Area Forecast Discussion - SouthCentral and SouthWest


FXAK68 PAFC 021431
AFDAFC

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
531 AM AKST MON MAR 2 2015
.ANALYSIS AND UPPER LEVELS...

THE WEST COAST OF THE NORTH AMERICAN UPPER RIDGE IS ESTABLISHED
OVER ALASKA AND THE GULF THIS MORNING...AND THE EASTERN HALF OF
THE BERING. THE RECENT MERGER OF TWO SYSTEMS HAS RESULTED IN A
TROUGH ACROSS THE BERING FROM EASTERN RUSSIA...ENDING IN A DOUBLE
UPPER LOW WITH THE NORTHERNMOST COMPONENT OVER THE EASTERN
ALEUTIANS. AT THE SURFACE...HIGH PRESSURE IS CENTERED OVER THE
ALCAN WITH ENOUGH CIRRUS TO DIMINISH THE COVERAGE AND SEVERITY OF
FOG ACROSS SOUTHCENTRAL...WHILE A BROAD TROUGH OF LOW PRESSURE IS
OVER THE CENTRAL BERING. A DECAYING OCCLUDED FRONT IS DRAPED
ACROSS THE AKPEN AND ALONG THE SOUTHWEST AK COAST...ACCOMPANIED BY
SOME WIND..RAIN...AND POSSIBLY SOME FREEZING RAIN OVER THE
SOUTHWEST ZONES.

&&
.MODEL DISCUSSION...

MODELS ARE IN GOOD AGREEMENT THAT THE DECAYING OCCLUDED FRONT WILL
MAKE ITS WAY INLAND OVER WESTERN AK TODAY AND TONIGHT BEFORE
DISSIPATING. THE EASTERN EXTENSION OF THIS FRONT WILL CROSS THE
GULF TODAY AND REACH THE NORTH GULF COAST TONIGHT BEFORE
DISSIPATING AS IT MOVES INLAND. INITIALLY THIS SYSTEM MAINTAINS A
MOIST TROPICAL CONNECTION...BUT THAT RAPIDLY DISCONNECTS AS THE
FRONT MOVES INLAND TONIGHT INTO TUE. WHILE THIS IS HAPPENING THE
BERING TROUGH MERGES WITH THE REMAINS OF THE FRONT...CROSSING INTO
THE MAINLAND TONIGHT AND WEAKENING INTO WED.

THE NEXT SYSTEM IS A FRONT ASSOCIATED WITH A KAMCHATKA LOW. THE
FRONT ENTERS THE WESTERN BERING AND ALEUTIANS EARLY TUE AND
REACHES AS FAR AS THE CENTRAL BERING BY WED. THE LOW APPROACHES
THE WESTERN EDGE OF THE FORECAST AREA DURING THE SAME TIME.

THE SHORT TERM FORECAST UTILIZES THE NAM AND GEM FOR FINE DETAIL IN
THE EAST AND THE EC/GFS OUT WEST.

&&

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

A WEAK UPPER LEVEL WAVE COMBINED WITH OVERRUNNING IS PRODUCING
LIGHT RAIN OVER KODIAK ISLAND AND THE WESTERN GULF. THE MAIN
WEATHER PRODUCER HOWEVER IS A MUCH STRONGER UPPER LEVEL WAVE 
AND SURFACE FRONT TO THE SOUTH OF THIS. KODIAK ISLAND AND THE 
GULF COAST WILL SEE THE BULK OF PRECIP AS THESE FEATURES MOVE
THROUGH THIS AFTERNOON THROUGH TONIGHT. LOW LEVEL SOUTHEASTERLY
FLOW ACROSS THE KENAI AND CHUGACH MOUNTAINS WILL LIMIT PRECIP IN
THE USUAL LEE SIDE LOCATIONS. HOWEVER...LOW LEVEL FLOW WILL SHIFT
AS THE UPPER WAVE MOVES THROUGH LATE TONIGHT WITH A WEAKER
TRAILING WAVE EXPECTED RIGHT BEHIND THE PRIMARY ONE. THUS...EXPECT
AT LEAST SOME LIGHT PRECIP FROM THE WESTERN KENAI UP TO ANCHORAGE
AND THE MAT VALLEY.

