Area Forecast Discussion
Issued by NWS Anchorage, AK

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FXAK68 PAFC 260021
AFDAFC

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
421 PM AKDT TUE AUG 25 2015

.ANALYSIS AND UPPER LEVELS...
THE MAIN RIDGE AXIS HAS SHIFTED EASTWARD...WITH SOUTHWESTERLY
ONSHORE FLOW IN PLACE ACROSS MUCH OF THE MAINLAND. A
STRENGTHENING SOUTHWESTERLY JET OVER THE BERING SEA IS RAPIDLY
PULLING SUBTROPICAL MOISTURE INLAND. AN UPPER WAVE MOVING INTO
SOUTHWEST ALASKA IS TAPPING INTO THE MOISTURE WITH RAIN HEADED
ONSHORE ALONG THE KUSKOKWIM DELTA. THE STRONG UPPER HIGH
REFLECTED AT THE SURFACE SOUTH OF THE CENTRAL ALEUTIANS IS
ALLOWING FOR THE PERSISTENCE OF WIDESPREAD SUBSIDENCE AND LOW-
LEVEL CLOUDS OVER THE EASTERN BERING SEA AND THE ALEUTIANS.
SATELLITE IMAGERY SHOWS AN UPPER WAVE IN THE WESTERN BERING SEA
ROTATING AROUND AN ELONGATED UPPER LOW OVER THE NORTHERN KAMCHATKA
PENINSULA...WHICH IS PROVIDING ADDITIONAL LIFT FOR RAIN OVER THE
BERING SEA AND WESTERN ALEUTIANS IN A MOISTURE RICH ENVIRONMENT.
THE PRESSURE GRADIENT OVER THE CENTRAL BERING SEA IS STRENGTHENING
AND ALLOWING FOR GUSTY SOUTHWESTERLY WINDS AS A SURFACE LOW UNDER
THE UPPER WAVE STRENGTHENS IN THE FAVORABLE LEFT EXIT REGION OF
THE JET. FURTHER EAST...SUBTROPICAL MOISTURE WRAPPING AROUND THE
RIDGE AXIS IS BRINGING PARTLY CLOUDY CONDITIONS TO MUCH OF
SOUTHCENTRAL.

