Small Craft Advisory 502, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
152 PM AKDT TUE JUN 24 2014

.ANALYSIS AND UPPER LEVELS...
A WEAKENING LOW PRESSURE SYSTEM REMAINS IN THE GULF...WITH
CYCLONIC UPPER LEVEL FLOW ENCOMPASSING THE SOUTHERN MAINLAND AND
GULF OF ALASKA. SHOWER ACTIVITY HAS DIMINISHED SIGNIFICANTLY
AROUND THE LOW CENTER ITSELF...WITH THE MOST SIGNIFICANT AREA OF
CLOUDS AND SHOWERS ASSOCIATED WITH AN EMBEDDED SHORTWAVE MOVING
THROUGH THE COPPER RIVER BASIN AND INTO THE SUSITNA VALLEY.
MID-LEVEL CLOUDS HAVE LARGELY THINNED OUT OVER THE WESTERN KENAI
AND ANCHORAGE AREAS...WITH DIURNAL CONVECTIVE CLOUDS QUICKLY
DEVELOPING IN CLEAR AREAS. FURTHER WEST...MOSTLY SUNNY CONDITIONS
OVER SOUTHWEST ALASKA ARE ALSO GIVING WAY TO CONVECTIVE
CLOUDINESS. DENSE FOG SURGED INLAND FROM THE COAST AGAIN OVERNIGHT
ACROSS MUCH OF BRISTOL BAY AND THE DELTA...AND IS NOW IN THE
PROCESS OF ONCE AGAIN RETREATING IN THE FACE OF SOLAR HEATING.
OVER THE BERING SEA AND ALEUTIANS...MARINE FOG/STRATUS REMAIN VERY
WIDESPREAD WITH WEAK SURFACE HIGH PRESSURE OVER THE NORTHERN
BERING HANGING TOUGH. A WEAK FRONT HAS STALLED OVER THE CENTRAL
AND EASTERN ALEUTIANS...WITH GUSTY WINDS AND INCREASED RAIN SHOWER
ACTIVITY. HOWEVER THIS FEATURE IS SO WEAK IT IS HAVING DIFFICULTY
EVEN DISRUPTING THE WIDESPREAD LOW CLOUDINESS ALREADY IN PLACE.

&&

.MODEL DISCUSSION...
A SHORTWAVE CURRENTLY MOVING INTO THE SOUTHERN PANHANDLE WILL BE
THE BIG PLAYER IN SENSIBLE WEATHER IMPACTS ACROSS SOUTHERN ALASKA
(AND SPECIFICALLY SOUTHCENTRAL ALASKA) IN THE COMING DAYS. ALL
MODELS DEPICT THIS WAVE RACING NORTHWARD TONIGHT AND PINWHEELING
AROUND THE NORTH SIDE OF THE RESIDUAL UPPER LOW IN THE GULF OF
ALASKA. THIS WILL BRING WIDESPREAD RAIN TO VIRTUALLY ALL POINTS
SOUTH AND EAST OF THE ALASKA RANGE OVER THE COMING DAYS. SOME
IMPORTANT MODEL DISCREPANCIES CONTINUE TO PRESENT A SIGNIFICANT
FORECAST CHALLENGE TODAY...WITH THE MAIN AREA OF CONTENTION
INVOLVING HOW FAR NORTH AND WEST THE AFOREMENTIONED SHORTWAVE
PULLS THE GULF UPPER LOW AS IT MOVES INTO THE INTERIOR ON FRIDAY.
THIS WILL CONTROL THE MOVEMENT OF THE ASSOCIATED SHIELD OF STEADY
RAIN ACROSS THE AREA. THE GFS IS THE MOST AGGRESSIVE PULLING THIS
FEATURE WESTWARD...PULLING THE STEADY RAIN ACROSS THE ALASKA RANGE
AND INTO THE KUSKOKWIM VALLEY. THE ECMWF/GEMGLB AND TO A LESSER
DEGREE THE 18Z NAM SEND MUCH OF THIS ENERGY UP TO THE BROOKS
RANGE BY FRIDAY MORNING...KEEPING THINGS MOSTLY DRY WEST OF THE
ALASKA RANGE BUT POTENTIALLY LINGERING A DEFORMATION BAND AND MORE
RAIN ACROSS NORTHERN COOK INLET AND THE SUSITNA VALLEY. AT THIS
POINT THE GFS IS BEING CONSIDERED AN OUTLIER...AND WILL OPT TO
STICK WITH THE MORE CONSISTENT (AND CLOSER TO THE CURRENT
FORECAST) ECMWF-BASED SOLUTION.

&&

.SHORT TERM FORECAST...

