Small Craft Advisory 39, NWS Anchorage, AK

AREA FORECAST DISCUSSION
SOUTHCENTRAL AND SOUTHWEST ALASKA
1245 PM AKST THU JAN 14 2010

.ANALYSIS AND UPPER LEVELS...LONG-WAVE TROUGH ENCOMPASSES ALL OF
ALASKA WITH A 200 KT PLUS JET DRIVING EASTWARD ACROSS THE NORTH
PACIFIC BETWEEN 35N AND 40N LATITUDE. WITH THE JET NOW TO THE
SOUTH OF THE ALEUTIAN STORM PRESSURE HAS BOTTOMED OUT AROUND
948 MB AND SYSTEM MOVEMENT HAS SLOWED TO A CRAWL. MEANWHILE THE
VERTICALLY STACKED LOW WHICH HAS BEEN NEARLY STATIONARY SOUTH OF
KODIAK ISLAND IS BEGINNING TO BE PICKED UP BY THE WESTERLIES AND
STARTED TO TRACK OFF TO THE EAST-NORTHEAST. A VORT-MAX WHICH ROUNDED
THIS LOW HAS MOVED INTO THE NORTHERN GULF THIS MORNING AND BECOME
MORE ELONGATED...HELPING TO PRODUCE A LARGE AREA OF SYNOPTIC LIFT
OVER SOUTHCENTRAL. IN ADDITION...THE REMNANTS OF A WEAK OCCLUDED
FRONT ARE NOW MOVING OVERHEAD...AND SOUTHERLY FLOW IS LEADING TO
ISENTROPIC LIFT WITH WARMER AIR OVERRIDING THE COLD SURFACE AIR IN
PLACE ACROSS THE REGION. THIS IS VERY EVIDENT ON THE 12Z ANCHORAGE
SOUNDING WITH A STEEP INVERSION FROM THE SURFACE TO ABOUT 2000 FEET.
ALL OF THIS IS WORKING TOGETHER TO PRODUCE AREAS OF SNOW ACROSS
SOUTHCENTRAL. ALSO OF NOTE...THERE IS AN AREA OF DIVERGENCE NOTED
IN THE MODELS IN THE MID LEVELS FOR THE ANCHORAGE/NORTHERN COOK
INLET AREA. THIS MAY BE CONTRIBUTING TO SOME OF THE INCREASE IN
INTENSITY THAT HAS OCCURRED THIS MORNING.

.MODEL DISCUSSION...MODELS INITIALIZED FAIRLY WELL WITH MAJOR
FEATURES...BUT SOLUTIONS QUICKLY DIVERGE...AND OVERALL FORECAST
CONFIDENCE IS LOW. SPECIFICALLY...THE NAM SEEMS TO HAVE A BETTER
HANDLE ON THE PLACEMENT OF THE VORT-MAX OVER THE NORTHERN
GULF...ALTHOUGH IT IS NOT DOING A VERY GOOD JOB WITH QPF/SNOW
AMOUNTS. WILL LEAN TOWARD THE NAM MODEL INITIALLY...BUT PRECIP
FORECAST WILL BE PARTIALLY BASED OFF OF ONGOING WEATHER AND
PERSISTENCE AS THE VORT-MAX CONTINUES TO ELONGATE AND MOVE VERY
SLOWLY ACROSS THE REGION THROUGH TONIGHT. ELSEWHERE...THE GFS MODEL
CONTINUES TO BE THE ODD MODEL OUT WITH ITS DEVELOPMENT AND TRACKING
OF TRIPLE POINT LOW SOUTH OF THE AKPENN...ALTHOUGH THE SOLUTION
IS CLOSER TO OTHER MODELS THAN IT HAS BEEN. THE NAM/EC/UKMET ALL
DEVELOP SURFACE LOWS...BUT IN DIFFERENT PLACES...AND UNLIKE THE
GFS THEY ALL DEVELOP THE LOW JUST BEHIND THE FRONT IN RESPONSE
TO AN UPPER SHORT-WAVE RIDING EASTWARD ALONG THE UPPER JET. MODELS
PERFORMANCE WITH FORMATION OF LOWS IN THIS GEOGRAPHICAL REGION IS
NOT GOOD...AND CONFIDENCE IN ANY PARTICULAR SOLUTION IS VERY LOW.
THEREFORE...WILL GO WITH A BLEND OF INHERITED FORECAST AND NAM
SOLUTION (WHICH IS CLOSEST TO EXISTING FORECAST) FOR THE SHORT-TERM
AND LEAVE FORECAST BEYOND SATURDAY AS IS.

.SHORT TERM FORECAST...

