National Weather Service United States Department of Commerce


158
FXAK68 PAFC 181449
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
549 AM AKST Mon Jan 18 2021

.ANALYSIS AND UPPER LEVELS...

The low pressure system anticipated to impact Southcentral AK
today has spun up as forecasted. The low center is currently
entering the Southern Gulf of Alaska and has as well defined
center that can be observed on satellite. Satellite also shows the
frontal system ahead of the low itself is just now moving ashore
in Southcentral AK. RADAR indicates precipitation with this front
is arriving along the coast. Out in the Bering, and elongated
front continues its eastward trek across the Aleutians, with it
currently being located in the Central Aleutians. This front extends
southeast from a stationary low located all the way by the Kamchatka
Coast. A shortwave ridge is present in the Central/Eastern Bering.

These surface features are being supported by an active synoptic
pattern. Ridging continues to exist over mainland AK. An elogated
trough exists in the Eastern Bering diving south into the North
Pacific with a shortwave ridge just behind it. Longwave troughing
is present again in the Western Bering. In relation to the
specific surface features mentioned, a closed 500 mb low just
south of Kodiak Island supports the surface low moving north
through the Western Gulf. Another closed 500 mb low off the
Kamchatka Coast has an upper level trough that extends southeast
from it into the Western Bering. These upper level features
support the frontal system working its way across the Aleutians.
Some other smaller features exist in this active and complicated
pattern, but the ones mentioned are the players for our weather.

&&

.MODEL DISCUSSION...

In Southcentral AK, models continue to track the low forecasted to
move north through the Western Gulf of Alaska. Models continue to
make adjustments to the exact track of the low center, but they
generally bring it across the Eastern Kenai with the low pushing
quickly inland. This leads to high confidence in model performance
for this system. Models are also consistent with the progression
of the frontal system moving across the Bering Sea, as well as the
generation of a weak low pressure system along this front tonight.
So high confidence in the Bering and overall for our forecast area.

&&

.AVIATION...
PANC...VFR conditions will be present until mid morning at the
airfield. A rain/snow mix is expected to develop over the airfield
this morning and will drop ceilings to MVFR conditions. Strong
southeast winds are still expected out of Turnagain Arm early this
morning before then bend south down Cook Inlet. Winds should stay
elevated, which will present low- level wind shear issues. This
is noted in the TAF.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2: Today
through Tuesday Night)...

The well-advertised winter storm has now begun across much of
coastal Southcentral. The well defined North Pacific low is
centered southeast of Kodiak Island. It is expected to track
north towards the Kenai Peninsula, with the center likely tracking
into Western Prince William Sound before barreling inland
tonight. Most of the impactful weather will occur ahead of the
center, through this afternoon. This includes heavy rain and snow
across all of the Gulf coast and adjacent mountains. Heavy rain is
occurring along the coast, with snow occurring inland, including
in Portage. With a warmer air mass moving in and solar heating,
the snow is expected to change over to rain later this morning
through Portage, Whittier, Valdez and eventually Turnagain Pass.
Snow levels are expected to rise to between 1,000 and 2,000 feet.
Thus, any snow that falls be contending with simultaneous melting,
or it will be a very heavy, wet consistency, which doesn`t
accumulate all that quickly. Thus, no low elevation locations are
expected to get very much out of this storm. Further inland, the
usual downslope shadowing will prevent much of the moisture from
impacting the Cook Inlet region, but for a time this morning some
mixed rain and snow are expected, due mostly to the sheer amount
of moisture overwhelming the downsloping working against it.
However, with above freezing temperatures expected area-wide, no
significant accumulation is expected today.

The one exception to the inland precipitation rule will be across
the mountains of the Copper River Basin, as the low center moves
inland, the moisture will also be drawn northward farther into the
mountains than many events that impact the coast. Thus, expect
snow along the Glenn Highway west of Glennallen and along the
Richardson Highway south of the Edgerton Highway junction, where
the Winter Storm Warning for heavy snow remains in effect. High
Wind Warnings also remain in effect through Portage Valley and
Eastern Turnagain Arm, and also for the Cordova area, where gusts
to 75 mph are expected. For Portage Valley this will be due to
funneling in between the mountains, for Cordova the hurricane
force barrier jet is expected to be pushed inland due to strong
southerly flow behind the leading front.

Once the low moves inland tonight, the heaviest of the
precipitation will be over. However, with the Cook Inlet region on
the back side of the low, a period of snow is expected tonight as
slightly cooler air (or better put, not as warm air) should allow
the atmosphere to cool enough to change everything over to snow.
There will not be all that much moisture with this portion of the
storm so most areas can expect around an inch of accumulation,
with up to 2 inches for higher elevations. The snow will persist
into Tuesday morning through Hatcher Pass, where up to 6 inches of
new snow are expected. The snow will end from southwest to
northeast early Tuesday morning.

