Area Forecast Discussion
Issued by NWS Anchorage, AK

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FXAK68 PAFC 271312
AFDAFC

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

.ANALYSIS AND UPPER LEVELS...

A BROAD UPPER LEVEL TROUGH NOW ENCOMPASSES ALL OF ALASKA AND THE
BERING SEA...WITH THREE DISTINCT CENTERS OF ACTION EMBEDDED WITHIN
THE TROUGH. RADAR AND SATELLITE IMAGERY DEPICT THE REMNANTS OF A
CLOSED LOW CURRENTLY MOVING OVER THE KENAI PENINSULA INTO THE
NORTHERN GULF. THIS IS THE LOW RESPONSIBLE FOR THE RECENT HEAVY
RAINFALL FOR PORTIONS OF SOUTHCENTRAL LAST NIGHT. THE ANCHORING
LOW FOR THE BROADER SCALE TROUGH IS CENTERED OVER THE BEAUFORT
SEA...WITH A FAST MOVING SHORTWAVE TROUGH IN NORTHERLY FLOW
TRACKING OVER THE PRIBILOF ISLANDS. THIS SYSTEM REPRESENTS THE
BACKSIDE OF THE UPPER TROUGH. WITH THE LOW OVER SOUTHCENTRAL NOW
FULLY OCCLUDED AND STACKED...THE ASSOCIATED WEAK SURFACE LOW WILL
WEAKEN AND REMAIN AS A TROUGH OF LOW PRESSURE OVER THE NORTHERN
GULF...WITH A GENERAL WEAK PRESSURE GRADIENT OVER THE REGION...AND
WEST-NORTHWESTERLY FLOW OVER SOUTHWEST ALASKA. WITH TIME...THE
CLOSED LOW SKIRTING THE NORTHERN GULF COAST WILL SLOWLY MOVE EAST
AND CONTINUE TO WEAKEN...WITH A SERIES OF NORTHERLY WAVES TRACKING
ALONG THE BACKSIDE OF THE UPPER LEVEL TROUGH INTO SOUTHWEST
ALASKA.

&&

.MODEL DISCUSSION...

THE NUMERICAL MODELS ARE NOW IN EXCELLENT AGREEMENT WITH THE LARGE
SCALE SYNOPTIC PATTERN THROUGH THE END OF THE SHORT TERM PERIOD AS
THE OUTLIER GFS HAS NOW COME INTO AGREEMENT WITH THE REMAINDER OF
THE MODEL SUITE. THE LARGEST UNCERTAINTY IN THE FORECAST ACTUALLY
REVOLVES AROUND PRECIPITATION PATTERNS GIVEN THE WEAK FLOW AND
HARD TO DIAGNOSE UPPER LEVEL WAVE FEATURES. THIS WILL LEND TO SOME
UNCERTAINTY IN THE FORECAST AS CLOUDS AND TEMPERATURES WILL BE
MORE CHALLENGING THAN NORMAL.

&&

.FIRE WEATHER...

ACROSS SOUTHWEST...BROAD UPPER LEVEL TROUGHING AND A SERIES OF
VERY SUBTLE WAVE DISTURBANCES MOVING INTO THE REGION FROM THE
BERING STRAIT WILL MODULATE THE WEATHER INTO MIDWEEK. ONSHORE FLOW
WILL LARGELY KEEP VERY MOIST CONDITIONS ACROSS MOST OF THE
REGION...WITH SHOWERS LINGERING LARGELY OVER THE BRISTOL BAY AND
LOWER KUSKOKWIM VALLEY REGIONS. WET THUNDERSTORMS WILL ONCE AGAIN
BE A THREAT...BUT WITH EXTENSIVE CLOUDS AND WEAKER
INSTABILITY...THESE WILL GENERALLY REMAIN ISOLATED.

