Graphical Hydro Meteorological Discussion

919
FXAK68 PAFC 211341
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
441 AM AKST Tue Jan 21 2025

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

Active weather returns to Southcentral Alaska today with the
potential for (a) snowfall rates approaching 1 inch per hour in
Turnagain Pass, and (b) gusts as high as 50-65 mph along parts of
Turnagain Arm and Turnagain Pass.

A deepening area of low pressure is currently shifting northward
across the AK Peninsula with the attendant frontal boundary
extending southeast into the Gulf of Alaska. Satellite imagery shows
this front currently pivoting northward across the Gulf, currently
bringing rainfall and high-elevation snow to Kodiak Island.
Southeast winds will continue to increase in speed to gale-force
intensity (w/ storm-force gusts) in response to the front swinging
northward and the pressure gradient between the frontal trough and a
highly-amplified ridge axis off to the east.

Like the last several fronts in recent weeks, mild temperatures will
result in low-elevation rainfall and high-elevation snowfall for the
Kenai Peninsula and areas adjacent to the Prince William Sound. The
largest impact along the `lower` elevations, or essentially along
the road system, will be that of Turnagain Pass where somewhere in
the vicinity of a foot of snowfall is expected. Precipitation rates
will reach peak intensity this afternoon and evening, where rates in
excess of 1"/hr will be possible, and therefore the potential for
significantly reduced visibility at times.

The most uncertain and challenging forecast aspect is that of
precipitation type in the areas surrounding Turnagain Pass.
Confidence is reasonably high that the pass should remain snow, but
immediate lower elevations around the pass, as well as Portage and
Whittier present uncertainty. Recent fronts have had enough
intensity such that Whittier, and to a lesser degree Portage, have
maintained snow for longer than expected despite temperatures
warming above freezing. This will be a similar case. Should Whittier
remain mostly snow, several inches to a foot cannot be ruled out...
though temperatures will no doubt be at or above freezing. If enough
rain mixes in, amounts will be closer to 0.

Strong wind will also be a hazard today in addition to the snowfall.
The majority of strong wind on land will be confined to higher
elevations with the exception of Whittier-Portage-Turnagain Arm gap,
as well as the Anchorage Hillside. While wind gusts should remain
lower than last week`s windstorm, gusts in excess of 60 mph can`t be
ruled out for any of these locations. For Anchorage, confidence is
strong wind will remain confined to the Upper Hillside thanks to a
fairly strong down-inlet wind component. The trough passage will
occur around 9PM AKST this evening, of which when winds will likely
be the strongest for some of the lower elevations as we quickly lose
the down-inlet gradient and the SE winds out of the Turnagain Arm
may bend into town a bit.

The trough passage itself will also bring a chance of a rain and
snow mix to the lower elevations including Anchorage, as well as
the Mat- Su, though accumulation will be limited to non-existent.
The Alaska High Resolution Ensemble Forecast (AK-HREF) supports
this as probabilities of 0.5" snowfall are near 0 until reaching
the Talkeetna Spur Rd. To the north in Susitna Valley from
roughly just north of Trapper Creek to Broad Pass is most likely
to see accumulating snowfall through Wednesday between a
combination of this evening`s trough and then subsequent low-level
upslope snow coincident with further shortwave trough support.
The highest snow totals look to be Curry to K`esugi Ridge and the
Parks Highway on the northwest side of this ridge system.

Thompson Pass and Valdez are also likely to see accumulating
snowfall, with snowfall/precipitation quickly ending just north of
the Chugach Mountains. Convective snow showers moving from the
northern Gulf into the central and eastern Chugach are likely to
continue into Wednesday behind the front. Additional accumulation
should be fairly limited, though quick bursts of snow, and thus,
visibility-reductions will continue to be possible.

Following a brief lull Wednesday night, the tail end of a warm
front extending out from a low in the Bering will swing into
Southcentral Alaska and the Gulf. Broad lift owing to warm air
advection underneath shortwave troughing aloft should allow for
the development of loosely organized scattered-to-widespread
showers. Mild temperatures will once again result in low-elevation
rain and high-elevation snow, with snow levels gradually rising
through the day given the stream of warm southeast flow. At this
time it doesn`t appear this wave will produce significant and
widespread hazardous weather, though the possibility of any rain
on untreated ice- and snow-covered roads always presents the
possibility of additional slickness.

