Area Forecast Discussion
Issued by NWS Anchorage, AK

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26
000
FXAK68 PAFC 202136
AFDAFC

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
136 PM AKDT SAT SEP 20 2014

.ANALYSIS AND UPPER LEVELS...

A BAGGY YET HIGHLY AMPLIFIED TROUGH PERSISTS OVER THE MAINLAND OF
ALASKA EXTENDING SOUTH INTO THE MID-LATITUDES OF THE PACIFIC OCEAN
ALONG 150W. NUMEROUS STRONG CONVECTIVE SHOWERS WITH ISOLATED
LIGHTNING STRIKES ARE OCCURRING OVER THE NORTHERN GULF AND PRINCE
WILLIAM SOUND GIVEN IMPRESSIVE INSTABILITY IN THIS REGION. HIGH
PRESSURE OVER THE CENTRAL BERING HAS MOVED INTO THE EASTERN BERING
TODAY...WITH A SECOND TROUGH ON THE NORTHEAST SIDE OF THE RIDGE
CURRENTLY DIGGING INTO NORTHERN AND CENTRAL ALASKA TOWARD THE
BASE OF THE LONGWAVE TROUGH OVER THE MAINLAND. OUT WEST...A
KAMCHATKA LOW IS MOVING TOWARD THE FAR WESTERN ALEUTIANS AND
WESTERN BERING.

&&

.MODEL DISCUSSION...

MODELS HAD BEEN STRUGGLING WITH A SURFACE LOW THAT FORMS IN THE
MID-LATITUDES THIS AFTERNOON AND THEN RACES NORTH AND STRENGTHENS
AS IT ENTERS THE GULF SUNDAY MORNING AND THEN MOVES INLAND
SOMEWHERE OVER THE NORTH-CENTRAL OR NORTHEAST GULF. MODEL TRENDS
OVER THE LAST 24 HOURS HAVE MOVED FROM A STRONGER LOW MOVING INLAND
WEST OF YAKUTAT TO A WEAKER LOW MOVING INLAND NEAR OR EAST OF
YAKUTAT...THOUGH THERE ARE STILL NOTABLE OUTLIER SOLUTIONS. FOR
THIS TIME PERIOD DEFERRED TO NWS JUNEAU OFFICE WHICH PREFERRED THE
EUROPEAN (EC) MODEL...WHICH IN THE PAST HAS BEEN THE SUPERIOR
MODEL IN THIS TYPE OF SITUATION. OTHERWISE...MODELS ARE IN GOOD
AGREEMENT THROUGH THE REST OF THE SHORT TERM OTHERWISE.


&&

.SHORT TERM FORECAST...

SOUTHCENTRAL ALASKA...

WITH IMPRESSIVE INSTABILITY EVEN FOR THIS TIME OF THE YEAR OVER
THE NORTHERN GULF WILL KEEP ISOLATED THUNDERSTORM WORDING IN THE
FORECAST FROM EASTERN PRINCE WILLIAM SOUND INTO THE NORTHERN
GULF THROUGH SUNSET THIS EVENING. LIKEWISE...CONVECTIVE SHOWERS
ARE EXPECTED INLAND AS WELL TO LOCATIONS SUCH AS THE MATANUSKA
VALLEY AND ANCHORAGE THIS AFTERNOON AND EVENING UNDER THIS UPPER
LEVEL LOW.

AS LOW PRESSURE MOVES NORTH OVER THE EASTERN GULF
SUNDAY...NORTH TO WEST OFFSHORE WINDS SHOULD DEVELOP BY THE
AFTERNOON SUNDAY...AND COULD BE LOCALLY GUSTY WHERE THOSE WINDS
ARE CHANNELED THROUGH MOUNTAIN GAPS SUCH AS THOMPSON PASS AND NEAR
WHITTIER. IN ADDITION...THE DIGGING DISTURBANCE INTO THE BASE OF
THE LONGWAVE TROUGH OVER SOUTHCENTRAL WILL ALLOW FOR AN AREA OF
RAIN TO DEVELOP MAINLY OVER THE SUSITNA VALLEY TONIGHT INTO
SUNDAY...BUT THIS RAIN WILL BEGIN TO DISSIPATE THROUGH SUNDAY AS
DRIER AIR BEGINS TO MOVE INTO THE REGION AS A NORTH AND WESTERLY
OFFSHORE PRESSURE GRADIENT BEGINS TO DEVELOP. THIS OFFSHORE FLOW
WILL HELP INCREASE NORTHWEST WINDS INTO TURNAGAIN ARM AND EVEN
ALONG THE HILLSIDE...WHERE GUSTS 25 TO 30 MPH ARE POSSIBLE SUNDAY
AFTERNOON INTO THE OVERNIGHT HOURS.

