000
FXAK68 PAFC 181239
AFDAFC
SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
500 AM AKDT SAT MAY 18 2013
.ANALYSIS AND UPPER LEVELS...
NEGATIVE TILTED UPPER LEVEL TROUGH CONTINUES TO HANG FROM FROM
NORTHWEST ALASKA SOUTHEASTWARD THROUGH THE WESTERN GULF OF ALASKA.
THE FIRST VORTICITY MAXIMUM IN THE TROUGH IS PUSHING INLAND OVER THE
NORTH GULF COAST EARLY THIS MORNING WHILE THE SECOND IS DIGGING INTO
THE GULF. THE EDGE OF HIGHER MOISTURE VALUES AHEAD OF THE FIRST
DISTURBANCE IS EVIDENT ON WATER VAPOR IMAGERY...WHICH SIGNALS THE
DECLINE OF DYNAMIC LIFT FROM SOUTHWEST TO NORTHEAST OVER THE
SOUTHCENTRAL. THE REMAINING SNOWFALL WEST OF THE CHUGACH WILL
PRIMARILY BE DRIVEN BY COLD ADVECTION FROM THE WEST/NORTHWEST. THE
SOUTHWEST MAINLAND IS ENTRENCHED IN THE GUSTY NORTHWESTERLY FLOW
BEHIND THE ARCTIC FRONT WITH ISOLATED SHOWERS AND BELOW NORMAL
TEMPERATURES.
.MODEL DISCUSSION...
GUIDANCE REMAINING IN GREAT AGREEMENT WITH THIS LATE SEASON SNOW
EVENT AND COLD OUTBREAK. VERY SIMILAR TIMING AND PROGRESSION OF THE
UPPER TROUGH BRIEFLY CLOSING OFF THEN SHIFTING TO THE EAST AS A RIDGE
BUILDS OVER THE SOUTHWEST MAINLAND AND SOUTHCENTRAL. SOME SLIGHT
DIFFERENCES CONCERNING A WEAK SURFACE LOW IN THE GULF BUT SHOULD NOT
HAVE ANY IMPLICATIONS ON THE CURRENT FORECAST. THE NAM WAS PREFERRED
IN THE SHORT TERM.
.SHORT TERM FORECAST...
SOUTHCENTRAL ALASKA...LIGHT SNOWFALL WILL CONTINUE FROM THE SUSITNA
VALLEY SOUTHWARD TO THE ANCHORAGE BOWL THROUGH THE MORNING
HOURS...GRADUALLY TAPERING OFF BY AFTERNOON. SNOWFALL WILL LINGER
LONGEST AT HIGHER ELEVATIONS OF THE CHUGACH AND TALKEETNA MOUNTAINS.
UNSEASONABLE COLD TEMPERATURE WILL PERSIST TODAY WITH BRISK
NORTHWESTERLY WINDS CONTINUING THROUGHOUT THE DAY. THE NEXT FORECAST
CONCERN IS HOW LOW TEMPERATURES FALL TONIGHT INTO SUNDAY MORNING.
WITH DECREASING CLOUDS THROUGHOUT THE DAY AND A VERY COLD AIR MASS
ALOFT LOW TEMPERATURES ARE EXPECTED TO DROP RAPIDLY INTO THE 20S
EARLY SUNDAY MORNING...FLIRTING WITH RECORD LOWS. RIDGING OVERHEAD
INTO EARLY NEXT WEEK WILL KEEP DRY CONDITIONS WITH MODERATE
TEMPERATURES BACK TO MORE REASONABLE LEVELS BUT STILL SLIGHTLY BELOW
NORMAL.
SOUTHWEST ALASKA...NORTHWESTERLY FLOW WILL BEGIN TO SLACKEN TODAY
OVER THE KUSKOKWIM DELTA AS RIDGING BUILDS IN FROM THE BERING SEA
BRINGING SOUTHERLY WINDS AND MODERATING TEMPERATURES. THE BRISTOL BAY
REGION WILL SEE CONTINUED GUSTY NORTHWESTERLY FLOW SLOWLY LIGHTENING
THROUGH THE DAY WITH PARTLY SUNNY SKIES. A BERING SEA FRONT WILL
RETURN RAIN AND SNOW TO THE KUSKOKWIM DELTA ON MONDAY WHILE DRY
CONDITIONS WILL PERSIST FOR THE BRISTOL BAY AREA.
BERING SEA AND ALEUTIANS...HIGH PRESSURE OVER THE EASTERN BERING WILL
KEEP PARTLY SUNNY SKIES AND DRY CONDITIONS ON HAND WITH SOME
LINGERING SNOW SHOWERS ALONG THE ALASKA PENINSULA. A NORTH PACIFIC
LOW WILL SEND A WARM FRONT OVER THE WESTERN ALEUTIANS ALONG WITH
STRONG SOUTHEASTERLY WINDS AND RAIN. THE FRONT WILL PROGRESS TO THE
EASTERN BERING BY MONDAY RETURNING WARMER TEMPERATURES...RAIN AND
STRONG SOUTHERLY WINDS TO MOST OF THE CHAIN.
.LONG TERM FORECAST...
A STRONG NORTH PACIFIC SYSTEM MOVING INTO THE BERING SEA WILL PROMOTE
RIDGING OVER THE SOUTHERN MAINLAND. DRY CONDITIONS WILL PERSIST WITH
NEAR NORMAL TEMPERATURES EAST OF THE ALASKA RANGE WITH WETTER
CONDITIONS WEST OF THE ALASKA RANGE.
.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...WINTER WEATHER ADVISORY 111 145
MARINE...
FIRE WEATHER...NONE
MTL MAY 13
000
FXAK68 PAFC 172136
AFDAFC
SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
130 PM AKDT FRI MAY 17 2013
.ANALYSIS AND UPPER LEVELS...
