Small Craft Advisory 857, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
200 PM AKST THU NOV 24 2011

.ANALYSIS AND UPPER LEVELS...
A VERY LARGE AND COMPLEX AREA OF LOW PRESSURE PERSISTS OVER MUCH OF
THE STATE WITH A BUILDING RIDGE OF HIGH PRESSURE OVER THE KAMCHATKA
PENINSULA AND WESTERN BERING SEA. THE JET STREAM IS ACTIVE...BUT IS
WELL SOUTH OF OUR DOMAIN AND IS NOT PLAYING A SIGNIFICANT ROLE IN OUR
WEATHER.

THE CHALLENGE IN THE FORECAST IS THE NUMEROUS UPPER LEVEL WAVES AND
SURFACE LOWS THAT ARE FLOWING AROUND THE BASE OF THE TROF. SEVERAL
LOW LEVEL FEATURES EVIDENT ON SATELLITE AND RADAR THIS MORNING IN THE
NORTHWESTERN GULF THAT ARE BRINGING SOUTHCENTRAL WIDESPREAD LIGHT
SNOW. THE MOST PERSISTENT FEATURE IS AN AREA OF CONVERGENCE DOWN COOK
INLET THAT BENDS TOWARD KACHEMAK BAY. THIS COMMON WINTER CONVERGENCE
ZONE HAS BROUGHT SIGNIFICANT SNOW TO KACHEMAK BAY AND WILL CONTINUE
TO DO SO UNTIL THE GRADIENT SWITCHES LATER THIS WEEKEND.

.MODEL DISCUSSION...NOT SURPRISING THE MODELS ARE HAVING A DIFFICULT
TIME PICKING OUT THE MULTIPLE VORTICES SPINNING AROUND THE LARGER
PARENT CIRCULATION. THIS IS LEADING TO QUITE A BIT OF DISAGREEMENT IN
THE FIRST 48 HOURS THAT WILL HAVE LARGE IMPACTS ON THE FORECAST.
GENERALLY AGREE WITH THE NAM/GFS...EVEN MORE SO WITH THE STRONGER
NAM...DEVELOPING A LOW AND SWINGING IT NORTHWEST TOWARD MIDDLETON
ISLAND. THE ECMWF AND UKMET KEEP THIS LOW A BIT WEAKER AND TAKE IT
EAST OF CORDOVA. THE 18Z NAM IS NOW PICKING UP ON A SECONDARY LOW
THAT DEVELOPS AND FOLLOWS A SIMILAR TRACK AS THE EC/UK. THE WEAKER
FEATURE MAY BE WHAT THE LOWER RESOLUTION MODELS ARE FIXATING
ON...MOST LIKELY MISSING ON THE STRONGER OF THE TWO LOWS MOVING
TOWARD MIDDLETON ISLAND.

.SHORT TERM FORECAST...SHORT TERM FORECAST IS LARGELY FOCUSED ON THE
THREAT FOR ADDITIONAL LIGHT SNOW. THE COLD TEMPERATURES AND
RELATIVELY DRY AIRMASS (PRECIPITABLE WATER = .15 IN) IS RESULTING IN
VERY SMALL SNOW FLAKES. THE SNOW RATIO EARLIER THIS MORNING WAS A
SURPRISING 13 TO 1...IMPRESSIVE FOR THE DRY AIRMASS IN PLACE. IF
SNOWFALL INTENSITY AND RATIO SHOULD CONTINUE THROUGH THE NIGHT AND
PART OF THE DAY FRIDAY...IT WOULD RESULT IN SEVERAL MORE INCHES OF
SNOW ACROSS SOUTHCENTRAL. THE CHALLENGE WILL BE THE MESOSCALE LOWS
THAT THE MODELS ARE NOT INITIALIZING WELL ON. IT IS A SAFE BET TO
CONTINUE THE LIGHT SNOW WORDING WITH MINIMAL ACCUMULATION THROUGH
FRIDAY UNTIL THE NORTHERN GULF LOW FINALLY BEGINS TO BE DISPLACED BY
THE NEXT SYSTEM.

THE NEXT SYSTEM...A RELATIVELY WEAK FRONT THAT WILL MOVE ACROSS THE
ALEUTIANS THROUGH THE EARLY PART OF THE WEEKEND. A TRIPLE POINT LOW
DEVELOPS SOUTH OF KODIAK EARLY SUNDAY AND MOVES TOWARDS THE NORTH
GULF COAST BY THE END OF THE WEEKEND. AFTER A BRIEF BREAK IN THE
ACTION SATURDAY AND SUNDAY...LOOKS LIKE MORE LIGHT OVERRUNNING
MOISTURE TO RETURN TO SOUTHCENTRAL AND CONTINUED THREAT FOR SNOW
THROUGH EARLY NEXT WEEK WITH THE LOW REMAINING IN THE SAME REGION.

THE SILVER LINING IS THAT TEMPERATURES WILL MODERATE DURING THE
WEEKEND...REBOUNDING ACROSS MUCH OF THE AREA BACK TOWARD NORMAL FOR
THIS TIME OF YEAR.

.SOUTHWEST AK AND BERING SEA...RELATIVELY BENIGN WEATHER OUT WEST
WITH STRATUS AND FLURRIES OVER PORTIONS OF THE SOUTHWEST MAINLAND.
OTHERWISE MOSTLY CLEAR SKIES PREVAIL WITH CONTINUED COLDER THAN
AVERAGE TEMPERATURES. WIDESPREAD SNOW SHOWERS PERSIST OVER THE
MAJORITY OF THE BERING SEA WITH SOME HEAVIER SHOWERS OCCASIONALLY
BEING REPORTED ALONG THE ALASKA PENINSULA.

HIGH PRESSURE BUILDS OVER THE FAR WESTERN BERING SEA AND RAPIDLY
TRAVERSES EAST TOWARD THE ALASKA PENINSULA BY SATURDAY AFTERNOON.
THIS IS AS RESULT OF THE NEXT LOW INVADING FROM THE WEST...AND THE
ASSOCIATED FRONTAL BOUNDARY CHASING THE TRANQUIL WEATHER AWAY. THE
PARENT LOW EVENTUALLY OCCLUDES AND DISSIPATES BY THE END OF THE
WEEKEND...KEEPING THE MAJORITY OF THE MOISTURE ALONG THE CHAIN AND
NOT SPREADING IT INLAND. THE AFOREMENTIONED TRIPLE POINT LOW DEVELOPS
NEAR KODIAK SUNDAY AFTERNOON AND COLD AIR ADVECTION SNOW SHOWERS
RESUME ACROSS THE BERING SEA EARLY NEXT WEEK.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...NONE.
MARINE...GALE 132 150 155 170 172 175 176 180 185 411 412 413 414.
FIRE WEATHER...NONE.

AB NOV 11

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