Small Craft Advisory 233, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
453 PM AKDT SUN MAR 15 2015

.ANALYSIS AND UPPER LEVELS...

THE UPPER LEVEL PATTERN SHOWS ZONAL FLOW IN THE NORTHERN PART OF
THE STATE WITH A LOW OVER THE EASTERN BERING SEA. THIS IS BRINGING
SOUTH TO SOUTHWEST FLOW ALOFT OVER SOUTHCENTRAL ALASKA WITH WEAK
FLOW ALOFT OVER SOUTHWEST ALASKA. THE 250 MB JET STREAM IS WELL
SOUTH OF THE AREA BUT IS PEAKING AT A VERY IMPRESSIVE 200 KT
BETWEEN 35N AND 40N. THIS PATTERN WILL KEEP THE JET WELL SOUTH OF
THE AREA FOR THE REMAINDER OF THE WEEK. AT THE SURFACE A STRONG
LOW AROUND 970 MB IS CENTERED NEAR THE WESTERN TIP OF THE ALASKA
PENINSULA. AN OCCLUDED FRONT IS REACHING INTO THE KUSKOKWIM DELTA
AND BRISTOL BAY FROM THIS LOW AND THEN THE FRONT CONTINUES OVER
SOUTHCENTRAL ALASKA. THE ARCTIC FRONT REMAINS OVER SOUTHCENTRAL
ALASKA AND WILL BE PUSHED NORTHWARD OVER THE NEXT DAY AS WARM
SOUTHEAST FLOW AT THE SURFACE CONTINUES.

&&

.MODEL DISCUSSION...

MODELS DID INITIALIZE WELL AND ARE IN GOOD AGREEMENT OVERALL WITH
THE MAJOR FEATURES. BY TUESDAY THE AGREEMENT IN THE GULF OF ALASKA
AREA BREAKS DOWN SIGNIFICANTLY IN THE SOUTHEAST FLOW. THE ECMWF
AND GEM REGIONAL ARE KEEPING GENERALLY SOUTHEAST FLOW OVER THE
GULF WITH NO EMBEDDED WAVES OF SIGNIFICANCE. THE GFS IS TRYING TO
EXTEND A TROUGH INTO THE EASTERN GULF AT THIS TIME WHILE THE 18Z
NAM HAS GONE FOR A CLOSED LOW APPROACHING MIDDLETON ISLAND...A
SOLUTION THAT DOES NOT SEEM VERY LIKELY. WHILE THE ECMWF WAS THE
PREFERRED SOLUTION...THE MAIN POINT IS THAT THERE IS A HIGHER
DEGREE OF UNCERTAINTY THAN USUAL WITH SUCH A LARGE SPREAD OF
SOLUTIONS IN THE SHORT TERM PERIOD OF THE MODEL RUNS. THE MODELS
ARE ALSO GIVING A RATHER LARGE SPREAD IN THE EXACT PLACEMENT OF
THE ELONGATED LOW IN THE EASTERN BERING SEA TUESDAY AS WELL.
HOWEVER THIS WILL HAVE LESS IMPACT ON THE FORECAST AS THE AXIS OF
THE TROUGH IS VERY SIMILAR ON ALL MODELS.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 AND 2)...
THE WEATHER FRONT ASSOCIATED WITH THE NORTH PACIFIC LOW OVER THE
EASTERN ALEUTIANS IS POSITIONED NEAR THE BARREN ISLANDS EXTENDING
INTO THE WESTERN GULF. THE WARM AIR HAS MOVED INTO KODIAK ISLAND
AND THE PRECIPITATION HAS QUICKLY CHANGED FROM SNOW/RAIN MIXTURE
TO ALL RAIN WITH GUSTY EASTERLY WINDS. THIS FRONTAL BOUNDARY WILL
MOVE INTO THE EASTERN KENAI COASTAL AND INLAND AREAS THIS
EVENING. THIS WILL BRING MODERATE TO HEAVY SNOWFALL WITH GUSTY
WINDS AND BLOWING SNOW TO THE EASTERN KENAI PENINSULA TONIGHT AND
MONDAY. GUSTY WINDS ACCOMPANYING SNOWFALL WILL PRODUCE BLIZZARD
CONDITIONS MAINLY PORTAGE VALLEY THROUGH TURNAGAIN ARM
TONIGHT AND MONDAY...THOUGH SOME LOCALIZED BLIZZARD CONDITIONS
COULD BE EXPERIENCED ALONG THE SEWARD HIGHWAY FROM MOOSE PASS
NORTH. BY MONDAY MORNING THE WARMER AIR WILL MOVE INTO THE INLAND
AREAS WITH THE PRECIPITATION TURNING TO ALL RAIN. THE WINDS WILL
GRADUALLY DIMINISH AS THE WEATHER FRONT MOVES FARTHER INLAND AND
FALLS APART. THE ANCHORAGE BOWL AND THE MAJORITY OF WESTERN KENAI
WILL "DOWN-SLOPE" OUT WITH DRIER CONDITIONS BUT THERE WILL BE SOME
SNOW SHOWERS SPILLING OVER THE MOUNTAINS. THE SUSITNA VALLEY WILL
BEGIN TO HAVE SNOW PUSHING UP THE VALLEY FROM THE SOUTH WITH THE
WESTERN VALLEY GETTING UPSLOPE CONDITIONS RESULTING IN ADVISORY
LEVEL SNOWFALL AMOUNTS. THEREFORE...A SNOW ADVISORY WAS ISSUED
WEST OF THE PARKS HIGHWAY BEGINNING LATE SUNDAY NIGHT THROUGH
MONDAY NIGHT. THE SYNOPTIC SURFACE WIND FLOW WILL PRIMARILY BE OUT
OF THE SOUTH GOING INTO TUESDAY BRINGING WARMER TEMPERATURES TO
THE REGION.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)...
THE MAIN IMPACTS THIS EVENING WILL BE LIGHT SNOW WITH AREAS OF
BLOWING SNOW OVER THE KUSKOKWIM DELTA AS A FAIRLY POTENT
DISTURBANCE MOVES QUICKLY NORTH AND WEST THROUGH MIDNIGHT TONIGHT.
AT THIS TIME NOT EXPECTING TO MEET BLOWING SNOW ADVISORY CRITERIA
BUT IT IS POSSIBLE IN ISOLATED LOCATIONS FOR BRIEF PERIODS. THE
OTHER AREA OF CONCERN IS THE POTENTIAL FOR LOCALLY HEAVY SNOWFALL
OVER THE NORTHEASTERN BRISTOL BAY ZONE MAINLY FROM ILIAMNA
NORTH...AND EXTENDING INTO THE KUSKOKWIM VALLEY TOWARD THE
COMMUNITIES OF STONY RIVER AND SLEETMUTE. THE MAIN CAUSE OF THIS
HEAVIER BAND OF SNOW WILL ANOTHER APPROACHING DISTURBANCE LATE
THIS EVENING THROUGH TUESDAY MORNING COMBINING WITH A FAVORABLE
MID-LEVEL JET DYNAMICS WITH GOOD MOISTURE ADVECTION ALOFT IN THE
SNOW GROWTH LAYER. THE 12Z GFS IS DEPICTING AS MUCH AS 8 INCHES
OVERNIGHT TOWARD STONY RIVER IN THE KUSKOKWIM RIVER VALLEY BUT
WILL PLAY IT MORE CONSERVATIVE IN THE 2 TO 5 INCH RANGE.

