Area Forecast Discussion

FXAK67 PAJK 151617
Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
717 AM AKST Fri Dec 15 2017
.SHORT TERM.../Today through Saturday night/As of 0600 this
morning, an inverted trough extends over the eastern gulf and SE
Alaskan panhandle, while an upper level ridge builds over the 
west-central gulf in response to a strengthening shortwave near 
the Aleutian Islands. At the surface, a weak area of lower 
pressure has developed near the Baranof Island coastline, as a 
ridge of high pressure strengthens behind it. Both of these 
features will continue to quickly push east through today before a
weather front associated with the developing system near the 
Aleutians lifts northeast tonight and continues into Saturday 
morning. Colder air aloft is contributing to a more shower like 
regime and is providing some instability to some of the cells 
developing near the Baranof Island coastline. Lightning detection
algorithms did register a few lightning strikes near the southern
tip of Baranof Island early this morning.
Winds should continue to diminish today, and precipitation chances
should also decrease, as the ridge builds in. Some breaks in 
cloud cover may be possible for some areas, but for the most part,
onshore flow should persist. However, winds and precipitation 
should pick back up as the front moves through the area beginning 
late this evening. Winds will be high gale force along eastern 
waters, and a barrier jet will form along the coastline near 
Yakutat Bay. In the inside waters, winds will increase to Small 
Craft Advisory levels throughout most areas and to Gale Warning in
Clarence Strait. 
In addition, colder air aloft and additional cold air advection 
with this system may assist with mixing rain with snow in some 
areas, and sufficient cold air over more northern areas should
help to provide more significant snow amounts, especially for the
Haines and Klondike Highways, where we have issued Winter Storm 
Warnings. We also decided to expand a wider area of possible 
scattered thunderstorms Saturday afternoon into the overnight
hours, mainly near the Yakutat coastline and then extending along
the eastern gulf marine zones. Forecast soundings and model CAPE 
and lifted index continue to show a relatively unstable air mass 
with this system as the circulation head of the shortwave 
progresses east. 
We primarily used the NAM for adjustments to the forecast. Some 
high resolution models were incorporated into the wind forecast 
along the gulf waters and throughout the inner channels. Winds for
land areas were also adjusted using MOS guidance. Most areas in
northern portions of the panhandle have already seen their high
temperatures for the day, so temperatures have subsequently been
adjusted down. In addition, PoPs and QPF were updated with a 
combination of the ECMWF and the NAM to better time the onset of 
precipitation with the frontal passage.
.LONG TERM.../Sunday through Thursday/ As of 10pm Thursday 
Main highlight of the long term period is the potential for a
break, albeit short, in the seemingly endless progression of warm
and wet frontal systems which have made the first half of December
feel more like early fall than meteorological winter lately. 
Prior to this above-mentioned break, cool upper trough will drop 
into the northern Gulf on Sunday with onshore flow and a showery
regime continuing into at least the first half of Monday. Low
levels will cool as chilly air from the Bering Sea gets advected
around the trough. This air will be cool but not cold as it will
likely be modified by a moderate trajectory over the Gulf. May
just get cool enough over the northern inner channels for the
showers to be in the form of snow with a mix down towards the
Juneau area. Due to the showery nature of the precip and
borderline temperatures, not expecting significant accumulations 
at this time. 
The trough will drop quickly southeast on Monday as yet another
strong upper ridge re-asserts itself over the Gulf. The difference
between this ridge and the one that has parked itself over the
west coast of North America over the past 2 weeks is that the mean
ridge position looks to set up more over the central Gulf. This
will provide a period of offshore flow starting Monday night and
lasting through Tuesday night. This will likely promote clearing
skies and diminishing precip Monday night with the break in the
precip lasting until at least early Wednesday. Operational models
then disagree on the amount of Pacific energy that is able to make
it over the ridge and into the Panhandle for mid week while
ensembles indicate more ridging maintaining or re-asserting itself
by late week.
.AJK Watches/Warnings/Advisories...
PUBLIC...Strong Wind late tonight for AKZ027.
     Winter Storm Warning from 9 PM this evening to 9 AM AKST 
     Saturday for AKZ019.
     Winter Storm Warning from midnight tonight to 3 PM AKST Saturday 
     for AKZ018.
MARINE...Gale Warning for PKZ022-036-041>043-051-052. 
     Small Craft Advisory for PKZ011>013-021-031>035-053. 
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