Small Craft Advisory 757, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
541 AM AKDT MON SEP 28 2015

.ANALYSIS AND UPPER LEVELS...
RAIN AND LOW CLOUDS PERSIST ACROSS MUCH OF MAINLAND ALASKA THIS
MORNING BENEATH MOIST SOUTHWESTERLY FLOW ALOFT. AN EMBEDDED
SURFACE LOW PUSHING INTO THE CENTRAL INTERIOR IS HELPING TO
GENERATE STRONG SOUTHEASTERLY GAP WINDS THROUGH FAVORED AREAS OF
SOUTHCENTRAL ALASKA. PRECIPITATION HAS TAPERED OFF OVER THE
KUSKOKWIM DELTA BEHIND A TRAILING COLD FRONT WHICH EXTENDS ACROSS
THE SOUTHWEST MAINLAND INTO THE EASTERN BERING SEA ACCOMPANIED BY
GUSTY NORTHWEST WINDS. FARTHER UPSTREAM...A POTENT TROUGH HAS
DROPPED INTO THE NORTHERN BERING OUT OF EASTERN RUSSIA AND IS
POISED TO BRING COLDER TEMPERATURES AND POTENTIAL SNOWFALL TO
SOUTHCENTRAL AK OVER THE COMING DAYS AS IT AMPLIFIES AND SWINGS
ACROSS THE MAINLAND.

&&

.MODEL DISCUSSION...
THE DOMINANT FEATURE IMPACTING THE SHORT TERM FORECAST WILL BE THE
TROUGH CURRENTLY ENTERING THE NORTHERN BERING SEA. MODELS CONTINUE
TO HAVE A GOOD HANDLE ON THE SYNOPTIC EVOLUTION OF THIS SYSTEM AS
IT AMPLIFIES OVER THE WESTERN MAINLAND TONIGHT AND THEN SWINGS
THROUGH SOUTHCENTRAL AK BY TUESDAY NIGHT...HOWEVER THEY CONTINUE
TO STRUGGLE WITH SEVERAL DETAILS WHICH WILL HAVE LARGE
IMPLICATIONS ON THE POTENTIAL FOR SNOW FOR INTERIOR PORTIONS OF
SOUTHCENTRAL INCLUDING THE ANCHORAGE AREA. THE TREND OVER THE
PAST 24 HOURS HAS BEEN TO BRING THE TROUGH SIGNIFICANTLY FARTHER
SOUTH AND TRY TO DEVELOP A CUTOFF LOW OVER THE WESTERN GULF. THIS
CALLS INTO QUESTION THE TIMING OF THE TROUGH SWINGING THROUGH
SOUTHCENTRAL AND HOW QUICKLY THE COLDER AIR ARRIVES...WITH THE NAM
ON THE FASTER END OF THE ENVELOPE AND THE GFS/EC SOLUTIONS
LAGGING BEHIND. CONFIDENCE REMAINS MODERATE TO HIGH THAT LOCATIONS
AT SEA LEVEL WILL SEE SNOW AT SOME POINT ON TUESDAY...BUT THE
CONTINUED QUESTIONS REVOLVING AROUND THESE DETAILS LEND LOW
CONFIDENCE FOR SNOW AMOUNTS.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 AND 2)...
RAIN WILL CONTINUE IN THE SOUTHWEST FLOW ALOFT INTO TUESDAY. AT
THIS TIME NO FLOODING IS EXPECTED BUT STREAMS WILL BE RISING DUE
TO THE PERSISTENT RAINFALL THE PAST FEW DAYS.

THE BIG STORY COMES TOMORROW WITH THE ARRIVAL OF COLD AIR FROM
THE WEST AND A RAPIDLY DEVELOPING LOW MOVING INTO THE REGION FROM
THE NORTH PACIFIC. THIS LOW IS CURRENTLY SOUTH OF THE EASTERN
ALEUTIANS AND IS LOOKING QUITE IMPRESSIVE ON THE WATER VAPOR
SATELLITE IMAGERY. THIS IS A TRICKY SITUATION TRYING TO HIT THE
PRECIPITATION TYPE AND THE TIMING OF IT ALL. IN THE END IT ALL
BOILS DOWN TO THE COLD AIR AND WHEN IT GETS COLD ENOUGH TO
SNOW...AND IF THERE IS SUFFICIENT MOISTURE AT THAT TIME.

