Winter Storm in Alaska, November 24 to 25, 2017

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
320 PM AKST Fri Nov 24 2017

.SHORT TERM.../Through Saturday night/ A complex low pressure area
will wobble around the eastern gulf through Sat night while slowly
weakening. High pressure will build over the Yukon late tonight
and Sat. USed the 12z GFS to handle things as it had best handle
on the main low and second wrap feature that will affect mainly the
central outer coast tonight.

Main forecast concern is snowfall, especially with the second wrap
feature. Models were a bit spread out on how far E this would get
inland tonight, but based on satellite the GFS (with EC close as
well) had best handle on it. The heavier showers with the band
should run up the central outer coast and western parts of the N
inner channels...with the most persistent showers around zone 22.
There have been TSTMS in this band as well which should stay over
the gulf this evening, but they are indicative of how unstable the
airmass is. Given intensity of the showers and likely training of
cells over the central coast, have issued a WS warning for zone 22
for 4 to 8 inches tonight. It is possible that even more snow will
fall in some spots. Issued a WW advy for zones 20-21 as eastern
part of the band should clip them tonight, but the showers will
not be as heavy or frequent, but still expect 2 to 5 inches in the
advy areas. Other locations surrounding the advy area should see
generally 1 to 3 inches of snow tonight, with lesser amounts
further south as precip will be more mixed there.

Expect the rain and snow showers to gradually decrease Sat into
Sat evening as low moves a bit further offshore and increased E-NE
flow develops. However, another system approaching the central BC
coast could spread more rain and snow into the far SE corner of
the area by late Sat night.

Strongest winds will be associated with second wrap feature...with
35-45 KT winds along it. There will be increasing N and E flow
over the N inner channels tonight and Sat morning as high builds
some to the N, and gales are expected in Lynn Canal, far N
Stephens Passage, and out of Cross Sound. SCA level winds should
occur elsewhere tonight over the inner channels. Winds should drop
off some over the S channels and E gulf Sat and Sat night but
remain up over the N half of the inner channels.

.LONG TERM.../Sunday through Friday/ Upper level trough over the
eastern AK gulf on Sunday begins to shift to the northeast through
Sunday while a low tracks toward Haida Gwaii from the Pacific.
High pressure ridge replaces the trough over the gulf but that
quickly moves to the east was well by Sunday evening as another
broad upper low moves over the Aleutians and into the western gulf
Monday. Part of this wave follows similar track crossing into the
eastern gulf then into BC followed by another building  ridge.
This active pattern repeats though the rest of the week: low
tracks in from the west, followed by a ridge, followed by another
low.

At the surface Sunday will be a transition as one of the last
with cold air over the central and northern panhandle due to a
shift to more southerly flow and warm air advection. Precip will
be limited to the southern panhandle Sunday due to the low moving
south of Haida Gwaii. The next frontal band advances across the
gulf ahead of the parent low near the Aleutians Monday night
reaching the panhandle early Monday. Points north of Frederick
Sound will still be mostly snow with rain to the south and along
coastal areas. Potential of a developing low moving into the far
SE gulf behind the front but low confidence on this feature.
Parent low will weaken as it moves into the NW gulf Monday night.
Potential break in precip Tuesday into Tuesday night would be
short lived as the next deep low moves over the Aleutians with a
front once again crossing over the gulf. Indications that this low
center will move northward in to the AK interior, which would
push more southerly flow and warmer air over the panhandle.
Starting to see snow levels rising into the 2000 to 3000 ft range
towards the end of the week, so rain is now the likely precip type
for all but the far northern panhandle. Any place staying as
snow, or at least mostly snow could see a few more inches due to
higher QPF amounts from the warmer air mass. Besides then main
change to more southerly flow for most of the panhandle by Monday,
expecting barrier jet formation along frontal boundaries moving
in with small craft winds over most of the coastal waters
extending along the front.

ECMWF/GFS were falling more in line for the mid range. GFS was
showing best run to run consistency lately. Main change from
previous models and forecast was having the low center further
north, which emphasized the warming trend more. Nudge toward
GFS/ECMWf blend then more WPC later in the period. Confident of
the active pattern and warming trend but still uncertainty with
exact track of the mid range features.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...Winter Storm Warning until 6 AM AKST Saturday for AKZ022.
     Winter Weather Advisory until 6 AM AKST Saturday for AKZ020-021.
MARINE...Gale Warning for PKZ012-013-022-031-036-042-043-051>053.
     Small Craft Advisory for PKZ011-021-032>035-041.

&&

RWT/PRB

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PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE JUNEAU AK
1226 PM AKST FRI NOV 24 2017

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0800 AM     SNOW             HOONAH                  58.11N 135.42W
11/24/2017  M4.0 INCH                           AK   CO-OP OBSERVER

            24 HOUR SNOW TOTAL AS OF 8AM

0900 AM     HEAVY SNOW       MUD BAY                 59.15N 135.37W
11/24/2017  M9.0 INCH                           AK   TRAINED SPOTTER

            24 HOUR SNOWFALL TOTAL AS OF 9 AM.

0900 AM     SNOW             8 N JUNEAU              58.50N 134.13W
11/24/2017  M5.9 INCH                           AK   TRAINED SPOTTER

            FROM 9AM ON 1123 TO 9AM ON 1124

0900 AM     SNOW             2 SSE MENDENHALL GLACIE 58.39N 134.52W
11/24/2017  M6.4 INCH                           AK   NWS EMPLOYEE

            24 HOUR SNOW TOTAL

0900 AM     SNOW             HAINES                  59.24N 135.44W
11/24/2017  M7.2 INCH                           AK   CO-OP OBSERVER

            24 HOUR SNOW TOTAL AS OF 9 AM

1000 AM     SNOW             GUSTAVUS                58.41N 135.80W
11/24/2017  M5.5 INCH                           AK   CO-OP OBSERVER

            24 HR SNOW TOTAL AS OF 10 AM

&&

JBYRD