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000
FXAK67 PAJK 181413
AFDAJK

SOUTHEAST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE JUNEAU AK
613 AM AKDT SAT MAY 18 2013

.SHORT TERM...EARLY MORNING SATELLITE IMAGE SHOWS A LOW...ABOUT
996 MB...NEAR PRINCE WILLIAM SOUND WITH AN OCCLUDED FRONT
STRETCHING SOUTHEASTWARD ACROSS THE PANHANDLE. A WEAK SURFACE
TROUGH DEVELOPS ACROSS THE EASTERN GULF BEHIND THE TRAILING EDGE
OF THE OCCLUSION. A COLD FRONT IS EXTENDING SOUTHWARD FROM THE
LOW ACROSS THE CENTRAL GULF AND SLOWLY PROPAGATING EASTWARD.
AHEAD AND ALONG THE OCCLUDED FRONTAL PASSAGE...SOME GUSTY EAST TO
SOUTHEAST WINDS ARE SWEEPING ACROSS THE PANHANDLE WITH STEADY
RAIN. OVERNIGHT TEMPERATURES REMAIN MOSTLY IN THE 40S.

 MODELS ARE IN FAIR AGREEMENT AND QUITE PERSISTENT WITH THE LOW
NEAR PRINCE WILLIAM SOUND MOVES NORTHEASTWARD INTO YUKON AND
WEAKENS THROUGH LATE TONIGHT. THE FRONTAL PASSAGE WILL CONTINUE
TO MOVE ACROSS THE PANHANDLE THIS MORNING...THEN PUSH THE LIKELY
TO CATEGORICAL POPS CLOSER TO COAST MOUNTAIN RANGE...LEAVING
SCATTERED RAIN SHOWERS BEHIND ACROSS THE PANHANDLE THIS
AFTERNOON. REFRESHED THE INHERITED FORECAST PACKAGE WITH AN
ENSEMBLE APPROACH SINCE ALL MODELS ARE DRAWING SIMILAR SOLUTIONS.
A WEAK LOW WILL DEVELOP IN THE NORTHERN GULF TONIGHT THEN REMAIN
THERE THROUGH SUNDAY MORNING. HIGH PRESSURE BUILDING OVER THE
WESTERN GULF TONIGHT WILL EXPAND NORTHEASTWARD TO THE CENTRAL
GULF THROUGH SUNDAY MORNING.

 ALONG THE COLD FRONT...STRONG UPPER LEVEL VORT MAX WILL MOVE
TOWARD NORTHEASTERN GULF COAST WITH GENERALLY UNSTABLE
ATMOSPHERIC CONDITIONS. COMBINED WITH UNUSUAL LATE SEASON COLD
AIR ALOFT...SLEET MAY MIX WITH RAIN SHOWERS ACROSS THE
NORTHEASTERN GULF COAST. EXPECT SCATTERED SNOW SHOWERS NEAR
YAKUTAT LATE TONIGHT. THE WATER LEVEL AT SITUK RIVER NEAR YAKUTAT
WILL CONTINUE TO RISE THROUGH LATE THIS MORNING THEN BEGIN
FALLING THIS AFTERNOON. THE COLD AIR ALOFT WILL CONTINUE TO
SPREAD SOUTHEASTWARD THROUGH LATE TONIGHT. INCREASING SOUTHERLY
PRESSURE GRADIENT WILL MAINTAIN SMALL CRAFT LEVEL WINDS OVER THE
NORTHERN INNER CHANNELS AND CENTRAL OUTER COASTAL WATERS TODAY.
LONG PERIOD SOUTHWEST TO WEST SWELLS WILL PRODUCE SMALL CRAFT
SEAS MUCH OF THE OUTSIDE WATERS.

.LONG TERM...GRAND 500 MB TROUGH POSITIONED OVER THE NORTHEAST
GULF SUNDAY MORNING WILL DIG SOUTHWARD AS IT SLOWLY MOVES EAST INTO
BC...THEREBY ALLOWING DRIER NORTHWEST FLOW TO BEGIN INFILTRATING
THE PANHANDLE LATE SUNDAY NIGHT AND LASTING THROUGH MIDWEEK. THIS
WILL DICTATE THE THREE MAIN WEATHER STORIES FOR THE LONG TERM,
HIGHLIGHTED BY THE CLEAR FROST MORNINGS EARLY NEXT WEEK FOR
YAKUTAT, THE VILLAGES IN THE CROSS SOUND/GLACIER BAY/ICY STRAIT
AREA, AND EAST INTO THE MENDENHALL VALLEY OF JUNEAU.

