Small Craft Advisory 576, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
459 AM AKDT SUN JUL 19 2015

.ANALYSIS AND UPPER LEVELS...
THE UPPER LEVEL LOW IS NOW CENTERED OVER NORTHEASTERN ALASKA AND
WILL CONTINUE EAST INTO CANADA AS THE BUILDING UPPER LEVEL RIDGE
OVER THE EASTERN BERING AND ALASKA PENINSULA CONTINUES TO AMPLIFY
AND SHIFT EAST IN RESPONSE TO A STRENGTHENING WEST SOUTHWESTERLY
JET STREAM IN THE NORTHERN PACIFIC. ADDITIONALLY PROPELLED BY THE
STRENGTHENING RIDGE...AN UPPER LEVEL TROUGH SOUTH OF THE CENTRAL
GULF WILL SWING EAST NORTHEAST TOWARDS THE SOUTHEASTERN GULF.
FURTHER TO THE WEST A LARGE UPPER LEVEL LOW WILL MOVE SLOWLY EAST
ACROSS THE WESTERN ALEUTIANS.

&&

.MODEL DISCUSSION...
MODELS REMAIN IN GOOD AGREEMENT WITH REGARDS TO THE BUILDING OF
THE RIDGE OVER SOUTHWEST AND THEN MAINLAND ALASKA AS WELL A WITH
THE DEVELOPMENT OF OFFSHORE FLOW OVER SOUTH CENTRAL ALASKA TONIGHT
AND MONDAY. MODEL DIFFERENCES BECOME MORE SIGNIFICANT AS THE RIDGE
STARTS TO FLATTEN TUESDAY (WHICH APPEARS TO BE HAPPENING FASTER
THAN IT DID IN YESTERDAYS MODEL RUNS) AND MOISTURE FROM THE LOW
CROSSING THE BERING BEGINS TO OVERRUN IT. ONE QUESTION WILL BE HOW
FAST AND HOW FAR THE EAST THE INITIAL FRONTAL SYSTEM MAKE IT
TUESDAY AND ANOTHER HOW MUCH MOISTURE AND INSTABILITY FROM THE
INTERIOR ALASKA AND YUKON TERRITORY THERMAL TROUGH WILL MAKE IT
OVER THE MOUNTAINS TO DEVELOP INTO AFTERNOON CONVECTION. WITH THE
DIRECTION THE MODELS ARE TRENDING A FASTER BUT WEAKER AND MORE
QUICKLY SHEARED APART FRONT TUESDAY COMING IN FROM THE WEST SEEMS
REASONABLE WITH THE MORE COHERENT AND STRONGER SYSTEM PUSHING
INTO BOTH DOMAINS WEDNESDAY.

&&

.FIRE WEATHER...
THE BUILDING RIDGE WILL BRING A BRIEF SOMEWHAT WARMER...DRIER AND
SUNNIER BREAK OVER THE NEXT COUPLE OF DAYS WITH OFFSHORE FLOW
DEVELOPING ALONG THE NORTHERN GULF COAST TONIGHT AND MONDAY. THE
OFFSHORE WINDS WILL BECOME ESPECIALLY GUSTY NEAR THE OUTLETS OF
CHANNELED TERRAIN GAPS IN THE COASTAL MOUNTAINS IN LOCATIONS SUCH
AS WHITTIER AND PASSAGE CHANNEL...AROUND THE VALDEZ AREA AND THOMPSON
PASS AND ALONG THE COPPER RIVER DELTA. ISOLATED WET THUNDERSTORMS
ARE POSSIBLE ALONG NORTHEASTERN PORTIONS OF THE COPPER RIVER BASIN
THIS AND MONDAY AFTERNOON AND EVENING.

A WEAK FRONTAL SYSTEM WILL SPREAD SPOTTY LIGHT RAIN ACROSS
SOUTHWEST ALASKA ON TUESDAY. A STRONGER AND WETTER SYSTEM WILL
FOLLOW SHORTLY THEREAFTER...PUSHING INTO SOUTHWEST ALASKA TUESDAY
NIGHT AND SOUTH CENTRAL ALASKA WEDNESDAY.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 AND 2)...
AN UPPER LEVEL TROUGH REMAINS OVER SOUTHCENTRAL THIS MORNING...BUT
THE FLOW IS SHIFTING TO NORTHERLY AS AN UPSTREAM RIDGE OVER THE
EASTERN BERING SEA APPROACHES. AS OFFSHORE FLOW STRENGTHENS TODAY
MOST OF THE CLOUDS WILL CLEAR OUT LEAVING SUNNY SKIES AND WARMING
TEMPERATURES. THE ONE EXCEPTION IS THE COPPER RIVER BASIN WHERE
CYCLONIC FLOW ALOFT WITH WEAK EMBEDDED IMPULSES WILL COMBINE WITH
WEAK SURFACE BASED INSTABILITY TO PRODUCE SOME SHOWERS AND POSSIBLY
A FEW THUNDERSTORMS...ESPECIALLY OVER THE EASTERN HALF OF THE BASIN.

