029  
FXAK68 PAFC 201427  
AFDAFC  
 
SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE ANCHORAGE AK  
527 AM AKST THU FEB 20 2020  
   
ANALYSIS AND UPPER LEVELS
 
 
A VERTICALLY STACKED LOW OVER THE BRISTOL BAY AREA IS LIFTING TO  
THE NORTHEAST AND MOVING INLAND. NORTHERLY FLOW OVER THE WESTERN  
BERING IS PULLING DOWN COLD ARCTIC AIR WHICH IS WRAPPING AROUND  
THE BACKSIDE OF THE LOW. THE ASSOCIATED FRONT OVER THE GULF IS  
LIFTING INLAND OVER SOUTHCENTRAL. WHILE AREAS ALONG THE NORTHERN  
GULF COAST HAVE SEEN ADDITIONAL SNOWFALL, MIDLEVEL  
SOUTHEASTERLY WINDS HAVE LARGELY KEPT THE ANCHORAGE BOWL DRY WITH  
DOWNSLOPING OCCURRING OVERNIGHT.  
 

 
   
MODEL DISCUSSION
 
 
MODEL SOLUTIONS CONTINUE TO REMAIN IN GOOD AGREEMENT WITH SYNOPTIC  
FEATURES THROUGH FRIDAY NIGHT. THE BIGGEST FORECAST CHALLENGE WILL  
BE FIGURING OUT DETAILS INCLUDING PRECIPITATION TYPES AND THE TIMING  
OF ARRIVAL OF COLDER AIR OVER THE SOUTHERN MAINLAND THURSDAY  
NIGHT INTO FRIDAY. BY FRIDAY NIGHT, MODELS SHOW A LOW DEVELOPING  
SOUTH OF THE WESTERN ALEUTIANS AND TRACKING EASTWARD FOR SATURDAY,  
THOUGH THERE IS VAST DISAGREEMENT ON THE STRENGTH AND TRACK OF  
THIS LOW. OVERALL, FORECAST CONFIDENCE IS NEAR NORMAL.  
 

 
   
AVIATION
 
 
PANC...VFR CONDITIONS AND LIGHT WINDS WILL PERSIST THROUGH  
THE MORNING. SOUTHEASTERLY WINDS ALOFT WILL PRODUCE WIND SHEAR  
THIS MORNING BUT THESE MIDLEVEL WINDS ARE EXPECTED TO TAPER BY MIDDAY.  
A FRONT LIFTING INLAND OVER SOUTHCENTRAL WILL BRING MVFR  
CONDITIONS IN SNOW THIS AFTERNOON THROUGH MIDDAY FRIDAY.  
 

 
   
SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 AND 2)
 
 
(TODAY THROUGH FRIDAY EVENING)  
 
NOT TOO MUCH HAS CHANGED FROM THE PREVIOUS FORECAST PACKAGE, WITH  
JUST A FEW MINOR CHANGES MADE THIS MORNING. THE FIRST CHANGE WAS  
TO DROP THE HIGH WIND WARNING FOR THE TURNAGAIN ARM AND UPPER  
HILLSIDE AREAS. MCHUGH CREEK DID SEE PEAK GUSTS OF 77 MPH, WITH  
THE UPPER HILLSIDE HAVING TOPPED OUT IN THE MID 60 MPH RANGE. THE  
PRESSURE GRADIENT WILL CONTINUE TO WEAKEN OVER THE NEXT FEW HOURS,  
WITH SEVERAL OBSERVATION SITES IN THE PREVIOUSLY WARNED AREA  
SHOWING A CONTINUED DOWNWARD TREND IN THE WIND SPEEDS.  
 
OUR ATTENTION NOW TURNS TO GETTING A BETTER ESTIMATE FOR THE  
ANCHORAGE BOWL AND WESTERN KENAI PENINSULA REGARDING SNOWFALL  
TIMING OF INCREASED COVERAGE AND INTENSITY. EARLY THIS MORNING,  
THE KENAI DOPPLER RADAR SHOWS A WELL PRONOUNCED BOUNDARY  
STRETCHING FROM NEAR CLAM GULCH, TO NEAR SOLDOTNA, TO EAST OF  
KENAI. THIS IS LIKELY AN ARTIFACT OF THE DOWN INLET FLOW  
PRODUCING A SURFACE PRESSURE TROUGH/CONVERGENCE AXIS, WHICH HAS  
ALLOWED SOME LOCALIZED HEAVIER BURSTS OF SNOW WITH SOME  
INTERMITTENT VISIBILITY REDUCTIONS. HOWEVER, THE LATEST GUIDANCE  
CONTINUES TO SHOW THE "PRECIPITATION SHADOW" IN THE LEE OF THE  
CHUGACH MOUNTAINS CONTINUING FOR A FEW MORE HOURS.  
 
