Storm Warning 26, NWS Juneau, AK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
229 PM AKDT Mon Sep 21 2020

.SHORT TERM...Showers have been numerous today along the outer
coast and more scattered further inland as the showers get
shadowed by the mountains as they move across. Still think that a
more organized band could move inland to the Juneau area and even
up to Haines/Skagway this evening. Winds have been primarily out
of the E- SE around 10-15mph, but with some higher gusts out of
channels open to the east, like at Scull Island.

The story of the week is a very large low pressure system, currently
south of the Aleutians, with a huge occluded front wrapping around
it and extending to the central gulf. The front will progress
eastward tonight, with gale force winds of 35-45kt over the offshore
and outside waters by Tuesday morning. But rain and and the wind
looks to only just tag the outer coast land areas on Tuesday. This
is where the forecast starts to change. Models have come into better
agreement on the main low center taking a more southern track, but
with the occlusion remaining extended outward along the periphery of
the gulf. This should keep the front from making very far inland, so
have lowered POP for the inner channels and kept it highest for
Prince of Wales, Baranof Island and up to Yakutat.

Then on Tuesday night a new low center/trough forms along the front
off of Dixon Entrance. This will cause the front to back off the
coast and become oriented more E-W. The new, small low then rotates
about the parent low, through the gulf, taking a band of showers
with it through Wednesday morning. Some models give more weight to
the new low, making it the dominant one, but generally this pattern
of small lows rotating about the larger circulation (Fujiwara
effect) continues into Thursday. Expect multiple bands of showers
over the next few days with the highest POP over the southern
panhandle and lower POP for the northern inner channels. As each
band moves through, winds will likely become gusty for a period, or
even vary in wind direction. The further southern track of the
parent low will cause winds to shift out of the N-NE for most of the
northern and central inner channels.

Forecast confidence is below average for timing of specific bands
of showers.

.LONG TERM.../Wednesday through Monday/ As of Sunday evening. In
the upper levels, a broad area of troughing will dominate the
Gulf, with vorticity evident in the mid-levels. Closer to the
surface, a large corresponding low will move into the Gulf, the
remains of a strong autumn front lie along the panhandle and
embedded shortwaves moving towards the panhandle on Wednesday. The
front is spreading in more from the south than the west so the
better rains and winds are over the south, while a north or
northeast flow will be found over the northern panhandle. This
trend has continued so lowered pops for Wednesday to early
Thursday across the Lynn Canal area.

Beyond this event, forecast confidence degrades as it becomes
difficult to determine just how long the main low will remain in
the Gulf. The general trend is to keep most of the panhandle under
generally rainy conditions through the rest of the week. Some of
the ensembles are hinting at the possibility of another system
over the weekend, but still too far out to be very confident. Only
minor changes made to temperatures. Forecaster confidence is
average.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...Strong Wind from Tuesday morning through late Tuesday night for
     AKZ027.
     Strong Wind from Tuesday morning through Tuesday evening for
     AKZ023.
MARINE...Gale Warning for PKZ041>043-051-052.
     Small Craft Advisory for PKZ021-022-031>036-053.

&&

Ferrin/Bezenek

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