Heavy Freezing Spray Warning 47, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
1150 AM AKST THU JAN 19 2012

.ANALYSIS AND UPPER LEVELS...AN ANOMALOUSLY STRONG BLOCKING
UPPER LEVEL HIGH IS CENTERED OVER EASTERN RUSSIA WITH A
CORRESPONDINGLY DEEP UPPER LOW OVER NORTH CENTRAL CANADA. THIS
IS A DRY AND COLD PATTERN FOR ALASKA. IT IS ALSO A FAVORABLE
SET-UP FOR BORA WIND EVENTS ACROSS THE SOUTHERN MAINLAND AS SURGES
OF POLAR AIR MOVE SOUTHWARD BETWEEN THE HIGH AND THE LOW. WATER
VAPOR IMAGERY SHOWS A SHORT-WAVE MOVING SOUTH-SOUTHWESTWARD ACROSS
SOUTHWEST ALASKA THIS MORNING...PRODUCING MAINLY MID LEVEL CLOUDS
OUT AHEAD OF IT. WEAK COLD ADVECTION BEHIND IT COMBINED WITH
SYNOPTIC SCALE SUBSIDENCE WILL HELP KICK UP WINDS AS IT MOVES
THROUGH.

BECAUSE OF THE BLOCKING HIGH THE SUBTROPICAL JET IS SITUATED WELL
SOUTH OF ALASKA...REMAINING SOUTH OF 40 DEGREES NORTH FOR MOST OF
THE DISTANCE ACROSS THE NORTH PACIFIC. AN UPPER TROUGH AXIS EXTENDS
FROM THE CANADIAN LOW TO THE GULF OF ALASKA AND SOUTH OF THE
ALEUTIAN CHAIN. THE ONLY SYSTEM OF INTEREST TO ALASKA IS A DEEP
LOW OVER THE NORTHEAST PACIFIC WHICH IS TRACKING SLOWLY TOWARD
THE EAST-NORTHEAST. PRESSURE GRADIENTS ARE JUST BEGINNING TO
TIGHTEN ACROSS SOUTHCENTRAL ALASKA AND THE GULF IN RESPONSE TO
THIS LOW. EXPECT WINDS TO BEGIN TO STRENGTHEN TODAY IN RESPONSE
TO THE TIGHTENING GRADIENT...ESPECIALLY THROUGH COASTAL BAYS AND
PASSES.

.MODEL DISCUSSION...MODELS ARE GENERALLY IN GOOD AGREEMENT OVER
THE NEXT FEW DAYS AND FORECAST CONFIDENCE IS ABOVE AVERAGE. THE
ONE DIFFERENCE OF NOTE IS THE PLACEMENT OF UPPER LOW OVER THE
WESTERN GULF ON SUNDAY. THIS WILL AFFECT LOCATION OF SURFACE
LOW DEVELOPMENT WHICH IN TURN WILL AFFECT WHICH AREAS SEE SNOW.
THE ECMWF IS THE MOST CONSISTENT AND A NICE COMPROMISE BETWEEN
THE NAM AND GFS SOLUTIONS.

.SHORT TERM FORECAST...

SOUTHCENTRAL ALASKA...A RIDGE AXIS OVER THE BEAUFORT SEA WILL
MOVE SOUTHWARD OVER THE NORTHERN MAINLAND TONIGHT WHICH WILL
CAUSE THE JET MOVING AROUND THE UPPER HIGH TO SHIFT CLOSER TO
SOUTHCENTRAL AND CAUSE AMPLIFICATION OF THE TROUGH WHICH EXTENDS
FROM THE YUKON TO THE GULF. THIS WILL LEAD TO ENHANCED DOWNWARD
MOTION AS INDICATED BY THE MODEL DEPICTION OF A STRENGTHENING
POTENTIAL VORTICITY ANOMALY. MEANWHILE THE PRESSURE GRADIENT WILL
TIGHTEN SIGNIFICANTLY AS NORTHEAST PACIFIC LOW TRACKS TOWARD
SOUTHEAST ALASKA. WINDS WILL DEVELOP IN THE USUAL MOUNTAIN GAPS
ALONG THE GULF COAST. THE UPPER LEVEL FORCING MEANS WINDS WILL
MAKE IT DOWN TO THE SURFACE ACROSS PORTIONS OF INTERIOR
SOUTHCENTRAL...INCLUDING THE MATANUSKA AND SUSITNA VALLEYS.
WILL ISSUE A WIND CHILL ADVISORY FOR THE NORTHERN SUSITNA VALLEY
WHERE CONDITIONS ARE ALREADY CLOSE TO ADVISORY CRITERIA.

