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FXAK67 PAJK 211426
AFDAJK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
526 AM AKST Wed Feb 21 2018

.SHORT TERM...The short term forecast will bring a return of
considerable snow to portions of Southeast Alaska. This morning
begins with a weak low pressure system tracking southeast over the
southern Panhandle and Haida Gwaii. At this time all that is left
impact wise from the system are some lower broken layers of
clouds between about 1500 and 3000 ft over the far southern
Panhandle. Winds over the inner channels will follow a weak
northerly outflow pattern this morning.

Thursday is the primary forecast concern of the short term. A
weather front will track east across the gulf bringing
considerable snow into portions of the Panhandle. Initial onset
of precipitation will be early Thursday morning near Yakutat and
late morning for the northern Panhandle. Precipitation will
quickly spread southeast through the afternoon. The rain snow line
will be the biggest factor in snowfall totals with this system.
Currently, the forecast carries a rain snow line that includes a
transition to all rain south of Pelican and extending southeast
to Ketchikan. In addition to the snow, a return of gale force
winds over the gulf and within Cross Sound and Lynn Canal will be
associated with the front.

On Friday, the remnants of the system will transition into a
showery regime which could cause localized additional snow
accumulations where temperatures are still cool enough to support
snow.

A winter weather watch is now in effect between Thursday morning
and Thursday night for the Juneau, Petersburg, and Wrangell areas.
Possible snow totals are between 3 and 9 inches for these areas.
The current forecast does include snow totals between late Thursday
afternoon through Thursday night that will warrant warnings for
Juneau and Petersburg if confidence remains high in the current
snow total forecast. Near Juneau expect highest accumulations
within Mendenhall Valley and at higher elevations.

A large sum of factors went into adjustments to the snow forecast
for Thursday. The low level structure of the atmosphere
including: temperatures at the surface, 925 mb, & 850 mb,
thickness values between the surface and 850 mb with 1280m and
less being the the primary cutoff for all snow, and low level wind
profiles were all used in determination of the rain snow line
over the Panhandle. The rain snow line was pushed a bit further
east than previously forecast.

Models of choice in the short term were the Canadian, NAM, and
GFS for different elements of the forecast. Models as a whole are
in good agreement with the timing of the Thursday system. The
Canadian`s lower snow ratios were favored as the lowest of all
models due to the expected wet nature of the snow. PoPs were left
very similar to the inherited forecast and continued agree well
with the 06Z runs of the NAM and GFS. The 06Z NAM was the primary
model used to determine the rain snow line over the Panhandle
Thursday. Overall forecast confidence is above average on timing
with average confidence on snowfall totals.

.LONG TERM...Friday through Wednesday...As of 9pm Tuesday, we
appear to be heading into a slightly more active and progressive
pattern for the long term. The upper level ridge that will have
been a constant feature up until this point will retreat further
into western Canada and weaken as an upper level trough
strengthens over the panhandle on Friday and takes its place.
According to potential temperature analysis, a weak surface low
will develop along the northeast gulf coast using energy from
this upper level feature and slowly move southeast along the SE
Alaska coastline. This should keep some lingering convective
precipitation going across the panhandle, as forecast soundings
indicate steep lapse rates and plenty of cold air advection as the
low pushes through. An upper level ridge will then fill in behind
this feature Friday night before another shortwave develops out of
mainland Alaska on Saturday and impacts the panhandle Saturday
night into Sunday. Deterministic models have come into better
agreement regarding this developing shortwave, but we were
cautious about really going that route with the forecast, as there
is still plenty of uncertainty regarding the position, strength,
and timing of this low. This will be something that bears
watching as we receive more data.

Generally, we opted not to make very many substantial changes to
the long term forecast due to the uncertainty regarding
deterministic model solutions. We predominantly worked with the
12Z Friday to 12Z Saturday period using a combination of the GFS
and NAM. We also added in NBM for changes to PoP and to decrease
temperatures in the later periods to be in line with model
spreads, and we used GEM for snow ratios and subsequent snow
amounts.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...Winter Storm Watch from Thursday morning through late Thursday
     night for AKZ025-026.
MARINE...Small Craft Advisory for PKZ041-042.

&&

$$

JB/Voveris

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