-Brown

&&


.SHORT TERM FORECAST SOUTHWEST ALASKA AND THE BERING
SEA/ALEUTIANS (Today through Friday morning)...


The active weather pattern looks to continue across Southwest
Alaska, the Bering Sea, Alaska Peninsula (AKPEN), and Aleutian
Chain through the end of the work week. The main culprit for the
active weather across Southwest, at least this morning through
Wednesday morning, is a North Pacific low which will lift
northward into the eastern Bering and over mainland Southwest
through Wednesday morning. Precipitation across the Kuskokwim
Delta and Bristol Bay will remain rain, but transition to a
rain/snow mix Tuesday evening and end as snow Wednesday morning as
cold westerly flow works in off the Bering.

The next system to follow is currently taking shape this morning
in the form of a Kamchatka low and front. The front is currently
pushing eastward across the Western Aleutians and western Bering
this morning as Shemya reports light snow. This front along with
precipitation, and strong gusty winds will continue eastward
across the Central Aleutians and will reach the Eastern Aleutians
and Pribilof Islands by Wednesday. Gusty southerly to
southwesterly winds and precipitation will work from the Eastern
Aleutians and Pribilofs to the AKPEN and mainland Southwest by
Wednesday night into the day on Thursday.

Meanwhile, another strong North Pacific low and its front will
lift northward to the Aleutian Chain by Thursday. A strong push of
warm southerly flow will accompany this system with moderate to
heavy rain likely across the Central and Eastern Aleutians for
Thursday. Storm force southerly winds are also likely across the
the North Pacific south of the Eastern and Central Aleutians and
northward into the Bering potentially reaching as far north as the
Pribilof Islands Thursday evening. Precipitation and gusty
southerly winds make it to the mainland Southwest by Thursday
afternoon with most precipitation falling as rain due to the warm
southerly flow. One thing to follow in addition to the
precipitation and gusty winds across mainland Southwest, is that
this setup could bring increased water level up Kuskokwim Bay that
could affect the communities of Kwigillingok and Kongiganak
Thursday evening into Friday. A strong cold air push will build in
behind this strong low pressure, now located near Saint Lawrence
Island, beginning on Friday with the Western Aleutians and Central
Aleutians seeing off and on convective snow showers through the
day. Stay tuned to the forecast as more details are fine-tuned
with regards to the track and impacts of the low traveling from
the North Pacific Thursday to Saint Lawrence Island Friday.

&&
.LONG TERM FORECAST (Days 4 through 7: Friday through Monday)...

KEY POINTS:

- A highly amplified and potentially impactful weather pattern is
expected to affect much of the forecast area from Friday into
this weekend.

- Strong winds, heavy precipitation and very warm temperatures
will be possible over much of southern Alaska from Friday to
Monday as a strong frontal system moves across the region.

- Much colder air will move in from the northwest behind the heavy
precipitation early next week.

An extremely active, highly unusual and potentially highly
impactful pattern will be well underway at the start of the period
on Friday morning. A very strong longwave ridge is expected to be
in place over the Northeast Pacific on Friday morning, with the
crest of this ridge extending well up into Mainland Alaska. To the
northwest, a deep longwave trough with an embedded closed low is
expected to be moving up into the far northern Bering Sea and up
into far northeastern Russia. Two dramatically contrasting air
masses will be building under the main upper trough and Pacific
ridge. A very warm air mass will be in place under and towards the
upstream side of the ridge, including much of the southern
Mainland out to the eastern Aleutians. Meanwhile, a much colder,
Arctic air mass will be building under the longwave trough across
much of the northern and western Bering up into Siberia.

The gradient in terms of temperature and pressure fields between
the two synoptic features outlined above, both near the surface
and the upper levels, will be quite impressive for even Alaska
standards. For example, the highest surface pressure under the
ridge in the northeast Pacific could be as high as 1045 mb,
contrasting with a surface low moving into the Bering that could
deepen to around 980 mb or lower - a difference of at least 65
mb. A strong baroclinic zone (an area of strong temperature and
pressure gradients) will exist between the main high pressure and
the low/trough in the Bering Sea, initially in place Friday
morning along an axis from the central Aleutians to the coastline
of western Alaska. This effective frontal zone will slowly advance
southeast with time throughout the weekend into early next week,
and this will largely be the focus of the most inclement
conditions with this pattern. The strong pressure gradient setting
up along this boundary, plus at least two distinct stronger areas
of low pressure that will move north along it, will support
intense southwest winds, initially over the eastern Aleutians and
Southwest. Stronger winds could also affect parts of Southcentral
on Saturday and Sunday as the second low moves up from the Pacific
into Southwest, but this potential for strong winds will be
highly dependent on a still very uncertain low track.