A STRONG SHORT-WAVE MOVING OVER TOP OF THE RIDGE TODAY WILL DROP
SOUTHWARD ACROSS MAINLAND ALASKA FRIDAY AFTERNOON THROUGH FRIDAY
NIGHT RESULTING IN FORMATION OF AN UPPER LOW BY SATURDAY. THE BULK
OF POLAR AIR WILL BE SHUNTED TOWARD SOUTHWEST ALASKA AS THE UPPER
LOW FORMS...ALTHOUGH SOUTHCENTRAL WILL SEE SOME COOLING. FORMATION
OF THE UPPER LOW ALONG WITH SOME COLD ADVECTION OUT INTO THE GULF
WILL LEAD TO DEVELOPMENT OF A SURFACE LOW OVER THE NORTHWEST GULF
BY LATE SATURDAY. ANOTHER ROUND OF MORE WIDESPREAD WINDS WILL
ACCOMPANY THE INITIAL ARRIVAL OF THE UPPER TROUGH FRIDAY NIGHT
THROUGH EARLY SATURDAY. UPPER FLOW WILL THEN SHIFT TO SOUTHERLY
ON SATURDAY WHICH MEANS LOW LEVEL FORCING WILL DOMINATE...WHICH
FAVORS WINDS PRIMARILY THROUGH THE MOUNTAIN GAPS. AS SURFACE FLOW
DEVELOPS SOUTHERLY FLOW WILL BECOME MUCH DEEPER AND MOISTURE WILL
BE TRANSPORTED FROM THE GULF TO SOUTHCENTRAL...BRINGING SNOW BACK
INTO THE FORECAST SATURDAY NIGHT INTO SUNDAY. UNCERTAINTY IN THE
LOW POSITION MEANS WILL HAVE TO WAIT TO FINE TUNE WHERE THE SNOW
WILL FALL AS WELL AS HOW MUCH WILL ACCUMULATE.

SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...SHORTWAVE OVER SOUTHWEST
ALASKA WILL CONTINUE TO HEAD SOUTHWESTWARD AND REACH THE ALEUTIANS
BY FRIDAY AFTERNOON. COLD ADVECTION BEHIND IT WILL PRODUCE STRONGER
WINDS AND INCREASE SNOW SHOWERS OVER THE OPEN WATERS OF THE BERING
SEA. A MUCH MORE IMPRESSIVE SURGE OF POLAR AIR WILL ACCOMPANY NEXT
SHORT-WAVE DROPPING SOUTHWARD ACROSS MAINLAND ALASKA. EXPECT THE
ZONE OF STRONGEST COLD ADVECTION ACROSS SOUTHWEST ALASKA ON SATURDAY
AND THEN ACROSS THE EASTERN BERING SEA/EASTERN ALEUTIANS/ALASKA
PENINSULA SATURDAY NIGHT. WIND CHILL ADVISORIES WILL LIKELY BE
NEEDED FOR PORTIONS OF SOUTHWEST ALASKA. STRONG WINDS AND COLD AIR
ADVECTION OVER THE EASTERN BERING WILL KICK SNOW SHOWERS INTO A
HIGHER GEAR.

LONG TERM FORECAST...DEEP TROUGH THAT SETS UP OVER MAINLAND ALASKA
AND THE GULF THIS WEEKEND WILL PERSIST THROUGH THE MIDDLE OF NEXT
WEEK. A SURFACE LOW OR LOWS WILL PERSIST OVER THE GULF BRINGING
OCCASIONAL SNOW ESPECIALLY TO COASTAL LOCATIONS WHILE MAINTAINING
STRONG GAP WINDS. MEANWHILE THE BLOCKING RIDGE WILL SINK SOUTHWARD
TO THE WESTERN BERING BY EARLY NEXT WEEK AND THEN SLOWLY WEAKEN
THROUGH THE WEEK. MODELS ALMOST ALWAYS TRY TO BREAK THESE STRONG
RIDGES DOWN TOO FAST...BUT BASED ON THE LATEST SOLUTIONS COULD SEE
A RETURN TO MORE ACTIVE WEATHER ALONG THE ALEUTIANS BY THE END OF
NEXT WEEK.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...NONE.
MARINE...GALE WARNING...127 128 130 132 150 176.
         HEAVY FREEZING SPRAY WARNING...121 126 127 128 129 130
                                        138 140 141 179 185.
FIRE WEATHER...NONE.

SEB JAN 12