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Southeast Alaska forecast discussion 
National Weather Service Juneau Alaska
552 am akst Friday Mar 6 2015

Short term...weak upper disturbance located from the Skagway area
into northern British Columbia/southern Yukon will become absorbed by westerlies aloft this
morning. Meanwhile...upper pv anomaly over the northern Pacific is becoming
detached from northern stream flow regime...and will generally remain
stationary through Friday night. As this takes place...another
trough will begin to amplify as it moves from the Bering Sea into
western portions of interior Alaska. Surface pressure pattern over the
Panhandle will become dominated by a ridge of high pressure
building north during the day. The high combined with low pressure
over northwestern Canada...and possible Lee-side pressure falls downwind
of the chilkat mountains under west-southwest flow aloft...will result in a
locally tight pressure gradient over northern Lynn Canal...with southerly
winds near small craft levels occurring through this afternoon
into tonight. Closer to the high center...winds will be weaker for
central/southern areas. Resultant reduction in boundary layer mixing and
a dry/subsident midlevel environment could support patchy fog
development tonight.

Otherwise...satellite derived tpw guidance shows values near 1
inch extending northward from the subtropics into the northestern Pacific. Precipitable water
values from 0.6-0.8 inch are forecast to spread northward from the
subtropical plume during Friday night...and move into the southern
Panhandle. The increase in moisture will aid in the development of
light rain for southern areas tonight. Farther north...the combination of
increasing large-scale ascent ahead of the Bering Sea trough...and
southerly low-level flow emanating from the Gulf /resulting in orographic
ascent in vicinity of the St Elias mountains/ will yield potentially moderate
rainfall amounts west of the Yakutat area/akz017. Decided to assign
shower wording for this area of precipitation due to relatively
steep lapse rates in the 925-700 mb layer.

Nudged pressure grids toward the NAM/ECMWF/Gem in order to
strengthen the area of high pressure over the central/southern
Panhandle. Changes to winds were minimal...with the exception of
northern Lynn Canal and Skagway...where speeds were manually bumped up.
Pop and quantitative precipitation forecast were refreshed with an ensemble of coarse and hi-res
guidance. Few changes were made to temperatures...with values
during the short term period forecast to be similar to what was
observed Thursday.

Long term...the colder air we have been advertising will come in
two batches. Batch one can be traced from the Bering Sea trough
alluded to above. This trough is currently pushing eastward from
the waters southwest of St. Lawrence Island to arrive in the
northern Gulf late Saturday night. We think precipitation over
Yakutat will remain mainly liquid through Saturday night, as pre-
frontal winds will be mainly marine in origin with some weak warm
air advection in particular Saturday evening. However behind the
trough winds will back to westerly, allowing cold air advection to
begin in earnest. This will support a mix of snow for Sunday over
Yakutat with likely a complete changeover Sunday night. Likewise,
the northern Panhandle including Juneau will experience these same
changes but as much as 12 to 18 hours later with greater impacts
from batch two.

The second batch of cold air, originating over northeast Siberia at
this time will follow a shortwave on a similar track as the first,
swinging down into the Bering Sea Saturday evening on its way to
the northern Gulf by Monday morning. This will reinforce the cold
air mass that arrived with the first trough. The theory is that as
these short-waves drop southwest from the Lee of the Alaska Range
into the western Gulf, they will help spin up a surface feature
that will enhance precipitation on its forward side. The first
feature will be fed by a longer Pacific fetch and hence will be
more juicy than the second which has been starved from moisture by
the first system. Nevertheless, the pattern is such that
confidence is moderate that snow will fall over the northern
Panhandle ahead of this system as it progresses eastward. But quantitative precipitation forecast
is not so impressive so at this point, we are talking
accumulations of potentially from 2 to 4 inches Monday through
Monday night for Haines/Juneau. But a lot of this depends on
temperatures. Model guidance suggesting cooler temperatures for
this area Monday, however, MOS not so much. Did cool the northern
Panhandle under this system by a couple of degrees for Monday
which helped to enhance snowfall from previous forecast. But at
this point, this event looks sub-advisory and only significant
because of our lack of snowfall this winter.

Current forecast has cold air sticking around southeast with
zonal flow from the Bering Sea keeping temperatures below normal.
However, uncertainty is rising about a deepening longwave trough
midweek over the North Pacific perhaps kicking a cut-off
low northeastward to nudge the Arctic front away from the
Panhandle. Thus mention of snowfall in jeopardy from Wednesday
into Thursday for much of the Panhandle as southerly flow begins
to take root. No changes at this time as we will wait and see how
successive runs handle this change.

Windspeeds were raised into the weekend for some of the northern
Panhandle north-south passages as well as Juneau and Skagway to
account for increasing gradients associated with stronger low
pressure crossing into Canada and a building ridge Sunday morning
ahead of monday's system.

Used a blend of ecwmf and GFS to better handle second system in
the Gulf and nudged to European model (ecmwf) to handle the potential of a warmer
disturbance moving north into the southeast Gulf Tuesday/Tuesday
night. Otherwise only small tweaks. Temperatures may need to be
warmed middle-week Onward. Forecast confidence good through
Monday...falling rapidly afterwards.

&&

Ajk watches/warnings/advisories...
public...none.
Marine...Small Craft Advisory for pkz012.

&&

$$

Garner/Adkins

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