Area Forecast Discussion
Issued by NWS Anchorage, AK

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FXAK68 PAFC 251250
AFDAFC

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
450 AM AKDT SAT JUL 25 2015

.ANALYSIS AND UPPER LEVELS...

THE PROGRESSIVE AND SHORT WAVELENGTH UPPER AIR PATTERN CONTINUES
ACROSS THE REGION AS THE LAST LOW TO IMPACT SOUTHERN ALASKA
CONTINUES TO MOVE INTO THE YUKON. A TRAILING SHORTWAVE RIDGE IS
BUILDING INTO SOUTHCENTRAL ALASKA AHEAD OF ANOTHER BERING
LOW...CURRENTLY CENTERED OVER SAINT MATTHEW ISLAND IN THE NORTHERN
BERING SEA. WATER VAPOR IMAGERY IS ALSO DEPICTING A LARGE PLUME OF
HIGH CLOUDS ORIGINATING FROM THE NORTHEAST PACIFIC. THIS SATELLITE
FEATURE IS TIED TO A SUBTROPICAL WARM FRONT...WHICH IS CURRENTLY
RAPIDLY MOVING ACROSS THE CENTRAL AND EASTERN ALEUTIANS AS IT
MERGES WITH THE AFOREMENTIONED BERING LOW. WITH TIME...THE
SHORTWAVE RIDGE OVER SOUTHCENTRAL WILL CONTINUE TO MOVE EAST AS
THE COMBINED WARM FRONT/UPPER LOW OVER THE BERING SEA MAKES
EASTWARD PROGRESS THROUGH TONIGHT. THIS WILL USHER IN A PERIOD OF
CLOUDIER...COOLER...AND WETTER CONDITIONS FOR SOUTHERN ALASKA
THROUGH THE WEEKEND.

&&

.MODEL DISCUSSION...

THE NUMERICAL MODELS ARE IN EXCELLENT AGREEMENT IN THE SHORT TERM
PERIOD WITH THE LARGE SCALE SYNOPTIC PATTERN...AND REMAIN SO
THROUGH MIDWEEK BEFORE THEY BEGIN TO DIVERGE. THERE REMAINS SOME
NOTABLE SHORT TERM MESOSCALE DISCREPANCIES WITH RESPECT TO
PRECIPITATION DISTRIBUTION...ESPECIALLY WHERE AND HOW HEAVY THE
MOST INTENSE AXIS OF PRECIPITATION IS ACROSS PORTIONS OF SOUTHWEST
AND SOUTHCENTRAL. GIVEN THE DEVELOPMENT OF THE UPPER LOW RIGHT
OVER SOUTHERN ALASKA TODAY AND TONIGHT...THE LARGE TERRAIN BARRIERS
SUCH AS THE ALASKA RANGE AND CHUGACH RANGE WILL GENERALLY RESULT
IN LARGE NUMERICAL MODEL ERRORS GIVEN THE COMPLEX TERRAIN. RAPID
DEVELOPMENT OF THE SYSTEM AND THE POTENTIAL FOR CONVECTION AS THIS
SYSTEM DEVELOPS FURTHER COMPLICATES THE PRECIPITATION DISTRIBUTION
TOMORROW AND EARLY SUNDAY. WITH THAT SAID...THE MODELS HAVE COME
INTO MUCH BETTER AGREEMENT FROM PREVIOUS DAYS...WITH WIDESPREAD
RAIN AND SHOWERS EXPECTED ACROSS MOST OF THE REGION.
PRECIPITATION AMOUNTS WILL BE MUCH HARDER TO NAIL DOWN DUE TO
REASONS NOTED ABOVE...WITH THE GREATEST DISCREPANCIES NOTED
DOWNWIND OF THE CHUGACH MOUNTAINS AND ALASKA RANGE.
HOWEVER...GIVEN THE IMPRESSIVE MESOSCALE JET DYNAMICS AND
POTENTIAL ENHANCEMENT FROM LOCAL MOUNTAIN TERRAIN FORCING AND
AFTERNOON INSTABILITY...THERE IS A CHANCE FOR A HEAVY RAIN AXIS TO
FORM SOMEWHERE NEAR ANCHORAGE TOWARD THE NORTHERN KENAI PENINSULA
TOMORROW AFTERNOON AND EVENING.