AN UPPER LEVEL TROUGH ACCOMPANIED BY MUCH-ADVERTISED UNSEASONABLY
COLD AIR IS MOVING THROUGH SOUTHERN ALASKA THIS AFTERNOON. A WEAK LOW
PRESSURE SYSTEM IN THE GULF HAS SPREAD WIDESPREAD PRECIPITATION
ACROSS ALL OF SOUTHCENTRAL ALASKA...WHICH IS FALLING GENERALLY IN THE
FORM OF RAIN AT LOWER ELEVATIONS AND SNOW AT HIGHER ELEVATIONS.
SATELLITE IMAGERY SHOWS THE UPPER TROUGH BECOMING INCREASINGLY
NEGATIVELY TILTED...WITH AN UPPER LEVEL LOW NEAR THE BERING STRAIT
DROPPING INTO THE BASE OF THE TROUGH. FURTHER WEST...THE COLD FRONT
HAS PASSED THROUGH MOST OF THE SOUTHWEST MAINLAND...CLEARING SKIES
AND ALLOWING TEMPERATURES TO DROP INTO THE TEENS EARLIER THIS MORNING
FROM THE KUSKOKWIM DELTA TO THE WESTERN KUSKOKWIM VALLEY. HIGH
TEMPERATURES THIS AFTERNOON WILL STRUGGLE TO REACH THE UPPER 20S IN
PARTS OF THE DELTA...WHERE THE AVERAGE HIGH THIS TIME OF YEAR SHOULD
BE UPWARDS OF 50 DEGREES. WAY OUT WEST...HIGH PRESSURE REMAINS IN
COMMAND OF THE BERING SEA REGION...THOUGH HIGH CLOUDS IN ADVANCE OF A
NORTH PACIFIC LOW ARE APPROACHING THE FAR WESTERN ALEUTIANS.
.MODEL DISCUSSION...
MODEL AGREEMENT HAS CONTINUED TO IMPROVE TODAY WITH THE NAM JOINING
THE OTHER SOLUTIONS IN THE TIMING AND EVOLUTION OF THE UPPER LEVEL
TROUGH OVER THE MAINLAND TODAY (NAM HAD BEEN FASTER WITH SPEED OF THE
UPPER TROUGH IN PREVIOUS DAYS). THUS CONFIDENCE HAS CONTINUED TO
INCREASE THAT AN UNUSUAL LATE-SEASON SNOWFALL EVENT WILL BE UNFOLDING
ACROSS SOUTHCENTRAL ALASKA TONIGHT AND CONTINUING THROUGH SATURDAY
MORNING. THE NAM WAS THE PRIMARY MODEL USED TODAY...THOUGH NO STRONG
PREFERENCES WERE NOTED AND NO MODELS WERE UNUSABLE.
.SHORT TERM FORECAST...
EAST OF THE ALASKA RANGE...COLD AIR IS SLOWLY BLEEDING INTO THE AREA
BUT THIS TREND WILL ACCELERATE DRAMATICALLY BY LATE THIS AFTERNOON.
AS THE ATMOSPHERIC COLUMN COOLS THE RAIN/SNOW MIX WILL TURN TO ALL
SNOW EVEN AT LOWER ELEVATIONS. HIGHER ELEVATIONS WILL SEE THE MOST
SNOWFALL...LIKELY APPROACHING 6 INCHES IN SOME LOCATIONS...WITH LOWER
ELEVATIONS SIGNIFICANTLY LESS. AS SKIES CLEAR ON SATURDAY AFTERNOON
TEMPERATURES SHOULD DROP SIGNIFICANTLY OVERNIGHT INTO SUNDAY MORNING.
RECORD LOW TEMPERATURES IN ANCHORAGE ARE IN THE MID 20S...AND THESE
RECORDS MAY BE THREATENED IF WINDS DIMINISH ENOUGH TO ALLOW
SIGNIFICANT OVERNIGHT COOLING. EVEN IF THE RECORD LOWS ARE NOT
REACHED...A WIDESPREAD HARD FREEZE IS EXPECTED ACROSS MUCH OF THE
REGION FROM THE INLAND KENAI PENINSULA NORTHWARD. TEMPERATURES WILL
START TO SLOWLY REBOUND ON SUNDAY AS HIGH PRESSURE MOVES IN FROM THE
WEST.
WEST OF THE ALASKA RANGE...BEHIND THE COLD FRONT TEMPERATURES DROPPED
WELL BELOW FREEZING THIS MORNING AT MANY INLAND LOCATIONS OVER THE
MAINLAND. MOSTLY CLEAR SKIES WILL ALLOW TEMPERATURES TO DROP AGAIN
TONIGHT INTO THE LOWER TEENS...BEFORE A STEADY WARMING TREND ENSUES
FOR THE LATTER HALF OF THE WEEKEND. HIGH PRESSURE CURRENTLY OVER THE
BERING SEA WILL DRIFT EASTWARD...CONTINUING BENIGN CONDITIONS AND
MOSTLY SUNNY SKIES. MARINE FOG/STRATUS OVER THE BERING AND ALEUTIANS
WILL START TO SLOWLY DISSIPATE SATURDAY AS A GALE-FORCE FRONT
APPROACHES THE WESTERN ALEUTIANS. THIS WEATHER FRONT AND ASSOCIATED
RAIN SHIELD WILL MOVE TO THE EASTERN BERING BY LATE SUNDAY.
.LONG TERM FORECAST...
A MORE PROGRESSIVE WEATHER PATTERN WILL MOVE INTO THE REGION STARTING
SUNDAY. THIS WILL SEE MOST AREAS RETURN TO NEAR-NORMAL TEMPERATURES
BY THE MIDDLE OF NEXT WEEK...WITH RAIN MOVING INTO THE SOUTHWEST
MAINLAND BUT SOUTHCENTRAL REMAINING MOSTLY DRY.
.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...SNOW ADVISORY 101 111 145
MARINE...GALE 130 132 150 172 175 176
FIRE WEATHER...NONE
DIXON MAY 13
000
FXAK68 PAFC 171245
AFDAFC
SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
500 AM AKDT FRI MAY 17 2013
.ANALYSIS AND UPPER LEVELS...
