Small Craft Advisory 489, NWS Juneau, AK

SOUTHEAST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE JUNEAU AK
322 PM AKDT SUN SEP 2 2012

.SHORT TERM...SOMEWHAT QUIET DAY ACROSS THE AREA. SCATTERED
SHOWERS AROUND THE AREA IN THE MOIST ONSHORE FLOW MAINLY IN THE
NORTHERN HALF OF THE PANHANDLE. THE SOUTHERN HALF EXPERIENCED A
FEW MORE BREAKS IN THE CLOUD COVER EARLIER THIS MORNING BUT MORE
CLOUDS FROM A WEAK TROUGH ARE STARTING TO SPREAD OVER THE OUTER
COAST WITH MORE STRATIFORM LIGHT PRECIP SHOWING UP ON RADAR JUST
OFFSHORE. WINDS TODAY ARE NOT MUCH OF A PROBLEM EITHER WITH MOST
PLACES REPORTING LESS THEN 20 KT. THE EXCEPTION IS NORTHERN LYNN
CANAL WHERE SOUTHERLY WINDS TO 25 KT HAVE BEEN BLOWING MOST OF
THE MORNING.

INTO TONIGHT AND TOMORROW, SHOWERS WILL GIVE WAY TO MORE
STRATIFORM PRECIP THIS AFTERNOON AND EVENING AS THE WEAK TROUGH
MOVES COMPLETELY ONSHORE. NOT MUCH MOISTURE NOR UPPER LEVEL
SUPPORT ASSOCIATED WITH IT SO RAINFALL FROM THIS FEATURE WILL BE
LIGHT AND WINDS WILL BE LOW. LATER TONIGHT IT WILL GET HUNG UP ON
THE COAST MOUNTAINS AND WEAKEN AWAY.

ALMOST IMMEDIATELY ON THE TROUGHS HEELS IS A WARM FRONTAL FEATURE
THAT WILL BRING HIGHER POPS AND QPF TO THE AREA LATE TONIGHT AND
INTO TOMORROW. A BIT MORE MOISTURE AND UPPER LEVEL SUPPORT WITH
THIS FEATURE AS THERE IS A WEAK VORTICITY MAX AND SLIGHT TROUGH AT
500 MB. THERE IS ALSO A DISTINCT BOUNDARY AS FAR AS THICKNESS IS
CONCERNED AS THERE IS A 10 DAM INCREASE IN 1000 TO 500 MB
THICKNESS ACROSS IT. SO RAINFALL AMOUNTS WILL BE A BIT MORE THEN
WHAT THE PREVIOUS TROUGH DROPPED. WINDS WILL ALSO BE A BIT HIGHER
BUT NOT OVERLY SO. HIGHEST WINDS EXPECTED WITH IT WILL BE IN THE
AREA OF CAPE SUCKLING WHERE A BARRIER JET WITH 30 TO 35 KT WINDS
WILL FORM EARLY TOMORROW. OTHERWISE MOST WINDS WILL BE 25 KT OR
LESS.

MODEL AGREEMENT IS GOOD TODAY WITH NOT MUCH DIFFERENCE NOTED
BETWEEN THE VARIOUS MODELS. THE MOST NOTABLE DIFFERENCES WAS GFS
CONTINUED TO BE A BIT TOO FLAT REPRESENTING THE RIDGE OVER THE
PANHANDLE AND THE INCOMING TROUGH FROM THE GULF. DECIDED ON USING
THE NAM AND EC FOR UPDATES AS BOTH SEEMED TO DO WELL ON
REPRESENTING THE CURRENT FEATURES. FORECAST CONFIDENCE IS ABOVE
AVERAGE.

&&

.LONG TERM...TROUGH OVER SOUTHEAST ALASKA MOVES INTO CANADA MONDAY
NIGHT. A WAVE ALONG THE TRAILING BAROCLINIC ZONE OVER THE SOUTH
CENTRAL GULF LIFTS NE AND ACROSS SOUTHEAST ALASKA EARLY TUESDAY. A
FLAT RIDGE BUILDS OVER THE PANHANDLE TUESDAY NIGHT AND WEDNESDAY.
NEXT FRONT MOVES IN THE EASTERN GULF WEDNESDAY AND THEN WEAKENS AS
IT DROPS SOUTHEAST THROUGH SOUTHEAST ALASKA WEDNESDAY NIGHT. A
FLAT RIDGE REBUILDS ACROSS THE GULF AND SOUTHEAST ALASKA THURSDAY
AND FRIDAY.

RAIN WILL CONTINUE ACROSS ALL SOUTHEAST ALASKA MONDAY NIGHT INTO
EARLY TUESDAY AS TROUGH AND WAVE MOVE THROUGH. RAIN TAPERS OFF TO
A CHANCE OF SHOWERS TUESDAY AFTERNOON THROUGH WEDNESDAY MORNING
WITH ONSHORE FLOW OVER FLAT RIDGE. WITH RIDGE TO SOUTH AND LOW
OVER YUKON SOUTHERLY GRADIENTS BUILD TO SMCR LEVELS IN NORTHERN
LYNN CANAL TUESDAY NIGHT AND TO GALE FORCE ON WEDNESDAY.

NEXT FRONT WILL PUSH INTO THE EASTERN GULF TUESDAY NIGHT WITH A SE
BARRIER JET SETTING UP ACROSS THE NORTHERN GULF. THIS WEAKEN TO A
GENERAL SE SMCR IN OUR COASTAL WATERS WEDNESDAY. FRONT MOVES
ONSHORE WEDNESDAY NIGHT WITH RAIN DEVELOPING FOR NORTHERN AREAS BY
EARLY EVENING. FRONT WEAKENS AS IT DROPS SOUTHEAST SO JUST GAVE
SOUTHERN ZONES A CHANCE OF RAIN.

ONCE THIS FRONT MOVES THROUGH A FLAT RIDGE DOMINATES THE GULF WITH
A SW SMCR AND SEAS BUILDING IN THE 9 TO 13 FEET RANGE BY THURSDAY.
WITH ONSHORE FLOW WILL KEEP CLOUDS AND A CHANCE CHANCE OF SHOWERS
GO FOR THE AREA. THE MAIN EXCEPTION WILL BE OVER THE SOUTHEAST
CORNER OF THE PANHANDLE WHERE A NW WIND WILL HELP KEEP THE AREA
DRY WITH ENOUGH BREAKS TO GO PARTLY CLOUDY.

GOOD MODEL AGREEMENT THROUGH THE LONG TERM. USED A NAM/EC BLEND
THROUGH TUESDAY THEN A GFS/EC/HPC BLEND BEYOND.

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...GALE WARNING FOR PKZ052.
         SMALL CRAFT ADVISORY FOR PKZ012-043-051.
         SMALL CRAFT ADVISORY DUE TO SEAS FOR PKZ041-042.

&&

EAL/MM