&&

.FIRE WEATHER...

ACROSS SOUTHWEST...MOISTURE STREAMING IN AHEAD OF A BERING LOW
WILL BRING WETTING RAINS AND CONTINUED HUMID CONDITIONS TO MOST OF
THE REGION THROUGH SUNDAY. THE BEST CHANCES FOR WETTING RAINS WILL
BE ALONG THE ALASKA RANGE AND INTO THE LOWER KUSKOKWIM VALLEY AS A
STRONG JET MAXIMA MOVES OVER THAT AREA.

ACROSS SOUTHCENTRAL...THE DAY WILL START OUT LARGELY DRY BEFORE A
ROBUST LOW PRESSURE SYSTEM FROM THE BERING MOVES INTO SOUTHCENTRAL
AND THE GULF OF ALASKA. SHOWERS AND POSSIBLY THUNDERSTORMS WILL
FORM OVER THE HIGHER TERRAIN OF THE ALASKA RANGE AND TALKEETNA
MOUNTAINS BY MIDDAY...WITH RAIN AND SHOWERS OVER THE CHUGACH AND
KENAI RANGES. THUNDERSTORMS WILL ALSO FORM ACROSS THE COPPER BASIN
AS MUCH DRIER AND WARMER CONDITIONS REMAIN OVER THAT AREA.
SOUTHERLY WINDS WILL ALSO INCREASE IN THE AFTERNOON ACROSS THE
COPPER BASIN...BUT ARE EXPECTED TO REMAIN BELOW CRITICAL
CRITERIA...ALTHOUGH MIN RH VALUES WILL BE NEAR THE 30 PERCENT
RANGE. BY SUNDAY...MORE MOIST AND WET CONDITIONS WILL OVERSPREAD
ALL OF SOUTHCENTRAL...INCLUDING THE COPPER BASIN AS THE LOW TRACKS
OVER THE AREA.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 THROUGH 3)...
THE CLEAR SKIES AROUND THE SOUTHCENTRAL WILL COME TO AN END THIS
AFTERNOON AS A LOW IN THE EASTERN BERING BRINGS A STRONG PUSH OF
RAIN TO THE AREA. HOWEVER...THERE IS SOME UNCERTAINTY AS TO HOW
QUICKLY THE HEAVIER RAIN WILL MAKE IT TO AREAS AROUND ANCHORAGE.
THERE IS STRONGER SUPPORT FOR HEAVIER RAIN COMING IN EARLIER IN
THE AFTERNOON BUT SOME SOLUTIONS HOLD OFF ON THE HEAVIER RAIN
UNTIL THE EVENING. FOR LOCATIONS AROUND THE SUSITNA VALLEY AND
FOLLOWING THE ALASKA RANGE TO THE EAST THERE WILL BE ENOUGH
INSTABILITY IN THE AFTERNOON FOR THUNDERSTORMS TO DEVELOP BEFORE
THE BIGGER PUSH OF WIDESPREAD RAIN. THE RAIN WILL CONTINUE THROUGH
THE WEEKEND AS THE LOW TRACKS INTO THE GULF BEFORE PUSHING OFF TO
THE EAST AND WEAKENING. AS THE LOW DEPARTS A SHOWERY REGIME WILL
COME BACK TO THE SOUTHCENTRAL MONDAY.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 THROUGH 3)...
RAIN WILL REMAIN THE DOMINATE WEATHER FEATURE FOR SOUTHWEST
ALASKA THROUGH SUNDAY NIGHT AS THE LOW NEAR NUNIVAK ISLAND THIS
MORNING PUSHES INLAND AND MOVES INTO THE NORTHERN GULF OF ALASKA
SUNDAY AFTERNOON. THE FIRST ROUND OF RAINFALL IS TRANSITING THE
AREA THIS MORNING AS BANDS OF SHOWERS AS THE INITIAL SHORTWAVE
MOVES NORTHEASTWARD. AS THE LOW APPROACHES THE COAST LATER THIS
AFTERNOON...THIS WILL TRANSITION TO MOSTLY STRATIFORM RAIN. AS THE
LOW MOVES TO THE NORTHERN GULF COAST ON SUNDAY THE FLOW OVER THE
REGION WILL TURN NORTH TO NORTHWESTERLY...TURNING THE FLOW
PERPENDICULAR TO THE ALASKA/ALEUTIAN RANGES AND KEEPING THE REGION
GENERALLY UNSTABLE. THIS WILL RESULT IN MAINLY STRATIFORM RAINFALL
ALONG THE MOUNTAINS WITH SHOWERS OVER THE REMAINDER OF THE REGION.
PRECIPITATION WILL BEGIN TO TAPER OFF SUNDAY NIGHT MAINLY WEST OF
THE KILBUCK MOUNTAINS AS THE LOW MOVES FURTHER AWAY FROM THE
REGION AND DRIER AIR ADVECTS IN FROM THE NORTH.