A LARGE LONG-WAVE TROUGH/CLOSED UPPER LEVEL LOW LIES CENTERED ABOVE
THE CHUKCHI SEA EXTENDING WELL SOUTH INTO THE MID-LATITUDES. A
SHORTWAVE TROUGH EMBEDDED IN THE FLOW IS DIGGING OVER THE SOUTHWEST
MAINLAND PULLING A COLD FRONT ALONG FOR THE RIDE...STRETCHING FROM
THE ALASKA RANGE SOUTHWEST THROUGH THE ALEUTIANS BRINGING VERY
UNSEASONABLE ARCTIC AIR BEHIND IT. MOIST SOUTHERLY FLOW IS EVIDENT ON
THE FRONT SIDE OF THE TROUGH AS WARM ADVECTION HAS OVERSPREAD RAIN
OVER MOST OF SOUTHCENTRAL ALASKA. MORE SOUTHERLY FLOW ALOFT RATHER
THAN SOUTHEASTERLY ALONG WITH THE CORE OF THE STRONGEST WINDS
REMAINING ABOVE THE CHUGACH/KENAI MOUNTAINS HAVE MADE DOWN-SLOPE
DRYING A NON-ISSUE FOR MOST OF THE AREA SAVE THE MATANUSKA VALLEY.
EXTENSIVE CLOUD COVER AND RAIN HAVE KEPT TEMPERATURES WELL BELOW
NORMAL FOR MOST OF THE AREA. MOIST ONSHORE FLOW ACROSS THE NORTHERN
GULF COAST HAS KEPT THE AREA IN STEADY HEAVY PRECIPITATION AS WELL AS
PROMOTED STRONG WINDS THROUGH TURNAGAIN ARM.
.MODEL DISCUSSION...
GUIDANCE REMAINING IN GREAT SYNOPTIC AGREEMENT WITH THE MAJOR PLAYERS
OF THE FORECAST. CONCERNING THE UPPER TROUGH...THE MODELS ARE
CONSISTENT WITH SHOWING THE INITIAL TROUGH SHEARING JUST TO THE WEST
OF THE SOUTHCENTRAL WITH THE JET DIGGING DOWN THE BACKSIDE OF THE
TROUGH PROMOTING A CLOSED LOW DEVELOPMENT IN THE NORTHERN GULF
KEEPING THE TROUGH AXIS FROM THE ALASKA/ALEUTIAN RANGE THROUGH THE
WESTERN GULF THROUGH SATURDAY AFTERNOON. A LITTLE MORE SPREAD EXISTS
WITH THE ARRIVAL OF THE COLD AIR THIS AFTERNOON. THE NAM CONTINUES TO
BE THE MOST AGGRESSIVE WITH THE COLD AIR MOVING EASTWARD OWING TO THE
LEADING TROUGH STAYING A LITTLE MORE POSITIVE TILT. GENERALLY ALL
GUIDANCE PUSHES THE RAIN/SNOW LINE EAST OF TALKEETNA-ANCHORAGE-KENAI
BY MID-AFTERNOON.
.SHORT TERM FORECAST...
SOUTHCENTRAL ALASKA...PRECIPITATION WILL INCREASE IN INTENSITY THIS
MORNING AS THE SURFACE LOW MOVES CLOSER TOWARD THE EASTERN KENAI
PENINSULA...CUTTING OFF ANY REMAINING CROSS-BARRIER FLOW AND
DOWNSLOPE. COLD AIR ADVECTION WILL INCREASE IN INTENSITY FROM THE
WEST TODAY FACILITATING A CHANGE FROM RAIN TO SNOW DURING THE
AFTERNOON HOURS AND WILL BE THE MAIN DRIVER OF PRECIPITATION THROUGH
THE OVERNIGHT HOURS. THE GREATEST ACCUMULATIONS WILL BE AT HIGHER
ELEVATIONS THOUGH MOST LOCATIONS FROM THE SUSITNA VALLEY THROUGH THE
NORTHERN KENAI PENINSULA WILL SEE AT LEAST SOME ACCUMULATION OF SNOW
AT SEA LEVEL. SNOW WILL LINGER THROUGH SATURDAY MORNING AND TAPER OFF
BY AFTERNOON.
SOUTHWEST ALASKA...THE DIGGING UPPER TROUGH IS PRODUCING LIGHT SNOW
ACROSS THE BRISTOL BAY AREA WITH THE COLD FRONT RIGHT ON ITS HEELS.
LIGHT SNOW WILL CONTINUE EARLY THIS MORNING AND SLOWLY TAPER OFF BY
AFTERNOON...LINGERING THE LONGEST OVER THE ALASKA PENINSULA. BEHIND
THE COLD FRONT GUSTY NORTHWESTERLY WINDS WILL REMAIN WITH SCATTERED
SNOW SHOWERS ACROSS THE REGION. HIGH PRESSURE WILL BUILD INTO THE
AREA ON SATURDAY WITH LIGHTER WINDS AND RECOVERING TEMPERATURES.
BERING SEA AND ALEUTIANS...BRISK NORTHERLY FLOW OVER THE CENTRAL AND
EASTERN BERING WILL KEEP SCATTERED SNOW SHOWERS ACROSS THE AREA. THE
WESTERN BERING WILL REMAIN UNDER HIGH PRESSURE AND LIGHT WESTERLY
WINDS. HIGH PRESSURE WILL BUILD OVER THE EASTERN BERING OVER THE
WEEKEND WHILE A NORTH PACIFIC LOW WILL APPROACH THE WESTERN ALEUTIANS
BRINGING RAIN AND STRONG SOUTHEASTERLY WINDS.
.LONG TERM FORECAST...
A DISSIPATING UPPER LOW WILL MIGRATE INTO THE BERING EARLY NEXT WEEK
BUILDING AN UPPER RIDGE AHEAD TO THE EAST...WHICH WILL BRING MOST OF
THE MAINLAND BACK TO REASONABLE CLIMATOLOGICAL TEMPERATURES WITH
DRIER WEATHER.
.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...WINTER WEATHER ADVISORY 101 111 145
MARINE...HEAVY FREEZING SPRAY 180
GALE 120 130 132 150 175 176
FIRE WEATHER...NONE
MTL MAY 13
000
FXAK68 PAFC 162313
AFDAFC
SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
315 PM AKDT THU MAY 16 2013
.ANALYSIS AND UPPER LEVELS...
