High Wind in Alaska, November 8 to 10, 2019

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
159 PM AKST Fri Nov 8 2019

.SHORT TERM...Complex and potentially high impact weather
scenario unfolding over the next 24 to 36 hours for Southeast
Alaska. Main forecast concerns over the next 2 days will be strong
outflow and mountain wave event for the northern Panhandle, with
a narrow area of heavy snowfall for the central Panhandle. So lets
break down the impacts one at a time:

1. Outflow winds. Arctic front currently pushing south into the
far northern Panhandle as of this writing. Deep cold air
advection, combined with a increasing northerly surface gradient
between a 1040mb high in the Yukon and a sub-995mb low in the
southeast Gulf will lead to northerly winds approaching storm
force in Lynn Canal overnight and Saturday. Other gaps will also
experience strong gale conditions such as Cross Sound and Taku
Inlet. Those planning travel on the water over the northern
Panhandle through Saturday should make alternate plans until the
outflow winds subside late Saturday night and Sunday.
Additionally, a classic Taku Wind scenario will unfold late
tonight and Saturday as strong CAA increases over Salisbury Ridge
near downtown Juneau. Northern stream shortwave trough will drop
southward almost directly over the Juneau area by Saturday morning
and provide an excellent critical level for reflecting mtn wave
energy to the surface. Wind gusts of 70 mph or higher are not out
of the question with this type of scenario so residents in the
Juneau/Douglas area are encouraged to make preparations for a
period of strong wind gusts through Saturday.

2. Heavy snowfall. As the northern stream shortwave sinks south on
Saturday morning, models have come into better agreement on
partially phasing it with a southern stream shortwave over the
central Panhandle Saturday afternoon. Surface low will track along
a persistent low level frontal zone which will be draped over the
southern Panhandle through Saturday. Low level thicknesses are
progged to drop to critical levels by early Saturday morning over
the Angoon area and by early Saturday afternoon as far south as
Wrangell. Strong low to mid level frontogenesis will lead to the
development of one or perhaps several heavy bands of precipitation
just north of the surface low track. One concerning feature noted
in some of the guidance is the presence of a 1-2C warm nose near
850mb. While freezing rain is rare in SE Alaska, this scenario has
the potential to produce some icing especially at the onset of the
changeover from rain to snow as the arctic air begins to undercut
the warming aloft. Think the low to mid level dynamic cooling and
melting will eventually lead to a deep isothermal near freezing
layer north of the track of the 850mb low. This scenario would
likely lead to a period of 1 inch per hour snowfall rates
somewhere over the central Panhandle. Thus decided to issue a
Winter Storm Warning for the area from Angoon to Wrangell for the
period of early Saturday morning through Saturday evening.

As with most snowfall forecasts, there is the potential for major
bust depending on the exact track of the 850mb low and the timing
of the low level cold air. Do not typically like heavy snow
forecasts when the low level cold air is in the process of coming
in (as opposed to already in place for overrunning) but can't
ignore the amount of lift and forcing that will be in place near
the arctic boundary.

Overall forecast confidence is high in regards to the wind and
lower in regards to snowfall amount and timing.

.LONG TERM.../Sunday through Thursday/...As of 10 PM Sunday
night. With a bit more consistency between operational forecast
models, it appears that the dry conditions over the weekend in the
northern panhandle won't last long as precipitation returns
Monday morning. This break from the wet weather will not likely
last long as precipitation is expected to return by Monday. Beyond
this system, large variations in model solutions from the split-
stream jet introduce significant uncertainties into the forecast.

By Sunday, the upper level high will be positioned over the
Yukon. At the same time trof progged to be over the southern Gulf
will begin to move North, with its impact being felt in the
panhandle by Monday. Although pinning down where the exact center
of the system at the surface will be located remains difficult,
models and ensembles are in fair agreement that Monday will see
the return of wet weather as a front wraps around the low and into
the area, though exact timing remains an issue. The GFS and EURO
have both come into better agreement that the system will roll in
Monday morning, while the Canadian continues to lag behind. The
airmass looks like it will be slightly more moderate than
previously anticipated, likely as a result of some minor ridging
ahead of the system helping to weaken the grasp of the cooler
airmass slated to enter over the weekend. As a result, favoring
rain for most areas during the forecast period, though can't rule
out a flake for Haines/Skagway with snow possible for the Northern
highways. and snow possible along the northern highways. Warm air
advection will likely follow the system, bringing rising
temperatures on Monday and, in conjunction with onshore flow,
helping to keep rain over much of the area on Tuesday and
Wednesday. WPC and NBM guidance were used Sunday through early
next week. Overall forecast confidence remains average to below
average through the long range.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...Winter Storm Warning from 3 AM Saturday to midnight AKST
     Saturday night for AKZ024-026.
     High Wind Warning from 6 PM this evening to 3 PM AKST Saturday
     for AKZ025.
     Strong Wind from 9 PM AKST this evening through late tonight for
     AKZ021.
     Strong Wind from 6 PM AKST this evening through late tonight for
     AKZ019.
     Strong Wind from midnight AKST tonight through late tonight for
     AKZ018.
MARINE...Gale Warning for PKZ012-013-022-043.
     Small Craft Advisory for PKZ011-021-031>036-041-042-051>053.

&&

DEL/GFS

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