Small Craft Advisory 753, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
403 PM AKDT SAT SEP 26 2015

.ANALYSIS AND UPPER LEVELS...
A PROGRESSIVE UPPER LEVEL PATTERN IS SITUATED OVER THE MAINLAND AND
EASTER BERING THIS AFTERNOON WITH A JET CORE OF 110KT EXTENDING FROM
THE BERING STRAIT TO BRITISH COLUMBIA. ALONG THIS BOUNDARY A LOT OF
LIGHT PRECIPITATION WAS PREVALENT THIS MORNING ACROSS SOUTHCENTRAL.
WITH ONSHORE FLOW ACROSS SOUTHWEST ALASKA...PRECIPITATION REMAINS
PREVALENT. IN THE ZONAL FLOW...AREAS OF PRECIPITATION ARE
PROGRESSING ACROSS SOUTHWEST ALASKA AND INTO SOUTHCENTRAL TODAY
WITH ANOTHER PUSHING ONSHORE NOW. WHERE IT IS NOT RAINING...A LOT
OF LOW STRATUS REMAINS THIS AFTERNOON. A MORE ORGANIZED LOW IS
PRESENT OVER THE WESTERN ALEUTIANS/BERING SEA THIS AFTERNOON WITH
PRECIPITATION WRAPPING AROUND THE CENTRAL AND WESTERN BERING SEA.

&&

.MODEL DISCUSSION...
NUMERICAL GUIDANCE IS IN VERY GOOD AGREEMENT SYNOPTICALLY THROUGH
MONDAY. THERE ARE SLIGHT DIFFERENCES THAT START TO SHOW UP
TUESDAY. AT UPPER LEVELS...ZONAL FLOW OVER THE MAINLAND AND A LOW
AMPLITUDE TROUGH ARE CURRENTLY PRESENT OVER THE AREA. THE TROUGH
OVER THE BERING IS PROGRESSIVE AND WILL MOVE TO THE EASTERN BERING
SUNDAY WITH A POLAR LOW DROPPING INTO THE NORTHERN CHUKCHI SEA.
THIS IN TURN PUSHES A LOW AMPLITUDE RIDGE ACROSS THE MAINLAND.
HOWEVER...ENOUGH MOISTURE AND AN ADVANCING VORT MAX ON THE BACK
SIDE OF THE TROUGH THAT RAIN WILL REMAIN OVER THE AREA SUNDAY. IN
FACT...AS IT SWINGS THROUGH SOUTHWEST ALASKA AND THE REMAINDER OF
THE MAINLAND SUNDAY AND SUNDAY NIGHT...RAINFALL WILL BE HEAVY AT
TIMES. IN THE MEANTIME...A TROUGH DIGS DEEP OVER THE EASTERN
BERING WITH LOTS OF COLD AIR...CHANGING THE AIR MASS DRASTICALLY
FOR SOUTHWEST AND SOUTHCENTRAL ALASKA MONDAY AND TUESDAY. AS THE
BASE OF THE TROUGH SWINGS TO THE EAST INTO THE GULF OF ALASKA A
CLOSED LOW DEVELOPS IN THE COOK INLET REGION/NORTHERN GULF OF
ALASKA. THIS IS WHERE WE START TO SEE THE DIFFERENCES IN THE
NUMERICAL GUIDANCE. THE NAM IS A BIT FASTER IN TIMING OF THE
CLOSED LOW INTO THE VICINITY WHEREAS THE CANADIAN IS THE LAST. THE
EC DOES NOT HOLD ONTO THE CLOSED LOW FEATURE AS LONG AS THE GFS
INTO WEDNESDAY. EITHER WAY...IT LOOKS LIKE SNOW IS POSSIBLE FOR
TUESDAY ACROSS SOUTHCENTRAL ALASKA.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 AND 3)...
ZONAL FLOW ALOFT IS PROVIDING A FEW UPPER LEVEL DISTURBANCES
ACROSS THE AREA. WITH PLENTIFUL MOISTURE IN THE AIR
MASS...RAINFALL WILL KEEP IN PLACE THROUGH TONIGHT WITH AN
OVERRUNNING PRECIP SCENARIO. PRECIP WILL LIKELY BE LESS THAN HALF
AN INCH THROUGH TONIGHT. A LARGER...MORE ORGANIZED SYSTEM WITH A
SURFACE LOW REFLECTION WILL BE PUSHING INTO WESTERN ALASKA SUNDAY
AFTERNOON AND EVENING WHICH IS CURRENTLY LOCATED NEAR ADAK.
IT IS A FAST MOVING SYSTEM BUT FLOW WILL BE OUT OF THE SOUTHWEST
WITH SOME UPPER LEVEL JET SUPPORT FOR HEAVIER RAINFALL POSSIBLE
SUNDAY NIGHT. IN ADDITION...SURFACE GRADIENTS SET UP FOR STRONG
GAP WINDS ESPECIALLY THROUGH TURNAGAIN ARM. THIS GRADIENT DOES NOT
SUBSIDE UNTIL THE LOW PASSES TO THE EAST SUNDAY NIGHT. AS THE
ASSOCIATED SURFACE LOW CROSSES THE MAINLAND MONDAY...FLOW TURNS
MORE SOUTHERLY AS THE TROUGH DIGS ACROSS THE EASTERN BERING. A
SHORT WAVE TROUGH IN THE SOUTHWEST FLOW WILL BRING MORE PRECIP TO
THE AREA MONDAY AND MONDAY NIGHT TO SOUTHCENTRAL. THE EASTERN
BERING TROUGH STARTS TO SWING INLAND MONDAY NIGHT AND DEVELOPS A
CLOSED LOW OVER THE COOK INLET REGION TUESDAY MORNING AND SWINGS
TO THE PRINCE WILLIAM SOUND TUESDAY AFTERNOON. COLD AIR ARRIVES
WITH THIS AND BRINGS A GOOD CHANCE OF SNOW TO THE REGION AND
UNSEASONABLE COLD AIR. THE CHANCE FOR SNOW REMAINS THROUGH TUESDAY
WITH A POSSIBLE DEFORMATION ZONE SETTING UP OVER SOUTHCENTRAL.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)...
STEADY RAIN WILL CONTINUE OVER SOUTHWEST ALASKA THROUGH SUNDAY
NIGHT AS THE OCCLUDED FRONT PUSHES ON SHORE TONIGHT AND A SECOND
FRONT APPROACHES THE REGION MONDAY. THE FLOW ALOFT WILL REMAIN
SOUTHWESTERLY THROUGH MONDAY WHICH WILL CONTINUE TO PUMP A VERY
MOIST AIR MASS INTO THE AREA. SATELLITE IMAGERY THIS AFTERNOON
INDICATES AN EXTREMELY MOIST AIR MASS OVER MOST OF SOUTHWESTERN
ALASKA AND THE BERING SEA WITH PW VALUES IN EXCESS OF 200 PERCENT
OF NORMAL FOR THIS TIME OF YEAR. WITH THIS MOIST OF AN AIR MASS AND
THE STRONG SOUTHWESTERLY TRANSPORT OF MOISTURE...RAINFALL MAY BE
HEAVY AT TIMES ON SUNDAY ALONG PORTIONS OF THE BRISTOL BAY REGION
AND THE ALASKA RANGE.

