Area Forecast Discussion - Southeastern


FXAK67 PAJK 182314
AFDAJK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
214 PM AKST Thu Jan 18 2018
.SHORT TERM...Northerly gradients across Southeast Alaska is 
showing signs of easing as the low well to our south, in fact,
south of Haida Gwaii is now weakening. However, to allow slower
easing of winds, the low is forecast to approach Southeast via
Hecate Strait through Saturday and eventually die an uneventful
death. Bands of showers that have moved north through the day
across our central region have steadily weakened and have merely 
resulted in thickened clouds across much of the north-central 
Panhandle. Thus, Juneau and points north will remain dry. Showers
will end across the south-central Panhandle tonight including
Sitka, Wrangell, and Petersburg. But Ketchikan will continue with
rain chances through Friday. 

As the weakening low progresses northward into Friday night, we 
expand rain chances yet again across the south. And northerlies 
through Lynn Canal and some other north-south channels will begin 
to rebuild from a lighter period on Friday. 

Saturday, a new more vigorous system will move into the outer
central gulf and spawn a wave in the southeast gulf that will 
quickly kill off the previous feature described above. This system
may induce some breezy conditions for the far south including
Hydaburg and Craig and probably will spread showers farther north
up the coast into the Elfin Cove area including Sitka, Petersburg,
and Wrangell. Due to some weak cold air advection, we do 
anticipate that much of the shower activity though weak will 
support snow across much of the interior and northern 
locations. As the system glides northwest across the Panhandle, 
we foresee the chance of more breezy conditions developing across 
Downtown Juneau Saturday night. In addition, the tighter northerly
pressure gradient across Southeast will result in stronger winds 
with small crafts likely in Lynn Canal, Stephens, and Sumner at 
some point Saturday. 

We preferred the GFS/ECMWF solutions consensus with the lows' 
tracks through the period. Forecast confidence is generally good.
Details, such as the proper strength of some of these wind set-ups 
will emerge later. 
.LONG TERM...Saturday through Thursday...We'll be heading into a 
relatively active, but tricky, pattern for the Alaskan panhandle
in the long term. Potential temperature analysis currently 
indicates the development of an Omega Block, with the upper level 
ridge axis centered over the Bering Sea, which will influence our
weather during this period. The broad upper level trough 
associated with this blocking pattern will largely develop over 
mainland Alaska and extend south into the Gulf of Alaska. This 
will leave all of mainland Alaska and the SE panhandle as the 
prime target for a much colder, arctic air mass to move in aloft. 
Model 850 mb temperature analysis continues to show falling 
temperatures into the weekend, with the mainland receiving the 
heavier dose of arctic-like temperatures. We've left surface 
temperatures in the forecast average to below average accordingly 
for this time of year, meaning temperatures should start to feel 
like winter again for SE Alaska.

Embedded shortwaves within the larger trough, and their associated
surface lows, will largely remain offshore of the panhandle in 
the east-southeast gulf. A subsequent ridge will build over 
western Canada, being squeezed by the stronger trough over the 
gulf. This should provide a brief moment through the weekend of 
offshore flow, implying drier conditions, especially for portions 
of the northern panhandle. Due to the uncertainty regarding the 
exact position of the lows, the southern panhandle may not remain 
as dry, so we've left slightly higher chances of precipitation 
there through the weekend. By the early part of next week, models 
indicate that the broad upper level trough will begin to break 
down, allowing the shortwaves to track more northeastward towards 
the panhandle. This should gradually increase PoPs throughout the 
panhandle going into the middle of next week.

Generally, a lot of uncertainty exists with this forecast. Models
agree on upper level features, at least into the early part of next
week before disagreement ensues. However, there is quite a bit of
model spread regarding surface features, which will have an 
impact on precipitation chances. The cold air mass will largely 
remain entrenched, so any precipitation that does fall, would most
likely fall as snow. Due to outflow conditions through the 
weekend, we anticipate most of the northern inner channel winds to
remain around 20 kt or higher, as a strong pressure gradient and 
temperature gradient will provide brisk northerly flow throughout 
the inside waters.

We elected to use a GFS/ECMWF model blend for the earlier portion
of the long term (12Z Sat through 12Z Sun) and decided to remain 
with WPC guidance past this timeframe due to the model spread and 
uncertainty.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...None.
MARINE...Small Craft Advisory for PKZ012-041. 
     Small Craft Advisory due to Seas for PKZ042-043-051-052. 

&&

$$

JWA/Voveris

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