Small Craft Advisory 215, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
509 AM AKDT TUE MAR 10 2015

.ANALYSIS AND UPPER LEVELS...

AN UPPER LEVEL TROUGH IS OVER THE ALCAN THIS MORNING WITH ANOTHER
DEEPENING TROUGH SOUTH OF THE CENTRAL ALEUTIANS. ZONAL FLOW IS
OVER THE GULF OF AK WITH A 100 KT JET MOVING ACROSS SE AK. A SHARP
RIDGE IS OVER KAMCHATKA AND THE WESTERN BERING..WITH A STRONG JET
DOWN THE EASTERN FLANK HELPING TO DEEPEN THE TROUGH SOUTH OF THE
ALEUTIANS. AT THE SURFACE...LOW PRESSURE OVER THE NORTHERN GULF IS
EXITING INTO CANADA WITH STRONG OUTFLOW WINDS OUT OF CHANNELED
TERRAIN IN ITS WAKE. A FRONT ASSOCIATED WITH A SURFACE LOW WELL
SOUTH OF THE ALEUTIANS IS SPREADING GALE FORCE WINDS AND A MIX OF
RAIN AND SNOW OVER THE CENTRAL ISLANDS. DRY AND COLD CONDITIONS
COVER THE MAINLAND ZONES.

&&

.MODEL DISCUSSION...

MODELS REMAIN IN EXCELLENT AGREEMENT THROUGH THE SHORT TERM. THE
GULF LOW MOVES INTO CANADA LATER TODAY. THE LOW SOUTH OF THE
ALEUTIANS WILL DRIFT SOUTH AND DRAG THE ASSOCIATED FRONT AWAY FROM
THE ALEUTIANS. THIS LOW WILL THEN MERGE WITH A ANOTHER LOW
BETWEEN 40N AND 50N SOUTH OF THE GULF TUE NIGHT. AS THIS LOW
PIVOTS IN THE NORTH PACIFIC IT WILL BRING A FRONT INTO THE EASTERN
GULF WED NIGHT. IN THE WEST...SURFACE HIGH PRESSURE WILL SHRINK
AND MOVE TO THE CENTRAL BERING BY WED NIGHT. THE NEXT KAMCHATKA
FRONT WILL THEN ENTER THE WESTERN BERING WED NIGHT. WHILE THIS IS
HAPPENING...THE MAINLAND ZONES OF THE FORECAST AREA WILL REMAIN
UNDER A COLD AND DRY AIRMASS. THE FORECAST IS WELL REPRESENTED BY
THE GFS AND EC...WITH THE GEM REGIONAL WIND FIELD CAPTURING THE
OUTFLOW WINDS.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (TUESDAY AND WEDNESDAY)...

CLOUDS AND SNOW SHOWER ACTIVITY WILL REMAIN CONFINED TO THE
EASTERN COPPER BASIN TODAY AS AN UPPER LOW MEANDERS AROUND THE
ALCAN BORDER WITH A CORRESPONDING SFC LOW ALONG THE NORTHEAST GULF
COAST. AS THIS SFC LOW WEAKENS AND MOVES INTO THE NORTHERN
PANHANDLE TODAY THE PRESSURE GRADIENTS ACROSS OUR REGION WILL
SLOWLY RELAX. GAPS AROUND PRINCE WILLIAM SOUND HAVE BEEN THE MOST
ACTIVE...AND WILL LIKELY CONTINUE TO GUST INTO THE 40-50 MPH RANGE
FOR MUCH OF THE DAY TODAY.

WHILE WIND SPEEDS WILL COME DOWN SOMEWHAT HEADING INTO
WEDNESDAY...THE OVERALL WEATHER REGIME WILL REMAIN THE SAME AS
COLD AND SUNNY CONDITIONS PREVAIL FOR MOST AREAS. ONE AREA OF
CONCERN IS THE POTENTIAL FOR SOME OCEAN-EFFECT SNOW SHOWER
ACTIVITY TO DEVELOP AROUND SOUTHERN COOK INLET/NORTHERN KODIAK
ISLAND IN THE WEDNESDAY/THURSDAY TIMEFRAME. TEMPERATURE
DIFFERENCES BETWEEN THE AIRMASS AND OCEAN TEMPERATURE ON THE ORDER
OF 20C WILL BE MORE THAN ENOUGH TO GENERATE SUFFICIENT VERTICAL
MOTION TO GET THE BALL ROLLING ON BOUNDARY LAYER CONVECTION. THE
MAIN CHALLENGE IS THAT THIS PHENOMENON IS HIGHLY SENSITIVE TO
SUBTLE CHANGES IN THE WIND FIELD (BOTH SPATIALLY AND WITH RESPECT
TO TIME). THE BOTTOM LINE IS THAT SOMEWHERE BETWEEN SELDOVIA AND
NORTHERN AFOGNAK ISLAND (KODIAK) STANDS A GOOD CHANCE OF SEEING A
PERSISTENT BAND OF SNOW SHOWERS SET UP BY MIDWEEK. HAVE INTRODUCED
CHANCE POPS WITH THE MORNING FORECAST...BUT WILL NEED TO MONITORED
CLOSELY AS WE GET CLOSER IN TIME.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (TUESDAY AND WEDNESDAY)...A
BROAD UPPER LEVEL TROUGH WILL PERSIST OVER MAINLAND ALASKA WITH
NORTH TO NORTHWEST LOW LEVEL FLOW FROM SURFACE HIGH PRESSURE OVER
THE ARCTIC COAST AND KAMCHATKA PENINSULA KEEPING SW ALASKA WELL
SUPPLIED WITH COLD AIR FOR THE NEXT SEVERAL DAYS. WITH THE
EXCEPTION OF A FEW SNOW SHOWERS POSSIBLY CLIPPING THE WESTERN
CAPES AND NORTH SIDE OF THE ALASKA PENINSULA CONDITIONS WILL
REMAIN DRY AND MOSTLY SUNNY.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (TUESDAY AND WEDNESDAY)...
THE FRONTAL SYSTEM STRETCHED OVER THE CENTRAL ALEUTIANS WILL
SLIDE SLOWLY SOUTH FOLLOWING ITS PARENT LOW TONIGHT AND WEDNESDAY.
THIS WILL ALLOW MORE OF THE COLD BERING AIR TO ALSO SAG ACROSS AND
SOUTH OF THE ALEUTIANS. THE NEXT FRONTAL SYSTEM WILL APPROACH THE
WESTERN ALEUTIANS WEDNESDAY AFTERNOON AND EVENING THEN CONTINUE
EAST TO REACH 180 BY THURSDAY MORNING SPREADING RAIN AND SNOW WITH
IT.