A LONGWAVE TROUGH REMAINS CENTERED OVER THE WEST COAST OF ALASKA WITH
BROAD SOUTHWESTERLY FLOW IN PLACE OVER THE SOUTHERN AND EASTERN
MAINLAND. A FAIRLY WEAK LOW HAS MOVED NORTHWARD INTO THE GULF OF
ALASKA...BRINGING WIDESPREAD RAIN/SNOW TO MOST LOCATIONS EAST OF THE
ALASKA RANGE. FURTHER WEST...A VIGOROUS SHORTWAVE DROPPING INTO THE
LONGWAVE TROUGH IS USHERING IN UNSEASONABLY COLD AIR TO THE SOUTHWEST
MAINLAND AND IS EN ROUTE TO SOUTHCENTRAL ALASKA TOMORROW. AFTER A
PERIOD OF RAIN AND SNOW EARLIER THIS MORNING THE INCREASING
CROSS-BARRIER WINDS HAVE ALLOWED A CLASSIC DOWNSLOPE PATTERN TO
DEVELOP OVER THE COOK INLET REGION THIS AFTERNOON. STRONG WINDS ARE
BLOWING THROUGH TURNAGAIN ARM AND THE PRINCE WILLIAM SOUND COASTLINE
IS SEEING SUSTAINED HEAVY PRECIPITATION.
.MODEL DISCUSSION...
THE MODELS REMAIN IN GOOD OVERALL SYNOPTIC AGREEMENT. THE NAM
CONTINUES TO BE FASTER THAN THE OTHER SOLUTIONS PROGRESSING THE
MIDLEVEL TROUGH TO THE EAST. THIS IS RELATED TO THE FACT THAT THE NAM
DOES NOT DEVELOP A WEAK CLOSED LOW IN THE BASE OF THIS TROUGH LATE
FRIDAY NIGHT AS THE OTHER MODELS DO. THIS ALSO MEANS THE NAM BRINGS
THE COLD AIR INTO THE ANCHORAGE AREA FASTER THAN THE OTHER MODELS.
THE GFS AND ECMWF ARE CLOSER TOGETHER WITH A SLOWER SOLUTION FOR THE
ARRIVAL OF THE COLD AIR AND DEVELOPMENT OF A WEAK CLOSED UPPER LOW.
THE ECMWF IS A TOUCH FASTER THAN THE GFS...AND REPRESENTS A GOOD
MIDDLE-GROUND SOLUTION FOR THE PRODUCTION OF THE FORECAST TODAY.
.SHORT TERM FORECAST...
SOUTHCENTRAL...DOWNSLOPING WILL COME TO THE END BY LATE THIS EVENING
AS SURFACE LOW PRESSURE MAKES PROGRESS NORTHWARD TOWARD PRINCE
WILLIAM SOUND. THIS SHOULD ALLOW RAIN (MIXED WITH SNOW AT HIGHER
ELEVATIONS) TO RETURN TO THE WESTERN KENAI...ANCHORAGE...AND MAT-SU
AREAS. COLD AIR BEGINS TO BLEED INTO THE AREA AFTER MIDNIGHT
TONIGHT...AND SHOULD BE ENOUGH TO PRODUCE A RAIN/SNOW MIX AT SEA
LEVEL FROM ANCHORAGE NORTHWARD DURING THE DAY TOMORROW. BY TOMORROW
AFTERNOON THE COLD AIR ADVECTION WILL INTENSIFY DRAMATICALLY AS THE
UPPER TROUGH APPROACHES FROM THE WEST. THIS SHOULD LEAD TO INCREASED
PRECIPITATION RATES AND A CHANGE FROM RAIN/SNOW MIX TO ALL SNOW EVEN
AT SEA LEVEL. HIGHER ELEVATIONS WILL SEE THE THE MOST
ACCUMULATION...BUT MEASURABLE ACCUMULATION IS LIKELY EVEN AT LOWER
ELEVATIONS. CONSIDERING THE EXTREME RARITY OF AN EVENT OF THIS
MAGNITUDE AT THIS TIME OF YEAR...WILL BE ISSUING SNOW ADVISORIES FOR
THE ANCHORAGE AND MAT-SU REGIONS WITH THE UPCOMING FORECAST PACKAGE.
BY SATURDAY MORNING...SNOW WILL TAPER OFF AS MOST OF THE DYNAMIC
SUPPORT SHIFTS AWAY FROM THE AREA AS THE UPPER LOW DEVELOPS SOUTH OF
THE KENAI PENINSULA AND COLD AIR CONTINUES TO MOVE IN FROM THE WEST.
SOUTHWEST MAINLAND...UNSEASONABLY COLD AIR MOVE IN FROM WEST TO EAST
THROUGH FRIDAY AFTERNOON. FRONTAL PASSAGE WILL BE MOSTLY DRY ACROSS
THE REGION...WITH THE MOST SIGNIFICANT PRECIPITATION OCCURRING OVER
THE FAR EASTERN BRISTOL BAY AND KUSKOKWIM VALLEY AREAS. THESE AREAS
WILL SEE RAIN TURN TO SNOW TONIGHT AND LIKELY SEE SOME SNOW
ACCUMULATION BEFORE SNOW ENDS FRIDAY EVENING. A RIDGE OF HIGH
PRESSURE BUILDS IN FROM THE WEST LATE SATURDAY...CLEARING OUT CLOUDS
AND MODERATING TEMPERATURES.
BERING SEA AND ALEUTIANS...COLD AIR BEHIND THE FRONT WILL DELIVER A
GLANCING BLOW TO THE EASTERN BERING SEA THROUGH SATURDAY MORNING
BEFORE A RIDGE MOVES BACK INTO THE AREA. WIDESPREAD FOG AND STRATUS
WILL LIKELY CONTINUE THROUGH THE WEEKEND AT MANY LOCATIONS. A LOW
PRESSURE CENTER AND ASSOCIATED FRONT WILL MOVE INTO THE WESTERN AND
CENTRAL ALEUTIANS SATURDAY NIGHT WITH GALE-FORCE WINDS AND RAIN.
