768  
FXAK68 PAFC 180042  
AFDAFC  
 
SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE ANCHORAGE AK  
442 PM AKDT THU SEP 17 2020  
   
ANALYSIS AND UPPER LEVELS
 
 
A VERTICALLY STACKED LOW WHICH WAS OVER BRISTOL BAY THIS MORNING  
IS TRACKING INLAND ACROSS SOUTHWEST ALASKA THIS AFTERNOON. THE  
ASSOCIATED OCCLUDED FRONT IS APPROACHING THE NORTH GULF COAST,  
LEADING TO A BARRIER JET WITH STORM FORCE WINDS ALONG THE COAST  
AND GALE FORCE WINDS IN PRINCE WILLIAM SOUND. ADDITIONALLY,  
WIDESPREAD GALES ARE BEING OBSERVED IN THE WAKE OF THE LOW AS  
COOLER MORE UNSTABLE AIR MOVES IN AND PRESSURE RISES. THIS IS  
PRIMARILY AFFECTING COASTAL WATERS ALONG THE ALASKA PENINSULA  
AND BRISTOL BAY ON INTO THE WESTERN GULF. GUSTY WINDS ALSO MOVED  
ACROSS MUCH OF SOUTHCENTRAL TODAY, AHEAD OF THE FRONT. THE HIGHEST  
WINDS WERE OBSERVED THROUGH PORTAGE VALLEY, TURNAGAIN ARM, THE  
HIGHER ELEVATIONS OF ANCHORAGE, AND ALONG THE COPPER RIVER. GUSTS  
OF 65 TO 75 MPH WERE REPORTED AT MULTIPLE STATIONS ON THE UPPER  
HILLSIDE IN ANCHORAGE. THIS STORM SYSTEM IS ALSO BRINGING  
WIDESPREAD RAIN TO SOUTHWEST AND SOUTHCENTRAL ALASKA, WITH THE  
HEAVIEST RAIN ALONG THE COASTAL REGIONS. STORM TOTAL RAINFALL  
AMOUNTS THUS FAR GENERALLY RANGE FROM 0.5 TO 1.5 IN. WHILE RIVER  
LEVELS ARE ELEVATED, FLOOD RISKS ARE MINIMAL.  
 

 
   
MODEL DISCUSSION
 
 
MODELS ARE IN GOOD SYNOPTIC AGREEMENT OVER THE NEXT FEW DAYS.  
MODEL AGREEMENT (AND FORECAST CONFIDENCE) HAS IMPROVED FOR THE  
FAST MOVING LOW TRAVERSING EASTWARD ALONG THE ALEUTIAN CHAIN  
FRIDAY AND SATURDAY. HOWEVER, SOLUTIONS DIVERGE SIGNIFICANTLY  
AS THE LOW CROSSES THE GULF, SO THERE IS UNCERTAINTY IN THE  
WIND/PRECIP FORECAST FOR THE GULF COAST AND SOUTHCENTRAL.  
LASTLY, MODELS ARE ALL TRENDING STRONGER WITH THE LARGE LOW  
CROSSING THE BERING SEA SATURDAY THROUGH SUNDAY, SO THE  
FORECAST WILL FOLLOW THE SAME TREND.  
 

 
   
AVIATION
 
 
PANC...VFR CONDITIONS WILL CONTINUE, THOUGH THERE COULD BE A VERY  
BRIEF PERIOD OF MVFR CEILINGS AS BATCHES OF SHOWERS MOVE THROUGH.  
GUSTY WINDS WILL PERSIST THROUGH THE FORECAST PERIOD.  
 

 
   
SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 THROUGH 3)
 
 
A LOW HAS TRACKED WELL INTO THE INTERIOR OF WESTERN ALASKA WITH  
ITS ASSOCIATED FRONT MOVING ONSHORE ALONG THE GULF COAST.  
CONSEQUENTLY, GUSTY EASTERLY WINDS OVER TURNAGAIN AND KNIK ARMS  
WILL CONTINUE TO DIMINISH OVERNIGHT INTO FRIDAY MORNING. RAIN WILL  
TAPER OFF ACROSS ANCHORAGE AND THE MATANUSKA VALLEY AS WELL,  
THOUGH PRECIPITATION WILL CONTINUE TO REMAIN WIDESPREAD NEAR  
HIGHER TERRAIN THROUGH THE WEEKEND. AN UPPER LEVEL JET WILL DIP  
WELL TO THE SOUTH OF ALASKA ON SATURDAY BRINING MUCH COOLER  
TEMPERATURES. WHILE LOW TEMPERATURES OVERNIGHT SATURDAY INTO  
SUNDAY WILL GENERALLY BE MODERATED BY CLOUD COVER, THEY ARE  
EXPECTED TO FALL TO NEAR FREEZING OVER THE CHUGACH AND THUS MINOR  
SNOW ACCUMULATIONS AT HIGHER ELEVATIONS CANNOT BE RULED OUT.  
WEAKER FLOW WILL ALLOW FOR A HIGHER CHANCE OF SHOWERS IN THE  
ANCHORAGE AREA LATE SATURDAY INTO SUNDAY AS WELL.  
 

