Small Craft Advisory 994, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
432 PM AKST SUN DEC 13 2015

.ANALYSIS AND UPPER LEVELS...

THE SYNOPTIC PATTERN IS RATHER STRAIGHTFORWARD...WITH AN UPPER
RIDGE OVER THE CENTRAL GULF EXTENDING THROUGH THE EASTERN ALASKA
MAINLAND...AND A VERY POWERFUL BENT-BACK OCCLUSION OVER THE
BERING SEA. BUOYS HAVE ANALYZED THIS STORM INTO THE HIGH 920 MB RANGE
EARLIER TODAY...MAKING THIS POWERFUL STORM ONE OF THE STRONGEST ON
RECORD IN THE BERING SEA...YET IS NOW STEADILY WEAKENING INTO THE
LOWER 940S THIS AFTERNOON AS AN OCCLUDED LOW. MOREOVER...ASCAT
SATELLITE PASSES FROM EARLIER TODAY HAVE REMOTELY MEASURED
SUSTAINED WIND SPEEDS OVER OPEN WATERS IN EXCESS OF 80
KNOTS...VERY IMPRESSIVE FOR A NON-TROPICAL CYCLONE. SEAS HAVE BEEN
MEASURED BY BUOYS IN PARTS OF THE NORTH PACIFIC IN EXCESS OF 50
FEET OVERNIGHT INTO THIS MORNING. ELSEWHERE...WIDESPREAD STORM
FORCE WINDS CONTINUE TO IMPACT THE CENTRAL ALEUTIANS AND PRIBILOF
ISLANDS...AND MOST OF THE REMAINDER OF THE BERING EXCEPT THE
WESTERN ALEUTIANS...WHERE THE LOW IS MOVING AWAY AND WINDS HAVE
DIMINISHED BELOW WARNING CRITERIA. ACROSS SOUTHWEST ALASKA...AN
OCCLUDED FRONT CONTINUES TO MOVE INLAND BRINGING SNOW AND GUSTY
WINDS. TOWARD SOUTHCENTRAL...A WARM FRONT CONTINUES TO MOVE NORTH
AND EAST...HAVING JUST MOVED ACROSS THE KODIAK ISLAND IN THE
AFTERNOON AND NOW MOVING TOWARD THE SOUTHCENTRAL MAINLAND. THIS
WARM FRONT IS BEING FOLLOWED BY THE COLD FRONT MOVING EAST ACROSS
KODIAK AND THE GULF OF ALASKA. THESE FEATURES ARE MOVING NEARLY IN
TANDEM WITH ONE ANOTHER CURRENTLY...WHICH MEANS THAT THE OCCLUDING
PROCESS IS PROGRESSING.

&&

.MODEL DISCUSSION...

