Graphical Hydro Meteorological Discussion

441
FXAK68 PAFC 070132
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
432 PM AKST Fri Dec 6 2024

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


Key Points:

* Another round of freezing rain and drizzle is possible late
tonight into early morning Saturday for Anchorage and the Mat-
Su Valleys.

* A wintry mix of rain, freezing rain, and snow is possible along
the Seward Highway corridor from Turnagain Pass to Moose Pass
Saturday due to cooler temperatures and snow levels dropping to
around 1,000 ft.

* Despite a brief, dry and cold day Saturday, the unusually warm
and wet pattern will continue to bring a wash, rinse, and repeat
cycle for Southcentral into the week ahead. The next system
arriving Sunday through Tuesday.

Discussion:

Weak, southeasterly flow across Southcentral is leading to
showers for the Kenai Mountains and Prince William Sound. Upslope
flow against terrain is the key feature allowing precipitation on
the windward side of the mountains and for the leeside to be
fairly dry. Precipitation has redeveloped along the Alaska Range
of the Susitna Valley. Much of this precipitation is running into
drier air nearer the surface and is not reaching the ground for
most of western Kenai Peninsula, Anchorage, and MatSu Valley. A
second wave, along with the remnants of the surface front, may
bring another round of very light freezing rain to these same
areas later tonight into early Saturday morning.

For the eastern Kenai Peninsula, this second wave will also
advect slightly cooler air both at the surface and in the mid-
levels, helping to drive snow levels down to around 1,000 ft.
Although there is a definite trend in cooler temperatures for the
eastern Kenai Peninsula, the extent of colder air is still
uncertain. This is leading to a tricky precipitation type forecast
starting this evening along the Seward Highway corridor,
specifically whether any rain and/or freezing rain changes over to
snow.

Looking ahead, a shortwave ridge could bring another round of
patchy fog for Saturday, though confidence at this time is low.
Cloud cover could stunt development. Yet another system is
scheduled toward the end of the weekend, reaching Kodiak on Sunday
morning. Gale force winds are expected along the front and ample
moisture looks to be associated with this system. Again, warm
southeasterly air looks to advect into Southcentral. Recent model
runs have slowed the progression slightly and added a slightly
southerly component to the flow ahead of the front. Confidence in
the timing of its passage could change, though Sunday to Sunday
afternoon appears likely for Kodiak and Kenai Peninsula, spreading
across Southcentral Sunday afternoon and night. Warm air advection
is also likely to raise snow levels to around 2000 feet or higher. Midlevel
temperatures also vary and precipitation type challenges seem
moderately likely. Though, this system appears to have more
energy involved and precipitation changes could switch more
quickly, especially for areas susceptible to southeasterly and
warmer onshore flow. If precipitation rates for areas close to
freezing fall heavy enough, temperatures could cool to freezing
and fall as snow. This is a challenging forecast ahead, please
stay winter weather prepared and check for updates to the
forecast.

-TM/Rux

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

A col in the Central Bering is flanked by two weak low pressure
systems, with one currently south of the western and central
Aleutians and the other just north of Nunivak Island. A chance for
brief showers across the Southwest mainland through Saturday
remains as the trough over the region continues to weaken and
meander slowly northward out of the area. In the Bering, the
surface low just south of Amchitka Island is producing small craft
winds and light rain in the western and central Aleutians. This
low will continue to slowly shift eastward before beginning to
dissipate and becoming absorbed by a northward moving North
Pacific low. A ridge building over the Alaska Peninsula and
southwest Alaska Saturday will allow for calmer and drier
conditions. This ridging and break in the weather will be brief
and is quickly pushed out Sunday morning into Sunday afternoon by
the aforementioned North Pacific low that will continue to head
northwards into the Bering by Sunday night.

This North Pacific low will send bands of precipitation across
Southwest Alaska, the Eastern Aleutians, and the Alaska Peninsula.
The Bering could see a brief period of mixed rain/snow before
warm air advection occurs and transitions the precipitation fully
to rain. Gale force winds are expected accompany this low and will
set up on the southern coasts of the Aleutians and the Alaska
Peninsula as well as in Bristol Bay. Locations such as Cold Bay
are expected to see a period of strong winds with gusts to 50-60
mph possible by late Saturday night/Sunday morning.

