491  
FXAK68 PAFC 280040  
AFDAFC  
 
SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE ANCHORAGE AK  
440 PM AKDT MON MAR 27 2017  
   
ANALYSIS AND UPPER LEVELS
 
 
THE LARGE SCALE PATTERN CONTINUES TO SHIFT AS A CLOSED UPPER LEVEL  
HIGH OVER THE CENTRAL BERING SEA SLOWLY WEAKENS AND RETROGRADES  
WEST. AN UPPER LEVEL TROUGH IS NOW FIRMLY ENTRENCHED OVER  
MAINLAND ALASKA AND THE NORTH PACIFIC, SUPPORTED BY A ROBUST  
SUBTROPICAL JET. A SERIES OF LOWS ARE STRUNG OUT SOUTH OF THE  
ALEUTIAN ISLANDS AND ON UP INTO THE GULF OF ALASKA. A WEAK  
COMPACT LOW OVER THE NORTHERN GULF OF ALASKA IS PRODUCING LIGHT  
SNOW TODAY, PRIMARILY IN THE PRINCE WILLIAM SOUND REGION, THOUGH  
SOME SNOW MAY MAKE IT OVER THE COASTAL MOUNTAINS. THE FOCUS OF THE  
FORECAST IS WITH THE COMPLEX LOW PRESSURE SYSTEM SOUTH OF THE  
ALASKA PENINSULA. A DEVELOPING TRIPLE POINT LOW IS HEADED  
NORTH INTO THE GULF OF ALASKA WITH AN ALREADY ESTABLISHED LOW  
OVER THE NORTH PACIFIC BEGINNING TO CURVE NORTHEAST TOWARD THE  
GULF OF ALASKA. THESE TWO SYSTEMS WILL BE THE MAJOR PLAYERS IN  
PRODUCING WIDESPREAD SNOW ACROSS SOUTH CENTRAL ALASKA THE NEXT  
COUPLE DAYS.  
 

 
   
MODEL DISCUSSION
 
 
THERE HAS BEEN QUITE A BIT OF SPREAD IN HANDLING OF LOWS CROSSING  
THE GULF OF ALASKA. THE 12Z MODEL RUNS ARE GROUPED CLOSER  
TOGETHER, WITH A MEDIAN TRACK OF THE NEXT TWO LOWS TOWARD WESTERN  
PRINCE WILLIAM SOUND. ENSEMBLES ALSO SUPPORT THIS GENERAL IDEA.  
THE NAM IS THE ONE NOTABLE OUTLIER WITH A FARTHER EAST TRACK AND  
WILL BE IGNORED. BASED ON INCREASING CONFIDENCE IN THE GENERAL LOW  
TRACK, WE WILL AGGRESSIVELY INCREASE THE PROBABILITY OF SNOW FOR  
MUCH OF THE AREA, INCLUDING THE POPULATION CENTERS OF ANCHORAGE  
AND THE MATANUSKA VALLEY. CONFIDENCE IN THE LOCATION OF HEAVIER  
PRECIPITATION WITH THESE SYSTEMS IS LOW. THUS, WE WILL REMAIN  
CONSERVATIVE WITH SNOW ACCUMULATION FORECAST FOR NOW.  
 

 
   
AVIATION
 
 
PANC...THERE IS INCREASING CONFIDENCE IN DEVELOPMENT OF MVFR  
CONDITIONS AS SNOW MOVES IN ON TUESDAY. THERE IS STILL QUITE A BIT  
OF UNCERTAINTY IN THE EXACT TRACK OF LOWS CROSSING THE GULF OF  
ALASKA WHICH WILL GREATLY AFFECT THE INTENSITY OF SNOWFALL. IF  
HEAVIER SNOW DOES DEVELOP WE COULD EASILY SEE IFR/LIFR CONDITIONS  
TUESDAY NIGHT INTO WEDNESDAY.  
 

