868  
FXAK68 PAFC 210034  
AFDAFC  
 
SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE ANCHORAGE AK  
434 PM AKDT WED SEP 20 2017  
   
ANALYSIS AND UPPER LEVELS
 
 
THERE ARE TWO MAIN FEATURES IMPACTING THE AREA. THE FIRST IS A  
SHORTWAVE MOVING WEST TO EAST THROUGH THE SUSITNA VALLEY AREA  
HEADED TOWARDS THE COPPER RIVER BASIN. MOST OF THE RAIN WITH THIS  
FEATURE IS STAYING NORTH OF THE GREATER ANCHORAGE AREA. THE OTHER  
FEATURE IS A LOW SOUTH OF THE CENTRAL ALEUTIANS AND ITS  
ASSOCIATED FRONT THAT STRETCHES ACROSS THE SOUTHERN BERING AND  
OVER THE ALASKA PENINSULA. THE FRONT IS CONTINUING ITS TRACK TO  
THE NORTHEAST TOWARDS THE BRISTOL BAY AND KODIAK ISLAND AREAS.  
 

 
   
MODEL DISCUSSION
 
 
THERE ARE SEVERAL AREAS THAT THE MODELS ARE STRUGGLING TO LOCK IN  
ON WITH THE LOW NEAR THE ALEUTIANS AND ITS ASSOCIATED FRONT. ONE  
OF WHICH IS A LOW FORMING ON THE FRONTAL BOUNDARY SOUTH OF THE  
KENAI PENINSULA THURSDAY. MODELS ARE STRUGGLING WITH THE TIMING  
AND POSITIONING OF THIS LOW. THEN ON FRIDAY THERE ARE DIFFERENCES  
WITH HOW MUCH RAIN TO BRING TO THE KENAI PENINSULA. OUT WEST THERE  
ARE ISSUES WITH THE MAIN LOW LATE THURSDAY AND INTO FRIDAY AS IT  
APPROACHES THE ALASKA PENINSULA AND EVENTUALLY MOVES INTO THE  
BRISTOL BAY AREA. TIMING IS THE BIGGEST CONCERN FOR THIS CHANGE.  
 

 
   
AVIATION
 
 
PANC...VFR CONDITIONS AND LIGHT WINDS WILL PERSIST.  
 

 
   
SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 AND 2)
 
 
CLOUDS WILL DIMINISH THIS EVENING ACROSS THE ANCHORAGE AND MAT-SU  
AREAS AS A SHORT WAVE TROUGH PUSHES OUT OF THE AREA AND A SHORT-  
LIVED RIDGE BUILDS IN BEHIND IT. TEMPERATURES WILL QUICKLY COOL OFF  
TONIGHT BEFORE STABILIZING EARLY IN THE MORNING AS CLOUD COVER  
WILL RAPIDLY INCREASE FROM THE SOUTH. LOWS SHOULD NOT BE AS COLD  
AS THEY WERE LAST NIGHT, WITH THE EXCEPTION OF THE NORTHERN  
SUSITNA VALLEY AS CLOUD COVER WILL NOT REACH THERE UNTIL THE  
EARLY MORNING HOURS.  
 
A LOW PRESSURE SYSTEM CURRENTLY CENTERED SOUTH OF THE ALEUTIANS  
WILL SEND A WARM FRONT NORTH INTO SOUTHCENTRAL OVERNIGHT. WARM  
MOIST AIR BEHIND THE FRONT SUPPORTED BY AN UPPER LEVEL SOUTHERLY  
JET WILL LEAD TO STEADY PRECIPITATION SPREADING ACROSS THE REGION.  
RAIN WILL BEGIN IN KODIAK LATER THIS EVENING AND MOVE UP INTO THE  
ANCHORAGE AND MAT-SU AREAS BY THE MORNING HOURS, WHILE REACHING  
THE NORTHERN SUSITNA VALLEY BY NOON. RAIN WILL BE HEAVIEST ALONG  
THE GULF COAST AND PRINCE WILLIAM SOUND BEFORE SHIFTING EAST AS  
THE UPPER LEVEL JET SUPPORT TAKES A MORE SOUTHWESTERLY JIVE.  
PRECIPITATION WILL CONTINUE ACROSS MOST OF THE REGION THROUGH  
FRIDAY AS SEVERAL UPPER LEVEL WAVES MOVE DOWN INTO SOUTHCENTRAL,  
REINFORCING THE UPPER LEVEL TROUGH OVER THE STATE. THE FORECAST  
BECOMES MORE TRICKY BY FRIDAY EVENING AS AN UPPER LEVEL WAVE DIGS  
SOUTH INTO THE SOUTHERN GULF CHANGING TO LOW LEVEL FLOW TO  
SOUTHEASTERLY. THIS WILL LEAD TO HEAVIER PRECIPITATION ON THE EAST  
SIDE OF THE KENAI COAST BUT ALSO LOOKS TO DRY OUT THE LEEWARD  
SIDE SOME, INCLUDING THE WESTERN KENAI PENINSULA AND THE ANCHORAGE  
BOWL, THOUGH THE EXTENT OF DOWNSLOPING IS STILL UNCERTAIN AT THIS  
TIME.  
 

