Small Craft Advisory 1019, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
400 PM AKST WED DEC 18 2013

.ANALYSIS AND UPPER LEVELS...A VERY DYNAMIC PATTERN IS SETTING UP
OVER THE FORECAST AREA AS AN UPPER LEVEL TROUGH OVER THE BERING SEA
PUSHES INTO THE UPPER LEVEL RIDGE OVER THE GULF OF ALASKA. THIS IS
CREATING A STRONG JET STREAM OVER THE ALASKA PENINSULA INTO SOUTH
CENTRAL ALASKA....WHILE THERE IS A STRONG NORTHWEST JET OVER THE
EASTERN GULF ON THE OTHER SIDE OF THE RIDGE AXIS. THIS IS HELPING A
SURFACE LOW TO DEVELOP NEAR SAND POINT ALONG THE FRONT THAT IS MOVING
NORTHWARD INTO THE SURFACE RIDGE THAT HAS BEEN ENTRENCHED OVER THE
INTERIOR OF THE STATE. THIS FRONT IS ELONGATING AS IT MOVES NORTHWARD

WHICH IS KEEPING THE PRESSURE GRADIENT STRONG FROM BRISTOL BAY
EASTWARD INTO THE GULF OF ALASKA. THE SURFACE LOW DEVELOPING NEAR
SAND POINT WILL MOVE QUICKLY IN THIS JET AND BE IN THE NORTHERN GULF
LATER TONIGHT BEFORE CONTINUING EASTWARD TOWARD THE YAKUTAT AREA
TOMORROW.

.MODEL DISCUSSION...MODELS ARE DOING MUCH BETTER OVERALL TODAY WITH
THE TRACK OF THE LOW DEVELOPING ALONG THE FRONT IN THE SOUTHWEST OF
THE GULF THIS AFTERNOON AND THEN MOVING ACROSS THE NORTHERN GULF
TONIGHT. THIS LOW WILL BE A FAST MOVING BUT INTENSE SYSTEM AND THAT
IS SOMETHING THE MODELS USUALLY STRUGGLE WITH. THE NAM ECMWF AND GEM
ARE ALL IN VERY GOOD AGREEMENT WITH THIS TRACK OVERNIGHT WHILE THE
GFS IS TOO FAR SOUTH AND HAS THE ASSOCIATED UPPER LEVEL WAVE TOO
WEAK. THE NAM MAY BE A LITTLE TOO STRONG AS IT IS A GOOD 8 MB LOWER
THAN EITHER ECMWF OR GEM. THESE FACTORS WILL ALL PLAY INTO THE
WEATHER FOR TONIGHT INTO TOMORROW MORNING. GFS IS ALSO A BIT
DIFFERENT IN ITS TRACK OF THE LOW FARTHER WEST. THE TRACK OF THIS LOW
THAT WILL ELONGATE WITH A PART REMAINING NEAR THE ALEUTIANS AND
ANOTHER BREAKING OFF AND HEADING TOWARD THE KUSKOKWIM DELTA FRI
NIGHT. THE GEM NAM AND ECMWF ONCE AGAIN HAVE A DECENT LOCK ON THE
TRACK BUT THE SPECIFICS ON HOW THE LOW WILL ELONGATE ARE STILL UP IN
THE AIR A BIT. THIS MAY CAUSE SOME CHANGES IN THE FORECASTS IN THE
KUSKOKWIM DELTA AND BRISTOL BAY REGION LATER IN THE WEEK DEPENDING ON
HOW THAT PLAYS OUT.

