Area Forecast Discussion

FXAK67 PAJK 192252
Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
252 PM AKDT Mon Mar 19 2018
Afternoon WV imagery shows a zonal upper jet nosing into SE Alaska
from the west as a shortwave trough in the mid levels is beginning
to amplify over the northern Gulf. This shortwave will continue 
to amplify through Tuesday, evolving into a closed mid/upper low 
along the eastern Gulf coast as more jet energy over the Bering 
Sea carves out a mean trough over the Gulf. At the surface, a 
weak front will continue to push through the Panhandle this 
evening. A weak surface low will develop over the NE Gulf Tuesday 
morning and sink slowly southward along the Gulf coast through 
Tuesday night.
Main forecast concerns in the short term revolve around marine
winds and possibly some late season snowfall. Stable atmosphere
over the inner channels this afternoon has limited the southerly
winds to 25-30kt in Lynn Canal with 20kt or less elsewhere. Expect
the gradient to slowly relax tonight and thus expect a decreasing
trend in the southerly winds especially over Lynn Canal by late
tonight. Over the Gulf, stronger CAA under the amplifying upper
trough will aid gale force WNW'ly winds in developing tonight and
continuing into Tuesday morning. Some stronger W'ly winds of 25 to
30 kt will make it into Cross Sound and western parts of Icy
Strait as well by late tonight. As the surface low pulls to the
south by late Tuesday night, N'ly outflow will begin to develop
over the northern channels. Delayed the onset of the outflow a bit
as models have slowed the progression of the surface low to the
south a bit compared to previous runs. Still expect high end SCA
to low end gales by late Tuesday night/Wed over the northern inner
As the low to mid level low develops along the NE Gulf
coast/northern channels tomorrow morning, a mid level deformation
zone will pivot over the northern channels through the day
Tuesday. Steep lapse rates approaching 7 to 7.5 C/Km will exist
underneath the upper cold pool aloft. As the lower atmosphere
cools, expect snow to begin mixing into the rain showers across
much of the area. There is the potential for some more potent
banding of the showers in the Lynn Canal/Icy Strait area tomorrow
morning. Weak surface winds may allow for any persistent heavier
showers aligned with the mid level deformation zone to drive some
accumulating snow to sea level especially in the Haines to Juneau
corridor. Warm boundary layer and low snow ratios will limit
accumulation unless the rates become heavy enough. Hard to
indicate this in a deterministic forecast so went with
accumulations of 1 to 2 inches in the above-mentioned areas with
the caveat that this will likely be highly variable depending on
where any persistent snow banding sets up. Later shifts can
monitor this potential and update once the situation become
Overall model agreement was decent through Tuesday night although
placement of features on the mesoscale exhibited subtle
difference. Forecast confidence is average.
.LONG TERM.../Wednesday through Monday/ of 10 pm Sunday.
Overall upper level pattern has not changed all that much through
the late week period compared to yesterday. The upper low that
moves through the panhandle Tue and Tue night ends up stalling in
the NE pacific through Fri night. Though it is farther south, 
taking up space W of Vancouver Island rather then Haida Gwaii. The
upstream ridge over Western Alaska is still there as well, but 
guidance is still in flux on when it will break down. Current 
scenarios have significantly slowed the timing of the break down 
by 12 to 24 hours with a timing more toward Sat rather then the 
Fri that was suggested yesterday.
So what does this mean for the forecast? With the upper low
settling much further south for the latter half of the week the 
panhandle has trended toward a drier forecast. Most areas should 
be done with shower activity as early as Wed, through the southern
panhandle may still see a slight chance of precip through Fri as 
some short waves from the upper low get north enough to affect 
that area. Ptype will mainly be a mix of rain and snow especially
at night with the cool air mass in place. What has not changed is
the cold northerly outflow that will be in the area for much of 
the late week period. 25 to 40 kt northerly winds are still 
expected in the usual outflow areas Wed through Fri with the 
highest winds expected Thu. Temperatures were also not changed 
much and will be hovering right near normal for this time of year.
Now for the later break up time for the upstream ridge. The delay
of nearly a day means that the more zonal flow that sets up in its
wake will wait until the late weekend to arrive. The result is a
more quiet early weekend forecast followed by a more active 
pattern for early next week as several systems move east across 
the gulf. It is too early to get exact timing for individual 
systems as model agreement is very poor this far out, especially 
after an overall pattern change, but most are trending toward a 
warmer and wetter scenario for the panhandle.
.AJK Watches/Warnings/Advisories...
PUBLIC...Strong Wind until 10 PM AKDT this evening for AKZ018.
MARINE...Gale Warning for PKZ043-051-052. 
     Small Craft Advisory for PKZ012-013-021-022-031-041-042. 
     Small Craft Advisory due to Seas for PKZ053. 
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