Small Craft Advisory 876, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
500 AM AKST FRI NOV 8 2013

.ANALYSIS AND UPPER LEVELS...

A WEAK CUTOFF LOW CONTINUES TO SPIN OVER THE NORTHERN GULF WHICH HAS
BROUGHT LIGHT PRECIPITATION TO PART OF THE ALASKA GULF COAST AS IT
SLOWLY MOVES SOUTH TOWARD THE NORTH PACIFIC. AT THE SAME TIME...AN
UPPER LEVEL RIDGE OF HIGH PRESSURE CONTINUES TO AMPLIFY AND BUILD
ACROSS THE ALASKA MAINLAND. A NORTH PACIFIC (NPAC) LOW IS BEGINNING
TO INTERACT WITH A LOW OVER THE RUSSIAN FAR EAST AND WILL DRIVE A
WARM FRONT ACROSS THE WESTERN BERING SEA AND THE ALASKA MAINLAND.
SATELLITE DERIVED PWAT VALUES ASSOCIATED WITH THE NORTH PACIFIC LOW
RANGE BETWEEN THREE QUARTERS OF AN INCH TO AN INCH AND A QUARTER.
THESE VALUES ARE WELL OVER 200% OF THE NORMAL RANGE FOR THIS TIME OF
THE YEAR. THE LONG-WAVE TROUGH OVER KAMCHATKA IS BEING INFLUENCED BY
THESE TWO LOWS AND HAS BEGUN TO AMPLIFY. BECAUSE OF THE HIGH PWAT
VALUES ASSOCIATED WITH THIS SYSTEM AND THE QUICKLY AMPLIFYING
TROUGH...THE WARM FRONT THAT WILL CLASH WITH THE RIDGE OF HIGH
PRESSURE WILL ADVECT WARM AND MOIST SUBTROPICAL AIR TOWARD THE
MAINLAND OF ALASKA...POTENTIALLY LEADING TO A SIGNIFICANT RAINFALL
EVENT ON SUNDAY THROUGH EARLY MONDAY MORNING.

.MODEL DISCUSSION...

MODELS ARE IN GOOD AGREEMENT THROUGH THE SHORT TERM AS A RIDGE OF
HIGH PRESSURE BUILDS ACROSS SOUTHWEST AND SOUTHCENTRAL ALASKA
THROUGHOUT THE DAY. THE RIDGE WILL SHUT-OFF THE COLD AIR DIVING DOWN
FROM THE ARCTIC AS WINDS WILL BECOME MORE WESTERLY TO SOUTHWESTERLY
ALOFT. THE NPAC LOW THAT IS CURRENTLY INTERACTING WITH THE LOW OVER
THE RUSSIAN FAR EAST WILL PUSH A FRONT ACROSS ALASKA ON FRIDAY AND
THE EARLY PARTS OF SATURDAY. THIS FRONT WILL BE COLLIDING UP AGAINST
THE AMPLIFYING RIDGE AND PUSH IT EAST TOWARD CANADA AND THE WEST
COAST OF THE CONUS. AS THE RIDGE PUSHES EAST...AN UPPER LEVEL TROUGH
WILL AMPLIFY RAPIDLY OVER THE BERING SEA AND CENTRAL ALEUTIANS
CAUSING SUBTROPICAL AIR TO GET ADVECTED NORTHWARD TOWARDS
ALASKA...BRINGING ABOVE AVERAGE TEMPERATURES TO THE REGION. A COLD
FRONT WILL PUSH THROUGH ON SUNDAY WITH COLD AIR FILTERING IN BEHIND
QUICKLY. BEFORE THE COLD FRONT PUSHES THROUGH AND COLD AIR SETTLES
INTO THE REGION...RAIN WILL BE EXPECTED ACROSS MUCH OF THE SOUTHWEST
AND SOUTH CENTRAL DUE TO THE SUBTROPICAL AIR MOVING NORTHWARD AHEAD
OF THE FRONT. MODELS DISAGREE ON HOW FAST THE COLD AIR MOVES IN
BEHIND THE FRONT...THEREFORE IT IS DIFFICULT AT THIS TIME TO
DETERMINE HOW MUCH RAIN/SNOWFALL WILL BE RECEIVED. MODELS AGREE THAT
RAINFALL BETWEEN ONE HALF OF AN INCH TO AN INCH AND A QUARTER WILL BE
POSSIBLE SUNDAY MORNING THROUGH LATE SUNDAY NIGHT ACROSS SOUTHWESTERN
AND SOUTHCENTRAL ALASKA. THERE SHOULD BE A TRANSITION TO SNOW ON THE
BACKSIDE OF THE COLD FRONT...BUT THE EXACT DETAILS ARE A LITTLE TOO
FAR OUT TO PINPOINT.

.SHORT TERM FORECAST...

