Small Craft Advisory 121, NWS Juneau, AK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
533 AM AKST Tue Feb 28 2017

.SHORT TERM...Upper level trough has now moved over the AK gulf
from the AK interior and will remain over that area for the
foreseeable future. Two closed centers in the broad trough will
form today, one over the eastern panhandle coast and the other
near Prince William Sound. The eastern center will weaken in place
while the western center drops to the south. A gale force low
formed over the northern AK gulf overnight with associated frontal
band currently moving into the panhandle under first 500 mb vort
max. Second vort max on south side of the upper level trough
crosses over the southern panhandle later today. Still some
uncertainty with the exact track of the surface low but the
expected path for the low is to move southeastward, center to the
west of cross sound later this morning then begin to weaken later
tonight and track to the west.

Frontal band was bit further to the south this morning and tracked
in somewhat slower. Two fold results from this, snow over Yakutat
did not last as long as expected and snow began over the western
panhandle a couple of hours later then expected. Now that snow
has started, it has stated in earnest. Getting some intense
precip rates with visibilities around 1 mile or less and these
will last into the late morning. Kept snow accumulations as is
for now in the 3 to 7 inch range. Issues impacting total snow
could be some warmer temps, especially over the south, keeping a
wetter snow pack with less accumulation however as precip rates
intensify evaporational cooling could keep temps lower and thus
bump up snow totals. Heaviest snow fall will occur though the
morning then begin to lighten by the afternoon, but expect snow to
continue into the night with additional accumulations ranging
from 2 to 5 inches.

Gale force winds over the AK gulf will diminish as the low
weakens. Getting fair amount of easterly outflow winds near
Yakutat Bay and Cross sound with gust upwards of 50 kt. Some
gusty winds as the front moves inland this morning. Tightening
northerly pressure gradient will increase winds for the northern
inner channels tonight with outflow winds continuing.

ECMWF/Canadian remain models of choice with latest GFS falling
more in line. NAM is the outlier early this morning tracking the
low center further inland and thus stopping precip over the
central and northern panhandle earlier. For tonight as second vort
max moves in spread develops between ECMWF/Canadian on how far
north the next precip band moves with Canadian pushing the wave as
far norther as Skagway. Forecast confidence is average.

.LONG TERM...Snow will continue into the extended range forecast
period with amounts of 1 to 7 inches forecast for Wednesday.
Current forecast is for the heaviest snow over the central inner
channels - Juneau, Gustavus, Hoonah. Additional advisories or
possibly winter storm warnings will likely be issued once the
current advisories/warning expire or are cancelled. Generally,
lesser amounts from Wednesday night onward, but snow remains in
the forecast for the remainder of this week.

Significant changes made overnight to the pressure and wind
forecast in the extended range forecast period. A compact gale
force low is forecast to approach the central outer coast on
Thursday and high winds are a possibility for the Sitka area. High
pressure over the Yukon at the same time will result in a very
strong northerly pressure gradient with high winds currently
forecast for the Skagway area and 60 kt storm force winds over
northern Lynn Canal Wednesday night through Friday afternoon.
Pressure pattern and gradient also favors storm force winds out of
Cross Sound on Thursday.

Used ECMWF for updates to pressure and wind Wednesday through
Saturday night. PoP and QPF from a blend of SREF, Canadian NH, and
GFS. No changes to temperatures. Overall forecast confidence
limited to average due to large changes made to the wind forecast.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...Winter Storm Warning until noon AKST today for AKZ022>024-
     026>028.
     Winter Weather Advisory until noon AKST today for AKZ020-021-025.
     Winter Weather Advisory until 6 AM AKST Wednesday for AKZ029.
MARINE...Gale Warning for PKZ022.
     Small Craft Advisory for PKZ011>013-031-041>043-051>053.
&&

PRB/Fritsch

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