.LONG TERM FORECAST...AFTER COLD TEMPERATURES EARLY THIS
WEEKEND...LONG RANGE MODELS ARE IN GOOD AGREEMENT ON A WARMING TREND
EARLY NEXT WEEK (ALTHOUGH TEMPERATURES WILL STILL STAY AT OR BELOW
NORMAL FOR THIS TIME OF YEAR. THIS WILL BE ACCOMPANIED BY A MORE
PROGRESSIVE WEATHER PATTERN THAT IS CENTERED MORE OVER THE BERING SEA
THAN THE CURRENT STORM TRACK.
.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...SNOW ADVISORY 101 111 145
MARINE...GALE 120 130 132 150
HEAVY FREEZING SPRAY WARNING 180
FIRE WEATHER...NONE
DIXON MAY 13
000
FXAK68 PAFC 161609 CCA
AFDAFC
SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION...CORRECTED
NATIONAL WEATHER SERVICE ANCHORAGE AK
500 AM AKDT THU MAY 16 2013
CORRECTED TYPO
.ANALYSIS AND UPPER LEVELS...
A LARGE COLD CORE UPPER LEVEL LOW ENCOMPASSES THE WHOLE OF ALASKA
WITH THE MAIN BELT OF WESTERLIES WELL TO THE SOUTH IN THE
MID-LATITUDES. THE MAIN CYCLONIC CIRCULATION HAS MULTIPLE CENTERS
OVER THE CHUKCHI SEA WHILE A BAGGIER UPPER LEVEL TROUGH HAS BEEN
ABSORBED INTO THE LONG-WAVE. AN AREA OF MOISTURE IS SURGING OVER THE
WESTERN GULF OF ALASKA AHEAD OF A SHORTWAVE TROUGH WITHING THE
SOUTHWESTERLY FLOW. FURTHER UPSTREAM TO THE NORTHWEST...A STRONG
SHORTWAVE IS EMBEDDED IN THE FLOW STRETCHING FROM THE SEWARD
PENINSULA THROUGH EASTERN RUSSIA IS DIGGING TOWARD THE WEST COAST.
MOIST SOUTHWEST FLOW ALOFT HAS SPREAD CLOUD COVER OVER MOST OF THE
SOUTHCENTRAL AS A FRONTAL BOUNDARY APPROACHES THE NORTH GULF COAST.
HIGH PRESSURE OVER THE BERING SEA CONTINUES TO PROMOTE LOW CLOUDS AND
STRATUS FROM THE ALEUTIANS THROUGH THE WEST COAST.
.MODEL DISCUSSION...
GUIDANCE REMAINS IN GOOD SYNOPTIC AGREEMENT WITH THE NATURE OF THE
UPPER TROUGH SHIFTING OVER THE SOUTHWEST MAINLAND THROUGH THE KENAI
PENINSULA. THE TREND ACROSS ALL GUIDANCE HAS BEEN FROM A CONSOLIDATED
NEGATIVE TILT TROUGH TO A STRETCHING/SPLITTING OF THE ENERGY TO A
CLOSED CIRCULATION DEVELOPING IN THE GULF. SYNOPTICALLY THIS MAKES
SENSE GIVEN THE MAIN UPPER LOW CENTER WILL STAY CENTERED NORTH OF THE
BERING STRAIT WHILE THE JET STREAK ON THE EQUATORWARD SIDE OF THE
SYSTEM DIGS TO THE SOUTHEAST. THE EXACT NATURE OF THE TROUGH
PROGRESSION AND VORTICITY REDEVELOPMENT TO THE WEST AS THE JET DIGS
WILL HAVE LARGE IMPLICATIONS ON THE PRECIPITATION FORECAST AND THE
AMOUNT OF LIFT AVAILABLE AS THE TROUGH INTERACTS WITH THE MOISTURE
CURRENTLY MOVING OVER THE SOUTHCENTRAL. THE SECOND FACET TO THE
FORECAST IS THE PROGRESSION OF THE ARCTIC AIR FROM THE NORTHWEST.
CURRENTLY THE NAM IS THE OUTLIER IN TERMS OF QUICKLY MOVING THE
ARCTIC AIR THROUGH THE ALASKA RANGE...THIS IS LIKELY ASSOCIATED WITH
KEEPING THE INITIAL UPPER TROUGH INTACT LONGER BEFORE THE NEW
VORTICITY MAXIMUM/CLOSED LOW DEVELOPS IN THE WESTERN GULF. THE MAIN
THEME TO TAKE FROM THE MODEL GUIDANCE REGARDLESS OF EXACT DETAILS IS
A RESPECTABLE SLUG OF ARCTIC AIR COMBINED WITH A NEGATIVE TILT TROUGH
TO THE WEST OF THE SOUTHCENTRAL BODES VERY WELL FOR SNOWFALL ACROSS
THE AREA.
.SHORT TERM FORECAST...
SOUTHCENTRAL ALASKA...WARM FRONT WILL CONTINUE TO PROGRESS NORTHWARD
IN THE GULF TODAY SPREADING RAIN OVER THE NORTH GULF COAST. HIGHER
ELEVATIONS OF THE CHUGACH COULD SEE A MIX OF RAIN AND SNOW IN THE
MORNING BEFORE CHANGING OVER TO RAIN. RAIN WILL GRADUALLY SPREAD UP
INTO THE SUSITNA VALLEY TODAY WHILE AREAS FROM THE KENAI PENINSULA
THROUGH THE MATANUSKA VALLEY WILL REMAIN MOSTLY DRY AND DOWNSLOPED AS
SOUTHEASTERLY FLOW CROSS-BARRIER REMAINS. LATE TONIGHT INTO FRIDAY
MORNING AS THE SURFACE LOW PRESSURE AND FRONTAL BOUNDARY PUSH INLAND
STEADY RAIN WILL SPREAD OVER THE PREVIOUSLY DOWNSLOPED AREAS. DURING
THE DAY ON FRIDAY THE UNSEASONABLE ARCTIC AIR WILL BEGIN TO FILTER
THROUGH THE ALASKA RANGE. PRECIPITATION WILL LIKELY STAY AS RAIN
THROUGH THE AFTERNOON THEN BEGIN TO MIX WITH AND CHANGE OVER TO SNOW
DURING THE EARLY EVENING HOURS FOR THE COOK INLET REGION WITH A MIX
LINGERING IN THE COPPER RIVER BASIN AND THE NORTHERN GULF COAST.