MODELS HAVE BEEN VERY CONSISTENT FOR THREE DAYS IN BRINGING AN
ACCUMULATING SNOWFALL INTO SOUTHCENTRAL ALASKA SOMETIME TUESDAY
INTO TUESDAY NIGHT. THE GFS AND NAM MODELS IN PARTICULAR HAVE BEEN
TRYING TO TURN THE RAIN TO SNOW EARLY TUESDAY FROM COOK INLET
NORTHWARD THROUGH THE MATSU VALLEYS...INCLUDING THE ANCHORAGE
AREA. THIS DOES NOT SEEM OVERLY REASONABLE LOOKING AT THE TRACK TO
COLD AIR IS TAKING AND THE TIMING OF THE SYSTEMS. WHAT LOOKS MOST
LIKELY AT THIS TIME IS THAT THE AREAS OF SOUTHCENTRAL NEAR SEA
LEVEL WILL SEE ALL RAIN IN THE MORNING AND THEN MIX WITH SNOW IN
THE AFTERNOON AS THE TEMPERATURES DROP. BY LATE AFTERNOON OR EARLY
EVENING THERE SHOULD BE SUFFICIENT COLD AIR TO TURN PRECIPITATION
TO ALL SNOW. THERE SHOULD BE ENOUGH SNOW THROUGH THE EVENING TO
ACCUMULATE IN THESE AREAS BEFORE IT TAPERS OFF TO SNOW SHOWERS
AFTER MIDNIGHT.

HIGHER ELEVATIONS ABOVE 1000 TO 2000 FT ARE A DIFFERENT STORY
WITH THE TIMING. THESE AREAS ARE LIKELY TO SEE THE PRECIPITATION
TURN TO ALL SNOW BY TUESDAY AFTERNOON WITH PERHAPS MUCH HIGHER
SNOW ACCUMULATION AMOUNTS. THERE MAY BE ENOUGH ACCUMULATION TO
WARRANT A SNOW ADVISORY FOR HIGHER ELEVATIONS BUT THERE IS STILL
ENOUGH UNCERTAINTY TO ACTUAL AMOUNTS AT THIS TIME TO HOLD OFF ON
THAT FOR NOW.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)...
THE COLD FRONT THAT HAS BROUGHT WIDESPREAD RAIN TO THE REGION
SINCE YESTERDAY WILL CONTINUE TO BRING RAIN TO BRISTOL BAY AND THE
KUSKOKWIM VALLEY THIS MORNING. COLD AND DRY AIR MOVING IN BEHIND
THE FRONT WILL ALLOW FOR AN AN ABRUPT END TO THE PRECIPITATION IN
THE DILLINGHAM AREA BY MID MORNING...FOLLOWED BY KING SALMON BY
THIS AFTERNOON. AN UPPER LEVEL WAVE QUICKLY MOVING IN BEHIND THIS
FRONT WILL CAUSE SHOWERS TO REFORM OVER THE AREA THIS AFTERNOON
AND EVENING...AND WITH TEMPERATURES COOLING OFF SIGNIFICANTLY
BEHIND THE FRONT THIS PRECIPITATION WILL LIKELY FALL IN THE FORM
OF RAIN MIXED WITH SNOW. THIS UPPER LEVEL TROUGH WILL PASS OVER
THE REGION AS IT DIVES TOWARDS THE GULF ON TUESDAY...KEEPING RAIN
AND SNOW SHOWERS IN THE AREA THROUGH TUESDAY EVENING. IT DOES NOT
APPEAR AT THIS POINT HOWEVER THAT THESE WILL BE MORE THAN ISOLATED
TO SCATTERED IN COVERAGE...SO SIGNIFICANT ACCUMULATIONS ARE NOT
EXPECTED IN THE LOWER ELEVATIONS. WESTERN SLOPES OF THE ALASKA
RANGE HOWEVER WILL EXPERIENCE DEEP NORTHWESTERLY FLOW INTO
WEDNESDAY...ALLOWING FOR MORE SIGNIFICANT ACCUMULATIONS.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...
THE COLD FRONT WILL CONTINUE TO DISSIPATE OVER THE EXTREME
SOUTHEAST BERING SEA AND ALASKA PENINSULA EARLY THIS
MORNING...ALLOWING FOR RAIN TO PERSIST. BY LATE THIS MORNING...THE
FRONT WILL HAVE PASSED THE AREA...PRODUCING NORTHWESTERLY FLOW
THAT WILL PERSIST IN THE REGION THROUGH TUESDAY. THE UPPER LEVEL
TROUGH CURRENTLY APPROACHING THE PRIBILOFS WILL CAUSE RAIN AND
SNOW SHOWERS TO REFORM BEHIND THE FRONT...AND WITH STRONG COLD AIR
ADVECTION BEHIND THE TROUGH THESE SHOWERS WILL PERSIST INTO MID
WEEK. OVER THE WESTERN BERING/ALEUTIANS RIDGING WILL KEEP WINDS
GENERALLY LIGHT WITH OTHERWISE QUIET WEATHER INTO TUESDAY.
STARTING TUESDAY EVENING A FRONTAL SYSTEM ASSOCIATED WITH A LOW
CENTERED NEAR KAMCHATKA WILL BEGIN TO APPROACH THE WESTERN
ALEUTIANS...BRINGING RAIN AND STRONG SOUTHERLY WINDS TO THE
WESTERN BERING.