&&

.MODEL DISCUSSION...
MODELS HAVE COME INTO BETTER AGREEMENT WITH THE SYSTEM MOVING
INTO THE MAINLAND TOMORROW. THE RIDGE WILL FLATTEN OUT OVERNIGHT
AND A STRONG ZONAL JET WILL QUICKLY MOVE THE SYSTEM INLAND...
BRINGING THE HEAVIEST PRECIPITATION NORTH OF THE AOR WHERE THE
JET DYNAMICS FAVOR THE STRONGEST LIFT. THE SYSTEM WILL QUICKLY
MOVE EAST IN ADVANCE OF THE UPPER JET THURSDAY. THE EC BECOMES THE
OUTLIER THU MORNING AS A WAVE ROTATING AROUND THE BASE OF A
DIGGING ARCTIC TROUGH INTERACTS WITH REMNANT TROPICAL ENERGY IN
THE WESTERN BERING SEA. THE EC IS MOVING THE WAVES THROUGH
SOUTHCENTRAL AND INTO THE GULF FASTER THAN THE AMERICAN MODELS.
THE TEMPORAL DIFFERENCES BETWEEN THE MODELS HANDLING OF THE
SHORTWAVES WILL IMPACT THE TIMING OF THE ONSET OF AN UNSEASONABLY
STRONG OUTFLOW WIND EVENT TOWARDS THE END OF THE WORK WEEK...WITH
THE EC THE FASTEST IN THE ONSET AND THE NAM THE SLOWEST. A GFS/NAM
SOLUTION WAS THE PREFERRED CHOICE FOR THE TIMING OF THE WAVES
THURSDAY AND FRIDAY.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 AND 2)...
WHILE SOME PLACES ACROSS SOUTHCENTRAL ALASKA HAD SOME CLEARING
SKIES DURING THE DAY TODAY...CLOUD COVER WILL RETURN BACK TO THE
AREA TONIGHT AS THE UPPER LEVEL FLOW BECOMES MORE ZONAL AND
ADDITIONAL MOISTURE IS ADVECTED INTO THE AREA. THE ZONAL FLOW IS A
RESULT OF THE FLATTENING RIDGE OVER THE BERING SEA WHERE STRONG
HIGH PRESSURE WILL REMAIN OVER THE NORTH PACIFIC AND LOW PRESSURE
OVER THE ARCTIC/INTERIOR OF ALASKA. DUE TO THE TIGHT GRADIENT
ALOFT...A VERY POTENT JET WILL SET UP AND EJECT SOME POTENT
SHORTWAVES ACROSS SOUTHCENTRAL ALASKA WEDNESDAY AND THURSDAY. THE
FIRST WAVE WILL ENTER SOUTHCENTRAL ALASKA WEDNESDAY MORNING AS IS
CROSSES THE ALASKA RANGE. HOWEVER...IT APPEARS DOWNSLOPING WILL
INITIALLY LIMIT PRECIPITATION ACROSS PARTS OF THE SUSITNA VALLEY...
KENAI PENINSULA AND ANCHORAGE. HOWEVER...THE STRONG WESTERLY FLOW
WILL QUICKLY LEAD TO DEVELOPMENT OF A LEE SIDE TROUGH WHICH WILL
THEN BEND WINDS AROUND TO SOUTH TO SOUTHWEST AND PRODUCE UPSLOPE
RAINFALL ALONG THE TALKEETNA AND CHUGACH MOUNTAINS. THIS WILL
RESULT IN HIGHER PRECIPITATION AMOUNTS CLOSER TO THE MOUNTAINOUS
TERRAIN. AS THIS WAVE PROPAGATES FURTHER EAST...RAIN WILL
OVERSPREAD THE COPPER RIVER BASIN BY WEDNESDAY AFTERNOON/EVENING
AND WITH THE LACK OF DOWNSLOPING SEEN FURTHER WEST...SOME DECENT
RAINFALL AMOUNTS WILL BE OBSERVED. THE UPPER LEVEL JET APPEARS
STRONGER THAN YESTERDAY WHICH WILL HELP MOVE THE PRECIPITATION
ACROSS SOUTHCENTRAL ALASKA QUITE RAPIDLY.

THURSDAY WILL BE A SHOWERY DAY ACROSS MUCH OF SOUTHCENTRAL
ALASKA...WITH THE COPPER RIVER BASIN LIKELY TO SEE A CONTINUATION
OF STEADY RAINFALL. TWO SEPARATE WAVES WILL PUSH ACROSS
SOUTHCENTRAL ALASKA ABOUT 6 HOURS APART AND AS MENTIONED WILL KEEP
SHOWERS IN THE FORECAST. THESE TWO WAVES LOOK TO PHASE AS THEY
PUSH SOUTH INTO THE GULF OF ALASKA THURSDAY NIGHT WHICH WILL HELP
DEVELOP A SURFACE LOW WITH CONTINUED STRATIFORM PRECIPITATION
ALONG THE NORTHERN GULF COAST AND COPPER RIVER BASIN. COLDER AIR
WILL START FILTERING IN ON THE BACK SIDE OF THE LOW WITH DEEP
NORTHWESTERLY FLOW ALOFT.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)...
A STRONG UPPER LEVEL SHORT-WAVE AND JET STREAK COMBINED WITH
ABUNDANT TROPICAL MOISTURE WILL BRING WIDESPREAD RAIN TO
SOUTHWEST AK THROUGH WED. MOIST CONDITIONS WILL PERSIST BEHIND
THIS ON THURSDAY...THOUGH WITH A LACK OF UPPER LEVEL FORCING
PRECIP WILL BECOME MORE SHOWERY. GUSTY WIND WILL ACCOMPANY THE
RAIN...GRADUALLY DIMINISHING ON THU.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...
HIGH PRESSURE WILL KEEP THE EASTERN ALEUTIANS CLOUDY WITH SOME
FOG THROUGH THU...ALTHOUGH THE FRONT AFFECTING SOUTHWEST AK WILL
ALSO BRING GUSTY WINDS AND RAIN TO THE PRIBILOFS THAT WILL TAPER
OFF THROUGH THU. SHOWERS OVER THE CENTRAL ALEUTIANS WILL DIMINISH
WHILE THE FRONT KEEPS RAIN OVER THE WESTERN PORTION OF THE CHAIN
THROUGH THU. SMALL CRAFT WINDS WILL BE WIDESPREAD.

