614
FXAK67 PAJK 211405
AFDAJK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
605 AM AKDT Sun Jul 21 2024

.SHORT TERM...Low stratus decks, a few showers, and fog across
parts of SE AK will give way through the morning to a more
elevated cloud deck and significant breaks, enabling daytime
heating to bring temperatures into the upper 60s and 70s for much
of the area. The improving conditions will abruptly reverse across
much of the northern panhandle late Sunday afternoon through
Sunday evening as a quick moving easterly wave dives out of
British Columbia before rapidly advancing N, bringing with it
chances of showers and even some thunderstorms to areas around the
Icy Strait Corridor and northward.

Satellite and radar imagery depict a broad area of low pressure
continuing to rotate over the central Gulf as of the time of
writing. This low, which has been responsible for the prevailing
flow pattern for the past few days, will begin to slowly move east
towards the panhandle through the day. As it moves E, synoptic
flow will increasingly turn offshore as the NE flank of the low`s
circulation moves over the area. This, alongside a wedge of drier
air moving over the area (evident in water vapor imagery), as well
as moisture being dragged away from the panhandle by an embedded
shortwave within the broader low situated over the Gulf, will
contribute to significant clearing taking place across much of SE
AK through the day. This clearing will be most pronounced during
the late morning through afternoon time frame.

The drier weather will be brought to an end during the late
afternoon and evening timeframe for the Icy Strait Corridor and
northward as an easterly wave moves into the area. Operational
guidance has come into broad agreement that the wave will survive
passage across the Coast Mountains, though where exactly it
emerges is still not yet entirely certain. It is possible that
the wave could bring showers, and potentially a few thunderstorms,
as far south as Juneau and the Icy Strait Corridor, though the
best chances for more significant convection will be further
north, in the vicinity of Haines and Skagway. The most favorable
vorticity will move through the far northern panhandle, along with
by far the best CAPE Values (daytime heating will contribute to
CAPE reaching potentially in excess of 700 J/KG), and so the
potential for thunderstorm development will need to be closely
watched.

.LONG TERM...The long range forecast is broken up into two
distinct periods, Monday, marked additional cloudiness working
its way northward with lighter winds, and Tuesday, with a potent
easterly wave bringing widespread showers.

Monday continues to be the calmer day, with increased cloudiness
and a modest increase in winds due to clearing skies and solar
radiation in British Columbia. The interesting situation is on
Tuesday, where a rather potent backdoor warm front from a low in
British Columbia moves over SE AK from the northeast. A somewhat
unusual situation where a major moisture source from continental
air is overrunning entrenched drier, cooler air to the south in SE
AK. This will lead to light to moderate rainfall for
approximately 24 hours. NE flow aloft moving over the Coast
Mountains could causing drying from downsloping winds. In
addition, dry air from the south will be drawn in via pressure
gradient flow at the surface. Therefore, actual rainfall is far
less confident than any one model would imply, being anywhere
between 0.50 - 2.00 inches. The EFI tables have tended to assert
caution, with a majority of the shift of tails over the eastern,
mountainous portion of SE AK and much less model consensus over
even the far inner channels. Similarly, NBM confidence remains
low, with the highest six hour period of rain being anywhere
between 0.25 - 0.75 inches of rainfall.

&&

.AVIATION...Low cloud deck along the outer coast will persist
Sunday morning with ceilings at times falling below 1000 ft before
rising through the day. Inside waters will see continued ceilings
of 3000+ ft through the daytime hours and overall VFR conditions
after some patchy fog dissipates. Only real concern is the
potential for thunderstorms moving across the far northern inner
channels from an easterly wave that is set to blow through
starting late Sunday afternoon. These storms could potentially
feature moderate rain to reduce visibilities quickly, along with
erratic gusty winds. The highest likelihood for these storms to
develop will be late Sunday afternoon and through the evening. No
significant wind concerns aside from localized effects near
thunderstorms as mentioned above.


&&

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...None.

&&

$$

SHORT TERM...GFS
LONG TERM....NC
AVIATION...STJ

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