104
FXAK67 PAJK 172224
AFDAJK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
224 PM AKDT Tue Sep 17 2024

.SHORT TERM.../through Wednesday night/...
Quick notes:
-The brief sunny and dry weather comes to an end tonight.
-Passing front tonight will give SE AK passing light to moderate
 rain.
-Wind speeds will increase as the front approaches. Strongest wind
 speeds will be in the gulf and in Lynn Canal.

Details:
500mb and surface ridge will break down and slide southeast
tonight as a front approaches from the west. This front, and the
associated parent low, will be our weather maker for the next
couple of days.

The parent low will slide along the coast towards Anchorage then
move eastward into Canada. As it does, the initial front will move
in tonight, giving the area rain and increasing the winds.
Rainfall amounts aren`t all that impressive for this time of year,
with forecasted 24 hour amounts around 0.50 to 1 inch. 90th
percentiles in the EURO and GFS ensembles are closer to 1.5 inches
over 24 hours, but amounts like that are still within normal for
this time of year and have a 10%, or less, chance of happening.

The wind speeds will increase from the south beginning tonight as
well. Land wind speeds will be strongest in Skagway where gusts
could be near 40 to 45 mph late tonight into tomorrow. The rest of
the panhandle would see gusts upwards of 30 mph.

Marine winds will increase to around 20 to 30 knots with the
strongest winds in the gulf and in Lynn Canal. That being said,
Stephen`s Passage and Chatham Strait could get upwards of 15 to
25 knots. More details in the Marine section.


.LONG TERM...The long-term forecast is characterized by a
plethora of storms moving across the Gulf into the Panhandle
bringing wind and rain concerns from Thursday into the weekend.
Touching on rain, an atmospheric river will impact Queen Charlotte
Sound, with the southern Panhandle on the northern edge of the
higher moisture, potentially limiting more significant impacts.
For mariners operating in our area, especially in the inner
channels, expect changes on wind speeds in some inner channels as
guidance shows better agreement through the week.

More details:
At 500mb, a longwave trough will eject into Canada Thursday, with
the accompanying surface low in the Gulf beginning to weaken as
it loses support aloft, allowing surface winds speeds and rainfall
to diminish. Thursday afternoon into early Friday are now looking
like the only window for a break past the weekend. Early Friday
ensembles highlight another system moving into the southeast Gulf,
deterministic runs show decent divergence from the mean, with the
GFS and ECMWF trending towards gale force southerly winds for
Clarence. Furthermore, the trends of both deterministic are moving
the low further north with a drop in surface pressure and agree
in location. For now, the forecast reflects near-gale force
southerly winds impacting the southern coast, Dixon, and Clarence
Strait by Friday, with elevated easterly outflow winds shooting
east out of Cross Sound, and north winds in Lynn Canal. For now,
the highest winds Friday to Saturday look to impact the central
and southern Panhandle. By late Sunday or Monday, a broad low is
expected in the Gulf.

Switching to rainfall, the jet will begin to push a moderate atmospheric
river into Haida Gwaii and Queen Charlotte Sound with the
southern Panhandle on the northern edge of the AR. Expect rainfall
through the weekend into next week, resulting in about 1 to 2
inches per day, heaviest rainfall in the southern Panhandle. With
that said long range guidance is struggling with the location,
magnitude, and extent of the atmospheric river which could swing
more rainfall than anticipated further north.


&&

.AVIATION...Beautiful flying weather across nearly all of
Southeast Alaska will deteriorate overnight as the next front
approaches. Still, widespread conditions below 1000/3 associated
with the front not expected. Highest probability for instrument
conditions will be vicinity Gustavus and Haines mid to late
morning. Conditions getting down close to minimums along the
northeast gulf coast during the overnight hours as well. LLWS
will become an issue overnight, spreading from west to east
beginning along the northeast gulf coast overnight and into the
central Panhandle tomorrow morning. Worst conditions will be in
the post frontal environment but will remain offshore Wednesday
night.

&&

.MARINE...Wind speeds largely remain on the lower side until
tonight across most of the area. By late tonight, the next front
will move in from the west, causing wind speeds to increase. 20 to
30 knots in the gulf with 10 to 20 knots in the inner channels.
The exception for the inner channels is Lynn Canal where wind
speeds will increase to 20 to 30 kts by late Tuesday night with
elevated winds lasting into Wednesday night.

Seas will get up to 3 to 5 feet in the inner channels with gulf
seas getting up to 10 to 15 feet for late tonight into Wednesday.

&&

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Strong Wind late tonight for AKZ318.
MARINE...Small Craft Advisory for PKZ012-013-641-661-662-671-672.

&&

$$

SHORT TERM...GJS
LONG TERM....AP
AVIATION...Fritsch
MARINE...GJS/GFS

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