 
   
SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 AND 2)
 
 
THE CURRENT LOW OVER THE NORTHERN GULF OF ALASKA WILL DISSIPATE  
AS IT HEADS INLAND ACROSS THE KENAI PENINSULA TONIGHT. THE BULK OF  
SNOW WILL REMAIN IN THE PRINCE WILLIAM SOUND AREA THIS AFTERNOON  
THROUGH THIS EVENING, THEN TAPERING OFF AFTER THAT. LOW LEVEL DRY  
AIR IS THE LIMITING FACTOR FOR SNOWFALL AS MOISTURE ADVECTS ACROSS  
THE COASTAL MOUNTAIN RANGES TO THE REST OF SOUTH CENTRAL ALASKA.  
WE HAVE GENERALLY STUCK WITH CHANCE OR SLIGHT CHANCE FOR THESE  
INLAND AREAS. IF SNOW DOES FALL IT WILL BE LIGHT WITH LITTLE OR NO  
ACCUMULATION.  
 
MORE SIGNIFICANT AND WIDESPREAD SNOWFALL IS ON TAP FOR TUESDAY  
THROUGH WEDNESDAY. THE LATEST MODEL GUIDANCE IS BEGINNING TO LOCK  
INTO SOME VERY FAVORABLE FEATURES FOR SNOWFALL INCLUDING A SERIES  
OF FAIRLY WEAK LOWS MOVING UP FROM THE GULF OF ALASKA TO PRINCE  
WILLIAM SOUND AREA, A STRONGLY NEGATIVELY TILTED UPPER LEVEL  
TROUGH, AND COLD AIR ADVECTION FROM THE BERING SEA TO THE GULF OF  
ALASKA AND SOUTH CENTRAL ALASKA. THE QUESTION OF THE DAY IS  
"WHERE WILL THE SURFACE LOWS TRACK TO?". THIS IS THE KEY TO  
DETERMINING HOW MUCH SNOW WILL FALL AND HOW LONG IT WILL FALL.  
BASED ON MODEL TRENDS, THE LATEST FORECAST WILL LEAN PRETTY  
HEAVILY TOWARD A LOW TRACK TO WESTERN PRINCE WILLIAM SOUND, WHICH  
WOULD BRING A LONG DURATION SNOW EVENT TO PRINCE WILLIAM SOUND,  
THE EASTERN KENAI PENINSULA, ANCHORAGE, AND PORTIONS OF THE  
MATANUSKA AND SUSITNA VALLEYS. THIS WOULD ALL CHANGE WITH A SMALL  
SHIFT IN LOW TRACK.  
 
ALTHOUGH WE HAVE TRENDED UP WITH THE LIKELIHOOD OF SNOW ACROSS  
THE REGION, WE HAVE GONE WITH A CONSERVATIVE SNOWFALL  
ACCUMULATION FORECAST UNTIL THERE IS MORE CONFIDENCE IN THE LOW  
TRACK. FOR NOW, THE BIG MESSAGE IS THAT THE WEEKS OF DRY WEATHER  
FOR MUCH OF THE REGION ARE COMING TO AN END AND PEOPLE SHOULD  
PREPARE FOR A RETURN TO WINTER WEATHER AND WINTER DRIVING  
CONDITIONS.  
 

 
   
SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)
 
 
THE FORECAST PERIOD COMMENCES WITH A SURFACE LOW 200 MILES SOUTH  
OF DUTCH HARBOR RESULTING IN WIDESPREAD EASTERLY WINDS FROM THE  
KENAI PENINSULA TO ATTU ISLAND. AS TIME ELAPSES EXPECT THE LOW TO  
DROP SOUTH TAKING THE ENHANCED PRESSURE GRADIENT FORCE WINDS WITH  
IT SHIFTING THE STRONGEST GUSTS TO THE EAST FOLLOWED BY A BRIEF  
STINT OF NORTHERLY WINDS. BY TUESDAY NIGHT A NEW LOW WILL ENTER  
THE GULF OF ALASKA RESULTING IN GALE FORCE WINDS AND INCREASED  
CHANCE OF PRECIPITATION.  
 