SOUTHCENTRAL ALASKA...
THICK MID-LEVEL CLOUDS ARE GIVING WAY TO SUNNY BREAKS AND SETTING
THE STAGE FOR ANOTHER DAY OF CONVECTION ACROSS THE REGION.
SHOWERS ARE NOT EXPECTED TO BE AS STRONG AS YESTERDAY...WITH
MORNING CLOUDINESS KNOCKING TEMPERATURES DOWN SEVERAL DEGREES
ALONG WITH ASSOCIATED INSTABILITY. BEST CHANCES FOR THUNDERSTORMS
TODAY WILL BE FROM PALMER NORTHWARD...WHERE THE LINGERING THERMAL
TROUGH WILL BE THE STRONGEST. AS MENTIONED IN THE MODEL
DISCUSSION...BEGINNING LATE TONIGHT AND TOMORROW ALL EYES TURN TO
ANOTHER STRONG EASTERLY WAVE MOVING INTO THE REGION. WITH MODELS
TRENDING FASTER WITH THIS PROGRESSION ALL AREAS INCLUDING
ANCHORAGE AND THE SUSITNA VALLEY ARE NOW EXPECTED TO SEE ANOTHER
ROUND OF STEADY MODERATE RAIN MOVING IN FROM THE NORTHEAST. THE
ACTION BEGINS FIRST OVER THE COPPER RIVER BASIN AFTER MIDNIGHT
TONIGHT...THEN SPREADING QUICKLY TO THE MATANUSKA VALLEY DURING
THE MORNING. ANCHORAGE WILL LIKELY SEE RAIN BEGIN BY MIDDAY ALONG
WITH THE SUSITNA VALLEY AND FINALLY THE KENAI PENINSULA. RAINFALL
AMOUNTS ACROSS THE MAINLAND WILL BE WIDESPREAD IN THE HALF INCH
TO ONE INCH RANGE BY FRIDAY...WITH LOCALIZED AMOUNTS UPWARD OF 2
INCHES POSSIBLE MAINLY IN THE SUSITNA VALLEY.

SOUTHWEST ALASKA...
SUNSHINE AND A STRENGTHENING THERMAL TROUGH CURVING DOWN FROM
INTERIOR ALASKA ARE A RECIPE FOR CONVECTION TODAY OVER INLAND
AREAS OF SOUTHWEST ALASKA. MEANWHILE...EXTENSIVE MORNING
FOG/STRATUS AND CONTINUING ONSHORE FLOW WILL IRONICALLY KEEP THE
COASTLINES MOSTLY DRY. CONVECTION IS ALREADY EVIDENT ON SATELLITE
IMAGERY ALONG THE KILBUCK MOUNTAINS AND ALONG THE ALASKA
RANGE...WITH A STRENGTHENING TREND EXPECTED THROUGH THE EARLY
EVENING. CANNOT RULE OUT A FEW BOLTS OF LIGHTNING EACH OF THE NEXT
FEW DAYS...SO WILL KEEP MENTION IN THE AFTERNOON FORECAST. AS
MENTIONED ABOVE...THERE IS SOME UNCERTAINTY REGARDING HOW FAR WEST
THE STEADY RAIN WILL SPREAD ACROSS THE ALASKA RANGE THURSDAY
AFTERNOON THROUGH FRIDAY. AT THIS POINT THE MOST LIKELY SCENARIO
WOULD KEEP THIS RAIN MAINLY UP ALONG THE ALASKA RANGE...BUT
INTERESTED PARTIES IN THE LOWER KUSKOKWIM VALLEY AND EVEN FAR
NORTHERN BRISTOL BAY ZONE SHOULD PAY CLOSE ATTENTION TO
SUBSEQUENT FORECAST UPDATES. GENERALLY INCREASING OFFSHORE FLOW
OVER THE NEXT 36 HOURS WILL MAKE OVERNIGHT COASTAL FOG LESS OF A
PROBLEM THAN PREVIOUS DAYS...BUT WILL NOT BE STRONG ENOUGH TO
TOTALLY DISCOUNT SOME LOCALIZED ISSUES. BY THURSDAY OVERALL FLOW
BECOMES ONSHORE AND FOG WILL LIKELY BECOME MORE WIDESPREAD ONCE
AGAIN.

BERING SEA/ALEUTIANS...
LOW PRESSURE CURRENTLY SOUTH OF ADAK WILL HAVE OVERALL MINIMAL
IMPACTS ON THE ALEUTIAN CHAIN AS THE SYSTEM WEAKENS AND STAYS
SOUTH OF THE ISLANDS. HIGH PRESSURE BUILDS IN FROM THE WEST...AND
WILL KEEP MUCH OF THE AREA UNDER BENIGN CONDITIONS. THE MAIN
FORECAST CHALLENGE WILL REMAIN EXTENT AND SEVERITY OF MARINE
FOG/STRATUS.

&&

.LONG TERM FORECAST...
UNCERTAINTY REIGNS IN THE EXTENDED PERIOD AS GUIDANCE VARIES
WILDLY WITH HOW THE UPPER LEVEL LOW...CURRENTLY IN THE GULF OF
ALASKA...TRACKS ACROSS THE INTERIOR. SOLUTIONS VARY FROM NORTHWARD
ALONG THE ALASKA/CANADA BORDER NORTHWARD TO TOWARD THE SEWARD
PENINSULA. DESPITE THOSE MAJOR DIFFERENCES...THE TWO MAJOR PLAYERS
IN THE EXTENDED FORECAST LOOK TO BE A LINGERING NORTHEAST PACIFIC
LOW SETTING UP SHOP NEAR THE PANHANDLE AND TROUGHING NEAR THE
CHUKCHI PENINSULA. RIDGING LOOKS TO OCCUPY THE YUKON/NW TERRITORY
REGION AS WELL AS THE BERING SEA. THAT SOLUTION LEAVES MUCH OF THE
SOUTHCENTRAL/SOUTHWEST MAINLAND UNDER SLACK FLOW WITH THE
OPPORTUNITY FOR SHOWERS ALONG THE MOUNTAINS. HOWEVER...SOME
DISTURBANCES COULD PROPAGATE THROUGH THE WEAKER NW FLOW WHICH
WOULD AID IN PRECIPITATION COVERAGE. GENERALLY LOOK FOR NEAR TO
ABOVE NORMAL TEMPERATURES WITH MOSTLY DRY CONDITIONS BUT THE
CHANCE FOR SHOWERS ALONG THE MOUNTAINS.

&&

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

AMD/ML JUN 14