A very brief break in the weather is expected for the day on
Tuesday, where some periods of sunshine are likely at times for
much of Southcentral. Temperatures will generally be in the low to
mid 30s for most of the area. The next storm follows very quickly
on its heels Tuesday afternoon in Kodiak and expanding along all
of the Gulf Coast by Tuesday night. Unlike the current storm,
however, there are several attributes to this second low that will
make it much less impactful. First, the low will be weaker.
Second, it will track towards the east-northeast, parallel to the
coast, instead of plowing full throttle into the coast. Third, it
won`t have nearly as much moisture. While the eventual track is
still somewhat uncertain, it seems likely any associated
precipitation will be confined to the coast and the mountains,
with very little if anything making it inland. Thus, other than
more clouds, things stay quiet, especially from Anchorage north.
Conditions will quickly improve from west to east on Wednesday.

&&

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

A fairly active pattern exists across portions of Southwest
Alaska today as a low pressure system near Kodiak Island brings
snow and windy conditions to much of the AKPEN south of King
Salmon. A Winter Weather Advisory remains in effect for snow and
blowing snow along the AKPEN south of King Salmon. Blowing snow
can reduce visibility to one half of a mile at times. Total snow
accumulation will range between 4-6" with locally higher amounts
in the mountains of the Aleutian Range. Winds are expected to ease
this evening and tonight which will help improve visibility as
there will be less blowing snow. Precipitation type will remain as
snow since the low will remain southeast of the region which will
allow for northeasterly offshore flow as the predominant flow
pattern during this event. Temperatures will be on a warming trend
over the next few days as winds change from a northeasterly
offshore flow pattern to a southerly onshore flow pattern. This
onshore flow pattern will increase the likelyhood of fog in areas
along the Kuskokwim Delta and Bristol Bay as dewpoints will rise
due to more moisture in the atmosphere.

A second storm will approach the Kuskokwim and Bristol Bays on
Wednesday evening. There is lower confidence with respect to the
precipitation type with this second system as some models suggest
above freezing 850 mb temperatures. These warmer temperatures
cannot be ruled out, especially in coastal areas as the southerly
onshore flow pattern will advect warmer air into the area.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...

A fairly complex pattern is expected to continue across the
Bering/Aleutians over the next few days. A weakening frontal
boundary over the Central Bering/Aleutians will continue to move
northeastward while weakening. This will bring a quick shot of
rain/snow to the Aleutian and Pribilof Islands today and on
Tuesday. A strong North Pacific low is expected to move
northeastward across the Central Aleutians into the Central
Bering Tuesday night. There is a high confidence of widespread
gale force winds with a smaller area of storm force winds on the
southern and southeastern flanks of this system. This storm will
produce large areas of seas over 20ft Wednesday and Thursday.
Winds are expected to be highest in areas prone to funneling from
the south along the Aleutian Chain.

&&

.MARINE (Days 3 through 5): Wednesday through Friday...

Gulf of Alaska: There are no significant storms expected for the
Gulf except periods of gale conditions. Gales associated with the
next North Pacific low will stick around through early Wed morning
before tapering off around noon. The next episode of gales will
arrive Thu morning, ushered by a strong frontal boundary latched
to a deep storm over the Bering. This event is expected to be
short-lived. Hazardous seas are expected mainly near Kodiak waters
and south of Prince William Sound in the coming days. Hazardous
seas will be much higher for the first low, which could elevate
sea heights between 20 to 25 ft Wed morning, with seas reaching up
to 20 ft over the coastal area from Kodiak Island through the
south of Resurrection Bay in the early hours of Wed morning. Seas
are expected to decline to below 20 ft by noon. Large seas up to
18 ft will spread through the Southwestern Gulf Thu morning. Seas
will gradually subside to 15 ft on Fri morning and trending
downward through the rest of Fri.

Aleutians/Bering: A large swell and seas from a deepening North
Pacific storm will generate monstrous swells and seas up to 30 ft
with occasional higher sets starting Wed morning. The central
Aleutians and Pribilof Islands lie in the path of strongest winds
and swells, and as the storm moves north through Thu morning, sea
heights will gradually subside to 15 ft near the waters of
Pribilof Islands on Thu morning. Essentially, it`ll be a very
active forecast period in the first two days until the low moves
farther north of the Central Bering.

&&

.LONG TERM FORECAST (Days 3 through 7): Wednesday through Sunday...

