Heavy Freezing Spray Warning 71, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
458 PM AKDT MON MAR 28 2016

.ANALYSIS AND UPPER LEVELS...
A STRONG BLOCKING RIDGE IS NOW FIRMLY ENTRENCHED FROM THE EASTERN
PACIFIC NORTHWARD TO SOUTHEAST ALASKA AND UP INTO WESTERN CANADA.
A LARGE CLOSED LOW IS EMBEDDED WITHIN A CORRESPONDINGLY HIGH AMPLITUDE
TROUGH OVER THE NORTH CENTRAL PACIFIC. A 150 KT SUBTROPICAL JET IS
RIDING NORTHWARD BETWEEN THE RIDGE AND TROUGH (BETWEEN 150W AND
160W LONGITUDE)...HELPING TO ADVECT WARM MOIST AIR NORTHWARD TO
MAINLAND ALASKA. A CORRESPONDING BAROCLINIC ZONE CAN BE FOUND IN
THE LOW LEVELS UNDERNEATH THE JET.

THE FRONTAL SYSTEM WHICH ORIGINATED FROM THE LOW SITTING OVER THE
NORTH CENTRAL PACIFIC STALLED ACROSS BRISTOL BAY AND THE NORTHERN
GULF OF ALASKA YESTERDAY. THERE HAS BEEN SOME WARMING OF THE LOW
LEVELS OUT AHEAD OF THE FRONTAL SYSTEM...BUT MUCH WARMER AIR IS
CURRENTLY STALLED TO THE SOUTH ACROSS THE GULF OF ALASKA AND
NORTHEAST PACIFIC. THERE WERE QUITE A FEW AREAS OF SNOWFALL THIS
MORNING...ACROSS THE SUSITNA VALLEY AND NORTHERN COPPER RIVER
BASIN ALONG WITH THE VALDEZ AREA. MANY OF THESE AREAS HAVE WARMED
JUST ENOUGH TO CHANGE PRECIP OVER TO SNOW. RAIN IS FAIRLY
WIDESPREAD ACROSS SOUTHCENTRAL WITH THE EXCEPTION OF THE COPPER
RIVER BASIN WHICH IS LARGELY BEING DOWNSLOPED. SURFACE RIDGING
ALONG THE NORTH GULF COAST IS PRODUCING LOCALLY STRONG AND GUSTY
WINDS THROUGH THE NORMAL MOUNTAIN GAPS.

THE PRECIPITATION SHIELD IS SPREADING NORTH AND WEST TOWARD SOUTHWEST
ALASKA AS A SERIES OF UPPER WAVES RIDING UP THE RIDGE APPROACH THE
REGION. THERE IS ALSO A WEAK SURFACE LOW SOUTH OF KODIAK ISLAND
TRACKING SLOWLY NORTHWARD. WITH MARGINALLY COLDER AIR MOVING IN
BEHIND THIS LOW...THE WARMEST AIR WILL REMAIN OVER THE GULF OF
ALASKA. CONSIDERING THE HIGH AMPLITUDE PATTERN...THE EXTENT OF
WARM AIR ADVECTION INTO MAINLAND ALASKA IS LESS THAN EXPECTED.

MEANWHILE...A SERIES OF WEAK UPPER WAVES COMING OUT OF RUSSIA HAS
REINFORCED ARCTIC AIR ACROSS MOST OF THE BERING SEA...WITH COOLER
AIR FILTERING EASTWARD INTO SOUTHWEST ALASKA. AS PRECIP SPREADS
INTO SOUTHWEST ALASKA TODAY THIS WILL LEAD TO A NARROW TRANSITION
AREA FROM RAIN IN THE SOUTH AND EAST TO SNOW AS YOU HEAD NORTH AND
WEST.

&&

.MODEL DISCUSSION...
MODELS REMAIN IN GOOD AGREEMENT WITH THE LARGE SCALE PATTERN AND
MAJOR FEATURES MOVING THROUGH IT. ALL SOLUTIONS HAVE BACKED OFF
THE EXTENT OF WARM ADVECTION ACROSS SOUTHCENTRAL TODAY AND
TONIGHT...SO WILL FOLLOW THAT TREND AND ADJUST TEMPS AND PRECIP-
TYPE WHERE NECESSARY. THE KEY FORECAST CHALLENGE IS THE TRACK OF
THE WEAK SURFACE LOW AS IT HEADS NORTH TONIGHT. THIS WILL GREATLY
IMPACT EXTENT OF WARMING ACROSS THE SUSITNA VALLEY AND EASTERN
PORTIONS OF SOUTHWEST ALASKA...AND ULTIMATELY DETERMINE PRECIPITATION
TYPE. GENERALLY PREFER THE MORE EASTERN TRACK OF THE LOW...UP
COOK INLET AND INTO THE SUSITNA VALLEY. THIS MEANS A TREND TOWARD
COOLER TEMPS AND MORE SNOW OVER PORTIONS OF THE SUSITNA VALLEY.
OVERALL FORECAST CONFIDENCE IS FAIRLY HIGH. CONFIDENCE IS LOWER
ACROSS THE CORRIDOR OF PRECIP-TYPE TRANSITION...MAINLY FROM THE
SUSITNA VALLEY WESTWARD INTO INTERIOR SOUTHWEST ALASKA.