.SHORT TERM FORECAST...WHEN A STRONG FRONT MOVES INTO SOUTHERN ALASKA
FROM THE SOUTH IT OFTEN WEAKENS AS IT PUSHES INTO AN ESTABLISHED
RIDGE OVER THE MAINLAND PART OF THE STATE WITH VERY COLD
TEMPERATURES. IF IT IS STRONG ENOUGH TO PUSH INTO SOUTHERN ALASKA
THERE IS GENERALLY ENOUGH WARM AIR WITH IT TO CHANGE COASTAL
PRECIPITATION TO RAIN. THAT IS NOT THE CASE WITH THIS SYSTEM...WHICH
IS CAUSING CONCERN ABOUT THE SNOW AND BLOWING SNOW POTENTIAL FROM
BRISTOL BAY EASTWARD. A WINTER WEATHER ADVISORY FOR SNOW AND BLOWING
SNOW IS IN EFFECT FOR BRISTOL BAY AND THE WESTERN KENAI PENINSULA
WITH A BLIZZARD WARNING FOR THE EASTERN KENAI PENINSULA. THE BIG
UNCERTAINTY IN THIS WHOLE EVENT IS THE STRENGTH OF THE LOW MOVING
INTO THE NORTHERN GULF TONIGHT AS WELL AS HOW FAR NORTH THE TRACK
WILL BE. THE EASTERN KENAI PENINSULA LOOKS TO BE IN AN IDEAL SPOT TO
GET WIDESPREAD BLIZZARD CONDITIONS AS HEAVY SNOW AND STRONG WINDS
MOVE OVER THE AREA TONIGHT. THESE CONDITIONS SHOULD ONLY LAST FROM
THREE TO SIX HOURS AS THE LOW WILL BE MOVING QUITE FAST...BUT WILL
CAUSE LOW VISIBILITIES AS IT DOES MOVE THROUGH. LESS CERTAIN IS HOW
STRONG THE WINDS IN THE WESTERN KENAI PENINSULA WILL BE AND HOW
WIDESPREAD THE REDUCED VISIBILITIES WILL BE. IT DOES NOT LOOK LIKE
WIDESPREAD BLIZZARD CONDITIONS IN THE WESTERN KENAI BUT GUSTY WINDS
AND SNOWFALL WILL LOCALLY REDUCE VISIBILITIES THERE AS WELL
OVERNIGHT. PLACES ELEVATED A LITTLE BIT SUCH AS THE HOMER BLUFF ARE
AT INCREASED RISK OF FULL SCALE BLIZZARD CONDITIONS IN THIS SET UP.
THIS BRINGS US TO THE ANCHORAGE AREA WHERE THERE IS CONCERN FOR
SIGNIFICANTLY REDUCED VISIBILITIES ALONG THE HILLSIDE AND TURNAGAIN
ARM THIS EVENING TO A LITTLE PAST MIDNIGHT. THE WINDS WILL BE GUSTY
AT THE SURFACE AND WILL CAUSE SOME LIMITED VISIBILITIES AT LOWER
ELEVATIONS AS WELL...BUT HIGHER ELEVATIONS AND TURNAGAIN ARM MAY SEE
STRONG WINDS IN CONJUNCTION WITH THE SNOWFALL CAUSING GREATLY REDUCED
VISIBILITIES.

.LONG TERM FORECAST...BY THE END OF THE WEEKEND THERE WILL BE ONE LOW
EXITING THE EASTERN HALF OF THE STATE WITH ANOTHER ONE MOVING UP OVER
THE WESTERN ALEUTIANS. IN BETWEEN THESE WILL BE A SHORTWAVE RIDGE
PROVIDING A BRIEF PERIOD OF DRY CONDITIONS. MODELS INDICATE THAT THIS
RIDGE WILL START TO AMPLIFY NORTHWARD MONDAY. THERE IS SOME MODEL
DISCREPANCY AS TO THE ORIENTATION OF THIS RIDGE WITH THE GFS GIVING
IT A MUCH GREATER NEGATIVE TILT. THIS WOULD KEEP THE MAINLAND DRY AND
COLD A BIT LONGER. BUT EITHER WAY...IT APPEARS TO BE A TEMPORARY
FEATURE WITH SOUTH TO SOUTHWEST FLOW RETURNING BY MID WEEK.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...BLIZZARD WARNING 125.
         SNOW AND BLOWING SNOW ADVISORY 101 121 135 161
MARINE...STORM 119 130 131.
         GALE 120 132-140 150 160 175 177-185.
         HEAVY FREEZING SPRAY 130 139 140 141 181 185.
FIRE WEATHER...NONE.

DEC 13