730  
FXAK68 PAFC 221241  
AFDAFC  
 
SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE ANCHORAGE AK  
341 AM AKST MON JAN 22 2018  
   
ANALYSIS AND UPPER LEVELS
 
 
THE LONGWAVE TROUGH OVER MAINLAND ALASKA AND THE GULF HAS EVOLVED  
SOMEWHAT OVER THE PAST 24 HOURS AND IS NOW ANCHORED BY TWO  
DISTINCT LOW CENTERS, ONE OVER THE NORTH SLOPE AND ANOTHER OVER  
THE FAR SOUTHERN GULF. SOUTHCENTRAL AK IS CURRENTLY UNDER THE  
INFLUENCE OF WEAK CYCLONIC FLOW BETWEEN THESE TWO FEATURES,  
YIELDING MOSTLY CLOUDY SKIES AND ISOLATED TO SCATTERED SNOW  
SHOWERS. THESE SHOWERS ARE A LITTLE MORE PREVALENT ALONG THE GULF  
COAST OWING TO A DISTURBANCE TRANSITING THE NORTHERN GULF.  
MEANWHILE, MOSTLY CLEAR SKIES PREVAIL ACROSS THE SOUTHWEST  
MAINLAND IN BRISK NORTHERLY FLOW BETWEEN THE LONGWAVE TROUGH AND A  
STOUT, PERSISTENT RIDGE OVER THE BERING SEA. GUSTY WINDS AIDED BY  
A MERIDIONAL JET STREAK MOVING DOWN THE WESTERN PERIPHERY OF THE  
TROUGH ARE CONTRIBUTING TO VERY LOW WIND CHILL VALUES,  
PARTICULARLY THROUGH THE KUSKOKWIM DELTA WHERE A WIND CHILL  
ADVISORY IS POSTED THROUGH THIS MORNING. THE COLD NORTHERLY FLOW  
IS ALSO INITIATING WIDESPREAD CONVECTIVE SNOW SHOWER ACTIVITY  
ALONG THE NORTH SIDE OF THE ALASKA PENINSULA AND EASTERN  
ALEUTIANS, WITH MORE BENIGN CONDITIONS PREVAILING ACROSS THE REST  
OF THE BERING BENEATH THE RIDGE.  
 

 
   
MODEL DISCUSSION
 
 
MODELS CONTINUE TO SHOW GOOD SYNOPTIC AGREEMENT IN THE SHORT TERM  
AS THE LONGWAVE TROUGH PIVOTS AND TAKES ON A LITTLE MORE OF A  
NEGATIVE TILT, EVENTUALLY SWINGING THE TROUGH AXIS THROUGH THE  
COOK INLET REGION BY TUESDAY. THERE IS STILL SOME MINOR  
DISAGREEMENT ON THE NUMEROUS SMALLER SCALE FEATURES ROTATING  
THROUGH THE GULF IN THIS TIMEFRAME, BUT THERE IS A GENERAL  
CONSENSUS ON THE POTENTIAL SNOWFALL DEVELOPING FROM THE MAT-SU  
THROUGH ANCHORAGE AND THE KENAI PENINSULA LATE THIS AFTERNOON  
THROUGH TUESDAY MORNING. LARGELY STAYED THE COURSE WITH THE  
INHERITED FORECAST FOR THE MORNING PACKAGE GIVEN GOOD RUN TO RUN  
CONSISTENCY.  
 

 
   
AVIATION
 
 
PANC...VFR CONDITIONS AND LIGHT NORTH WINDS WILL PERSIST THROUGH  
THIS AFTERNOON. SNOW WILL BEGIN TO SPREAD INTO THE TERMINAL BY  
LATE AFTERNOON (AROUND 02Z), WITH CEILINGS LOWERING TO AROUND 5000  
FT. SNOW WILL INCREASE IN INTENSITY BY LATE EVENING AND PERSIST  
THROUGH TUESDAY MORNING AS MVFR CONDITIONS DEVELOP AFTER 08Z.  
 