THE CLEARING AND DRYING TREND CONTINUES THROUGH THE EARLY PART OF
THE WEEK EXCEPT AROUND THE PRINCE WILLIAM SOUND REGION WHERE A
CHANCE OF SHOWERS WILL PERSIST. THIS WILL RESULT IN DRAMATICALLY
COOLER NIGHTTIME AND MORNING TEMPERATURES ACROSS THE
AREA...WITH LOWS INTO THE UPPER 20S TO UPPER 30S INLAND BY TUESDAY
MORNING.


SOUTHWEST ALASKA...

CONVECTIVE SHOWERS ARE EXPECTED THIS AFTERNOON AND EVENING ALONG
AND JUST WEST OF THE ALASKA AND ALEUTIAN RANGES...BUT HIGH
PRESSURE BUILDS IN LATE TONIGHT INTO THE EARLY PART OF THE WEEK
AND WILL BRING MAINLY TRANQUIL WEATHER AND LIGHT WINDS...EXCEPT A
BIT GUSTIER WINDS OVER INTERIOR BRISTOL BAY AREAS NEAR OUTFLOW
CHANNELS TOWARD THE GULF SUNDAY INTO MONDAY. THIS CLEARING TREND
WILL ALLOW FOR A NOTABLE COOLING TREND IN TEMPERATURES OVER THE
SOUTHWEST MAINLAND FOR THE NEXT SEVERAL NIGHTS.


BERING SEA/ALEUTIANS...

HIGH PRESSURE BUILDING EASTWARD CONTINUES TO TAPER THE NORTHWEST
FLOW AND SHOWERS OVER THE EASTERN BERING AND ALASKA PENINSULA
THROUGH THIS EVENING. A WEAKENING FRONT AND LOW PRESSURE SYSTEM
OVER THE WESTERN ALEUTIANS WILL LINGER THROUGH THE WEEKEND.
OTHERWISE RELATIVELY TRANQUIL CONDITIONS WILL PERSIST OVER THIS
REGION THROUGH EARLY NEXT WEEK.

&&

.LONG TERM FORECAST (WED SEPT 24 - SAT SEPT 27)...

BEGINNING THE EXTENDED PERIOD ON WEDNESDAY...THE REGION WILL BE
UNDER THE INFLUENCE OF A SPLIT JET STREAM REGIME...WITH A DISTINCT
POLAR JET TO OUR NORTH...AND A MUCH STRONGER AND ACTIVE
SUBTROPICAL JET STREAM ACROSS THE NORTH PACIFIC. THIS WILL BE A
SOMEWHAT LARGE SHIFT FROM THE PREVIOUS TWO WEEKS WHERE THE
SUBTROPICAL JET STREAM WAS IN PHASE WITH THE POLAR JET...HELPING
TO PROMOTE A VERY ACTIVE PATTERN FOR ALASKA. THIS SPLIT JET STREAM
PATTERN WILL LINGER THROUGH FRIDAY...WITH UPPER LEVEL DISTURBANCES
TRACKING THROUGH NORTHERN AND CENTRAL ALASKA. THE NET EFFECT WILL
BE COOLER AND DRY AIR OCCASIONALLY DRIVING SOUTHWARD INTO THE GULF
OF ALASKA. OTHERWISE...THIS WILL LIKELY REPRESENT THE DRIEST AND
OVERALL MOST PLEASANT STRETCH OF WEATHER IN SEPTEMBER OVER THE
PAST THREE YEARS. OUTFLOW GAP WINDS INTO THE GULF OF ALASKA WILL
PEAK WEDNESDAY AND THEN POSSIBLY AGAIN ON SATURDAY. EXPECT THE
COLDEST TEMPERATURES OF THE YEAR ACROSS MANY LOCATIONS AS
OVERNIGHT TEMPERATURES PLUMMET WITH FAVORABLE RADIATIONAL COOLING.
BY FRIDAY...THE ECMWF IS HINTING AT A NORTH PACIFIC LOW TRACKING
INTO SOUTHWEST AND EVENTUALLY SOUTHCENTRAL...BUT CURRENTLY THAT IS
A LOW PROBABILITY SOLUTION.


&&

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

&&

$$

CC/JA SEP 14



USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.