Graphical Hydro Meteorological Discussion

791
FXAK68 PAFC 191334 CCA
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
529 AM AKDT Thu Sep 19 2024

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

The powerful low that tracked across the southern potion of the
state the past few days has now moved on shore and into Canada.
The final impact from it around the Gulf is some sea heights
which are still at small craft advisory levels today. With the
next low southwest of Kodiak and looking to take a track across
the far southern Gulf and then into the eastern Gulf,
Southcentral is under some ridging today which has helped clear
out skies from Kodiak through Cook Inlet and to Prince William
Sound. These clear skies have dropped temperatures into the 30s
in many locations that have not been that cold so far this
season. However, all is not clear skies mostly due to the upper
level pattern. The 500 mb trough associated with that strong low
is near the AlCan border early this morning, and while it is
moving eastward, another short wave can be seen on satellite
moving through the northern Susitna Valley and into the Copper
River Basin. While this short wave should mostly bring just some
cloud cover to these areas today, a stronger wave will move into
the Susitna Valley and to the Copper River Basin tonight into
Friday. This wave will tap into some low level moisture from the
Gulf and bring in some rain and snow showers tonight into Friday
night for the Talkeetna Mountains southeastward into Prince
William Sound as the Copper River Basin. This is where most of the
forecast uncertainty lies for tonight into Saturday.
Precipitation type is beginning to become more of a question this
time of year and higher elevations along the highways, such as
near Paxson and near Eureka may see the rain mixing with or
turning to snow during the overnight hours. Any accumulations are
expected to be light at this time, but it does bear watching. The
Prince William SOund region will be all rain.

How far east this wave dig will also make a big difference for
whether Anchorage and the Matanuska Valley will showers along the
mountains for Friday or if it will clear out Friday night into
Saturday morning and bring below freezing temperatures to east
Anchorage and colder areas such as Willow and the Butte. The
western Kenai Peninsula should be mostly clear Friday night so
Kenai, Soldotna, and other colder areas will likely get below
freezing Friday night into Saturday morning.

&&


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

Generally colder and quieter weather is on tap for most of the
Bering Sea and Southwest Alaska as a ridge builds in over the
Bering Sea. While dry weather and 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
the Kuskokwim Valley Friday through this weekend. This should be
very minor and not widespread.

Farther west, a front accompanying a Kamchatka low is lingering
over the western Aleutians and western Bering Sea. Progression of
the front is being suppressed by the ridge over the Bering Sea,
keeping rainfall likely over the western Aleutians until Friday
morning. South of the ridge along the Aleutian Chain, a surface
low is moving eastward. Chances for rain showers could skirt the
eastern Aleutians this morning, but models have continued to trend
the low more south so little precipitation is expected. With the
ridge in place and its strength dictating the synoptic flow, a
series of troughs moving toward the Aleutians will be diverted to
along this Aleutian track. The next low approaching the western
Aleutians arrives tonight and deflects eastward, spreading
precipitation to the central Aleutians before quickly dying off in
the southern Bering Sea for Friday night. Yet another, stronger
low tracks behind the previous for Saturday morning. Precipitation
is likely for Saturday across much of the Aleutians. Models are
in reasonably good agreement for the low center to track east
along the Chain, reaching the Alaska Peninsula for Sunday and into
the Gulf of Alaska by Monday. The Pribilof Islands should remain
mostly dry until Saturday night when chances increase to around 30
to 40 percent. Precipitation on Sunday will depend on how quickly
the former low dies off in the southern Bering and then also on
the low center position of the weekend system shifting slightly
north or south. Winds are expected to be stronger with this
weekends low. Small craft winds increase to gales over the weekend
near the eastern Aleutians and Alaska Peninsula. Storm force gusts
out of the north and northeast are possible near Unalaska Sunday
morning.

rux

&&

.LONG TERM FORECAST (Days 4 through 7: Sunday through Wednesday)...

The upper level Alaska Weather map shows a mildly cyclonic flow
with a number of perturbations from Eastern Asia into Canada.
These support surface lows and fronts that will sweep over the
Aleutians and the Alaska Peninsula. These systems will become more
or less stationary and weaken across the Northern Gulf of Alaska
through the forecast period. A ridge over the Bering through
Monday flattens out through midweek. The ECMWF is running more
energetically with greater amplitude, so the GFS/Canadian models
are preferred. Locally heavy rains spread from the Central
Aleutians, over the AKPEN and Kodiak Island and across the Gulf
through Monday. Locally heavy rains occur over the Southcentral
coast, with lesser amounts pushing inland. The associated surface
also brings near gale force winds with gale force gusts over the
Southern Bering, diminishing Monday. Widespread gale force winds
extend along the Pacific side of the Aleutians and AKPEN. The
surface low continues into the Gulf just East of Kodiak Island,
winds intensify to high end gales and storm force gusts. Worst
conditions are expected through the Barren Island, lower Cook
Inlet and Shelikof Strait Monday, diminishing Tuesday. A gale
force barrier jet forms along the North Gulf Coast for Monday,
diminishing late.

- Kutz

&&

.AVIATION...

PANC...There is high confidence VFR conditions prevail to the
weekend. There is a small chance for low-lying morning fog early
this morning, but light winds mixing the boundary layer are making
it more unlikely.

