Small Craft Advisory 1012, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
400 PM AKST SAT NOV 20 2010

.MODEL DISCUSSION...MODELS ARE DOING FINE OVERALL. IN THE EASTERN
PART OF THE FORECAST AREA THEY HOLD TOGETHER WELL THROUGH THREE DAYS.
IN THE WEST THERE ARE SOME SIGNIFICANT DIFFERENCES IN THE BEHAVIOR OF
A LOW MOVING TOWARD THE EASTERN ALEUTIANS AND AK PENINSULA MONDAY AND
TUESDAY. THIS LOW WILL ELONGATE ALONG A FRONTAL BAND AND THIS WILL
LIKELY NOT SHAKE OUT FOR ANOTHER DAY OR SO IN TERMS OF THE MODELS
CONVERGING ON A COMMON SOLUTION...IF AT ALL.

.ANALYSIS AND UPPER LEVELS...VERY WARM ALOFT AND STEEP INVERSIONS.
THIS IS THE BASIC SUMMARY OF THE AREA TODAY. THERE IS A STRONG UPPER
LEVEL WARM RIDGE IN THE NORTHERN PACIFIC WITH ITS AXIS OVER KODIAK
ISLAND AND THEN OVER THE ALASKA RANGE AND INTO THE INTERIOR. THIS IS
THE CAUSE OF THE WARM TEMPERATURES ALOFT. THE STRONG INVERSIONS ARE
THE RESULT OF RADIATIVE LOSS WITH SUCH LITTLE SUNLIGHT THIS TIME OF
YEAR. SOME OF THESE INVERSIONS ARE QUITE IMPRESSIVE. THE KING SALMON
SOUNDING FROM THIS MORNING HAD A SURFACE TEMPERATURE OF 5 ABOVE WHICH
WARMED TO FREEZING AT ONLY 390 FEET. IT CONTINUED TO WARM TO ABOUT 48
DEGREES AT 4200 FEET AND THEN BEGAN TO COOL OFF AGAIN WITH HEIGHT.
THE PATTERN IS SIMILAR OVER MOST OF THE AREA. THIS WILL HAVE A LARGE
IMPACT ON THE PRECIPITATION TYPE AS THE SYSTEM MOVES INTO SOUTHWEST
ALASKA TONIGHT AND THEN INTO SOUTH CENTRAL ALASKA SUNDAY NIGHT AND
MONDAY.

.SHORT TERM FORECAST...THE BIGGEST CONCERN IS THE THREAT OF FREEZING
RAIN IN THE BRISTOL BAY REGION AND KUSKOKWIM VALLEY AFTER MIDNIGHT
TONIGHT. THE SYSTEM MOVING INTO SOUTHWEST AREA WILL BRING RAIN TO THE
KUSKOKWIM DELTA AND ALASKA PENINSULA BUT THE INVERSION IN THE BRISTOL
BAY ZONE WILL LIKELY CAUSE THE PRECIPITATION TO BE FREEZING RAIN
UNTIL THE INVERSION BREAKS AND PRECIPITATION TURNS TO ALL RAIN. THIS
WILL OCCUR FIRST ALONG THE COAST EXCEPT FOR THE DILLINGHAM AREA WHICH
TENDS TO SEE FREEZING RAIN PERSIST LONGER THAN THE OTHER AREAS. THE
KUSKOKWIM VALLEY IS A T RISK FOR A LONGER DURATION OF FREEZING RAIN
SINCE THE LAYER OF BELOW FREEZING AIR IS MUCH THICKER. THE MCGRATH
SOUNDING SHOWED A FREEZING LEVEL OF 2800 FEET THIS MORNING AND THAT
LAYER WILL HAVE TO BE WARMED FOR THE FREEZING RAIN TO TURN TO RAIN.
THE WARM AIR LAYER IS TOO THICK FOR AREAS TO SEE SNOW FROM THIS
SYSTEM...EXCEPT FOR PLACES AT LEAST 6000 FT ABOVE SEA LEVEL.

FOG IS A DISTINCT POSSIBILITY AGAIN TONIGHT IN SOUTH CENTRAL ALASKA
AS THERE WILL BE ONLY HIGH CLOUDS AROUND WHICH MAY NOT BE ENOUGH TO
INHIBIT DEVELOPMENT.

.LONG TERM FORECAST...THE FREEZING RAIN THREAT MOVES INTO SOUTH
CENTRAL ALASKA SUNDAY NIGHT AND MONDAY. IT IS LOOKING INCREASINGLY
LIKELY THAT THERE WILL BE FREEZING RAIN FOR A GOOD PART OF SOUTH
CENTRAL ALASKA FROM THE KENAI PENINSULA INTO THE SUSITNA VALLEY. THE
BIG QUESTION IS HOW FAST IT WILL TURN INTO RAIN. THE FREEZING LEVEL
WHERE THE TEMPERATURE WARMED ABOVE FREEZING IN ANCHORAGE THIS MORNING
WAS AROUND 1850 FEET. THIS LAYER WILL NEED TO BE DISSIPATED FOR THE
FREEZING RAIN TO TURN TO ALL RAIN. THAT WILL BE THE KEY TO HOW LONG
THE FREEZING RAIN PERSISTS IN ANY GIVEN AREA.
A SIGNIFICANT ITEM TO NOTE IMPACT WISE IS THAT SOUTH CENTRAL ALASKA
HAS BEEN IN THE DEEP FREEZE FOR MANY DAYS. THERE IS SO MUCH COLD IN
THE GROUND THAT THE RAIN WILL FREEZE ON CONTACT WITH ITEMS SUCH AS
ROADWAYS AND TREES FOR A WHILE AFTER AIR TEMPERATURES ARE ABOVE
FREEZING. IN SHORT ICE ACCUMULATION IS EXPECTED WITH THIS FREEZING
RAIN EVENT LATE SUNDAY NIGHT INTO MONDAY. THE BIG UNCERTAINTY IS HOW
MUCH.

BY MONDAY THE FREEZING RAIN SHOULD BE ALL RAIN IN SOUTHWEST ALASKA.
OVERALL THE PATTERN FOR NEXT WEEK SUPPORTS WARMER THAN NORMAL
TEMPERATURES OVER THE FORECAST AREA.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...NONE.
MARINE...GALE 160 170 176 180 185.
FIRE WEATHER...NONE.

EZ NOV 10