Blizzard Watch 6, NWS Anchorage, AK

AREA FORECAST DISCUSSION
SOUTHCENTRAL AND SOUTHWEST ALASKA
230 PM AKST TUE JAN 27 2009

.ANALYSIS AND UPPER LEVELS...
LONG WAVE UPPER LEVEL TROF EXTENDS FROM THE BEAUFORT SEA NORTH OF THE
ALASKA/CANADA BORDER SOUTHWEST ACROSS BRISTOL BAY AND INTO THE NORTH
PACIFIC NEAR 175W. THE SUBTROPICAL JET STREAM REMAINS SOUTH OF OUR
FORECAST DOMAIN FLOWING AROUND THE DEEPENING UPPER LEVEL TROF. THE
POLAR JET IS BECOMING MORE ACTIVE...HELPING TO SEND A COLD CORE UPPER
LEVEL LOW CENTERED OVER THE EASTERN BROOKS RANGE SOUTH INTO NORTH
CENTRAL ALASKA THIS AFTERNOON.

AT THE SURFACE...THERE ARE VERY FEW SURFACE FEATURES ACROSS ALASKA
MAINLAND THIS AFTERNOON...ONLY A WEAKENING LOW MOVING INLAND NEAR
SITKA. ONE COULD MAKE AN ARGUMENT FOR A SURFACE COLD FRONT MOVING
SOUTHEAST ACROSS THE KUSKOKWIM VALLEY AS TEMPERATURES ARE COOLING
INTO THE SINGLE DIGITS AND TEENS WITH WINDS INCREASING FROM THE
NORTHWEST...ALTHOUGH THERE IS LITTLE TO NO PRESSURE GRADIENT ACROSS
THE REGION SO THIS IS MAINLY A RESULT OF THE UPPER LEVEL FEATURE.
FINALLY...THE MOST SIGNIFICANT SYSTEM IS A 993 MB LOW MOVING EAST
ALONG THE EASTERN ALEUTIANS. A FRONTAL BOUNDARY EXTENDS SOUTH WITH A
DEVELOPING TRIPLE POINT LOW IN THE NORTH PACIFIC.

.MODEL DISCUSSION...
ALTHOUGH MODELS ARE IN RELATIVELY GOOD AGREEMENT ON THE EVOLUTION OF
THE LOW MOVING ALONG THE EASTERN ALEUTIANS...THEY DO NOT INITIALIZE
WELL ON FEATURES OVER THE NORTH PACIFIC. WATER VAPOR IMAGERY IS
INDICATING A DEEPER TROF THAN ANY OF THE MODELS THINK IT SHOULD BE AT
THIS POINT. THIS MAY TRANSLATE TO A MORE AMPLIFIED PATTERN DOWNSTREAM
AND THUS INCREASED THERMAL ADVECTION AND DEEPER SURFACE FEATURES. ONE
SUCH FEATURE THAT IS HIDDEN BY THE CIRRUS SHIELD AT THIS POINT IS THE
DEVELOPING TRIPLE POINT LOW NEAR 16OW 45N. THIS FEATURE WILL PLAY A
CRITICAL ROLE IN THE FIRST 48 HOURS...AS THIS LOW MOVES INTO THE
WESTERN GULF. THE 12Z RUN OF THE GFS BEST DEPICTS THE SURFACE AND
UPPER LEVEL FEATURES AND EVOLUTION SO THE FORECAST IS LARGELY BASED
OFF OF THIS MODEL...ALTHOUGH CONFIDENCE IS NOT AS HIGH AS IT COULD BE
GIVEN THE INITIALIZATION ISSUES.

.SHORT TERM FORECAST...

.SOUTHCENTRAL...
BENIGN WEATHER COMING TO AN END OVER SOUTHCENTRAL AS SOUTHWESTERLY
FLOW ALOFT USHERS IN MOISTURE FROM THE LOW MOVING INTO THE WESTERN
GULF OVERNIGHT. THE FIRST OCCLUSION CURRENTLY LOCATED NEAR COLD BAY
WILL MOVE RAPIDLY OVERNIGHT INTO THE GULF. OVERRUNNING MOISTURE WILL
SPREAD SNOW NORTH AND EAST ALONG THE GULF COAST OVERNIGHT AND FARTHER
INLAND WEDNESDAY MORNING. THE 18Z RUNS OF THE GFS AND NAM ARE
CURRENTLY COMING IN AND ARE FURTHER DECREASING CONFIDENCE OF SNOW
MAKING IT TOO FAR INLAND OVERNIGHT. THE AFTERNOON FORECAST PACKAGE
WILL DELAY THE ONSET OF SNOW ABOUT 6 HOURS FROM THE PREVIOUS FORECAST
GIVEN THE LATEST TRENDS IN THE MODELS AND BASED OFF OF THE POOR
INITIALIZATION. BY WEDNESDAY AFTERNOON...A SECOND OCCLUSION WILL MOVE
ACROSS THE GULF WHILE THE MAIN SURFACE LOW WEAKENS AND BECOMES A
BROAD TROF EXTENDING FROM THE ALASKA PENINSULA UP THE WESTERN GULF.
SHORT WAVES EMBEDDED WITHIN THE SOUTHWESTERLY FLOW ALOFT WILL
CONTINUE TO SPREAD MOISTURE ACROSS SOUTHCENTRAL.

