Area Forecast Discussion
Issued by NWS Anchorage, AK

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356
FXAK68 PAFC 300158
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
458 PM AKST Fri Nov 29 2019

.ANALYSIS AND UPPER LEVELS...
A deep low a few hundred miles south of Kodiak Island is weakening
as it exits eastward. This is allowing the upper trough which has
been nearly stationary over the eastern Bering Sea to begin to
progress eastward. While the upper level deformation that had set
up between the two systems has ended, there is still decent
vertical motion ahead of the upper trough. This is bringing light
snow to portions of interior Bristol Bay (mainly Iliamna
northward) as well as the west side of the Alaska Range and across
the middle Kuskokwim Valley (near Sleetmute). An occluded front
which extends from the low south of Kodiak up and around the
northwest Gulf is lifting northward toward Prince William Sound
leading to persistent rain near sea level and some mountain snow.
A barrier jet is beginning to form ahead of the front along the
north Gulf coast, with winds strengthening from Cape Suckling
to Hinchinbrook Entrance. Upslope flow continues to produce
precipitation along the west side of Cook Inlet on up to the
Susitna Valley, though it is much lighter than the past couple
days. Low level southerly flow has pushed all the way up the
valley changing precipitation to all rain as far north as Chulitna.

Out west, an elongating and weakening front extending from a low
over the northwest Bering Sea is approaching the Alaska Peninsula
and Southwest Alaska coast. Expect a narrow band of light
precipitation with this front. Cold air advection showers prevail
behind this front across the Bering Sea and Aleutians.

Lastly, a strong upper jet and atmospheric river are crossing the
north central Pacific. The flow is fairly zonal right now, but
expect some amplification tonight as a short-wave digs southward
out of Russia tonight. This will lead to cyclogenesis and
development of the next storm to affect the southern mainland
later this weekend into early next week.

&&

.MODEL DISCUSSION...
There are some minor timing and precipitation type differences
among the models with precipitation event over Southcentral on
Saturday. In particular, most of the forecast soundings from the
western Kenai to Anchorage show a quick transition from rain to
snow as an upper trough approaches during the day Saturday.
However, with southerly winds moving up the Inlet into Anchorage,
this usually maintains warmer surface temperatures. Thus, tend to
prefer the warmer guidance and a slower transition from rain to
snow.

Models have mostly converged on a common solution with deepening
low tracking from the eastern Aleutians to Bristol Bay Saturday
night and into the Gulf on Sunday. The NAM is the one big outlier
with a weaker low and farther south track. Forecast changes will
be based off of the GFS/ECMWF which show a very similar storm
track.

&&

.AVIATION...
PANC...The tail end of the short-wave trough crossing Southwest
Alaska will move up Cook Inlet this evening while a weak wave from
the Gulf is pulled northward across the Kenai Peninsula. This
could bring some light precipitation to the terminal as early as
09Z, but with above freezing temperatures in place and persistent
southeast flow across the Kenai and Chugach Mountains, any
precipitation that does faLL will be in the form of rain. Thus,
expect VFR conditions to prevail.

A second stronger trough arrives in Cook Inlet tomorrow morning.
This will intensify precipitation, perhaps enough to cool the
column (via melting) and change precipitation to snow. However,
a south wind off of the warm waters of Cook Inlet should keep
surface temperatures above freezing. Thus, if it does change to
snow, it will be wet snow with little or no accumulation. For now,
have indicated a mix of rain and snow in the TAF with a trend
toward lowering ceilings. If it does snow, expect the ceiling and
visibility to both decrease markedly.

&&

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

Remnants of a weakening trough continue to bring light
precipitation to the Susitna Valley. Temperatures have been warm
enough for a change to rain across the southern Susitna Valley
this afternoon, and warm air is expected to linger longer than
previously thought. Southeasterly cross-barrier flow has allowed
for drier conditions over the western Kenai Peninsula and
Anchorage Bowl, however as the next upper level wave approaches
Saturday, these midlevel winds will weaken and become more
southerly. This will allow for an increase in precipitation
throughout the day on Saturday. While the northern Susitna Valley
will see snow, most other locations will start as rain with a
possible rain/snow mix by midday. By the end of the weekend,
another upper level low moves into the Gulf and brings another
chance of snowfall for Southcentral, though there are still many
uncertainties regarding the placement of the low center as well as
the timing of the colder air.

&&

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

An active pattern continues for the area as a pair of systems
affects the region. The first of which will be an Arctic front
pushing into the coast from the Kuskokwim Delta to the Western
Capes. A narrow band of snow will accompany this feature with some
light accumulations possible. This front will continue to weaken
some through the next 24 hours, as it continues to outrun the
better upper level support associated with a eastern Siberia low
(the better jet dynamics and ascent/ventilation remains over the
northern Bering).

