT THE TEMPERATURES WILL STAY JUST BELOW FREEZING ALONG
THE ALASKA PENINSULA AND PRIBILOF ISLANDS AS THE PRECIPITATION
ARRIVES SATURDAY. THIS COULD LEAD TO BLIZZARD CONDITIONS...MORE SO
OVER THE PRIBILOFS THAN THE PENINSULA.

.OUTLOOK...
THE LARGE KAMCHATKA LOW DRIFTS EAST THROUGH THE WEEKEND AND EARLY
NEXT WEEK AND SLOWLY FILLS. ANOTHER NORTH PACIFIC LOW APPROACHES THE
EASTERN ALEUTIANS BY TUESDAY NIGHT AS HIGH PRESSURE BUILDS OVER THE
MAINLAND. THIS COULD RESULT IN A PATTERN CHANGE...MEANING NORMAL
TEMPERATURES RETURNING TO SOUTHCENTRAL AND SOUTHWEST ALASKA.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...BLIZZARD WATCH 195
MARINE...STORM 185 412
         GALE 130 132 150 155 160 165 170 171 172 175 176 179 180 411
              413 414.
FIRE WEATHER...NONE.

AB NOV 11

SPECIAL WEATHER STATEMENT
NATIONAL WEATHER SERVICE ANCHORAGE AK
135 PM AKST FRI NOV 25 2011

AKZ101-111-260300-

...SNOW RETURNS TO ANCHORAGE LATE THIS AFTERNOON AND EVENING...

SNOW DIMINISHED OVER MUCH OF ANCHORAGE THIS MORNING AND EARLY
AFTERNOON. AN AREA OF LOW PRESSURE IS APPROACHING THE NORTHWESTERN
GULF OF ALASKA WHICH WILL BRING SNOW BACK TO THE REGION THIS EVENING.
EXPECT CLOUDS TO RETURN AND SNOW TO BEGIN FALLING IN THE NEXT SEVERAL
HOURS.

SNOW WILL GENERALLY BE LIGHT WITH 1 TO 2 INCHES ADDITIONAL SNOW
ACCUMULATION EXPECTED DURING THE LATE AFTERNOON AND EARLY EVENING.
THE SNOW ARRIVING APPROXIMATELY THE SAME TIME AS RUSH HOUR COULD LEAD
TO TREACHEROUS DRIVING CONDITIONS WITH SLIPPERY ROADS AND REDUCED
VISIBILITY.

AB NOV 11