On the mainland, a brief period of a rain/snow mix is possible in
Bristol Bay Sunday morning, but a quick transition to plain rain
is expected given the strong downsloping southeasterly winds
quickly warming the region to above freezing. The strength of the
southeasterly downsloping will limit the chances for precipitation
for much of the Bristol Bay region, particularly for the King
Salmon area. As the low lifts northward Sunday morning into
Monday, a favorable pressure gradient will increase the winds
through Kamishak Gap into portions of Bristol Bay with wind gusts
up to 40-50 mph. In the Kuskokwim Delta, mainly snow is expected,
but a small possibility of a short period of snow mixed with
freezing rain is possible in the Bethel region Sunday afternoon
before quickly transitioning to rain for any precipitation that
continues to fall. However, this is highly dependent on how fast
and how warm the temperatures can rise and how much downsloping
the area experiences once the winds turn southeasterly. Snow mixed
with freezing rain also remains possible for an area
approximately from Aniak to Sleetmute through Monday. Regarding
the warm-up expected with this system, the high temperatures for
most of Southwest will be near 25 degrees F Saturday, whereas
Sunday`s high temperatures are expected to be near 35 degrees,
with a trend towards warmer temperatures possible heading into the
workweek.

-JH/CL

&&

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

Operational models continue to show negatively tilted 500 mb
troughing across the Aleutians into the Gulf with its associated
upper low moving across the Aleutians. Main weather story is a
deep frontal occluded low tracking the Aleutians. Sufficient upper
level forcing will support rain/mixed precip (locally heavy)
along the coast with snow farther inland. There is some degree of
uncertainty that needs fine tuning along the coast where it is
possible these areas could see more snow than expected. These
areas include Alaska Range, Eastern Kenai Peninsula and across
Prince William Sound to the Canadian border. Pressure gradient
tightens through the period in which wind gusts will felt up to
gale force from Kodiak Island extending northeastward to Prince
William Sound. Overall impacts should remain confined closer to
the coast and away from the northern mtns. The relatively warm
southerly flow complicates precip types during the period,
especially just inland from the coast.

&&.AVIATION...

PANC...VFR conditions and light northerly winds will prevail. Cannot
completely rule out a chance of freezing rain this evening, but
confidence and coverage remain much too low to include in TAF at
this time.

&&


$$

626
FXAK69 PAFG 071222
AFDAFG

Northern Alaska Forecast Discussion
National Weather Service Fairbanks AK
322 AM AKST Sat Dec 7 2024

.SYNOPSIS...
Relatively mundane weather ongoing today with areas of light snow
from St. Lawrence Island to the Seward Peninsula and northeast
into the Brooks Range. Later this afternoon, a band of light snow
moves from south to north in the Western Interior. The North
Slope turns drier but there is still a chance for some fog or
mist. Temperatures will stay above normal for much of Northern
Alaska through next week. Our eyes then turn to a low in the
southern Bering on Sunday which brings a mess of snow, rain,
freezing rain, sleet and some blowing snow to Western Alaska.

&&

.DISCUSSION...
Key Weather Impacts by Region...

Central and Eastern Interior...
- Warm temperatures continue through the middle of next week with
high temperatures peaking around the mid-upper 30s on Monday
from the AK Range to FNSB.

- Gap winds weaken today, then strengthen tomorrow afternoon and
evening. Winds will likely gust near or over 70 mph, especially
in Isabel Pass. Windy Pass may approach 70 mph sunday night but
there is less confidence. Strong winds continue through Monday.

- A front will move over the AK Range on Monday, there is a slight
chance for snow in Fairbanks, though FNSB will likely be
chinooked enough to keep it dry. North of the area has the
greatest probabilities for light snow.

West Coast and Western Interior...
- Light snow from St. Lawrence Island to the Brooks Range. A front
will move from south to north this afternoon and evening
providing light snow for most of the Western Interior. This
front arrives near Ambler by late Sunday night.
- 0.5 - 1.5 inches of snow is expected for most of the area.

