Small Craft Advisory 420, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
252 PM AKDT TUE MAY 27 2014

.ANALYSIS AND UPPER LEVELS...

THE AMPLIFICATION OF THE UPPER AIR PATTERN IS COMMENCING THIS
AFTERNOON...WITH A DEEP POSITIVE TILT LONGWAVE TROUGH EXTENDING
FROM THE NORTH CENTRAL PACIFIC TO AN ARCTIC VORTEX OVER THE
BEAUFORT SEA. A SOUTHWEST FLOW 120 KT JET STREAK EXISTS FROM THE
DATELINE AND 35N TO THE SOUTHWEST GULF OF ALASKA...WITH NUMEROUS
LOW AMPLITUDE WAVE DISTURBANCES PROPAGATING ALONG IT. THE FIRST OF
THESE WEAKENING AND SHEARING WAVES IS TRACKING INTO SOUTHERN
ALASKA AND THE GULF OF ALASKA...WITH A TRAILING WAVE JUST SOUTH OF
DUTCH HARBOR. THIS FIRST WAVE/FRONT IS SUPPORTING PERIODS OF LIGHT
RAIN ACROSS ALL OF SOUTHERN ALASKA WITH THE EXCEPTION OF THE
COPPER BASIN. THE CLOUDY AND COOLER PATTERN WILL PERSIST THROUGH
THE REMAINDER OF THE WEEK AS THE TROUGH SHIFTS EAST AND
STRENGTHENS. BY LATE THURSDAY AND FRIDAY...A MORE SUBSTANTIAL
NORTH PACIFIC LOW WILL TRACK INTO THE GULF...BRINGING WIDESPREAD
RAIN AND WINDS TO SOUTHCENTRAL AND THE GULF. SOME DISCREPANCY
REMAINS WITH THE EXACT DETAILS OF THE LATE WEEK SCENARIO (SEE
MODEL DISCUSSION FOR DETAILS).

&&

.MODEL DISCUSSION...

THE MODELS ARE IN EXCELLENT AGREEMENT WITH THE LARGE SCALE
SYNOPTIC PATTERN HEADING INTO THE WEEKEND...BUT THEY CONTINUE TO
DIFFER WITH MORE SUBTLE DETAILS AS THE NORTH PACIFIC SHORTWAVE
TROUGH PHASES WITH THE PARENT BERING SEA UPPER CLOSED LOW. THESE
DETAILS EVENTUALLY YIELD VERY LARGE DIFFERENCES IN THE ORIENTATION
OF THE TROUGH AND ASSOCIATED FRONT AS IT TRACKS INTO SOUTHCENTRAL.
THE NAM...GFS...AND CANADIAN GDPS SUPPORT A FULLER VORTEX-TROUGH
MERGER...EVENTUALLY RESULTING IN A DEEP NEGATIVE TILT TROUGH OVER
SOUTHCENTRAL...AND A STRONG SURFACE LOW TRACKING INLAND FRIDAY.
THE ECMWF...HOWEVER...DEPICTS A MUCH WEAKER PHASE WITH SUBSTANTIAL
ENERGY TRACKING EAST OF PRINCE WILLIAM SOUND. THIS SOLUTION WOULD
YIELD A FASTER FRONTAL PROGRESSION...WITH A MUCH WEAKER LOW
TRACKING INLAND. THE DIFFERENCES BECOME RATHER STARK BY LATE
FRIDAY WITH BOTH TIMING AND IMPACTS. AS A RESULT...THE FORECAST
WILL CONTINUE TO TAKE A BLENDED APPROACH...WITH SOME DETAILS OF
THE NAM...CANADIAN GDPS...AND ECMWF. THIS WILL HIGHLIGHT MORE
WIDESPREAD RAIN THURSDAY AND FRIDAY...WITH INCREASING SOUTHEAST
WINDS TRACKING INTO THE GULF AND PRINCE WILLIAM SOUND...AND
INCREASING OUT OF LOCAL GAPS SUCH AS TURNAGAIN ARM AND ALONG
PORTIONS OF THE KENAI PENINSULA.

&&

.SHORT TERM FORECAST...

SOUTHCENTRAL ALASKA...
CLOUDS AND SHOWERS WILL CONTINUE TONIGHT AND TOMORROW AS NUMEROUS
WEAK DISTURBANCES TRACK OVERHEAD. THE BEST CHANCE FOR
LIGHT ACCUMULATING RAIN WILL COME OVERNIGHT AS A DISTURBANCE NEAR
THE EASTERN ALEUTIANS TRACKS INTO COOK INLET...WITH MORE ISOLATED
TO SCATTERED SHOWERS DURING THE DAY...AND A DISTINCT BREAK IN
ACTIVITY DURING THE AFTERNOON AND INTO THE OVERNIGHT FOR INLAND
LOCATIONS. VISIBILITIES HAVE ALSO IMPROVED NORTH OF THE FUNNY
RIVER FIRE AS RAIN AND WEAKER STEERING WINDS HAVE PUT A DAMPER ON
THE NORTHWARD TRANSPORT OF SMOKE. THIS GENERAL PATTERN SHOULD
CONTINUE INTO THE WEEKEND...WITH MUCH IMPROVED VISIBILITIES ACROSS
THE NORTHERN COOK INLET AND MAT-SU VALLEYS.

