Winter Storm in Alaska, March 10 to 11, 2018

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
257 PM AKST Sat Mar 10 2018

.SHORT TERM...Snow, and in most areas rain, continues across
portions of the panhandle this afternoon. At the synoptic level, a
deepening trough over the gulf is tightening the gradient between
it and an upper level ridge strengthening over British Columbia in
western Canada. This is helping to maintain a jet streak aloft,
which is subsequently advecting additional moisture over the
panhandle with an almost subtropical origin. Models continue to
show substantial moisture transport through the mid and lower
levels with this moisture plume. In addition, strong onshore flow
has helped to warm temperatures through most areas, so we saw
most locations transition to rain. One of the outliers is the
Juneau area, which remains mostly as snow. This is probably due to
the heavier rates helping temperatures to hover around freezing.
Once these rates decrease this evening, the snow should transition
fully to rain. However, we still expect colder temperatures to
remain along the highways from Haines and Skagway, so snow should
continue overnight tonight and through Sunday.

Beginning tonight, an embedded shortwave within the broad upper
level trough over the gulf will lift out of the central gulf and
strengthen while swinging northeast. The left exit region of the
jet streak mentioned previously should provide some divergence
aloft to help in the development of this feature. An associated
front with this system will impact the panhandle late tonight,
providing us with enough lift to give us an extra shot of
precipitation that will continue through the day Sunday. A low
level jet with this feature will create some concerns for wind. We
have Strong Wind headlines for a few areas, including Sitka,
Yakutat, and Elfin Cove and Pelican, as we'll see wind increase on
land as it mixes down to the surface. Inner channel marine waters
will see at least SCA winds through most areas, and we'll see
strong gales along coastal and offshore marine zones. In addition,
low level wind shear will also be a concern for most sites
pertaining to aviation purposes. Another possible concern may be
the development of isolated thunderstorms over offshore waters and
marine zones 51 and 52 late in the day on Sunday. This will be a
very unstable air mass near the center of the circulation with
this low, and coupled with relatively steep lapse rates, increased
vorticity, and cooler air wrapping in aloft, we may see some
cells produce lightning.

Monday may provide a brief respite from the heavier precip,
especially in southern areas, from Ketchikan to the Wrangell-
Petersburg area as the low weakens and lifts out of the gulf
Sunday night. The front may stall, which would leave some
lingering precip, mostly limited to northern areas, such as Juneau
and Haines and Skagway. The next system will be another developing
low over the central gulf with another front that will impact the
panhandle beginning Monday afternoon, increasing precip from
south to north as the front lifts north.

Generally, we used the NAM for forecast updates. Temperatures, for
the most part, were tweaked slightly to account for increasing
overnight due to the onshore flow, warm air advection, and cloud
cover. For winds, we also utilized the NAMnest in addition to hand
analysis for local effects.

.LONG TERM.../Monday through Saturday evening/...Extended range
forecast begins with a deep, negatively tilted upper trough that
will extend from the western interior, south across the central
gulf and into the northern Pacific. A strong vorticity lobe near
the base of the trough will spin up an upper low south of 50N
which will then cut off Tuesday morning and remain south of the
forecast area. As the low cuts off, an upper ridge will build in
over the gulf briefly from the southwest, but then get displaced
by an upper short wave that will move over the eastern gulf by
early Friday morning. In the lower levels, warm air advection from
the south on Monday will spread north through Tuesday afternoon,
pushing temperatures at 850mb, or approximately 5000 ft altitude,
above freezing. Weak cold air advection from the west will
displace the warmer air on Wednesday, but more warm air advection
from the southwest, courtesy of the upper ridge, will move in on
Thursday. More cold air will move in again from the west by next
weekend. This patterns strongly suggests migration of the Polar
Front jet north and a beginning of the transition season - Winter
to Spring - resulting in more rain and less snow.

At the surface, weak low pressure over the eastern gulf will
persist through Tuesday night with shower activity rotating north
over the eastern gulf and panhandle from a surface low associated
with the upper cut off low to the south. By wednesday morning, a transient
surface ridge associate with the upper level feature will have
built in and cause precipitation to dry up for a while. The ridge
will be displaced to the east as a strong front moves into the
western on Thursday morning. Precip chances remain elevated
through St. Patrick's Day.

Model agreement is fair through mid week, but then model solutions
diverge. Overall forecast confidence is average through midweek,
but then trends below average.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...Strong Wind until 6 PM AKST this evening for AKZ027.
     Strong Wind from Sunday morning through Sunday afternoon for
     AKZ022.
     Strong Wind from 9 PM AKST this evening through Sunday morning
     for AKZ023.
     Winter Storm Warning until 1 PM AKDT Sunday for AKZ019.
     Winter Weather Advisory from 9 PM AKST this evening to 7 AM AKDT
     Monday for AKZ018.
     Strong Wind from Sunday morning through Sunday afternoon for
     AKZ017.
MARINE...Gale Warning for PKZ022-036-041>043-051-052.
     Small Craft Advisory for PKZ011>013-021-031>035-053.

&&

Voveris/Fritsch

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