Small Craft Advisory 755, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
458 AM AKDT SUN SEP 27 2015

.ANALYSIS AND UPPER LEVELS...
A PROGRESSIVE AND MOISTURE RICH ZONAL FLOW PATTERN IS KEEPING
LIGHT RAIN AND WIDESPREAD LOW CLOUDS OVER THE BULK OF THE SOUTHERN
MAINLAND THIS MORNING AS FLOW BEGINS TO SHIFT FROM THE WEST TO
THE SOUTHWEST AHEAD OF A LOW AMPLITUDE UPPER TROUGH LIFTING
NORTHEASTWARD ACROSS THE BERING SEA. AN ASSOCIATED SURFACE LOW IS
BEGINNING TO SPREAD HEAVIER RAIN OVER THE KUSKOKWIM DELTA AND
OTHER PORTIONS OF SOUTHWEST ALASKA AS IT TRACKS TOWARDS NUNIVAK
ISLAND...WITH SOUTHEASTERLY GAP WINDS BEGINNING TO INCREASE ACROSS
FAVORED PORTIONS OF SOUTHCENTRAL AK IN RESPONSE TO THE TIGHTENING
PRESSURE GRADIENT.

&&

.MODEL DISCUSSION...
GUIDANCE REMAINS IN STRONG SYNOPTIC AGREEMENT IN THE SHORT TERM AS
THE CURRENT MOIST ZONAL FLOW PATTERN GIVES WAY TO A HIGHLY
AMPLIFIED TROUGH DIVING OUT OF EASTERN RUSSIA EARLY IN THE WEEK.
THE LOW AMPLITUDE BERING TROUGH AND ASSOCIATED SURFACE LOW WILL
CONTINUE TO SUPPORT WIDESPREAD RAIN AND CLOUDS THROUGH TONIGHT AS
THEY PUSH INTO THE INTERIOR AND YIELD SOUTHWEST FLOW OVER THE
SOUTHERN MAINLAND. ATTENTION THEN TURNS TO NEXT TROUGH AS IT
SWINGS ACROSS THE BERING AND DEEPENS OVER WESTERN ALASKA ON MONDAY
BEFORE TAKING ON A NEGATIVE TILT AS IT PUSHES THROUGH SOUTHCENTRAL
AK ON TUESDAY. THIS WILL ALLOW UNSEASONABLY COLD AIR TO FILTER
INTO THE REGION AS A SURFACE LOW FORMING IN THE VICINITY OF
PRINCE WILLIAM SOUND PROVIDES A SOURCE OF MOISTURE. THE STRONG
OVERALL AGREEMENT IN THIS SCENARIO LENDS INCREASING CONFIDENCE
THAT INLAND PORTIONS OF SOUTHCENTRAL AK WILL SEE SNOWFALL AT SEA
LEVEL MONDAY NIGHT INTO TUESDAY. HOWEVER...THE SPECIFIC DETAILS OF
THE FORECAST REMAIN UNRESOLVED AT THIS TIME GIVEN MODEST
DIFFERENCES IN TIMING AND INTENSITY OF THESE FEATURES.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 AND 2)...
THE DUE WEST UPPER LEVEL FLOW HAS CAUSED SLOW BUT STEADY WARMING
OF THE TEMPERATURES ALOFT OVER THE PAST DAY RESULTING IN A
WIDESPREAD LOW CLOUDS AND LIGHT RAIN OVER MUCH OF SOUTHCENTRAL
ALASKA. AS THIS FLOW ALOFT BECOMES SOUTHWEST TODAY AND THE LOWER
LEVEL WIND BECOME GENERALLY SOUTHEAST IT WILL INCREASE THE
RAINFALL RATES AND ALSO LIFT THE CLOUD CEILINGS SOME AS WELL. THE
SOUTHWEST FLOW ALOFT USHERING IN LARGE AMOUNTS OF MOISTURE AND
LIFT WILL CONTINUE THROUGH MONDAY. THERE MAY BE A BRIEF BREAK
MONDAY NIGHT OVER THE COOK INLET REGION BEFORE THE COLD FRONT
MOVES INTO THE AREA TUESDAY.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)...
STEADY RAIN WILL OVER SOUTHWEST THIS MORNING AS THE OCCLUDED
FRONT MOVES THROUGH THE AREA. THE HEAVIEST RAIN WILL LIKELY OCCUR
ALONG THE FRONT FROM THE EASTERN KUSKOKWIM DELTA TO THE LOWER
KUSKOKWIM VALLEY EARLY THIS MORNING...AND AGAIN LATER THIS MORNING
THROUGHOUT THE AREA AS THE RAIN IS REINVIGORATED FROM A SHORTWAVE
MOVING OVER THE SOUTHWEST COAST FROM THE BERING. THE LOW WILL THEN
MAKE LANDFALL OVER THE KUSKOKWIM DELTA THIS AFTERNOON...KEEPING
RAIN IN THE AREA INTO THIS EVENING.

