Small Craft Advisory 811, NWS Juneau, AK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
337 PM AKST Sat Dec 24 2016

.SHORT TERM...Things continue to not add up for snow across much
of Southeast for Christmas Day. While guidance still appears cool,
winds from the southeast are a sign of warm air advection always
for our region. Therefore, we continue to serve two masters: trying
to honor the guidance, by taking the warmest of them all, the
Canadian, regarding snow levels and also continuing to adjust the
rain-snow line quicker inland and northward just from experience.
At this point, Ketchikan may see a few wet flakes at precipitation
onset, but with such a well-mixed layer and winds arriving from
Hecate Strait we see a transition to rain pretty quickly by
evening. Given these winds, we could see an earlier transition to
rainfall across much of the southern and south-central Panhandle
by tomorrow afternoon. We have snowfall down to 1 inch for all
areas where snow starts. But for the coast and Ketchikan, we think
accumulations will be far less than 1 inch, if at all. Finally,
models generally are too wet with such a northwest/southeast
frontal orientation, and may at times under-account for any
drying in the boundary layer. In other words, much of this model
suggested precipitation that falls through the boundary layer will
not only cool the atmosphere, but will also evaporate, thus
leading to lighter amounts.

Confidence is falling in lower temperatures tonight with cirrus
shield topping over the ridge. We see signs that the air mass is
already moderating, and this does not bode well for snow-worthy
web-bulb temperatures persisting into Sunday.

Strong winds in the forecast were kept for the coast and the
southern areas. Gale to storm force winds of 40 to 50 kt look
reasonable for the outside gulf waters with the front's
progression toward the coast. Likewise gales for the southern
Inner Channels and small craft southerlies for the central Inner
Channels look reasonable for Sunday as well. To the north, we
have relaxed winds given northerly, but near parallel gradient.
Any angling of the gradient to allow southerlies through the
northern and north central Panhandle will greatly impact the
current forecast towards rain and kibosh the snow.

It must be stressed that our confidence for Sunday is low, and
small things could change to cause great havoc on our delicate
forecast.

.LONG TERM...As mentioned above, very tough and low confidence
forecast for Christmas night into Monday. However, there are some
significant trends emerging in guidance this afternoon. Model
guidance the past few days has been hinting at the development of
a surface wave crossing the southern panhandle Monday. This is
becoming more of a certainty as models have come into good
agreement on this feature. There does remain differences in the
exact track and strength of this feature, but its development will
have implications for areas to the north. Strong easterly flow
with the front orientation combined with the increased easterly
flow as the wave passes to the south has led to a lowering of
snowfall amounts for the central and northern panhandle. The 18z
GFS confirmed this thinking as it came in noticeable drier with
QPF amounts over the northern panhandle. Generally 2-6 inches of
snow is expected in a 36 hour period for these locations. With
energy focused to the south and cold air locked into place, the
Hyder area is expected to see significant snowfall from Sunday
evening through Monday evening. A Winter Storm Watch has been
issued with around a foot of snow possible. The other concern is
exact track of the wave on Monday as areas immediately to the
north will be in a zone of very strong ascent and weak low level
flow. If enough cold air is maintained in the low levels, a
period of moderate to heavy snow is likely. With the current track
suggested by model guidance, the Petersburg and Wrangell areas
would be most likely to be affected by this. Later shifts will
continue to fine tune.

Winds will be strongest across the southern Inner Channels with
a strong low level jet crossing Sunday evening. 40-50 mph gusts
are likely with 30-45 kt in the inside marine waters. Another
round of wind is expected on Monday as the wave crosses the area.
Winds will then quickly shift westerly as the low pushes inland.

The remainder of the extended range will feature showers gradually
diminishing through the week as flow shifts to the northwest.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...Strong Wind Sunday afternoon for AKZ026-027.
     Strong Wind from Sunday morning through Sunday afternoon for
     AKZ017-023.
     Strong Wind Sunday afternoon for AKZ028.
     Winter Storm Watch from Sunday evening through Monday evening
     for AKZ029.
MARINE...Storm Warning for PKZ043-051-052.
     Gale Warning for PKZ022-033-035-036-041-042.
     Small Craft Advisory for PKZ021-031-032-034-053.
&&

JWA/TPS

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