PRECIPITATION LOOKS TO CONTINUE THROUGH SATURDAY MORNING BEFORE
TAPERING OFF.
SOUTHWEST ALASKA...A TALE OF TWO SYNOPTIC REGIMES OVER THE SOUTHWEST
TODAY. EASTERN BRISTOL BAY AND ALONG THE ALASKA/ALEUTIAN RANGE
REMAINS UNDER THE INFLUENCE OF THE WESTERN GULF LOW PRESSURE BRINGING
THE CHANCE FOR LIGHT RAIN THIS AFTERNOON. FURTHER OUT WEST A COLD
FRONT IS APPROACHING THE KUSKOKWIM DELTA AND WILL BRING VERY LIGHT
SNOW TODAY. THE FRONT WILL MOVE INTO THE BRISTOL BAY AREA TONIGHT AND
BRING SCATTERED SNOW SHOWERS AND A PERIOD OF GUSTY NORTHWESTERLY
FLOW.
BERING SEA AND ALEUTIANS...A COLD FRONT WILL DROP SOUTHWARD OVER THE
BERING TODAY BRINGING A PERIOD OF LIGHT SNOW FIRST TO THE PRIBILOF
ISLANDS THEN TO THE EASTERN ALEUTIANS AND ALASKA PENINSULA TONIGHT.
MOSTLY CLOUDY CONDITIONS WILL PREVAIL ALONG THE BERING SIDE IN
NORTHERLY FLOW.
.LONG TERM FORECAST...
AFTER THE UNSEASONABLY COLD SYSTEM A BIT OF A PATTERN CHANGE IS IN
ORDER. RIDGING LOOKS TO BUILD OVER THE MAINLAND WITH A STRONG UPPER
LOW MOVING INTO THE BERING SEA FROM THE SOUTHWEST. PRECIPITATION WILL
BE PREDOMINATELY CONFINED TO THE WEST WITH MODERATING TEMPERATURES IN
THE EAST.
.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...NONE
MARINE...HEAVY FREEZING SPRAY...180
GALE 120 125 130 132 150
FIRE WEATHER...NONE
MTL MAY 13
000
FXAK68 PAFC 161246
AFDAFC
SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
500 AM AKDT THU MAY 16 2013
.ANALYSIS AND UPPER LEVELS...
A LARGE COLD CORE UPPER LEVEL LOW ENCOMPASSES THE WHOLE OF ALASKA
WITH THE MAIN BELT OF WESTERLIES WILL TO THE SOUTH IN THE
MID-LATITUDES. THE MAIN CYCLONIC CIRCULATION HAS MULTIPLE CENTERS
OVER THE CHUKCHI SEA WHILE A BAGGIER UPPER LEVEL TROUGH HAS BEEN
ABSORBED INTO THE LONG-WAVE. AN AREA OF MOISTURE IS SURGING OVER THE
WESTERN GULF OF ALASKA AHEAD OF A SHORTWAVE TROUGH WITHING THE
SOUTHWESTERLY FLOW. FURTHER UPSTREAM TO THE NORTHWEST...A STRONG
SHORTWAVE IS EMBEDDED IN THE FLOW STRETCHING FROM THE SEWARD
PENINSULA THROUGH EASTERN RUSSIA IS DIGGING TOWARD THE WEST COAST.
MOIST SOUTHWEST FLOW ALOFT HAS SPREAD CLOUD COVER OVER MOST OF THE
SOUTHCENTRAL AS A FRONTAL BOUNDARY APPROACHES THE NORTH GULF COAST.
HIGH PRESSURE OVER THE BERING SEA CONTINUES TO PROMOTE LOW CLOUDS AND
STRATUS FROM THE ALEUTIANS THROUGH THE WEST COAST.
.MODEL DISCUSSION...
GUIDANCE REMAINS IN GOOD SYNOPTIC AGREEMENT WITH THE NATURE OF THE
UPPER TROUGH SHIFTING OVER THE SOUTHWEST MAINLAND THROUGH THE KENAI
PENINSULA. THE TREND ACROSS ALL GUIDANCE HAS BEEN FROM A CONSOLIDATED
NEGATIVE TILT TROUGH TO A STRETCHING/SPLITTING OF THE ENERGY TO A
CLOSED CIRCULATION DEVELOPING IN THE GULF. SYNOPTICALLY THIS MAKES
SENSE GIVEN THE MAIN UPPER LOW CENTER WILL STAY CENTERED NORTH OF THE
BERING STRAIT WHILE THE JET STREAK ON THE EQUATORWARD SIDE OF THE
SYSTEM DIGS TO THE SOUTHEAST. THE EXACT NATURE OF THE TROUGH
PROGRESSION AND VORTICITY REDEVELOPMENT TO THE WEST AS THE JET DIGS
WILL HAVE LARGE IMPLICATIONS ON THE PRECIPITATION FORECAST AND THE
AMOUNT OF LIFT AVAILABLE AS THE TROUGH INTERACTS WITH THE MOISTURE
CURRENTLY MOVING OVER THE SOUTHCENTRAL. THE SECOND FACET TO THE
FORECAST IS THE PROGRESSION OF THE ARCTIC AIR FROM THE NORTHWEST.
