Small Craft Advisory 574, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
405 PM AKDT SAT JUL 18 2015

.ANALYSIS AND UPPER LEVELS...
THERE IS AN UPPER LOW OVER CENTRAL ALASKA THAT IS SLOWLY DRIFTING
NORTH AND EAST. THERE IS A DEEP UPPER LOW CENTERED JUST WEST OF
THE WESTERN ALEUTIANS. A RIDGE OF HIGH PRESSURE IS BUILDING
AHEAD OF THIS LOW OVER THE EASTERN BERING AND SOUTHWARD ACROSS
THE ALASKA PENINSULA INTO THE NORTH PACIFIC...JUST STARTING TO
NUDGE INTO SOUTHWEST ALASKA. A RESIDUAL TROUGH REMAINS OVER SOUTH
CENTRAL ALASKA. AT THE LOWER LEVELS THE FLOW IS ONSHORE FOR ALMOST
ALL OF THE SOUTHERN ALASKA MAINLAND.

&&

.MODEL DISCUSSION...
THE NUMERICAL MODELS ARE IN GOOD AGREEMENT THROUGH ABOUT 60 HOURS
INTO THE FORECAST (MONDAY AFTERNOON). AFTER THIS THEY BEGIN TO
DIVERGE ON THE SPEED AND STRENGTH OF A FRONT MOVING INTO THE
CENTRAL/EASTERN ALEUTIANS AND SOUTHERN BERING SEA...BUT NOT BY
LARGE AMOUNTS. FORECAST CONFIDENCE IS NEAR NORMAL...THE GOOD
MODEL AGREEMENT IS OFFSET BY THE UNCERTAINTY IN JUST HOW FAST THE
SENSIBLE WEATHER WILL BE AFFECTED BY THE PATTERN CHANGE.

&&

.FIRE WEATHER...
A BIT OF A DRYING TREND IS IN STORE FOR THE NEXT FEW DAYS AS A
RIDGE OF HIGH PRESSURE WILL BUILD INTO SOUTHERN ALASKA. SOME
OFFSHORE WINDS ARE EXPECTED AS THIS RIDGE BUILDS IN...BUT WILL
MAINLY BE LIMITED TO THE "PREFERRED" AREAS SUCH AS WHITTIER AND
SEWARD. WITH CONDITIONS BEING SOMEWHAT WET FROM RECENT
RAINS...EXPECT IT WILL BE MONDAY (AT LEAST) BEFORE ANY HUMIDITIES
GET DOWN BELOW 30 PERCENT...AND THIS ONLY AT AREAS WELL INLAND
SUCH AS THE SUSITNA AND COPPER RIVER BASINS. THIS DRY TREND WILL
BE SOMEWHAT SHORT LIVED...WITH CONDITIONS MOISTENING UP AGAIN OVER
SOUTHWEST ALASKA ON TUESDAY/TUESDAY NIGHT AND IN SOUTH CENTRAL
PROBABLY ON WEDNESDAY. THIS WILL BE IN RESPONSE TO AN UPPER LOW
MOVING INTO SOUTHERN ALASKA FROM THE BERING SEA.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 AND 2)...
A WEAK UPPER LEVEL TROUGH OVER THE SOUTHWEST MAINLAND WILL
QUICKLY TRACK SOUTHEAST THROUGH THE GULF OVERNIGHT...BEFORE A
BUILDING RIDGE TO THE WEST WEAKENS THIS SYSTEM AS IT PUSHES IT
FURTHER TO THE SOUTHEAST ON SUNDAY. RESIDUAL WEAK UPPER LEVEL
DISTURBANCES ROUNDING THE BASE OF THE UPPER LOW OVER THE NORTHERN
INTERIOR WILL TRACK THROUGH INTERIOR SOUTHCENTRAL THROUGH TOMORROW
MORNING...WHICH WILL HELP INITIATE DIURNAL CONVECTION THROUGH THIS
EVENING IN A MARGINALLY UNSTABLE ATMOSPHERE. THE BULK OF THESE
SHOWERS ARE EXPECTED FROM THE MAT-SU TO THE COPPER BASIN...WITH
DRIER AND MORE STABLE CONDITIONS ALONG THE COASTAL AREAS. ISOLATED
THUNDERSTORMS CAN DEVELOP DURING THE AFTERNOON AND EVENING HOURS
THROUGH MONDAY...AS CLEARING CONDITIONS WILL LEAD TO INCREASED
DAYTIME HEATING.

AN AMPLIFYING RIDGE BEGINS TO BUILD IN FROM THE WEST EARLY SUNDAY
MORNING...AND WILL BEGIN TO SHIFT THE OVERALL PATTERN TO A MORE
CLEAR AND DRY OFFSHORE FLOW WITH WARMING TEMPERATURES THROUGH
MONDAY. THE COPPER RIVER BASIN WILL SEE THE BULK OF ANY CONVECTIVE
SHOWERS OR ISOLATED THUNDERSTORM DEVELOPMENT DURING THE AFTERNOON
AND EVENING HOURS THOUGH MONDAY...AS INCREASED CLEARING AND
DAYTIME HEATING WILL LEAD TO INCREASED INSTABILITY. THE OFFSHORE
FLOW WILL HELP BUILD LOWER PRESSURE OVER THE GULF...WHICH WILL
ACT TO TIGHTEN THE PRESSURE GRADIENT OVER THE AREA. AS A
RESULT...LOCALIZED OUTFLOW GAP WINDS WILL BECOME GUSTY ALONG
PRINCE WILLIAM SOUND SUNDAY EVENING THROUGH MONDAY.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)...
LIGHT PRECIPITATION WILL GRADUALLY DIMINISH ACROSS THE REGION
TONIGHT AS A BUILDING RIDGE USHERS IN A WARMING AND DRYING TREND
THROUGH MONDAY. PATCHY FOG WILL ALSO BE POSSIBLE WITH WEAK ONSHORE
FLOW BENEATH THE RIDGE.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...
RAIN AND GUSTY SOUTH WINDS WILL CONTINUE ACROSS THE WESTERN HALF
OF THE BERING THROUGH SUNDAY AS THE FRONT MOVES SLOWLY EASTWARD
AND THEN STALLS OVER THE CENTRAL ALEUTIANS. AN UPPER WAVE WILL
THEN ARRIVE FROM THE NORTH PACIFIC ON SUNDAY NIGHT AND HELP TO
PUSH THE FRONT INTO THE EASTERN BERING...WITH RAIN SPREADING
ACROSS THE EASTERN ALEUTIANS/PRIBILOFS AND AKPEN THROUGH MONDAY.
SHOWERS WILL ALSO CONTINUE ACROSS THE WESTERN BERING AS THE PARENT
LOW LINGERS NEAR ATTU.

&&

.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 TRACK THROUGH THE
SOUTHWEST ALASKA REGION BY MIDWEEK BEFORE ENTERING THE
SOUTHCENTRAL AREA. THEN...EXIT 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 BRIER DRIER CONDITIONS WITH SUNNY
SKIES AND WARMER TEMPERATURES FOR THE SOUTHCENTRAL ALASKA NEXT
WEEK AS WERE HEADED INTO THE WEEKEND. WHILE...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...GALE 175 176 177.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...BL
SOUTHCENTRAL ALASKA...TP
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CB
LONG TERM...PD