BY LATE THIS EVENING...A COLD FRONT DRAPED BEHIND THE LOW WILL
CAUSE THE WINDS TO SHIFT TO NORTHWESTERLY AND DRAG COLD SIBERIAN
AIR BEHIND IT. 850 MB TEMPERATURES WILL PLUMMET TO AROUND 8C BY
EARLY MONDAY MORNING NEAR BETHEL...MORE THAN COLD ENOUGH TO CAUSE
RAIN SHOWERS IN THE AREA TO SWITCH OVER TO SNOW. THIS COLD AIR
WILL THEN RAPIDLY SPREAD EASTWARD INTO THE KUSKOKWIM
DELTA...CAUSING RAIN TO MIX OR SWITCH OVER TO SNOW. HOWEVER WITH
THE WARM AND MOIST AIRMASS CURRENTLY OVER THE AREA...SURFACE
TEMPERATURES ARE NOT EXPECTED TO FALL ENOUGH TO ALLOW FOR
SIGNIFICANT ACCUMULATIONS...BUT LOCALIZED AREAS OF AROUND AN INCH
WOULD CERTAINLY BE POSSIBLE. AN UPPER LEVEL TROUGH WILL THEN
APPROACH THE WEST COAST ON MONDAY AFTERNOON...KEEPING RAIN AND
SNOW SHOWERS IN THE AREA THROUGH MONDAY NIGHT.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...
RAIN AND SOUTHWESTERLY WINDS WILL CONTINUE TODAY FOR MUCH OF THE
EASTERN BERING AND ALEUTIANS AS THE LOW OVER THE NORTHERN BERING
PROGRESSES TOWARDS THE COAST. BEHIND THIS LOW...A COLD FRONT WILL
CAUSE WINDS THROUGHOUT THE AREA TO SHIFT TO NORTHWESTERLY BY LATE
TONIGHT...SETTING UP A SHOWERY COLD AIR ADVECTION REGIME THAT WILL
CONTINUE THROUGH MONDAY. THERE WILL ALSO BE ENOUGH COLD AIR BEHIND
THE FRONT TO CAUSE RAIN TO MIX WITH SNOW FROM THE PRIBILOFS INTO
THE AKPEN...HOWEVER WITH THE WARM SEA SURFACE TEMPERATURES THIS IS
NOT EXPECTED TO ACCUMULATE. AN UPPER LEVEL TROUGH DIVING INTO THE
BERING FROM SIBERIA WILL REINVIGORATE RAIN AND SNOW SHOWERS AS IT
PASSES OVER. THIS TROUGH WILL ALSO BRING INCREASED NORTHWESTERLY
WINDS TO THE AKPENN ON MONDAY NIGHT AS IT PASSES OVERHEAD...BUT
WITHOUT GOOD JET SUPPORT OR PRESSURE RISES THESE ARE NOT EXPECTED
TO REACH STRONG WIND CRITERIA.

&&

.LONG TERM FORECAST (DAYS 3 THROUGH 7)...
AN ARCTIC TROUGH OVER THE SOUTHWEST MAINLAND MONDAY NIGHT WILL
BEGIN TO SHIFT EASTWARD OVER SOUTHCENTRAL ALASKA BY TUESDAY...
WHILE A COLD UPPER LOW BEGINS TO TAKE SHAPE OVER THE NORTHERN
GULF. THIS DYNAMIC LOW WILL CONTINUE TO SUPPORT AREAS OF SNOW AND
SNOW SHOWERS INLAND...ESPECIALLY ALONG THE CHUGACH MOUNTAINS INTO
THE COPPER RIVER BASIN. THIS SYSTEM WILL RAPIDLY DEPART
WEDNESDAY...LEAVING BEHIND COLD AIR AND MOUNTAIN SNOW
SHOWERS...ALTHOUGH LOWER ELEVATION SNOW SHOWERS WILL REMAIN ON
WEDNESDAY ACROSS THE COPPER BASIN. A BRIEF REPRIEVE IN WEATHER
WILL LAST ACROSS SOUTHERN ALASKA FROM WEDNESDAY NIGHT THROUGH
THURSDAY AFTERNOON...AS RIDGING OVER THE BERING SEA AND ALEUTIAN
CHAIN SHIFTS EASTWARD OVER THE SOUTHERN MAINLAND BY THURSDAY
MORNING. ANOTHER TROUGH OVER THE FAR WESTERN BERING WILL BEGIN
MOVING A STRONG FRONT ACROSS THE BERING AND INTO SOUTHERN ALASKA
THURSDAY EVENING THROUGH FRIDAY...BRINGING ANOTHER ROUND OF RAIN
AND CLOUDS TO SOUTHERN ALASKA.

MODELS BEGIN TO STRUGGLE MUCH MORE WITH THE OVERALL SHORTWAVE
PATTERN BY THE TRANSITION INTO NEXT WEEKEND. THEY ARE HAVING A
HARD TIME WITH THE TIMING AND STRENGTH OF ANOTHER SYSTEM NEAR THE
SOUTHERN KAMCHATKA PENINSULA...AND ITS ASSOCIATED FRONT TRACKING
INTO THE WESTERN ALEUTIANS/BERING BY THE END OF THE WEEK/EARLY
WEEKEND. HOWEVER...THE FLOW LOOKS TO REMAIN PROGRESSIVE WITH
CHANCES FOR PRECIPITATION LASTING THROUGH THE WEEKEND....AND
TEMPERATURES MODERATING BACK TO NEAR NORMAL BY THE END OF THE
WEEK.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...GALE WARNING 179 180 181 185.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...CB
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...DEK
LONG TERM...TP/JA