Small Craft Advisory 373, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
436 PM AKDT THU APR 30 2015

.ANALYSIS AND UPPER LEVELS...
THERE IS A VERTICALLY STACKED LOW OVER THE CENTRAL/EASTERN GULF
THAT IS MOVING TO THE EAST. THERE IS AN UPPER LOW OVER THE CENTRAL
BERING SEA WITH AN EXTENSION TO THE ALASKA PENINSULA. THIS LOW
AND ITS EXTENSION EACH HAS A WEAK SURFACE REFLECTION. ZONAL FLOW
CONTINUES OVER THE NORTHERN PACIFIC TO THE SOUTH OF THESE LOWS.
THE FLOW ALOFT OVER SOUTHERN ALASKA IS STILL WEAKLY CYCLONIC. THIS
IS COMBINED WITH SOME MOISTURE TO CREATE UNSTABLE CONDITIONS. THE
MOST UNSTABLE CONDITIONS ARE OVER SOUTHWEST ALASKA WHERE THE
INTERIOR REMAINS QUITE UNSTABLE AS THE AIR MASS HAS NOT CHANGED
MUCH SINCE WEDNESDAY. THE COASTAL SECTIONS ARE GETTING SOME
ONSHORE FLOW SO THEY ARE A MORE STABLE.

&&

.MODEL DISCUSSION...
THE NUMERICAL MODELS CONTINUE TO BE IN VERY GOOD AGREEMENT IN THE
SHORT TERM. THERE ARE OF COURSE SOME MINOR DIFFERENCES IN THE
MOISTURE/STABILITY FIELDS BUT THESE ARE FAIRLY EASILY HANDLED IN
THE FORECAST. THEREFORE FORECAST CONFIDENCE IS HIGHER THAN NORMAL.

&&

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

WITH THE MAIN STORM TRACK SHUNTED SOUTH IN THE NORTH PACIFIC THE
REGION LARGELY REMAINS UNDER THE INFLUENCE OF VERY WEAK UPPER
LEVEL FLOW WITH NO DISTINCT OR WELL DEFINED UPPER LEVEL DISTURBANCES.
THE MOST ROBUST AND WIDESPREAD SHOWERS ARE ACROSS THE SUSITNA
VALLEY INTO THE COPPER BASIN. TOWARD COASTAL LOCATIONS SEA BREEZES
ARE KEEPING THOSE AREAS MORE STABLE AND FREE OF SHOWERS.

EVENTUALLY WEAK HIGH PRESSURE WILL BEGIN TO BUILD OVER SOUTHCENTRAL
ALASKA THROUGH SATURDAY BRINGING GRADUALLY WARMER AND DRIER
CONDITIONS WITH EACH SUCCESSIVE DAY. DIURNAL SEA BREEZES WILL
REMAIN ALTHOUGH THERE WILL BE A BRIEF SHOT OF COASTAL OUTFLOW
WINDS TONIGHT BEFORE THE PRESSURE GRADIENT BECOMES VERY WEAK
FRIDAY AND SATURDAY.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)...
MOIST SOUTHWESTERLY FLOW WILL CONTINUE TO DEVELOP OVER SOUTHWEST
ALASKA TONIGHT INTO FRIDAY AS AN UPPER LEVEL TROUGH SWINGS
THROUGH THE REGION ON FRIDAY. THIS TROUGH IS SHOWING LITTLE IF ANY
REFLECTION AT THE SURFACE AND WILL MAINLY AID IN THE DEVELOPMENT
OF SHOWERS AND SOME ISOLATED THUNDERSTORMS THROUGH TOMORROW NIGHT.
THE BEST DYNAMICS FOR THUNDERSTORMS TONIGHT WILL BE ALONG ELEVATED
TERRAIN IN THE REGION AS THERE IS LITTLE UPPER LEVEL FORCING TODAY
UNLIKE YESTERDAY WHERE A SHORTWAVE TRAVELING WEST ALONG THE ALASKA
RANGE PROVIDED ADDITIONAL FORCING. AS THE UPPER LEVEL TROUGH
PUSHES INTO THE REGION TOMORROW THE SOUTHWESTERLY FLOW WILL HELP
TO STABILIZE THE ATMOSPHERE ALONG THE BRISTOL BAY COAST SO THUNDERSTORMS
SHOULD GENERALLY LIMITED TO THE LOWER KUSKOKWIM VALLEY. COASTAL
FOG IS ALSO EXPECTED TO DEVELOP TONIGHT ALONG THE ENTIRE SOUTHWEST
COASTLINE.

