Graphical Hydro Meteorological Discussion

502
FXAK68 PAFC 181350
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
550 AM AKDT Wed Sep 18 2024

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

The strong low that tracked across the Bering Sea has crossed
into southern Cook Inlet early this morning. While not as strong
as it was earlier, it still is packing a punch with areas of heavy
rainfall and strong winds around it. This occluded low was
triple-wrapped by its front last evening as seen on satellite
imagery and these multiple waves will bring the periods of
enhanced rainfall today. There is a bit of cold air aloft
associated with this low center which is bringing in unstable air
with it. This instability kicked off a number of thunderstorms in
southwest Alaska yesterday, but it is marginal whether there will
be an isolated thunderstorm or two in Southcentral today as this
low passes through. Most likely areas will be right under the
upper low center as it tracks across southern Cook Inlet through
Prince William Sound. This low is continuing to move rather
quickly so it should move past the AlCan border this evening then
most of the precipitation should subside for Southcentral. However
there should be some stiff north to west winds behind the low
which will be strongest near the coast from Kodiak to Whittier.
One of the big questions will be whether these winds will be
enough to keep the lower atmosphere mixed enough after the
rainfall, or if all that moisture will result in widespread fog
tonight into Thursday.

The 12Z sounding this morning for Anchorage showed the freezing
level down to 4166 ft. Therefore expect the dusting of snow along
the higher peaks of the Chugach front range to hove turned into a
solid line of snow or elevations above 4000 ft when the some brief
breaks in the clouds develop late today and into tonight for
Anchorage and the Mat-Su.

In general, Thursday should see some breaks in the clouds for much
of SOuthcentral,with areas farther south such as Kodiak and Homer
seeing a good bit of sunshine. While there should be some sunny
breaks tomorrow for most o Southcentral, the wrench in it all is
that the longwave upper level trough really does not go
away. Another upper level short wave looks to dig into the
Susitna Valley Thursday night and stall over the Talkeetna
Mountains for Friday. This might result in the Copper River Basin
and Prince William Sound seeing some rain on Friday as a result of
this trough. With the cold air aloft, it is possible for some of
the mountain passes, mainly near Eureka and Paxson, to see snow
get close to the highway on Friday.

&&


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

Winds are continuing to gradually decrease as it shifts
northwesterly across Southwest Alaska and the eastern Bering Sea
this morning. Lingering showers will remain possible across
Southwest Alaska for today with likely chances near mountains and
the through the Kuskokwim Valley. Cooler air is expected to linger
through the rest of this week with widespread low temperatures
across Southwest Alaska hovering near freezing.

A ridge currently over the western Bering Sea will build toward
Southwest Alaska tonight. This should help to decrease chances for
precipitation for Thursday. Current satellite imagery under the
ridge indicates widespread low stratus across the Bering Sea and
most recent surface observations at Atka indicate that patchy fog
is developing as well. Looking ahead, there is low confidence for
how much extent this fog will develop across the Aleutians and the
Bering Sea. With light onshore flow and pressure building, it is
possible that the Kuskokwim Delta and Bethel could have patchy fog
for Thursday morning. While weak subsidence is likely for
Southwest Alaska, there is a low potential for embedded shortwaves
to bring brief periods of precipitation (rain, snow, or a mix) to
Southwest Alaska Friday and this weekend. This should be very
minor and not widespread.

Farther west, a front accompanying a Kamchatka low is expected to
move into the western Aleutians this afternoon. The ridge will
help slow the fronts progression, keeping rain chances likely for
the western Aleutians. This feature is expect to interact with a
North Pacific low approaching the western Aleutians and could be
the vehicle to help spread precipitation east and a track for
additional troughs. With a strong ridge in place, the trough is
expected to be somewhat deflected to the east and track light
showery activity along the Aleutians to the Alaska Peninsula
through Friday. Meanwhile, as the Kamchatka low lingers, another
low will quickly follow into the western Aleutians for Thursday
night, though model agreement begins to fall apart for the low
trajectory and how it interacts with other synoptic features for
Friday and this weekend. Models hint at continued troughing for
the Aleutians, while high pressure sits in the northern Bering
Sea.

