Storm Warning 27, NWS Juneau, AK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
838 PM AKST Thu Dec 22 2022

.SHORT TERM...A front associated with a low pressure system in the
Gulf is making its way towards the Southern Panhandle this
evening. As it does so, snow chances steadily increase through
tonight. The forecast challenge for this event revolves around
offshore flow limiting support for snow during the onset of
precipitation. This will be on ongoing concern for all communities
as snow approaches. At this time, expecting dry air to erode away
at the incoming moisture which will hold back snow amounts at
first, but the moisture will eventually win out. Main changes to
the forecast included holding back on beginning of precipitation
as well as increasing winds through inner channels with the
outflow conditions.

By Friday morning, snow will just being making its way through the
Central Panhandle. Same story, dry air will keep any significant
accumulations at first but once the atmosphere saturates, heavier
snow will begin late Friday afternoon. Late Friday night, snow
changes to rain across the southern panhandle and the northern
and central panhandle remain well into the snow event. Snow was
increased across Yakutat during this time as heavy snow looks more
favorable.

An additional forecast challenge included timing of multiple bands
of heavier precip. This has made for a very complex forecast as it
can be very challenging nailing down exactly when these heavier
bands will move through the panhandle. It is also worth noting
that there will be a decent break and decreasing trend in snow
late Friday night and into Saturday morning. Models have been all
over the place trying to pinpoint where exactly the dry break
develops. For now, expecting the northern and parts of the central
panhandle to dry out Saturday morning before another round of
heavy snow arrives Saturday afternoon.

As for temperatures, not much change for today. Leaning towards
MOS for this forecast as models are suggesting colder
temperatures. 850mb temperatures do decrease through the period
but not thinking those will greatly influence locations at the
surface. Windy conditions will also keep the atmosphere mixed and
allow temperatures to be on the warmer side.

.LONG TERM.../Sunday through Tuesday/...The next Gale force frontal
system will move through Southeast Alaska from the south starting on
Sunday. It will bring a mix of precipitation and strong winds over
the Panhandle and eastern Gulf. This system looks to bring
additional appreciable snow accumulations primarily across the
northern half of the Panhandle and the Hyder area through the
period.

Warmer air will gradually advect through the Panhandle from south to
north, enabling a transition to either all rain or a wintry mix as
the pressure gradient angle changes to allow southerly flow to
infiltrate the Inner Channels. This is further backed up by model
forecast 850mb positive temperature advection over the area. The
thing that may hinder this over the Icy Strait Corridor, northward
is a NE to SW oriented pressure gradient from an area of high
pressure poised to the NE of the Panhandle, advecting colder air
over the area, which may act to slow down or block warm enough
air above freezing to enable widespread P-Type change over that
area. So, stay tuned.

For the southern half of the Panhandle, any significant rain falling
on top of significant snow will cause snow loading, which will
cause the snow to become very heavy and hard to manage.

As for winds, Small Craft magnitudes of up to around 30 knots are
possible across the Inner Channels as the front approaches and
pushes through the area. Over the eastern Gulf waters, Gales of
up to around 45 knots and enhanced seas are also possible
associated with the tightest pressure gradient near the front. At
this time, it looks as if winds will diminish for the latter half
of the period, and precipitation rates look to also greatly
diminish on Tuesday.

Forecast confidence is average for Sunday. As we move through the
rest of the timeframe, weather forecast model agreement
significantly diverges. Ensemble spread becomes wide, as well.
Therefore, for midday Monday, onward, only minor changes were
made, primarily blending toward WPC Guide.

Beyond Monday, ENS spread is very vague about what will happen.
500mb ENS mean heights are mostly onshore with times of troughing
and ridging - so no real help there besides "warmer and wetter."
850mb temps between the EURO and GEFS are very different. The EURO
wants to warm the entire panhandle up while the GEFS wants to
keep the northern Lynn Canal area cold. This difference is made
very clear by looking at the surface temp anomalies, the EURO
wants to warm the entire panhandle up to above normal while the
GEFS wants to keep Icy Strait and north below normal. Differences
like this would be the difference between rainy weather and
continued wintry mix. So at this point, confidence is low and
excitement is high, in the extended forecast.

&&

.AVIATION...Clouds will continue to lower throughout the evening from
south to north. Snow will increase across the south quickly this
evening leading to MVFR and eventually IFR conditions by late
Friday afternoon. Heavier bands of snow could lead to LIFR
conditions. Farther north, expect MVFR conditions to hold on
longer as snow takes longer to move into these areas.

&&

.MARINE...Gale force winds will continue through Lynn Canal, as
well as, gap terrain through the inside waters. There is also the
potential for some light freezing spray through N Lynn Canal.
Gale force winds continue through the Central Gulf before moving
north towards the NE Gulf Coast Friday. Along the Central and
Southern Gulf Coast, gales force winds will also remain steady as
a front pushes north. Seas will be building during this time
before decreasing from south to north during the day tomorrow.

&&

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Small Craft Advisory from midnight tonight to 6 AM AKST Friday
     for AKZ317.
     Winter Storm Warning from 6 PM Friday to 9 AM AKST Saturday for
     AKZ317.
     Wind Chill Advisory until 3 AM AKST Friday for AKZ318.
     Strong Wind until 3 AM AKST Friday for AKZ318.
     Winter Storm Watch from late Friday night through Saturday
     afternoon for AKZ318-319.
     Winter Storm Warning from 3 PM Friday to 3 PM AKST Saturday for
     AKZ320>322-324-325.
     Winter Weather Advisory from midnight tonight to 9 PM AKST
     Friday for AKZ323.
     Strong Wind through Friday afternoon for AKZ325.
     Winter Storm Warning from midnight tonight to 6 AM AKST Saturday
     for AKZ326>332.
MARINE...Gale Warning for PKZ012-013-041>043-051-052.
     Small Craft Advisory for PKZ021-022-031>036-053.

&&

SHORT TERM...CC
LONG TERM....JLC/GJS
AVIATION...SF
MARINE...SF/CC

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