Small Craft Advisory 758, NWS Anchorage, AK

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

.ANALYSIS AND UPPER LEVELS...

A POTENT UPPER LEVEL TROUGH IS POISED JUST TO THE WEST OF MAINLAND
ALASKA... AND STRETCHES FROM NORTHEAST ALASKA INTO THE SOUTHERN
BERING. TO THE EAST...A QUITE BROAD RIDGE COVERS MAINLAND ALASKA
AND STRETCHES INTO NORTHWEST CANADA. A SECOND RIDGE COVERS THE
WESTERN BERING. AT THE SURFACE...A LOW PRESSURE SYSTEM IS
PROGRESSING INTO THE WESTERN GULF OF ALASKA. A POTENT COLD FRONT
CURRENTLY STRETCHES ACROSS THE ALEUTIAN ISLANDS AND JUST NORTH OF
THE ALASKA RANGE. A STEADY STREAM OF MOISTURE IS SURGING NORTHWARD
TOWARDS ALASKA FROM THE TROPICS...AND WILL CONTINUE TO SUPPORT
WIDESPREAD RAINFALL ACROSS CENTRAL ALASKA OVER THE SHORT TERM.

&&

.MODEL DISCUSSION...
IN THE SHORT TERM...THERE IS SOME INCONSISTENCY WITH MODELS...WITH
THE NAM BRINGING IN COLDER AIR MUCH FASTER THAN ANY OTHER
MODEL...AND INDICATING WIDESPREAD SNOW ACROSS SOUTHCENTRAL.
HOWEVER...THE NAM IS CURRENTLY AN OUTLIER...AND THE MORE
CONSISTENT AND REALISTIC GFS MODEL WAS CHOSEN.  MEANWHILE...TO
THE WEST OF THIS LOW PRESSURE SYSTEM...HIGH PRESSURE WILL CONTINUE
TO BUILD ACROSS THE BERING AND AKPEN. THE CONTRAST BETWEEN THESE
TWO POTENT FEATURES IS LIKELY TO SUPPORT PARTICULARLY GUSTY
WINDS...ACROSS THE AKPEN AND KODIAK ISLAND...TUESDAY MORNING
THROUGH AFTERNOON. GUSTY WINDS ARE LIKELY IN SEWARD WEDNESDAY
MORNING INTO AFTERNOON.&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 AND 2)...
ALL ATTENTION IS FOCUSED ON DEVELOPMENT OF A MAJOR STORM AND THE
POTENTIAL FOR THE FIRST SNOWFALL OF THE WINTER SEASON FOR THE
POPULATION CENTERS OF SOUTHCENTRAL. THERE ARE STILL SOME SMALL BUT
VERY SIGNIFICANT MODEL DIFFERENCES IN TRACK OF SURFACE LOW AND IN
SYSTEM STRUCTURE WHICH WILL GREATLY AFFECT PLACEMENT OF HEAVIEST
PRECIPITATION AND ARRIVAL TIME OF COLD AIR (TO CHANGE PRECIP OVER
TO SNOW). THUS...FORECAST CONFIDENCE IN WHETHER SNOW WILL
ACCUMULATE NEAR SEA LEVEL REMAINS LOW.

BASED ON THE LATEST GUIDANCE THE LATEST FORECAST WILL TREND TOWARD
A WARMER SOLUTION...WITH LESS PROBABILITY OF ACCUMULATING SNOW IN
THE LOW LANDS OF SOUTHCENTRAL. WITH A LOW ALREADY UNDERGOING
DEEPENING OVER THE NORTH PACIFIC CONFIDENCE IS HIGH THAT THIS WILL
DEVELOP INTO A STRONG SURFACE LOW AS IT INTERACTS WITH UPPER
TROUGH AND COLD AIR CURRENTLY DROPPING SOUTHEASTWARD ACROSS THE
BERING SEA. AS THIS LOW TRACKS INTO THE WESTERN GULF THE BERING
TROUGH WILL DIG UNDERNEATH THE LOW AND PULL ITS TRACK BACK
WESTWARD... ULTIMATELY TRACKING TO THE EASTERN KENAI PENINSULA.
WARM AIR WRAPPING AROUND THE LOW CENTER WILL ADVECT WESTWARD
ACROSS THE KENAI PENINSULA. THIS LOW TRACK ALSO MEANS THE
DEFORMATION BAND ON THE BACK SIDE OF THE LOW WILL SET UP WEST OF
ANCHORAGE FROM THE WESTERN KENAI/COOK INLET NORTHWARD TO THE
SUSITNA VALLEY. THIS WILL DELAY THE BEST LOW LEVEL COLD ADVECTION
UNTIL THE STORM SYSTEM BEGINS TO EXIT TO THE EAST TUESDAY NIGHT.
NOT ALL MODELS DEPICT THIS...BUT CONCEPTUAL MODELS AND EXPERIENCE
SUPPORT THIS IDEA.