 
   
SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 THROUGH 3)
 
 
(TONIGHT THROUGH SUNDAY)  
 
AN UPPER LOW OVER KUSKOKWIM DELTA HAS BROUGHT A LOT OF RAIN AND  
MOISTURE WEST OF ALASKA RANGE, WHILE ISOLATED THUNDERSTORMS WERE  
OCCURRING SOUTH OF KUSKOKWIM VALLEY THIS AFTERNOON. SHOWERY  
CLOUDS ASSOCIATED WITH THIS SYSTEM WILL GRADUALLY TAPER OFF LATE  
TONIGHT, AND COOLER TEMPERATURES WILL LINGER OVER THE REGION  
TONIGHT AND FRIDAY NIGHT. THE NEXT MAJOR SYSTEM FROM THE ALEUTIANS  
COULD BRING MORE RAINFALL, BUT THE TIMING MAY BE A CHALLENGE AS  
MODELS DELAY THE FORWARD MOTION OF THIS EXPECTED SYSTEM. OVERALL,  
IT WILL BE A QUIET FORECAST PERIOD WITH ONLY ISOLATED SHOWERS IN  
THE COMING DAYS.  
 

 
   
SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 THROUGH 3)
 
 
(TONIGHT THROUGH SUNDAY)  
 
THE NEXT DEVELOPING SYSTEM WILL ELEVATE SEAS AND WINDS TO GALE  
FORCE CONDITIONS, BUT THAT MAY ALL CHANGE SHOULD THIS NEW SYSTEM  
INTENSIFY AS IT MOVES ACROSS THE BERING AND THE ALEUTIANS OVER THE  
WEEKEND. AS A RESULT, ANTICIPATE ANOTHER WET PATTERN TO RETURN  
BEFORE THE END OF THE WEEK, WITH MARINE WARNINGS TO REFLECT THE  
EXPECTED SITUATION.  
 

 
   
MARINE (DAYS 3 THROUGH 5: SUNDAY THROUGH TUESDAY)
 
 
A STRONG LOW ENTERING THE BERING SEA ON SATURDAY SEEMS TO BE  
TRENDING STRONGER WITH THE NEWER MODEL RUNS. THUS, EXPECT GALE  
FORCE WINDS WILL MORE WIDESPREAD IN NATURE ACROSS THE BERING SEA  
AND ALEUTIANS.SATURDAY NIGHT THROUGH MONDAY. THERE IS ALSO THE  
POSSIBILITY OF A BRIEF PERIOD OF STORM FORCE WINDS ALONG THE  
ALEUTIAN CHAIN AS COLDER FEEDS INTO THE BACK SIDE OF THE  
STORM SATURDAY NIGHT INTO SUNDAY. BY TUESDAY THE LOW CROSSES  
OVER THE ALEUTIANS AND MOVES INTO THE NORTHEAST PACIFIC/GULF OF  
ALASKA. THERE IS DISAGREEMENT ON THE TRACK, BUT WITH THE LATEST  
MODELS GENERALLY FARTHER SOUTH, SUSPECT THE BULK OF SMALL  
CRAFT/GALE FORCE WINDS MAY BE CONFINED TO THE SOUTHERN GULF/KODIAK  
ISLAND.  
 

 
   
LONG TERM FORECAST (DAYS 4 THROUGH 7: MONDAY THROUGH THURSDAY)
 
 
A STRONG LOW IN THE VICINITY OF THE EASTERN ALEUTIANS/ALASKA  
PENINSULA ON MONDAY WILL CONTINUE SOUTHEASTWARD INTO THE  
GULF/NORTHEAST PACIFIC BY TUESDAY. THUS, RAIN AND WIND OVER  
THE CENTRAL TO EASTERN BERING WILL SHIFT SOUTH AND EASTWARD TO  
KODIAK ISLAND AND THE GULF. COLD AIR ADVECTION BEHIND THE LOW  
WILL MAINTAIN ROBUST NORTHERLIES ACROSS THE EASTERN BERING AND  
SOUTHWEST ALASKA COASTAL WATERS ALL THE WAY THROUGH WEDNESDAY.  
MUCH OF SOUTHWEST AND SOUTHCENTRAL LOOK DRIER ON THE NORTHERN  
PERIPHERY OF THE LOW MONDAY THROUGH TUESDAY, WITH STEADY RAIN  
LIKELY CONFINED TO THE GULF COAST. HOWEVER, IT MAY NOT BE  
COMPLETELY DRY OVER SOUTHCENTRAL, AS MOST MODELS INDICATE A  
SHORT-WAVE TROUGH MOVING THROUGH SOMETIME MONDAY.  
 
THE FORECAST FOR LATE NEXT WEEK LOOKS POTENTIALLY "WINTER-LIKE".  
MOST MODELS SHOW A TROUGH DROPPING OUT OF THE ARCTIC MID WEEK,  
THOUGH THERE IS DISAGREEMENT ON HOW FAR SOUTH THE TROUGH WILL  
MAKE IT. THE GFS IS MOST AGGRESSIVE IN DRIVING COLD AIR (DOWN  
TO -10 DEGREES C AT 850 MB) DOWN ACROSS SOUTHWEST ALASKA, THEN  
POTENTIALLY INTERACTING WITH ENERGY BEING PULLED FROM THE  
GULF/NORTHEAST PACIFIC UP TO SOUTHCENTRAL. AT A MINIMUM, COULD  
BE A ROUND OF MOUNTAIN SNOW. STAY TUNED, AS FORECAST CONFIDENCE  
IS QUITE LOW AT THIS POINT IN TIME.  
 

 
 
   
AFC WATCHES/WARNINGS/ADVISORIES
 
 
PUBLIC...NONE.  
MARINE...GALE WARNING 130 138 139 160 180.  
FIRE WEATHER...NONE.  
 

 
 

 
 
SYNOPSIS AND MODEL DISCUSSION...SEB  
SOUTHCENTRAL ALASKA...AP  
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CB  
MARINE/LONG TERM...KM  
 
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