However, the southern portion of the front will begin to stall on
Saturday, as a wave of low pressure develops along it. This low
is expected to bisect Bristol Bay as it heads towards King Salmon
by Sunday afternoon. Snow will develop near both the front and
the low, with the heaviest snow developing just north of the storm
track. Given the model differences in strength and track, and
considering this is still 36 to 48 hours out, decided to hold off
on an Winter Weather Advisory. However, this may need to be
issued with the next forecast package for any combination of
snowfall exceeding advisory criteria (6 inches in 12 hours/12
inches in 24 hours) snowfall, or for a combination of accumulation
snowfall with reduced visibilities due to winds of 25 mph or
more.

&&

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

A gale force front extends from the eastern Bering Sea through
the tip of the AKPEN this afternoon. Winds are subsiding while
becoming westerly behind the front. This feature will be a key
player in the weather over the next few days, as rapid
cyclogenesis (low pressure development) commences across the west
central Aleutians. A disturbance from Siberia will dive into the
base of a Bering trough, aided by a fairly strong upper-level jet
streak. The models continue to hold their ground, with the
majority of them strengthening and showing a track from near Atka
to King Salmon.

However, there are still differences in the forecast strength,
with the European model dropping the low to 969mb on the stronger
side, to 980 mb on the GFS, with the Canadian in between but
closer to the European.

The NAM is an outlier as it`s weaker, faster, and further
south/east. As such, it was discounted. Overall, it`s looking more
and more likely that sustained winds will be either high-end gale
or low-end storm force system, with gusts into the storm force
range either way. Snow will change to rain for much of the
Aleutians and Alaska Peninsula (AKPEN), as warmer air moves into
the region.

&&

.MARINE (Days 3 through 5)...

On Monday, low pressure will move into the central Aleutians,
although there is some uncertainty if this low will track north or
south of the Aleutians as it continues eastward. If this low
deepens enough, there is a chance that storm force winds may develop
around the core. For now, it looks to produce westerly small craft
winds with embedded gales from the western Aleutians Monday towards
the AKPEN Tuesday. A small craft to possibly gale force low in
the northern Bering looks to drop towards the southeast Monday
then gradually weaken Tuesday with uncertainty as to when it will
dissipate or where exactly its remnants will end up.

A front associated with a low over Prince William Sound will
produce small crafts and gales Monday into Tuesday. While the
front then weakens and moves out of the area, offshore flow will
continue to produce locally strong winds near terrain gaps along
the Gulf coast.

&&

.LONG TERM FORECAST (Days 3 through 7)...

By Monday, a cold upper level trough over Southwest Alaska will be
shifting east into Southcentral with the accompanying strong cold
air advection supporting snow showers and strengthening offshore
north to northwesterly winds. Once the trough reaches the northern
Gulf coast, a cutoff upper low will vertically stack over the
surface low near Prince William Sound. Both features will then
track slowly to the east southeast Monday night and Tuesday with
snow showers gradually tapering off and skies clearing. The
resulting cold, dry pattern will persist through midweek with
another upper level trough swinging in from the northwest around
Wednesday and prolonging the offshore flow.

To the west, a more active storm track will persist near the
Aleutians supported by cold air over the Bering and a strong
westerly jet. A developing surface low near the central Aleutians
on Monday will continue east through the southern Bering Monday
night with upper level ridging out ahead of it building in over
the Southwest Alaska coast. The surface low will then turn
southeastward, crossing the western Alaska Peninsula Tuesday
morning and digging south of the central Gulf Wednesday. As this
low phases with the cold upper trough over southern Alaska, an
upstream high amplitude upper level ridge will build in over the
western Bering Monday night and Tuesday, moving east to be
centered over the central and eastern Bering Tuesday night and
Wednesday.

A warm front pushing up the west side of the ridge will reach the
eastern Bering Wednesday night and southwest Alaska coast Thursday
morning. The associated cold front will follow more slowly due to
several waves developing along it and lifting north through the
Aleutians Wednesday and Thursday. A cold upper level trough
swinging off of the Kamchatka Peninsula Wednesday night and over
the western Aleutians Thursday will spin up a deep vertically
stacked low over the Bering Thursday night and Friday. As this
larger and more consolidated low develops it will strengthen the
frontal system along the Southwest Alaska coast and push it inland
Thursday night and Friday.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Storm Warning 172 174 176
     Gale Warning 119 132 414 150-170 173 180 181.
 Heavy Freezing Spray Warning 181.
FIRE WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...SEB
SOUTHCENTRAL ALASKA...KO
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD
MARINE/LONG TERM...CK/JER



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