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FXAK68 PAFC 230533
AFDAFC
SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
833 PM AKST FRI NOV 22 2024
UPDATE TO THE AVIATION SECTION
PREV DISCUSSION
/ISSUED 518 PM AKST FRI NOV 22 2024/
SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 THROUGH
3: FRIDAY AFTERNOON THROUGH MONDAY AFTERNOON)...
NOT MUCH HAS CHANGED WITH THE FORECAST THROUGH EARLY SUNDAY
MORNING. DRY CONDITIONS CONTINUE AS A RIDGE LINGERS OVER SOUTHWEST
ALASKA. WARM AIR ALOFT STILL REMAINS OVER MUCH OF SOUTHCENTRAL,
HOWEVER THE WARMEST CORE OF AIR WILL BEGIN TO SHIFT SOUTHWARD
THROUGH SATURDAY AS THE RIDGE AXIS MOVES TOWARDS THE GULF. THE
STEEP TEMPERATURE INVERSION IN PLACE, ALONG WITH A TIGHTENED
PRESSURE GRADIENT OVER THE KENAI AND CHUGACH MOUNTAINS, IS STILL
PRODUCING GUSTY GAP WINDS ALONG THE GULF COAST, ESPECIALLY IN
WHITTIER AND SEWARD WHERE WIND GUSTS COULD STILL GET UP TO 40-50
MPH THROUGH SATURDAY AFTERNOON.
A LOW CURRENTLY OVER THE WESTERN BERING WILL SLOWLY WEAKEN AND
BEGIN TO MOVE THE BLOCKING RIDGE THROUGH THE WEEKEND. IN
RESPONSE, THE WEAKENING PRESSURE GRADIENT SHOULD LEAD TO
DIMINISHING GAP WINDS, AND TEMPERATURES WILL GRADUALLY MODERATE,
ESPECIALLY AS WEAKER WAVES BRING IN GREATER CLOUD COVER. WITH THE
INVERSION WEAKENING, WARMER TEMPERATURES ALOFT OVER THE MOUNTAINS
WILL BEGIN TO TREND DOWNWARD, FALLING TO NEAR OR JUST ABOVE
FREEZING SUNDAY AFTERNOON. ADDITIONALLY, INLAND VALLEYS COULD SEE
A SLIGHT WARM UP FROM BITTER COLD TEMPERATURES, THOUGH STAYING
BELOW FREEZING. HOW COLD OVERNIGHT LOW TEMPERATURES FALL SATURDAY
AND SUNDAY MORNING ACROSS INTERIOR SOUTHCENTRAL WILL BE HIGHLY
DEPENDENT ON THE AMOUNT OF CLOUD COVER SLIPPING OVER THE RIDGE
FROM THE NORTH AS THE RIDGE MOVES SOUTH. OVERALL, FORECAST
CONFIDENCE CONTINUES TO BE LOW WITH TEMPERATURES. WHILE THERE'S
MODERATE CONFIDENCE IN THE GENERAL TREND, THE EXACT NUMBERS HAVE
BEEN DIFFICULT FOR MODELS TO HONE IN ON AS TEMPERATURES HAVE
LARGELY BEEN DICTATED BY DIURNAL (RADIATIONAL WARMING AND COOLING)
RATHER THAN DYNAMIC (ADVECTION OF COOLER OR WARMER AIR MASSES)
EFFECTS, WHICH MODELS STRUGGLE WITH. AS SUCH, KEEP AN EYE MORE
TOWARDS TEMPERATURE TRENDS THAN EXACT NUMBERS.
WITH A WEAK SURFACE LOW FORMING IN THE GULF, DEVELOPING
SOUTHWESTERLY, UP-INLET FLOW AS EARLY AS SUNDAY MORNING COULD LEAD
TO DENSE FOG. CONFIDENCE IS LOW WITH WHEN OR WHERE DENSE FOG
MIGHT FORM, BUT THE INTRODUCTION OF MOIST AIR INTO AN OTHERWISE
DRY AND THERMALLY STABLE AIR MASS PROVIDES A GOOD WEATHER PATTERN
FOR FOG.
MOVING AHEAD TO SUNDAY EVENING, THE ENTRANCE OF A LOW INTO CENTRAL
AND INTERIOR ALASKA WILL BRING AN END TO OUR RECENT SPELL OF DRY
WEATHER. COLD TEMPERATURES OVER SOUTHCENTRAL SHOULD KEEP
PRECIPITATION AS ALL SNOW, WITH A HANDFUL OF INCHES EXPECTED
ACROSS MUCH OF THE AREA. SNOW AMOUNTS COULD BE 6 INCHES OR HIGHER
FOR SUSITNA VALLEY, WHICH COULD BEAR WATCHING. MODELS TEND TO HAVE
DIFFICULTY WITH HOW QUICKLY BLOCKING RIDGES ARE BROKEN DOWN AND
SHUNTED DOWNSTREAM, SO STAY TUNED AS THE SNOW FORECAST COULD
CHANGE.
-TM/BS/CHEN
SHORT TERM FORECAST SOUTHWEST ALASKA AND THE BERING
SEA/ALEUTIANS (DAYS 1 THROUGH 3: TODAY THROUGH MONDAY MORNING)...
THE CORE OF THE LARGE BLOCKING RIDGE WILL CONTINUE TO SLIDE SOUTH
INTO NORTH PACIFIC TODAY THROUGH SUNDAY. QUIET WEATHER WILL
CONTINUE TO BE EXPECTED ACROSS MAINLAND SOUTHWEST ALASKA THROUGH
LATE SATURDAY NIGHT. THE PATTERN WILL BEGIN TO CHANGE ON SATURDAY
AS UPPER- LEVEL FLOW TURNS MORE SOUTHWESTERLY AND COLDER OFF THE
BERING SEA. A SURFACE LOW PRESSURE OVER THE WESTERN AND CENTRAL
ALEUTIANS CURRENTLY PRODUCING RAIN AND GUSTY WINDS FROM SHEMYA TO
ATKA WILL ALSO HELP DRIVE THE PATTERN SHIFT.