&&

.VOLCANO...
SATELLITE IMAGERY INDICATES THAT PAVLOF VOLCANO LOCATED ON THE
ALASKA PENINSULA CONTINUES TO ERUPT CONTINUOUSLY...AS IT HAS BEEN
SINCE SUNDAY EVENING. WITH PERSISTENT LOW LEVEL FLOW FROM THE
SOUTH-SOUTHWEST...MUCH OF THIS ASH HAS BEEN CARRIED TO THE NORTH
TOWARDS NELSON LAGOON WHERE WE HAVE REPORTS OF ASHFALL ON THE
COMMUNITY. AS IT TYPICALLY THE CASE WITH VOLCANIC
ERUPTIONS...THERE REMAINS UNCERTAINTY IN HOW MUCH ASH IS BEING
CARRIED AWAY FROM THE VOLCANO TO ACCUMULATE ON THE AREA. AFTER
COORDINATING WITH THE ALASKA VOLCANIC OBSERVATORY AND MONITORING
AREA WEBCAMS TODAY...THIS IS MOST LIKELY LIGHT ACCUMULATION (LESS
THAN ONE-QUARTER INCH)...BUT MORE THAN THE TRACE THAT WAS
INITIALLY REPORTED. AS A RESULT...WE UPGRADED THE SPECIAL WEATHER
STATEMENT TO AN ASHFALL ADVISORY TODAY TO REFLECT THIS MORE
SIGNIFICANT ACCUMULATION. STARTING LATE TONIGHT...LOW LEVEL WINDS
ARE EXPECTED TO SHIFT TO MORE OF A WESTERLY DIRECTION AS AN UPPER
LEVEL TROUGH APPROACHES THE AKPEN FROM THE EASTERN BERING...WHICH
(ASSUMING ERUPTIONS CONTINUE) WOULD SHIFT THE AREA OF EXPECTED
ASHFALL TO THE EAST...POSSIBLY EXTENDING INTO THE SAND POINT AREA
BY TUESDAY MORNING.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 AND 2)...
WET AND WARM WEATHER WILL CONTINUE OVER MOST OF SOUTHCENTRAL
ALASKA THROUGH WEDNESDAY AS A HIGH AMPLITUDE RIDGE REMAINS IN
PLACE OVER THE WESTERN COAST OF NORTH AMERICA...KEEPING THE
REGION IN MOIST SOUTHERLY FLOW. TONIGHT...PRECIPITATION WILL
GRADUALLY START TO DIMINISH FROM SOUTHWEST TO NORTHEAST AS A WEAK
SURFACE LOW AND ITS ASSOCIATED UPPER WAVE MOVES UP SHELIKOF
STRAIGHT INTO COOK INLET AND THE SUSITNA VALLEY. PRECIPITATION
TYPE OVER THE UPPER SUSITNA VALLEY WILL BE TRICKY WITH THIS LOW
WITH THE ACTUAL TRACK BEING THE DETERMINING FACTOR IN HOW MUCH
WARM AIR MOVES INTO THE NORTHERN VALLEY. FROM SKWENTNA ALONG THE
ALASKA RANGE TO THE PARKS HIGHWAY NORTH OF TALKEETNA...PRECIPITATION
WILL LIKELY BE MOSTLY SNOW...MIXING BACK AND FORTH WITH RAIN AS
BITS OF WARM AIR MOVE IN AND OUT OF THE AREA. ANY SNOW
ACCUMULATION WILL BE IN THESE AREAS...WITH ABOUT 2 TO 4 INCHES OF
WET SNOW POSSIBLE. WINDS TONIGHT OVER TURNAGAIN ARM AND PORTAGE
VALLEY WILL CONTINUE TO INCREASE THROUGH THE EVENING AS THE
GRADIENT TIGHTENS WITH THE APPROACH OF THE LOW. WINDS WILL
DECREASE AS THE LOW MOVES NORTHEASTWARD OVERNIGHT. OVER THE
WESTERN KENAI PENINSULA AND THE ANCHORAGE AREA...WINDS ALOFT WILL
GRADUALLY SHIFT THROUGH THE EVENING HOURS..BRINGING A DOWNSLOPE
EFFECT INTO THE REGION AND TAPERING PRECIPITATION DOWN THROUGH THE
EVENING.

