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FXAK67 PAJK 242347

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
347 PM AKDT Fri Mar 24 2017

.SHORT TERM...Another day, another difficult, intricate weather
pattern complicated by our complex terrain lies before us. One
short-wave is situated over Haida/Gwaii. A surface low lies
underneath causing northerlies and easterlies over much of the
Panhandle today. But it has driven a moist air mass aloft across
much of the southern Panhandle. Ketchikan has been raining most of
the day, while farther north where drier air has been entrained,
precipitation rates decrease more so than what the models have
shown, but evaporative cooling has allowed Petersburg to snow
much of the day, with only minor accumulations if any. To the
north and west, insolation in the month of March despite mainly
cloudy skies make for the slow continuation of break-up.

The pattern`s evolution becomes intriguing tonight as a longwave
deformation band moves in quickly from the west. The interaction
of these two elements will help draw moisture from the warm bands
of the first system and combine it with the dynamics of the system
to the west. Thus, we expect snow to develop, especially across
the Juneau area tonight for up to 3 inches of accumulation through
tomorrow morning. It is possible that the deformation band could
extend snow into Skagway and White Pass overnight, but given a
drier boundary layer forced by northerlies, we have kept snow
amounts lower here.

The trough pushes east tomorrow to slowly feed us with a cooler
air mass aloft and more instability. Onshore flow and increasing
instability will keep showers through the forecast through
tomorrow afternoon. Snow will be possible through much of the
central Panhandle throughout tomorrow though, while interior
southern Panhandle areas could see a mix late tonight, but no real
accumulation is forecast outside of Juneau and Skagway areas.

The 06Z High Res NMM model had good positioning of the current
forecast to allow for finer edits, and was the driest model in QPF
on the cooler side of the deformation band across the south
central Panhandle. For these reasons, we preferred its version
over others regarding the issue of the day...snow potential for
tonight/Saturday morning.

.LONG TERM...A very gradual retrogression of the long wave trof
from the gulf of AK to the AKPEN area will occur through next
week. This will mean milder temps and more chances for precip. A
low should move NW through the E gulf into the N gulf Sun night
into Mon. The associated occluded front should move across SE AK
during that time. Looking like another low will try to move NE
into the W gulf by midweek, although models differ some on timing
and strength of the low. By late week, a much larger and stronger
system will affect the area, with main low likely moving into SW
AK or the far W gulf while the occluded front moves into the E
gulf and SE AK. Still some model timing differences with this
feature but all agree it will be a large and fairly strong system.

Main forecast adjustments were to raise low temps for Sat night
through Mon night as airmass moderates, and to speed up timing of
Sun night/Mon system. Did speed up transition from showers to
stratiform precip as occluded front moves N across the area.
Looking like most of the precip with this system will be rain
although some snow could mix in at times especially over the N.
Systems later in the week will probably have milder air especially
at night so later shifts may need to raise min temps from Tue
night through the end of the week and decrease mention of snow.


.AJK Watches/Warnings/Advisories...
MARINE...Small Craft Advisory for PKZ012-013-041.
     Small Craft Advisory due to Seas for PKZ042.



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