ACROSS SOUTHCENTRAL...EXTENSIVE MOISTURE AND CLOUDS WILL REMAIN
OVER THE AREA AS A CLOSED UPPER LOW SLOWLY WEAKENS AND MIGRATES
INTO THE GULF OF ALASKA. THIS WILL KEEP THE REGION UNDER THE
CONSTANT THREAT OF SHOWERS...AIDED IN THE AFTERNOON PERIODS BY
DIURNAL WARMING. HOWEVER...INSTABILITY WILL GENERALLY REMAIN
WEAK...WITH WET THUNDERSTORMS REMAINING ISOLATED.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 THROUGH 3)...

THE REGION WILL REMAIN UNDER THE INFLUENCE OF A WEAKENING UPPER
LEVEL LOW THROUGH TUESDAY BEFORE WEAK UPPER LEVEL RIDGING BUILDS
IN WEDNESDAY. EXTENSIVE CLOUDS AND MOISTURE WILL KEEP SHOWERS
OVER MOST OF SOUTHCENTRAL...ENHANCED IN THE AFTERNOON PERIODS AS
DIURNAL HEATING INCREASES INSTABILITY. HOWEVER...INSTABILITY WILL
GENERALLY REMAIN LIMITED...KEEPING THUNDERSTORMS LARGELY RELEGATED
TO THE TALKEETNA MOUNTAINS AND ALASKA RANGE MONDAY AND TUESDAY
AFTERNOON. GIVEN THE EXTENSIVE LOW LEVEL MOISTURE...PATCHY
OVERNIGHT FOG WILL BECOME A THREAT. THE ATMOSPHERE WILL SLOWLY
BEGIN TO DRY ON WEDNESDAY AS WEAK UPPER LEVEL RIDGING BUILDS
IN...RESULTING IN LESS WIDESPREAD SHOWERS...AND DIMINISHING
INSTABILITY.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2...MON AND
TUE)...THE PATTERN WILL BE DOMINATED BY BROAD NORTHWESTERLY
SYNOPTIC FLOW COMING DOWN FROM EASTERN SIBERIA AND DIVING INTO THE
BASE OF THE LONGWAVE TROUGH IN THE GULF OF ALASKA. THIS WILL LEAVE
THE MAJORITY OF THE AREA OPEN TO WEAK WAVES THAT WILL RIDE THROUGH
THE FLOW AND FAVOR UPSLOPE ON NORTHWEST FACING SLOPES. SOME
SHOWERS CONTINUE THIS MORNING. ADD IN A LITTLE DAYTIME HEATING
THIS AFTERNOON...ESPECIALLY ALONG THE MOUNTAINS...AND
THUNDERSTORMS SHOULD DEVELOP. TUESDAY THE AFTERNOON AND EVENING
THUNDERSTORMS SHOULD BE A BIT MORE WIDESPREAD AS AN UPPER LEVEL
TROUGH PASSES OVER THE AREA AND PROVIDES THE TRIGGER IN A
RELATIVELY UNSTABLE ENVIRONMENT. WEST TO NORTHWEST FLOW WILL ALSO
CONTINUE TO BRING MARINE STRATUS AND FOG TO THE WEST COAST LATE
EACH NIGHT INTO THE MORNING HOURS.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2...MON AND
TUE)...HIGH PRESSURE WILL CONTINUE TO BUILD IN OVER THE WESTERN
BERING. NORTHWEST FLOW WILL CONTINUE OVER THE EASTERN BERING.
THESE TWO THINGS WILL LEAD TO CALM WINDS AND FOG FROM ESSENTIALLY
ATKA WEST AND SOMEWHAT GUSTY NORTHWEST WINDS AND SHOWERS FOR THE
EASTERN ISLANDS. THE HEAVIEST AND MOST PERSISTENT PRECIPITATION
WILL FAVOR THE BERING SIDE. BY LATE TUESDAY AND EARLY
WEDNESDAY...A NEW LOW WITH ABUNDANT SUB-TROPICAL MOISTURE TRIES TO
MOVE NORTHWARD INTO THE WESTERN ALEUTIANS. MODELS VARY QUITE
WIDELY IN HOW FAR NORTH THE PRECIPITATION AND SMALL CRAFT WINDS
ACTUALLY MAKE IT WITH HIGH PRESSURE OUT AHEAD OF IT. USED A MIDDLE
GROUND SOLUTION AT THIS TIME KEEPING SOME LIGHT RAIN AND MODEST
WINDS IN THE FORECAST.