A RIDGE WILL BUILD OVER THE CENTRAL ALEUTIANS ON SATURDAY WHILE A
FRONT ASSOCIATED WITH A LOW IN THE SOUTHWEST GULF OF ALASKA SWINGS
NORTHWARD TO A POSITION SOUTH OF KODIAK ISLAND ON SATURDAY AFTERNOON.
THIS WILL KEEP THE FLOW ALOFT SOUTHERLY AND BRING ANOTHER ROUND OF
RAIN TO THE REGION BEGINNING LATE SATURDAY AFTERNOON.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...
THE BERING SEA AND ALEUTIANS WILL REMAIN IN A WEAK AND BROAD CYCLONIC
FLOW THROUGH FRIDAY AFTERNOON AS A RIDGE BUILDS IN FROM THE WEST. THIS
WILL HELP PRODUCE AREAS OF FOG MAINLY IN THE WESTERN BERING AND
ADJACENT COASTAL WATERS TONIGHT THROUGH SATURDAY. PRECIPITATION IS
EXPECTED TO REMAIN MOSTLY RAIN WITH SOME AREAS SEEING A RAIN/SNOW
MIX.

A FRONT ASSOCIATED WITH A DEVELOPING LOW OFF THE KAMCHATKA PENINSULA
WILL BEGIN TO IMPACT THE WESTERN ALEUTIANS FRIDAY AFTERNOON. THIS FRONT
WILL BRING LOWER END GALE FORCE WINDS TO THE WESTERN ALEUTIANS AND SOUTHWEST
BERING SEA FRIDAY AFTERNOON THROUGH SATURDAY.

&&

.LONG TERM FORECAST (DAYS 3 THROUGH 7)...
A NARROW RIDGE WILL BUILD OVER THE EASTERN HALF OF THE MAINLAND BY THIS
WEEKEND WHICH LOOKS TO KEEP RELATIVELY BENIGN CONDITIONS CHARACTERIZED
BY DIURNAL SHOWER ACTIVITY OVER SOUTHCENTRAL AK THROUGH EARLY NEXT WEEK.
THE COMPLICATING FACTOR WILL BE A WEAK UPPER TROUGH THAT EVOLVES INTO
A CUTOFF LOW OVER THE SOUTHWEST MAINLAND BY MONDAY. THE GFS IS MORE
AGGRESSIVE IN MOVING THIS FEATURE EASTWARD AND BRINGING ADDITIONAL
PRECIPITATION INTO SOUTHCENTRAL AK WHEREAS MOST OF THE GUIDANCE KEEPS
THE LOW FARTHER BACK TO THE WEST AS IT GRADUALLY WEAKENS. THE FORECAST
REFLECTS THE LATTER SCENARIO FOR NOW WHICH WOULD KEEP MORE WIDESPREAD
SHOWER ACTIVITY CONFINED TO SOUTHWEST AK AND LIMIT IMPACTS FOR SOUTHCENTRAL
AK PRIMARILY TO INCREASED CLOUD COVER.

MEANWHILE A BROAD UPPER LOW WHICH WILL BE LOCATED OVER THE WESTERN
ALEUTIANS THIS WEEKEND WILL MOVE TO NEAR THE ALASKA PENINSULA BY THE
MIDDLE OF NEXT WEEK. THIS WILL RESULT IN WINDY AND RAINY CONDITIONS
ACROSS MUCH OF THE ALEUTIAN CHAIN WITH A PERIOD OF WETTER CONDITIONS
ALSO RETURNING TO SOUTHCENTRAL AK AND ESPECIALLY THE GULF COAST BY
THE MIDDLE OF THE WEEK AS A FRONT PUSHES INTO THE GULF.

&&

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

&&

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