Area Forecast Discussion
Issued by NWS Anchorage, AK

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FXAK68 PAFC 271301
AFDAFC

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
501 AM AKDT MON APR 27 2015

.ANALYSIS AND UPPER LEVELS...
A SOMEWHAT CHAOTIC PATTERN IS BEGINNING TO EMERGE ALOFT. THE UPPER
LEVEL JET IS IN A CONVERGENT SPLIT FLOW JUST SOUTH OF THE
ALEUTIANS AROUND 140 KTS BEFORE TURNING TO THE SOUTH AROUND THE
GULF/NORTHEAST PACIFIC STORM SYSTEM. THE FEATURES ON THE POLEWARD
SIDE OF THE JET ARE NOW IN A WEAKENING AN MEANDERING STATE
WITHOUT THE MEAN FLOW TO GUIDE THEM. THE BIGGEST PLAYER IS THE
UPPER LOW TAKING UP RESIDENCE IN THE GULF. THE FRONT AND SURFACE
SYSTEM ASSOCIATED WITH THE UPPER LOW CONTINUES TO WEAKEN...THE
MAIN ACTIVITY WITH THIS SYSTEM WILL BE PIECES OF ENERGY COMING
AROUND THE TROUGH THE CURRENTLY STILL HAVE JET SUPPORT TOWARD THE
PANHANDLE. THE OTHER CYCLONIC CIRCULATION IS SPINNING OVER THE
BERING SEA ALONG WITH A FRONT STRETCHING OUT FROM THE PRIBILOF
ISLANDS THROUGH THE END OF THE ALASKA PENINSULA. SKIES ARE MOSTLY
CLEAR OVER THE SOUTHWEST MAINLAND WHILE EASTERLY FLOW IS BRINGING
PLENTY OF MIDDLE AND HIGH CLOUDS OVER THE SOUTHCENTRAL. WINDS
AROUND THE AREA ARE GENERALLY LIGHT WHILE TEMPERATURES ARE
HOVERING SLIGHTLY ABOVE NORMAL FOR THIS TIME OF YEAR.

&&

.MODEL DISCUSSION...
GUIDANCE IS BEGINNING TO HANDLE THE COMPLEXITIES OF THE UPCOMING
PATTERN A LITTLE BIT BETTER THAN PREVIOUS DAYS. SYNOPTICALLY...THE
MAIN DIFFERENCES BETWEEN SOLUTIONS ARE HOW A SHORTWAVE TROUGH
COMING OFF THE EAST ASIAN JET INTERACTS FIRST WITH THE BERING SEA
SYSTEM AND THEN WITH THE GULF SYSTEM. ON SHORTER TIME AND SMALLER
SCALES...THE MAIN DIFFERENCES ARE PRECIPITATION CHANCES ACROSS THE
MAINLAND WITH ANY EASTERLY WAVE COMING AROUND THE UPPER LOW.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (TODAY THROUGH TUESDAY)...

EASTERLY DISTURBANCES MOVING FROM THE GULF COAST INLAND WILL
SLOWLY CONTINUE TO MOISTEN THE ATMOSPHERE AND BRING THE NUMEROUS
SHOWERS TO COASTAL LOCATIONS AND ISOLATED TO SCATTERED SHOWERS ON
THE LEE SIDE SLOPES (ANCHORAGE AND THE MAT VALLEY). INITIAL DRY
AIR WILL BE VERY TOUGH TO OVERCOME AT FIRST BUT AS THE DAY
PROGRESSES CLOUD BASES WILL GRADUALLY LOWER. MORE WIDESPREAD RAIN
AND SNOW IS EXPECTED TO DEVELOP ESPECIALLY ACROSS THE EASTERN COPPER
RIVER BASIN AS A WEAK EASTERLY SHORTWAVE MOVES IN AND TAKES
ADVANTAGE OF SOME OF THE EXISTING INSTABILITY OF THE AIRMASS. THE
MOISTURE WITH THIS WAVE WILL HAVE A HARD TIME GETTING OVER THE
CHUGACH RANGE BY TUESDAY...SO ONLY ISOLATED SHOWERS ARE EXPECTED
AT THIS TIME. HOWEVER...UP-INLET SOUTHWEST FLOW DEVELOPING TONIGHT
OVER THE BARREN ISLANDS AND KAMISHAK BAY REGIONS WILL PUSH A
STRATUS DECK ALL THE WAY TO ANCHORAGE FOR TUESDAY...RESULTING IN
CLOUDY AND COOL CONDITIONS LIKELY UNTIL EARLY EVENING.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (TODAY THROUGH TUESDAY)...