WINDS WILL REMAIN ELEVATED OVER THE COASTAL REGIONS THROUGH
TOMORROW AS HIGH BUILDS INTO THE NORTH PACIFIC SOUTH OF THE
ALEUTIAN CHAIN.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 THROUGH 3)...
WINDS WILL REMAIN AT SMALL CRAFT LEVELS THROUGH THE PRIBILOF
ISLANDS AND SOUTH OF THE EASTERN ALEUTIANS/ALASKA PENINSULA
THROUGH TONIGHT AS THE LOW NEAR NUNIVAK ISLAND MOVES INLAND AND
THE RIDGE CONTINUES TO BUILD OVER THE AREA. STEADY RAINFALL IS
EXPECTED OVER MOST OF THE ALEUTIANS TODAY BEFORE TAPERING OFF LATE
TONIGHT. SHOWERS WILL REMAIN THROUGH SUNDAY OVER THE EASTERN HALF
OF THE AREA IN NORTHWEST FLOW BEHIND THE LOW AS IT MOVES INTO THE
NORTHERN GULF. A SECOND SHORTWAVE WILL DIVE THROUGH THE CENTRAL
BERING SEA AND THE PRIBILOF ISLANDS SUNDAY NIGHT...BRINGING MOSTLY
RAIN WITH SOME WINDS TO 25 KT OVER THE WESTERN BERING SEA.

HIGH PRESSURE WILL BUILD OVER THE WESTERN ALEUTIANS THROUGH
SUNDAY...BRINGING DRIER CONDITIONS AND LIGHT WINDS FROM ATKA
WESTWARD.

&&

.LONG TERM FORECAST (DAYS 4 THROUGH 7)...

THE BERING AND ALEUTIANS WILL STAY IN A RELATIVELY QUIETER
PATTERN THROUGH TUESDAY. AN UPPER LEVEL RIDGE BUILDS ACROSS THE
WESTERN BERING WITH SCATTERED SHOWERS STILL POSSIBLE ACROSS
EASTERN AREAS DUE TO WEAK SHORTWAVES MOVING ALONG THE BACKSIDE OF
THE MAIN LOW MOVING INTO THE GULF. FORECAST CONFIDENCE BEGINS TO
DIP BELOW AVERAGE AS MODELS BEGIN TO STRUGGLE WITH AN UPPER LEVEL
LOW TRACKING THROUGH THE NORTHERN MAINLAND TUESDAY NIGHT AND INTO
THE EASTERN BERING BY THE END OF NEXT WEEK. THE ECMWF IS THE MOST
BULLISH OF THE MODELS WITH THIS FEATURE...WITH THE CANADIAN MODEL
IN MOST RECENT RUNS STARTING TO TREND TOWARD THE EC SOLUTION. THE
SURFACE REFLECTION OF THIS IS A SURFACE CIRCULATION TRACKING
THROUGH THE EASTERN ALEUTIANS BY NEXT WEEKEND...WITH AN
ASSOCIATED FRONTAL SYSTEM TRACKING INTO THE NORTHERN GULF.

&&


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

&&

$$

SYNOPSIS AND MODEL DISCUSSION...AHSENMACHER
SOUTHCENTRAL ALASKA...DWK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...RF
LONG TERM...TP


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