Heavy Snow in Alaska, December 11 to 12, 2022

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
608 AM AKST Sun Dec 11 2022

.SHORT TERM...Winter weather is once more the big story as the
next system approaches the panhandle, and accumulating snow
is expected through much of the area. Aloft, a ridge moving
across the gulf and over the panhandle as of the time of writing
is being displaced by a narrow trough advancing E out of the
Aleutians. The southernmost quadrant of this trough will continue
to elongate, ultimately becoming a cut off low in the Northern
Pacific. The northern flank of this trough however, will provide
the necessary upper level divergence to help fuel the development
of a low immediately S of the Alaska Peninsula which will
intensify as it races NE into the Gulf. This low will see a strong
warm front rotates around its E flank, advancing into the
panhandle Sunday night into Monday, before the trailing cold
frontal feature catches up and moves through the area late on
Monday.

The warm front's arrival into the panhandle will initiate a broad
overrunning event, especially across the N half of SE AK, as
warmer air and integrated moisture transport progged to be
approaching 100 gm/kgs in some areas begins to push into the
area. Given the N outflow event that previously took place
earlier in the weekend, the requisite cold air will be entrenched
in many locations to enable the initiation of efficient snowfall
processes. A swath of strong 700- 500 mb omega alongside strong
equiv pot temp adv in the same level - especially along outer
coastal areas, are indicative of the potential for significant
snowfall accumulations in places such as Yakutat, where a Winter
Storm Warning has already been issued.

Further inland, calculating exact snow totals will remain a bit more
challenging, though expect many areas in the N half of the
panhandle could see snowfall totals exceeding 6 inches,
significantly so in some locations. Near the surface, winds will
attempt to pivot to the S as WAA moves to push out the cold air,
but from 850mb to 500 mb, operational guidance from the GFS is
indicative of a SW flow pattern. Such a flow pattern would
potentially indicate the possibility that parts of the Haines and
Klondike Highways could be shielded from the absolute worst of any
potential snowfall accumulations - though still expect reasonable
snowfall in these areas - particularly the Haines Highway.
Conversely, this means that winds in downtown Haines and towards
the Chilkat Peninsula (and to a lesser extent, downtown Skagway),
could be more inclined to remain out of the N for a longer period
of time. Such a setup would favor significantly more snowfall for
these areas in the immediate vicinity of Lynn Canal, and it is
entirely possible that places like Mud Bay (in the Chilkat
Peninsula) could yield surprisingly high snowfall amounts.

Along the Icy Strait Corridor, anticipate that snow will continue
through much of Monday morning before WAA results in a change over
from snow to rain through Monday afternoon and evening. Some areas -
such as the Mendenhall Valley (Juneau), and various parts of the
broader Gustavus area and Hoonah areas may see snow continue for a
slightly longer time as localized topographical influences
contribute to keeping sufficient cold air in place to prevent a
changeover in precipitation. Ultimately however, the surge of WAA
will prove to be brief, as the increasing snow levels and rising
temperatures are sent tumbling back down by the arrival of the cold
front Monday night through Tuesday, which will also see
precipitation convert from stratiform to convective given
increasing lapse rates in the low-to-mid levels. For more
information on this feature, please refer to the long term
forecast discussion.

Expect elevated winds across many locations near sea level,
especially in the N half of the panhandle and along the outer gulf
coast, with strong winds possible in some locations, especially
Monday through Monday night.

Lingering forecast challenges will be how far S the accumulating
snow extends past the Icy Strait Corridor as well as the exact
timing of the anticipated changeover from snow to rain. Sitka in
particular could yield higher snowfalls than model data would
indicate - particularly in areas that aren't in the immediate
vicinity of Sitka airport. Think that a period where some freezing
rain could be a concern Monday morning may not be out of the
question, but this remains low confidence at the time. As the
forecast approaches Tuesday, exact temperatures start becoming
harder to assess, though think that many areas will reach the mid
30s at least given the previous WAA and the relative weakness of
the cold front. Forecaster confidence is above average in the
chances of measurable snow, though exact snow amounts are not yet
set in stone.

.LONG TERM...A weather front will move into SE AK as the parent low
reaches the north central gulf. A wave will try to develop along the
front as it drifts into the eastern gulf. With that said,
precipitation will continue to fall across the region with most of
this precipitation falling as snow across the central and northern
panhandle while coastal areas and the southern panhandle will see
more rain or a rain snow mix during the day on Monday. Went ahead
and issued a winter storm watch for the Icy Strait corridor and N
Lynn Canal locations for the potential of accumulating snow
throughout the day on Monday of 6 to 12 inches. Precipitation type
will switch over to rain as a wave rotates around the parent low for
most locations through late Monday afternoon as winds mix down to
scour out any cold air that was left near the surface. Some
locations will change later that will affect snow accumulations.
Winds during this time will strengthen as they are able to mix
throughout the area Monday evening with south marine winds
increasing along with land wind gusts. Winds offshore will be Gales
throughout the day Monday before weakening behind the front. The
exception with regards to precipitation will be locations that do
not see the winds mix into the area which will stay as a mix or
straight snow during this time. As the front moves through,
precipitation will become more showery during the day on Tuesday.
Expect to see a switch back to mix or straight snow throughout the
day Tuesday across the Panhandle with the exception of the far
Southern Panhandle which will remain as rain.

Throughout the week, the pattern looks to remain unsettled with
showers remaining across the region. An exception to this right now
looks to be the Yakutat region as precipitation looks to be more
uniform for this area and less showery in nature. Towards the end of
the week, the pattern changes with most places drying out. Current
CPC guidance points towards a cooler and drier pattern for the next
six to ten days for SE AK.

&&

.AVIATION...Clear/VFR conditions and decreasing northerly outflow
winds will give way to an approaching front over the next 24hrs.
PAYA will see gradually lowering ceilings through the daylight
hours and snow beginning around the 00z this afternoon. LLWS is
likely as surface flow will remain NE-E while winds aloft increase
out of the SE with the front. As snow begins, conditions will
quickly lower to MVFR then VIS falling to IFR. Similar trends
expected as the front/snow moves into the northern TAF sites
closer to 6z. Freezing levels rise along the outer coast Monday
morning with transition to mix/rain. Some models indicating
potential for brief freezing rain/mix at all TAF sites except PAHN
and PAGY, but lower confidence in that and will depend on timing
of surface wind shift to the S-SE.

&&

.MARINE...Strong N outflow winds across the inner channels and
various outlets along the outer coast will give way to
strengthening S and SE flow by the end of Sunday. Gale force
winds will arrive along the NE Gulf coast through the day on
Sunday, before extending along much of the the outer coast through
Sunday night. Storm force winds are anticipated west of Yakutat
along the outer gulf coast. Strengthening S flow in the inner
channels will ultimately see small craft winds in many locations
by Monday, before winds begin flipping back out of the N through
Tuesday in the wake of the cold front's passage.

&&

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Storm Warning from 3 PM this afternoon to 6 AM AKST
     Monday for AKZ317.
     Winter Storm Watch from this evening through Monday evening for
     AKZ318>322-325.
MARINE...Storm Warning for PKZ052.
     Gale Warning for PKZ012-042-043-051.
     Small Craft Advisory for PKZ013-021-022-031-033-041-053.

&&

SHORT TERM...GFS
LONG TERM....ABJ/SEF
AVIATION...Ferrin

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