Small Craft Advisory 574, NWS Juneau, AK

SOUTHEAST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE JUNEAU AK
348 PM AKDT SAT AUG 22 2015

.SHORT TERM...GALE FORCE LOW SPINNING NEAR 57N/141W AT THE
MOMENT...MOVING SLOWLY EASTWARD. SECONDARY VORT MAX AND FRONT
ACCOMPANIED BY RAPIDLY COOLING/CONVECTIVE TOPS HAS JUST REACHED
ONSHORE OVER THE NORTHERN PANHANDLE. IT IS PROGRESSING STEADILY
NORTHEASTWARD. HAVE NOT, SURPRISINGLY, SEEN ANY LIGHTNING STRIKES
ON THE NETWORK THIS AFT DESPITE MODERATELY STRONG CONVECTIVE
SHOWERS EVIDENT ON RADAR AND COOLING TOPS. INCLUDED ISOLATED
THUNDERSTORMS IN ZONES 310, 42 AND 43 THROUGH 18Z SUN. MODELS HAVE
THIS SECONDARY PUSH INLAND TONIGHT AND ENSEMBLE APPROACH YIELDED
2-2.5" IN GENERAL (INCLUDING THE SITKA AREA) OVER MUCH OF THE
NORTHERN PANHANDLE (LOW ELEV) THROUGH 03Z MON WITH LOCAL WIND
EFFECTS INCLUDED. THIS SEEMS REASONABLE AND DEPENDING ON LOCATIONS
OF HEAVY SHOWERS MONTANA/JORDAN CREEKS MAY INDEED FLOOD...LITTLE
TO NO CONFIDENCE FOR MENDENHALL LAKE WITH GLACIER RELEASE
BACKGROUND...AND THE TAIYA WILL BE IN OR NEAR ACTION STAGE BY LATE
TOMORROW. KEY WILL BE OROGRAPHICS AND ASIDE FROM A FEW
MEASUREMENTS THIS IS CONJECTURE...DOMINANT FLOW IS ABOUT 150-160
DEGREES FOR MUCH OF THE AREA SO AREAS THAT DECELERATE AND/OR STEER
THE FLOW UPWARD WILL SEE THE MOST RAINFALL. FIRST GUESS WOULD BE
HAINES ZONE FOR BEST OROGRAPHIC ENHANCEMENT THAT INCLUDES A
POPULATED RIVER DRAINAGE.

MEANWHILE WINDS WILL CONTINUE TO BE A BIG PROBLEM WITH GALES
OFFSHORE ANTICIPATED THROUGH 09Z AND VARIOUS SCAS WHERE WINDS ARE
BELOW THE 35KT THRESHOLD... AND FOR A FEW INNER CHANNELS
CONTINUING INTO TOMORROW AFTERNOON. STRONGEST WINDS SHOULD BE
OVER THE EAST- WEST ORIENTED CHANNELS ALONG WITH SOME 30KT WINDS
OVER THE SOUTHERN PORTIONS OF THE SOUTHERNMOST N-S CHANNELS. RIGHT
NOW STRONGEST WINDS MEASURED ARE WESTERLIES OFFSHORE SOUTH OF THE
LOW...SOUTHEASTERLY WINDS AHEAD OF THE LOW... AND LOCAL INNER
ACCELERATION AT SCULL ISLAND. WENT WITH GALE IN STEPHENS PASSAGE
THROUGH 03Z...WITH STRONGEST WINDS MENTIONED SOUTH OF DOUGLAS
ISLAND. LATE TONIGHT SHOULD BEGIN TO SEE GRADUAL DECREASE IN WINDS
OFFSHORE AND A FEW HOURS LATER FOR THE WESTERN INNER
CHANNELS...SPREADING EASTWARD THROUGH THE DAY. SEAS 16+ FT ARE
WIDESPREAD IN TONIGHT'S OFFSHORE FORECAST.

MODELS WERE IN CLOSE AGREEMENT FOR THE PERIOD ON BOTH DYNAMICS
AND THERMODYNAMICS AND ONLY HAD TO TWEAK A FEW WIND LOCATIONS
UPWARDS FOR EXPOSED AREAS. WE ARE ALREADY IN THE SHOWER MODE AFTER
THE INITIAL FRONT LAST NIGHT AND DECIDED TO STICK WITH THAT
THROUGH THE PERIOD BASED ON DOMINANCE OF OBSERVED CELLULAR
CONVECTION ON SATELLITE/RADAR.

