Small Craft Advisory 603, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
140 PM AKDT SUN AUG 10 2014

.ANALYSIS AND UPPER LEVELS...THE UPPER LEVEL PATTERN LOOKS VERY
LATE SUMMER-LIKE THIS MORNING WITH A STRONG AMPLIFIED SUB-TROPICAL
JET STREAM TAPPING INTO TROPICAL MOISTURE FROM THE WEST PACIFIC.
A BROAD AND MUCH WEAKENED UPPER LEVEL LOW IS CENTERED SOUTH OF
KODIAK ISLAND WITH THE JET ON THE EAST SIDE OF THE LOW DRIVING A
POTENT SHORT-WAVE NORTHWARD INTO THE GULF. WEAKER WAVES OUT AHEAD
OF THIS ARE PRODUCING ISOLATED TO SCATTERED CONVECTION FROM
SOUTHCENTRAL TO BRISTOL BAY...BUT MOST AREAS ARE DRY AND STABLE
THIS MORNING. MEANWHILE A MUCH STRONGER LOW OVER THE FAR SOUTHWEST
BERING SEA HAS BECOME VERTICALLY STACKED AND IS BEGINNING TO
OCCLUDE AND WRAP UP WHILE THE JET DRIVES THE FRONTAL SYSTEM
EASTWARD ACROSS THE BERING AND ALEUTIANS.

&&

.MODEL DISCUSSION...THERE ARE SOME MINOR DIFFERENCES IN TIMING AND
PLACEMENT OF GULF SHORT-WAVE AS IT ROTATES WESTWARD ACROSS THE
KENAI PENINSULA THIS AFTERNOON AND EVENING. NAM LOOKS JUST A LITTLE
BIT SLOW AND A LITTLE FARTHER NORTH THAN OTHER SOLUTIONS...BUT
THE IMPACTS WILL BE SIMILAR. THE NAM DOES BECOME A LARGER OUTLIER
ON MONDAY IN LIFTING THE REMNANTS OF THE UPPER LOW NORTH-NORTHWEST
OVER COOK INLET INTO THE KUSKOKWIM VALLEY. FOR NOW WILL STICK WITH
OTHER SOLUTIONS WHICH ARE FARTHER EAST AND QUICKER TO OPEN THE
UPPER LOW INTO A BROAD TROUGH. THE TRACK AND INTENSITY OF THIS
WILL IMPACT THE CONVECTIVE FORECAST MONDAY AFTERNOON AND
EVENING...WITH POTENTIAL FOR FAIRLY WIDESPREAD THUNDERSTORMS.
FORECAST CONFIDENCE IS AVERAGE FOR THE SOUTHERN MAINLAND. MODELS
SEEM TO BE VERIFYING RELATIVELY WELL WITH THE BERING SYSTEM AND
DEPICT A VERY SIMILAR EVOLUTION OVER THE NEXT FEW DAYS...SO
FORECAST CONFIDENCE IS ABOVE AVERAGE.

&&

.SHORT TERM FORECAST...

SOUTHCENTRAL ALASKA...SATELLITE IMAGERY INDICATES A LINE OF
CONVECTION RIGHT OUT AHEAD OF THE SHORT-WAVE MOVING INTO THE GULF.
CONTRIBUTIONS TO GREATER INSTABILITY ARE COLDER AIR ALOFT (IN
PROXIMITY TO UPPER LOW) AND MID-LEVEL DRY AIR NOTED IN WATER VAPOR
IMAGERY (WHICH CAN LEAD TO STEEPER LAPSE RATES AS MOISTURE IS
INTRODUCED INTO THE DRY LAYER). ALL OF THIS IS AIDED BY FORCED
LIFT FROM THE SHORT-WAVE ITSELF. VERTICAL MOTION WILL BECOME EVEN
MORE ENHANCED AS THIS AREA OF GREATER INSTABILITY REACHES PRINCE
WILLIAM SOUND AND THE KENAI PENINSULA THIS AFTERNOON AND EVENING
AND INTERACTS WITH THE TERRAIN (UPSLOPE FLOW). THE END RESULT WILL
BE A PERIOD OF HEAVY RAIN AS THE SHORT-WAVE ROTATES WESTWARD
ACROSS THE KENAI PENINSULA. SOME OF THIS HEAVIER RAIN MAY MAKE IT
AS FAR NORTH AS ANCHORAGE AND THE MAT-VALLEY...ALTHOUGH IT SHOULD
ONLY LAST A FEW HOURS AT MOST. PRESSURE RISES OF A MILLIBAR OR
MORE PER HOUR BEHIND THE WAVE WILL LEAD TO A BRIEF PERIOD OF GUSTY
WINDS...ESPECIALLY THROUGH EAST-WEST ORIENTED MOUNTAIN GAPS (SUCH
AS TURNAGAIN ARM).

THE UPPER LEVEL LOW WILL OPEN INTO A TROUGH AND LIFT NORTHWARD
ACROSS SOUTHCENTRAL MONDAY AFTERNOON AND EVENING. THERE IS SOME
UNCERTAINTY IN THE STRENGTH AND TRACK...BUT WITH THE COLD AIR
ALOFT AND ARRIVAL AT TIME OF PEAK HEATING CONDITIONS WILL BE
FAVORABLE FOR DEVELOPMENT OF THUNDERSTORMS. RIGHT NOW THE PRIME
AREA LOOKS TO BE FROM THE CHUGACH AND TALKEETNA MOUNTAINS
NORTHWESTWARD ACROSS THE MATANUSKA AND SUSITNA VALLEYS. DEEP
SOUTHEASTERLY FLOW IS NOT FAVORABLE FOR ANCHORAGE TO SEE
THUNDERSTORMS...BUT IF THEY FORMED OVER THE CHUGACH EAGLE RIVER
UP TO EKLUTNA WOULD HAVE A GOOD CHANCE OF OBSERVING THEM.

