Heavy Snow in Alaska, January 23 to 24, 2020

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
547 AM AKST Thu Jan 23 2020

.SHORT TERM.../Through Friday night/ Active period in store for
the area as complex stream interactions lead to potentially
hazardous weather. Upper trof will amplify over the western gulf
today, then drift into the central gulf for tonight into Fri
night. A series of shortwave trofs will move out of the southern
stream into the far eastern gulf and SE AK through the period.
At the sfc, a low over the far eastern gulf will come onshore
across the northern panhandle today, and weaken tonight. Another
low will move N into the area late tonight and Fri. There remain
some important detail differences in the models as far as this
system's evolution goes, making for a very tricky forecast,
especially for precip type and amounts. A sharp cold front will
swing E across the area Fri into Fri evening.

Given some of the issues mentioned above, ended up making only
limited changes through Fri night. Ended up making some adjustment
to the winter weather advisory for Juneau. Precip stayed rain
overnight, although there were pockets of freezing rain going on.
Ended up shortening the advisory to 18z, and made main cause light
freezing rain. It has just started snowing here at the office, but
expect it to go back to rain/freezing rain as warmer air aloft
moves in by around 16z on increasing SE flow. The winter storm
warnings for zones 18 and 19 were kept as is, with snow increasing
at this time around Haines. Skagway should get into the snow by
18z.

The other main adjustment was to put in more snow to the left of
the current forecasted track of the low for late tonight and Fri.
Precip will increase on the cold side of this system with
increasing frontogenesis and lift from the upper trof over the
gulf. This would cause snowmelt cooling especially in areas with
lighter winds. Based on current forecasted low track N through
the inner channels along or just E of Chatham Strait, this would
put the PAGS to PASI areas in favorable location for snow.
However, any change to the low track would shift this area in the
direction that the low track deviates. Also, models differ on just
how strong the lift on the cold side of the system will be,
affecting QPF/snowfall forecast. At this point, have kept this
area of snowfall below advy levels, but later shifts will need to
monitor for potential update to the amount and location of this
snowfall.

Once the sharp cold front passes by the area, precip will become
showery and be mainly snow showers, as airmass is very cold
(especially aloft), even with the over-water trajectory. The far
SE corner may end up only going to mixed precip though, as the
deeper cold airmass will be further N. The snow showers will be
most frequent/heavy across the NW half of the area, especially
along the NE gulf coast.

Winds should not get above SCA levels with any of the systems
through Fri night, although there could be gusts over 40 kt at
times especially after the sharp cold front moves through.

.LONG TERM.../Saturday through Thursday as of 10 PM Wednesday/
Complex and quite stormy pattern will dominate the weather over
the Panhandle through the long term.

Upper ridging will be in place over the Bering Sea this weekend
and will slowly migrate westward through mid week. The downstream
result will be deep troughing over mainland AK and the Gulf of
Alaska. Very cold airmass will be in place over much of the state
but will generally be centered to the north and west of the
Panhandle through the period. To the south, shortwave energy
riding an active Pacific jet will periodically interact with the
deep northern stream trough and arctic airmass to generate several
potent storms. These storms will track towards the Panhandle in
the SW to S flow aloft. As usual with complex stream interactions,
operational guidance is in complete disarray in regards to tracks
of frontal waves along the tight baroclinic zone in the eastern
Gulf through mid next week. As stated yesterday, precip type may
be a huge issue as the pattern favors bouts of quite significant
precipitation along with shots of modified arctic air ejecting
across the Gulf and into SE Alaska. Will likely see precipitation
across the northern half of the Panhandle alternate between rain
and snow several times through the long term while the southern
Panhandle remains warm enough for mainly rain.

Given the strong baroclinic zone in place over the eastern
Gulf, periods of strong winds are also possible especially if
phasing between northern and southern streams can occur.

All in all, way too early to nail down details of any particular
system so left the forecast generally unchanged. The potential is
there for one or two significant winter storms with strong winds
and heavy precipitation (potentially heavy snow for the north)
from the weekend onward which is hard to reflect in a
deterministic forecast at this time.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...Winter Weather Advisory until 9 AM AKST this morning for AKZ025.
     Winter Storm Warning until 6 PM AKST Friday for AKZ018-019.
MARINE...Small Craft Advisory for PKZ012-013-031-033>036-041>043-051-052.

&&

RWT/DEL

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