Gale Warning 797, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
100 PM AKST WED NOV 13 2013

.ANALYSIS AND UPPER LEVELS...THE HIGH AMPLITUDE RIDGE CENTERED
OVER THE NORTH CENTRAL PACIFIC HAS BECOME POSITIVELY TILTED WITH
THE RIDGE AXIS NOW EXTENDING NORTHEASTWARD TO THE WESTERN GULF OF
ALASKA. VERY WARM AND MOIST TROPICAL AIR CONTINUES TO SURGE UP
THE WEST SIDE OF THE RIDGE ACROSS THE BERING SEA TO MAINLAND ALASKA.
WIDESPREAD STRATUS AND FOG IS BEING OBSERVED FROM THE ALEUTIANS TO
SOUTHWEST ALASKA. THE WARM FRONT IS MAKING ITS WAY FROM SOUTHWEST
ALASKA INTO SOUTHCENTRAL ALASKA THIS MORNING. SURFACE TEMPERATURES
HAVE RISEN ABOVE FREEZING ACROSS MOST OF SOUTHWEST ALASKA SO ANY
PRECIP THAT IS FALLING IS IN THE FORM OF RAIN. TEMPERATURES HAVE
BEGUN A VERY SLOW AND GRADUAL RISE OVER SOUTHCENTRAL. THE BULK OF
MOISTURE WITH THE WARM FRONT HAS SHIFTED NORTHWARD INTO INTERIOR
ALASKA OR IS GETTING HUNG UP ON THE WEST SIDE OF THE ALASKA RANGE.
THEREFORE...SOUTHCENTRAL IS DRY AT THE MOMENT.

.MODEL DISCUSSION...MODELS ARE NOW IN PRETTY GOOD AGREEMENT WITH
HANDLING OF WARM FRONT MOVING THROUGH SOUTHCENTRAL TODAY FOLLOWED
BY THE COLD FRONT ON THURSDAY. THE FORECAST CHALLENGE CONTINUES
TO BE QPF AND PRECIP-TYPE WITH GENERALLY WEAK VERTICAL MOTION
AS THE VARIOUS FEATURES MOVE THROUGH. THERE IS MUCH GREATER
UNCERTAINTY IN TRACK OF THE LOW PRESSURE SYSTEM OVER THE NORTHWEST
PACIFIC AS IT MOVES ACROSS THE BERING SEA TONIGHT AND THURSDAY TO
SOUTHWEST ALASKA THURSDAY NIGHT AND FRIDAY. EACH MODEL IS QUITE A
LITTLE DIFFERENT AND RUN-TO-RUN CONSISTENCY IS NOT VERY GOOD. FOR
NOW WILL USE AN AVERAGE POSITION OF THE 12Z RUNS. THERE WILL NO
DOUBT BE PRECIPITATION TYPE ISSUES AS THIS SYSTEM ARRIVES IN
SOUTHWEST ALASKA...BUT THERE IS TOO MUCH UNCERTAINTY IN TIMING
TO CONSIDER ISSUING ANY ADVISORIES AT THIS TIME.

.SHORT TERM FORECAST...

SOUTHCENTRAL ALASKA...THERE WILL BE A QUICK TRANSITION FROM COLD
TO WARM TO COLD DURING THE NEXT 18 TO 24 HOURS. THE MELTING AND
FREEZING THAT GO ALONG WITH IT WILL CAUSE ICY AND SLIPPERY
CONDITIONS ACROSS MUCH OF THE REGION. THE WORST CONDITIONS WILL
BE FROM ANCHORAGE TO THE SOUTHERN SUSITNA VALLEY WHERE FREEZING
RAIN ADVISORIES ARE IN EFFECT FOR LATER TODAY/TONIGHT THROUGH
TOMORROW MORNING. THE BEST CHANCE OF ACCUMULATING ICE WILL BE
FROM THE MATANUSKA VALLEY TO THE SOUTHERN SUSITNA VALLEY WHERE
TEMPERATURES WILL BE SLOWEST TO WARM TO FREEZING OR ABOVE AND
WHERE PRECIPITATION IS EXPECTED TO DEVELOP FIRST. CONDITIONS ARE
DRY RIGHT NOW...BUT AS THE WARM FRONT CONTINUES INTO SOUTHCENTRAL
EXPECT LOWERING CEILINGS AND SOME VERY LIGHT OVER-RUNNING PRECIP
TO DEVELOP. WESTERLY FLOW ACROSS THE ALASKA RANGE WILL ALSO
INCREASE WHICH IN TURN WILL INDUCE A LEE SIDE TROUGH OVER COOK
INLET. THIS WILL RESULT IN INCREASING SOUTHERLY WINDS WILL WILL
PROVIDE UPSLOPE FLOW INTO THE MAT-SU TO AID IN GENERATING PRECIP
AND IN TRANSPORTING WARMER AIR NORTHWARD.