 
   
SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 AND 2)
 
 
THE TWO MAIN FORECAST IMPACTS THROUGH TUESDAY ARE OUTFLOW WINDS  
OVER THE WESTERN GULF AND SNOW POTENTIAL FROM THE WESTERN KENAI  
PENINSULA ACROSS THE ANCHORAGE BOWL TO OVER THE MAT-SU VALLEYS.  
STRONG PRESSURE AND THERMAL GRADIENTS WILL CONTINUE ALONG THE  
ALEUTIAN RANGE. AS A RESULT, STRONG GAP FLOWS WITH GALES TO STORM  
GUSTS ARE EXPECTED TO CONTINUE ACROSS THE WESTERN GULF MARINE  
AREAS THROUGH TUESDAY NIGHT. GAP FLOWS ACROSS PRINCE WILLIAM SOUND  
HOWEVER SHOULD DIMINISH TODAY THROUGH TUESDAY, BUT COULD SEE AN  
INCREASE AGAIN LATE TUESDAY NIGHT AS PRESSURE GRADIENTS START TO  
BUILD AGAIN WITH COLD ADVECTION.  
 
SCATTERED SHOWERS ARE EXPECTED ACROSS MANY GULF AREAS, COOK INLET,  
ANCHORAGE BOWL AND SUSITNA VALLEY TODAY. THINGS GET A LITTLE MORE  
INTERESTING THIS EVENING AND TUESDAY AS A LONG WAVE UPPER TROUGH  
SETS UP JUST WEST OF SOUTHCENTRAL. WHILE THIS OCCURS A FAIRLY  
DEFINED WAVE MOVES WEST ACROSS THE GULF AND FEEDS LOW LEVEL  
MOISTURE WEST OF THE CHUGACH. THIS RESULTS IN A BROAD DEFORMATION  
ZONE FROM THE KENAI PENINSULA TO THE MAT-SU VALLEYS THIS EVENING  
WHICH WILL PROVIDE FOR STEADY SNOWFALL. SNOW WILL LIKELY CONTINUE  
FAIRLY STEADY INTO TUESDAY AFTERNOON BEFORE TAPERING OFF TO  
FLURRIES OR SNOW SHOWERS TUESDAY NIGHT. AT THIS TIME WE ARE  
THINKING SUB-ADVISORY SNOWFALL, BUT THIS SETUP BEARS WATCHING FOR  
ANY ADDITIONAL CLUES FOR CHANGES IN INTENSITY AND DURATION OF  
SNOWFALL.  
 

 
   
SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)
 
 
WITH MOSTLY CLEAR CONDITIONS AND ARCTIC AIR CONTINUING TO FILTER  
IN OVER THE SOUTHWEST MAINLAND, TEMPERATURES WILL CONTINUE TO DROP  
AS WINDS CONTINUE TO INCREASE DURING THE OVERNIGHT HOURS THROUGH  
THIS MORNING. THE MODELS ARE STILL NOT IN GREAT AGREEMENT WITH  
REGARDS TO HOW LOW THE WIND CHILL VALUES WILL REACH TODAY, WHICH  
HAS CAUSED SOME UNCERTAINTY IN THE FORECAST. HOWEVER, BASED ON  
OBSERVATIONS DURING THE OVERNIGHT AND EARLY MORNING PERIOD, SOME  
OBSERVATIONS AROUND THE KUSKOKWIM DELTA ARE APPROACHING WIND CHILL  
VALUES OF -25 F TO -35 F. THUS, THE WIND CHILL ADVISORY REMAINS  
IN EFFECT THROUGH NOON TODAY AS TEMPERATURES ARE EXPECTED TO  
CONTINUE DROPPING THIS MORNING. FURTHER UPSTREAM INTO THE  
KUSKOKWIM VALLEY, TEMPERATURES WILL REMAIN MUCH COLDER THAN THE  
SURROUNDING BRISTOL BAY AND KUSKOKWIM DELTA REGIONS. HOWEVER, WIND  
SPEEDS ARE STRUGGLING TO REACH 15 MPH IN THE VALLEY, KEEPING WIND  
CHILL VALUES ABOVE ADVISORY CRITERIA.  
 
OTHERWISE, A PERSISTENT AREA OF HIGH PRESSURE ACROSS THE BERING  
WILL CONTINUE TO FUNNEL COLDER AIR INTO THE AREA IN AN OFFSHORE  
FLOW REGIME, WITH NO SENSIBLE CHANGE IN THE PATTERN SEEN THROUGH  
THE NEXT FEW DAYS.  
 