- A stronger storm moving into the southern Bering on Sunday will
provide widespread mixed precipitation.
- Expect snow amounts upwards of 6-10 inches in the Nulato Hills
with around 1-3 inches in the Western Interior by Sunday night.
- St. Lawrence Island and the Seward Peninsula will begin to see
snow Sunday evening/night with 2-4 inches in the lower
elevations and upwards of 6-8 inches in the terrain by Monday
night.
- Precipitation looks to turn to a mix with and change to rain in
SW AK Sunday evening. It changes over to rain in the southern
Seward Peninsula and St. Lawrence Island Monday
afternoon/evening.
- A glaze of ice is likely for most areas.

- Precipitation looks to taper on Tuesday, but drizzle and light
rain will be possible in most valleys south of the Seward
Peninsula while the terrain remains snow.

- Above freezing temperatures are very likely across almost the
whole area Monday and Tuesday.

North Slope and Brooks Range...
- Light snow in the Brooks Range today, around 1-2 inches of snow
expected.

- Strong Chinook conditions Monday and Tuesday will bring well
above normal temperatures and strong south winds through Brooks
Range passes.
- Snow chances Sunday night into Monday have the potential to mix
with freezing rain, sleet and rain Monday night, though it will
be drying out across most of the Slope.

- Strong easterly gales develop along the coast Sunday night into
Monday. Some blowing snow is possible, especially near Kaktovik,
but visibilities should remain around 1 mile.

Analysis and Forecast Confidence...
The low that has been near Nunivak Island the last couple of days
is gradually shifting northwest but will still provide light snow
to most of Western Alaska from the Brooks Range south today
through tomorrow morning. This low, coupled with a high in SE AK
is providing southerly flow over the Interior. This has resulted
in some gap winds which will be weakening today. Otherwise, dry
weather persists in the Central/Eastern Interior. The North Slope
has a chance for some light snow near Kaktovik, but any snow will
be brief and light. Otherwise, a high in the Arctic is bringing
some clear skies to the coast. It will cloud up once again this
afternoon into tonight as a shortwave drops south from about 80N.

We then look towards Sunday where a storm enters the southern
Bering from the Northern Pacific. This will be coming with plenty
of moisture. As the moisture passes over the mountains, it will
get wrung out but there will still be snow in SW AK by Sunday
morning. This progresses north through the afternoon and to the
Brooks Range by Sunday night. Warm, above freezing air, will
intrude from the south all the way to the Seward Peninsula
changing the snow to rain. Given the below freezing ground
surfaces, any rain will likely freeze on contact. This will lead
to a glaze of ice across much of the area from Nome south/east.
The only exception will be in the terrain where cold air likely
remains in place and keeps precipitation all snow.

For more details on timing, check out the specific warnings/advisories
in your area on our website weather.gov/afg .

Extended Forecast Days 4-7...
Warm weather continues for much of the state. The low in the
Bering Sea will be weakening and drifting northwest on Tuesday,
but light rain and snow will still be around Western Alaska.
Otherwise, everyone along the West Coast will dry out by Wednesday
afternoon. From here, there will be chances for snow on the North
Slope Tuesday - Thursday as the remnant front progresses northward,
though this looks to be mainly light. Lastly, we will be
monitoring the chance for snow in the Central/Eastern Interior
Wednesday and Thursday as a low in the Gulf of Alaska throws a
front over the Alaska Range providing southeast flow and moisture.
Confidence levels for this are low, but snow chances for the
Interior will return next week as an active pattern persists.

Coastal Hazard Potential Days 3 and 4...Nome may see water levels
1 to 3 feet above the normal high tide line on Monday night and
Tuesday, otherwise no coastal impacts are expected from this storm
system.

&&

.AFG WATCHES/WARNINGS/ADVISORIES...
AK...High Wind Warning for AKZ849.
Winter Storm Watch for AKZ817.
Winter Weather Advisory for AKZ822-824.
Winter Storm Warning for AKZ823-829.
Winter Weather Advisory for AKZ825-826-830.
Wind Advisory for AKZ837-847.
PK...Heavy Freezing Spray Warning for PKZ806.
Brisk Wind Advisory for PKZ811-812.
Small Craft Advisory for PKZ816.
Heavy Freezing Spray Warning for PKZ816-817-851.
Small Craft Advisory for PKZ817-854.
Small Craft Advisory for PKZ851.
Heavy Freezing Spray Warning for PKZ854.
Brisk Wind Advisory for PKZ856.
Gale Warning for PKZ857-858.
&&

