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SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
509 AM AKDT TUE SEP 1 2015

.ANALYSIS AND UPPER LEVELS...THE POTENT NORTHERN STREAM OF ENERGY
CONTINUES TO POUR INTO THE GULF OF ALASKA AT ABOUT 90 KTS AT
25,000 FT. IT THEN DIVES DOWN TO SUPPORT THE UPPER LEVEL LOW OVER
SOUTHEAST ALASKA. OUT OVER THE WESTERN BERING...THE SAME UPPER
LEVEL LOW THAT HAS BEEN OUT THERE FOR OVER A WEEK CONTINUES TO
SPIN NEAR THE KAMCHATKA PENINSULA. IN BETWEEN THESE TWO LOW
PRESSURE CENTERS...A MODEST EXTENSION OF THE NORTH PACIFIC RIDGE
IS STARTING TO BUILD OVER THE ALASKA PENINSULA. THIS RIDGE OF HIGH
PRESSURE WILL BE THE MAIN WEATHER DRIVE FOR THE NEXT FEW DAYS. THE
NORTH TO SOUTH JET OVER THE MAINLAND IS HELPING TO KEEP GUSTY
WINDS BLOWING TONIGHT...ESPECIALLY FOR THE FAVORED AREAS SUCH AS
SEWARD AND VALDEZ. WHERE THE WINDS ARE BLOWING...IT IS HELPING TO
KEEP THE ATMOSPHERE SLIGHTLY MORE MIXED AND THUS...KEEP OVERNIGHT
TEMPERATURES A FEW DEGREES WARMER. A NEW AREA OF LOW PRESSURE IS
DEVELOPING THIS MORNING NEAR ATKA ALONG A REMNANT FRONTAL
BOUNDARY. AS THIS LOW TAKES SHAPE TODAY...IT WILL BRING SOME LIGHT
TO MODERATE RAIN TO THE CENTRAL ALEUTIANS AND SOME SMALL CRAFT
WINDS OVER MUCH OF THE EASTERN BERING.

&&

.MODEL DISCUSSION...ALL MODELS WERE IN DECENT AGREEMENT THIS
EVENING WITH THE EXCEPTION OF THE NAM. IT WAS MUCH MORE AGGRESSIVE
IN THE STRENGTH OF THE EXTENSION OF THE NORTH PACIFIC RIDGE...AND
THUS...WAS MUCH SLOWER AND WEAKER WITH THE FRONT/PRECIPITATION
MAKING IT THROUGH SOUTHWEST AK. SO A BLEND OF THE GEM REGIONAL AND
GFS/EC WERE USED FOR THIS FORECAST PACKAGE. THE GENERAL IDEA
REMAINS THE SAME WITH THE PRECIPITATION ASSOCIATED WITH THE
DEVELOPING FRONT OUT WEST REACHING THE COAST ALONG BRISTOL BAY
EARLY WEDNESDAY MORNING AND PUSHING FROM SOUTHWEST TO NORTHEAST.
THE ENERGY THEN SPLITS WITH SOME OF IT GOING NORTH INTO THE BERING
STRAIT WHILE SOME TRANSLATES ITS WAY TOWARD THE GULF OF ALASKA.
EACH MODEL HAS ITS OWN RENDITION OF HOW THIS OCCURS...BUT NET
RESULT IS WET WEATHER FOR SOUTHCENTRAL AK FOR THE LATTER HALF OF
THE WEEK.