An anomalous moisture plume, or atmospheric river, is also likely
to stream north along and ahead of the main frontal zone,
initially expected to be impacting parts of Southwest and much of
the Alaska Peninsula and eastern Aleutians on Friday. This deep
moisture tap will help support rather intense precipitation rates
for much of Southwest and later Southcentral compared to what is
typically possible for late January. Temperatures in many areas of
Southwest close to sea level will likely initially be warm enough
for rain at lower elevations, and this will likely also be the
case as the atmospheric river and front shift into Southcentral
between Saturday and Monday. There could be a transition over to
snow across both Southwest and Southcentral as cooler air begins
to move in behind the front between Sunday and Monday.

Lastly, it looks increasingly likely for a significant, sudden
temperature drop to advance west to east across the southern
Mainland early next week as the front moves through and as the
Arctic air mass cascades south and east. There is still a fairly
large spread in terms of when it looks like the colder air will
arrive for any given location, but there is a strong signal in
both deterministic and ensemble guidance that this cold air mass
will eventually make it into our outlook area early next week.
This could lead to more weather impacts due to the sudden switch
from warm temperatures and rain/snow to bitter cold. Stay tuned as
we follow this very active pattern and resolve details for
timing, precipitation amounts and wind magnitude for this setup
in the coming days.

-AS

&&

.AVIATION...

PANC... Northerly winds will continue to increase atmospheric
mixing through the day keeping VFR conditions through 08Z Wed.
As a front moves through the Gulf today, northerly winds change
to a more northeasterly/easterly direction mid- afternoon. Wind
gusts will range from 20 to 30 knots. Wind shear is likely from
this morning through 08Z Wed. Scattered rain rain/snow showers
are expected after 08Z Wed into 12Z Wed though VFR conditions
return afterward.


&&

$$

470
FXAK69 PAFG 202259
AFDAFG

Northern Alaska Forecast Discussion
National Weather Service Fairbanks AK
159 PM AKST Mon Jan 20 2025

.SYNOPSIS...
A series of storms will impact the west coast through the week. A
strong front will set up over the west coast Tuesday into
Wednesday. A heavy band of snow will set up from the Y-K Delta to
the Kobuk Valley. Winter Storm Warnings have been issued for the
west coast. A stronger storm will impact the west coast Thursday
into Friday bringing high winds, heavy snow, and some mixed
precipitation south of the Seward Penn. Chances for snow increase
over the Western and Central Interior this weekend, then turning
much colder towards the end of the month over much of the area.

.DISCUSSION...

Key Weather Messages:

Central and Eastern Interior...

- Temperatures are beginning to warm in the Interior this morning.
The highs throughout the week will be in the 20s.

- Southerly gap winds will ramp up again through Isabell Pass
Tuesday into Wed. There could be gusts up to 60 mph and gusts
to 40 mph near Delta Junction. Wind Advisories are in effect.

- Fairbanks picked up 1-2" of snow overnight. Snowfall will
tapering off this afternoon.

West Coast and Western Interior...

- A strong front will set up over the west coast Tuesday into
Wednesday. A heavy band of snow will set up from the Y-K Delta
to the Kobuk Valley. Winter Storm Warnings have been issued for
portions of the west coast, including the Seward Penn. Snow
totals could be up to a foot in some spots depending on snow
ratios. Winter Storm Warnings have been issued for these areas.

- The system on Tuesday looks to northeasterly wind gusts up to 35
mph from the Seward Peninsula to the Y-K Delta. We could see
significant visibility restrictions in areas that are receiving
the heaviest snow as well as gusty winds.

- After this next system moves out of the area Wednesday morning,
there will be another more impactful system Thursday and Friday.
This system looks to be bring strong winds and heavy snow
initially, then mixed precipitation as it moves northeast
through Saturday. Gusts over 60 mph are possible along the
coast. Another foot of snow is possible along the Nulato Hills,
Seward Penn and Kobuk Valley.

North Slope and Brooks Range...