HOWEVER, WITH THE WINDS HAVING DECREASED EARLIER THAN ORIGINALLY  
THOUGHT, ONE MAY THINK A QUICKER ONSET OF SNOWFALL WILL COMMENCE  
AS THE WINDS ALOFT ALSO WEAKEN. THAT IS IN FACT WHAT THE LATEST  
HIGH RESOLUTION MODELS NOW SHOW, WITH SOME OF THE LONGER RANGE  
MODELS ALSO CONVERGING ON THIS TREND. THIS IS IN PART DUE TO A  
SERIES OF DISTURBANCES IN THE SOUTHWEST FLOW ALOFT MOVING IN  
FASTER, WITH THE FIRST SUCH DISTURBANCE LOCATED OVER KODIAK ISLAND  
NOW. WITH WINDS ALOFT WEAKENING AND THE DISTURBANCE PROVIDING AN  
EARLIER ONSET OF ASCENT/LIFT ABOVE THE SURFACE, THE RADAR IMAGERY  
SHOULD BE INCREASING WITH PRECIPITATION COVERAGE BY LATE MORNING.  
AS SUCH, PRECIPITATION CHANCES WERE INCREASED TO CATEGORICAL  
(ABOVE 80%) FOR THE ANCHORAGE BOWL/WESTERN KENAI NORTHWARD INTO  
THE MATANUSKA VALLEY.  
 
FOR THE REMAINDER OF THE AREA, THE FORECAST REMAINS ON TRACK WITH  
THE HIGHEST SNOW TOTALS EXPECTED OVER THE NORTHERN AND WESTERN  
SUSITNA VALLEY (ROUGHLY FROM SKWENTNA WEST AND BROAD PASS NORTH),  
WHERE AROUND 6 INCHES OF ACCUMULATION IS ANTICIPATED. THIS IS JUST  
BELOW ADVISORY CRITERIA, AND WILL CONTINUE TO BE MONITORED. FOR  
THOMPSON PASS, WE STILL EXPECT PERHAPS A BRIEF REDUCTION IN  
VISIBILITIES TO NEAR 1/2 MILE, BUT THIS SHOULD BE LIMITED  
SUFFICIENTLY ENOUGH IN DURATION TO PRECLUDE ANY HEADLINE ISSUANCE.  
FINALLY, FOR THE WESTERN KENAI PENINSULA THROUGH THE MATANUSKA  
VALLEY, SOME WARM AIR MAY STILL ALLOW FOR A BRIEF MIX, AS FORECAST  
SOUNDINGS DO SHOW A SHALLOW/NARROW AREA OF ABOVE FREEZING  
TEMPERATURES. THAT SAID, THE PREVIOUSLY MENTIONED INCREASING  
ASCENT SHOULD COMPENSATE ENOUGH FOR WET BULB-COOLING EFFECTS TO  
KEEP THAT LAYER ISOTHERMAL WITH BETTER SNOWFLAKE PRODUCTION, WITH  
2 TO 4 INCHES OF SNOWFALL EXPECTED.  
 

 
   
SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)
 
 
THE WINTER WEATHER ADVISORY FOR BRISTOL BAY HAS EXPIRED THIS  
MORNING AS THE HEAVIEST SNOWFALL HAS PASSED, ALLOWING LOW  
VISIBILITY TO IMPROVE OVER THE REGION IN LINGERING SNOW SHOWERS.  
LATER THIS AFTERNOON, ANOTHER BAND OF SNOWFALL SETS UP ACROSS THE  
ALASKA RANGE. THIS COULD BRING ANOTHER ROUND OF LIGHT SNOWFALL  
ACCUMULATIONS OVER INTERIOR BRISTOL BAY ON THE BACKSIDE OF THE LOW  
THAT HAS DEPARTED INTO SOUTHCENTRAL. A SECOND BAND WILL ALSO SET  
UP OVER THE KUSKOKWIM MOUNTAINS THIS AFTERNOON, GIVING AREAS FROM  
ANIAK TO BETHEL A BRIEF PERIOD OF LIGHT SNOWFALL THROUGH LATE  
TONIGHT.  
 
