Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 200119
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
419 PM AKST Fri Jan 19 2018

.ANALYSIS AND UPPER LEVELS...
A large scale pattern change is well underway with a strengthening
upper level ridge over the Bering Sea and a short-wave trough
rounding the top of the ridge and into northwest Alaska. The tail
end of the trough is beginning to drop southward down the west
coast of Alaska, with Arctic air moving in behind it. This is the
beginning of what will be a prolonged cold air outbreak for the
Eastern Bering Sea and Mainland Alaska.

Out ahead of the ridge there is a remnant trough along the eastern
Aleutians and southern Alaska Peninsula producing some garden
variety snow showers. This trough is headed southward into the
Pacific. An upper level low is nearly stationary over the Kenai
Peninsula, with some mid level clouds and a few snow showers near
Cook Inlet. Weak low level flow under temperature inversions did
lead to development of widespread fog last night, from the southern
Susitna Valley to the Matanuska Valley, Anchorage, and the Western
Kenai Peninsula. The combination of the mid-level clouds swirling
overhead and a slight increase in low level flow is leading to
some improvement, but there doesn`t seem to be enough forcing to
get rid of all the fog. Thus, local areas of low ceilings and/or
fog have persisted through the day.

West of the strengthening upper level ridge, a gale force low and
leading occluded front are approaching the western Aleutians, with
strengthening easterlies and warming temperatures along the
westernmost islands.

&&

.MODEL DISCUSSION...
Models are in very good agreement and forecast confidence is
generally above average. One of the biggest forecast challenges is
determining how long fog/stratus will hold in over Southcentral,
which is something models usually handle very poorly. This will
have a dramatic effect on the temperature forecast, with much
lower temps expected if skies clear out.

&&

.AVIATION...
PANC...This is an extremely challenging forecast. It appears
likely fog and low clouds will remain in the vicinity of the
terminal until low level flow strengthens more significantly
during the day Saturday. However, it`s very tough to say whether
fog will settle over the terminal at any given point in time. For
now, have gone with the idea of prevailing IFR conditions for
tonight.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2: Saturday
and Sunday)...

Tonight`s forecast is definitely a tricky one as regards the
area`s stratus and fog conditions. The skies cleared out quite a
bit this afternoon around Anchorage and the Matanuska Valley.
However, the fog and stratus have persisted isolated, sheltered
locations, including the Knik River Valley and the Copper River
Basin. Expect some redevelopment of the fog tonight, particularly
during the evening hours with light winds and the persistent
inversion, which is still very much intact per the 00Z Anchorage
sounding. From midnight onward, slow weak cold advection,
strengthening gap flows, and gradual drying should reduce the fog
threat in favor of more stratus beneath the weakening inversion.
The trickiest aspect to the forecast lies in the potential for no
stratus to develop in some areas at all. Where that occurs,
temperatures will plummet to levels much lower than currently
forecast due to radiational cooling, whereas areas where there is
very persistent fog and low stratus will keep temperatures warmer.
The fog is expected to be more persistent through the Copper
River Basin through tonight, with just enough moisture inflow out
of the south to perhaps cause some flurry activity through the
central part of the basin.

As an upper level trough digs southward across Southwest Alaska
this weekend, the slow cooling of the lower atmosphere over
Southcentral area-wide will slowly invigorate many of the gap
winds, as the increasingly cold air drains through them. With the
cold air building the most nearest the trough axis over Southwest
Alaska, the strongest and most persistent gap winds will be
through Kamishak Bay, and the bays and passes that open into
Shelikof Strait. Nevertheless, all of them will be gusting
strongly right through Sunday. Another aspect to the forecast
resulting in the increasing winds is the expected development of a
low over the northern Gulf by Sunday. This low will increase the
chances of snow through Prince William Sound and the north Gulf
Coast during the day. A few of the models are suggesting some of
that moisture gets into Anchorage and the Matanuska Valley Sunday
afternoon and evening. However, that possibility is very low
confidence, and even if it did happen, would not amount to much in
the way of accumulations.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA...

For the next two days, the main story for Southwest will be
continued offshore flow and cold advection as arctic air pours
into the region. This will continue to push both daytime highs and
lows downward with increasing threats for bitter wind chills
approaching -40F. This will be most likely where the winds are
the strongest, so it is likely the Kuskokwim Delta will realize
the coldest wind chills. The one change in this forecast is it
seems more likely that a weak low will move into the Kuskokwim
Delta, bringing some light snow to the coast Saturday evening.
Otherwise, cold and windy conditions with clearing skies will
resume on Sunday.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS...

High pressure settling over the central part of the Bering Sea
will induce more of an easterly flow component as cold air pours
over the eastern Bering Sea and a gale force front moves into the
western Aleutians then stalls. This front looks like it will not
make further progress than the western Aleutians, so increasing
winds to gale force will be the biggest impact with this front.
With cold air pouring over the Bering Sea, expect freezing spray
to be a threat from the Pribilof Islands eastward.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Colder than normal temperatures continue through the next week as
a ridge pattern dominates the Bering. Low pressure systems wobble
on either side of the ridge, with one along the gulf and the
other just south of the western Aleutians. This pattern keeps a
drier pattern across the southern mainland with scattered showers
mainly along area waters. Confidence is higher than normal for
the extended forecast as models are trending toward similar
solutions. WPC solutions incorporated use of the operational
ensembles of the GFS and ECMWF which maintains consistency along
the active low centers.


&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 130 131 150 155 177 178 180.
         Heavy Freezing Spray 130 131 138 139 160 180 181.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...SEB
SOUTHCENTRAL ALASKA...JW
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JA
LONG TERM...KH



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