&&

.FIRE WEATHER DISCUSSION...BIG PICTURE IS THAT WE ARE NEARING THE
END OF THE WINDY AND QUITE DRY OFFSHORE FLOW PATTERN. BY THE
LATTER HALF OF THE WEEK...RAIN CHANCES GO UP MARKEDLY. GUSTY NORTH
TO NORTHWEST WINDS WILL HANG AROUND AT LEAST THROUGH THE EARLY
THIS AFTERNOON BEFORE SUBSIDING TONIGHT. THESE WINDS WILL PUSH
RELATIVE HUMIDITIES RATHER LOW AGAIN TODAY...ESPECIALLY FOR THIS
NORMALLY WET TIME OF YEAR. BUT AS THE WINDS SUBSIDE BY WEDNESDAY
...AFTERNOON HUMIDITIES SHOULD START TO CLIMB BACK INTO THE 30-50
PERCENT RANGE. THE RIDGE OF HIGH PRESSURE FROM THE NORTH PACIFIC
WILL KEEP THE SOUTHCENTRAL MAINLAND DRY THROUGH WEDNESDAY. BUT A
WARM FRONT WILL BRING WETTING RAIN TO MOST PLACES STARTING WITH
KODIAK LATE WEDNESDAY AND SPREADING UP THROUGH THE COOK INLET BY
LATE THURSDAY. MOIST SOUTHWEST FLOW WILL KEEP SHOWERS IN FORECAST
AS WE HEAD INTO THE WEEKEND.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 AND 2)...
A POTENTIAL VORTICITY ANOMALY DROPPING THROUGH SOUTHCENTRAL AND
INTO THE GULF EARLY THIS MORNING WILL PROVIDE ONE MORE UPWARD 
BUMP IN WINDS...PRIMARILY THROUGH COASTAL GAPS...INCLUDING SEWARD
WHITTIER AND VALDEZ. PRESSURE GRADIENTS WILL THEN BEGIN TO
SLACKEN LEADING TO WEAKENING WINDS BEGINNING THIS AFTERNOON.
INLAND AREAS WHICH HAVE DECOUPLED FROM THE BOUNDARY LAYER OR AT
LEAST LIGHTENED UP OVERNIGHT WILL ALSO SEE A LITTLE UPTICK IN
WINDS AS TEMPS WARM AND LOW LEVEL LAPSE RATES STEEPEN ONCE AGAIN.

AN UPPER LEVEL RIDGE WILL BUILD IN FROM THE WEST BEGINNING TONIGHT
WHICH WILL ALLOW WINDS TO GO LIGHT ACROSS MUCH OF INTERIOR ALASKA.
THIS WILL RESULT IN A CHILLY NIGHT ACROSS THE REGION...WITH
WIDESPREAD LOWS IN THE UPPER 20S TO MID 30S EARLY WEDNESDAY
MORNING. FREEZE WARNINGS HAVE BEEN ISSUED FOR ANCHORAGE AND THE
MATANUSKA VALLEY. THERE HAVE BEEN LOCALIZED AREAS OF SUB-FREEZING
TEMPERATURES OVER THE LAST SEVERAL NIGHTS...BUT THIS WILL BE MORE
WIDESPREAD AND THE COLDEST AREAS WILL LIKELY SEE A HARD FREEZE
WITH AS MANY AS FIVE HOURS WITH TEMPERATURES BELOW FREEZING. THERE
WILL BE VERY LOCALIZED WINDS COMING OUT OF THE MATANUSKA RIVER
VALLEY INTO PALMER AND ALONG THE INLET AND WEST ANCHORAGE. THIS
WILL KEEP THESE AREAS ABOVE FREEZING. MEANWHILE...GAP WINDS WILL
LINGER ALONG THE GULF COAST/PRINCE WILLIAM SOUND WHICH WILL
MAINTAIN MUCH WARMER TEMPERATURES ALONG THE IMMEDIATE COAST. WINDS
WILL FINALLY COMPLETELY DIE OFF WEDNESDAY AS THE RIDGE MOVES
OVERHEAD.

THE UPPER LEVEL RIDGE WILL NOT STAY VERY LONG...QUICKLY EXITING 
TO THE EAST WEDNESDAY NIGHT INTO THURSDAY. CONFIDENCE HAS GREATLY
INCREASED THAT RAIN WILL OVERSPREAD MOST (IF NOT ALL) OF
SOUTHCENTRAL AS A SYSTEM MOVES UP FROM THE PACIFIC ON THURSDAY. 