BASED ON THIS PREFERRED SOLUTION WILL DELAY THE WIDESPREAD
CHANGEOVER IN PRECIP FROM RAIN TO SNOW UNTIL TOMORROW NIGHT FOR
LOWER ELEVATIONS OF ANCHORAGE...THE WESTERN KENAI...AND MUCH OF
THE MAT-SU. HIGHER ELEVATIONS WILL BE A DIFFERENT STORY WITH SNOW
LEVELS DROPPING TOMORROW AFTERNOON DUE BOTH TO ADVECTION AND THE
PROCESS OF MELTING SNOW (WHICH REMOVES HEAT FROM THE ENVIRONMENT).
THUS THERE IS STILL POTENTIAL THAT HIGHER ELEVATIONS OF THE
ANCHORAGE AREA WILL SEE SOME SNOW ACCUMULATION BEGINNING TOMORROW
AFTERNOON. BY THE TIME CONDITIONS COOL ENOUGH FOR SNOWFALL AT
LOWER ELEVATIONS THE STRONG DYNAMICAL LIFT WILL BE COMING TO AN
END. LOW LEVEL UPSLOPE PROCESSES WILL START UP ON THE WEST SIDE OF
THE MOUNTAINS WHICH WILL GENERATE SOME PRECIP. THE BIG QUESTION IS
WHETHER THERE IS ENOUGH PRECIP TO ACCUMULATE MUCH IF ANY SNOW.
WILL HAVE TO CONTINUE TO WORK OUT THE DETAILS AS NEW GUIDANCE
COMES IN BEFORE BEING ABLE TO ANSWER THIS QUESTION WITH ANY
CONFIDENCE.

MEANWHILE...THIS WILL BE AN IMPRESSIVE WIND EVENT ACROSS THE
WESTERN GULF AND KODIAK ISLAND. THE CURRENT FORECAST ALREADY
INDICATES HIGH END GALES TO STORM FORCE WINDS FOR THIS AREA BASED
ON INITIAL COLD ADVECTION FOLLOWED BY STRONG DOWNWARD MOTION (AND
MIXING DOWN OF STRONGER WINDS FROM ALOFT) WITH PASSAGE OF THE
UPPER TROUGH. WITH HIGH CONFIDENCE IN A DEEPER SURFACE LOW (AND
TIGHTER SURFACE PRESSURE GRADIENTS) WILL UP WINDS EVEN MORE...WITH
HURRICANE FORECAST GUSTS NOW LIKELY IN THE KAMISHAK BAY AREA (AND
POSSIBLY OTHER GAPS ALONG THE ALASKA PENINSULA).

AS THE STORM SYSTEM EXITS ON WEDNESDAY GENERALLY EXPECT IMPROVING
CONDITIONS ACROSS THE REGION. AS COLD AIR CONTINUES TO ADVECT
EASTWARD ACROSS SOUTHCENTRAL DO EXPECT THE USUAL GAP WINDS ALONG
THE COAST. AS WINDS DIE DOWN WEDNESDAY AND WEDNESDAY NIGHT ACROSS
MUCH OF THE REST OF SOUTHCENTRAL EXPECT SOME VERY CHILLY TEMPERATURES.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)...
PASSAGE OF A LOW LEVEL COLD FRONT HAS RESULTED IN DEVELOPMENT OF
GUSTY NORTHERLY WINDS ACROSS MUCH OF SOUTHWEST ALASKA. MUCH
STRONGER WINDS ARE IN STORE LATER TONIGHT THROUGH TUESDAY AS THE
UPPER TROUGH MOVES THROUGH...RESULTING IN STRONG DOWNWARD MOMENTUM
IN THE ATMOSPHERE WHICH WILL MIX DOWN STRONGER WINDS FROM ALOFT.
IN ADDITION A STRONG STORM TRACKING INTO THE GULF OF ALASKA WILL
TIGHTEN PRESSURE GRADIENTS ACROSS THE REGION. WITH HIGH CONFIDENCE
HAVE STRENGTHENED WINDS AREA-WIDE IN THE LATEST FORECASTS. SOME OF
THE STRONGEST WINDS WILL BE THROUGH GAPS OF THE ALASKA PENINSULA
DUE TO A VERY FAVORABLE ORIENTATION OF LOW LEVEL FLOW AS WELL AS
TRACK OF THE SURFACE LOW ACROSS THE WESTERN GULF. WINDS WILL
GRADUALLY DIMINISH TUESDAY NIGHT AND WEDNESDAY AS THE STORM SYSTEM
IN THE GULF EXITS TO THE EAST AND HIGH PRESSURE BUILDS IN FROM THE
CENTRAL BERING TO SOUTHWEST ALASKA.

