Small Craft Advisory 343, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
513 PM AKDT SUN APR 19 2015

.ANALYSIS AND UPPER LEVELS...
A HIGH AMPLITUDE TROUGH IS CENTERED OVER THE BERING SEA AND NORTH
CENTRAL PACIFIC SUPPORTED BY A CONTINUOUS JET STREAM WHICH DIVES
SOUTHEASTWARD FROM EASTERN RUSSIA....ROUNDS THE BOTTOM OF THE
TROUGH THEN HEADS BACK NORTHWARD ACROSS THE GULF OF ALASKA AND
MAINLAND ALASKA. ONE NOTEWORTHY ASPECT OF THIS TROUGH IS THE EXTENT
OF COLD AIR ALOFT...WITH 500 MB TEMPERATURES OF -35 TO -43 DEGREES
CELSIUS OVER MUCH OF THE REGION. THIS IS SOME OF THE COLDEST AIR
OF THE PAST SIX MONTHS. THIS IS LEADING TO GENERAL INSTABILITY AS
NOTED BY THE CONVECTIVE NATURE OF CLOUDS (AS SEEN IN SATELLITE
IMAGERY) OVER MUCH OF THE SOUTHERN MAINLAND AND BERING SEA. THERE
ARE A SERIES OF SHORT-WAVES EMBEDDED WITHIN THE LARGE CYCLONIC
FLOW AROUND THE TROUGH WHICH ARE HELPING TO FOCUS PRECIP. OF MOST
SIGNIFICANCE IS A SHORT-WAVE LIFTING NORTHWARD ACROSS SOUTHWEST
ALASKA WITH A SURFACE COLD FRONT PUSHING ONSHORE BRINGING STRONG
GUSTY WINDS. A WINTER WEATHER ADVISORY FOR BLOWING SNOW IS IN
EFFECT FOR THE PORTIONS OF THE KUSKOKWIM DELTA COAST WHERE THE
COMBINATION OF FALLING AND BLOWING SNOW IS LEADING TO SIGNIFICANTLY
REDUCED VISIBILITIES. WARMER AIR OUT AHEAD OF THE COLD FRONT IS
LEADING TO MAINLY RAIN FOR INTERIOR SOUTHWEST ALASKA ON OVER TO
SOUTHCENTRAL ALASKA. WIDESPREAD STRONG GUSTY WINDS ARE PREVALENT
ACROSS SOUTHCENTRAL WITH INCREASING INSTABILITY IN THE WAKE OF AN
OCCLUDED FRONT WHICH MOVED THROUGH THIS MORNING ALONG WITH TIGHT
PRESSURE GRADIENTS BETWEEN THE TROUGH AND A DOWNSTREAM RIDGE. BACK
BEHIND THE COLD FRONT (AND ON THE BACK SIDE OF THE UPPER TROUGH)
GUSTY NORTHWESTERLIES AND SNOW SHOWERS DOMINATE THE BERING
SEA...ALEUTIANS...AND SOUTHERN ALASKA PENINSULA.

