Small Craft Advisory 557, NWS Juneau, AK

SOUTHEAST ALASKA FORECAST DISCUSSION...CORRECTED
NATIONAL WEATHER SERVICE JUNEAU AK
350 PM AKDT FRI SEP 28 2012

.SHORT TERM...LOW ALOFT OVER THE SEWARD PENINSULA PROVIDES
CYCLONIC FLOW ON A LARGE SCALE THAT INCLUDES OUR ENTIRE AREA OF
RESPONSIBILITY. THE POLAR FRONT ZONE WILL REMAIN S OF OUR AREA
THROUGH SATURDAY. THE CONVECTIVE FIELD WILL CONTINUE TO COVERED
OUR ENTIRE AREA THROUGH SAT.

NONE OF THE MODEL INITIALIZATIONS HAD IMPORTANT PROBLEMS AND
THEIR CONSENSUS REMAINS USEFUL.  CHANGES TO THE FORECAST PACKAGE
ARE FEW. THE VAST AREAL COVERAGE OF CONVECTIVE CELLS, AND THEIR
TENDENCY TO NOT BE ORGANIZED PROVIDES A LACK OF DETAIL TO BE
USED FOR TRACKING MODEL FEATURES ALOFT.

THIS CONVECTIVE AIRMASS WILL HAVE A SMALL CONVECTIVE ENHANCEMENT
MOVING FROM THE EAST GULF TO ACROSS THE PANHANDLE TO CANADA
BY SATURDAY MORNING. ISOLATED THUNDERSTORMS CAN OCCUR IN THE
EASTERN
GULF TONIGHT AND CAN EXTEND AS FAR SOUTH AS DIXON ENTRANCE WITH
THE CONVECTIVE BLOOM. THE ECMWF AND SREF POINTED TO THIS CONVECTIVE
BLOOM.

&&

.LONG TERM...COMPLEX AND UNCERTAIN EXTENDED WEATHER PATTERN IN
STORM FOR SOUTHEAST AK. AT MID LEVELS: 500 MB LOW DROPS FROM THE
ARCTIC INTO BRISTOL BAY THEN OVER THE ALEUTIANS AND INTO THE
NORTHWESTERN AK GULF SATURDAY INTO SUNDAY. THIS LOW WILL WEAKEN TO
AN OPEN WAVE TROUGH AS IT MOVES OVER THE PANHANDLE THROUGH MONDAY
THEN DROPS SOUTH OVER BRITISH COLUMBIA TUESDAY. TO THE SOUTH
FRONTAL BARRIER IS EVIDENT FROM 300 MB JET STREAM AND 500 MB VORT
MAX. MEANWHILE RIDGE OF HIGH PRESSURE WILL BUILD OVER THE WESTERN
AK GULF THEN TRACK EASTWARD. RIDGE AXIS WILL BE OVER THE EASTERN
GULF MONDAY EVENING THEN OVER THE PANHANDLE TUESDAY EVENING. BEYOND
TUESDAY LARGE UNCERTAINTY REGARDING SHORTWAVES TO THE WEST OF THE
RIDGE AND THE TIMING OF THE EVENTUAL RIDGE BREAK DOWN.

AT THE SURFACE: A RE DEVELOPING LOW IN THE NORTHWEST GULF
ASSOCIATED WITH THE UPPER LEVEL LOW WILL TRACK ACROSS THE NORTHERN
GULF THROUGH MONDAY. SHOWER ACTIVITY WILL CONTINUE OVER THE
PANHANDLE BEFORE THIS LOW WEAKENS AND EXITS THE REGION TO THE EAST.
IN THE NORTH PACIFIC A SECOND SHORT WAVE MOVES TO THE NORTHEAST
TOWARD HAIDA GWAII. THIS WAVE IS ASSOCIATED WITH THE FRONTAL
BOUNDARY AND WHILE MODELS ARE IN AGREEMENT THAT THE LOW EXISTS
THERE ARE DIFFERENCES IN THE STRENGTH AND POSITION OF POSSIBLE
MULTIPLE LOW CENTERS. HOWEVER THESE SYSTEMS ARE TRENDING TO REMAIN
JUST SOUTH OF THE REGION. BY LATE MONDAY HIGH PRESSURE RIDGE AT THE
SURFACE WILL BECOME THE DOMINANT FEATURE AND TREND TOWARD A DRY
PERIOD. STABLE AIR AND ONSHORE FLOW MAY PROMOTE DEVELOPMENT OF
MARINE LAYER. BY MID WEEK INDICATION OF NEXT LOW PRESSURE SYSTEM
MOVING IN...BUT AGAIN MODELS ARE SHOWING LARGE VARIATIONS ON TIMING
AND STRENGTH OF THIS SYSTEM. EXPECT THE FORECAST FORM WEDNESDAY
ONWARD TO UNDERGO QUITE A FEW CHANGES.

NAM/ECMWF WERE IN FAIR AGREEMENT WITH THE GFS BIT MORE OF AN
OUTLIER AT THE START OF THE LONG RANGE FORECAST. GFS CONTINUED ITS
TREND OF BEING INCONSISTENT RUN TO RUN AND WITH OTHER
MODELS...ESPECIALLY STARTING WITH THE 12Z RUN BY BOMBING A LOW IN
THE WESTERN GULF BY TUESDAY. OPERATIONAL ECMWF WHILE A BIT BETTER
HAS NOT BEEN VERY CONSISTENT RUN TO RUN EITHER. REFRESHED THE
FORECAST TO DAY 3 USING 12Z ECMWF/NAM BLEND WITH ONLY MINOR
CHANGES. WIND WILL TRANSITION FROM SOUTHERLY TO NORTHERLY SUNDAY
INTO MONDAY WITH THE BUILDING RIDGE BUT AS THEY WILL BE LIGHT AND
GRADIENT WILL BE WEAK BIT OF UNCERTAINTY REGARDING THE TIMING. FOR
THE REMAINDER OF THE LONG RANGE USED UPDATED HPC FOR AN ENSEMBLE
APPROACH. FORECAST CONFIDENCE IS BELOW AVERAGE.

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...STRONG WIND FROM 7 PM AKDT THIS EVENING THROUGH LATE
         TONIGHT FOR AKZ018.
MARINE...GALE WARNING FOR PKZ012.
         SMALL CRAFT ADVISORY FOR PKZ011-013-022-036-052.
         SMALL CRAFT ADVISORY DUE TO SEAS FOR PKZ041>043-051-053.

&&

JBT/PRB