Area Forecast Discussion
Issued by NWS Anchorage, AK

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FXAK68 PAFC 281232
AFDAFC

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
432 AM AKDT MON JUL 28 2014

.ANALYSIS AND UPPER LEVELS...

AN OMEGA BLOCK PATTERN CONTINUES TO DEVELOP ACROSS THE BERING SEA
AND THE FAR WEST COAST OF ALASKA THIS MORNING. AN UPPER LEVEL LOW
(THE FIRST ANCHOR POINT OF THE OMEGA BLOCK) LOCATED BETWEEN THE
KAMCHATKA PENINSULA AND THE WESTERN ALEUTIANS HAS BEGUN TO PHASE
WITH A POTENT SHORTWAVE DROPPING SOUTH OUT OF RUSSIA. THIS PHASING
HAS ALREADY BEGUN TO SHOW SIGNS OF AMPLIFYING THE LONGWAVE TROUGH
ALONG THE WESTERN PERIPHERY OF THE OMEGA BLOCK. ANOTHER UPPER
LEVEL LOW (THE SECOND ANCHOR POINT OF THE OMEGA BLOCK) IS CURRENTLY
LOCATED IN THE GULF OF ALASKA. SCATTERED RAIN SHOWERS HAVE CONTINUED
ALONG THE NORTHERN GULF COAST WITH A WEAK EASTERLY WAVE ROTATING
AROUND THE AFOREMENTIONED LOW. A POTENT SHORTWAVE DROPPING OUT OF
THE ARCTIC HAS ALREADY MOVED INLAND OVER NORTHWESTERN ALASKA WHICH
IS DEPICTED WELL BY BOTH INFRARED AND WATER VAPOR SATELLITE
IMAGES. RAINFALL AND INCREASED CLOUDINESS WILL BE OBSERVED ACROSS
THE MAJORITY OF THE WESTERN HALF OF THE STATE AS THE WAVE CONTINUES
TO PUSH SOUTH DURING THE MORNING HOURS. THIS WAVE HAS BEGUN TO
INTERACT WITH THE LOW IN THE NORTHERN GULF AND SHOULD PHASE WITH
THE UPPER LOW LATE THIS AFTERNOON AND INTO THIS EVENING.

AN ANOMALOUSLY STRONG RIDGE (THE FINAL COMPONENT OF THE OMEGA
BLOCK) IS CENTERED ACROSS THE CENTRAL BERING SEA THIS MORNING.
THE 12Z RAOB FROM PASN (SAINT PAUL) SHOWS THE 500 MB HEIGHTS AT
584 DECAMETERS...WITH EVEN HIGHER HEIGHTS EXPECTED FURTHER SOUTH
NEAR THE CENTRAL ALEUTIANS. THE RIDGE OF HIGH PRESSURE HAS
CONTINUED THE PERIOD OF LOW STRATUS AND FOG ACROSS THE MAJORITY OF
THE BERING SEA AND ALEUTIAN ISLANDS BECAUSE OF THE STABLE AIR MASS
IN PLACE. EVEN WITH THE ABUNDANT CLOUD COVER...TEMPERATURES HAVE
STILL MANAGED TO CLIMB INTO THE UPPER 50S TO LOWER 60S...WHICH IS
SLIGHTLY UNCOMMON WITH A STRATUS DECK THAT IS THOROUGHLY ENTRENCHED.

&&

.MODEL DISCUSSION...

MODELS APPEAR TO BE ON THE SLOW SIDE WITH THE UPPER LEVEL SHORTWAVE
DROPPING SOUTH FROM THE ARCTIC OVER THE WESTERN HALF OF THE
ALASKA MAINLAND. WHEN COMPARING THE 400 MB RELATIVE HUMIDITY
FIELDS TO THE WATER VAPOR IMAGES AT BOTH 00Z AND 06Z...A DISCREPANCY
OF 3 TO 6 HOURS WAS OBSERVED. SURFACE OBSERVATIONS ALONG THE
NORTHWESTERN COAST HAVE PROVIDED GROUND TRUTH THAT HELPED VERIFY
THAT THE MODELS ARE INDEED ON THE SLOW SIDE. THE 06Z RUNS HAVE
STARTED TO CATCH ONTO THIS TREND BUT ARE STILL NOT QUITE CAUGHT UP
AT THIS TIME. DUE TO THIS NEW DATA...THE DECISION WAS MADE TO
SPEED UP THE PRECIPITATION MOVING ACROSS THE WESTERN HALF OF
ALASKA DURING THE DAY TODAY BY 3 TO 6 HOURS.

