Small Craft Advisory 343, NWS Juneau, AK

SOUTHEAST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE JUNEAU AK
132 PM AKDT SAT MAY 14 2011

.SHORT TERM...DIFFERENTIAL HEATING AND SEA BREEZE EFFECTS WILL
MATTER  TODAY AND SUNDAY. A INVERTED TROUGH ALONG THE PANHANDLE
IS PROVIDING A PRESSURE GRADIENT FOR NW WINDS, AND CASES WHERE
THE SEA BREEZE IS IN CONCERT CAN DEVELOP NW WINDS TO 20 KT.
TODAY'S DIURNAL CYCLE OF WINDS WILL BE APPROXIMATELY REPEATED FOR
THE COASTAL CHANNELS AND LAND MASSES FOR SUNDAY. FROST IS
EXPECTED ON RAISED SURFACES LATE TONIGHT AND AN ADVISORY IS BEING
PREPARED: SEE TABLE BELOW.

 ON THE GOES VISIBLE AND IR, NEARLY ALL THE AREA OF
RESPONSIBILITY (OAR) ONSHORE IS CLEAR OR HAS ONLY THIN CLOUDS.
RADIATIVE COOLING WILL OCCUR FREELY TONIGHT, WITH THE YAKUTAT
ZONE 17 EXPECTED TO GET PATCHY FOG.

 SHORT-WAVE TROUGHS ARE MOVING TOWARD THE SEN NHL FM THE SSE AND
WILL INCREASE THE CLOUD COVER OVER THE SOUTHERN PANHANDLE. FOR
THE ONSET OF  PRECIP IN THE PANHANDLE, SEE THE LONG RANGE SECTION
BELOW.

 &&

 .LONG TERM...STILL SEEING SIGNIFICANT DIFFERENCES, FROM RUN TO
RUN AND BETWEEN THE MODELS BOTH AT THE SURFACE AND ALOFT. DOES
NOT BODE WELL FOR CONFIDENCE IN THE EXTENDED RANGE FORECAST SO FAR
AS THE DETAILS ARE CONCERNED. SYNOPTIC SETUP AT THE BEGINNING OF
THE LONG RANGE FORECAST WILL FEATURE A SHARPLY DEFINED RIDGE OVER
THE CENTRAL GULF SEPARATING A BROAD AREA OF LOW PRESSURE OVER
BRITISH COLUMBIA AND AN OCCLUDED LOW OVER THE ALEUTIANS WEST OF
BRISTOL BAY THAT WILL BE TRACKING EAST AND WEAKENING. THE RIDGE
AXIS WILL RE-ORIENT IN A MORE NORTHWEST-SOUTHEAST FASHION AND TAKE
UP STATION OVER THE PANHANDLE. THIS FEATURE IS EXPECTED TO BLOCK
THE ADVANCE OF THE OCCLUDED ALEUTIAN LOW AS WELL AS SEVERAL
FOLLOW-ON SURFACE LOWS OVER THE COURSE OF THE NEXT WEEK PAST THE
CENTRAL GULF. BIGGEST UNCERTAINTY INSOFAR AS IT AFFECTS THE PUBLIC
ZONES FORECAST WILL BE HOW MUCH SHOWER ACTIVITY SETS UP OVER THE
PANHANDLE WHILE THE RIDGE IS IN PLACE DURING THE MIDDLE OF NEXT
WEEK, AND HOW EFFECTIVELY A FRONT LATE IN THE WEEK WILL BE AT
DISPLACING THE SURFACE RIDGE OFF TO THE EAST.

HAD A PREFERENCE FOR GFS FOR THIS LATE WEEK FRONT BASED ON
STRENGTH OF THE LOW AND THE VORTICITY PATTERN, SO FAVORED THAT
MODEL IN A BLEND WITH ECMWF FOR PRESSURE AND WIND. MADE SOME CHANGES
TO THE POP FIELD THAT WERE PROGRESSIVELY SMALLER AND SMALLER
THROUGH WEDNESDAY NIGHT AND MADE NO CHANGES TO THE TEMPERATURE GRIDS
EXCEPT FOR SUNDAY NIGHT. DECREASED OVERNIGHT LOWS BY A FEW DEGREES
BASED ON EXPECTED PARTLY CLOUDY TO MOSTLY CLEAR SKIES. RESULTING
TEMPS PROMPTED THE INCLUSION OF AREAS OF FROST FOR THE PUBLIC
ZONES NORTH OF TRACY ARM AND PERIL STRAIT.

HAVE A SHORT LIVED SMALL CRAFT EVENT FOR THE SOUTHERN PORTION OF
CHATHAM STRAIT, THE WESTERN AND CENTRAL PORTIONS OF SUMNER STRAIT,
AND THE SOUTHERN PORTION OF CHATHAM STRAIT FOR SUNDAY EVENING.
ALSO ENHANCED THE LATE AFTERNOON/EARLY EVENING WESTERLIES IN CROSS
SOUND AND ICY STRAIT FOR SUNDAY AND MONDAY TO REFLECT THE DIURNAL
PATTERN THAT OFTEN OCCURS THERE UNDER MOSTLY CLEAR AFTERNOON
SKIES.

FORECAST CONFIDENCE IS AVERAGE.

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...FROST ADVISORY FROM 1 AM TO 7 AM AKDT SUNDAY FOR AKZ017>021-025.
MARINE...SMALL CRAFT ADVISORY FOR PKZ033-036-041-042.

&&

JBT/FRITSCH