&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 THROUGH 3)...
AN UPPER LEVEL RIDGE BUILDING IN FROM THE WEST WILL BRING WARMER
AND DRIER CONDITIONS TO SOUTHWEST ALASKA TODAY. THE NEXT
APPROACHING FRONTAL SYSTEM WILL REACH THE WESTERN CAPES LATE
TONIGHT AND START SPREADING PRECIPITATION ONSHORE WEDNESDAY
MORNING. THE FRONTAL SYSTEM WILL SLOW OVER THE KUSKOKWIM DELTA
AND BRISTOL BAY AREAS WEDNESDAY AND WEDNESDAY NIGHT AS A FRONTAL
WAVE LIFTS NORTHEAST FROM THE WESTERN TIP OF THE ALASKA PENINSULA
ACROSS WESTERN BRISTOL BAY. ON THURSDAY A TROUGH SWINGING ALONG
THE ALASKA PENINSULA AND INTO THE WESTERN GULF WILL DRAG THE FRONT
FURTHER INLAND INTO SOUTHWEST ALASKA WHERE IT WILL REMAIN AS A
DEFORMATION BAND THROUGH FRIDAY.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 THROUGH 3)...
THE BROAD UPPER LEVEL LOW CENTERED TO THE EAST OF THE KAMCHATKA
PENINSULA WILL REMAIN A DOMINANT FEATURE DRIFTING EAST INTO THE
WESTERN BERING WEDNESDAY AND THURSDAY AS THE UPPER LEVEL RIDGE
SHIFTS EAST AND INLAND OVER MAINLAND ALASKA. A NORTH PACIFIC SURFACE
LOW WILL CROSS THE CENTRAL ALEUTIANS THIS MORNING THEN LIFT NORTH
NORTHEAST THROUGH THE BERING TODAY AND TONIGHT PUSHING AN
ACCOMPANYING FRONTAL SYSTEM EAST ACROSS THE EASTERN BERING AND
BRISTOL BAY. A TROUGH WILL SWING ACROSS THE WESTERN AND CENTRAL
ALEUTIANS WEDNESDAY AND INTO THE BERING THURSDAY OUT AHEAD OF THE
LARGE UPPER LOW COMING IN FROM THE WEST.

&&

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

THE FRONT MOVING INLAND ACROSS SOUTHWEST ALASKA WILL SLOW DOWN A
LITTLE BIT ON THURSDAY AS IT BEGINS TO PUSH UP AGAINST SFC HIGH
PRESSURE OVER MUCH OF THE ALASKA MAINLAND. THIS WILL KEEP MUCH OF
SOUTHWEST AND SOUTHCENTRAL ALASKA IN A FAVORABLE AREA TO SEE
RAINFALL THROUGH THE MAJORITY OF THE WEEKEND. RAIN CHANCES WILL
DIMINISH FROM WEST TO EAST SUNDAY MORNING WITH THE PASSAGE OF THE
UPPER LEVEL TROUGH. SOME TIMING ISSUES ARE PRESENT AS TWO UPPER
LOWS (ONE OVER THE NORTHERN BERING AND THE OTHER IN THE NORTH
PACIFIC) MAY OR MAY NOT INTERACT WITH ONE ANOTHER HEADING INTO THE
WEEKEND. THE GFS HAS THE LOW IN THE BERING DROPPING SOUTH AND
INTERACTING WITH THE LOW MOVING TOWARD THE GULF OF ALASKA WHICH
WILL KEEP THE UPPER LEVEL FLOW IN A MORE ZONAL PATTERN. THE
ECMWF...HOWEVER...KEEPS THESE TWO LOWS SEPARATE WITH THE NORTHERN
BERING LOW CUT OFF FROM THE MEAN FLOW AND SLOWLY MEANDERING
EASTWARD. THIS WOULD ALLOW AN UPPER LOW COMING OFF THE KAMCHATKA
TO AMPLIFY THE LONGWAVE PATTERN LEADING TO A STRENGTHENING RIDGE
OVER MUCH OF THE MAINLAND. THE GFS FAILS TO AMPLIFY THE PATTERN AS
THE ZONAL FLOW AND STRONG JET WOULD SHUNT THIS UPPER LOW INTO
ALASKA BEFORE IT HAS TIME TO AMPLIFY THE PATTERN. THE ECMWF
SOLUTION STILL LOOKS TO BE THE MORE FAVORABLE SOLUTION ALTHOUGH
MAYBE NOT THE EXTREME THE MODEL IS CURRENTLY DEPICTING. THERE IS
STILL QUITE A BIT OF UNCERTAINTY AT THIS TIME SO IT IS TOUGH TO
REALLY BUY INTO ANY ONE SOLUTION BUT IT APPEARS MORE LIKELY THAT
THE PATTERN RE-AMPLIFIES HEADING INTO NEXT WEEK. SHOULD THE
RIDGING DEVELOP AS ANTICIPATED...A RETURN OF SOUTHWESTERLY FLOW
WOULD LEAD TO A BRIEF PERIOD OF SLIGHTLY ABOVE NORMAL TEMPERATURES
RETURNING TO NEAR NORMAL BY MONDAY.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...FREEZE WARNING 101 111. 
MARINE...GALE WARNING 180 181.
FIRE WEATHER...NONE.

&&

$$

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

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