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 Mon Feb 19 2018

.ANALYSIS AND UPPER LEVELS...
A ridge of high pressure moving over the Mainland is providing
enough subsidence to bring widespread stratus with some fog mixed
into much of the southern Mainland and Gulf of Alaska today. The
worst conditions have occurred around Kodiak Island where periods
of dense fog occurred earlier this morning, but this is beginning
to diminish as westerly flow develops as well as the daytime
heating. Over the eastern edge of the ridge axis, a strong
northerly jet is bringing widespread upper level cloud cover
stretching from Southcentral into the Gulf, which is slowly moving
to the east in advance of an incoming front over the Eastern
Bering. Over the Kuskokwim Delta coast the first signs this gale
force front is beginning to appear with rain/snow showers along
with strong southerly winds. This is the same front that brought
rain and widespread storm force winds along with a brief round of
high winds to the Aleutians yesterday. It has now weakened some
as it detaches from the upper level low centered well to the west
near the Siberian Coast.

&&

.MODEL DISCUSSION...
As has been the case over the past few days, the models remain in
very good synoptic agreement into mid week, leading to generally
good forecast confidence throughout the area. The biggest forecast
concern in the short term forecast surrounds the potential for
snowfall from the weak shortwave that will ride along the
flattening ridge over the North Pacific late for Tuesday morning.
At this point the highest confidence for accumulating snow is
over the Northern Susitna Valley and Copper River basin where
there is good model consensus of another quick shot of light
snowfall. More uncertainty exists along the southern edge of this
shortwave over Anchorage and the Matanuska Valley where the
models bring little (if any) snowfall, leading to much lower
forecast confidence in the potential for snowfall here.

&&

.AVIATION...
PANC...After a brief break in the LIFR stratus and patchy fog this
afternoon and evening, continued subsidence from high pressure
moving over the area will likely allow for its redevelopment
by late this evening. The one piece of good news is that these
LIFR conditions will likely diminish much quicker (around 15z)
than had been the case over the past few days as some mixing from
a shortwave approaching from the west pushes through. There is an
outside chance that this shortwave could bring a brief light snow
shower, but at this point it appears that the biggest impact from
it will be to keep ceilings hovering around 5000 ft for Tuesday
morning.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Fairly benign conditions will prevail cross much of Southcentral
through tonight beneath the ridge, keeping patchy fog across the
Cook Inlet region and widespread low stratus over the Copper
River Basin during the overnight hours. A weak shortwave will then
briefly flatten out the ridge as it traverses the northern half
of the area Tuesday morning. This will likely bring a dusting of
snow to portions of the Susitna Valley from about Talkeetna north,
with snow then spreading into the northern Copper River Basin
through early Tuesday afternoon. Areas farther south including
Anchorage and the Matanuska Valley may see a brief snow shower
towards daybreak, but the threat of precipitation will quickly
come to an end as the shortwave exits to the east. This will usher
in another period of quiet weather through Wednesday as ridging
is quickly re-established over the mainland, with the familiar
potential for patchy fog and stratus returning to the area by
Tuesday night.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
(Tonight through Thursday evening)

An active pattern looks to continue for the next few days, as a
strong cold front is currently moving shore. Widespread rain and
snow showers have developed ahead of this feature, with winds
gusting over 40 mph along the majority of the coastal locations.
This front will continue its eastward trek through the night,
reaching the Alaska Range by morning. Colder air advecting back
across the mainland will quickly change any mixed precipitation
back to all snow later tonight. However, only minor accumulations
are anticipated.

Otherwise, look for a transient ridge of high pressure to build
across the region once again for Tuesday afternoon, before another
warm front brings precipitation back to the area late Tuesday
night, followed by a cold front for Wednesday, with more rain and
snow showers. Although the cold front looks to clear the area on
Thursday, a trough axis extending south from a low near the Seward
Peninsula will keep showery-type precipitation lingering across
the region through the evening.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
(Tonight through Thursday evening)

A gale force front currently extends from the far eastern Bering
through the AKPEN, with widespread rain and snow showers
accompanying it. This low is attached to a 965 mb low over the
Chukchi Sea, with storm force winds observed over the northwest
Bering on the latest scatterometer pass. Here, seas of 35 to 40
feet and sustained winds of 55 knots or more can be expected.
Higher wind gusts are also likely given the very cellular
signature seen in satellite imagery.

Further south, a ridge of high pressure will develop from the
east central Aleutians through the central portions of the sea
over the next 18 to 24 hours, bringing a brief period of drier
weather with decreasing winds. This will be short-lived however,
as one storm enters the southwest Bering Tuesday evening, sending
a cold front eastward through the Aleutian chain/Bering on
Wednesday as the parent low lifts northeast. Yet another storm is
waiting in the wings for Thursday evening.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The pattern in the long term looks to be an active one with
multiple systems moving through the area over the next week. The
main low centers look to track through the Bering with the fronts
still making it over the southern mainland. However, with the
associated low centers further away, the fronts do not look to be
very strong by the time they make it to the southern mainland.
There is decent confidence amongst the models on the overall
pattern but they are struggling with exact placement of the main
lows. Further out in time, ensemble means were preferred over the
deterministic solutions.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 174-178 181.
 Storm 185.

&&
$$

SYNOPSIS AND MODEL DISCUSSION...DEK
SOUTHCENTRAL ALASKA...CB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD
LONG TERM...DK



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