Small Craft Advisory 480, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
427 PM AKDT SAT JUN 6 2015

.ANALYSIS AND UPPER LEVELS...
THERE IS ONE MAJOR FEATURE IN THE PATTERN OVER THE ALASKA AREA.
THAT IS A CLOSED LOW OVER THE BERING STRAIT REGION WITH A BROAD
TROUGH EXTENDING FROM THE SEA OF OKHOTSK TO THE GULF OF ALASKA. A
STRONG JET OF 160 KT IS OVER THE NORTH PACIFIC FROM THE BASE OF
THE TROUGH IN THE SEA OF OKHOTSK OUT TO THE DATE LINE. THIS IS
ACTING TO PUSH A FEW SHORT WAVE TROUGHS AND ASSOCIATED SURFACE
WARM FRONTS INTO THE WESTERN ALEUTIANS. ANOTHER TROUGH AXIS IS
SITUATED ALONG 150W WITH A JET OF 120 KT ORIENTED TOWARD THE
PANHANDLE. AROUND THE BERING STRAIT LOW A FEW VORT MAXES/SHORT
WAVE TROUGHS ARE ROTATING IN THE BERING SEA AND WESTERN GULF OF
ALASKA. THE STRONGEST OF THESE WITH A SURFACE REFLECTION IS OVER
THE WESTERN GULF OF ALASKA. COMBINED WITH THE JET IN THE EASTERN
GULF...THE SURFACE LOW WILL CONTINUE TO LIFT TOWARD THE NORTH
COAST THIS EVENING. AS A RESULT...MODERATE WINDS WILL AFFECT THE
NORTHERN GULF COAST THROUGH THE EVENING. IN ADDITION...THIS WILL
KEEP MOISTURE PUMPING INTO SOUTHCENTRAL ALBEIT SOME RAIN SHADOW
EFFECT OVER THE ANCHORAGE BOWL...MAT VALLEY AND PORTIONS OF THE
WESTERN KENAI. BEHIND THE UPPER LOW OVER THE WESTERN GULF OF
ALASKA....A DEFORMATION ZONE EXISTS ACTING A BIT AS A FOCUS
MECHANISM FOR SHOWER ACTIVITY ALONG THE ALASKA RANGE AND INTO
BRISTOL BAY INLAND AREAS TO THE KUSKOKWIM VALLEY. THE AIR MASS
ALONG THE TALKEETNA AND ALASKA RANGE MOUNTAINS IS ENOUGH FOR
POSSIBLE ISOLATED WET THUNDERSTORMS TODAY AND TONIGHT.

AS THE SHORT WAVE TROUGH LIFTS NORTH OVER SOUTHCENTRAL ALASKA
TOMORROW...HIGH PRESSURE WILL MOVE IN ALOFT AND AT THE SURFACE
OVER SOUTHCENTRAL AND SOUTHWEST ALASKA. HOWEVER...ENOUGH
INSTABILITY WILL REMAIN THROUGH MUCH OF THE DAY FOR SHOWERS AND
POSSIBLE ISOLATED THUNDERSTORMS IN THE SAME AREAS AS TODAY. ON
SUNDAY AS WELL...THE LOW THAT WAS SOMEWHAT ANCHORED IN THE SEA OF
OKHOTSK WILL START TO SWING INTO THE WESTERN ALEUTIANS WITH A 150
KT JET.

AS MONDAY ROLLS AROUND...THE WESTERN ALEUTIAN LOW MOVES INTO THE
CENTRAL ALEUTIANS BUILDING THE RIDGE IN THE GULF OF ALASKA AT THE
SURFACE AND ALOFT AND BRINGING A RETURN OF MOISTURE TO SOUTHWEST
ALASKA AND EVENTUALLY SOUTHCENTRAL ALASKA. IT ALSO BUILDS A GOOD
SURFACE RIDGE ALONG THE COAST PICKING UP GAP WINDS ALONG THE NORTH
COAST AND CREATING THE NORMAL PRECIP SHADOW FOR THE KENAI
PENINSULA TO THE SUSITNA VALLEY. THIS PATTERN ALSO LENDS ITSELF TO
A DRIER AND WINDIER FLOW THROUGH THE COPPER RIVER BASIN FOR A
HEIGHTENED AWARENESS OF FIRE WEATHER CONCERNS.

&&

.MODEL DISCUSSION...
WITH A LONG WAVE TROUGH IN PLACE OVER THE BERING SEA TRANSITIONING
TO A BIT MORE PROGRESSIVE IN THE NEXT COUPLE DAYS...IT IS
SURPRISING THE NUMERICAL GUIDANCE IS IN SUCH GREAT AGREEMENT
THROUGH MONDAY. AFTER THAT TIME FRAME MODELS ARE IN FAIRLY DECENT
AGREEMENT INTO TUESDAY AND THEN THE DIFFERENCES BETWEEN THE
EC/CANADA VERSUS THE GFS START TO DEVELOP.