AS THE UPPER RIDGE CONTINUES TO BUILD IN TONIGHT INTO MONDAY THE
STRENGTHENING OFFSHORE FLOW WILL LEAD TO PRESSURE FALLS OVER THE
NORTHERN GULF. THE TIGHTENING PRESSURE GRADIENT WILL LEAD TO
DEVELOPMENT OF STRONG GUSTY GAP WINDS ALONG PRINCE WILLIAM SOUND
AND THE NORTH GULF COAST. THE NORTHERLY FLOW WILL LIMIT OR IN SOME
CASES COMPLETELY PREVENT SEA BREEZE DEVELOPMENT. THUS EXPECT WARM
TEMPERATURES NEARLY EVERYWHERE THE NEXT COUPLE DAYS...WITH SOME
COASTAL LOCATIONS (SUCH AS SEWARD) WARMING ALMOST AS MUCH AS
INTERIOR LOCATIONS.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)...

EXCEPT FOR A FEW SHOWERS FROM THE COAST TO NEAR ANIAK TODAY...WARMER
AND DRIER CONDITIONS WILL AFFECT THE AREA THROUGH MONDAY. BY
MONDAY NIGHT SOME RAIN WILL BE APPROACHING THE COAST...WHICH THEN
WILL MAKE ITS WAY ACROSS TO THE ALEUTIAN AND ALASKA RANGES ON
TUESDAY.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...

AS THE FRONT MOVES EAST ACROSS THE BERING...RAIN AND GALE FORCE
WIND WILL AFFECT THE CENTRAL ALEUTIANS TODAY AND THE EASTERN ALEUTIANS
TONIGHT THROUGH MONDAY. AREAS OF FOG WILL PERSIST...ESPECIALLY IN
THE SOUTHEAST FLOW PRECEDING THE FRONT. DRYING OVER THE WESTERN
PORTION OF THE CHAIN MONDAY NIGHT WILL REACH THE CENTRAL ISLANDS
ON TUESDAY.

&&

.LONG TERM FORECAST (DAYS 3 THROUGH 7)...
THE MAIN FEATURE WILL BE THE BROAD LOW PRESSURE SYSTEM IN THE
CENTRAL BERING SEA THROUGH NEXT WEEK. THIS SYNOPTIC FEATURE WILL
BRING A SHOWERY WEATHER REGIME AS THIS SYSTEM TRACKS THROUGH THE
SOUTHWEST ALASKA REGION BY MIDWEEK BEFORE ENTERING THE
SOUTHCENTRAL AREA...THEN EXITS THE AOR INTO THE YUKON TERRITORY
BY END OF NEXT WEEK. THERE WILL BE MINOR RIDGES OF HIGH PRESSURE
SURROUNDING THE LOW PRESSURE SYSTEM AS IT MOVES THROUGH THE
REGION. THIS WILL GIVE WAY TO BRIEF DRIER CONDITIONS WITH SUNNY
SKIES AND WARMER TEMPERATURES FOR THE SOUTHCENTRAL ALASKA AS WE
HEAD INTO THE WEEKEND. MEANWHILE...THE SOUTHWEST AND BERING SEA
WILL HAVE A WET SHOWERY PATTERN ASSOCIATED WITH THE LOW PRESSURE
SYSTEM BEFORE IT MOVES INLAND. AS THIS SYSTEM MOVES OUT OF THE
BERING IT WILL GIVE WAY TO SOUTHWESTERLY FLOW WITH SCATTERED
SHOWERY PRECIPITATION WITH MARINE LAYER STRATUS AND FOG THROUGH
THE WEEK. THE GFS/ECMWF MODELS CONTINUED TO HAVE DISCREPANCIES
WITH THE TIMING OF THE LOW AS IT TRACKS EASTWARD OUT OF THE BERING
SEA. THEREFORE...WPC HAS USED THE MEAN EC ENSEMBLES TO WORK OUT
THE DIFFERENCES WITH THE MODELS.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...GALES 170 175 176 412 413.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...JER
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...DS
LONG TERM...PD