- Well above normal temperatures continue today from the Brooks
Range to the Arctic Coast. Expect temperatures mainly in the
20s, with colder air filtering back in from west to east through
Wednesday.

- Falling snow along with southerly gap winds through the Brooks
Range passes up to 40 mph will cause blowing snow and low
visibility through Tuesday. A Winter Weather Advisory is in
effect.

Forecast Analysis and Confidence...

Upper level pattern shows an upper level low near Wrangel Island
will continue to track to the NE over the Arctic. Another upper
low will move into Bristol By on Tuesday. Southwestly flow will
set up over the west coast and Interior Thursday and into the
weekend.

A strong front will set up over the west coast Tuesday into
Wednesday. There will be rapid frontogenesis along this boundary
creating a very narrow band of heavy snowfall. Heavy band of snow
has shifted slightly to the west to include the Seward Penn. Have
upgraded the last of the watches to warnings. We are trending to
the ECMWF solution as the ECMWF/GFS ensemble support that solution
as well as the Nam. Snow totals will be 8 to 12" of snow along
this boundary. Models are also in disagreement on the timing of
the Thursday/Friday system over the west coast as well as the
timing of the precip moving across the western Interior and
Central Interior over the weekend.

Extended Forecast Days 4-8...

A major pattern shift is likely by the end of the week as flow
turns more southwesterly and a number of strong low pressure
systems enter the Bering Sea. While the exact tracks of these lows
are still uncertain, the overall pattern is favorable to bring an
abundance of warm air and precipitation northward over western
Alaska and possibly into the central and eastern Interior. Expect
a strong storm with high winds across the West Coast on Thursday
and Friday, with increasing chances for heavy precipitation in the
Interior Friday through Sunday as well as gusty winds over higher
terrain of the Interior. Models indicate an increasing probability
for very cold temperatures early next week behind this system.

Coastal Hazard Potential Days 3 and 4...Impactful storm will
produce near Storm-force winds offshore as well as possible water
moving over the ice along the coastlines of the Y-K Delta and
Norton Sound.


&&

.AFG WATCHES/WARNINGS/ADVISORIES...
AK...Wind Advisory for AKZ847-849.
Winter Storm Warning for AKZ813-819.
Winter Storm Warning for AKZ816-822-823.
Winter Storm Warning for AKZ824>826-829-830.
Winter Weather Advisory for AKZ809.
PK...Heavy Freezing Spray Warning for PKZ801.
Small Craft Advisory for PKZ801.
Heavy Freezing Spray Warning for PKZ802.
Small Craft Advisory for PKZ802.
Brisk Wind Advisory for PKZ803-817.
Brisk Wind Advisory for PKZ804-805-816-851-852.
Brisk Wind Advisory for PKZ806.
Brisk Wind Advisory for PKZ811.
Brisk Wind Advisory for PKZ812-813.
Brisk Wind Advisory for PKZ814.
Brisk Wind Advisory for PKZ815-861.
Brisk Wind Advisory for PKZ850.
Brisk Wind Advisory for PKZ853.
Brisk Wind Advisory for PKZ854.
Brisk Wind Advisory for PKZ857.
Brisk Wind Advisory for PKZ858-859.
Brisk Wind Advisory for PKZ860.
&&

$$

JK

480
FXAK67 PAJK 211003 AAA
AFDAJK

Southeast Alaska Forecast Discussion...UPDATED
National Weather Service Juneau AK
103 AM AKST Tue Jan 21 2025

.UPDATE...
Reduced visibilities in Petersburg have prompted the issuance of a
dense fog advisory for the Petersburg Borough. Dense fog will
persist in parts of the Borough until around 9 AM AKST Tuesday
morning.

&&


.PREV DISCUSSION......ISSUED AT 319 PM AKST MON JAN 20...

SHORT TERM...

Key Points:
-Brief break in widespread precipitation Tuesday as weak
ridging moves aloft.
-Next system with a gale force front pushes into the Gulf
Tuesday night, into the Panhandle early Wednesday morning.
-Snow accumulations up to 4" possible for the upper elevations of
the Haine`s and Klondike Highways on Wednesday.

