Small Craft Advisory 1002, NWS Anchorage, AK

AREA FORECAST DISCUSSION
SOUTHCENTRAL AND SOUTHWEST ALASKA
500 PM AKST WED DEC 9 2009

.ANALYSIS AND UPPER LEVELS...
LARGE OMEGA BLOCK REMAINS ENTRENCHED OVER THE ALASKA REGION TODAY.
STRONG UPPER LEVEL HIGH REMAINS TO THE EAST WITH A WEAKENING UPPER
LEVEL LOW IN THE WEST OVER THE BERING SEA. THIS PATTERN IS REFLECTED
VERY WELL TO THE SURFACE...WITH HIGH PRESSURE DOMINATING OVER THE
EAST AND A SERIES OF FAIRLY WEAK SURFACE LOWS ROTATING AROUND EACH
OTHER IN THE BERING SEA.  THE MOST ACTIVE WEATHER CAN BE FOUND ALONG
THE BORDER BETWEEN THE HIGH AND THE LOW THROUGH THE ALASKAN
PENINSULA. AS THE WEEK PROGRESSES THE PATTERN WILL GRADUALLY SHIFT
WESTWARD...SHIFTING THE WEATHER WESTWARD AS WELL.  THIS WILL BRING
COLDER AIR INTO THE SURFACE LOWS IN THE WESTERN ALEUTIANS AND PRODUCE
STRONG SYSTEMS...BUT NO WARNINGS ON THE HORIZON.

.MODEL DISCUSSION...
MODELS WERE IN GOOD AGREEMENT IN THE SHORT TERM TODAY...WITH SOME
MINOR DIFFERENCES ON THE SURFACE LOWS WOBBLING THROUGH THE ALEUTIANS.
THE FIRST MAJOR DIFFERENCES DO NOT SHOW UP UNTIL 84 HOURS WHERE THE
GFS AND THE ECMWF DIFFER ON THE TRACK OF THE NEXT LOW...HOWEVER THIS
PERIOD IS BEYOND OUR IMMEDIATE CONCERN. FOR THE SHORT TERM
FORECAST...GFS WAS PREFERRED FOR THE WESTERN ZONES FOR LOCATION OF
THE LOWS IN THE ALEUTIANS...WHILE NAM WAS PREFERRED IN THE EAST FOR
BETTER RESOLUTION ON TEMPERATURES AND FOG.

.SHORT TERM FORECAST...
OVER THE MAINLAND...MAIN CONCERNS TODAY ARE TEMPERATURES AND
FOG/STRATUS ALONG THE COAST.  FOG IS THE MAIN CHALLENGE WITH VERY
LIGHT FLOW IN MOST REGIONS.  ALONG THE KENAI PENINSULA AND GULF COAST
THE FOG IS MAINLY ADVECTING IN FROM THE WATER SO IT IS HIGHLY
DEPENDENT ON THE DIRECTION OF THE WINDS.  AS THE HIGH MOVES OVERHEAD
IT WILL BE EVEN MORE DIFFICULT TO PREDICT THE DIRECTION OF THE FOG.
BY THURSDAY AFTERNOON SLIGHT DOWN INLET FLOW SHOULD DEVELOP...HELPING
TO PUSH THE FOG BACK OUT TO SEA.  HOWEVER...THIS PATTERN HAS BEEN
RELUCTANT TO SET UP OVER THE PAST SEVERAL DAYS.  IN THE ANCHORAGE
AREA FOG SEEMS TO BE FAIRLY DIURNAL WITH DENSE FOG INCREASING IN THE
MORNING HOURS WITH INCREASED MIXING FROM THE SUN AND EARLY MORNING
TRAFFIC.  HIGH TIDE HAS ALSO BEEN PROLONGING THE FOG WITH THE
POSSIBILITY OF THE MORNING FOG LASTING ALL DAY UNTIL THE DOWN INLET
FLOW SETS UP.  OTHER AREAS IN SOUTHCENTRAL HAVE BEEN LUCKY TO ESCAPE
THE FOG...SUCH AS THE SUSITNA VALLEY WHICH HAS BEEN EXPERIENCING A
SLIGHT DRAINAGE FLOW FROM THE NORTH...OR THE COPPER RIVER BASIN WHERE
THE MOISTURE SOURCE IS LACKING.  WESTERN AREAS ARE NOT HAVING AS MUCH
TROUBLE WITH THE FOG TODAY...WITH THE EXCEPTION OF SOME PATCHES IN
THE BRISTOL BAY REGION.  THIS IS DUE TO THE FACT THAT THE FLOW IS
PRIMARILY OFFSHORE...ALTHOUGH LAKE ILIAMNA REMAINS OPEN WATER WHICH
IS CONTRIBUTING TO THE LOW LEVEL MOISTURE IN THE BRISTOL BAY AREA.

AS FOR TEMPERATURE CONCERNS...THESE ARE MAINLY OUT WEST WHERE
TREMENDOUS TEMPERATURE INVERSIONS HAVE SET UP IN THE VALLEYS.  THE
TEMPERATURE INVERSION AT MCGRATH THIS MORNING WAS 54 DEGREES
FAHRENHEIT...AS A RESULT TEMPS ALONG THE MOUNTAINS WILL BE MUCH
HIGHER.

BERING SEA AND ALEUTIANS...
A STRONG GRADIENT HAS SET UP BETWEEN THE HIGH PRESSURE OVER THE
MAINLAND AND THE LOWS IN THE ALEUTIANS AND THIS IS BRINGING STRONG
WINDS TO THE ALASKA PENINSULA TODAY.  AS THE CURRENT SURFACE LOW
WEAKENS SLIGHTLY AND MOVES OFF TO THE WEST EXPECT WINDS TO DECREASE
TOMORROW MORNING. SOUTHWEST FLOW HAS BROUGHT UNSEASONABLY WARM
TEMPERATURES TO MOST OF THE BERING...SO EXPECT PRIMARILY RAIN NOT
SNOW THROUGH THE PERIOD.

.LONG TERM FORECAST...
NOT TOO MUCH CHANGE IN THE LONGTERM...ALTHOUGH PERHAPS A BIT OF A
SHIFT AT THE BEGINNING OF NEXT WEEK AS THE HIGH MOVES WESTWARD.  THIS
SHOULD ALLOW COLDER AIR TO MOVE IN FROM CANADA AND OFFSHORE FLOW MAY
RETURN THE BERING TO MORE SEASONABLE TEMPERATURES.  FOR THE MAINLAND
THE DRY PATTERN WILL CONTINUE THROUGH THE WEEKEND...WITH THE FIRST
SHOT AT PRECIPITATION AT THE EARLY PART OF NEXT WEEK.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...NONE
MARINE...GALES 160 165 170 179 180 185
FIRE WEATHER...NONE

LEWF DEC 09