.LONG TERM...THE LOW CURRENTLY CAUSING WIND AND HEAVY RAIN ISSUES
WILL REMAIN IN THE FORECAST FOR THE EARLY PART OF THE EXTENDED
RANGE FORECAST PERIOD, ALBEIT ON A WEAKENING TREND. DIFFERING
MODEL DETAILS ON TRACK AND INTENSITY AS CAN BE EXPECTED, BUT
OVERALL AGREEMENT IN THE BIG PICTURE IS QUITE GOOD. THE LOW AND
ASSOCIATED PRECIPITATION DRIFTS SOUTHEAST OUT OF THE FORECAST AREA
ON TUESDAY WITH THE LAST OF THE PRECIP CLEARING THE SOUTHERN ZONES
BY LATE TUESDAY AFTERNOON. DIMINISHING CLOUDS AND LIGHTER WINDS
WILL FOLLOW IN THE WAKE OF THIS LOW WITH TUESDAY LOOKING LIKE A
NICE DAY FOR THE NORTHERN TWO THIRDS AND PARTLY CLOUDY WITH LIGHT
WINDS FOR ALL LOCATIONS ON WEDNESDAY. POSSIBILITY OF SEA BREEZES
FOR BOTH DAYS WHERE PARTLY CLOUDY TO MOSTLY CLEAR SKIES PREVAIL
AND A CHANCE OF SMALL CRAFT WINDS OVER CROSS SOUND IN THE LATE
AFTERNOON/EARLY EVENING FOR BOTH DAYS. THESE TWO DETAILS HAVE
NOT, AS OF YET, BEEN BUILT INTO THE FORECAST.

MODEL DIVERGENCE AGAIN BECOME AN ISSUE LATE IN THE WEEK WITH THE
APPROACH OF A STRONG LOW. CURRENT MODELS IN AGREEMENT ON THIS
SYSTEM SPINNING UP AS A CLOSED LOW NEAR PRINCE WILLIAM SOUND BUT
DIFFER ON TIMING. THE 18Z GFS IS FASTER, STAMPING A LOW AS EARLY
AS THURSDAY MORNING, THEN TRACKING IT AND THE ASSOCIATED FRONT
RAPIDLY TO THE EASTERN GULF BEFORE TRACKING IT DUE SOUTH. ECMWF IS
SLOWER, BOTH WITH THE DEVELOPMENT OF THE LOW AND FRONTAL PROGRESS
TO THE EAST. CURRENT FORECAST REFLECTS A BIAS TOWARDS THE ECMWF
BUT WITH POP VALUES LIMITED TO LIKELY DUE TO MODEL TIMING
UNCERTAINTY. ENORMOUS QPF IN BOTH MODELS SUGGESTING A STRONG
TROPICAL CONNECTION. HERE TOO, FORECAST QPF AMOUNTS HEAVY
TRUNCATED DUE TO MODEL UNCERTAINTY. WOULD NOT BE SURPRISED TO SEE
BOTH POP AND QPF NUMBERS RISING WITH SUBSEQUENT FORECASTS AND,
HOPEFULLY, BETTER MODEL AGREEMENT.

WIND DIRECTIONS OVER THE GULF FROM A BLEND OF GEM AND ECMWF
PRESSURES WITH INHERITED WIND SPEEDS RETAINED. POP AND QPF FROM A
BLEND OF ECMWF AND GEM AS WELL, WITH THE AFOREMENTIONED CAVEATS.
REMAINDER OF THE FORECAST HEAVILY WEIGHTED TOWARDS WPC. OVERALL
FORECAST CONFIDENCE ABOVE AVERAGE THROUGH WEDNESDAY NIGHT, THEN
BELOW AVERAGE.

&&

.HYDROLOGY...2-2.5" ESTIMATED QPF VALUES MAY PUT MONTANA
CREEK...TAIYA RIVER...JORDAN CREEK AND THE MENDENHALL INTO THE
"ACTION" CATEGORY OR EVEN INTO MINOR FLOODING TOMORROW
AFTERNOON...WITH THE HIGHEST RISKS CURRENTLY ESTIMATED TO BE THE
TAIYA (CURRENTLY LESS THAN 1 FOOT BELOW ACTION LEVEL) AND JORDAN
CREEK (LESS THAN 1 FOOT BELOW ACTION).

&&

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...STRONG WIND UNTIL 10 PM AKDT THIS EVENING FOR AKZ025.
         STRONG WIND UNTIL 4 PM AKDT THIS AFTERNOON FOR AKZ022.
         STRONG WIND UNTIL 10 PM AKDT THIS EVENING FOR AKZ023.
MARINE...GALE WARNING FOR PKZ013-022-031-041>043-051-052.
         SMALL CRAFT ADVISORY FOR PKZ011-012-021-032>036-053.

&&

WESLEY/FRITSCH

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