THE SUITE OF NUMERICAL WEATHER MODELS ARE IN EXCELLENT AGREEMENT
WITH THE POWERFUL CYCLONE OVER THE BERING...AND WITH THE OVERALL
SYNOPTIC PATTERN HEADING INTO LATE WEEK BEFORE THEY BEGIN TO
DIVERGE WITH MORE IMPORTANT DETAILS OF THE SYNOPTIC PATTERN. THERE
IS DISAGREEMENT PARTICULARLY WITH RESPECT TO TIMING OF INDIVIDUAL
DISTURBANCES MOVING THROUGH THE SOUTHCENTRAL REGION THE NEXT
COUPLE OF DAYS...THOUGH THERE IS NOT MUCH DIFFERENCE WITH RESPECT
TO THE SPEED AND STRENGTH OF THESE WEAK DISTURBANCES. THERE IS
ALSO DISAGREEMENT WITH A COMPACT AND FAST MOVING SHORTWAVE TROUGH
FORMING AT THE BASE OF THE BERING CYCLONE...WHICH IS PROGGED BY
THE NAM AND ECMWF TO MOVE INTO THE NORTHERN GULF OF ALASKA AND
PRINCE WILLIAM SOUND BY TUESDAY. THIS DISCREPANCY HAS AN IMPACT
FOR THE SOUTHCENTRAL FORECAST...AS WELL AS THE POSSIBILITY OF
GALE-FORCE WINDS IN THE NORTHERN GULF TOWARD MID-WEEK. THE
BIGGEST IMPACT WOULD BE SNOW THREATS ALL ALONG THE COAST INTO
PORTIONS OF INTERIOR SOUTHCENTRAL. WITH THE GFS BEING A LARGE
OUTLIER...AND WITH THE SENSITIVITY OF THE PATTERN TO SMALL MODEL
ERRORS...THIS FEATURE WILL BE HANDLED CONSERVATIVELY FOR NOW.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 AND 2)...
THE WEATHER FRONT IN THE WESTERN GULF HAS MOVED PASS KODIAK
ISLAND AND IS ON TRACK TO MOVE INTO THE NORTHERN GULF THIS
EVENING. THIS WILL INCREASE THE WINDS THROUGH EASTERN TURNAGAIN
ARM AND THE PORTAGE AREA ACCOMPANIED BY SNOW RESULTING IN BLIZZARD
CONDITIONS WITH BLOWING SNOW THROUGH TONIGHT INTO EARLY MONDAY
MORNING. THE ANCHORAGE BOWL WILL SEE SOME SNOW ACCUMULATION WITH
THE 700 MB SHOWING A MORE SOUTHERLY FLOW WHICH WILL ALLOW MORE
MOISTURE TO MOVE UP THROUGH COOK INLET INTO THE ANCHORAGE AREA.
BY MONDAY THE WINDS OVER THE AREA WILL WEAKEN AS AN UPPER LEVEL
WEAK RIDGE BUILDS INTO THE REGION. THIS WILL BE SHORT LIVED AS AN
UPSTREAM TROUGH WILL ENTER THE SOUTHWESTERN GULF BRINGING MORE
MOISTURE BY MONDAY NIGHT IN THE LATE EVENING.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)...
THE FRONT HAS PASSED OVER MUCH OF THE MAINLAND USHERING IN AN
UNSTABLE SHOWERY REGIME. THESE CONDITIONS WILL LAST THROUGH MOST
OF THE SHORT TERM...SLOWLY DECREASING IN COVERAGE OVER TIME. WINDS
WILL TURN MORE SOUTHERLY TONIGHT BRINGING IN A BIT WARMER AIR AT
THE SURFACE AND ALLOW FOR SNOW TO MIX WITH RAIN AT TIMES. LATE
MONDAY NIGHT A DISTURBANCE WILL RIDE UP THE ALASKA PENINSULA AND
ONTO THE MAINLAND. THIS WILL BRING A QUICK SHOT OF SNOW...RAIN...AND
WIND TUESDAY MORNING INTO THE AFTERNOON...HOWEVER IT WILL QUICKLY
MOVE OUT OF THE AREA TUESDAY AFTERNOON. BY THIS TIME...SHOWER
COVERAGE OVER MOST OF THE MAINLAND WILL HAVE DIMINISHED TO
ISOLATED OR ZERO ALTOGETHER.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...
WINDS HAVE PEAKED AND ARE DIMINISHING IN MOST AREAS THIS
AFTERNOON...WITH THE STRONGEST WINDS LINGERING OVER THE PRIBILOF
ISLANDS TONIGHT WHERE THEY SHOULD PEAK THIS EVENING BEFORE SLOWLY
DIMINISHING OVERNIGHT. THE OVERALL STORM SYSTEM WILL CONTINUE TO
LOSE STRENGTH THE NEXT COUPLE DAYS WHILE LEAVING MUCH OF THE
BERING AND CHAIN IN A SHOWERY REGIME. ON MONDAY AFTERNOON AND
EVENING...A QUICK MOVING DISTURBANCE WILL CLIP THE UNALASKA AREA
AS WELL AS THE ALASKA PENINSULA...WHICH WILL BRING A QUICK SHOT OF
RAIN AND SNOW BEFORE RETURNING TO THE SHOWERY REGIME.

&&

.LONG TERM FORECAST (DAYS 3 THROUGH 7)...
THE BERING LOW WILL HAVE WEAKENED AND PUSHED OFF TO THE NORTH
HEADING INTO THE MID TERM LEAVING A SHOWERY PATTERN BEHIND IT. TWO
NEW SYSTEMS WILL APPROACH THE AREA MID TO LATE WEEK. THERE IS
DECENT AGREEMENT WITH THE WESTERN SYSTEM WHICH MOVES INTO THE
WESTERN BERING THURSDAY. THE EC IS A LITTLE BIT FASTER THAN THE
OTHERS BUT FROM AN IMPACTS STANDPOINT THEY ALL HAVE THE FRONT FROM
THIS SYSTEM MAKING IT TO THE WEST COAST FRIDAY. THERE IS MORE
DISAGREEMENT WITH THE LOW APPROACHING KODIAK WEDNESDAY. THE NAM
AND GEM TAKE THE LOW INTO SHELIKOF STRAIT WHILE THE GFS STAYS ON
THE SOUTHEASTERN SIDE OF THE ISLAND WITH THE EC EVEN FURTHER TO
THE SOUTH. THESE DIFFERENT SOLUTIONS OFFER A WIDE VARIATION ON
IMPACTS OF WINDS AND PRECIPITATION FOR THE GULF AND SOUTHCENTRAL
WHICH LEADS TO A LOW CONFIDENCE FORECAST BY LATE WEEK. AN
ENSEMBLE MEAN SOLUTION WAS CHOSEN WHICH HELPED GET A MIDDLE GROUND
SOLUTION THAT BRINGS THE LOW INTO THE SOUTHERN GULF WEDNESDAY
NIGHT.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...BLIZZARD WARNING 125 (PORTAGE AND TURNAGAIN ARM).
 HIGH WIND WARNING 195.
MARINE...STORM WARNING 165 170 173 174 175 179.
 GALE WARNING 119 125 129 132 138 150 155 160 171 172 176 177 178
180 181 185.
 HEAVY FREEZING SPRAY 185.

&&

SYNOPSIS AND MODEL DISCUSSION...CC
SOUTHCENTRAL ALASKA...PD
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...ML
LONG TERM...DK