&&

.LONG TERM FORECAST (DAYS 3 THROUGH 7)...
WIDESPREAD SHOWERY CONDITIONS THROUGH MIDWEEK WILL BEGIN TO
DIMINISH THROUGH THE END OF THE WEEK...AS THE TROUGH OVER MUCH OF
SOUTHERN MAINLAND WILL BEGIN TO WEAKEN AND SHIFT EASTWARD. THE
EXTENT OF CLOUD BREAKS AND DAYTIME WARMING CONTINUE TO BE LESS
CERTAIN...AS MODELS CONTINUE TO STRUGGLE WITH THE FINER DETAILS
REGARDING THE SHORTWAVES CIRCULATING AROUND THE TROUGH. THIS HAS
ALSO RESULTED IN LESS THAT NORMAL CONFIDENCE WITH RESPECT TO
THUNDERSTORM CHANCES ACROSS THE SOUTHERN MAINLAND. SHOWERS BECOME
CONFINED TO THE MOUNTAINS OVER THE SOUTHERN INTERIOR BY THE
WEEKEND...AS AN UPPER LEVEL RIDGE OVER THE SOUTHWEST MAINLAND
BEGINS TO PUSH THE UPPER LEVEL TROUGH FURTHER EASTWARD OVER THE
GULF.

FURTHER WEST...AN UPPER LEVEL RIDGE BUILDS ACROSS THE WESTERN AND
CENTRAL 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. BY MIDWEEK THE MODELS BEGIN TO
STRUGGLE WITH THE TIMING AND TRACK OF A NORTH PACIFIC LOW
APPROACHING THE WESTERN ALEUTIANS...AS WELL AS THE TIMING AND
STRENGTH OF A SHORTWAVE TRACKING THROUGH THE NORTHERN/EASTERN
BERING THROUGH THURSDAY. THE AMERICAN MODELS CONTINUE TO STALL THE
NORTH PACIFIC SYSTEM CLOSER TO THE ALEUTIAN CHAIN THROUGH
SATURDAY...KEEPING PRECIPITATION AND MILDLY GUSTY WINDS ALONG THE
WESTERN/CENTRAL ALEUTIANS SLIGHTY LONGER. ON THE OTHER HAND...ALL
OTHER MODELS ARE QUICKER TO BRING THE NORTH PACIFIC SYSTEM FURTHER
SOUTH WITH A STRONGER RIDGE BUILDING OVER THE BERING...RESULTING
IN THE PRECIPITATION AND GUSTY WINDS REMAINING FURTHER SOUTH AND
AWAY FROM THE ALEUTIAN CHAIN. THE MOST RECENT RUNS OF THE AMERICAN
MODELS ARE ALSO KEYING IN ON A STRONGER WAVE TRACKING THROUGH THE
EASTERN BERING AND ALEUTIANS FROM THE END OF THE WEEK AND INTO THE
WEEKEND...WHERE THE OTHER MODELS ARE SHOWING A MORE LIKELY
SOLUTION OF A WEAKER/ELONGATED WAVE QUICKLY TRACKING SOUTHWARD
PRIOR TO THE WEEKEND. WITH THAT SAID...THE ECMWF AND WPC SOLUTIONS
WERE UTILIZED FOR THIS FORECAST PACKAGE. HOWEVER...FORECAST
CONFIDENCE DOES REMAIN ON THE LOWER END...AS THERE CONTINUES TO BE
A FAIR MODEL SPREAD THROUGH NEXT WEEKEND.

&&


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

&&

$$

SYNOPSIS AND MODEL DISCUSSION...JA
SOUTHCENTRAL ALASKA...JA
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MSO
LONG TERM...TP



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