Blizzard and High Wind in Alaska, December 18, 2012

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
400 PM AKST MON DEC 17 2012

.ANALYSIS AND UPPER LEVELS...
THE PREVAILING WEATHER PATTERN ACROSS THE ALASKA DOMAIN REMAINS QUITE
PROGRESSIVE...WITH EMBEDDED DISTURBANCES STEERED BY A 125+ KT JET
STREAM QUICKLY MOVING THROUGH THE AREA. THE ZONAL JET STREAM FLOWS
JUST SOUTH OF THE ALEUTIANS AND ALASKA PENINSULA BEFORE JET REMAINS
GENERALLY JUST SOUTH OF THE ALEUTIAN ISLANDS BEFORE TURNING SOUTHWARD
TOWARD THE COAST OF BRITISH COLUMBIA. THIS IS LEAVING SOUTHCENTRAL
ALASKA UNDER MOSTLY CLEAR SKIES...WITH SOME OF THE COLDEST
TEMPERATURES OF THE SEASON SO FAR OCCURRING AT MANY PLACES THIS
MORNING. OUT WEST...A LOW IN THE MID 980S NEAR THE BERING STRAIT IS
SENDING A WEAKENING ASSOCIATED FRONT THROUGH THE SOUTHWEST
MAINLAND...WHERE THE MAIN IMPACT IS LOW CLOUDS AND A BIT OF SNOW. THE
MAIN FEATURE ON THE MAP IS ANOTHER MID 980S LOW LOCATED JUST WEST OF
DUTCH HARBOR THAT IS STRENGTHENING AS IT MOVES TO THE EAST. THIS LOW
WILL CONTINUE TO STRENGTHEN AS IT HEADS TOWARD KODIAK ISLAND...AND
WILL BE THE PRIMARY FORECAST CHALLENGE FOR THE NEXT 48 HOURS ACROSS
THE DOMAIN.

.MODEL DISCUSSION...
THE MODELS ARE HANDLING THE EVOLUTION OF THE BERING SEA LOW FAIRLY
WELL. ALL MODELS SHOW THE STORM FOLLOWING A TRACK ACROSS THE ALASKA
PENINSULA THIS EVENING AND SAGGING SLIGHTLY SOUTHWARD TO NEAR THE
SOUTHERN TIP OF KODIAK ISLAND BY TUESDAY MORNING...STEADILY
STRENGTHENING AS IT PROGRESSES. THE LOW THEN STALLS IN THE SOUTHERN
GULF FOR ABOUT 12 HOURS BEFORE RESUMING AN EASTWARD PATH. CONFIDENCE
IS HIGH THAT COLD AIR BEHIND THE LOW WILL CHANGE A RAIN/SNOW MIX IN
KODIAK BACK TO ALL SNOW TUESDAY AFTERNOON. THIS WILL LEAD TO
SUBSTANTIAL SNOWFALL AND AREAS OF BLOWING SNOW AROUND KODIAK THAT
WILL PERSIST THROUGH THE EVENING HOURS. THE OTHER SIGNIFICANT IMPACT
TO THE REGION WILL BE STRONG NORTHWESTERLY WINDS THAT DEVELOP ACROSS
THE ALASKA PENINSULA BEHIND THE LOW. WITH PLENTY OF COLD AIR MOVING
INTO THE REGION...PRESSURE RISES FROM THE APPROACHING RIDGE IN THE
BERING...AND ENHANCED DOWNWARD VERTICAL VELOCITY DUE TO AN UPPER
LEVEL WAVE DROPPING DOWN FROM THE NORTH...WINDS WILL LIKELY REACH
HURRICANE FORCE IN CHANNELED AREAS STARTING LATE TONIGHT.
OVERALL...THE GFS HAS BEEN THE MOST CONSISTENT MODEL WITH THE TIMING
AND STRENGTH OF THE BERING STORM...AND WAS THE FAVORED MODEL WEST OF
THE ALASKA RANGE. TO THE EAST...THE SUPERIOR RESOLUTION OF THE NAM
WAS FAVORED FOR GAP WINDS AROUND SOUTHCENTRAL.