&&


$$

711
FXAK69 PAFG 190744
AFDAFG

Northern Alaska Forecast Discussion
National Weather Service Fairbanks AK
1144 PM AKDT Wed Sep 18 2024

.SYNOPSIS...
Wet and cool over the Interior through today with continued rain
showers. An arctic trough digs into the Brooks Range and Arctic on
Friday, bringing widespread accumulating snowfall to the Brooks
Range and Arctic Plain. That same arctic trough will dig across
the Interior and also bring chances for mixed rain and snow to
parts of the E Interior from the White Mtns down to the Alaska
Range.

&&

.DISCUSSION...

Upper Levels and Analysis...
The Pacific storm track is now well south of the Mainland. A
weakening low is now moving into the Panhandle with a broad trough
moving into the West Coast and Arctic. Weak ripples of energy are
moving across the Interior, bringing rain showers to the S
Interior and Alaska Range.

Forecast and Model Discussion...
Models are tightly clustered as a trough forms across the Mainland
over the next three days. Rain showers will gradually shift east
and then colder air will move across the Brooks Range into the
Interior, lowering snow levels down to 1000 feet by this weekend.
A good early season accumulating snowfall is expected for the
Brooks Range and Arctic Plain with several inches expected Friday
and Saturday. Snow accumulations, albeit low, are expected over
the White Mtns and into the Yukon-Tanana Uplands. The Alaska Range
will also see light snow accumulations. A blend to the GFS, NAM
Nest, and NBM was favored, although there were minimal changes to
forecast thinking.

Central and Eastern Interior...
Light rain showers will continue through today which will keep it
wetter and cooler from the White Mtns southward. Light snow
accumulations are expected in the Alaska Range with snow levels
dropping to 2000 feet by tonight, albeit light snow, enough to
possible coat the ground. An arctic trough digs southward Friday
and Saturday, and will keep precip chances in the forecast from
the White Mtns to the Yukon-Tanana Uplands, and down over the
Alaska Range. The big question mark will be the Tanana Valley,
where rain is possible, but snow levels could drop below 1000 feet
Saturday. Right now, little to no lower elevation snow is
expected, but it will be getting cooler as the trough digs south.
The Hills around Fairbanks could see some white ground by Sunday
morning.

West Coast and Western Interior...
Light onshore flow will keep periodic light rain showers over the
region, with mixed snow showers over the NW Arctic and W Brooks
Range, through Friday. After that, north winds behind the arctic
trough will dry things out and high pressure builds in for the
weekend with offshore flow.

North Slope and Brooks Range...
Light northwest winds will accompany an arctic trough moving into
the region tonight, and moving east through the weekend. This
will bring accumulating snow to the Brooks Range and Arctic
Plain, with several inches of accumulating snowfall expected.
Right now, 48+ hour totals of 6+ inches are likely from Umiat
east to the Dalton Hwy corridor south of Prudhoe Bay. High
pressure over the W Arctic will keep that side of the region on
the drier side after Sat.

Extended Forecast Days 4-7...
Sunday and beyond, arctic troughing will be in place over the
Interior with a strong low moving into the Gulf of AK.
Eventually warmer air will move back into the Gulf and will try
and move north. This will likely keep rain/snow showers possible
over the Mainland, especially the eastern half of the Interior.
The W Interior looks drier with north winds, while the arctic air
persists from the Brooks Range north.

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

&&

.HYDROLOGY...
No concerns.

&&

.AFG WATCHES/WARNINGS/ADVISORIES...
AK...None.
PK...None.
&&

$$

Ahsenmacher

209
FXAK67 PAJK 191314
AFDAJK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
514 AM AKDT Thu Sep 19 2024

.SHORT TERM...Post frontal showers will continue through mainly
Thursday morning before clearing north to south through the day.
Looking at satellite, the low responsible for generating much of
yesterday`s showers has largely disintegrated. While numerous to
widespread showers are still present, expect these to diminish
quite rapidly over the course of the day. The deepening trough
upstream from the incoming low on Friday, discussed in the long
term section, will promote upper level ridging near the NE gulf
coast. From this ridging, mid level drying and clearing will
spread from north to south down the panhandle as the ridge
strengthens.

Therefore, main changes to the forecast included decreasing PoP
across the CWA. Furthermore, due to lack of forcing, reduced winds
across the inner channels tonight to 10 knots or less. Finally,
introduced widespread fog across the panhandle.

.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...Showers are the rule of the morning for just about the
entire panhandle this morning. So far VFR or MVFR conditions
prevail though some brief periods of IFR in heavier showers are
being observed. The lowest conditions are along the outer coast
and up against the coast mountain range. Winds have quieted down
overnight with the Skagway area still seeing some gusts to 30 kt.
Improving conditions expected today as showers diminish and exit
the area from NW to SE. Expect mostly VFR conditions by this
evening and into tonight though some areas could see fog develop
late with the accompanying IFR vis and ceilings. Conditions
expected to deteriorate again Friday as the next system begins to
move into the area.

&&

.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...None.
MARINE...Small Craft Advisory for PKZ012-022-053-641>644-651-652-661>664-
671-672.

&&

$$

SHORT TERM...NC
LONG TERM....AP
AVIATION...EAL
HYDROLOGY...AP

Visit us at http://www.weather.gov/Juneau