AS THE ACTIVE PATTERN MOVES INTO SOUTHCENTRAL, SNOW SHOWERS BEGIN  
TO TAPER OFF WITH DRY CONDITIONS AND COLDER TEMPERATURES MOVING  
BACK INTO SOUTHWEST ALASKA FOR THE WEEKEND. WIND CHILLS OF 40  
BELOW ZERO FOR THE KUSKOKWIM DELTA COAST TONIGHT INTO FRIDAY HAS  
LED TO THE ISSUANCE OF A WIND CHILL ADVISORY AS GUSTY WINDS MOVE  
BACK INTO THE AREA. THE MAIN AREAS AFFECTED WILL BE ALONG THE  
COAST FROM HOOPER BAY TO TOKSOOK BAY, INCLUDING MEKORYUK.  
 

 
   
SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)
 
 
A LOW OVER BRISTOL BAY THIS MORNING WILL CONTINUE ITS PATH  
EASTWARD OUT OF THE REGION EARLY TODAY. ON THE BACKSIDE OF THIS  
LOW, EXPECT STRONG WINDS AND GUSTS UNTIL LATE TONIGHT ALONG THE  
EASTERN ALEUTIANS AND ALASKA PENINSULA, ESPECIALLY THROUGH THE  
GAPS, AS COLDER AIR MOVES IN ACROSS THE BERING SEA. THE GUSTY  
WINDS AND WIDESPREAD SNOW SHOWERS COULD CREATE BLOWING SNOW  
CONDITIONS ALONG THESE AREAS, BUT SHOULD NOT HAVE ANY SIGNIFICANT  
VISIBILITY REDUCTIONS.  
 
OVER THE WESTERN BERING, QUIET CONDITIONS PERSISTS THROUGH FRIDAY  
MORNING WHEN A SMALL LOW TRIES TO SPIN UP OVER THE CENTRAL BERING.  
MODELS ARE STILL STRUGGLING ON THE STRENGTH AND PLACEMENT OF THIS  
FEATURE AND WHETHER A SECOND ONE TRIES TO DEVELOP SHORTLY AFTER.  
THE NEXT SIGNIFICANT SYSTEM MOVES OVER THE WESTERN AND CENTRAL  
ALEUTIANS ON SATURDAY. STAY TUNED FOR ADDITIONAL FORECAST UPDATES.  
 

 
   
MARINE (DAYS 3 THROUGH 5 SAT THROUGH MON)
 
 
WESTERLY GALE FORCE WINDS ARE EXPECTED ON SATURDAY IN THE GULF,  
WHICH WILL WEAKEN ON SUNDAY. A LOW WILL MOVE FROM THE WESTERN AND  
CENTRAL ALEUTIANS ON SATURDAY EASTWARD ACROSS THE EASTERN  
ALEUTIANS SUNDAY AND INTO THE GULF ON MONDAY. GALE FORCE WINDS ARE  
EXPECTED WITH THIS LOW. CONFIDENCE ON THE TIMING OF THIS LOW IS  
FAIRLY HIGH.  
 

 
   
LONG TERM FORECAST (DAYS 4 THROUGH 7 SUN THROUGH WED)
 
 
ON SUNDAY MORNING THE PATTERN WILL CONSIST OF AN UPPER LEVEL  
TROUGH OVER SOUTHCENTRAL ALASKA EXTENDING SOUTH INTO THE GULF OF  
ALASKA, AN UPPER LEVEL RIDGE OVER THE CENTRAL AND EASTERN BERING  
SEA, AND AN UPPER LOW OVER THE ALEUTIANS. THE TROUGH OVER  
SOUTHCENTRAL WILL SLOWLY SLIDE EAST, MOVING INTO CANADA SUNDAY  
NIGHT. THE BERING RIDGE WILL THEN BUILD OVER SOUTHERN ALASKA  
SUNDAY NIGHT, SLIDING SLOWLY EASTWARD THROUGH WEDNESDAY WHEN THE  
AXIS SHOULD BE IN NORTHWEST CANADA. THE ALEUTIAN LOW WILL MOVE  
EAST AND A BIT NORTH THROUGH WEDNESDAY, UNDERCUTTING THE RIDGE  
OVER SOUTHERN ALASKA.  
 

 
 
   
AFC WATCHES/WARNINGS/ADVISORIES
 
 
PUBLIC...WIND CHILL ADVISORY 155.  
MARINE...STORMS 119 125.  
GALES 128 132 150 155 160 352.  
HEAVY FREEZING SPRAY 150 160 165 179 180 185 411 412 414.  
FIRE WEATHER...NONE.  
 

 
 

 
 
SYNOPSIS AND MODEL DISCUSSION...KO  
SOUTHCENTRAL ALASKA...PD  
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AH  
MARINE/LONG TERM...BL  
 
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