IN THE SHORTER TERM...SOME SLIGHT DIFFERENCES IN WIND AND
PRECIPITATION PATTERNS IN THE GULF OF ALASKA WITH THE LOW LED TO A
PREFERENCE OF THE NAM. OVERALL...FORECAST CONFIDENCE IS HIGH IN
THE SHORT TERM...THROUGH MONDAY...RIGHT NOW.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 AND 2...SUNDAY
AND MONDAY)...SUNDAY HAS THE POTENTIAL TO BE ONE OF THOSE NICE
DAYS EMBEDDED WITHIN A MOSTLY COOL AND WET PATTERN. AS THE UPPER
LEVEL LOW CURRENTLY OVER THE GULF SLIDES OFF TO THE NORTHEAST...A
WEAK RIDGE OF HIGH PRESSURE WILL TAKE ITS PLACE. THIS SHOULD BE
ENOUGH TO KEEP MOST AREAS DRY FOR THE DAY. BUT WITH COLDER
TEMPERATURES ALOFT...THE ATMOSPHERE WILL STILL BE UNSTABLE ENOUGH
TO SUPPORT SOME SHOWERS ALONG THE MOUNTAINS. THE LONE EXCEPTION TO
THIS WILL BE ALONG THE EASTERN PORTIONS OF THE ALASKA RANGE AND
THE WRANGELLS WHERE A THUNDERSTORM OR TWO STILL CANNOT BE RULED
OUT. WHERE THE SUN DOES POKE THROUGH...EXPECT SEA BREEZES TO PICK
UP IN THE AFTERNOON.

BY MONDAY WE SHIFT OUR FOCUS TO A NEW FRONT APPROACHING THE
WESTERN GULF. WHILE THE MAIN PRECIPITATION SHIELD SHOULD ONLY JUST
BE REACHING KODIAK LATE IN THE AFTERNOON...A STRONG COASTAL RIDGE
IS EXPECTED TO BUILD OUT IN FRONT OF THIS SYSTEM. THIS WILL MAKE
FOR GUSTY SOUTH TO SOUTHEAST WINDS FOR PLACES LIKE TURNAGAIN ARM
AND THE COPPER RIVER BASIN THAT WILL ONLY INCREASE THROUGH THE
DAY. SOUTHERLY FLOW WILL ALSO BE CAPABLE OF BRINGING NUMEROUS
SHOWERS TO COASTAL LOCATIONS THROUGH THE DAY AHEAD OF THE MAIN
CORRIDOR OF RAIN. BY MONDAY NIGHT...DEEP LAYER FLOW TURNS MORE
SOUTHEASTERLY. THIS SHOULD SET-UP THE CLASSIC DOWNSLOPE HOLE
KEEPING AREAS DOWNSTREAM OF THE KENAI AND CHUGACH MOUNTAINS MOSTLY
DRY.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)...
ISOLATED TO SCATTERED SHOWERS WILL CONTINUE TO DEVELOP TONIGHT
OVER THE SOUTHWEST AS A WEAK TROUGH LIFTS NORTH. ISOLATED
THUNDERSTORMS ARE POSSIBLE THIS EVENING AND AGAIN LATE SUNDAY
ACROSS THE WESTERN ALASKA RANGE ALONG A WEAK BOUNDARY. A RIDGE
BUILDING ACROSS THE GULF OF AK SLIDES ACROSS THE SOUTHWEST ON
SUNDAY KEEPING THE AREA DRYER UNTIL THE NEXT FRONT MOVES TO THE
SOUTHWEST COAST MONDAY MORNING. LIGHT RAIN ALONG THE FRONT SLOWLY
SPREADS INLAND THROUGH TUESDAY.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...
LOW PRESSURE REMAINS THE DOMINANT WEATHER PATTERN INTO THE
BEGINNING OF THE WORK WEEK BRINGING CLOUDS AND INTERMITTENT LIGHT
RAIN THROUGH TUESDAY. THE FRONT CURRENTLY ALONG THE WESTERN HALF
OF THE ALEUTIAN CHAIN PROGRESSES EAST AND SOUTH THROUGH SUNDAY
AND IS QUICKLY FOLLOWED BY ANOTHER FRONT THAT PUSHES INTO THE
CENTRAL ALEUTIANS MONDAY MORNING. GUSTY SMALL CRAFT WINDS UP TO 30
KNOTS CAN BE EXPECTED ALONG THE FRONTAL PASSAGES WITH NORTHERLY
FLOW RETURNING TO THE BERING BEHIND THE SECOND FRONT.