&&

.MODEL DISCUSSION...ALL MODELS CONTINUE TO SHOW THE SAME GENERAL
SYNOPTIC EVOLUTION OVER THE NEXT COUPLE DAYS. AS WOULD BE EXPECTED
THERE ARE SOME MINOR TIMING DIFFERENCES WITH THE MYRIAD OF SHORT-
WAVES ROTATING FAIRLY QUICKLY AROUND THE TROUGH. OF PARTICULAR
INTEREST IS A SHORT-WAVE WHICH MOVES UP COOK INLET TOMORROW MORNING
ALONG WITH THE SURFACE COLD FRONT. THIS COULD BRING A BRIEF PERIOD
OF HEAVY WET SNOW TO ANCHORAGE DURING THE MORNING COMMUTE. WILL
TREND TOWARD THE NAM TIMING OF THIS WAVE...REPRESENTING A MIDDLE
GROUND SOLUTION AMONG ALL OF THE MODELS. AS USUAL WILL MAKE USE OF
HI-RES GUIDANCE FOR WINDS FORECAST ACROSS THE REGION...THOUGH WITH
WIDESPREAD INSTABILITY AND WINDS EXTENDING INLAND THE SYNOPTIC
MODELS ARE PRETTY GOOD AS WELL. FORECAST CONFIDENCE IS AVERAGE TO
SLIGHTLY ABOVE AVERAGE.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 AND 2)...
PRECIPITATION WILL SPREAD ACROSS THE COOK INLET REGION LATE
TONIGHT INTO MONDAY MORNING AS A COLD FRONT AND ASSOCIATED UPPER
TROUGH APPROACHES THE AREA FROM THE SOUTHWEST. THE MAIN FORECAST
CHALLENGES RELATED TO THESE FEATURES WILL BE THE TIMING/AMOUNT OF
SNOWFALL AS PRECIPITATION CHANGES OVER TO SNOW DURING THE EARLY
MORNING HOURS WITH TEMPERATURES AT SEA LEVEL REMAINING NEAR OR
SLIGHTLY ABOVE FREEZING. IT APPEARS AT THIS TIME THAT A BRIEF
PERIOD OF MODERATE TO HEAVY SNOW WILL IMPACT THE ANCHORAGE BOWL
DURING THE MORNING COMMUTE. HIGHER PRECIPITATION RATES DURING THIS
TIME WILL POTENTIALLY PRODUCE ENOUGH EVAPORATIVE COOLING TO
OVERCOME THE WARMER AMBIENT CONDITIONS AND YIELD A WINDOW OF A FEW
HOURS OF HEAVY/WET SNOW ACCUMULATION...WITH THE HIGHEST AMOUNTS
FOCUSED ALONG THE HILLSIDE. MINOR ACCUMULATIONS WILL ALSO BE
POSSIBLE INTO THE MAT-SU REGION...WITH HIGHER AMOUNTS OVER
NORTHERN PORTIONS OF THE SUSITNA VALLEY AND WEST FACING SLOPES OF
THE TALKEETNA MOUNTAINS. ANY SNOW THAT DOES ACCUMULATE WILL THEN
QUICKLY MELT DURING THE AFTERNOON AS TEMPERATURES WARM BACK INTO
THE 40S AND PRECIPITATION TAPERS OFF TO SHOWERS. A SHOWERY REGIME
WILL REMAIN IN PLACE THROUGH TUESDAY ACROSS SOUTHCENTRAL ALASKA IN
DEEP SOUTH TO SOUTHWESTERLY FLOW IN THE WAKE OF THE TROUGH.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)...
THE LARGE UPPER LEVEL LOW CENTERED OVER THE BERING WILL CONTINUE
TO SPIN A RATHER COLD AIR MASS UP OVER THE ALASKA PENINSULA AND
INTO SOUTHWEST ALASKA TONIGHT AND MONDAY. CURRENTLY THE LEADING
EDGE OF THE COLDER AIR IS ALONG THE WEST COAST WITH BREEZY
WESTERLY WINDS CREATING SOME AREAS OF BLOWING SNOW. ALSO
ASSOCIATED WITH THIS BOUNDARY IS A MORE ORGANIZED BAND OF SNOW
STRETCHING ACROSS KUSKOKWIM DELTA WHICH WILL SHIFT GRADUALLY
INLAND THIS EVENING. COOL SHOWERY UNSTABLE CONDITIONS WILL
CONTINUE TONIGHT AND MONDAY WITH ISOLATED THUNDERSTORMS FOLLOWING
THE COLD UPPER LEVEL SHORTWAVES ROTATING THROUGH. THE NEXT FRONTAL
SYSTEM WILL APPROACH THE ALASKA PENINSULA FROM THE SOUTH TUESDAY.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...
A SERIES OF SHORTWAVE TROUGHS ROTATING AROUND THE LARGE LOW IN THE
BERING WILL CONTINUE TO BRING MODERATE WESTERLY FLOW AND COOL SHOWERY
WEATHER TO THE BERING SEA AND ALEUTIAN ISLANDS THROUGH MONDAY NIGHT.
THE NEXT STRONGER FRONTAL SYSTEM WILL APPROACH THE WESTERN
ALEUTIANS TUESDAY.

&&

.LONG TERM FORECAST (WEDNESDAY THROUGH SUNDAY)...

THE LONG TERM FORECAST WILL BE HIGHLY DEPENDENT UPON SOME HIGH
AMPLITUDE FEATURES EXTENDING FROM KAMCHATKA INTO THE GULF OF
ALASKA. ON WEDNESDAY...TWO AREA OF LOW PRESSURES...ONE OVER THE
GULF OF ALASKA AND THE OTHER OVER THE WESTERN BERING SEA...WILL
BOTH BRING GALE FORCE WINDS TO THOSE LOCATIONS. THE LOW OVER THE
SOUTHERN GULF OF ALASKA WILL BE QUICK TO DISSIPATE AS THE LOW OVER
THE WESTERN BERING SEA BARRELS EASTWARD AND ABSORBS THE REMNANTS
OF THE GULF LOW ON THURSDAY. MODEL AGREEMENT CONTINUES TO INCREASE
WITH THESE TWO LOWS ALTHOUGH MODELS HAVE SPED THE SYSTEMS UP BY
ABOUT 12 HOURS DUE TO A SLIGHTLY LESS AMPLIFIED PATTERN. A JET
STREAK ON FRIDAY WILL HELP TO AMPLIFY THE LONGWAVE TROUGHING
DEVELOPING AS THE INITIAL LOW OVER KAMCHATKA ELONGATES AND GETS
TRAPPED ON THE EAST SIDE BY A BUILDING PACIFIC NORTH AMERICAN
RIDGE. PREVIOUS MODEL RUNS HINTED AT THIS RIDGE NOSING ACROSS
ALASKA BUT NOW IT LOOKS LIKE IT WILL STAY EAST OF ALASKA DUE TO
THE TRACK OF THE UPPER LEVEL LOW. A SECOND KAMCHATKA LOW WILL PUSH
OFF THE PENINSULA ON SATURDAY AND TAKE A SIMILAR PATH TO THE ONE
OBSERVED MIDWEEK. OVERALL...THE SYNOPTIC PATTERN DOES NOT LOOK TO
CHANGE MUCH IN THE COMING 7 TO 10 DAYS AS THE JET STREAM CONTINUES
TO STAY OVER THE NORTH PACIFIC.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...GALES...150 155 160 165 170 172 180
         HVY FRZG SPRY...180
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...SB
SOUTHCENTRAL ALASKA...CB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JR
LONG TERM...MC