THE BIGGEST FORECAST CHALLENGE WILL BE PRECIPITATION-TYPE. COOLER
AIR OVER THE NORTH CENTRAL PACIFIC WILL RAPIDLY CATCH UP WITH THE
WARMER AIR CURRENTLY MOVING INTO THE GULF...HELPING TO MODIFY THE
AIR MASS BEFORE IT REACHES SOUTHCENTRAL. THE END RESULT IS THAT
THE AIR MASS WHICH REACHES SOUTHCENTRAL COINCIDENT WITH THE MAIN
UPPER WAVE WILL BE ONLY MARGINALLY WARMER. TO COMPLICATE MATTERS
THE MOST SIGNIFICANT WARMING WILL LIKELY BE FROM DOWNSLOPE CONDITIONS...
PRODUCING MORE LOCALIZED WARM POOLS IN THE LEE OF THE KENAI AND
CHUGACH MOUNTAINS. SURFACE TEMPERATURES FROM THE WESTERN KENAI TO
ANCHORAGE AND THE MAT VALLEY WILL HOVER NEAR OR JUST BELOW
FREEZING TONIGHT...SO IF ANY PRECIP DOES FALL THERE IS A CHANCE OF
FREEZING RAIN. EXPECT PRECIP DURING THIS TIME WOULD BE SPOTTY...SO
THERE SHOULD NOT BE ANY SIGNIFICANT ICING PROBLEMS. AS LOW LEVEL
WINDS COME AROUND TO SOUTH ON TUESDAY TEMPERATURES OVER MUCH OF
THIS AREA SHOULD RISE ABOVE FREEZING AND PRODUCE PLAIN RAIN. THE
BEST CHANCE OF ACCUMULATING SNOW THROUGH ALL OF THIS IS OVER THE
NORTHERN SUSITNA VALLEY WHERE TEMPERATURES SHOULD REMAIN COLD
ENOUGH FOR ALL SNOW AND WHERE SOUTHERLY FLOW WILL ENHANCE PRECIP
AMOUNTS ON TUESDAY.
 

&&
.SHORT TERM FORECAST SOUTHWEST ALASKA...

ONLY MINOR IMPACTS EXPECTED THROUGH TUESDAY MORNING AS A SERIES
OF DISTURBANCES MOVE TOWARD SOUTHWEST ALASKA. RAIN WILL FALL OVER
THE BRISTOL BAY ZONE. RAIN...SNOW...AND POCKETS OF FREEZING RAIN
WILL FALL OVER THE KUSKOKWIM DELTA TODAY BEFORE SWITCHING TO RAIN
BY TONIGHT AND THEN BACK TO A MIX OF RAIN AND SNOW TUESDAY BEFORE
ENDING LATE TUESDAY. A WINTRY MIX WILL ALSO FALL OVER THE
KUSKOKWIM VALLEY MAINLY TONIGHT THROUGH EARLY TUESDAY
MORNING...WITH FREEZING RAIN MOST LIKELY THIS EVENING THROUGH
EARLY TUESDAY MORNING IN SHELTERED VALLEYS BETWEEN THE KUSKOKWIM
AND ALASKA MOUNTAIN RANGES. EVEN THE WINTRY MIX WILL CHANGE OVER
TO RAIN AS WARMER AIR MOVES INTO THE AREA TUESDAY MORNING. RAIN
AND SNOW WILL SCATTER OUT ACROSS THE AREA TUESDAY INTO TUESDAY
NIGHT...WITH ANY SNOW ACCUMULATIONS LESS THAN 1 INCH AND MAINLY
CONFINED TO THE KUSKOKWIM DELTA AND HIGHER ELEVATIONS OF THE
SOUTHWEST MAINLAND.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS...

WEAK LOW PRESSURE WILL MOVE NORTH THROUGH THE BERING SEA TODAY AND
TONIGHT PUSHING A WEST-EAST FRONT NORTH AND EAST TOWARD THE
MAINLAND. TO THE NORTH AND EAST OF THE LOW AND AHEAD OF THE
FRONT..."WARM" TEMPERATURES SUPPORT ALL RAIN. AS THE SYSTEM MOVES
NORTH AND EAST INTO TUESDAY...COOLER NORTHWESTERLY FLOW WILL PUSH
SCATTERED SNOW SHOWERS LATE TONIGHT THROUGH THE PRIBILOFS AND
RAIN TO THE EASTERN ALEUTIANS AND AKPEN TONIGHT INTO TUESDAY.

