Small Craft Advisory 584, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
457 AM AKDT TUE JUL 21 2015

.ANALYSIS AND UPPER LEVELS...

UPPER LEVELS... A LOW CENTERED OVER THE WESTERN ALEUTIANS IS
BEGINNING TO MOVE EAST. THIS MOVEMENT IS BEING DRIVEN BY A JET
STREAK MOVING OVER THE CENTRAL AND EASTERN ALEUTIANS. THE JET
STREAK IS WORKING TO DEAMPLIFY THE UPPER LEVEL RIDGE OVER THE
WESTERN GULF. THE JET STREAK WILL ALSO USHER IN A VERY PROGRESSIVE
WEATHER PATTERN INTO SOUTHERN ALASKA FOR THE FORESEEABLE FUTURE.

SURFACE ANALYSIS... A WEAK TROUGH OF LOW PRESSURE PERSISTS ALONG
THE NORTH GULF COAST. MEANWHILE...AN ELONGATING FRONT OVER
SOUTHWEST ALASKA IS BEING SHEARED APART. THUS IT IS ONLY PRODUCING
LIGHT SHOWER ACTIVITY IN COASTAL LOCALES WEST OF BETHEL. OVER THE
WESTERN BERING... A VERTICALLY STACKED SURFACE LOW IS ALSO
WEAKENING...WITH IMPULSES OF RAIN MOVING ALONG THE SOUTHEASTERN
PERIPHERY ALONG THE CENTRAL AND EASTERN ALEUTIANS AND OVER THE
SOUTHERN ALASKA PENINSULA.

&&

.MODEL DISCUSSION...

REGARDING THE EVOLUTION OF THE SYNOPTIC PATTERN...THE MODELS ARE
LARGELY IN AGREEMENT. THE UPPER LOW OVER THE BERING WILL PROGRESS
EASTWARD...MOVING OVER SOUTHWEST ALASKA WEDNESDAY AFTERNOON AND
REACHING THE GULF EARLY THURSDAY MORNING. IT THEN CONTINUES
MOVING TO SOUTHEAST ALASKA BY EARLY FRIDAY MORNING. AS THE LOW
TRANSITS A WEAK RIDGE DEVELOPS OVER THE BERING WHICH BUILDS INTO
WESTERN AK BY THE END OF THE WEEK.

PRECIPITATION DEPICTION BY ALL OF THE MODELS IS ALSO LARGELY IN
AGREEMENT. AFTER THE FRONT ALONG THE SOUTHWEST COAST DECAYS...THE
UPPER LOW BECOMES THE PRIMARY FORCING MECHANISM OVER SOUTHWEST
ALASKA DURING THE DAY WEDNESDAY AND OVER SOUTHCENTRAL WEDNESDAY
NIGHT. THIS WILL INDUCE SURFACE LOW FORMATION OVER MUCH OF THE
MAINLAND PORTIONS OF THE FORECAST AREA AND ALSO OVER THE WESTERN
GULF WEDNESDAY NIGHT.

WEDNESDAY NIGHT IS WHEN SIGNIFICANT DIFFERENCES IN THE MODELS
ARISE. THE ECMWF KEEPS A BROAD TROUGH ALONG THE NORTH GULF COAST
WHILE THE NAM AND GFS DEVELOP A STRONGER CLOSED SURFACE LOW. THE
ECMWF ALSO SMOOTHS THE VORTICITY FIELDS TOO MUCH. THE GFS WAS THE
PREFERRED SOLUTION HERE OWING TO ITS MIDDLE GROUND DEPICTION
BETWEEN THE WEAK EC AND THE STRONGER NAM. THE NAM AND GFS
GENERALLY AGREE WITH THE PRESENCE AND LOCATIONS OF INDIVIDUAL
VORTMAXES SPIRALING AROUND THE UPPER LEVEL LOW.

&&

.FIRE WEATHER...