AS THE UPPER TROUGH APPROACHES THE AREA ON MONDAY...THE FLOW WILL
BECOME NORTHWESTERLY BEHIND THE TROUGH BRINGING WITH IT A DECENT
AMOUNT OF COLD AIR ALOFT WITH TEMPERATURES AT 850MB DROPPING INTO
THE -6C TO -8C RANGE AND 925MB TEMPERATURES DOWN TO ABOUT -3C.
THIS WILL BE COLD ENOUGH TO ALLOW FOR PRECIPITATION TO SWITCH OVER
TO A RAIN/SNOW MIX BY MONDAY NIGHT...MAINLY OVER THE KUSKOKWIM
DELTA AND AND THE LOWER KUSKOKWIM VALLEY.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...
WINDS AND SEAS WILL REMAIN ELEVATED OVER THE BERING SEA AND THE
ALEUTIANS THROUGH THE PERIOD AS THE LOW MOVES INLAND NEAR NUNIVAK
ISLAND SUNDAY EVENING AND THE UPPER TROUGH SWINGS THROUGH BY
MONDAY NIGHT. WINDS ALONG THE BACK SIDE OF THE LOW WILL REMAIN
NEAR OR AT MINIMAL GALE FORCE THROUGH MONDAY NIGHT AS THE UPPER
TROUGH SWINGS THROUGH THE REGION. EXPECT WIDESPREAD RAIN ALONG
THE FRONT WHICH WILL MIX WITH SNOW ALONG THE NORTHERN PORTIONS OF
THE REGION AS COLD AIR ENTERS THE AREA WITH THE NORTHWESTERLY FLOW
ALOFT.

&&

.LONG TERM FORECAST (DAYS 4 THROUGH 7)...

BEGINNING THE EXTENDED PERIOD TUESDAY EVENING...THE COLD UPPER
LEVEL LOW WILL BE OVER SOUTHCENTRAL ALASKA. THIS DYNAMIC LOW WILL
BE CONTINUING 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 BEFORE 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 RAIN AND CLOUDS
TO SOUTHERN ALASKA. UNCERTAINTY DEVELOPS WITH THE SHORTWAVE
PATTERN BY NEXT WEEKEND...BUT THE FLOW LOOKS TO REMAIN PROGRESSIVE
WITH CHANCES FOR PRECIPITATION LASTING THROUGH THE WEEKEND.
TEMPERATURES WILL MODERATE BACK TO NEAR NORMAL AFTER THURSDAY.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...GALE WARNING 178 179 185.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...JN
SOUTHCENTRAL ALASKA...JN
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...RF
LONG TERM...JA