rux

&&


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

Latest operational models show little agreement by Friday, so
will lean more on the ensemble means for the long term forecast. A
broad upper low positioned over the Western Aleutians looks to
slide eastward into the western Gulf by Tuesday. Upper-level jet
support should remain on the south side of the low with the
strongest winds at the surface to remain largely south of the
Aleutians. This looks to promote a quieter period of weather for
both the Bering, Southwest Alaska, and Southcentral Alaska through
the weekend. Instead, drier and cooler conditions are forecast
and lowering snow levels as a cooler airmass slowly makes its way
from interior Alaska into Southwest Alaska by Tuesday. As the low
becomes anchored over the western Gulf early next week, some
potential will exist for the return of shower chances for
Southcentral as easterly waves may bring scattered showers to the
Copper River Basin, Prince William Sound, and Gulf coastal
regions.

-BL

&&

.AVIATION...

PANC...MVFR to IFR ceilings and light rain are expected through
Wednesday afternoon, then VFR conditions return Wednesday night
into Thursday. There is a chance for some fog Wednesday night if
the winds become light and clouds clear out significantly.

&&


$$

252
FXAK69 PAFG 182227
AFDAFG

Northern Alaska Forecast Discussion
National Weather Service Fairbanks AK
227 PM AKDT Wed Sep 18 2024

.DISCUSSION...

.SYNOPSIS...
A powerful occluded low is moving inland over Bristol Bay this
morning. That low shifts east today and eventually clouds and
shower chances increase over the Alaska Range and Southern
Interior into Thursday night. An arctic trough will bring colder
air and accumulating snow chances to the Brooks Range and Arctic
beginning Thursday. The arctic trough will dig south this weekend
bringing colder air and chances for snow even down into the Interior.

.DISCUSSION...

Upper Levels and Analysis...
A strong occluded low pressure system of around 530 dam is
transiting eastward across the Kenai Peninsula and will continue
to fill as it propagates across the Prince William Sound by later
this evening. There is also a weak shortwave transitioning slowly
across the NW Arctic. Winds will continue to weaken across the
eastern Alaska Range through the evening as the low pressure to
the south continues to pull away. Models have begin to show more
of a stronger trough deepening over the Mainland by this weekend,
which will allow for much colder temperatures to be advected in
from the north.

Forecast and Model Discussion...
As the low to the south further weakens as it moves into the Gulf
of Alaska, the loosening gradient will allow winds to will
continue to weaken over the E Alaska Range. The weak troughing
is expected to keep the chances of rain going across most of the
Mainland and Interior region later this evening and through
Thursday. There will be a stronger through moving in this weekend,
which will allow for colder air advection from the North Slope
and bring the chance of snow down in the Interior, especially by
Saturday night. GFS has continued to have a better and more
reliable handle of this and will continue to be blended in more.
Temperatures will likely continue to trend below the seasonal
average into early next week.

Central and Eastern Interior...
With residual moisture in place and weak troughing, the
possibility of on and off showers will continue over portions of
the Mainland, and this will likely prevail through Friday. There
will likely be snowfall accumulations across the Alaska Range,
especially for elevations around 2000 ft and above tonight. The
colder air moving in this weekend could support snowfall down to
the valley floor of the Interior, and especially light
accumulations around 1000 ft and above. It is more likely to see
accumulating snow by Saturday night.

West Coast and Western Interior...
There will still be the possibility of light showers across the
West Coast and Interior all the way into the late week with
continued flow being more westerly. There could be snowfall
across the western Brooks Range by later in the week and though
the weekend as the trough deepens from the north. There will be
some drier air moving in on Friday, and also though into Saturday
as more northerly winds become predominant will the deepening low
across the North Shore.

Extended Forecast Days 4-7...
With the deepening trough over the Mainland this weekend, and much
colder temperatures expected as a result, it could allow for some
of the first snowfall to make it down into the Interior, even for
the lower elevations. Models, and particularly the GFS, have also
been trending towards the arctic trough this weekend of becoming
stronger as it moves across the Brooks Range and then setting
setting up over the Central and Eastern Interior through Tuesday
of next week. The ECMWF has also continued to align more with this
solution and also agrees with the trend towards a colder airmass
advection which may support measurable snowfall down to the
valley floor of the Interior this weekend.