There will be a lot of weather activity over the Bering/Aleutians
as two separate North Pacific lows approach the area. Strong
gales and occasional higher gusts will spread over much of the
Aleutians through the forecast period. Warm temperatures will fill
the Aleutians east of 170W and over much of Southwest,
Southcentral and the Gulf of Alaska through the long term. For
this, perhaps rain will be mostly favorable as long as warm
temperatures from the coastal waters and warm southerly winds
persist through the week. A couple of shortwaves from two separate
frontal boundaries could increase sky cover and potential for
more rain over the forecast area. Overall, a very favorable
progressive pattern will persist through the long term.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Storm Warning: 141
High Wind Warning: 125 135
Winter Weather Advisory: 161
MARINE...Hurricane: 119
Storms: 120 125 130 131 139 351 352
Gales: 128 129 132 136-138 140 141 150 155 160 165
172-176 178 180 185 411-413
Heavy Freezing Spray: 160 185 412 414
FIRE WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...BJB
SOUTHCENTRAL ALASKA...JW
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...ED
MARINE/LONG TERM...CB



001
FXAK69 PAFG 182348
AFDAFG

Northern Alaska Forecast Discussion
National Weather Service Fairbanks AK
248 PM AKST Mon Jan 18 2021

.SYNOPSIS...
More southerly flow aloft today as a powerful hurricane force low
in the Gulf of Alaska has made "landfall" along the North Gulf
Coast and is moving inland. This is promoting strong chinook
warming and a strong easterly wind across the entirety of the
Tanana Valley. Temps have rocketed up into the 30s F across parts
of Fairbanks, with some areas nearing 40 F in town. Out west,
snow is falling from the YK Delta to the Northwest Arctic as an
arctic trough over the Bering Strait supports a quasi-stationary
frontal zone. The snow over the West Coast will shift east across
the Interior through tomorrow.

&&

.DISCUSSION...

Upper Levels and Analysis...
A very impressive atmospheric river stretching from south of
the Hawaiian Islands is supporting a powerful cyclone over the
Gulf of Alaska which is pushing inland along the Gulf Coast this
afternoon. Strong east winds over the Tanana Valley have pushed
temps over many Interior locations to the 30s F and even higher.
The low, analyzed around 968 mb in the Gulf of Alaska, is already
moving inland. There is another inverted surface trough across
the Lower Yukon/Middle Yukon Valley with a broad area of light
snow associated with southerly flow from Bristol Bay. Easterly
winds remain over the North Slope as a strong high latitude
surface high dominates the arctic flow.

Model Discussion...
The numerical models are in very good agreement with the current
low moving inland, the snow through Tuesday, and the next two
fronts including a strong low on Thursday which will impact the
West Coast. A blend of all models was favored.

Central and Eastern Interior...
Strong Tanana Valley winds are blowing which has pushed temps
into the 30s across a wide swath of the Valley and even parts of
the local hills and White Mountains. Strong winds continue to blow
through Delta Junction, too. These winds are also keeping much of
the Interior mostly dry with snow expected to move into the White
Mountains and higher elevations of the Dalton Highway this
evening and tonight. Blowing and drifting snow is possible,
although the temps in low 30s will likely limit this.

The front lifts north tonight into the Yukon Flats and Upper
Koyukuk Valley then weakens. The next phase moves into the
Interior Tuesday as cold air advection and southwest winds move
in, bringing in light accumulating snow everywhere, including the
Tanana Valley which has been in a minor snow drought for two
months. Accumulations will be light, however, with generally less
than 1-2" everywhere, higher over the higher elevations and
southwest facing aspects. Southwest winds of 20-30 mph will likely
result in some blowing and drifting snow before waning Tuesday
night. Weak colder high pressure settles in Tuesday night with a
return to more south gap winds in the AK Range and slightly
warming temps on Wednesday.

West Coast and Western Interior...
Snow is falling across much of the region as an arctic trough
slowly shifts east. Blowing snow is also occurring along the
immediate Bering Coast where north winds to 35 mph continue to
blow. Snow will be heaviest over the Lower/Middle Yukon and
Nulato Hills before it tapers off and shifts east by Tuesday
evening.

A quick hitting front will move into the YK Delta/Lower Yukon on
Tuesday night and early Wednesday, then quickly fizzle out as it
moves north into Norton Sound. Expect up to 1-2", mainly over the
Lower Yukon/Delta before snow tapers off Wednesday evening.

A much stronger low will move into the Bering Sea and push a warm
front up along the West Coast Wednesday evening and overnight,
bringing more snow and this time quite a bit of wind, too. Thus,
expect the threat for blowing and drifting snow to be rather
widespread, but details yet need to to be worked out with this
system. Yet another warm low will move into the region sometime
Thursday, bringing another surge of warmer air and mixed precip.

North Slope and Brooks Range...
Light snow and blowing snow will continue over the Western Arctic
as a front remains stalled over the Lisburne Peninsula. The
gradient tightens a little tonight so the normal blowholes like
Point Lay and Wainright will likely see a bit more blowing and
drifting snow. The front lifts offshore and winds weaken over the
whole Arctic coast Tuesday night. Light snow will be a chance on
and off thereafter as a a couple weakening shortwave troughs move
across the area. Expect otherwise normal weather with areas of fog
and stratus. Elsewhere, some light snow will fall over Atigun
Pass area of the Brooks Range tonight and tomorrow with light
accumulations anticipated.