A SOUTHWARD DIGGING TROUGH ACROSS THE WESTERN BERING WILL HELP TO
PULL PACIFIC MOISTURE NORTHWARD SATURDAY EVENING AND DELIVER A
ROUND OF STEADY RAIN TO THE PRIBILOF ISLANDS THROUGH SUNDAY
MORNING. SHOWERY PRECIPITATION MAKES IT TO THE KUSKOKWIM DELTA
COAST SATURDAY AFTERNOON AND TURNS MORE STEADY FOR SUNDAY.
PRECIPITATION, AT THIS TIME, IS EXPECTED TO INITIATE AS LIGHT
FREEZING RAIN LATE SATURDAY NIGHT/EARLY SUNDAY MORNING, THEN
TRANSITION QUICKLY TO MOSTLY PLAIN RAIN. THIS INITIAL FREEZING
RAIN OR DRIZZLE IS EXPECTED TO BE VERY LITTLE IF ANY AT ALL, WITH
ACCUMULATION TOTALS OF ONLY A TRACE TO A COUPLE HUNDREDTHS OF AN
INCH. BY MID-SUNDAY MORNING, PRECIPITATION SHOULD MOSTLY BE IN
THE FORM OF RAIN WITH TEMPERATURES HOVERING A LITTLE ABOVE
FREEZING.
STEADY PRECIPITATION IS LIKELY FROM THE EASTERN ALEUTIANS TO THE
KUSKOKWIM DELTA SUNDAY AFTERNOON INTO THE DAY ON MONDAY.
CONFIDENCE IN THE PROGRESSION OF THE FRONT AND MOISTURE SURGE ARE
RELATIVELY HIGH THROUGH SUNDAY MORNING WITH REGARDS TO THE
KUSKOKWIM DELTA. HOWEVER, CONFIDENCE DWINDLES TOWARDS SUNDAY
AFTERNOON INTO MONDAY WITH THE PROGRESSION AND TIMING OF THE FRONT
FURTHER INLAND WITH REGARDS TO PRECIPITATION ONSET TIME FOR
BRISTOL BAY AND THE LOWER KUSKOKWIM VALLEY. STAY TUNED FOR THE
LATEST FORECAST UPDATES ON THIS AS THE DRY PERIOD ACROSS SOUTHWEST
ALASKA LOOKS TO COME TO AN END THIS WEEKEND INTO EARLY NEXT WEEK.
-DN/CL
LONG TERM FORECAST (DAYS 4 THROUGH 7: TUESDAY THROUGH FRIDAY)...
BIG CHANGES ON THE UPPER LEVEL ALASKAN WEATHER MAP THROUGH THIS
FORECAST PERIOD. A MODERATELY STRONG SHORTWAVE STALLS OVER
INTERIOR ALASKA THROUGH THE WEEK. THIS EFFECTIVELY ANCHORS COLDER
AIR FURTHER SOUTH OVER THE MAINLAND THROUGH FRIDAY. THE UPPER
LEVEL RIDGE ACROSS THE SOUTHERN GULF OF ALASKA RETREATS INTO THE
EASTERN NORTH PACIFIC, AND IN ITS WAKE, A DEVELOPING LOW CENTER
MIGRATES FROM THE NORTH PACIFIC. THIS LOW MOVES TO THE SOUTHERN
ALASKA PENINSULA AND LINKS UP WITH THE MAINLAND TROUGH FOR THE END
OF THE WEEK. AN ECMWF / GFS / UKMET ENSEMBLE HOLDS THE INDIVIDUAL
STRENGTHS AND TRACKS PRETTY WELL. CANADIAN MODEL WAS A BIT OF AN
OUTLIER. IN THE WEST, AN EASTERN SIBERIAN RIDGE BUILD OVER THE
BERING, COMPLETING THE REVERSAL OF THE PREVIOUS WEATHER PATTERN.
A SURFACE FRONT AHEAD OF THE SHORTWAVE TROUGH PRESSES ACROSS
INTERIOR ALASKA THROUGH WEDNESDAY WITH AREAS OF SNOW ALMOST ALL
THE WAY TO THE SOUTHCENTRAL COAST. COASTAL ZONES EAST OF PRINCE
WILLIAM SOUND MAY SEE A MIX OF RAIN AND SNOW FOR WEDNESDAY. THE
APPROACHING NORTH PACIFIC LOW BRINGS GALE FORCE WINDS TO THE
BERING, AND MODERATE RAIN TO THE WESTERN AND CENTRAL ALEUTIANS
TUESDAY. THE WINDS DIMINISH OVER THE BERING FOR THURSDAY, BUT
DEVELOPS FURTHER EASTWARD, SPREADING SNOW MIXED WITH OR CHANGING
TO RAIN OVER THE EASTERN ALEUTIANS AND ALASKA PENINSULA TO KODIAK
ISLAND FOR LATE WEDNESDAY. ON THURSDAY AND FRIDAY, THE AREA OF
MIXED PRECIPITATION WILL SPREAD ALONG THE SOUTHCENTRAL COAST.
AVIATION...
PANC...VFR CONDITIONS AND LIGHT WINDS IS EXPECTED. HOWEVER, FOG
HAS BEEN REPORTED IN THE VICINITY AND HAS THE POTENTIAL TO MOVE
OVER THE TERMINAL. CONFIDENCE IS LOW FOR ITS OCCURRANCE AND TIMING.
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