FOR THE ANCHORAGE AREA...DON'T EXPECT PRECIP TO DEVELOP UNTIL THE
OVERNIGHT TO EARLY MORNING HOURS AS COLD FRONT APPROACHES FROM THE
NORTH AND WEST AND AN UPPER LEVEL TROUGH CURRENTLY OVER THE ARCTIC
BEGINS TO DIVE SOUTHEASTWARD ACROSS THE STATE. BY THE TIME PRECIP
BEGINS IN ANCHORAGE MANY PARTS OF THE CITY WILL BE ABOVE FREEZING.
HOWEVER...RAIN FALLING ON TO SNOW AND ICE COVERED SURFACES CAN CAUSE
THE SAME IMPACT AS FREEZING RAIN. SO THE BOTTOM LINE IS ITS LOOKING
LIKE A MESSY COMMUTE TOMORROW MORNING FROM ANCHORAGE TO THE MAT-SU.

OTHER AREAS OF SOUTHCENTRAL WILL SEE A BRIEF PERIOD OF PRECIP...BUT
DON'T EXPECT NEARLY THE IMPACT AS ANCHORAGE AND THE MAT-SU. THE
COLD FRONT WILL THEN RAPIDLY CROSS SOUTHCENTRAL ON THURSDAY CAUSING
TEMPERATURES TO FALL AND ANY LIQUID WATER TO RE-FREEZE. THE PASSAGE
OF THE UPPER TROUGH AND ARRIVAL OF A VERY STRONG UPPER LEVEL JET WILL
PROVIDE SUPPORT FOR STRONG WINDS ACROSS PRINCE WILLIAM SOUND AND THE
GULF. THE WEST TO NORTHWEST FLOW IS VERY FAVORABLE FOR HIGH WINDS IN
WHITTIER...SO PLAN TO ISSUE A HIGH WIND WARNING WITH THE AFTERNOON
FORECAST PACKAGE.

TEMPERATURES WILL REMAIN COLD AS WE HEAD INTO THE WEEKEND. THERE
IS HIGH UNCERTAINTY AS TO WHETHER ANY PRECIP FROM THE NEXT SYSTEM
CROSSING THE BERING SEA WILL SURVIVE BY THE TIME IT REACHES
SOUTHCENTRAL SOMETIME FRIDAY NIGHT OR SATURDAY. IF SO...THERE COULD
BE A BRIEF PERIOD OF LIGHT SNOW.

SOUTHWEST ALASKA...WIDESPREAD FOG (DENSE IN MANY AREAS) WILL PERSIST
OVER SOUTHWEST ALASKA UNTIL THE COLD FRONT PUSHES SOUTHWARD LATE
TONIGHT THROUGH THURSDAY. THE FRONT WILL STALL AS IT GETS TO BRISTOL
BAY THURSDAY AFTERNOON SO MOST OF THE ALASKA PENINSULA WILL REMAIN
IN THE WARM AND JUICY AIRMASS. WARM AIR WILL THEN BEGIN TO ADVECT
BACK NORTHWARD THURSDAY NIGHT AND FRIDAY. THERE IS LARGE UNCERTAINTY
IN WHEN PRECIP WILL MOVE BACK IN...BUT AS IT DOES THERE WILL BE A
VARIETY OF PRECIP-TYPES ACROSS THE REGION WITH PLAIN RAIN IN THE
SOUTH TRANSITIONING TO FREEZING RAIN AND SNOW AS YOU HEAD NORTHWARD.
THERE IS BETTER AGREEMENT THAT THIS NEXT SYSTEM CROSSING THE BERING
WILL EXIT SOUTHWEST ALASKA THIS WEEKEND WITH MUCH COLDER AND DRIER
AIR DROPPING DOWN OUT OF THE ARCTIC.

BERING SEA/ALEUTIANS...WARM AND MOIST CONDITIONS WILL PERSIST
THROUGH FRIDAY. A TROUGH DROPPING OUT OF THE ARCTIC WILL THEN
LEAD TO A PATTERN CHANGE AS ARCTIC AIR SPILLS SOUTHWARD ACROSS
THE BERING SEA THIS WEEKEND.

.LONG TERM FORECAST...MEDIUM RANGE MODELS ARE TRENDING TOWARD A
DRAMATIC CHANGE IN WEATHER ACROSS THE ALASKA REGION WITH A HIGH
AMPLITUDE RIDGE OVER THE WESTERN BERING AND TROUGH OVER MAINLAND
ALASKA ALLOWING MUCH COLDER AIR TO SETTLE IN. MODELS STILL INDICATE
A LOW SETTING UP IN THE GULF...BUT DISAGREE ON THE STRENGTH AND
POSITION. IT IS LOOKING FAVORABLE FOR SOUTHCENTRAL TO SEE SOME SNOW
DURING THE EARLY TO MIDDLE PART OF NEXT WEEK.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...FREEZING RAIN ADVISORY 101 111 145.
MARINE...GALE WARNING...119 120 121 125 129 130 131 177 178 181 185.
FIRE WEATHER...NONE.

SEB NOV 13