 
   
SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)
 
 
STEADY STRATIFORM RAIN WILL MOVE OVER COASTAL PORTIONS OF BRISTOL  
BAY THIS EVENING AND THEN SPREAD INLAND OVERNIGHT AS THE BERING  
FRONT MOVES INTO SOUTHWEST AK, PUSHING INTO THE LOWER KUSKOKWIM  
VALLEY BY THURSDAY MORNING. THE RAINY PATTERN WILL THEN CONTINUE  
THROUGH FRIDAY AS THE TROUGH OVER THE NORTHERN HALF OF THE  
MAINLAND PHASES WITH THE UPPER LOW SOUTH OF ADAK AND THE FRONT  
PIVOTS FROM E-W TO MORE OF A NE-SW ORIENTATION WHILE BROAD  
SOUTHWEST FLOW IS ESTABLISHED OVER BRISTOL BAY AND THE KUSKOKWIM  
VALLEY. THIS EVOLUTION WILL SUPPORT DRIER CONDITIONS ACROSS THE  
KUSKOKWIM DELTA IN NORTHEAST FLOW ON THE DRY SIDE OF THE FRONT,  
WITH AREAS FROM BETHEL WEST LIKELY SEEING LITTLE OR NO  
PRECIPITATION THROUGH FRIDAY EVENING.  
 

 
   
SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)
 
 
THE LOW SOUTH OF ADAK WILL CONTINUE TO BE THE PRIMARY FEATURE OF  
INTEREST THROUGH FRIDAY EVENING, PRODUCING RAIN AND GUSTY WINDS  
ACROSS THE SOUTHEAST BERING AS IT MOVES SLOWLY EASTWARD AND  
EVOLVES INTO A LONGWAVE TROUGH SPANNING THE WESTERN MAINLAND AND  
THE AKPEN. THE WESTERN HALF OF THE BERING WILL REMAIN IN A DRIER  
PATTERN IN NORTHEAST FLOW AS THE LOW MOVES TO THE EAST, WITH HIGH  
PRESSURE BUILDING INTO THE WESTERN ALEUTIANS BY FRIDAY.  
 

 
   
LONG TERM FORECAST (DAYS 3 THROUGH 7)
 
 
BEGINNING THE EXTENDED PERIOD FORECAST FRIDAY NIGHT, THERE WILL BE  
AN ARCTIC TROUGH CENTERED OVER EASTERN BRISTOL BAY WITH WAVES OF  
NORTH PACIFIC MOISTURE ROTATING NORTHWARD TOWARD THE GULF OF  
ALASKA. THIS PATTERN WILL SUPPORT WET WEATHER ACROSS THE REGION  
WITH THE HEAVIEST RAIN ALONG THE NORTH GULF COAST. SATURDAY NIGHT  
INTO SUNDAY MORNING, A STRONGER FRONT WILL MOVE TOWARD THE COAST,  
BUT THIS FEATURE NOW LOOKS MUCH WEAKER THAN IT POTENTIALLY WAS  
LOOKING THE PAST FEW DAYS. THE MAIN IMPACT WILL BE INCREASING  
WINDS ALONG THE COAST, ALONG WITH MORE RAIN ACROSS SOUTHCENTRAL.  
 
BY MONDAY AND TUESDAY, THE UPPER LOW WILL BEGIN TO DIG SOUTH INTO  
THE NORTH PACIFIC. THIS CHANGE WILL CUT OFF THE MAIN MOISTURE  
SUPPLY AND ALLOW FOR GRADUAL DRYING HEADING THROUGH THE MIDDLE OF  
THE WEEK.  
 

 
 
   
AFC WATCHES/WARNINGS/ADVISORIES
 
 
PUBLIC...FLOOD WATCH 121 125.  
MARINE...GALES 177 412 413.  
 

 
 

 
 
SYNOPSIS AND MODEL DISCUSSION...DK  
SOUTHCENTRAL ALASKA...KVP  
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CB  
LONG TERM...JA  
 
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