Area Forecast Discussion - Southeastern


FXAK67 PAJK 172329
AFDAJK

SOUTHEAST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE JUNEAU AK
329 PM AKDT FRI APR 17 2015
.SHORT TERM...SHOWERS CONTINUE TO BE THE RULE OF THE DAY ACROSS
THE PANHANDLE THOUGH NOT FOR MUCH LONGER. LOCAL UPPER AIR
SOUNDINGS SHOW AROUND 100 TO 200 J/KG OF CAPE BUT UPPER LEVEL
TEMPS ARE STARTING TO WARM UP BRING MORE STABILITY TO THE AREA.
THE WEAK TROUGH THAT BROUGHT THE STRONGER SHOWERS TO THE AREA THIS
MORNING IS STARTING TO MOVE INTO CANADA. BEHIND IT SHOWERS ARE
BECOMING FEWER AND FARTHER BETWEEN AS A SURFACE RIDGE BUILDS
ACROSS THE SE GULF AND DIFFLUENT FLOW ALOFT BEGINS TO DEVELOP.
CLOUDS ARE STARTING TO BREAK UP AS WELL WITH AREAS OF SUN
DEVELOPING. STILL THERE ARE ONE OR TWO STRONGER SHOWERS AROUND.
YAKUTAT HAD A SHOWER MOVE THROUGH AROUND 1130 AM THAT DROPPED A
QUARTER INCH OF SMALL HAIL AND ICE PELLETS ON THE AIRPORT.

THIS EVENING IS LOOKING MOSTLY DRY WITH SOME MOSTLY TO PARTLY
CLOUDY SKIES AS SHOWERS CONTINUE TO SHOW A DIMINISHING TREND. SOME
SCATTERED TO ISOLATED SHOWERS COULD STILL LINGER, ESPECIALLY NEAR
THE HIGHER TERRAIN. THESE SHOWERS SHOULD REMAIN LIGHT FOR THE
MOST PART AS WHAT UPPER SUPPORT THEY HAD HAS MOVED INTO CANADA
BY THIS POINT. WINDS TOO WILL BE ON THE DECREASING TREND. THE
GUSTY SOUTHERLY WINDS IN LYNN CANAL AND SKAGWAY WILL DIMINISH THIS
EVENING AS THE BUILDING RIDGE GET FAR ENOUGH NORTH THAT PRESSURE
GRADIENTS TURN MORE PARALLEL TO LYNN CANAL THEN PERPENDICULAR.

LATER TONIGHT AND INTO TOMORROW, FOCUS SHIFTS TO THE GALE FORCE
FRONT THAT IS ADVANCING ACROSS THE GULF. MOST OF THE GULF WILL
EXPERIENCE GALE FORCE WINDS WITH THE HIGHEST WINDS NEAR CAPE
SUCKLING DUE TO BARRIER JET EFFECTS. THESE WINDS WILL SPREAD INTO
THE NORTHERN PANHANDLE TOMORROW AS THE FRONT DRIFTS TO THE EAST.
NOT EXPECTING TO NEED STRONG WIND HEADLINES FOR THE NORTHERN OUTER
COAST AS BARRIER INFLUENCES KEEP MOST OF THE STRONG WIND OFFSHORE.
AS FOR RAIN, IT LOOKS TO COME IN TWO MAIN BURSTS. HOW FAR EAST
EACH BURST GETS WILL LIKELY DEPEND ON HOW STRONG THE RIDGE OVER
BRITISH COLUMBIA IS THAT IS SUPPRESSING THE EASTWARD MOVEMENT OF
THE FRONT AS A WHOLE. THE FIRST PULSE WILL BE LATE TONIGHT AS AN
INITIAL WARM FRONT CROSSES THE PANHANDLE. MOST AREAS EXCEPT FOR THE
EXTREME SOUTHERN PANHANDLE WILL LIKELY SEE RAIN OUT OF THIS
FEATURE WITH LIGHTER RAINFALL AMOUNTS THE FARTHER EAST AND SOUTH
YOU ARE. 850 MB TEMPS AHEAD OF THE FRONT ARE STILL AROUND -7 TO -6
C OVER THE EXTREME NORTHERN PANHANDLE SO RAIN MAY MIX WITH SNOW
AROUND YAKUTAT LATE TONIGHT BEFORE THE HEAVIER RAIN AND STRONGER
WARM AIR ADVECTION MAKES IT INTO THE AREA. PULSE NUMBER TWO IS A
TRAILING WAVE ON THE COLD FRONT THAT WILL PUSH THE FRONTAL RAIN
BANDS INTO THE PANHANDLE ON SAT. RIGHT NOW MOST OF THE RAIN
APPEARS TO BE ON A LINE FROM TAKU INLET DOWN TO FIVE FINGERS TO
JUST WEST OF KLAWOCK AND WESTWARD. EAST OF THAT LINE RAIN CHANCES
DROP TO CHANCE OR LESS FOR SAT AS THE BRITISH COLUMBIA RIDGE
BLOCKS ANY MORE EASTWARD MOVEMENT. HEAVIEST RAINFALL WILL MOSTLY
BE OVER THE YAKUTAT AREA AS THE MAIN MOISTURE PLUME IS AIMED IN
THAT DIRECTION AND EFFECTS FROM TERRAIN WILL JUST WRING EVEN MORE
RAIN OUT OF THE CLOUDS.