DUE TO THE STORM TRACKING INTO THE GULF THE FORWARD PROGRESSION
OF THE LOW LEVEL COLD FRONT AND THE UPPER LEVEL TROUGH WILL STALL
OVER SOUTHWEST ALASKA OVER NIGHT TONIGHT (WITH THE COLD FRONT
ALONG THE ALASKA RANGE AND THE UPPER TROUGH FARTHER WEST).
CONTINUED LIFT OUT AHEAD OF THE TROUGH (WITH SOME LIFT ALSO COMING
IN FROM THE EAST AHEAD OF THE GULF STORM) WILL COMBINE WITH LOW
LEVEL UPSLOPE FLOW ALONG THE ALASKA AND ALEUTIAN RANGES TO PRODUCE
STEADY RAIN THROUGH EARLY THIS EVENING...CHANGING TO STEADY SNOW
AS COLD AIR MOVES IN THROUGH THE EVENING AND OVERNIGHT HOURS. THIS
WILL RESULT IN SOME MINOR SNOW ACCUMULATION WEST OF THE MOUNTAIN
RANGES WITH MUCH HIGHER SNOWFALL AMOUNTS AS YOU RISE UP THE
WESTERN SLOPES. ELSEWHERE...A FEW SNOW SHOWERS ARE POSSIBLE WITH
MORE NUMEROUS SNOW SHOWERS ALONG THE IMMEDIATE COAST AS COLD AIR
MOVES OVER MUCH WARMER WATERS OF THE SOUTHWEST COAST AND BRISTOL
BAY.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...
THE PASSAGE OF THE UPPER TROUGH ACROSS THE EASTERN BERING THIS
AFTERNOON THROUGH THIS EVENING WILL COINCIDE WITH THE STRONGEST
WINDS AT THE SURFACE AS STRONGER WINDS FROM ALOFT MIX DOWN. THE
CONSTANT MOVEMENT OF COLD AIR ACROSS MUCH WARMER WATERS WILL
GENERATE A STEADY STREAM OF SHOWERS. THE WARM WATERS WILL HELP
MAINTAIN A FAIRLY SHALLOW LAYER OF ABOVE FREEZING AIR NEAR THE
SURFACE WHICH MEANS PRECIP-TYPE WILL REMAIN MAINLY RAIN DESPITE
THE FAIRLY COLD AIR ABOVE THIS BOUNDARY LAYER. TEMPERATURE
ADVECTION WILL BECOME NEUTRAL TO POSITIVE TUESDAY AS HIGH PRESSURE
BUILDS IN FROM THE WEST WHICH WILL LEAD TO DIMINISHING WINDS AND
SHOWERS. IT DOES LOOK LIKE A WEAK SURFACE LOW WILL MOVE OVER TOP
OF THE RIDGE BRINGING SOME LIGHT RAIN TO THE PRIBILOFS AND EASTERN
BERING. OTHERWISE ATTENTION WILL QUICKLY SHIFT TO A COMPLEX LOW
OVER THE NORTHWEST BERING. THIS REMAINS ON TRACK TO PUSH A STORM
FORCE FRONT EASTWARD ACROSS THE WESTERN ALEUTIANS AND WESTERN
BERING. IT WILL BE A FAIRLY NARROW AREA OF VERY STRONG WINDS AND
WILL BE ACCOMPANIED BY MODERATE TO HEAVY RAIN THANKS TO A STRONG
SOUTHERLY JET STREAK MOVING UP FROM THE PACIFIC.

&&

.LONG TERM FORECAST (DAYS 3 THROUGH 7)...

BEGINNING THE EXTENDED PERIOD ON WEDNESDAY EVENING...THE LEFTOVERS
OF AN ARCTIC AIR MASS WILL BE OVER SOUTHERN ALASKA AS THE BROADER
SCALE TROUGH MOVES EAST. LINGERING SNOW OVER THE COPPER BASIN WILL
END OVERNIGHT...WITH MOST OF SOUTHCENTRAL CLEARING OUT AT LEAST
PARTIALLY...ALLOWING FOR SOME UNSEASONABLY COLD TEMPERATURES TO
DEVELOP BY THURSDAY MORNING. WEAK AND FAST MOVING SHORTWAVE
RIDGING OVER THE BERING SEA WILL MOVE EAST THURSDAY...ALLOWING
FOR A REPRIEVE FROM RAIN AND CLOUDS BEFORE A STRONG STORM FORCE FRONT
OVER THE BERING MAKES EASTWARD PROGRESS THURSDAY NIGHT AND FRIDAY.
BESIDES RAIN AND CLOUDS...THIS FRONT WILL ALSO ALLOW FOR A RETURN
TO NEAR AVERAGE TEMPERATURES. THE FRONT WILL RAPIDLY WEAKEN BY
SUNDAY...WITH THE UPPER LEVEL FLOW BECOMING MORE ZONAL AND
DISCONNECTED FROM THE NORTH PACIFIC MOISTURE. ANOTHER WESTERN
BERING FRONT WILL MOVE EAST AND BEGIN IMPACTING SOUTHWEST ALASKA
MONDAY NIGHT AND TUESDAY WITH DRIER WEATHER OVER SOUTHCENTRAL.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...STORM 130 131 132 138 150 351 352. GALE 119 120 155 160
165 170 172 178 179 180 414.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...EN
SOUTHCENTRAL ALASKA...SB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...SB
LONG TERM...JA