WITH THE UPPER LEVEL LOW SPINNING IN THE GULF...MULTIPLE WEAK
EASTERLY WAVES ARE LIKELY TO TRAVERSE THE NORTHERN GULF COAST.
HOWEVER...MODEL CONFIDENCE GENERALLY TENDS TO LACK WITH THE TIMING
OF EACH WAVE AS THE POSITION OF THE LOW AND THE HEIGHT FALLS/POTENTIAL
VORTICITY ANOMALIES ASSOCIATED WITH THE SHORTWAVES BEING EJECTED
ARE NOT RESOLVED WELL BY MODELS GENERALLY PAST 24 TO 36 HOURS.

OVERALL THOUGH...THE MODELS ARE IN PRETTY GOOD SYNOPTIC AGREEMENT.
THERE ARE OF COURSE THE TYPICAL TIMING ISSUES WITH WAVES (AS
MENTIONED ABOVE) AND THE EXACT POSITIONS OF THE UPPER LOWS.
HOWEVER...THE OMEGA BLOCK PATTERN SEEMS TO BE LOCKED IN THROUGH
THE SHORT-TERM AND INTO THE LONG-TERM PENDING SOME DRASTIC SHIFT
IN THE LONGWAVE PATTERN...WHICH DOES NOT SEEM REALISTIC AT THIS
TIME. ENDED UP UTILIZING THE ECMWF FOR FORECAST UPDATES THIS
MORNING...DUE IN LARGE PART TO THE INITIALIZATION OF THE SHORTWAVE
THAT WILL PUSH ACROSS SOUTHWEST AND SOUTHCENTRAL ALASKA THIS
AFTERNOON AND EVENING.

&&

.SHORT TERM FORECAST...

SOUTHCENTRAL ALASKA...A POTENT SHORTWAVE WILL PUSH THROUGH THE
SUSITNA VALLEY THIS AFTERNOON AND THEN PUSH SOUTH TOWARD KODIAK
ISLAND OVERNIGHT TONIGHT. THIS WILL ALLOW STEADY RAINFALL TO
DEVELOP IN THE SUSITNA VALLEY BY THIS AFTERNOON AND SPREAD ACROSS
THE MATANUSKA VALLEY...THE ANCHORAGE BOWL AND KENAI PENINSULA
MID TO LATE AFTERNOON. THIS WAVE WILL BE FAST MOVING AND MOST
PLACES WILL SEE PRECIPITATION TAPER OFF OVERNIGHT TONIGHT...WITH
THE EXCEPTION OF KODIAK AND THE SOUTHERN KENAI PENINSULA WHERE
PRECIPITATION WILL LIKELY LINGER THROUGH TUESDAY MORNING. AN
EASTERLY WAVE WILL PUSH INTO THE COPPER RIVER BASIN ON TUESDAY AND
BRING ANOTHER ROUND OF RAINFALL TO THE BASIN. DUE TO INCREASED
CLOUD COVER FROM THE SHORTWAVE PUSHING THROUGH THE REGION
TODAY...THE ATMOSPHERE SHOULD HAVE LITTLE TO NO TIME TO
DESTABILIZE PRIOR TO THE ONSET OF PRECIPITATION ON TUESDAY.
HOWEVER...MODELS ARE STARTING TO HINT AT THE POTENTIAL FOR AN
ISOLATED THUNDERSTORM OR TWO TUESDAY EVENING ACROSS THE MATANUSKA
VALLEY...ANCHORAGE BOWL AND NORTHERN PORTIONS OF THE KENAI
PENINSULA AS A PORTION OF ENERGY ASSOCIATED WITH THE EASTERLY WAVE
PIVOTS ACROSS THE AREA.