SOUTHCENTRAL ALASKA...AREAS OF SNOW WILL PERSIST THROUGH TONIGHT
AS SOME WEAK SYNOPTIC LIFT ASSOCIATED WITH ELONGATING VORT-MAX
CONTINUES. HOWEVER...EXPECT SNOW INTENSITY TO WEAKEN AND SNOW TO
BEGIN TO TAPER OFF MOST PLACES AFTER MIDNIGHT...SO ADDITIONAL SNOW
ACCUMULATION WILL BE LIGHT. MEANWHILE...THE LOW SOUTH OF KODIAK
WILL OPEN UP INTO A TROUGH ALOFT AND TRACK NORTHWARD ACROSS THE
GULF FRIDAY...BRINGING ANOTHER ROUND OF PRECIP TO THE COAST. THIS
TROUGH WILL STRETCH AND WEAKEN AS IT CROSSES SOUTHCENTRAL FRIDAY
NIGHT...SO INLAND AREAS WILL HAVE JUST A CHANCE OF SNOW. BEYOND
THIS UNCERTAINTY IS VERY HIGH IN FORMATION AND TRACK OF NEXT
LOW...BUT CONFIDENCE IS HIGH THAT SOMETHING WILL MOVE UP TOWARD
SOUTHCENTRAL THIS WEEKEND. THEREFORE...PRECIP WILL CONTINUE ALONG
THE COAST. WILL HAVE TO WAIT UNTIL GET A BETTER HANDLE ON THIS
BEFORE DETERMINING IF PRECIP WILL SPREAD INLAND.

SOUTHWEST ALASKA...SOUTHWEST ALASKA WILL REMAIN DRY...COLD...AND
WINDY THROUGH THE WEEKEND AS PRESSURE GRADIENT TIGHTENS AHEAD OF
THE ALEUTIAN LOW AND AS A TROUGH DROPS DOWN FROM THE ARCTIC THIS
WEEKEND BRINGING ANOTHER ROUND OF COLD ADVECTION. THERE IS POTENTIAL
FOR REACHING WIND CHILL CRITERIA THIS WEEKEND ACROSS THE KUSKOKWIM
DELTA...SO WILL HAVE TO MONITOR THIS THE NEXT COUPLE DAYS.

BERING SEA/ALEUTIANS...THE ALEUTIAN STORM WILL PROGRESS VERY SLOWLY
OFF TO THE EAST...REMAINING SOUTH OF THE ALEUTIAN CHAIN. THERE ISN'T
MUCH SOUTHERLY PUSH BEHIND THE OCCLUSION SO IT WILL STALL OVER THE
SOUTHERN BERING ON FRIDAY...WHICH MEANS THE HEAVIER SNOW ALONG THE
FRONT WILL REMAIN SOUTH OF THE PRIBILOFS WHICH WILL PREVENT THEM
FROM OBSERVING BLIZZARD CONDITIONS...ALTHOUGH STRONG WINDS AND
LIGHT SNOW WILL PRODUCE BLOWING SNOW. ARCTIC AIR WILL ADVECT
SOUTHWARD BEHIND THE SYSTEM CHANGING ALL PRECIP ALONG THE ALEUTIANS
BACK TO SNOW BY LATE SATURDAY.

.LONG TERM FORECAST...WEATHER CALMS DOWN SIGNIFICANTLY IN THE
BERING SEA/ALEUTIAN REGION AFTER CURRENT LARGE LOW SHIFTS EASTWARD
THIS WEEKEND...BEING REPLACED BY COLD NORTHERLY FLOW. IN THE
GULF...THE PICTURE IS SIGNIFICANTLY MORE MUDDLED...WITH MOST LIKELY
SCENARIO BEING MULTIPLE REMNANT LOWS PINWHEELING AROUND THE GULF
THROUGH THE MIDDLE OF NEXT WEEK. COASTAL LOCATIONS WILL SEE PERIODS
OF PRECIPITATION THROUGH NEXT WEEK...WHILE CONFIDENCE IS LOWER IN
PRECIPITATION FORECAST FOR INTERIOR LOCATIONS. IT SEEMS LIKELY THAT
ANCHORAGE AND THE MAT-SU VALLEY WILL SEE SNOW AT SOME POINT LATE
THIS WEEKEND INTO NEXT WEEK...BUT TRYING TO PIN DOWN DETAILS AT
THIS POINT WOULD BE FUTILE.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...NONE.
MARINE...GALE WARNING...170 175 176 179 180 185.
         HEAVY FREEZING SPRAY...185.
FIRE WEATHER...NONE.

SEB JAN 10