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189
FXAK67 PAJK 221455
AFDAJK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
555 AM AKST Sun Jan 22 2017

.SHORT TERM...Upper trof over the far W gulf will move into the E
gulf tonight. A weak shortwave trof will move N into the SE gulf
today and continue into the NE gulf tonight. High pressure over
the Yukon will move E today. A weak low will form over the SE
gulf and move N toward Yakutat Bay tonight. This low will send a
weak occluded front onshore late tonight.

Main forecast concerns will be precip potential/type/amounts and
winds. An overrunning scenario is developing with warmer air to
the S moving N with weak shortwave trof. Appears to be fairly
moist airmass as well with dewpoints in the lower 40s. As this air
lifts N over the cold airmass, expect clouds to thicken and snow
to develop over the N third of the area during the afternoon.
There will be precip with the low and front as well, with this
mainly over the E gulf today then it will begin to move onshore
tonight. This will enhance the overall lifting environment with
support from upper trof moving in as well, so the snow over the N
will increase in intensity tonight. At this point, based on model
qpf and frontal placement, looking like the most snow will fall
in zones 17, 19, and 20 tonight. Decided to issue a winter
weather advisory for these zones starting at 00z this afternoon.
Looks like 3 to 6 inches of snow should fall in these areas by
daybreak Mon. Lighter snow should fall further E in the Juneau
area with about 1-3 inches likely by daybreak Mon. Further S,
airmass will moderate more quickly so most precip will be rain,
but the precip threat over the SE will be lower until the main
frontal band moves in late tonight. Exception will be early this
morning as weak shortwave moves through with some light rain over
the S.

Winds will weaken through the day over the N channels as the
pressure gradient weakens with high moving away from the area.
Looks like wind with the low/front will be 20-25 KT over the E
gulf today into this evening. May also see some 25 KT wind in
Clarence Strait this afternoon.

Otherwise, airmass will be moderating slowly through tonight.
Temps over the N may remain steady or rise slowly tonight. May see
some temp drop over the S tonight with more breaks in clouds for
that area, but the clouds will thicken again later tonight and
stop temp drop. Basically kept low temps tonight a bit above MOS
over the S and in line with a blend of MOS numbers over the N.

.LONG TERM...Period begins with the first in a series of fronts
moving into the panhandle. Models continue to support warming low
level temperatures Mon into Tue. However, the challenge still
remains when ptype will transition from SN to RA for the central
and N panhandle. Heaviest precip with this system will be into
AKZ017 as the surface low moves NW along the outer coast and into
the N gulf coast near Yakutat Bay. A shortwave ridge behind this
wave will allow precip to diminish late Mon into Tue, but
continued onshore flow and orographic forcing will maintain precip
across the panhandle. Precip chances increase again Tue afternoon
as the a gale force low south of the Aleutians moves into the W
gulf. This system will be more potent with NAEFS IVT increases to
1-2 standard deviations above normal and GEFS M-climate QPF
indicate MAX values beginning Tue afternoon. Feel this system will
be the one to provide sufficient WAA for precip at sea level to
transition to liquid across the entire panhandle. This system will
bring gales to the gulf, a developing barrier jet and seas
building to near 20ft over the gulf. Model solutions begin to
diverge late in the week but are in general agreement on an even
stronger storm system impacting SE AK Wed into Thu. This system
progged to have more moisture available as NAEFS IVT values
increase to 2-3 standard deviations above normal and modeled 6hr
QPF values approach 1-1.5 inches for 12-24 hours. Upper flow will
remain SWly ahead of this system and with continued WAA, expect
precip to be liquid at sea level. This system will also help
maintain seas greater than 15ft in the gulf into the weekend.
Operational model solutions diverge significantly heading into the
weekend, but ensembles and WPC support continued active weather
pattern with troughing over the W gulf and deep onshore SW flow
across SE AK through next weekend.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...Winter Weather Advisory from 3 PM this afternoon to 9 AM AKST
     Monday for AKZ017-019-020.
     Wind Chill Advisory until 10 AM AKST this morning for AKZ018.
MARINE...Small Craft Advisory for PKZ013-022-036-041>043-051.
     Small Craft Advisory due to Seas for PKZ032-052-053.
&&

$$

RWT/BC

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