Small Craft Advisory 225, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
543 AM AKDT FRI MAR 13 2015

.ANALYSIS AND UPPER LEVELS...

ON THE EARLY MORNING WEATHER MAP...THERE ARE THREE MAJOR FEATURE
OF NOTE. WE WILL START OUT OVER THE WESTERN BERING SEA WHERE AN
AREA OF LOW PRESSURE HAS BEGUN TO DEVELOP NORTHWEST OF ADAK WITHIN
A BROADER AREA OF LOW PRESSURE THAT EXTENDS FROM THE CENTRAL
ALEUTIANS TO KAMCHATKA. WATER VAPOR SHOWS A POTENT SHORTWAVE
ROTATING AROUND THE MAIN UPPER LEVEL LOW OVER EASTERN RUSSIA AS
WELL AS GOOD JET STREAK SUPPORT (100 KT AT 500 MB AND 185KT AT 200
MB). THE COMBINATION OF JET STREAK AND POTENTIAL VORTICITY
ADVECTION HAS ALREADY BEGUN TO DROP THE 500 MB HEIGHT FIELDS OVER
FAR WESTERN BERING SEA/NORTH PACIFIC...WHICH WILL HAVE IMPLICATIONS
ON THE FORECAST IN THE NEXT 24-36 HOURS. THE NEXT FEATURE WE WILL
LOOK AT IS AN OCCLUDED FRONT OVER THE CENTRAL BERING SEA THAT
EXTENDS WESTWARD TO THE KAMCHATKA PENINSULA WHERE IT IS CONNECTED
TO A WEAKENING AREA OF LOW PRESSURE. SCATTEROMETER DATA SHOWS 30
KT WINDS SHIFTING EASTWARD AS THE FRONT DRIFTS EASTWARD WITH SOLID
GALES FROM ATKA TO NIKOLSKI. THE LAST (AND NOT LEAST) FEATURE OF
NOTE THIS MORNING IS THE BROAD UPPER LEVEL TROUGH ENCOMPASSING THE
ALASKA MAINLAND. MULTIPLE WEAK SURFACE BASED LOW RESIDE IN THE
TROUGH WITH THE MOST POTENT ONE MOVING INLAND NEAR YAKUTAT BAY
THIS MORNING. A WEAK MESOSCALE LOW DEVELOPED NORTH OF MIDDLETON
ISLAND DURING THE EVENING YESTERDAY AND MOVED INTO PRINCE WILLIAM
SOUND AND BROUGHT SOME SNOW SHOWERS TO THE NORTHERN GULF COAST. IN
ADDITION...AS THE LOW MOVED INTO THE SOUND...A FAVORABLE GRADIENT
SET UP ACROSS TURNAGAIN ARM AND BROUGHT SOME GUSTY WINDS INTO ANCHORAGE.

&&

.MODEL DISCUSSION...

MODELS ARE IN GOOD AGREEMENT THROUGH SUNDAY MORNING WITH THE
EXCEPTION OF THE NAM WHICH STRUGGLES WITH A DEVELOPING LOW IN THE
NORTH PACIFIC STARTING LATE FRIDAY NIGHT. THERE ARE SOME MINOR
DISCREPANCIES WITH HOW QUICKLY THE DEVELOPING TRIPLE POINT LOW
CLOSES OFF BUT THAT IS TO BE EXPECTED WITH THE COMPLEX NATURE OF
THE SURFACE/UPPER LEVEL LOWS AND LOTS OF RANDOM BITS OF ENERGY
PUSHING THROUGH THE UPPER LEVELS. FORECAST PACKAGE THIS MORNING
WAS TRENDED TOWARD THE GFS WITH GOOD RUN-TO-RUN CONSISTENCY AND
GOOD INITIALIZATION OF THE 00Z AND 06Z SYNOPTIC CYCLES.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 AND 2)...

THE UPPER LEVEL LOW OVER COOK INLET HAS REACHED ITS MAXIMUM
SOUTHWARD EXTENT...WITH SATELLITE IMAGERY REVEALING THE BEGINNINGS
OF A NORTHWARD MOVEMENT IN THE LAST FEW HOURS. AS EXPECTED...SOME
MIDLEVEL CLOUDS MOVED INTO THE ANCHORAGE AND MAT-SU AREA
OVERNIGHT...BUT LIGHT PRECIPITATION DID NOT MAKE IT ANY FARTHER
WEST THAN PORTAGE. DEEP MOISTURE/DYNAMIC LIFT FROM THE NOW-
NEGATIVELY TILTED TROUGH LIFTING NORTHWARD IS TEAMING UP WITH
LOW-LEVEL CONVECTIVE PROCESSES OVER THE NORTHERN GULF TO PRODUCE
ENHANCED SNOW SHOWER ACTIVITY AROUND CORDOVA AND VALDEZ. SNOWFALL
HAS EVEN SPREAD INTO THE COPPER RIVER BASIN EARLY THIS
MORNING...AND IS LIKELY TO CONTINUE THROUGHOUT MUCH OF THE DAY.
THE ONLY SNOWFALL IN THE COOK INLET AREA CONTINUES TO BE A NARROW
BAND OF SNOW SHOWERS AFFECTING THE SOUTHERN END OF KACHEMAK BAY.
THE INTENSITY AND MOVEMENT OF THIS FEATURE IS (AND WILL CONTINUE
TO BE) HIGHLY SENSITIVE TO SUBTLE CHANGES IN WIND SPEED AND
DIRECTION...SO WHILE SOME LOCALES MAY SEE A FEW INCHES OF
ACCUMULATION MANY OTHERS WILL SEE MINOR OR EVEN NO ACCUMULATIONS.
THIS PATTERN WILL WIND DOWN AS GAP WINDS AND THE MULTI-DAY COLD
AIR "OUTFLOW" EVENT SLOWLY COMES TO AN END BY SATURDAY NIGHT.