PRECIPITATION WILL GRADUALLY DECREASE DURING TUESDAY OVER MOST OF
THE INTERIOR AREAS BEFORE A SECOND UPPER WAVE MOVING ALONG THE
WESTERN EDGE OF THE RIDGE INTERACTS WITH AN UPPER TROUGH MOVING
THROUGH THE INTERIOR. THIS WILL BRING RAINFALL BACK INTO THE
FORECAST FOR MOST OF THE REGION WITH THE HEAVIEST AMOUNTS EXPECTED
ALONG COASTAL AREAS AND THE WESTERN ALASKA RANGE. THE FLOW ALOFT
WILL REMAIN SOUTHERLY...AND TEMPERATURES OVER THE REGION ARE
EXPECTED TO RESPOND IN KIND WITH HIGHS IN THE MID 40S TO LOWER
50S POSSIBLE ON WEDNESDAY OVER MUCH OF THE REGION. WINDS WILL ONCE
AGAIN INCREASE TUESDAY AFTERNOON AND EVENING OVER TURNAGAIN ARM
AND PORTAGE VALLEY AS THE SURFACE HIGH ALONG THE WESTERN NORTH
AMERICAN COAST SHIFTS INTO SOUTHEAST ALASKA...TIGHTENING THE
CROSS-GULF GRADIENT.

&&

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

TWO SEPARATE WEATHER SYSTEMS WILL DRIVE THE WEATHER ACROSS THE
SOUTHWEST MAINLAND IN THE SHORT TERM. A WEAK LOW PRESSURE SYSTEM
SPINNING WEST OF BRISTOL BAY IS RESULTING IN BROAD SOUTHWESTERLY
FLOW AND WIDESPREAD SHOWERS ALONG THE BRISTOL BAY COAST AND
KUSKOKWIM DELTA. THIS LOW WILL CONTINUE TO BRING SHOWERS TO THE
KUSKOKWIM DELTA THROUGH TUESDAY AS THIS LOW PUSHES INLAND AND
DISSIPATES. MEANWHILE...A FRONT THAT IS CURRENTLY LOCATED ALONG
THE WESTERN GULF OF ALASKA...WILL PUSH WESTWARD
OVERNIGHT...REINFORCING WIDESPREAD SHOWERS ACROSS THE EAST BRISTOL
BAY AND EAST KUSKOKWIM VALLEY TUESDAY. A SECOND MORE POTENT FRONT
WILL PUSH ONSHORE LATE TUESDAY NIGHT AND REINFORCE SHOWERS AND
WARMER AIR ACROSS THE REGION...INCLUDING THE ALASKA PENINSULA
THROUGH WEDNESDAY.

&&

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

BROAD NORTHEASTERLY FLOW ACROSS MUCH OF THE WEST AND CENTRAL
BERING IS RESULTING IN WIDESPREAD COLD AIR ADVECTION. WHILE
PARTLY CLOUDY SKIES ARE OCCURRING FROM THE ICE EDGE TO THE
PRIBILOFS...A BANK OF STRATUS CONTINUES TO LINGER ALONG THE
NORTHERN COASTS OF THE ALEUTIAN ISLANDS. BROAD NORTHWESTERLY FLOW
IS EXPECTED TO PERSIST THE NEXT FEW DAYS...RESULTING IN RATHER
QUIET CONDITIONS.

&&

.LONG TERM FORECAST (DAYS 3 THROUGH 7)...
TEMPERATURES WILL REMAIN ABOVE SEASONAL NORMAL VALUES THROUGH THE
END OF THE WORK WEEK AS WARM AND MOIST AIR CONTINUES TO STREAM
NORTHWARD INTO SOUTHERN ALASKA FROM THE SUBTROPICAL PACIFIC ON THE
PERIPHERY OF A STOUT UPPER LEVEL RIDGE OVER THE NORTHEAST PACIFIC.
SOUTHCENTRAL AK WILL SEE DRIER CONDITIONS ON THURSDAY AS THE RIDGE
AXIS SHIFTS TO THE NORTHWEST AND THE BULK OF THE PRECIPITATION
BECOMES FOCUSED ON THE SOUTHWEST MAINLAND AND EASTERN BERING.
PRECIPITATION WILL THEN QUICKLY RETURN TO SOUTHCENTRAL AK BY
THURSDAY NIGHT AS A FRONT ASSOCIATED WITH A SYSTEM APPROACHING THE
AKPEN MOVES INTO THE GULF AND ONSHORE FLOW IS RE-ESTABLISHED ALONG
THE GULF COAST. TEMPERATURES WILL BEGIN TO MODERATE DURING THE
WEEKEND AS THE WEAKENING RIDGE IS DISPLACED EASTWARD AND THE
SUBTROPICAL CONNECTION IS REPLACED BY MORE OF A SOUTHWESTERLY FLOW
PATTERN AHEAD OF A BROAD UPPER TROUGH ENCOMPASSING THE EASTERN
BERING. THIS WILL SUPPORT THE CONTINUATION OF A FAIRLY WET PATTERN
ACROSS THE SOUTHERN MAINLAND INTO THE BEGINNING OF NEXT WEEK.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...ASH FALL ADVISORY 181.
MARINE...GALES 119 125 150 165.

&&

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