Heavy Freezing Spray Warning 221, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
500 AM AKST TUE DEC 4 2012

.ANALYSIS AND UPPER LEVELS...
TWO UPPER LEVEL LONGWAVE TROUGHS ARE IN THE PROCESS OF CUTTING OFF
THE BERING SEA RIDGE. THE LONG-WAVE TAKING SHAPE FROM THE
SOUTHCENTRAL MAINLAND THROUGH THE NORTHEASTERN PACIFIC HAS DUAL UPPER
LOW CENTERS WITH A NUMBER OF EMBEDDED SHORTWAVE TROUGHS. THE
LONG-WAVE OVER KAMCHATKA IS ITSELF IN THE PROCESS OF BECOMING CUT OFF
BETWEEN THE CHUKCHI HIGH PRESSURE AND A BUILDING RIDGE TO THE WEST.
AT THE SURFACE...A COMPLEX SURFACE LOW RESIDES OVER THE GULF OF
ALASKA AND NORTHEASTERN PACIFIC...WITH NUMEROUS SURFACE CIRCULATIONS.
HIGH PRESSURE OVER THE INTERIOR CONTINUES TO FORCE OUTFLOW WINDS IN
FAVORED LOCATIONS. OUT WEST...WEAK HIGH PRESSURE STRETCHES FROM THE
BERING STRAIT SOUTHWARD THROUGH THE CENTRAL ALEUTIANS. A SLOWLY
FILLING KAMCHATKA LOW KEEPS THE WESTERN ALEUTIANS IN A SOUTHWESTERLY
FLOW WHILE A FRONTAL BOUNDARY CONTINUES TO WEAKEN OVER THE CENTRAL
ALEUTIANS.

.MODEL DISCUSSION...
GUIDANCE IS IN GREAT AGREEMENT WITH THE MAJOR SYNOPTIC FEATURES
THROUGH THE SHORT TERM. THE DIGGING UPPER LOW OVER THE AKPEN WILL
BECOME THE DOMINANT UPPER LOW CENTER WITH WEAKER TROUGHING EXTENDING
BACK OVER THE ALEUTIANS WHICH IS A REMNANT OF THE KAMCHATKA TROUGH
AFTER IT CUTS OFF LATE THIS EVENING. A POTENT SHORTWAVE UNDERNEATH
THE WESTERN PACIFIC JET WILL PHASE WITH THE WESTERN ALEUTIAN TROUGH
AND RESHAPE THE PREVAILING WEATHER PATTERN. HERE IS WHERE SOME
DISCREPANCY AMONG THE MODELS ARISES. AT THE SURFACE...THE COMPLEX
SURFACE LOW IN THE GULF WILL EXIST THROUGH LATE WEEK. WEAK HIGH
PRESSURE WILL RESIDE OVER THE BERING SEA WITH WEAK TROUGHING OVER THE
ALEUTIANS THROUGH EARLY WEDNESDAY. GUIDANCE ALL AGREE ON A STRONG
SURFACE LOW DEVELOPING IN THE NORTHERN PACIFIC AND MOVING SOUTH OF
THE CENTRAL/EASTERN ALEUTIANS ON THURSDAY BUT THE EVOLUTION OF THE
SYSTEM AS IT WEAKENS IS STILL UP IN THE AIR. GFS ENSEMBLE GUIDANCE AS
WELL AS SREF PREFER A MORE WESTWARD SOLUTION OF THE SURFACE LOW WHILE
THE DETERMINISTIC GUIDANCE HAS BEEN TRENDING EASTWARD.

.SHORT TERM FORECAST...
SOUTHCENTRAL ALASKA...COLD DRY OUTFLOW DRIVEN PATTERN CONTINUES. THE
ONLY PRECIPITATION IN THE FORECAST WILL AFFECT KODIAK ISLAND TODAY AS
A LOW LEVEL TROUGH APPROACHES FROM THE EAST. OTHERWISE GUSTY WINDS
WILL CONTINUE FOR THE MATANUSKA VALLEY...MAINLY FROM SUTTON TO
WASILLA AS WELL AS VALDEZ/THOMPSON PASS. WHITTIER AND SEWARD WILL
CONTINUE WITH THE GUSTY WESTERLY AND NORTHERLY WINDS RESPECTIVELY.
THE COMPLEX SURFACE LOW IN THE GULF WILL KEEP SPOTTY SHOWERS ALONG
THE GULF COAST THROUGH LATE WEEK...BUT THE FORECAST WILL BE MAINLY
DRY.

SOUTHWEST ALASKA...CLEAR DRY NORTHERLY FLOW CONTINUES FOR THE
SOUTHWEST. THE BRISTOL BAY REGION WILL CONTINUE TO SEE GUSTY
WINDS...MAINLY ALONG THE COAST. THE AKPEN COULD SEE FLURRIES OR A FEW
SNOW SHOWERS AS COLDER AIR MOVES OFFSHORE WITH THE DIGGING TROUGH BUT
GENERALLY DRY CONDITIONS WILL PREVAIL WITH STRONGER WINDS THROUGH
CHANNELED TERRAIN.

BERING SEA AND ALEUTIANS...RAIN AND SNOW SHOWERS REMAIN POSSIBLE
THROUGH MID WEEK AS A DECAYING FRONTAL BOUNDARY AND UPPER TROUGH SIT
OVER THE WESTERN ALEUTIANS. THE EASTERN ALEUTIANS/BERING WILL REMAIN
UNDER WEAK HIGH PRESSURE WITH DRY CONDITIONS.

.LONG TERM FORECAST...
CONFIDENCE CONTINUES TO GROW WITH MAJOR PATTERN CHANGE UPCOMING. A
MUCH MORE PROGRESSIVE PATTERN SHAPING UP IN THE EXTENDED PERIOD. A
MUCH MORE ZONAL FLOW WILL SEND A NUMBER OF DISTURBANCES FROM THE
SOUTHWEST JUST SOUTH OFF THE ALEUTIANS UP THROUGH THE SOUTHERN
MAINLAND ON THE HEELS OF A DEEP LOW THAT WILL GIVE THE SOUTH CENTRAL
THE FIRST PRECIPITATION IN QUITE A WHILE THIS WEEKEND.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...NONE
MARINE...GALE WARNING...121 127 128 129 130 132 150 160
         HEAVY FREEZING SPRAY 121 126 127 128 129 130 138 140 141
                              150 160 165 185
FIRE WEATHER...RED FLAG WARNING 111

MTL DEC 12