Extended Forecast Days 4-7...
Beginning Thursday night, a warm and moisture rich low will be
moving north up the West Coast as a broad trough stretches from
the North Pacific to the Bering Strait. Stormy weather will mainly
be over the West Coast and Interior with strong southerly gap
winds off the AK Range keeping the Central/Eastern Interior on
the drier side. Some snow threats will move through the Interior
by the weekend. UNcertainty grows high early next week as the
models are uncertain if the storm track remains over the Bering
Sea or remains farther south.

Coastal Hazard Potential Days 3 and 4...None.

&&

.AFG WATCHES/WARNINGS/ADVISORIES...
Wind Advisory for AKZ223.

Winter Weather Advisory for AKZ213-AKZ215-AKZ216-AKZ226.

Brisk Wind Advisory for PKZ200-PKZ210-PKZ220-PKZ230-PKZ235-
PKZ240-PKZ245.

&&

$$

JAN 21



000
FXAK67 PAJK 181539
AFDAJK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
639 AM AKST Mon Jan 18 2021

.SHORT TERM...Satellite imagery and surface analysis indicate a
971mb storm force low moving into the SW gulf early this morning.
The frontal band associated with this feature is pushing into the
N gulf coast attm. Models remain in good agreement with this front
moving quickly across the N gulf coast and through the panhandle
today. This will bring high winds and heavy rains to most of SE AK
as it passes through. GPM rain rate indicate precip rates of
0.25-0.3 inches/hr moving N through the gulf supporting periods of
heavy precip approaching the region. Majority of the gulf will
experience storm force winds with inner channels seeing SCA to
gales. Most land areas will see gusts to 40-50mph with the front
the outer coast will experience gusts to 60mph with some potential
around Juneau as the front passes this afternoon and high wind
warnings remain in effect through this evening. WAA will help
keep ptype as rain near sea level with snow for higher elevations.
Snow levels rising to around 2kft will keep the threat of heavy
snow near White Pass where a winter storm warning remains in
effect through Tue morning. Conditions will gradually improve Tue
behind the front with winds and showers gradually diminishing. Minor
changes to focus heaviest precip with fropa and to add mention of
patchy fog to inner channels Tue and Wed night. Otherwise,
forecast represented this will with confidence in heavy precip and
strong winds remaining high.

.LONG TERM.../Wednesday through Monday as of 10 pm Sunday/ The
upper level pattern in the extended looks very different from what
we have been experiencing the last week or two. First off it is
noticeably less active with a Pacific storm track more focused on
the Bering Sea than the Gulf of Alaska. This is in large part due
to the second point which is there is more ridging over the Gulf
that will develop Wed into Fri. This ridge will not last as an
upper trough is still forecast to push the ridge east this
weekend, though the timing of that has slowed up a little
compared to yesterday.

Lower in the atmosphere, chances of rain still linger in the
panhandle on Wed, but diminish into Wed night as a low over the
gulf slides SE and weakens as it does. It will be the victim of
the building ridge over the gulf that will keep the panhandle
fairly dry into Fri. Precip returns by Fri night and Sat as the
next trough moves through the area. Forecast uncertainty is rather
high for the timing of the onset of precip from this feature
however, as the GFS wants to delay the precip until Sat afternoon
while the ECMWF has it come in as early as Fri evening.
Temperatures should be warm enough that most areas will be rain,
with the possible exception of the northern inner channels, but
precip amounts will be low as the system will have been weakened
from its fight with the ridge as it crossed the gulf. Following
this trough, guidance is still pointing toward a cold air
outbreak late weekend into early next week. It is a little delayed
from yesterday with not as much outflow expected until Mon at the
earliest now, but if it persists in future runs, it will be some
of the coldest air we have seen yet this year.

&&

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Strong Wind from 3 PM AKST this afternoon through this evening
for AKZ026.
Strong Wind from noon AKST today through this evening for AKZ027-
028.
Strong Wind from noon AKST today through this evening for AKZ020-
021-024.
High Wind Warning from noon today to 9 PM AKST this evening for
AKZ025.
High Wind Warning from 9 AM this morning to 9 PM AKST this
evening for AKZ023.
Strong Wind from 3 PM AKST this afternoon through this evening
for AKZ019.
Winter Storm Warning from 9 AM this morning to 6 AM AKST Tuesday
for AKZ018.
High Wind Warning from 9 AM this morning to 6 PM AKST this
evening for AKZ017.
MARINE...Storm Warning for PKZ043-051-052.
Gale Warning for PKZ012-013-022-031>036-041-042-053.
Small Craft Advisory for PKZ011-021.

&&

$$

BC/EAL

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