Small Craft Advisory 423, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
1204 PM AKDT WED MAY 28 2014

.ANALYSIS AND UPPER LEVELS...

A DEEP LONGWAVE TROUGH...VERY IMPRESSIVE FOR THIS TIME OF
YEAR...IS IN THE PROCESS OF CARVING ITSELF OUT OVER THE BERING
SEA...ANCHORED BY A COLD UPPER LEVEL VORTEX OVER EASTERN SIBERIA.
AT THE BASE OF THE TROUGH...NEAR 45N AND 177W...EXISTS A DIGGING
POTENTIAL VORTICITY ANOMALY WHICH WILL EVENTUALLY LIFT NORTHEAST
INTO THE GULF OF ALASKA THURSDAY AND FRIDAY ALONG A FRONTAL ZONE.
ACROSS SOUTHERN ALASKA...BROAD SOUTHWEST FLOW AND A DIFFLUENT JET
AXIS EXISTS...WITH NUMEROUS WEAK SHORTWAVE TROUGHS EMBEDDED WITHIN
THE FLOW SUPPORTING AREAS OF ISOLATED TO SCATTERED RAIN SHOWERS.
EVENTUALLY...DYNAMIC ASCENT WILL RAPIDLY INCREASE THURSDAY AND
FRIDAY AS THE BASAL POTENTIAL VORTICITY ANOMALY MERGES WITH THE
EASTERN SIBERIA VORTEX OVER SOUTHERN ALASKA...WITH THE FORMER
FEATURE TAPPING INTO SUBTROPICAL MOISTURE. THE NET EFFECT WILL BE
AN IMPRESSIVE RAIN EVENT FOR PORTIONS OF SOUTHERN
ALASKA...ESPECIALLY INTERIOR PORTIONS ACROSS SOUTHWEST ALASKA. A
SEASONABLY STRONG GALE FORCE FRONT WILL TRACK INTO THE NORTHERN
GULF AND PRINCE WILLIAM SOUND...WITH STRONG SOUTHEAST WINDS
ACCELERATING THROUGH FAVORED GAPS. A VIGOROUS COLD FRONT WILL
TRACK FROM WEST TO EAST LATE FRIDAY INTO SATURDAY...WITH
TEMPERATURES DROPPING BELOW NORMAL FOR THE REMAINDER OF THE
WEEKEND.

&&

.MODEL DISCUSSION...

THE MODELS ARE IN EXCELLENT AGREEMENT WITH THE SYNOPTIC PATTERN
THROUGH THE WEEKEND...AND HAVE COME INTO MUCH BETTER AGREEMENT
WITH THE SUBTLE SHORTER WAVELENGTH FEATURES. IN OTHER
WORDS...THERE IS VERY HIGH CONFIDENCE IN THE GENERAL WEATHER
PATTERN ACROSS SOUTHERN ALASKA AS THIS IMPRESSIVE STORM SYSTEM
TRACKS ACROSS THE REGION. THE CANADIAN RDPS WAS FAVORED IN THE
SHORT TERM...WITH A BLEND OF THE CANADIAN GDPS AND ECMWF FAVORED
FRIDAY THROUGH SUNDAY.

&&

.SHORT TERM FORECAST...

SOUTHCENTRAL ALASKA...
SOUTHCENTRAL REMAINS UNDER THE INFLUENCE OF SOUTHWESTERLY FLOW
ALOFT...WITH WEAKENING UPPER DISTURBANCES TRACKING OVER THE
REGION. THIS CONTINUES TO SUPPORT ISOLATED TO SCATTERED RAIN
SHOWERS AND OVERCAST SKIES. WEAK FRONTAL ZONES SITUATED IN PRINCE
WILLIAM SOUND WITH ANOTHER NEAR THE KENAI PENINSULA ARE HELPING TO
KEEP INTERIOR LOCATIONS ON THE DRIER SIDE DUE TO DOWNSLOPE
SHADOWING. THIS WILL CONTINUE INTO EARLY THURSDAY BEFORE MORE
WIDESPREAD RAIN MOVES INTO THE REGION FOR THURSDAY AND FRIDAY AS A
MUCH MORE VIGOROUS FRONT TRACKS INTO THE NORTHERN GULF. THE
HEAVIEST RAIN WILL FAVOR THE COASTAL LOCATIONS...BUT INTERIOR
LOCATIONS WILL LIKELY SEE PERIODS OF RAIN...WITH MORE SHOWERY
CONDITIONS FOR THE COPPER BASIN ON FRIDAY. SOUTHEAST WINDS WILL
INCREASE DRAMATICALLY ACROSS FAVORED GAPS LATE THURSDAY THROUGH
FRIDAY...WITH SOME OF THESE WINDS CLIPPING SOUTH ANCHORAGE AND THE
HILLSIDE.

EARLY SATURDAY...AN IMPRESSIVE COLD FRONT WILL TRACK OVER
SOUTHCENTRAL...USHERING IN COLDER TEMPS AND SNOW THREATS DOWN TO
2500 FEET. HOWEVER...A DRY SLOT WILL ACCOMPANY THE COLD FRONT...SO
THE ARRIVAL OF THE FRONT WILL ALSO USHER IN DIMINISHING
PRECIPITATION SATURDAY. EXPECT TEMPERATURES TO BE BELOW NORMAL
REGION WIDE SATURDAY AND SUNDAY...WITH CONTINUED EXTENSIVE CLOUD
COVER.

SOUTHWEST ALASKA...

RAIN WILL BE ON THE INCREASE ACROSS ALL OF SOUTHWEST ALASKA AS A
SERIES OF DISTURBANCES TRACKS INLAND. RAIN WILL INCREASE MARKEDLY
IN COVERAGE AND INTENSITY TONIGHT...LASTING THROUGH FRIDAY. A COLD
FRONT WILL SWEEP THROUGH FRIDAY NIGHT AND SATURDAY...WITH
CONTINUED RAIN SHOWERS BEHIND THE FRONT THROUGH THE WEEKEND.
LOCALLY HEAVY RAINFALL WILL BE POSSIBLE ACROSS PORTIONS OF THE
INTERIOR...WITH SOME LOCATIONS EXCEEDING 1 TO POSSIBLY 2 INCHES OF
RAIN BY THIS WEEKEND.

BERING SEA/ALEUTIANS...

A COLD FRONT WILL SWEEP FROM WEST TO EAST ACROSS THE BERING SEA
TONIGHT AND THURSDAY...WITH COOLING TEMPERATURES AND INCREASING
RAIN SHOWERS. RAIN AND SNOW SHOWERS WILL ALSO IMPACT THE PRIBILOF
ISLANDS...BUT NO ACCUMULATIONS ARE EXPECTED. PERIODIC SHOWERS WILL
CONTINUE THROUGH THE WEEKEND.

&&

.LONG TERM FORECAST...

MONDAY AND BEYOND...THE REMNANTS OF THE UPPER LOW WILL SLOWLY
WEAKEN AND TRACK EAST THROUGH TUESDAY...KEEPING THE REGION UNDER
THE INFLUENCE OF SHOWERS AND SLIGHTLY COOLER TEMPERATURES.
MODERATING TEMPERATURES AND DRIER CONDITIONS WILL DEVELOP
WEDNESDAY THROUGH THE END OF THE WEEK AS SHORTWAVE RIDGING BUILDS
IN BEHIND THE DEPARTING UPPER LOW.

&&

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

&&

AHSENMACHER MAY 14