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Southeast Alaska forecast discussion 
National Weather Service Juneau Alaska
331 PM akst Thursday Dec 25 2014

Short term...weak front is pushing into the Panhandle this
evening. This will cause Lee side troughing to develop on the
Yukon side of the pass to the north, and thus southerly gradient
will increase abruptly through Lynn Canal and other northern
passages this evening. Rain showers have already begun in earnest
over Yakutat. For the rest of the northern Panhandle, any rains will
transition to showers this evening. To the south, showers will be
enhanced by orographics near the Coast Mountains but should be
scattered in nature for points coastward. Friday a shortwave from
the Bering Sea will interact with another short-wave riding NE
within a very fast moving upper level jet. This will quickly spin
a low to enter the eastern Gulf on Friday. In addition, another
boundary will migrate eastward across the northern Gulf and will
enhance shower activity for Yakutat yet again tomorrow and spread
into the rest of the northern Panhandle Friday afternoon.

The southerly flow should transition the last sea level spots in
Southeast Alaska--Haines and Skagway--back to rain late this
afternoon, but it could happen early this evening instead, ie, a
little later. This holds huge ramifications on the forecast. As
currently our forecasted low temperatures are warmer than the
current temperatures. Point Retreat blowing at 20 knots from the
south now, so Eldred should follow soon. Some cooler air aloft
bleeding southeast with this northern Gulf shortwave will help
support the survival of a few flakes within the rain showers late
Friday afternoon for Haines/Skagway, Yakutat, and possibly
Gustavus. Finally, Stewart, British Columbia just now transitioned to rain, but
judging by webcams, there has been little to no accumulation near
Hyder.

Other than small craft seas and some small craft westerlies over
the far northern Gulf, no real wind issues tonight for outside
waters. Looking inside, northern Lynn will reach small craft as
the ridge builds behind the front.

Yesterday's Christmas snow forecast for Juneau turned out to be
big mess. Guidance offered a few conflictions, but with any
overrunning event, Wisdom from past events usually calls for US to
lean more towards the larger impacts, the snow in this case. Arw and
nmm high res models all advertised a good southerly push all night
into today, which itself suggests warming. However, other
guidance, namely ec and NAM was skeptical on this. In addition,
cloud cover developed over the northern Panhandle a little earlier
and helped to inhibit any cooling. The shallow cold air over the
Juneau area turned out to be much more shallow and fragile than we
thought. In addition, the northerly outflow winds through Lynn
Canal only rose to 15 kt, which is not a signal for a strong
defense against warm air intrusions from marine fronts. Finally,
model thicknesses seemed to warm from guidance of 24 hours
earlier, which suggested the rain-snow line was going to be close.
While out the Road, maybe an inch and a half fell, and parts of
Douglas Island away from the water may have received a half inch
of slush, the precipitation and temperatures in the low to middle
30s largely supported a little bit of rain. This is illustrative
of the fact, that even small changes in the forecast yield great,
even drastic changes, which given ramifications is a tough call to
make...over a little evidence that the paradigm will not hold. Still
MOS guidance suggested temperatures stabilizing all night with
dewpoints and thus wetbulbs remaining in the upper 20s until
Friday morning. This could be a study case, not only in factors
directing a forecast, but also of how a narrative of a white
Christmas contributed to any resistance against an attractive
paradigm.

Changes were confined to late Thursday night into Friday, mainly
for some Gulf winds with the new low moving approaching US, as
well as to diminish showers a little late tonight into tomorrow
for the Panhandle coast south of Cross Sound. Patchy fog added
mainly due to the visuals of the webcams...saturated and murky but
do not believe it will be a huge issue.

Long term...overall weather pattern does not change that much
over the next week. A building upper ridge over the Gulf this
weekend will gradually drift to the east into middle next week with
the ridge axis main situated over the southern Panhandle and
northwestern British Columbia. Only main change is a rather strong
trough driving in from the west that ruthlessly flattens the
ridge on Thursday. Otherwise the ridge returns for the next weekend
just a bit further to the west then is was.

In the lower layers, the Panhandle will be starting to dry and
clear out on Sat as the weak low from Friday moves off into Canada
and a strong 1040 mb high builds into the southern Yukon. The cold
air accompanying the high is not extremely cold for this time of
year with 850 mb temperatures only getting down to -12 c over the Yukon
and -4 to -8 c over the Panhandle. So I am not expecting any
extremely cold temperatures for the Panhandle this weekend. Coldest temperatures
will likely be Sat and Sun night mainly due to radiational cooling
from the clearer skies. Northerly outflow will not be that bad
either. While the surface pressures in the Yukon will be around
1040 mb there is no strong low in the Gulf to tighten the
gradient. As a result outflow winds are not expected to exceed small
craft criteria for Cross Sound and Lynn Canal this weekend.
However, some some brief periods of min gale could still occur in
these areas if more cold air spills over the mountains then
expected.

Into early next week temperatures start to warm up slightly as
southerly flow on the windward side of the upper ridge starts
bringing in some warmer air. Most of the warm air advection will
be to our west but some of that will wrap around the top of the
ridge and settle in aloft over the Panhandle next week. Surface
cold air will likely get trapped in the Inner Channels with no
major systems to push it out until middle week. So sea level temperatures
will likely not get out of the 30s for much of next week.

Next major system slowly crosses the Gulf middle week bringing more
warm air advection, precipitation and some wind. Main concern will be the
precipitation as the continued presents of low level cold air in the
Inner Channels may mean that most of the precipitation in the northern
half of the Panhandle may start out in the form of snow. The
question here remains if the cold air will be deep enough for that
or will the upper lever warm layer be dragged down enough by the
precipitation and warm air advection for most of that precipitation to change
to rain with little snow accumulation resulting. At the moment
going for a snow rain mix for the most part with a few inches of
accumulation possible for Wednesday for the northern half of the
Panhandle.

Overall long range models were in good agreement with most
features well represented and agreed upon. Main trends were the
continued slowing of the front trying to get across the Gulf next
week while keeping the Yukon high around just a bit longer.
Generally went with a European model (ecmwf)/NAM/GFS blend through the weekend then
transitioned to a European model (ecmwf)/wpc blend for next week.

&&

Ajk watches/warnings/advisories...
public...none.
Marine...Small Craft Advisory for pkz012-051-052.
Small Craft Advisory due to seas for pkz022-041>043-053.

&&

$$

Jwa/eal

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