WINDS WILL ALSO BE INCREASING ACROSS THE NORTHERN PANHANDLE AS
WELL AS THE FRONT GETS CLOSER. MOST AREAS SHOULD HAVE SOME GUSTY
WINDS BUT I DO NOT THINK STRONG WIND HEADLINES WILL BE NEEDED AS
850 MB WINDS ONLY REACH 35 TO 40 KT AND THE PARENT LOW IS WELL
WEST OF THE AREA. STILL LYNN CANAL, SOUTHERN CHATHAM STRAIT AND
CROSS SOUND WILL LIKELY SEE 25 TO 30 KT SMALL CRAFT WINDS
DEVELOPING TOMORROW.

SHORT RANGE MODELS WERE IN GOOD AGREEMENT WITH ONLY VERY MINOR
DIFFERENCES THAT DID NOT AFFECT THE OVERALL FORECAST. MAIN CHANGES
WERE SMALL LOCAL EFFECT CHANGES. OF NOTE IS THAT THE MODELS ARE
ONCE AGAIN UNDERESTIMATING THE SOUTHERLY PRESSURE GRADIENT THAT
DEVELOPED AROUND LYNN CANAL. MOST WERE 1 TO 2 MB TOO WEAK AND AS
SUCH TOO WEAK WITH WINDS. MAIN CHANGE FROM THIS WAS INCREASING WINDS
IN LYNN CANAL TO SMALL CRAFT FOR EARLY THIS EVENING AND TOMORROW
AFTERNOON.
.LONG TERM...MODELS CONSISTENT WITH PREVIOUS RUNS IN PRODUCING 2.5
TO 4 INCHES OF RAIN IN THE YAKUTAT AREA IN A 3-DAY PERIOD
BEGINNING LATE TONIGHT. 2-3 INCHES IS ALSO PREDICTED DURING THIS
TIME PERIOD IN JUNEAU AND AROUND SITKA WITH LESSER AMOUNTS IN
OTHER SEA LEVEL LOCATIONS AND OFFSHORE. THIS IS IN RESPONSE TO
WELL-ADVERTISED FRONT WHICH WILL PUSH INTO THE PANHANDLE TOMORROW.
THE SITUK RIVER IS STILL SHOWING LESS THAN 67 FEET...FLOOD IS AT
71.5...COORDINATED THINKING WITH RFC ON CONF CALL TODAY AND AGREE
WITH THEM TO MONITOR AND THAT BANKFULL IS IMPROBABLE AT THIS
POINT. THOUGH FREEZING LEVELS WILL RISE IN THAT AREA WILL STRUGGLE
EARLY IN THE SYSTEM TO GET ABOVE 3000 FEET AND WILL FALL ONCE THE
SLOW FRONT MOVES THROUGH. THIS WILL RESTRICT CONTRIBUTION OF
SNOWMELT TO RUNOFF. 