SUNDAY MORNING AT THE SURFACE, LOW PRESSURE OVER THE YUKON WILL
CONTINUE TO PULL AWAY TO THE NORTHEAST. AT THE SAME TIME A WEAK
SURFACE LOW WILL EASE EASTWARD ACROSS THE NORTHERN GULF ON
AFTERNOON AND DISSIPATE INTO NOTHING MORE THAN A ILL-DEFINED
TROUGH SUNDAY NIGHT. THIS WILL PROVIDE SOME DYNAMICS FOR SHOWERS
TO CONTINUE SUNDAY INTO SUNDAY EVENING FOR THE PANHANDLE. THE COLD
AIR MASS IN PLACE ALOFT...REPRESENTED BY -8 C TEMPS ACROSS THE
EASTERN GULF SUNDAY WILL PROVIDE SOME GOOD SOLID INSTABILITY OVER
BOTH THE WATER AND THE PANHANDLE SUNDAY. NORTHERN AND CENTRAL PANHANDLE
RESIDENTS WILL NOTICE TEMPERATURES COOLING INTO THE MIDDLE TO
UPPER 40S SUNDAY AFTERNOON. AND THERE IS A SMALL POSSIBILITY THAT
SOME SHOWERS MAY MIX WITH SOME SLEET. BY SLEET I MEAN SOME FAT
GLOBULAR RAIN DROPS THAT LOOK AND BEHAVE A LITTLE SOLID-LIKE WHILE
CARRYING A VERY LIQUIDY COATING. BUT THERE ARE GREATER REASONS WHY
IT WILL NOT OCCUR. IF IT DOES OCCUR, THINK IT WILL BE VERY LOCAL
AND WILL NOT LAST LONG. WHY? THE HIGHEST CAPE VALUES SEEM TO BE
CENTERED OFFSHORE OF THE PANHANDLE OVER THE EASTERN GULF WHERE THE
COLDEST AIR ALOFT IS SITUATED. AND EVEN THESE VALUES...JUST IN
EXCESS OF 100 JOULES PER KG...ARE NOTHING TOO MUCH TO GET EXCITED
ABOUT. ANOTHER LIMITING FACTOR IS THE WEAKENING SURFACE LOW WHICH
ALSO CORRESPONDS TO A WEAKENING VORT MAX AT 500 AS WELL.
FINALLY...NAM, GFS, AND ECMWF ALL SUGGESTING WINDS TURNING
OFFSHORE FAIRLY QUICKLY SUNDAY EVENING OVER THE PANHANDLE AND THIS
WILL SERVE TO DRY OUT THE MID-LEVELS AND SHEAR AWAY CLOUD
DEVELOPMENT...AT LEAST ON SUNDAY EVENING OVER THE NORTHERN
PANHANDLE. THESE ARE ALL REASONS WHY I CHOSE TO OMIT ANY FULL-
BLOWN MENTIONS OF SLEET FOR AREAS SOUTHEAST OF YAKUTAT. IN
YAKUTAT`S CASE, THE VORT MAX IS STRONGER AS IT PASSES OVER THE
NORTHEAST GULF SUNDAY MORNING AND TEMPERATURES ARE A BIT COOLER.

ONCE THIS OFFSHORE FLOW TAKES OVER AT MOST ALL LEVELS LATE SUNDAY
NIGHT...HEAT WILL RADIATE NICELY THEREBY GIVING MUCH OF THE
NORTHERN PANHANDLE SOME UNCHARACTERISTIC POST-MOTHER`S DAY MAY
FROST. PREPARE A SPOT FOR YOUR PLANTS NOW. SUNSHINE WILL WARM MOST
OF US BACK INTO THE 50S ON MONDAY...EVEN COLD AIR MASSES CAN ONLY
DO SO MUCH GIVEN MID-MAY SUN. BUT TEMPERATURES WILL COOL DOWN
QUICKER MONDAY NIGHT WITH THE GREATEST POTENTIAL FOR FROST
OCCURRING EARLY TUESDAY MORNING. BY EARLY WEDNESDAY MORNING THE AIR MASS
BEGINS TO MODERATE SOMEWHAT AND THINK THE EXPANSE OF FROST WILL
SHRINK FROM TUESDAY MORNING. IT MUST BE SAID...WHILE ADVERTISING
THIS COLD SNAP WITH A SPECIAL WEATHER STATEMENT OVER ZONES 17, 20,
21, 22, AND 25, NOT ALL OF THESE AREAS WILL EXPERIENCE FROST.
AREAS CLOSE TO THE INSIDE WATERS WILL ABSORB SOME HEAT FROM THE
WATERS AND KEEP TEMPERATURES ABOVE FREEZING. ALSO, IT IS POSSIBLE
THAT SOME AREAS AT HIGHER ELEVATIONS MAY NOT BE AS COLD AS THE
LOWER VALLEY BOTTOMS DUE TO SOME COLD AIR DRAINAGE...ESPECIALLY
DURING MONDAY NIGHT/TUESDAY MORNING AS WINDS WILL BE FAIRLY LIGHT
IN THESE LOCATIONS.

FINALLY THE BIG STORY NEXT WEEK WILL BE A RETURN TO DRY WEATHER
AND SOME BEAUTIFUL DAYS. A RIDGE BUILDS IN THE GULF EARLY NEXT
WEEK AND STRENGTHENS NORTHWEST WINDS OVER THE OUTSIDE WATERS AND
SOME NORTHERLY GRADIENTS OVER THE INSIDE PASSAGES BUT WILL ALSO
KEEP PRECIPITATION AT BAY...WELL TO THE WEST OF US. THERE MAY BE
SOME RISK THAT MOISTURE FROM AN EASTERLY WAVE MAY CREEP OVER THE
COAST MOUNTAINS LATE NEXT WEEK, BUT LITTLE REASON TO CHANGE THE
FORECAST.

MODELS ARE IN FAIR AGREEMENT THROUGH WEDNESDAY. MADE ONLY A FEW
LOCAL CHANGES TO WINDS, BUT UPDATES ON PRESSURE FIELD WOULD DEMAND
LITTLE CHANGES, SO ELECTED NOT TO. MAINLY SPENT TIME ON REFINING
TRANSITIONAL FORECAST FROM SUNDAY INTO MONDAY AS WELL AS THE
FROSTY TEMPERATURES INTO TUESDAY MORNING AND POSSIBLY INTO
WEDNESDAY MORNING. CONFIDENCE IN FORECAST AVERAGE TO GOOD.

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...SMALL CRAFT ADVISORY FOR PKZ012-013-036-043.
         SMALL CRAFT ADVISORY DUE TO SEAS FOR PKZ041-042-051-052.

&&

$$

AHN/JWA