$$

Bianco

357
FXAK67 PAJK 062358
AFDAJK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
258 PM AKST Fri Dec 6 2024

.SHORT TERM...The theme Friday afternoon into the early overnight
hours is much drier with lighter winds. A closed low currently
south of Kodiak will continue to transit east towards Haida Gwaii
overnight, which will drive our weather into Sunday. Main threat
is the combination of an occluding front at the surface, some
moisture, cold air advection at 700mb, and weak outflow winds in
the inner channels. Throwing these ingredients together expecting
another quick round of moderate to heavy precip for the central
and southern Panhandle. 24-hour rainfall amounts are not
impressive, around 0.5 to 1 inch, perhaps 1.5 inches under a
more potent shower.

On that note, expecting embedded convective showers given surface warm
air advection associated with the occluding front while cold air
advection occurs at 700mb. Historically, 24-hour rainfall amounts
can be a bit higher than expected underneath heavier showers.
Furthermore, we are not expecting widespread thunderstorms, just
the potential for isolated lightning, as shown in model sounding
profiles. Conditions look better for the southern Panhandle into
Dixon Entrance.

For Yakutat and the northern Panhandle, will see some fog/low-
clouds in the morning but enjoy the dry weather Saturday into Sunday
morning.


.LONG TERM.../Sunday through Friday/...High pressure ridge will move
over the AK Panhandle Sunday as a gale force front starts to track
into the western AK Gulf. As the front approaches Sunday morning
will be a transition from northerly to southerly flow. Overall Sunday
will be dry but there is a 500 mb vort max moving over that
afternoon which could set off some showers ahead of the main front
that afternoon into Monday. Temps will increase Sunday morning as
flow shifts to the south and then continue to rise overnight and
into Monday. Since there will be cold air in place from Saturday
night any precip on Sunday could fall a snow or a rain snow mix, but
then switch over the all rain for most sea level locations. Due to
limited moisture and QPF not much accumulating snow expected.
Question has been the timing of the approaching front with latest
operational models slowing it down and thus keeping the drier
weather in place through Monday. Ensemble timing did not change as
much so holding off on really cutting off the PoP yet. What does
look likely are the gale force winds over the eastern gulf along the
front with inner channel winds picking up by at least early
Tuesday. Moderate precip amounts expected especially for the NE Gulf
Coast from the front. The parent low pushes into the gulf late
Tuesday into early Wednesday with more gale force winds and rain.
Moderate to heavy precip for mid week will be more wide spread. with
the rising snow levels and increased rainfall will again be watching
for river rises.

&&

.AVIATION...Fairly quiet aviation weather as most areas are VFR
with low wind conditions. Some areas of fog or low clouds still
plague isolated areas of the southern panhandle this afternoon.
Into tonight main concern is the incoming system for the southern
panhandle. Rain is expected to spread across the south late
tonight and persist into the first half of Saturday bringing MVFR
(possible isolated IFR) conditions to most areas. Conditions start
to improve by Saturday evening as precipitation moves out, but
some low clouds and fog could develop overnight Saturday night.
Northern panhandle by contrast is looking to be mostly VFR through
Saturday at least as offshore flow takes hold there (though no
strong outflow this time).

&&

.MARINE...Buoys 46001, 46186, and 46084 continue to show
diminishing seas as the gulf fetch loses support from the parent
storm this morning. While 82 and 83 are offline, wave nomograms
and guidance highlight the potential for 15 to 18ft swell along
the northern coast tonight before dropping below 12 ft Saturday
morning. On top of the swell will be minor easterly wind waves
driven by 20 to 30 knot ESE winds. For the coast of Prince of
Wales and western Dixon, Saturday afternoon expect SW seas 14 to
17ft to move in.

Inner Channels: Current regime in the inner channels is light
onshore southerly winds. These will transition to northerly
moderate to strong breezes overnight as a low transits south of
the gulf into Haida Gwaii. Main threat for 20 to 30 knots of wind
is Lynn Canal, eastern Icy, and out of Cross Sound. Down south in
Clarence we will likely see southerly 20 to 30 knots Saturday as
the front moves over.


&&

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Small Craft Advisory for PKZ012-013-022-036-053-641>644-651-652-
661>664-671-672.

&&

$$

SHORT TERM...AP
LONG TERM....PRB
AVIATION...ABJ
MARINE...AP

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