CURRENTLY THE NAM IS THE OUTLIER IN TERMS OF QUICKLY MOVING THE
ARCTIC AIR THROUGH THE ALASKA RANGE...THIS IS LIKELY ASSOCIATED WITH
KEEPING THE INITIAL UPPER TROUGH INTACT LONGER BEFORE THE NEW
VORTICITY MAXIMUM/CLOSED LOW DEVELOPS IN THE WESTERN GULF. THE MAIN
THEME TO TAKE FROM THE MODEL GUIDANCE REGARDLESS OF EXACT DETAILS IS
A RESPECTABLE SLUG OF ARCTIC AIR COMBINED WITH A NEGATIVE TILT TROUGH
TO THE WEST OF THE SOUTHCENTRAL BODES VERY WELL FOR SNOWFALL ACROSS
THE AREA.
.SHORT TERM FORECAST...
SOUTHCENTRAL ALASKA...WARM FRONT WILL CONTINUE TO PROGRESS NORTHWARD
IN THE GULF TODAY SPREADING RAIN OVER THE NORTH GULF COAST. HIGHER
ELEVATIONS OF THE CHUGACH COULD SEE A MIX OF RAIN AND SNOW IN THE
MORNING BEFORE CHANGING OVER TO RAIN. RAIN WILL GRADUALLY SPREAD UP
INTO THE SUSITNA VALLEY TODAY WHILE AREAS FROM THE KENAI PENINSULA
THROUGH THE MATANUSKA VALLEY WILL REMAIN MOSTLY DRY AND DOWNSLOPED AS
SOUTHEASTERLY FLOW CROSS-BARRIER REMAINS. LATE TONIGHT INTO FRIDAY
MORNING AS THE SURFACE LOW PRESSURE AND FRONTAL BOUNDARY PUSH INLAND
STEADY RAIN WILL SPREAD OVER THE PREVIOUSLY DOWNSLOPED AREAS. DURING
THE DAY ON FRIDAY THE UNSEASONABLE ARCTIC AIR WILL BEGIN TO FILTER
THROUGH THE ALASKA RANGE. PRECIPITATION WILL LIKELY STAY AS RAIN
THROUGH THE AFTERNOON THEN BEGIN TO MIX WITH AND CHANGE OVER TO SNOW
DURING THE EARLY EVENING HOURS FOR THE COOK INLET REGION WITH A MIX
LINGERING IN THE COPPER RIVER BASIN AND THE NORTHERN GULF COAST.
PRECIPITATION LOOKS TO CONTINUE THROUGH SATURDAY MORNING BEFORE
TAPERING OFF.
SOUTHWEST ALASKA...A TALE OF TWO SYNOPTIC REGIMES OVER THE SOUTHWEST
TODAY. EASTERN BRISTOL BAY AND ALONG THE ALASKA/ALEUTIAN RANGE
REMAINS UNDER THE INFLUENCE OF THE WESTERN GULF LOW PRESSURE BRINGING
THE CHANCE FOR LIGHT RAIN THIS AFTERNOON. FURTHER OUT WEST A COLD
FRONT IS APPROACHING THE KUSKOKWIM DELTA AND WILL BRING VERY LIGHT
SNOW TODAY. THE FRONT WILL MOVE INTO THE BRISTOL BAY AREA TONIGHT AND
BRING SCATTERED SNOW SHOWERS AND A PERIOD OF GUSTY NORTHWESTERLY
FLOW.
BERING SEA AND ALEUTIANS...A COLD FRONT WILL DROP SOUTHWARD OVER THE
BERING TODAY BRINGING A PERIOD OF LIGHT SNOW FIRST TO THE PRIBILOF
ISLANDS THEN TO THE EASTERN ALEUTIANS AND ALASKA PENINSULA TONIGHT.
MOSTLY CLOUDY CONDITIONS WILL PREVAIL ALONG THE BERING SIDE IN
NORTHERLY FLOW.
.LONG TERM FORECAST...
AFTER THE UNSEASONABLY COLD SYSTEM A BIT OF A PATTERN CHANGE IS IN
ORDER. RIDGING LOOKS TO BUILD OVER THE MAINLAND WITH A STRONG UPPER
LOW MOVING INTO THE BERING SEA FROM THE SOUTHWEST. PRECIPITATION WILL
BE PREDOMINATELY CONFINED TO THE WEST WITH MODERATING TEMPERATURES IN
THE EAST.
.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...NONE
MARINE...HEAVY FREEZING SPRAY...180
GALE 120 125 130 132 150
FIRE WEATHER...NONE
MTL MAY 13
000
FXAK68 PAFC 152134
AFDAFC
SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
130 PM AKDT WED MAY 15 2013
.ANALYSIS AND UPPER LEVELS...
A LONGWAVE TROUGH CENTERED OVER THE WEST COAST OF ALASKA CONTINUES TO
ENCOMPASS THE EASTERN BERING SEA AND MOST OF THE MAINLAND. WARM AIR
ADVECTION IS BEGINNING ACROSS SOUTHCENTRAL AS A WEAK LOW SOUTH OF THE
ALEUTIANS IS BEING DRAWN UP INTO THE WESTERN GULF. AN AREA OF
PERSISTENT RAIN AND SNOW SHOWERS IS LOCATED OVER WESTERN PRINCE
WILLIAM SOUND AND THE CHUGACH MOUNTAINS...WITH MORE WIDESPREAD RAIN
APPROACHING KODIAK ISLAND FROM THE SOUTH. OUT WEST...HIGH PRESSURE IN
THE LOWER LEVELS CONTINUES TO MAINTAIN WIDESPREAD FOG AND STRATUS
ACROSS MUCH OF THE BERING SEA REGION. LIGHT NORTHWEST WINDS ARE
BRINGING SOME OF THESE CLOUDS INTO THE KUSKOKWIM DELTA. ELSEWHERE
OVER THE SOUTHWEST MAINLAND...SKIES ARE MOSTLY SUNNY WITH
BELOW-AVERAGE TEMPERATURES.
.MODEL DISCUSSION...
