Small Craft Advisory 992, NWS Anchorage, AK

AREA FORECAST DISCUSSION
SOUTHCENTRAL AND SOUTHWEST ALASKA
230 PM AKST SAT DEC 13 2008

.ANALYSIS AND UPPER LEVELS...

TWO LONG WAVE TROUGHS ARE ESTABLISHED ON BOTH SIDES OF ALASKA...ONE
OVER WESTERN CANADA AND A SECOND OVER EASTERN SIBERIA. A BLOCKING
RIDGE IS IN PLACE OVER THE MAINLAND OF ALASKA SENDING SYSTEMS
NORTHWARD INTO THE CENTRAL AND WESTERN BERING SEA. AT THE SURFACE...A
1054 MB HIGH PRESSURE IS CENTERED OVER NORTH CENTRAL ALASKA WHICH HAS
DRIED OUT THE REGION.

.MODEL DISCUSSION...GOOD MODEL AGREEMENT AND CONSISTENCY THROUGH
SUNDAY NIGHT LEADS TO HIGH FORECAST CONFIDENCE. BEYOND THAT
DIFFERENCES BEGIN TO SHOW UP OUT WEST. IT SEEMS THAT THE MODELS ARE
HAVING A DIFFICULT TIME WITH THE LONG FETCH OF MOISTURE IN THE NORTH
PACIFIC AND THE IMPULSES EMBEDDED WITHIN THE SOUTHERLY FLOW. THIS IS
MOST EVIDENT BEYOND 48 HOURS WHEN THE THE NAM AND GFS ARE IN RELATIVE
AGREEMENT...BUT THE UKMET AND ECMWF PLACE THE LOW 350 MILES TO THE
NORTHWEST. FROM THIS POINT...THE 4 MODELS DIVERGE WILDLY AS THE LOW
WEAKENS AND APPROACHES THE EASTERN ALEUTIANS. FURTHER WEST...A
FRONTAL BOUNDARY IS SEPARATING THE ARCTIC AIRMASS OVER EASTERN
SIBERIA FROM THE ALEUTIANS WILL MAKE SOME PROGRESS INTO THE WESTERN
BERING SEA. THE GFS IS TRENDING THIS FRONT STRONGER WITH EACH RUN SO
WE WILL BE NUDGING TOWARD THE GFS...WHICH WILL BUMP UP THE WINDS IN
THE BERING SEA OFFSHORES TO STORM FORCE. NO CHANGES ARE NEEDED IN THE
EAST DOMAIN...ONLY MINOR TWEAKS TO CLOUD COVER AND LOCAL WINDS.

.SHORT TERM FORECAST...

SOUTHCENTRAL ALASKA...
A STABLE WEATHER PATTERN IS CURRENTLY IN PLACE...THE CHALLENGE IN THE
FORECAST WILL BE DETERMINING WHERE FOG WILL DEVELOP AND THE STRENGTH
OF THE GAP WINDS. ALONG THE COAST...THE MAXIMUM PRESSURE GRADIENT IS
THIS MORNING SO WINDS WILL DIMINISH TODAY THROUGH MONDAY WHEN A WEAK
SHORTWAVE MOVES ACROSS SOUTH CENTRAL AND INCREASES THE PRESSURE
GRADIENT ACROSS THE MOUNTAINS. CLEAR SKIES WILL PREVAIL THROUGH
MONDAY WITH THE EXCEPTION BEING AREAS OF LOW CLOUDS AND FOG. FOG HAS
BEEN LIMITED TO THE ANCHORAGE BOWL AND WE ARE NOT SEEING ANY
INDICATIONS THAT IT WILL SPREAD ANYWHERE ELSE. CONDITIONS ARE SLOWLY
IMPROVING EARLY THIS AFTERNOON IN ANCHORAGE BUT DO NOT HAVE ANY GOOD
REASONS WHY MORE LOW CLOUDS AND FOG WILL RETURN LATER THIS EVENING.
THE CLOUDS ARE LIMITING OVERNIGHT LOW TEMPERATURES LOCALLY. OUTSIDE
OF ANCHORAGE...TEMPERATURES ARE FALLING TO 5 TO 30 BELOW.

MONDAY...A WEAK SHORT WAVE MOVES FROM THE NORTHWEST ACROSS
SOUTHCENTRAL ALASKA BRINGING SOME CLOUDS...BUT ONLY A SLIGHT CHANCE
OF SNOW IN THE SUSITNA VALLEY. SKIES WILL QUICKLY CLEAR OUT ON THE
BACK SIDE OF THE TROUGH AS HIGH PRESSURE SETTLES BACK OVER THE AREA.

SOUTHWEST ALASKA...
A STRONG SUBSIDENCE INVERSION IS KEEPING A STRATUS DECK BETWEEN 500
AND 1000 FEET OVER MOST OF THE KUSKOKWIM VALLEY. LIGHT SNOW IS
FALLING OUT OF THE STRATUS IN MANY PLACES BUT NO ACCUMULATION IS
EXPECTED. DENSE FOG HAS BEEN ENTRENCHED OVER THE KUSKOKWIM DELTA
ALTHOUGH NORTHEASTERLY WINDS ARE INCREASING WHICH WILL HELP ERODE
WHAT IS LEFT OF THE FOG. ONE FORECAST CHALLENGE IN THIS DOMAIN WILL
BE THE STRENGTH AND IMPACT OF THE SHORT WAVE MOVING ONSHORE SUNDAY
NIGHT. AM LEANING TOWARD THE GFS WHICH HAS MORE PRECIPITATION
SPREADING INLAND THROUGH MONDAY. THIS IS IMPORTANT BECAUSE IT COULD
LEAD TO BLIZZARD CONDITIONS ALONG THE COAST AS EARLY AS MIDNIGHT ON
SUNDAY.

ON THE BACK SIDE OF THE SHORT WAVE...THE MODELS DIVERGE SO CONFIDENCE
IS NOT HIGH BEYOND TUESDAY. IT APPEARS THAT THE HIGH PRESSURE WILL
REBUILD OVER THE INTERIOR WHICH WILL KEEP THE AREA MOSTLY CLEAR AND
COLD INTO MID WEEK.

BERING SEA/ALEUTIANS...A FRONT IS WEAKENING OVER THE EASTERN
ALEUTIANS. TEMPERATURES ARE WARM ENOUGH TO BRING RAIN TO DUTCH HARBOR
THIS AFTERNOON. THIS WILL CHANGE AS COLD AIR BEGINS TO MOVE FROM WEST
TO EAST ACROSS THE BERING SEA TONIGHT. 1036 MB HIGH PRESSURE WILL
BUILD OVER THE CENTRAL BERING AHEAD OF A STORM FORCE FRONT OVER THE
WESTERN ALEUTIANS SUNDAY NIGHT AND MONDAY. THIS FRONT WILL QUICKLY
WEAKEN AS IT PROGRESSES WEST AND ENCOUNTERS THE LONG WAVE BLOCKING
RIDGE OVER THE ALASKA MAINLAND.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...NONE.
MARINE...GALE WARNING...155 165 170 171 172 179 180.
         STORM WARNING...176.
FIRE WEATHER...NONE.

AB DEC 08