.SHORT TERM FORECAST...
SOUTHCENTRAL...HIGH CLOUDS FROM THE WEAKENING FRONT OVER SOUTHWEST
WILL MOVE INTO THE AREA LATE THIS AFTERNOON...BUT WILL HAVE LITTLE
IMPACT OTHER THAN TO HELP WARM TEMPERATURES A FEW DEGREES. MOST OF
THE SOUTHCENTRAL WILL REMAIN TOO FAR TO THE NORTH TO GET MORE THAN
MIDLEVEL CLOUDS FROM THE STORM THAT MOVES INTO THE WESTERN GULF
TONIGHT. AS MENTIONED ABOVE...KODIAK ISLAND WILL BE A DIFFERENT
STORY. SNOW SHOULD DEVELOP THIS EVENING IN ADVANCE OF THE
LOW...MIXING WITH RAIN AFTER MIDNIGHT AND INTO TUESDAY
MORNING...BEFORE THE WINDS SWITCH AND TEMPERATURES DROP AROUND MIDDAY
TUESDAY. THE MIX WITH RAIN OVERNIGHT WILL LIMIT ACCUMULATIONS...BUT
TUESDAY AFTERNOON AND EVENING SHOULD BRING SEVERAL HOURS OF MODERATE
TO HEAVY SNOW WITH BLOWING SNOW.

SOUTHWEST MAINLAND...RESIDUAL ONSHORE FLOW AND SNOW SHOWERS ACROSS
THE REGION WILL DIMINISH THIS AFTERNOON AS WINDS BECOME SOUTHEAST
AHEAD OF THE APPROACHING FRONT. WITH THE LOW TRACKING JUST SOUTH OF
THE BRISTOL BAY ZONE...AREAS AROUND DILLINGHAM WILL SEE SEVERAL
INCHES OF SNOW AS POSSIBLE DEFORMATION BAND MOVES THROUGH THE ZONE
DURING THE DAY ON TUESDAY. HOWEVER...THE MOST IMPACTFUL WEATHER WILL
OCCUR ALONG THE ALASKA PENINSULA WHERE GAP FLOWS THROUGH COMPLEX
TERRAIN WILL LIKELY REACH HURRICANE FORCE BY TUESDAY MORNING.

BERING SEA AND ALEUTIANS...AS THE CURRENT LOW NEAR DUTCH HARBOR MOVES
EAST INTO THE GULF THE BERING SEA REGION WILL SEE A SUBSTANTIAL RIDGE
OF HIGH PRESSURE DEVELOP AND BECOME NEARLY STATIONARY FOR MUCH OF THE
WEEK. THIS WILL LEAD TO THE RETURN OF NORTHERLY WINDS AND COLD
TEMPERATURES ACROSS THE EASTERN BERING...WITH GUSTY SOUTHERLY FLOW
AND WARMER TEMPERATURES OVER THE WESTERN BERING. A SERIES OF
GALE-FORCE FRONTS WILL IMPACT THE FAR WESTERN ALEUTIANS THIS
WEEK...BUT AT THIS POINT MODELS ARE SUGGESTING THE HIGH PRESSURE WILL
HOLD STEADY AND DEFLECT MOST OF THESE FRONTS NORTHWARD INTO RUSSIA.

.LONG TERM FORECAST...
THERE ARE QUITE A FEW ISSUES WITH THE LONG RANGE MODELS...BUT THE
GENERAL TREND THROUGH THE WEEKEND AND BEYOND IS FOR THE BERING RIDGE
TO BECOME A CLOSED UPPER LEVEL HIGH OVER FAR NORTHERN ALASKA AND THE
ARCTIC OCEAN. THIS MEANS A CONTINUATION OF A GENERALLY CLEAR AND COLD
PATTERN FOR MOST OF THE MAINLAND.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...BLIZZARD WARNING 171
         HIGH WIND WARNING 181
MARINE...HURRICANE FORCE WIND WARNING 150
         STORM WARNING 130 132 155 160 165 170 176
         GALE WARNING 120 127 138 140 171 172 175 179
FIRE WEATHER...NONE

AMD DEC 12