SOUTHCENTRAL AND SOUTHWEST...TODAY SHOULD BE ANOTHER CLEAR AND CRISP
DAY ACROSS THE SOUTHCENTRAL AS A RIDGE OF HIGH PRESSURE BUILDS INTO
THE REGION. HOWEVER...THESE CONDITIONS WILL NOT LAST LONG AS TWO
AREAS OF LOW PRESSURE WILL INTERACT AND DRIVE A WARM FRONT TOWARD THE
AREA LATE THIS AFTERNOON. OVER THE LOWER KUSKOKWIM VALLEY...POCKETS
OF FREEZING RAIN COULD BE SEEN THIS EVENING AS THE WARM AIR OVERRUNS
THE COLD AIR THAT HAS SETTLED IN THE VALLEY. LESS THAN A TENTH OF AN
INCH OF FREEZING RAIN IS EXPECTED AS TEMPERATURES WILL WARM ABOVE
FREEZING AROUND MIDNIGHT. THE RIDGE WILL BE SHIFTED EASTWARD BY THIS
WARM FRONT APPROACHING FROM THE WEST WHICH WILL BRING SUBTROPICAL AIR
ONCE AGAIN TO THE REGION. THIS WILL ALLOW TEMPERATURES TO WARM WELL
ABOVE AVERAGE (5 TO 10 DEGREES IN SOME AREAS) FOR SATURDAY AND
SUNDAY. AS THE SUBTROPICAL AIR GAINS A FOOTHOLD...A COLD FRONT WILL
BEGIN TO APPROACH LATE SATURDAY EVENING. AS THE TWO AIR MASSES
INTERACT...PERIODS OF MODERATE TO HEAVY PRECIPITATION WILL OCCUR. THE
PRECIPITATION WILL START OFF AS RAIN IN MOST PLACES DUE TO THE WARM
SUBTROPICAL AIR PUSHED INTO THE REGION...BUT WILL LIKELY TRANSITION
TO SNOWFALL ON THE BACK END OF THE COLD FRONT. THE ONE EXCEPTION WILL
BE PLACES EAST OF THE CHUGACH RANGE...WHERE COLD TEMPERATURES WILL
RESIDE AND ONLY SNOWFALL WILL BE SEEN. THE MAIN VARIABLE IN THIS CASE
WILL BE HOW FAST THE COLD AIR FILTERS IN BEHIND THE COLD FRONT.
RAINFALL AMOUNTS BETWEEN ONE HALF OF AN INCH AND AN INCH WILL BE
COMMON...WITH SOME AREAS SEEING HIGHER VALUES FROM SUNDAY MORNING
THROUGH MONDAY MORNING. THIS WILL BE AN UNUSUAL NOVEMBER HEAVY
RAINFALL...WITH SEVERAL PLACES LIKELY TO SET RECORD RAINFALL TOTALS
ON SUNDAY. SNOWFALL AMOUNTS AT THIS TIME ARE TRICKY WITH THIS
SITUATION AND FURTHER DETAILS WILL BECOME CLEARER AS THE EVENT GETS
CLOSER.

BERING SEA AND ALEUTIAN ISLANDS...THE AFOREMENTIONED LOWS WILL MOVE
ACROSS THE NORTHERN BERING SEA THIS AFTERNOON THROUGH LATE SATURDAY
EVENING. THIS WILL BRING A LARGE SWATH OF GALES AND A FEW AREAS OF
STORM FORCE WINDS TO THE AREA. SCATTERED SHOWERS WILL BE SEEN ACROSS
THE ALEUTIANS THROUGHOUT THE DAY AS WARM AND MOIST AIR MOVES INTO THE
REGION. BY LATE FRIDAY NIGHT...A COLD FRONT WILL BEGIN TO MOVES FROM
THE WESTERN ALEUTIANS TOWARDS THE EASTERN ALEUTIANS BRINGING PERIOD
OF MODERATE TO HEAVY RAINFALL. SNOW SHOWERS WILL LIKELY BE SEEN
BEHIND THE COLD FRONT SUNDAY AFTERNOON THROUGH MONDAY MORNING WITH
LITTLE TO NO ACCUMULATION.

.LONG TERM FORECAST...

AN OMEGA BLOCKING PATTERN WILL START OFF THE EXTENDED PERIOD WHICH
WILL BRING ABOVE NORMAL TEMPERATURES BACK TO THE REGION BY WEDNESDAY.
ONCE THE PATTERN BEGINS TO BREAK DOWN ON THURSDAY...COLD AIR WILL
DROP OUT OF THE ARCTIC AND BRING A SIGNIFICANT COOL DOWN TO THE
REGION. THERE IS STILL A BIT OF MODEL DISAGREEMENT ON HOW THE OMEGA
PATTERN BREAKS DOWN...BUT A COLD SURGE DOES APPEAR TO BE ON THE
HORIZON. AS THE COLD AIR SURGES DOWN BEHIND A COLD
FRONT...PRECIPITATION SHOULD BE PRETTY WIDESPREAD...LEADING TO RAIN
ON THURSDAY TRANSITIONING TO SNOWFALL BY FRIDAY ACROSS MOST OF THE
SOUTHWEST AND SOUTHCENTRAL.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...COASTAL FLOOD WARNING...155
MARINE...STORM...178
         GALE...165 170 172 173 174 175 176 177 179 180 181 185
FIRE WEATHER...NONE

MMC NOV 13