A STRONGER LOW AND ASSOCIATED FRONT WILL BEGIN IMPACTING
SOUTHCENTRAL THURSDAY AFTERNOON THROUGH FRIDAY...BRINGING MORE
WIDESPREAD RAIN ACROSS ALL LOCATIONS EXCEPT THE COPPER BASIN. A
STRONGER LOW AND ASSOCIATED FRONT WILL TRACK INTO THE GULF...WITH
INCREASING SOUTHEAST WINDS ACROSS FAVORED GAPS AND INTO THE GULF
COAST AND PRINCE WILLIAM SOUND. A COLD FRONT WILL TRACK THROUGH
SOUTHCENTRAL SATURDAY...WITH BELOW NORMAL TEMPERATURES SWEEPING
ACROSS SOUTHCENTRAL...BUT CONTINUED CLOUDS AND RAIN SHOWERS.

SOUTHWEST ALASKA...
DEEP-LAYER SOUTHWEST FLOW OVER THE REGION WILL KEEP THE AREA IN A
CLOUDY WET AND COOL PATTERN THROUGH THE END OF THE WEEK. NUMEROUS
EMBEDDED UPPER LEVEL SHORTWAVES WILL BRING MULTIPLE ROUNDS OF RAIN
TO A WIDE AREA...WITH THE STRONGEST AND MOST SIGNIFICANT OF THESE
BATCHES OF RAIN LIKELY AFFECTING THE AREA WEDNESDAY NIGHT AND
AGAIN SOMETIME THURSDAY EVENING/NIGHT INTO FRIDAY. THE THURSDAY
EVENT LOOKS INCREASINGLY LIKELY TO BE THE MOST SIGNIFICANT ROUND
OF MEASURABLE RAINFALL SO FAR DURING THIS NEW OVERALL PATTERN. IN
FACT...CONFIDENCE IS INCREASING THAT MOST AREAS WILL LIKELY SEE IN
EXCESS OF ONE HALF INCH OF RAINFALL DURING THE THURSDAY/FRIDAY
TIMEFRAME. FOG WILL CONTINUE TO AFFECT PRIMARILY COASTAL
AREAS...THOUGH CANNOT BE COMPLETELY DISCOUNTED EVEN AT INLAND
LOCATIONS DURING THE OVERNIGHT PERIODS IF SKIES ARE ABLE TO CLEAR
OUT AT ALL. MEANWHILE...AN UNSEASONABLY COLD SURGE OF AIR APPEARS
AS IF IT WILL MOVE IN FROM THE BERING SEA...DROPPING TEMPERATURES
INTO THE MID 30S ACROSS A LARGE AREA FOR THE START OF THE WEEKEND.

BERING SEA/ALEUTIANS...
THE MOST ACTIVE WEATHER WILL REMAIN OVER THE EASTERN BERING
SEA...AS THE AXIS OF THE LONGWAVE TROUGH (CURRENTLY OVER THE
WESTERN ALEUTIANS) MOVES SLOWLY EASTWARD. THIS WILL CONSIST OF
PERIODIC ROUNDS OF RAINFALL...TAPERING TO MORE SHOWERY
PRECIPITATION BETWEEN THE UPPER LEVEL SHORTWAVES. MEANWHILE...THE
WESTERN BERING/ALEUTIANS WILL SEE RELATIVELY DRY NORTHWESTERLY
FLOW CONTINUE ON THE UPSTREAM SIDE OF THE LONGWAVE RIDGE AXIS.
CONFIDENCE IS INCREASING THAT A SURGE OF COLD AIR WILL DROP DOWN
THROUGH THE NORTHERN AND CENTRAL BERING FOR THE END OF THE
WEEK...LIKELY DROPPING TEMPERATURES INTO THE LOW 30S AND RAISING
THE LIKELIHOOD OF SNOW MIXING WITH THE RAIN OVER THE PRIBILOF
ISLANDS AND EASTERN ALEUTIANS/ALASKA PENINSULA FOR THE START OF
THE WEEKEND. ONLY SHOWERY PRECIPITATION IS EXPECTED WITH THIS
BLAST OF COLD AIR...SO ANY SNOW ACCUMULATIONS SHOULD BE VERY
MINOR.

&&

.LONG TERM FORECAST...

SUNDAY AND BEYOND...THE CURRENT TROUGH MOVING INTO SOUTHCENTRAL
LATE THIS WEEK WILL STALL OUT OVER THE STATE...WITH SHOWERY AND
CLOUDIER CONDITIONS ACROSS MOST OF SOUTHERN ALASKA CONTINUING INTO
EARLY MIDWEEK. BY THE LATTER PORTIONS OF THE WEEK...ALL THE GLOBAL
ENSEMBLE GUIDANCE IS HINTING AT LONGWAVE RIDGING BUILDING IN
ACROSS WESTERN ALASKA AND THE EASTERN BERING...BRINGING A RETURN
TO QUIETER AND DRIER WEATHER.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...NONE.
FIRE WEATHER...NONE.

&&

AHSENMACHER/AMD MAY 14