 
   
SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)
 
 
THE BLOCKING PATTERN OVER THE BERING SEA WHICH HAS BEEN IN PLACE  
MOST OF MARCH WILL FINALLY BREAK DOWN OVER THIS FORECAST PERIOD.  
BY DAY TWO ANTICIPATE STORMY CONDITIONS TO MOVE INTO THE WESTERN  
PORTION OF THE BERING SEA AND ALEUTIANS COMPLETE WITH A SPIKE IN  
PRECIPITABLE WATER AND EASTERLY GUSTS.  
 

 
   
LONG TERM FORECAST (DAYS 3 THROUGH 7)
 
 
THE EXTENDED FORECAST LOOKS TO BE QUITE AN ACTIVE ONE FOR MUCH OF  
ALASKA. STARTING LATE WEDNESDAY INTO THURSDAY, AN UPPER LEVEL  
TROUGH EXTENDING OVER THE BERING SEA AND NORTHERN PACIFIC OCEAN  
WILL CONTINUE TO AMPLIFY AND ELONGATE AS AN INFLUX OF ENERGY FROM  
THE COAST OF RUSSIA MOVES INTO THE PACIFIC OCEAN. AS THE TROUGH  
AMPLIFIES, A WARMER AND MORE MOIST AIR MASS ORIGINATING IN THE  
SUBTROPICS WILL BEGIN TO BE ADVECTED NORTH TOWARD THE GULF COAST  
OF ALASKA. WHILE MODELS HAVE STRUGGLED RECENTLY WITH THE PATTERN  
CHANGE, MODELS ARE IN NEAR LOCK-STEP THROUGH FRIDAY BEFORE SOME  
DISCREPANCIES BEGIN TO DEVELOP.  
 
THE BIG STORY ON THURSDAY AND FRIDAY WILL BE A POWERFUL LOW  
PRESSURE SYSTEM MOVING NORTH TOWARD THE ALASKA PENINSULA WHICH  
WILL LIKELY SLAM THE GULF COAST WITH A MIXTURE OF RAIN AND SNOW.  
MODEL GUIDANCE SHOWS SURFACE TO 500 MB THICKNESSES INCREASING AND  
850 MB TEMPERATURES APPROACHING 0 DEGREES C WHICH INDICATES A  
CHANGE TO RAINFALL ALONG THE COAST. HOWEVER, INTERIOR LOCATIONS  
THAT SEE PRECIPITATION WILL LIKELY REMAIN SNOW. THE LOW TRACK WILL  
BE IMPORTANT AS THIS FAR WEST TRACK WILL LIKELY KEEP ANCHORAGE  
AND NORMAL DOWNSLOPED LOCATIONS ON THE DRY SIDE. HEADING INTO THE  
WEEKEND, MODELS BEGIN TO STRUGGLE WITH THE ORIENTATION OF THE  
UPPER LEVEL TROUGH AS THE ECMWF TRIES TO FLATTEN THE TROUGH OUT  
WHICH BRINGS ALL THE ENERGY TOWARD JUNEAU WHILE THE GFS KEEPS THE  
ENERGY IN THE GULF OF ALASKA. THE TAKE-AWAY STORY HERE IS THAT  
THIS ACTIVE PATTERN LOOKS TO BE HERE TO STAY FOR THE NEXT 7 TO 10  
DAYS BUT IT'S DIFFICULT TO SEE AS OF NOW WHICH AREAS MAY SEE THE  
MOST RAINFALL/SNOWFALL. STAY TUNED.  
 

 
 
   
AFC WATCHES/WARNINGS/ADVISORIES
 
 
PUBLIC...NONE.  
MARINE...NONE.  
 

 
 

 
 
SYNOPSIS AND MODEL DISCUSSION...SB  
SOUTHCENTRAL ALASKA...CB  
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PS/DEK  
LONG TERM...MC  
 
CLICK HERE TO GO TO PREVIOUS BULLETINS.

The Nexlab AK Page Main Text Page