&&

.LONG TERM FORECAST (DAYS 3 THROUGH 7)...
THE LOW PRESSURE SYSTEM IN THE NORTHERN GULF WILL TRACK TO EAST
AND WEAKEN RAPIDLY ALLOWING RAIN AND SNOW SHOWERS TO TAPER OFF
THROUGH THE DAY WEDNESDAY AND END WEDNESDAY NIGHT. WEST TO
NORTHWESTERLY GAP WINDS WILL START THE DAY RATHER BREEZY
WEDNESDAY THEN GRADUALLY DIMINISH THROUGH WEDNESDAY NIGHT.
MEANWHILE A STRONG FRONTAL SYSTEM WILL LIFT THROUGH THE WESTERN
ALEUTIANS WEDNESDAY MORNING AND TRACK TO THE CENTRAL ALEUTIAN AND
BERING WEDNESDAY NIGHT THROUGH THURSDAY MORNING BRINGING A SWATH
OF STORM FORCE WINDS WITH IT.

AS THE LOW IN THE BERING DEEPENS RAPIDLY IT WILL STRENGTHEN AN
UPPER LEVEL RIDGE OVER THE EASTERN ALEUTIANS WEDNESDAY. THE RIDGE
WILL CONTINUE TO AMPLIFY SHARPLY AS IT IS PUSHED EAST OVER THE
ALASKA PENINSULA AND BRISTOL BAY WEDNESDAY NIGHT...WESTERN ALASKA
THURSDAY AND INTO SOUTH CENTRAL ALASKA THURSDAY NIGHT. THE STRONG
FRONT IN THE BERING WILL REACH THE WEST COAST AROUND THURSDAY
AFTERNOON THEN PUSH INTO SOUTHWEST ALASKA THURSDAY NIGHT AND
FRIDAY.

MODELS REMAIN IN FAIR AGREEMENT THROUGH FRIDAY THEN RAPIDLY DIVERGE
THEREAFTER WITH SOLUTIONS RANGING FROM ZONAL WESTERLY FLOW WITH A
SERIES OF SYSTEMS CROSSING THE AREA TO A STRONG BLOCKING RIDGE
SETTING UP OVER THE WESTERN US AND EXTENDING NORTH THROUGH THE
YUKON TERRITORY AND INTO INTERIOR ALASKA AND KEEPING LOWS MORE
CONFINED TO THE BERING...WESTERN GULF AND SOUTHWEST ALASKA.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...GALE WARNING 119 120 132 136 137 138 150 155 160 165 170
            172 180 351 352.
         STORM WARNING 130 131.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...CB
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...DEK
LONG TERM...JER