THE OVERALL DRY PATTERN IS EXPECTED TO CONTINUE THROUGH THE BEGINNING
OF THE WEEK WITH AFTERNOON HIGH TEMPERATURES IN THE 50S AND RELATIVE
HUMIDITY VALUES DROPPING INTO THE 20 PERCENT RANGE. IT IS
BEGINNING TO LOOK MORE FAVORABLE FOR SCATTERED SHOWERS AND
ISOLATED THUNDERSTORMS TO DEVELOP WEST OF THE ALASKA RANGE THIS
AFTERNOON AND EVENING AS A SHORTWAVE PUSHES WESTWARD ACROSS THE
ALASKA RANGE. THE BIG QUESTION FOR THUNDERSTORM DEVELOPMENT WILL
BE HOW MUCH MOISTURE IS AVAILABLE AS A VERY DRY SURFACE LAYER IS
ANTICIPATED. THIS WOULD LIKELY EVAPORATE MUCH OF THE RAINFALL
PRIOR TO IT REACHING THE SURFACE...POSSIBLY LEADING TO A DRY
THUNDERSTORM OR TWO. SCATTERED RAIN SHOWERS WILL BE SEEN AGAIN ON
TUESDAY BUT CONDITIONS APPEAR LESS FAVORABLE FOR ANY THUNDERSTORM
DEVELOPMENT WITH LITTLE TO NO VERTICAL LIFT PRESENT.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (TODAY AND TUESDAY)...

AN AREA OF LOW PRESSURE OVER THE WESTERN ALEUTIANS WILL REMAIN
NEARLY STATIONARY DURING THE DAY TODAY WHILE PUSHING A FRONT
ACROSS THE PRIBILOF ISLANDS AND DUTCH HARBOR. THIS WILL BRING
ANOTHER SHOT OF RAIN (MAYBE A FEW FLURRIES NEAR ST PAUL) DURING
THE DAY TODAY. SOME AREAS OF FOG HAVE DEVELOPED IN THE STABLE AIR
MASS AHEAD OF THE FRONT BUT WILL LIKELY MIX OUT DURING THE
AFTERNOON HOURS TODAY. A SECOND AREA OF LOW PRESSURE WILL ENTER
THE BERING SEA THIS AFTERNOON AND ABSORB THE WEAKENING LOW OVER
THE WESTERN BERING SEA. THE WILL LEAD TO AN ELONGATED AREA OF LOW
PRESSURE OVER THE BERING SEA...BRINGING WIDESPREAD RAINFALL TO
THE ALEUTIAN ISLANDS THROUGH MIDWEEK.

&&

.LONG TERM FORECAST (WEDNESDAY THROUGH MONDAY)...
THE 130 KNOT ZONAL JET STREAM OVER THE ALEUTIANS WILL CONTINUE TO
TRACK TO THE EAST HEADING INTO THE WEEKEND. THIS WILL BRING
SEVERAL LOWS FROM THE BERING SEA INTO THE LOWER GULF THROUGH
WEDNESDAY. THESE SYSTEMS WILL BE ACCOMPANIED BY SHOWERS IN THE
GULF AND ALONG THE COASTAL AREAS. BY THURSDAY AND UPPER LEVEL
RIDGE WILL BUILD INTO THE WESTERN GULF. THIS RIDGE WILL BECOME
AMPLIFIED WITH A SOUTHEAST TO NORTHWEST PLACEMENT BY SATURDAY.
THIS SYNOPTIC FEATURE WILL BRING WARMER TEMPERATURES AND DRIER
CONDITIONS TO THE SOUTHCENTRAL REGION. THIS WILL INCREASE THE
CHANCES FOR AFTERNOON TO EVENING SHOWERY ACTIVITY FOR THE INLAND
AREAS.

THIS SYSTEM WILL BRING RAIN OUT AHEAD OF THIS SYSTEM AND RAIN
SHOWERS ON THE BACKSIDE OF THE LOW AS IT MOVES ALONG THE CHAIN. BY
WEDNESDAY ANOTHER WEAK LOW FORMS IN THE WESTERN BERING SEA BEFORE
DIMINISHING INTO A TROUGH OVER THE REGION THROUGH FRIDAY. THEN
ANOTHER NORTH PACIFIC APPROACHES THE WESTERN ALEUTIANS BY
SATURDAY. THIS WILL CONTINUE TO BRING WARMER TEMPERATURES AND RAIN
SHOWERS FOR THE BERING SEA AND THE ALEUTIAN CHAIN HEADING INTO THE
WEEKEND. THE MODELS WERE STRUGGLING WITH THE TIMING AND PLACEMENT
OF THESE LOWS AS THEY TRACK THROUGH THE BERING SEA. THIS RESULTED
IN HPC USING A BLEND OF THE ECMWF/GFS ENSEMBLE MEANS THROUGH THE
NEXT SEVERAL DAYS.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...GALE 176 177.
FIRE WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...MTL
SOUTHCENTRAL ALASKA...CC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MC
LONG TERM...PD


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