THE FRONTAL SYSTEM FROM THE BERING LOW WILL MAKE ITS WAY INTO THE
GULF AND SOUTHCENTRAL ON TUESDAY. THIS LOOKS LIKE A REPEAT OF THE
PATTERN FROM THE PAST SEVERAL DAYS WITH THE BULK OF RAIN FALLING
ALONG THE COAST...AND DOWNSLOPING IN THE LEE OF THE KENAI
MOUNTAINS AND CHUGACH WITH A CHANCE FOR RAIN AS STRONGER UPPER
WAVES MOVE THROUGH.

SOUTHWEST ALASKA...WITH GENERALLY STABLE CONDITIONS IN PLACE OVER
SOUTHWEST ALASKA EXPECT LITTLE IN THE WAY OF CONVECTION THIS
AFTERNOON AND EVENING. THERE IS SOME WEAK LIFT ASSOCIATED WITH
WAVES ROTATING IN FROM THE EAST...BUT SHOWER COVERAGE SHOULD BE
NO MORE THAN SCATTERED. THE STRONG SHORT-WAVE WHICH WILL AFFECT
SOUTHCENTRAL THIS AFTERNOON THROUGH TONIGHT WILL ROTATE BACK
AROUND THE LOW CENTER...POSSIBLY EFFECTING THE VERY EASTERN
PORTION OF SOUTHWEST ALASKA...ALONG THE ALASKA AND ALEUTIAN
RANGES. ARRIVAL OF THE BERING TROUGH ON MONDAY AND POSSIBLY SOME
IMPACT FROM THE GULF LOW LIFTING NORTHWARD WILL HELP INCREASE
INSTABILITY OVER SOUTHWEST ALASKA AND PROVIDE SLIGHTLY STRONGER
LIFT...SO EXPECT AN INCREASE IN SHOWER COVERAGE. IT DOESN'T LOOK
LIKE THERE WILL BE ENOUGH INSTABILITY TO SUPPORT THUNDERSTORMS...
ALTHOUGH IF THE NAM SOLUTION VERIFIED AND BROUGHT THE REMNANTS OF
THE UPPER LOW FARTHER WEST THE KUSKOKWIM VALLEY COULD SEE ISOLATED
THUNDERSTORMS. STABLE RAIN WILL MAKE ITS WAY INTO THE REGION
TUESDAY WITH THE ARRIVAL OF THE FRONTAL SYSTEM CROSSING THE
BERING SEA.

BERING SEA/ALEUTIANS...WITH A STRONG UPPER LEVEL JET AND
PROGRESSIVE FLOW ALOFT THE CURRENT STORM SYSTEM WILL BE QUICK TO
CROSS THE BERING SEA. AS WAS THE CASE WITH THE PRIOR STORM...IT
DOES LOOK LIKE THE UPPER TROUGH WILL AMPLIFY WITH THE UPPER LOW
STALLING SOUTH OF THE ALASKA PENINSULA MID WEEK.

&&

.LONG TERM FORECAST...
THIS OLD BERING/ALEUTIAN LOW LOOKS TO BE CENTERED NEAR THE AKPEN
ON THU AND MEANDER AROUND BRISTOL BAY THROUGH FRI. MODELS CONTINUE
TO HAVE FAIRLY GOOD AGREEMENT WITH THIS FEATURE. MORE SIZABLE
DIFFERENCES BEGIN TO ARISE REGARDING THE DEMISE OF THIS FEATURE AS
VIGOROUS SHORTWAVE ENERGY DROPPING OUT OF EASTERN RUSSIA BEGINS TO
ROUND THE LONGWAVE TROUGH AND INTERACT WITH SOME REMNANT ENERGY
FROM WHAT WAS TYPHOON HALONG WHICH HAS PASSED JAPAN. IN GENERAL
MODELS SHOW THAT THE REMNANT HALONG ENERGY AND THE SHORTWAVE WILL
HELP DEVELOP A SURFACE LOW JUST SW OF SHEMYA WHICH THEN SKIMS
ALONG THE PACIFIC SIDE OF THE CHAIN TOWARDS THE AKPEN AND SW
MAINLAND NEXT WEEKEND. ALSO AN ELEMENT IN THE MIX IS REMNANT
ENERGY FROM TYPHOON GENEVIEVE WHICH WILL BE DRIFTING NORTHWARD
TOWARDS THE JET STREAM AND THOSE OTHER AFORMENTIONED LOBES OF
ENERGY. RUN TO RUN MODEL CONSISTENCY HAS BEEN POOR BEYOND NEXT
THU...BUT THERE WAS SOME TRENDING TOWARDS A HEALTHY STORM POSSIBLY
IMPACTING THE GULF AND SOUTHERN MAINLAND NEXT WEEKEND. THIS WOULD
ONLY SERVE TO REINFORCE AND PROLONG A CLOUDY AND WET PATTERN MUCH
OF THE AOR WILL ALREADY BE EXPERIENCING. FORECAST CONFIDENCE IN
THIS EXTENDED FORECAST PERIOD IS AVERAGE TO LOW.

&&

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

SEB/JAM AUG 14