BY EARLY SUNDAY MORNING...THE WEATHER PATTERN WILL COMPLETELY
SHIFT TO A PREVAILING SOUTHERLY FLOW. THIS PATTERN HAS BECOME VERY
FAMILIAR THIS WINTER...AND DRAMATICALLY WARMING
TEMPERATURES...CLOUD COVER...AND HEAVY COASTAL PRECIPITATION WILL
DEVELOP LATE IN THE WEEKEND AND CONTINUE INTO NEXT WEEK.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)...

CLEAR SKIES AND COLD NORTHWESTERLY WINDS WILL CONTINUE THROUGH
THIS MORNING...ESPECIALLY FOR THE EASTERN BRISTOL BAY AREA WHERE
THESE WINDS WILL CONTINUE INTO THIS EVENING. STARTING TOMORROW
AFTERNOON A FRONTAL SYSTEM CURRENTLY MOVING THROUGH THE CENTRAL
BERING BRING WARMER SOUTHEASTERLY WINDS AND A CHANCE OF SNOW TO
THE AKPENN. THIS FRONT WILL THEN SPREAD THROUGHOUT BRISTOL BAY
WHERE IT WILL CONTINUE TO BRING A CHANCE OF SNOW AND GUSTY
EASTERLY WINDS INTO SUNDAY.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...

THE LOW CURRENTLY ENTERING THE SOUTHCENTRAL BERING WILL BRING SNOW
FROM THE EASTERN ALEUTIANS NORTHWARD INTO THE PRIBILOF ISLANDS
STARTING THIS AFTERNOON. WARM AIR ASSOCIATED WITH THIS SYSTEM
WILL CAUSE RAIN TO MIX WITH SNOW ALONG THE ALEUTIAN ISLANDS BY
THIS EVENING...WHICH WILL LIMIT SNOW ACCUMULATION TO AN INCH OR SO
OF WET SNOW. THE PRIBILOFS HOWEVER SHOULD REMAIN AS SNOW
THROUGHOUT THE EVENT...WHICH WILL CREATE AREAS OF BLOWING SNOW AS
STRONG SOUTHEASTERLY WINDS DEVELOP THROUGH SATURDAY. THIS MAY
BRIEFLY BRING VISIBILITIES LESS THAN ONE MILE...BUT WITH NO
CURRENT TRANSPORTABLE SNOW AND SOUTHEASTERLY WINDS BLIZZARD
CONDITIONS SHOULD NOT BE LONG LIVED OR WIDESPREAD. ANOTHER NORTH
PACIFIC LOW WILL BEGIN TO APPROACH THE AKPENN LATE
SATURDAY...WHICH WILL BRING RAIN AND SNOW TO THE EASTERN BERING
AND ALEUTIANS FOR SUNDAY.

&&

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

ALL IS STILL LOOKING ON TRACK FOR A MAJOR PATTERN SHIFT BEGINNING
EARLY NEXT WEEK. A FAIRLY DEEP VERTICALLY STACKED LOW WILL TRACK
SOUTH OF THE ALASKA PENINSULA PUMPING UP A SHARP UPPER LEVEL
RIDGE OVER THE GULF OF ALASKA SUNDAY MORNING. THE RIDGE WILL LIFT
NORTH INTO MAINLAND ALASKA SUNDAY AFTERNOON AND EVENING WITH
SOUTHERLY FLOW ALOFT PUSHING A FRONTAL SYSTEM TO THE NORTHERN
GULF COAST. INITIALLY AS PRECIPITATION SPREADS ONSHORE SUNDAY THEN
FURTHER INLAND SUNDAY NIGHT IT WILL FALL AS SNOW ALTHOUGH
LOCATIONS IMMEDIATELY ALONG THE GULF WILL MIX WITH AND TURN OVER
TO RAIN AS SOUTH TO SOUTHEASTERLY FLOW BRINGS IN WARMER AIR.

SOUTHERLY FLOW ALOFT WILL CONTINUE THROUGH MUCH OF NEXT WEEK AS
THE UPPER LEVEL RIDGE SHIFTS OVER BRITISH COLUMBIA AND THE YUKON
TERRITORY AND THEN STRENGTHENS. A VARIETY OF LOW PRESSURE SYSTEMS
WILL LIFT NORTH THROUGH THE GULF OF ALASKA AND INLAND BRINGING
INCREASING TEMPERATURES AND PERIODIC PRECIPITATION TO SOUTH
CENTRAL AND SOUTHWEST ALASKA THROUGH THE WEEK.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...GALE WARNING 121 127 129 130 131 132 137 150 165 170 172
173 174 176 177 179 185 351 352 411 412 413 414.
HEAVY FREEZING SPRAY WARNING 120 121 125 126 127 128 129 130 131
132 137 138 150 160 165 180 185 412 414.
FIRE WEATHER...NONE.

&&

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