&&

.LONG TERM FORECAST (DAYS 3 THROUGH 7)...
BEGINNING THE EXTENDED PERIOD THURSDAY EVENING...THERE WILL BE A
STOUT CENTRAL NORTH PACIFIC RIDGE EXTENDING THROUGH THE WESTERN
AND CENTRAL BERING SEA...AND A STRONG SHORTWAVE TROUGH DIGGING
THROUGH SOUTHCENTRAL INTO THE NORTHERN GULF OF ALASKA. THIS WILL
SET IN MOTION A MODERATE TO STRONG OUTFLOW WIND EVENT ACROSS
PORTIONS OF SOUTHERN ALASKA...AND WILL ALSO USHER IN RAPID DRYING
BY FRIDAY AS OFFSHORE FLOW AND DOWNSLOPE WINDS INCREASE. THIS
OUTFLOW WIND EVENT WILL BE RATHER ROBUST FOR THIS TIME OF THE
YEAR...AND WILL REPRESENT THE STRONGEST OF THE YEAR...AIDED BY A
STRONG 130 KNOT JET STREAK AND COLD AIR ADVECTION. THE STRONGEST
GAP WINDS WILL BE FROM THE BARREN ISLANDS EASTWARD...INCLUDING
LOCATIONS AROUND SEWARD...WHITTIER...AND VALDEZ. THIS LOW WILL
ALSO BRING IN SOME MODIFIED ARCTIC AIR...DROPPING TEMPERATURES
BELOW CLIMATOLOGY BY THE WEEKEND. OUTFLOW WILL WEAKEN FRIDAY
AFTERNOON BEFORE ANOTHER VIGOROUS SHORTWAVE TROUGH MOVES INTO
SOUTHERN ALASKA LATE SATURDAY AFTERNOON INTO SUNDAY. THERE IS
ENOUGH DISCREPANCY WITH HOW THE LOW DEVELOPS AND THE EXACT
TRACK...WHICH WILL DRAMATICALLY INFLUENCE THE PRECIPITATION
DISTRIBUTION AND STRENGTH OF SURFACE WINDS IN THE GULF. THE ECMWF
AND GFS DIFFER ON THESE DETAILS...WITH THE ECMWF CURRENTLY
SUGGESTING A FARTHER SOUTH TRACK THROUGH THE CENTRAL GULF...AND
THE GFS FAVORING A NORTHERN GULF SOLUTION. BOTH OF THESE SOLUTIONS
HINT AT A STRONG GALE FORCE LOW TRACKING THROUGH THE GULF...BUT
THE CURRENT FORECAST REMAINS MUCH MORE CONSERVATIVE GIVEN
UNCERTAINTY. EITHER WAY...THIS SYSTEM WILL CLEAR THROUGH BY SUNDAY
EVENING WITH DRIER WEATHER ON MONDAY. THEREAFTER MUCH UNCERTAINTY
WITH THE REMNANTS OF FORMER TYPHOON ATSANI CLOUDS THE FORECAST FOR
NEXT WEEK...WITH A MORE GENERAL WPC ENSEMBLE APPROACH FAVORING
FLAT RIDGING MOVING EAST ACROSS THE BERING.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...GALE WARNING 179 180 181 185.
FIRE WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...RD
SOUTHCENTRAL ALASKA...MC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...DS
LONG TERM...JA


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