 
   
SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)
 
 
AN AREA OF HIGH PRESSURE WILL REMAIN ANCHORED ACROSS CENTRAL  
PORTIONS OF THE BERING SEA/ALEUTIANS OVER THE NEXT FEW DAYS. THIS  
CREATES A RATHER STAGNANT PATTERN WITH A DECAYING STALLED FRONT  
ACROSS WESTERN PORTIONS OF THE CHAIN AND BERING, AND COLD SHOWERY  
CONDITIONS OVER EASTERN PORTIONS OF THE ALEUTIANS AND BERING. THIS  
WILL BRING CONTINUED FREEZING SPRAY CONCERNS TO THESE EASTERN  
AREAS THROUGH THE PERIOD. ANOTHER FRONT WILL CLIP THE WESTERN  
ALEUTIANS AND BERING WEDNESDAY, BUT MEET THE SAME FATE AS ITS  
PREDECESSOR, BEING BLOCKED BY THE BERING RIDGE AND NOT MAKING MUCH  
EASTWARD PROGRESSION PAST SHEMYA.  
 

 
   
LONG TERM FORECAST (DAYS 3 THROUGH 7)
 
 
BEGINNING THE EXTENDED PERIOD FORECAST TUESDAY NIGHT, THERE WILL  
BE A RATHER POTENT COLD UPPER LEVEL VORTEX OVER THE NORTHWEST PART  
OF ALASKA WITH ASSOCIATED TROUGHING EXTENDING INTO THE GULF OF  
ALASKA. LOW LEVEL CYCLONIC FLOW WILL PREFERENTIALLY WRAP AROUND  
THE ALASKA RANGE INTO THE BERING SEA. SNOW SHOWERS OVER PARTS OF  
SOUTHCENTRAL WILL QUICKLY BE ENDING OVERNIGHT (ALTHOUGH THEY MAY  
LINGER LONGER OVER THE COPPER BASIN) AS STRONG OFFSHORE FLOW  
INCREASES IN RESPONSE TO COLD AIR ADVECTION. TEMPERATURES WILL  
CONTINUE TO DROP ACROSS ALL OF SOUTHERN ALASKA AS 850 TEMPERATURES  
PLUMMET TO -20 TO -30C ON THURSDAY AS THE UPPER VORTEX MOVES SOUTH  
ACROSS THE REGION. AS THE UPPER LOW DIGS INTO THE NORTH PACIFIC,  
SECONDARY CYCLOGENESIS WILL DEVELOP IN THE GULF WITH INCREASING  
CHANCES OF COASTAL PRECIPITATION (IN THE FORM OF SNOW) THURSDAY  
INTO SATURDAY. SOME LOCATIONS SUCH AS KODIAK ISLAND MAY SEE THE  
POTENTIAL FOR GOOD ACCUMULATIONS SHOULD THE SETUP DEVELOP AS  
CURRENT GLOBAL MODELS DEPICT...STAY TUNED.  
 
OTHERWISE, THE PATTERN LOOKS TO REMAIN DOMINATED BY THE EVENTUAL  
LARGE TROUGH OVER THE NORTH PACIFIC INTO THE GULF OF ALASKA, ALONG  
WITH HIGH LATITUDE BLOCKING OVER EASTERN SIBERIA, FOR THE  
REMAINDER OF THE WEEKEND INTO EARLY NEXT WEEK. THIS WILL LIKELY  
RESULT IN DRY AND CONTINUED COLDER THAN AVERAGE INLAND WEATHER AS  
THE GENERAL STORM TRACK REMAINS SOUTH OF THE FORECAST AREA.  
 

 
 
   
AFC WATCHES/WARNINGS/ADVISORIES
 
 
PUBLIC...WIND CHILL ADVISORY 155  
MARINE...GALE WARNING 127 130 131 138 150 155 165 172 180  
HEAVY FREEZING SPRAY 130 131 138 139 160 165 180 414  
 

 
 

 
 
SYNOPSIS AND MODEL DISCUSSION...CB  
AVIATION...CB  
SOUTHCENTRAL ALASKA...RMC  
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...TP  
LONG TERM...AHSENMACHER  
 
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