INHERITED PRESSURE/WIND GRIDS WERE GOOD FOR
TOMORROW NIGHT AND SUNDAY BASED ON NEW MODEL DATA WITH JUST SOME
UPWARD TWEAKS IN THE CHANNELS WITH ORIENTATION PERPENDICULAR TO
ISOBARS. OFFSHORE GALES WILL BE COMMON THROUGH ABOUT SUNDAY
MORNING WITH THE FRONT WHILE SCA WINDS WILL PREVAIL IN CROSS AND
FAVORED SOUTHERN CHANNELS INTO SUNDAY ALSO. ONLY THREAT FOR
WEEKEND MIXED STRATIFORM PRECIP SHOULD BE UP THE ROADS AND AMOUNTS
OF WET SNOW SHOULD BE LIGHT AND CONFINED MOSTLY TO LATE NIGHT
PERIODS. I HAVE AN INCH OR SO AT WHITE PASS ON SATURDAY AND SUNDAY
NIGHTS. SLOW-MOVING FRONTAL PRECIP SHOULD DIMINISH MONDAY
AFTERNOON AND A SHOWERY PERIOD SHOULD BEGIN WITH LESS UPPER
FORCING BUT MORE SOLAR EFFECTS. WITH WEAKENING PRESSURE GRADIENTS
THE CONSENSUS ON MODEL MSLP FIELDS DETERIORATES BY MONDAY. WENT
WITH AN EC/NAM COMBO ALONG WITH INHERITED GRIDS THROUGH MONDAY BUT
WITHOUT A LOT OF CONFIDENCE FOR THE LATTER PERIOD. THEN USED
MOSTLY EC/INHERITED ON THE WAY TO WPC GUIDANCE FOR THE EXTENDED
PERIOD. THE PREVIOUSLY DRYING TREND BY MIDWEEK IS CERTAINLY IN
DANGER AT THIS POINT AS MODELS DELAY THE RIDGING ALOFT...ALONG WITH
PERSISTING -25 TO -35C TEMPS AT 500 THROUGH WEDNESDAY. THUS ANY
SURFACE WARMING WILL CERTAINLY LEAD TO SHOWERS. THIS IS REFLECTED
IN THE ZONE FORECASTS FROM TUE ONWARDS. NEW EC DETERMINISTIC RUN
NOW DELAYS UPPER RIDGING UNTIL THE END OF THE WEEK...A HIGH-
AMPLITUDE UPPER PATTERN IS LIKELY WHICH MODELS HAVE TYPICALLY MORE
PROBLEMS WITH...AND COULD BECOME SLOW-MOVING. AGREEMENT BETWEEN
THE DETERMINISTIC EC/GFS AT MID-LEVELS IS REASONABLE THROUGH
THURSDAY WITH BOTH ADVERTISING DEVELOPMENT OF A RIDGE OVER THE
WESTERN GULF AT THAT POINT. EC CHANGES VERY LITTLE BY FRI WHILE THE
GFS SHOWS A STRANGE ELONGATED NW-SE LOW OVER THE EASTERN GULF BY
FRI.

CAN SUMMARIZE THE EXTENDED WITH A STATEMENT OF LITTLE
CONFIDENCE...PROBABLY IS BETTER STATISTICALLY TO TRUST AN ENSEMBLE
APPROACH WHICH IS WHAT WPC CHOSE. A SIGNIFICANT DRYING/WARMING
PATTERN OVER THE PANHANDLE IS STILL IN THE CARDS...JUST LIKELY DELAYED
UNTIL LATE NEXT WEEK. FINALLY DECIDED TO INCREASE SKY COVER FOR
THE WED-THU PERIOD AS AFTERNOON SHOWER DEVELOPMENT IS LIKELY AS
LONG AS CYCLONIC COOL FLOW ALOFT PERSISTS...WITHOUT FRONTAL
FORCING, SURFACE HEATING WILL LIKELY BE A FACTOR. THIS MAY BE
EXTENDING CLOUDY CONDITIONS FOR TOO LONG BUT UPCOMING MODEL RUNS
WILL TELL. TRYING TO AVOID THE "PARTLY CLOUDY, CHANCE SHOWERS"
RUBBER STAMP.

&&

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...GALE WARNING FOR PKZ042-043-051-052. 
         SMALL CRAFT ADVISORY FOR PKZ012-022-033-041-053. 

&&

$$

EAL/WESLEY

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