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FXAK68 PAFC 180130

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
430 PM AKST Sat Feb 17 2018
High pressure continues to bring generally stable conditions to
much of the southern Mainland, while upper level cloud cover can 
be seen streaming over top of the ridge from a storm force front 
moving through the Eastern Bering. Patchy fog and stratus developed
again overnight in Cook Inlet from Kenai northward into Knik Arm 
and the Anchorage Bowl, which persisted for much of the morning 
today with continued subsidence and weak forcing. From the Eastern
Aleutians northward to the Pribilof Islands, storm force winds 
were observed this morning along the front along with widespread 
rain which mixed with snow toward the Pribilof Islands. This 
front is moving ever so slowly off to the east as quickly as the 
mainland ridge allows, but should begin impacting the west coast 
by late tonight.

The models are in very good agreement with good initialization in
the early term, and are beginning to come to a consensus with the
remnants of the front currently moving through the Eastern Bering
as it tracks into the Gulf/Southcentral late Sunday. Previous runs
had been very inconsistent in either bringing a weaker shortwave
further to the north over top of the flattening ridge over the 
Gulf, or digging an upper level low much more aggressively into 
the ridge well to the south, which would have diverted much of the
energy/moisture well to the south of Southcentral. This morning's
12z models have all come into agreement in the further north 
track, increasing the chances for snow from the Kenai Peninsula 
into northward into the Susitna Valley on Sunday evening.

PANC...After dissipating this afternoon, the potential for
the redevelopment of LIFR fog/stratus from Northern Cook Inlet 
into Knik Arm will be the primary forecast challenge through 
Sunday morning. There will be some additional upper level 
cloud cover that may inhibit the growth of the fog, but will 
also make tracking of it difficult on anything but area webcams, 
leading to lower short-term forecast confidence once it begins to
develop late this evening. A weakening front will then bring a 
short lived period of MVFR snow Sunday evening with only light 
accumulation expected.


At the onset of the forecast period, the long-wave pattern 
features a high amplitude ridge over Kodiak Island that extends 
north of the Brooks Range and a low vortex east of Kamchatka. The
upper level ridge will slowly move eastward across southern 
Alaska tonight and tomorrow. Patchy fog was detected along Cook 
Inlet and Knik Arm early Saturday morning and similar conditions 
are anticipated with weak flow in place. With the projected 
temperature/dewpoint spreads and the inversions over Southcentral,
patchy fog is possible again Sunday morning. On Sunday, a 
vigorous short wave will get kicked out of the vortex over the 
Bering and become negatively tilted. This feature will bring 
enhanced southwest flow directly up Cook Inlet. The PWs (precipitable
water) spike to nearly 3/4 of an inch near Kodiak, Homer and 
portions of Cook Inlet. Between the favorable dynamics and the 
moisture surge, bumped up pops this forecast package from Homer to
Palmer, including Anchorage. The timeframe for snowfall in 
Anchorage and Palmer is late Sunday and into the early morning 
hours of Monday. This is a fast moving shortwave and at this time 
significant accumulations are not expected. However, the models 
have been bouncing around with the track and the evolution of 
this Cook Inlet snow event, so stay tuned for more details in the 
event of last minute adjustments.


(Tonight through Tuesday evening)
Increasing winds and precipitation is the main item of interest 
for the next 24 to 36 hours, as a weak cold front moves ashore. 
Ahead of this feature, southeast down-slope winds have warmed the 
temperatures higher than model guidance indicated earlier this 
morning. Thus, the morning update for freezing rain across the 
western zones has been removed with this afternoon's package. 
With the warmed air, delayed the onset of blowing snow as well. 
Otherwise, look for widespread rain to develop with increasing 
winds as the front approaches. 

Strong wind gusts are already occurring along the coast, and this
will continue until frontal passage with some gusts in the 45 to 
60 mph range expected near the coast. The models have also trended
faster with the colder air moving in behind the front. Most 
locations should see 2 to 5 inches of snowfall, with the mountains 
northwest of Dillingham seeing perhaps a touch more. Conditions 
look to dry out for Sunday before another front brings more wind 
and precipitation back to the region by Monday.


(Tonight through Tuesday evening)
An active pattern will continue through early next week, with 
multiple lows/fronts affecting the region. Widespread rain and 
wind gusts to 60 mph will continue through Sunday morning before 
the system pushes east of the area. A shortwave ridge will quickly
advance from west to east for Sunday evening into Monday, bringing
lighter winds and some clearing. This will be short-lived though,
as a strengthening low pressure system moves from the north 
Pacific into the western Bering, sending another front eastward. 
High end gales will be fairly widespread with some locations 
seeing sustained storm force conditions. Given the repetitive 
pattern and good fetch length, seas will build into the 30 to 35 
feet range across the western Bering/coastal waters of the chain. 
Widespread moderate to locally heavy precipitation will accompany 
the front as well, with strong wind gusts likely over land as the
front progresses eastward.


.LONG TERM FORECAST (Days 3 through 7)...
The long term forecast beginning Monday night starts with dry 
conditions across Southcentral with a surface ridge across the 
eastern interior. To the west, an organized low tracking to the
northern Bering moves a front from the eastern waters to the 
Southwest, spreading light snow inland before diminishing just 
west of the Alaska Range early Tuesday. A ridge amplifies across 
the southern mainland late Tuesday into Wednesday ahead of the 
next approaching system.

Another storm system tracks quickly to the southwest Bering early
Tuesday on a similar track toward the Bering Strait. This system 
will be "the one to watch" this week as a significant short-wave 
travels to the southern mainland late Wednesday into Thursday, 
bringing increased chances for precipitation as it propagates to 
the Alcan border. The pattern remains active through the next work
week as a third storm system enters the western Bering Wednesday 
night pushing the upper long wave eastward. 

Models are in general agreement with the active forecast pattern
through the end of next week, however they differ with the
amplitude of the upper wave pattern and in the progression of the
storm systems. The updates for the current package maintain 
consistency, but higher uncertainty remains through the extended 
as models typically struggle with these active patterns.


MARINE...Storm Warning 173 175 176.
Gale Warning 150 155 160 165 170 171 172 174 177 178 179 180 181




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Page Modified: 03 Feb 2008 18:00 UTC
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