Discussion:
Starting of the short range on Monday night into Tuesday with a
brief break in widespread precip across the area as ridging moves
aloft then eastward through the day. Weather break doesn`t last
long as the main weather story will be an approaching Gale force
system from the Gulf by Wednesday morning, overspreading SEAK
through the day. Forecast remains on track with highest 24hr
rainfall totals coming across the NE Gulf with Yakutat forecast
around 1 to 2 inches Wednesday through Thursday morning. As front
pushes inland, have current forecast temps in the mid 30s so will
mostly likely see a rain/snow mix for much of the panhandle during
onset before gradually turning to predominate rain as
temperatures warm to the upper 30s through Wednesday afternoon.
Main forecast question will be snow accumulations for the far
northern panhandle and highways during frontal onset before the
changeover to rain via strong warm air advection. Adding to the
conundrum will be downsloping or "shadowing" limiting potential
precipitation totals for the Haine`s Highway, thus limiting
potential snowfall accumulations. With all this to say, current
forecast thinking has snow totals around 4" possible for the upper
elevations of the Haine`s and Klondike Highways, down to 2" of
snow-rain mix for Haines, Skagway, and Yakutat.

LONG TERM.../Wednesday though the early weekend/...
Diminishing winds and precipitation transitioning to weak showers
with additional shortwaves moving over the panhandle. Going into
late week, a very unusual pattern for this time of year will bring
much drier conditions.

On Wednesday, a weakening front will successfully push over the
panhandle and into Canada, bringing mainly rain with some snow
accumulations for the Haines and Klondike Highway. As spoken about
in the short term, WAA from the S-SW in the 850-700 mb layer
promotes relatively isothermal temperatures along with shadowing
along the Haines Highway look to limit both moisture and lift in
the dendritic growth zone. What this translates into is relatively
low snow totals. This is reflected in the NBM where there is an
80% chance to have 12 hour snow totals of around 2-4 inches. While
there may be locally higher amounts, confidence is not high
enough to issue winter weather products.

Going into Thursday and the weekend, an omega block looks to set
up, with run to run consistency of the GEFS and ensemble Euro
continuously strengthening the ridge. At the surface, this means a
high pressure, with approximately 90% chance of being above 1040
mb in the gulf. This will bring clearing skies, downward motion
aloft, and lighter winds for the inner waters. The main source of
uncertainty in regards to these patterns is how long the ridge
will stick around, which will determine when precipitation
returns. Upstream; however, has very cold arctic air advecting
down from the Arctic, which will assist in both strong systems
moving into the northern Gulf Coast, and pushing the stream of
moisture towards the NE Gulf Coast/Yakutat area. The stronger the
cold air advection, the quicker the block moves off.

AVIATION...
The showers slowly tapper off through this evening but MVFR
conditions will persist with low clouds. At the same time any
strong winds that area happening at this time will decrease with
most locations seeing light winds. There will be some breaks in
the cloud cover and areas and with the recent precipitation fog
may develop with IFR conditions due to low VIS and CIGS. These low
clouds will slowly rise through Tuesday morning but some
locations may take longer to break out. Overall there is average
forecaster confidence on the development of fog but lower on the
extent and severity of VIS and CIGS levels.

MARINE...
For the Outer and Coastal waters: Wave heights in the outer
coastal waters will subside down to 6 to 8 ft through Monday
night. Brief weather break through Tuesday morning as ridging
shifts eastward. By Tuesday afternoon, a Gale force front pushes
into the western Gulf before continuing into the panhandle by
Tuesday night. Anticipate seas building up to 15 to 20ft in the
northern Gulf by Tuesday night, especially near the coast from
Cape Fairweather westwards towards Cape Suckling as barrier jet
forms with front pushing inland. Seas gradually subside to 12ft
or less by Wednesday morning post-front.

For Inner Channels: Winds in the inner channels will gradually
diminish to a light to moderate breeze through Monday night. Winds
should remain subdued to gentle breeze or less through Tuesday
morning as weak ridging moves overhead. As the system approaches,
anticipating inner channel winds to increase up to Strong Breeze
with wave heights building to 4ft by Tuesday evening. Strongest
winds expected within Clarence Strait given orientation to the
front, reaching Near Gale by early Wednesday morning. Post-front,
inner channel winds will remain elevated near Strong Breeze
through the rest of the day Wednesday.

&&

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Dense Fog Advisory until 9 AM AKST this morning for AKZ326.
MARINE...Gale Warning for PKZ652-663-664-671-672.
Small Craft Advisory for PKZ053-641>644-651-661-662.

&&

$$

UPDATE...GFS
SHORT TERM...NM
LONG TERM....NC
AVIATION...ABJ
MARINE...NM

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