Winter Storm in Alaska, December 22 to 23, 2019

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
158 PM AKST Sun Dec 22 2019

.SHORT TERM...Complex forecast especially in regards to ptype
unfolding over the next 24 hours or so.

Currently, weak sfc trough extending from the far northern Gulf
eastward across the northern portion of the Panhandle is
dissipating as the next stronger frontal system to the SW impinges
on the eastern Gulf. S of the weak surface trough, winds have
switched to a S'ly direction as far N as Eldred Rock. Temperatures
there have warmed from the mid 20s to the mid 30s as the wind
went S'ly. To the N of this boundary, temperatures remain in the
low to mid 20s near sea level ranging to upper 10s on the Haines
and Klondike highways. Light snow has developed in Haines and
Skagway as moist isentropic upglide has commenced. Snowfall should
continue through the evening hours before increasing in intensity
over night as the stronger front to the SW pushes into the inside
waters.

Made relatively few changes to the inherited forecasts with all
winter weather statements remaining in place. Only changes were to
make the wind and precipitation type forecasts a bit more
internally consistent. Some definite issues with this system in
that the surface gradient and low level thermal profiles indicate
the lower atmosphere will be well-mixed ahead of the front. This
usually does not bode well for sea level snow in SE Alaska. There
will be some modified cool air making its way into the northern
Panhandle this evening ahead of the front. A glance at some of the
buoys under this airmass indicates surface temps in the mid to
upper 30s but dewpoints in the upper 20s to low 30s. This should
lead to low level thickness values falling briefly again this
evening over the far north, but will it be enough to overcome with
mixed boundary layer? Pros for significant snow near sea level
are:

1: The current cold, stable airmass in place in the Chilkat
Valley and Klondike where surface temps remain in the low to mid
20s even as temperatures at higher elevations are warming
slightly. Often these cool stable airmasses are harder to dislodge
than models indicate, leading to a delay in the mixing of the
boundary layer. 2: The cool stable airmass near the surface also
leads to a sloped isentropic surface to enhance forcing for ascent
as strengthening WAA aloft overruns the near surface stable
layer.

Cons for significant snow near sea level are:

1: The stronger surface gradient favoring E and SE winds. Forecast
profiles indicate most of the low level cold being mixed out this
evening. As the front approaches, the low level gradient does back
slightly, allowing for potentially more blocked flow in the Lynn
Canal area, but only provided it does not mix out this evening.

2: With the mixed boundary layer, there is less of a surface for
moist isentropic upglide, especially in the Chilkat Valley. Thus
the S flow aloft could lead to precip shadowing in the lee of the
Chilkat Range which would cut down on overall precip amounts even
up the highway.

For now, decided to cut back on snow amounts near sea level but
leave warning amounts of snowfall above 500 ft on the Haines and
Klondike highways. As temperatures warm, think snow ratios will
become increasingly unfavorable for significant accumulations as
the snow will be wet and may mix with rain late. For this reason
also took out the mention of blowing snow due to low snow ratios
and temperatures near or slightly above freezing. Up the highways,
think it will be hard to dislodge all the cool stable air so
overrunning still a good bet further up on the road system. Used
the NAM for these adjustments as it seemed to be handling the low
level cold airmass the best. A conservative 1285m thickness value
as the rain/snow line led to a transition to rain near sea level
late tonight in most northern locations. Elsewhere, expect a
rainy, windy morning Monday with some snow mixing in along the Icy
Strait corridor. Rain, heavy at times will continue into Monday
night for the southern Panhandle as the low level front slows and
becomes more aligned with the upper level flow. It will be windy
over the inside waters late tonight and early Monday with SE winds
increasing to 25 to 30 kt over the majority of the channels.

.LONG TERM...This extended range forecast, beginning on the
morning of Christmas Eve, features a large, negatively tilted and
progressive upper trough over the eastern half of the Gulf. By
that night, the trough will have shifted east over the Panhandle
and sharpened up considerably. On its heels, a large upper ridge
will move in and produce upper level northwesterly flow by
Christmas morning, then deep southwesterly flow by Christmas
night.

The implications of this at the surface are significant, and
represent the largest change to the long range forecast. Models
are fairly good agreement now on a storm force low and triple
point feature moving into the western Gulf by Christmas. High end
gale force winds will accompany the front as it approaches the
outer coast late Wednesday night. Winds in grids were increased
during this period as a result. Besides the gales, am expecting
strong winds for the outer coast and southern inner channels.
Model of choice was GFS and modeled QPF was really quite scary.
Took only half of the model values, but this still increased the
QPF grids quite a bit. Caveat here is that those values may have
to be increased if GFS remains consistent and the other models
fall in line with both timing and intensity of the front.

Long range forecast highs and lows remain within the
model/ensemble spread, so little change there except to decrease
temps for Haines and Skagway by a few degrees and for Juneau by 1
to 2 degrees. Elsewhere, temperatures were, and still are, warm
enough that snow from this front does not reach epic levels.
Probably will see another round of advisory snow for Haines &
Skagway Wednesday night with some sub-advisory amounts for
Gustavus, Juneau, and Yakutat.

Forecast confidence is average to below average due to the
significant changes made to winds and QPF.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...Strong Wind from Monday morning through Monday afternoon for
     AKZ027.
     Strong Wind from late tonight through Monday morning for AKZ023-
     025.
     Strong Wind late tonight for AKZ017-021-022.
     Strong Wind Monday afternoon for AKZ028.
     Winter Storm Warning from 6 PM this evening to noon AKST Monday
     for AKZ018-019.
     Winter Storm Warning until 5 AM AKST Monday for AKZ017.
MARINE...Gale Warning for PKZ036-041>043-051>053.
     Small Craft Advisory for PKZ012-013-021-022-031>035.

&&

DEL/Fritsch

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