&&

.LONG TERM FORECAST (DAYS 3 THROUGH 7)...
CONSENSUS IS FINALLY BEGINNING TO SHOW IN THE MODELS WITH THE LOW
APPROACHING THE GULF ON THURSDAY AFTER TODAYS RUNS. THEY ALL ARE
NOW IN MUCH BETTER AGREEMENT WITH THE PLACING OF A SLIGHTLY
WEAKER ARCTIC TROUGH OVER THE MAINLAND ON THURSDAY-FRIDAY...AND
NOW WITH THE 18Z GFS HAVE COME INTO MUCH BETTER AGREEMENT WITH A
SURFACE LOW TRACK NEAR YAKUTAT ON THURSDAY EVENING. THE GFS HAS
ALSO BECOME CONSISTENTLY WEAKER WITH THIS LOW INITIALLY WITH A
TRACK FURTHER TO THE EAST...WHICH WILL RESULT IN LESS WARM AIR
ADVECTION THAT IS NOW EXPECTED TO BE DIRECTED TOWARDS THE
PANHANDLE RATHER THAN THE SOUTH MAINLAND. THERE REMAINS SOME
DISAGREEMENT AS TO THE STRENGTH OF A WEAK TROUGH OR WAVE THAT WILL
FORM ALONG THE NORTH GULF COAST INTO PRINCE WILLIAM SOUND AS THIS
LOW APPROACHES THE COAST...BUT THIS SHOULD HAVE LITTLE IMPACT IN
THE OVERALL FORECAST OF SNOW SHOWERS DEVELOPING AND OUTFLOW WINDS
INCREASING ON FRIDAY. SNOWFALL FROM THIS SYSTEM AT THIS POINT
APPEARS MOST LIKELY ALONG THE NORTH GULF COAST AND COPPER RIVER
BASIN ON FRIDAY AND SATURDAY...WITH STRONG SUBSIDENCE AND
CONTINUED COLD ADVECTION ON THE COLD SIDE OF A STRONG UPPER LEVEL
JET THAT FORMS ALONG OR JUST EAST OF THE SPINE OF THE
CHUGACH/KENAI RANGES THAT WILL INHIBIT SNOW CHANCES FOR ANCHORAGE
AND OTHER LOCATIONS ON THE WEST SIDE OF THE MOUNTAINS.

OVER THE BERING...A GALE FORCE FRONT ENTERING THE WESTERN
ALEUTIANS WEDNESDAY NIGHT WILL GET CAUGHT UP IN THE BROAD ARCTIC
TROUGH DESCENDING THROUGH THE MAINLAND DURING THE SAME
TIME...CAUSING THIS SYSTEM TO QUICKLY SHEER APART AND WEAKEN BY
FRIDAY MORNING. THIS WILL KEEP RAIN AND SNOW SHOWERS WITH NOTABLY
WEAKER WINDS IN THE FORECAST AS IT SPREADS INTO THE CENTRAL BERING
BY EARLY THIS WEEKEND.

BY SUNDAY...THERE CONTINUES TO BE VERY GOOD AGREEMENT AMONG THE
MODELS THAT THE NORTHEAST PACIFIC RIDGE WILL BUILD THROUGH THE
PANHANDLE INTO THE EASTERN INTERIOR (A FAMILIAR PATTERN FROM THIS
WINTER). BECAUSE OF THIS...THERE IS GOOD CONFIDENCE THAT MORE
SEASONAL TEMPERATURES WILL RETURN TO THE SOUTHERN MAINLAND BY
MONDAY AND LIKELY PERSIST INTO MID WEEK. THIS WILL ALSO USHER IN A
RETURN TO WARM AND MOIST NORTH PACIFIC LOWS ON A NORTHERLY TRACK
TO THE MAINLAND AND EASTERN BERING FOR THE FIRST HALF OF NEXT
WEEK.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...GALES...170 172-178
 HVY FRZG SPRY....121 126-131 138 139 150 160 165 179 180
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...DS
SOUTHCENTRAL ALASKA...AD
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JR
LONG TERM...DEK/CC