MODELS REMAIN IN GOOD SYNOPTIC AGREEMENT. THE MAIN FORECAST CHALLENGE
ACROSS THE DOMAIN REMAINS HOW THE UNSEASONABLY COLD AIR MOVING INTO
THE REGION ON FRIDAY WILL INTERACT WITH THE MOISTURE LEFTOVER FROM
THE WEAK STORM CURRENTLY MOVING INTO THE GULF. ALL MODELS CONTINUE TO
SHOW SIGNIFICANT COOLING OF THE ATMOSPHERE FRIDAY AFTERNOON AND
EVENING...WITH EVEN SEA-LEVEL LOCATIONS CHANGING OVER TO ALL SNOW.
SNOW AMOUNTS WILL BE PROBLEMATIC AS POTENTIAL FOR ACCUMULATION IS
CLOSELY TIED TO PRECIPITATION RATES. THIS IS WHERE THERE REMAINS A
BIT OF MODEL DISAGREEMENT...AS THE MODELS ARE ALL HINTING AT A WEAK
LOW CENTER FORMING IN THE BASE OF THE UPPER TROUGH BY SATURDAY
MORNING. THE TIMING OF THIS DEVELOPMENT WILL DICTATE HOW STRONG THE
DEEP-LAYER LIFT IS ACROSS THE ANCHORAGE BOWL AND MAT-SU VALLEY. IF
THE LOW DEVELOPS EARLIER THAN EXPECTED...STRONG DYNAMIC LIFT WILL BE
DIMINISHED AND SNOWFALL RATES AROUND THE ANCHORAGE AREA WILL LIKELY
NOT BE ENOUGH TO RESULT IN MORE THAN MINOR SNOW ACCUMULATIONS. THE
MODELS ALL SHOW A SIMILAR TREND...THOUGH THE NAM IS A SLIGHT
OUTLIER WITH A FASTER SPEED OF THE LOW MOVING INTO THE GULF AND ALSO
WITH THE COLD AIR ARRIVAL ON FRIDAY. THUS WILL OPT TO UTILIZE A BLEND
OF THE GFS/ECMWF FOR MOST OF THE FORECAST TODAY.
.SHORT TERM FORECAST...
SOUTHCENTRAL...PERSISTENT SHOWERS OVER THE CHUGACH WILL DIMINISH AS
CROSS-BARRIER WINDS INCREASE THIS EVENING. A CLASSIC DOWNSLOPE
PATTERN WILL DEVELOP ON THURSDAY...KEEPING THE WESTERN KENAI UP TO
ANCHORAGE AND THE MATANUSKA VALLEY MOSTLY DRY. MEANWHILE...LOCATIONS
FURTHER EAST AROUND PRINCE WILLIAM SOUND AND THE NORTH GULF COAST
WILL SEE STEADY RAIN DEVELOP BY MIDNIGHT TONIGHT. BY LATE THURSDAY
NIGHT...DOWNSLOPING WINDS WILL DIMINISH AS THE SURFACE LOW WEAKENS
AND MOVES NORTHWARD. THIS WILL ALLOW RAIN TO SPREAD INTO THE REST OF
THE COOK INLET AREA. THE REAL FUN BEGINS BY FRIDAY AFTERNOON AS THE
COLD AIR ARRIVES FROM WESTERN ALASKA. SNOW SHOULD BEGIN MIXING IN
WITH RAIN BEFORE CHANGING TO ALL SNOW FRIDAY EVENING. THE ANCHORAGE
HILLSIDE WILL CHANGE OVER TO SNOW EARLIEST...AND WILL ALSO HAVE THE
BEST CHANCE TO SEE ACCUMULATION. IT IS IMPORTANT TO NOTE THAT
ACCUMULATING SNOWFALL IN THE ANCHORAGE AREA IS EXTREMELY RARE IN THE
LATTER HALF OF MAY...AND HAS ONLY BEEN RECORDED TWICE BEFORE.
THUS...A HIGHLY UNUSUAL EVENT APPEARS TO BE TAKING AIM AT
SOUTHCENTRAL ALASKA FOR THE START OF THE WEEKEND.
SOUTHWEST MAINLAND...MOSTLY CLEAR SKIES WILL GIVE WAY TO INCREASING
CLOUDS DURING THE DAY THURSDAY AS THE AFOREMENTIONED COLD FRONT
APPROACHES FROM THE NORTH. AVAILABLE ATMOSPHERIC MOISTURE WILL BE
PRETTY MINIMAL AS THE FRONT PASSES...SO ONLY EXPECTING SCATTERED RAIN
AND SNOW SHOWERS AT MOST LOCATIONS. THE EXCEPTION WILL LIKELY BE OVER
THE EASTERN HALF OF THE LOWER KUSKOKWIM VALLEY WHERE AVAILABLE
MOISTURE AND UPSLOPING WINDS WILL BRING THE POTENTIAL FOR SOME SNOW
ACCUMULATION IN THAT AREA ON THURSDAY NIGHT AND FRIDAY.
BERING SEA AND ALEUTIANS...A BROAD AREA OF HIGH PRESSURE WILL REMAIN
IN PLACE AND STRENGTHEN THROUGH THE START OF THE WEEKEND. THIS WILL
MAINTAIN THE WIDESPREAD MARINE FOG AND STRATUS THAT IS EVIDENT ACROSS
THE REGION ON SATELLITE IMAGERY. WINDS GENERALLY OUT OF THE NORTH
WILL ALLOW THIS CLOUDINESS TO CONTINUE TO IMPACT COMMUNITIES MAINLY
ALONG THE BERING SIDE OF THE ALASKA PENINSULA AND ALEUTIAN ISLANDS.
.LONG TERM FORECAST...
COLD AIR WILL MOVE INTO SOUTHERN ALASKA ON FRIDAY AND BY SUNDAY WILL
BE ON THE DECLINE. LONG-RANGE MODELS ARE SUGGESTING A PATTERN SHIFT
FOR THE START OF NEXT WEEK THAT WOULD BE CHARACTERIZED BY MORE ACTIVE
WEATHER OVER THE BERING SEA WITH MODERATING TEMPERATURES (BUT STILL
AT OR BELOW NORMAL) AND SUNNIER WEATHER ACROSS THE MAINLAND FOR NEXT
WEEK.
.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...NONE
MARINE...GALE 120
FIRE WEATHER...NONE
DIXON MAY 13
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