ANOTHER DISTURBANCE MOVES NORTH ACROSS THE WESTERN CAPES AND THE
KUSKOKWIM DELTA MONDAY NIGHT INTO TUESDAY...AND WILL BRING ANOTHER
COUPLE OF INCHES OF SNOW TO THESE AREAS. AREAS OF BLOWING SNOW
WILL LIKELY BE A CONCERN BUT AT THIS TIME IT APPEARS WINDS SHOULD
REMAIN LIGHT ENOUGH FOR THERE NOT TO BE ANY WARNING OR ADVISORY
CONCERNS.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...
THE MAIN IMPACTS FOR THE BERING SEA REGION WILL BE AN EXTENDED
PERIOD OF BLOWING SNOW WITH STRONG NORTH WINDS OF 30 TO 50 MPH FOR
THE PRIBILOFS...WITH THE POTENTIAL AT TIMES FOR VISIBILITIES TO
FALL BELOW 1 MILE. THE EC SOLUTION ESPECIALLY WOULD SUGGEST THE
POTENTIAL FOR BLIZZARD CONDITIONS TOWARD LATE MONDAY NIGHT INTO
TUESDAY...BUT THERE IS STILL ENOUGH DISAGREEMENT IN WHERE THE LOW
PRESSURE AND ASSOCIATED HEAVIER SNOW DEVELOP TO FOREGO ISSUING A
WATCH AT THIS TIME.

OTHERWISE...RAIN AND RAIN SHOWERS WILL IMPACT THE ALASKA PENINSULA
AND EASTERN ALEUTIANS FOR MOST OF THE NEXT FEW DAYS...WITH
SCATTERED SNOW SHOWERS IN COLD ADVECTION FLOW ACROSS THE CENTRAL
AND WESTERN BERING.

&&

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

MODELS ARE IN AGREEMENT THAT A BROADLY CYCLONIC JET WILL CURVE
THOUGH THE NORTH PACIFIC BETWEEN 30N AND 40N THIS WEEK. IT WILL
ALLOW AN UPPER LOW TO MOVE JUST SOUTH OF THE AKPEN AND REMAIN
ESSENTIALLY STATIONARY. WAVES WILL MOVE AROUND THE LOW AND AFFECT
THE EASTERN GULF OF AK. BEYOND THIS SYNOPTIC PICTURE THE MODELS
RAPIDLY FALL INTO DISARRAY CONCERNING ASSOCIATED SURFACE FEATURES.

THE WPC SOLUTION HEAVILY INCORPORATED THE EC ENSEMBLE WITH
SEVERAL OTHER COMPONENTS. THIS CONSOLIDATES A SURFACE LOW SOUTH OF
THE AKPEN TUE NIGHT THAT MOVES OVER THE EASTERN ALEUTIANS BY
THU..AND THEN DISSIPATES OVER THE EASTERN BERING ON SAT. LOWS AND
ASSOCIATED FRONTS MOVING AROUND THE UPPER LOW REACH THE NORTH GULF
COAST THU AND SAT. FINALLY...A NORTH PACIFIC LOW MOVES TO THE
CENTRAL ALEUTIANS ON SUN.

THE FORECAST UTILIZED WPC BUT ALSO ENHANCED THE WIND FIELD AND
OTHER FEATURES USING THE OPERATIONAL EC, WHICH OFTEN RESEMBLED
WPC GUIDANCE.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...BLIZZARD WARNING 125. SNOW ADVISORY 145.
MARINE...STORMS 119 139. GALES 120 125 131 150 155 160 170 173
174 175 176 177 179 180 185 351 352 412 413 414.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...EZ
SOUTHCENTRAL ALASKA...PD
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CC
LONG TERM...DS