A VERY DRY ATMOSPHERE AS INDICATED BY THE 12Z ANCHORAGE SOUNDING
SPELLS THE CONTINUATION OF DRY WEATHER FROM SOUTHCENTRAL ALASKA.
ENOUGH LINGERING MOISTURE OVER THE COPPER RIVER BASIN WITH THE
PASSAGE OF A FEW VORTMAXES MAY SET OFF AN ISOLATED THUNDERSTORM OR
TWO NEAR THE ALASKA RANGE AND THE WRANGELLS. SOUTHWEST FLOW AND
INCREASING CLOUDINESS WILL BRING MORE MOISTURE AND SEASONABLE
TEMPERATURES TO SOUTHWEST ALASKA TUESDAY AS THE FRONT ALONG THE
COAST MOVES INLAND...THOUGH VERY LITTLE IF ANY RAIN IS EXPECTED
FROM THE FRONT. DURING THE DAY WEDNESDAY...THE UPPER LEVEL LOW NOW
OVER THE BERING WILL MOVE INTO SOUTHWEST ALASKA. STRONG UPPER
LEVEL DIFFLUENCE WILL PROVIDE LIFT...AND COMBINED WITH A LARGE
INFLUX OF MOISTURE OFF THE BERING...WILL PRODUCE WIDESPREAD
WETTING RAINS TO ALL OF SOUTHWEST ALASKA. THE RAIN WILL MOVE EAST
WEDNESDAY NIGHT INTO MUCH OF SOUTHCENTRAL WITH DIMINISHING
PRECIPITATION AMOUNTS TOWARD THE ALASKA RANGE.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA...
WITH WEAK RIDGING MOVING THROUGH THE AREA CLOUD COVER WILL BE
HELD AT BAY IN THE MORNING AND EARLY AFTERNOON LEADING TO WARM
AFTERNOON TEMPERATURES. THIS WILL ALLOW FOR AFTERNOON CONVECTION
INCLUDING THE POSSIBILITY OF AFTERNOON AND EVENING THUNDERSTORMS
ALONG THE MOUNTAINS IN THE COPPER RIVER BASIN AND SUSITNA VALLEY
THROUGH WEDNESDAY. WEDNESDAY EVENING THE LOW MOVING INTO THE GULF
FROM SOUTHWEST ALASKA WILL BRING MORE WIDESPREAD RAIN TO THE
REGION. BY THURSDAY AFTERNOON THE LOW WILL HAVE MOVED OFF TO THE
SOUTHEAST TAKING MOST OF THE RAIN WITH IT RETURNING THE AREA TO A
SHOWERY PATTERN.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)...
RAIN WILL PERSIST OVER THE SOUTHWEST MAINLAND THROUGH WEDNESDAY
EVENING AS THE UPPER LEVEL LOW AND ITS ASSOCIATED SURFACE
FEATURES MOVE THROUGH THE REGION. THE FIRST ROUND OF RAINFALL
WILL BE ASSOCIATED WITH THE FRONT CURRENTLY MOVING THROUGH THE
AREA THIS MORNING. THIS SYSTEM WILL CONTINUE TO DISSIPATE AS IT
MOVES FURTHER INLAND TODAY...GENERATING MAINLY SCATTERED SHOWERS
OVER THE REGION. THE UPPER LOW WILL MOVE INTO THE REGION ON
WEDNESDAY MORNING STARTING THE SECOND ROUND OF RAINFALL. AS THE
LOW TRANSITS INTO THE GULF OF ALASKA...FLOW AT THE LOWER LEVELS
WILL FAVOR POTENTIALLY LOCALLY HEAVY RAINFALL WEDNESDAY
AFTERNOON/EVENING ALONG THE WESTERN SIDE OF THE ALEUTIAN RANGE.

OTHERWISE...EXPECT BREEZY CONDITIONS ALONG THE SOUTHWEST COAST
WITH THE STRONGEST WINDS BEHIND THE UPPER LOW ON WEDNESDAY.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...
WINDY AND WET CONDITIONS WILL PERSIST OVER THE REGION THROUGH
WEDNESDAY AS THE UPPER LEVEL LOW MOVES EASTWARD. WINDS WILL REACH
25 KT ALONG MOST OF THE ALEUTIAN CHAIN AND ALASKA PENINSULA BY
LATE THIS EVENING AND WILL PERSIST THROUGH WEDNESDAY EVENING AS
THE UPPER LOW EXITS INTO THE GULF OF ALASKA AND A NEW LOW MOVES
INTO THE FAR WESTERN BERING SEA. RAINFALL IS EXPECTED OVER MOST OF
THE REGION AS SHORTWAVES ASSOCIATED WITH THE UPPER LEVEL LOW WILL
SERVE AS THE FOCUS FOR AREAS OF STRATIFORM RAIN AND SHOWERS.

&&

.LONG TERM FORECAST (DAYS 3 THROUGH 7)...

THE LONG RANGE FORECAST STARTS OFF WITH A CLOSED UPPER LOW SLIDING
SOUTHEAST INTO THE GULF ON THURSDAY. A SURFACE LOW ASSOCIATED WITH
THIS SYSTEM PUSHES A FRONT FROM THE SOUTHWEST MAINLAND ACROSS TO
THE GULF AND SURROUNDING COASTAL AREAS WHICH SHOULD BRING RAINFALL
AMOUNTS UP TO ONE HALF OF AN INCH WITH HIGHER AMOUNTS ALONG THE
SOUTHERN KENAI AND WESTERN PRINCE WILLIAM SOUND. SOME
PRECIPITATION SCATTERS INLAND LATE THURSDAY INTO FRIDAY ACROSS
SOUTHCENTRAL...HOWEVER RAIN AMOUNTS WILL BE NEGLIGIBLE AS THE
SURFACE LOW QUICKLY SLIDES SOUTH TO THE EASTERN PACIFIC.

A PROGRESSIVE PATTERN SETS UP ACROSS THE BERING HEADING INTO THE
WEEKEND WITH SEVERAL WEAK DISTURBANCES MOVING AHEAD OF A
DEVELOPING TROUGH OVER KAMCHATKA. THERE IS STILL MODEL DISAGREEMENT
ON HOW THE UPPER LOW DEVELOPS AND TRACKS. ONLY MADE MINOR
ADJUSTMENTS FOR THE EXTENDED FORECAST USING THE ECMWF WITH A
CONSERVATIVE APPROACH. THE 12Z ECMWF MODEL RUN BRINGS THE
DEVELOPING LOW INTO THE GULF ON SATURDAY WITH A RIDGE BUILDING
OVER THE BERING IN ITS WAKE THROUGH THE BEGINNING OF NEXT WEEK.
THIS SOLUTION WOULD BRING THE HIGHEST PRECIPITATION ACROSS THE
GULF AND SURROUNDING COASTAL AREAS WITH SCATTERED SHOWERS INLAND.
THE BERING AND ALEUTIANS WOULD CONTINUE WITH CLOUDY AND PATCHY FOG
UNDER THE BUILDING RIDGE.

&&

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

&&

SYNOPSIS AND MODEL DISCUSSION...JW/DS
SOUTHCENTRAL ALASKA...DK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...RF
LONG TERM...KH