NWS Juneau, AK wrote its last area forecast discussion before the first event at Jan 21, 6:42 AM AKST, 8 minutes earlier.
Area forecast discussion, Jan 21, 6:42 AM AKST, 8 minutes before
Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
642 AM AKST Sun Jan 21 2024
.SHORT TERM...The main story through the next few days is the return of
accumulating snow to much of the panhandle from Sunday onwards as a
more active weather pattern resumes across SE AK.
After an extended period of time under a northerly outflow event,
the upper level pattern has begun to shift back towards a more
active storm cycle. Aloft, an area of high pressure over the Bering
Sea has been working in unison with a vertically integrated area of
low pressure SE of the Aleutians to contribute to the strengthening
of a trough situated along a NE to SW axis across the interior of
the state, and extending into the Yukon. The strengthening trough
has opened the gate for a system to move up from the S, into the
panhandle (and into a more favorable area of upper level divergence).
Although the system itself is not necessarily that impressive
insofar as its anticipated isobaric strength, its fairly close
position relative to the panhandle, along with the amount of
moisture associated with it, will contribute to widespread snow
across the panhandle. Warm air advection has already begun to move
into the area as of the time of writing, and an overrunning event
has already begun across parts of the area. For the southern
panhandle, temperatures - already largely above freezing - will
largely stay warm enough to keep rain as the predominant form of
precipitation, barring the morning hours of Sunday, where some snow
will mix in across various locations.
The central panhandle will prove to be a different story.
Sufficient cold air has become entrenched, due to the extended
duration outflow event, to prevent a quick changeover from rain to
snow. Winter Weather Advisories have been issued for Angoon
(Admiralty Island) as well as Kake (Kuiu Island). Given the
relative weakness of the warm air advection being driven northward
and the relative strength of the outflow event which preceded it,
determining where exactly the rain snow line will set up has
proven to be challenging. At this time, believe that the warm air
advection will have significant difficulty moving into Northern
Chatham Strait, past Angoon and Kake, not successfully causing a
changeover to rain until late Sunday night or Monday morning.
Snowfall rates will increase past noon on Sunday for these
locations, with the greatest rates Sunday evening and Sunday night.
Uncertainty continues to exist with regards to Petersburg and the
Mitkof Highway, where after a period of accumulating snow a
changeover to rain is anticipated during the afternoon or early
evening hours on Sunday across most of Kupreanof Island. However,
there is a second, less likely but still conceivable option,
which would see temperatures in Petersburg remain cool enough to
enable snow to accumulate through Sunday night. Should that be the
case, snow totals in Petersburg will likely be higher than are
currently forecast.
The greatest snowfall totals will be in the Icy Strait Corridor, and
Winter Weather Advisories have been issued for Pelican, Elfin Cove,
Gustavus, Hoonah, and Tenakee Springs. The relative weakness of the
WAA will enable a long duration overrunning event to occur in these
areas, with snow beginning during the morning hours, rates beginning
to pick up during the afternoon hours, and the heaviest rates
expected Sunday night through Monday morning. Have increased QPF
totals Sunday morning and afternoon as a wave has already begun to
move into the area as of the time of writing, and moved the
starting time of the advisories forward. Juneau will likely prove
to have the most favorable dynamics for this upcoming system, and
a Winter Storm Warning is in effect for Juneau from Sunday night
at 9 PM through Monday night at 9 PM, with 11 to 15 inches of snow
expected during the duration of the warning. Some accumulating
snow will begin before these times, and may extend past them,
should WAA prove insufficient to fully overcome the current
northerly outflow and cause a changeover to rain even as moisture
continues to enter the panhandle.
The overrunning event will have some difficulty successfully
reaching the far northern panhandle, but snow is expected to
eventually reach Skagway and Haines, albeit in lower amounts than
Juneau. The possibility does exist that snowfall, especially on
Monday, could prove to be significantly greater than is currently
progged for these locations, should the system fully advance past
the gale force outflow in Lynn Canal still present as of the time
of writing. Should this occur, respectable snow totals for
Haines, Skagway, and their respective Highways are likely, and the
situation will be closely monitored in the event that conditions
become more favorable for this outcome.
Snowfall will largely begin for many areas - especially those away
from the outer coast - at a higher snow ratio. This will result in
an initial round of lighter snow, before heavier wet snow becomes
increasingly prevalent as WAA begins to bring temperatures in the
lower levels of the atmosphere closer to freezing. Differences in
snow ratios for various locations could result in snow totals
differing to some degree from the forecast - as high snow ratios and
otherwise equivalent QPF totals would likely see greater snowfall
totals, with the inverse also proving true.
Winds will be on a diminishing trend across the panhandle by Sunday
evening as northerly outflow steadily weakens, though gales are
still anticipated for many of the inner channels and some of the
outer coastal waters. By Monday morning, small craft conditions will
be confined primarily to Lynn Canal, the outer coastal waters, and
ocean entrances. Strong winds for Skagway and downtown Juneau are
anticipated through much of Sunday and Sunday evening
respectively.
.LONG TERM...The main story for the long term is that the warming
trend will continue for the middle to latter half of the week.
Confidence in this warm up remains high as temperatures at the
surface as well at 850mb warm up. The main reason driving this
warm up is the return of an active pattern as Euro/Canadian
ensembles continue to show areas of low pressure moving up from
the N pacific bringing warmer, wetter air back to the area. With
the warmer air, precipitation is expected to switch back to rain
for most of the panhandle with the exception of the Lynn Canal
area which will likely remain as a mix of rain and snow or snow.
This warm up will allow for high temperatures across the area to
return to above freezing for areas that have been stuck below
freezing since the cold air moved in. While the models continue to
trend towards a warmer pattern, there is still some uncertainty
with regards to details that will need to be watched as we get
closer towards the middle part of the week
Warmer, wetter weather remains in the cards into the late week
period as southerly flow continues. Current trends (from the
ensembles and the Euro) for next weekend are pointing toward a
rather active storm track into the panhandle from a southerly
direction. The first front of which may arrive on Fri night with a
few subsequent waves or fronts coming in behind it. Not much
consistency on timing of individual features at this time though.
&&
.AVIATION...The front is currently draped across the central and
southern Panhandle, bringing IFR to MVFR conditions. Expect
conditions to worsen through the day across the central areas,
lifting north into Icy Strait as snow intensity and coverage
expands, with moderate rain and IFR cigs continuing in the south.
LLWS concerns later this afternoon into the evening over the
coastal areas Sitka south.
&&
.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Strong Wind until 3 PM AKST this afternoon for AKZ318.
Winter Weather Advisory from 6 PM this evening to 3 PM AKST
Monday for AKZ320>322.
Winter Weather Advisory from noon today to 9 AM AKST Monday for
AKZ324-327.
Strong Wind through this evening for AKZ325.
Winter Storm Warning from 9 PM this evening to 9 PM AKST Monday
for AKZ325.
MARINE...Gale Warning for PKZ012-013-643-651-664.
Small Craft Advisory for PKZ021-022-031-032-641-642-644-652-
661>663-671-672.
&&
SHORT TERM...GFS
LONG TERM....SF
AVIATION...AP
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