SOUTHWEST ALASKA...AREAS OF FOG WILL BE SEEN ALONG THE SOUTHWEST
COAST OF ALASKA THIS MORNING AS THE RIDGE OVER THE BERING SEA
BEGINS TO SLOWLY SLIDE EASTWARD. THIS WILL ALLOW WEAK ONSHORE FLOW
TO PERSIST OVER THE REGION OVER THE NEXT DAY OR TWO...BRINGING
PERIODS OF FOG TO MAINLY THE COASTAL AREAS OF SOUTHWEST ALASKA
DURING THE OVERNIGHT AND THE EARLY MORNING HOURS. A SHORTWAVE
DROPPING SOUTH FROM NORTHWESTERN ALASKA WILL BRING RAIN TO THE
LOWER KUSKOKWIM VALLEY AND EASTERN SECTIONS OF THE BRISTOL BAY
AREA DURING THE DAY TODAY. PRECIPITATION WILL BECOME MORE SHOWERY
LATE THIS EVENING BEFORE ULTIMATELY TAPERING OFF TUESDAY AFTERNOON.

BERING SEA/ALEUTIANS...AN UPPER LEVEL LOW WILL EJECT A SERIES OF
WAVES ACROSS THE WESTERN ALEUTIANS THROUGH THE SHORT-TERM AND
BRING SMALL CRAFT WINDS AND SEAS. PERIODS OF LIGHT TO MODERATE
RAINFALL WILL ALSO BE SEEN AS THESE UPPER LEVEL WAVES PROPAGATE
THROUGH THE AREA. ELSEWHERE...AN ANOMALOUSLY STRONG RIDGE WILL
ALLOW PATCHY/AREAS OF FOG AS WELL AS A LOW STRATUS DECK TO PERSIST
THROUGH THE SHORT-TERM. WITH THE LOW STRATUS DECK WELL ESTABLISHED...
WOULD NOT BE SHOCKED TO SEE SOME COLLISION COALESCENCE TAKE PLACE
AND ALLOW A FEW AREAS TO SEE SOME LIGHT DRIZZLE/SPRINKLES.

&&

.LONG TERM FORECAST...
AN OMEGA BLOCK SETTING UP SHOP OVER THE BERING SEA AND MAINLAND
WILL DOMINATE THE PATTERN...AT LEAST FOR THE BEGINNING OF THE
EXTENDED FORECAST. THE SAME HIGH AMPLITUDE RIDGE CURRENTLY OVER
THE BERING WILL CLOSE OFF AT THE UPPER LEVELS...BOOKENDED BY TWO
UPPER LOW PRESSURE CENTERS...ONE OVER THE GULF OF ALASKA AND THE
OTHER SOUTHWEST OF THE WESTERN ALEUTIANS. THE GULF SYSTEM WILL
CONTROL SENSIBLE WEATHER ACROSS THE SOUTHCENTRAL WITH THE MAIN
FEATURE BEING AN EASTERLY WAVE TOWARD THE BEGINNING OF THE
FORECAST. GUIDANCE HAS COME INTO BETTER AGREEMENT WITH PLACEMENT
AND TIMING OF THE FEATURE BUT THESE DISTURBANCES GENERALLY GIVE
GUIDANCE FITS WITH BOTH TIMING AND INTENSITY. THAT SHORTWAVE WILL
AFFECT THE SOUTHCENTRAL MAINLAND MID TO LATE WEEK. BEYOND THAT
TIME FRAME PRECIPITATION CHANCES REVOLVE AROUND DIURNAL CONVECTION
OVER THE MOUNTAINS...MOSTLY DRY AT THE LOWEST ELEVATIONS BUT CHANCES
FOR SHOWERS ALONG HIGHER TERRAIN. THE UPCOMING PATTERN LOOKS TO BE
VERY CLIMATOLOGICAL AS FAR AS TEMPERATURES AND PRECIPITATION GO.

&&

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

$$

MCLAY JUL 14



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