AT THIS TIME...THE THINKING IS THAT SNOW AMOUNTS WILL BE LIGHT ACROSS
THE ANCHORAGE BOWL AND MATANUSKA VALLEY THROUGH TUESDAY NIGHT. ON
WEDNESDAY...FURTHER DEEPENING OF THE SURFACE LOW AS A RESULT OF THE
INTERACTION OF THE COLD CORE UPPER LEVEL LOW AND ARCTIC AIR MASS WILL
LEAD TO THE POTENTIAL FOR HEAVIER SNOW ACROSS THE AREA. A LOW IN THE
UPPER 980S WILL LIKELY MOVE JUST SOUTH OF THE KENAI PENINSULA TOWARD
WESTERN PRINCE WILLIAM SOUND AS COLD AIR SPILLS ACROSS THE
ALASKA RANGE. THIS IS A FAVORABLE SETUP FOR SNOW FROM THE KENAI
PENINSULA THROUGH THE SUSITNA VALLEY. WE WILL CONTINUE TO ADVERTISE
AT LEAST A CHANCE OF SNOW THROUGH EARLY IN THE WEEKEND WHEN THE UPPER
TROF FINALLY SWINGS THROUGH AND COLD AIR SETTLES ACROSS THE REGION.

.SOUTHWEST...
A COLD FRONT ALONG THE KUSKOKWIM VALLEY IS MOVING SOUTHEAST THIS
AFTERNOON. THE WEST TO NORTHWESTERLY FLOW ALOFT IS FORCING AIR TO
RISE AND SNOW TO FALL ALONG THE RANGE. OTHERWISE...COOLING
TEMPERATURES AND A WIND SHIFT HAVE ACCOMPANIED THE FRONTAL PASSAGE.
THIS FIRST BURST OF COLD AIR WILL MAINLY MOVE EAST AND NOT MAKE IT
PAST THE RANGE. IT WILL BE A SECOND SURGE OF COLD AIR ASSOCIATED WITH
THE TROF AXIS SWINGING TO THE EAST LATER IN THE WEEK THAT WILL BRING
BITTERLY COLD WEATHER BACK TO SOUTHWEST ALASKA. THIS FORECAST PACKAGE
WILL BE LOWERING TEMPERATURES IN THE -20 TO -40 RANGE BY WEDNESDAY
NIGHT ALONG THE KUSKOKWIM VALLEY. STRONG NORTHERLY WINDS ACROSS THE
DELTA WILL RESULT IN NEAR WIND CHILL ADVISORY READINGS...WE WILL HINT
AT THAT IN THIS PACKAGE BUT THAT WILL LIKELY HAVE TO BE ISSUED WITH
TOMORROWS FORECAST PACKAGE.

SNOW ASSOCIATED WITH THE MAIN LOW MOVING INTO THE GULF WILL DEVELOP
ALONG THE ALASKA PENINSULA TONIGHT. SNOW AMOUNTS...BESIDES SOME
ISOLATED AREAS ALONG THE RANGE...WILL BE RELATIVELY LIGHT OR AT LEAST
BELOW ADVISORY CRITERIA. SOME BLOWING SNOW IS A POSSIBILITY ON THE
BACK SIDE OF THE LOW AS WINDS INCREASE AND TEMPERATURES DROP. BY THE
WEEKEND...AN ARCTIC AIRMASS WILL FIRMLY BE IN CONTROL OF THE REGION.

.BERING SEA AND ALEUTIANS...
THE LOW MOVING JUST SOUTH OF THE ALEUTIANS IS ALLOWING TEMPERATURES
TO REMAIN NEAR FREEZING FOR MOST OF THE CHAIN. RAIN AND SNOW WILL
CHANGE TO ALL SNOW IN THE NEXT 36 HOURS AS THE LOW MOVES INTO THE
GULF AND COLD AIR ADVECTION TAKES OVER THE REGION. THERE WILL LIKELY
BE AREAS OF BLOWING SNOW ACCOMPANYING THE SNOW BANDS DEVELOPING AS A
RESULT OF THE COLD AIR MOVING OVER THE RELATIVELY WARMER BERING SEA
WATERS. THE ICE EDGE MAY REDUCE THE LIKELIHOOD OF COLD AIR ADVECTION
SNOW SHOWERS ALONG THE ALASKA PENINSULA AND POTENTIAL FOR LOCAL
BLIZZARD CONDITIONS...BUT WILL NOT AFFECT FROM NELSON LAGOON
WESTWARD.

.LONG TERM FORECAST...
AS ALLUDED TO IN THE SHORT TERM FORECAST...A VERY COLD AIRMASS WILL
LIKELY SETTLE ACROSS THE MAJORITY OF ALASKA BY THE WEEKEND. ANOTHER
LOW WILL LIKELY DEVELOP SOUTH OF THE CHAIN LATE IN THE WEEK AND
FOLLOW A SIMILAR PATH TO THE CURRENT SYSTEM...BUT CONFIDENCE IN THE
TRACK AND TIMING OF THIS LOW REMAINS LOW.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...NONE
MARINE...GALES 170 172
         HEAVY FREEZING SPRAY 160 179
FIRE WEATHER...NONE

AB JAN 09