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

.HYDROLOGY...
Rainfall of 0.5 to 0.75 inches fell over the YK Delta and Lower
Yukon Valley/Middle Kusko Valley the last 24 hours. Smaller
stream and larger stem river rises are occurring, but will not be
significant. Elsewhere, no concerns as rivers are running low due
to fairly low snow levels.

&&

.AFG WATCHES/WARNINGS/ADVISORIES...
AK...None.
PK...Small Craft Advisory for PKZ801.
Small Craft Advisory for PKZ803-852.
Small Craft Advisory for PKZ804.
Small Craft Advisory for PKZ805.
&&

$$

417
FXAK67 PAJK 182325
AFDAJK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
325 PM AKDT Wed Sep 18 2024

.SHORT TERM.../Through Thursday night/...Post frontal showers will
prevail through this short term forecast period. As the remnants
of the front cross into Canada, gusty winds over the northern
inner channels will ease, but low level onshore flow will persist
through the period before backing to the southeast in advance of
the next front. Overnight lows through Thursday night will be
getting down to the lower 40s, so no frost concerned yet. With
the growing season behind us now, no frost or freeze warnings will
be issued, so pay attention to overnight lows lest your late
season outdoor plants get inadvertently damaged. Near White Pass,
overnight lows tonight and tomorrow should be getting low enough
that a mix of rain and snow is possible, though no accumulation is
expected. Daytime highs in the mid to upper 50s forecast for
tomorrow.

.LONG TERM...

Key Points:
Multiple lows impact the Panhandle through next week bringing wind
and rain. Most rainfall expected in the far southern region.
Potential break in rain Sunday.

For rainfall details see Hydrology section.

Early Friday a long wave trough will be departing the Panhandle
moving into western Canada, shutting off support to the surface
low positioned in the Gulf. With the loss of reinforcing dynamics,
expect the pressure gradient to weaken, allowing winds and rain
to diminish into Friday afternoon. This break will be short lived
as an extensive jet drops south of our region allowing multiple
embedded shortwave troughs to rapidly move into the Panhandle.
Current forecasts reflect a surface low associated with each of
these features, bringing elevated wind Friday into Saturday and
potentially Sunday. Snow levels are dropping but no impacts
expected at this time.

While the Panhandle is impacted by these lows this weekend, a
broad open long wave trough will begin to move into the Gulf.
Associated with this feature will be a 980 to 985mb surface low.
While the low takes residence near Cook Inlet Sunday the pressure
gradient will tighten over the Panhandle with winds increasing out
of the south again.

&&

.AVIATION...Flying conditions for the panhandle today have varied
depending on location. Some places continue to report IFR
conditions while other locations have improved to VFR while others
are still experiencing MVFR CIGs. CIGs are expected to improve
during the evening and overnight hours as showers continue to
dissipate. There is the potential though that fog could develop
during the overnight hours with the clearing skies. So that is
something to be mindful of as we head into the overnight hours.

&&

.HYDROLOGY...Multiple rounds of heavy rainfall through the next 7
days. SPS issued for Friday into Saturday in the south.

First round, Friday into Saturday: Long range guidance is still
flip-flopping on location, extent, and magnitude of an Atmospheric
River (AR) that will be carried by the jet stream for this
weekend. Current ensemble prognosis indicates a weak AR event with
some outliers reaching moderate. With that said, guidance the
ECMWF extreme forecast index continues to shift this first AR
north a touch from Hecate Strait/Haida Gwaii; simply put, heavier
rainfall is becoming more of a reality in the southern Panhandle,
potentially breaking some record 24 hour values with rivers
responding region wide. No flooding is anticipated at this time.


Second round Monday into Tuesday: As a broad longwave trough
positions itself in the western Gulf Monday and Tuesday, the jet
axis will become positive, which will steer a more potent
atmospheric river into the Panhandle. Guidance is showing some
agreement that this could be a moderate to strong AR with most
rainfall expected in the central and south. ECWMF EFIs are
highlighting a significant shift from climate normal over most of
the Panhandle with further trend analysis required over the next
few days. Simply put we could see a more significant rainfall event
Monday and Tuesday.

&&

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Strong Wind until 10 PM AKDT this evening for Akz018-319.
MARINE...Small Craft Advisory for
PCs-013-022-031-053-641>644-651-652- 661>664-671-672.

&&

$$

SHORT TERM...Fritsch
LONG TERM....AP
AVIATION...SF
HYDROLOGY...AP

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