THE NEXT WEATHER FRONT MOVES NORTH AND EAST INTO THE WESTERN
ALEUTIANS LATE TONIGHT AND THEN INTO THE CENTRAL ALEUTIANS AND
BERING BY LATE TUESDAY NIGHT. PRECIPITATION-TYPE WITH THIS SYSTEM
WILL BEGIN AS SNOW AT SHEMYA BUT VERY QUICKLY CHANGEOVER TO RAIN
WITH SOUTHERLY NEAR-GALE FORCE WINDS PUSHING WARMER AIR NORTHWARD.
ALL RAIN IS EXPECTED AS THE FRONT REACHES THE CENTRAL ALEUTIANS
TUESDAY NIGHT.

&&

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

THIS TIME-FRAME WILL FEATURE A BIT MORE ACTIVE WEATHER THAN WE
HAVE SEEN FOR THE PAST COUPLE OF WEEKS. THE TWO SYSTEMS THE KICK
OFF THE WEEK...THE DECAYING FRONT OVER THE WEST AND THE WARM
OCCLUSION THAT COMES ASHORE THROUGH GULF INTO SOUTH CENTRAL...WILL
BE TAPERING OFF. THE OCCLUSION WILL MAKE FOR SOME LINGERING
MOISTURE ACROSS MUCH OF SOUTH CENTRAL INTO WEDNESDAY...ESPECIALLY
EAST AND SOUTH OF THE CHUGACH MOUNTAINS. THE FRONT OUT WEST WILL
CONTINUE TO WEAKEN AND SNOW SHOWERS SHOULD BE CONFINED TO THE
ALASKA AND ALEUTIANS RANGES BY LATE WED. MODELS HAVE CHANGED THEIR
TUNE A BIT FROM LAST NIGHT NOW SHOWING MOST OF THE UPPER LEVEL
ENERGY ASSOCIATED WITH THAT FEATURE REMAINING NORTH OF THE AK
RANGE. SO WHILE CLOUDS WILL BE VERY PREVALENT ACROSS THE COOK
INLET AND MAT-SU VALLEY...PRECIPITATION WILL BE LIMITED. WEAK LOW
PRESSURE WILL MEANDER AROUND THE GULF OF AK ON THU WITH A LITTLE
ADDED SUPPORT FROM A CUT-OFF UPPER LEVEL LOW. THIS WILL KEEP
PRECIPITATION IN THE FORECAST ALONG THE GULF COAST AND INTO THE
PRINCE WILLIAM SOUND FOR MOST OF THE WEEK.

AS WE LOOK TOWARD NEXT WEEKEND...THE TRANSIENT WEATHER PATTERN
CONTINUES. THE DIFFERENCE IN THIS PERIOD IS THAT A 150 KT POLAR
JET POINTED FROM SHEMYA TOWARD THE WEST COAST WILL CARRY A LARGE
LOW PRESSURE CENTER TOWARD THE STATE. WHILE NOT PARTICULARLY
STRONG...IT WILL BE QUITE VAST IN ITS EXTENT. THIS SYSTEM WILL
COVER MOST OF THE BERING SEA WITH PRECIPITATION ON FRIDAY BEFORE
MAKING ITS WAY TOWARD THE SEWARD PENINSULA ON SATURDAY. IT WILL
PUSH A BROAD SWATH OF PRECIPITATION ACROSS JUST ABOUT ALL OF AK
THROUGH SATURDAY INTO SUNDAY. IT DOES PULL UP SOME WARMER AIR WITH
THE PRECIPITATION...BRINGING 850MB TEMPERATURES TO NEAR -5 TO -6C
AROUND THE COOK INLET. THIS COULD LEAD TO A RAIN/SNOW MIX FOR MANY
LOCATIONS. HOWEVER...BECAUSE IT IS SUCH A LARGE SYSTEM...IT DOES
PULL DOWN SOME IMPRESSIVELY COLD AIR FROM THE ARCTIC ON SUNDAY-
MONDAY. MODELS ARE IN ALMOST SCARY-GOOD AGREEMENT WITH THIS LOW
ALREADY. THUS...HAVE BUMPED UP WINDS AND PRECIPITATION WITH THIS
FORECAST PACKAGE.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...FREEZING RAIN ADVISORY 152 155
MARINE...GALE 119 120 130 137 138 139 177 178
         HEAVY FREEZING SPRAY 185
FIRE WEATHER...NONE.

&&

$$

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