&&

.LONG TERM FORECAST (DAYS 3 THROUGH 7)...
A LONGWAVE TROUGH COVERING MUCH OF THE BERING AND ALEUTIANS EARLY
IN THE WEEK WILL BEGIN TO SHIFT EASTWARD AND BECOME MORE
NEGATIVELY TILTED OVER THE SOUTHERN MAINLAND BY MID WEEK. A RIDGE
OVER THE GULF WILL CONTINUE TO BUILD EASTWARD INTO EASTERN
ALASKA/NORTHWEST CANADA THROUGH MID WEEK...AS A SERIES OF WEAK
WAVES ROTATE AROUND THE BASE OF THE TROUGH AND PUSH THE RIDGE
FURTHER EASTWARD. WEAK RIDGING THEN TRIES TO BUILD BACK OVER THE
BERING/ALEUTIANS FROM THE NORTH PACIFIC BY THE END OF THE
WEEK...BUT IS HALTED BY A POLAR TROUGH STRETCHING DOWN OVER THE
CENTRAL BERING FRIDAY. THIS TROUGH THEN REDEVELOPS OVER THE
WESTERN BERING AS IT COMBINES WITH AN UPPER LOW TRACKING IN FROM
THE NORTH PACIFIC...PUSHING THE WEAK BERING RIDGE BACK OVER THE
GULF/SOUTHEAST ALASKA. THIS PATTERN IS VERY SIMILAR TO THE ONE
THAT WILL SET UP OVER THE AREA EARLIER IN THE WEEK.

AT THE SURFACE...MOIST SOUTHERLY FLOW OVER THE SOUTHERN MAINLAND
WILL KEEP SHOWERY CONDITIONS IN THE FORECAST THROUGH MUCH OF THE
LONG TERM. THE BULK OF PRECIPITATION WILL BE CONFINED TO THE
NORTHERN AND WESTERN GULF COAST TUESDAY INTO WEDNESDAY...AS A
DECAYING LOW AND ASSOCIATED FRONT TRACK INTO THE THE WESTERN GULF.
DURING THIS TIME...SOUTHEASTERLY FLOW OVER THE CHUGACH AND
ALEUTIAN RANGE WILL KEEP AREAS JUST WEST OF THE RANGES DOWNSLOPED
FROM MUCH OF THE PRECIPITATION. GAP WINDS ALONG THE NORTHERN GULF
COAST AND COPPER RIVER WILL BEGIN TO DEVELOP AS RIDGING BUILDS
INTO THE GULF EARLY IN THE WEEK. WINDS THROUGH TURNAGAIN ARM ARE
EXPECTED TO BEND DOWN INLET AND STAY OUT OF THE ANCHORAGE
BOWL...AS THERE WILL BE A LIMITED AMOUNT OF BREAKS IN CLOUD COVER
(DIURNAL HEATING) OVER ANCHORAGE OR THE LOWER SUSITNA VALLEY AT
THIS TIME. OUT WEST...A LOW OVER THE CENTRAL ALEUTIANS WILL WEAKEN
AND TRACK EASTWARD OVER THE ALASKA PENINSULA BY MID WEEK...KEEPING
SHOWERY CONDITIONS OVER THE EASTERN HALF OF THE ALEUTIANS AND
BERING THROUGH MID WEEK. WEAK RIDGING BUILDS BACK OVER THE
BERING/ALEUTIANS MID WEEK...TAPERING OFF MOST OF THE SHOWERY
CONDITIONS OVER THE AREA THROUGH THE END OF THE WEEK. RAINY AND
WINDY CONDITIONS WILL PICK BACK UP OVER THE WESTERN/CENTRAL
ALEUTIANS FRIDAY INTO SATURDAY...AS A FRONTAL SYSTEM APPROACHES
FROM THE NORTH PACIFIC.

THE MODELS WERE IN MUCH BETTER AGREEMENT TODAY WITH THE OVERALL
SYNOPTIC PATTERN MENTIONED ABOVE. THE GFS HAS STARTED TO COME
INTO BETTER AGREEMENT WITH THE OTHER MODELS...WHICH HELPED RAISE
FORECAST CONFIDENCE HIGHER THROUGH MUCH OF THE LONG TERM.
HOWEVER...THERE ARE STILL SOME DIFFERENCES IN THE MODELS WITH THE
TIMING AND STRENGTH OF THE WAVES ROTATING AROUND THE BASE OF THE
TROUGH FROM MIDWEEK INTO THE WEEKEND...KEEPING OVERALL FORECAST
CONFIDENCE SLIGHTLY ABOVE AVERAGE.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...NONE.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...JN
SOUTHCENTRAL ALASKA...MO
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KH
LONG TERM...TP