Heavy Snow in Alaska, December 14, 2020

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
234 PM AKST Sun Dec 13 2020

.SHORT TERM...Satellite imagery this afternoon showing a 984 mb
low with associated gale force front tracking northward along the
Eastern AK Gulf. As the low center reaches the NE Gulf coast it
will weaken while the front moves over the AK Panhandle and into
western Canada through late Monday afternoon. By Monday evening
another strong low will move into the SE AK Gulf. With a low
center around 970 mb the associated occluded front will likely
produce gale force winds again. As of now this low is expected to
remain in the SE Gulf through Tuesday before it tracks east into
British Columbia while weakening.

Weather for Sunday night into Monday will be a complex wintry mix
for much of the central and northern Panhandle. We are expecting
a combination snow, freezing rain, sleet and rain. The following
elements are part of the tricky forecast: On Sunday afternoon
temperatures for the central and southern areas are mostly above
freezing, however dew points are still in the 20s due to the dry
air mass. If precipitations rates are high enough snow melt
cooling will drop temps and allowing more snow fall. Dendritic
growth zone is also saturated, which is conducive for snow fall.
The deep isothermal layer indicated by model soundings is just at
or near freezing for many points, so a degree fahrenheit
difference is enough for a change from snow to rain and back
again. A warm mid level air mass tracks in, which along with a
shallow cold air mass in place sets up for freezing rain. What is
working against snow or freezing precipitation is increasing winds
allowing the warm air to mix in and changing precip to more rain,
(except for the far northern panhandle which should stay all snow).
Temperatures will rise overnight and into the morning as the warm
air moves in. The front is also fairly fast moving and QPF
amounts are not especially high, which trends towards less
confidence for snow accumulation.

For now have Winter Weather Advisories for most of the central
and northern panhandle starting Sunday evening through Monday.
While snow accumulations at most maybe 4 to 6 inches, it will be
the periods of freezing rain and changing precipitation type which
will cause the most issues for Monday morning, even if snow
amounts are limited to an inch or two. For details on specific
areas see the Winter Weather Advisory.

The next low will bring rain for the southern panhandle while
offshore flow keeps the northern areas dry. Northerly winds
increase as this low moves in. Winds for the SE Gulf will hit max
gales with a chance to near storm force for a short time.

Kept with GFS/ECMWF as main guidance for synoptic pattern. NAM
becomes more of an outlier Tuesday over the western Gulf so did
not incorporate its winds. As stated above difficult part of the
forecast was tracking the temperature curve and resulting
precipitation type over the next 12 to 18 hours. Forecast
confidence is bit below average for exact weather details but have
confidence that this will be an impactful system through Monday.

.LONG TERM.../Tues through Sun as of 10 PM Sat/...After Monday's
wintry storm system, all eyes turn toward the next low pressure
expected to move into the area Tuesday and Wednesday. Models have
been indicating a strong system pushing up from the north with a
threat of winds across the entire region (probably southeasterlies
across the south and northerlies in the north). However, recent
trends are both slightly weakening the system and taking a more
easterly track. While we still could see impactful winds,
confidence is not at all high. However, this track may indicate a
more southerly rain-snow line that could reach
Kake/Wrangell/Petersburg.

Current model projections of the temperature profile brings more
rain with a rain and snow mix possible Tuesday night. Wednesday
looks like there will be more cold air in place so we may see more
snow or rain/snow mixes during the day in the leftover,
wraparound moisture falling on the backside of the exiting low.

The rest of the week, into next weekend, still holds on to the
overall wet pattern. While the details are few and confidence is
low, there is an emerging pattern in the model ensembles that
would give southeast AK continued rain and snow as another low
pressure system moves into the gulf and impacts the panhandle on
Thursday.

Temperatures next week are still expected to be on the cooler
side with highs mostly in the 30s, with 40s in the southern
regions, and lows mostly in the 20s.

&&

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Strong Wind until 9 PM AKST this evening for AKZ027.
     Winter Weather Advisory from 9 PM this evening to 3 PM AKST
     Monday for AKZ019-025.
     Winter Weather Advisory from 9 PM this evening to noon AKST
     Monday for AKZ020>022.
     Strong Wind until 3 PM AKST this afternoon for AKZ018.
     Winter Weather Advisory from 3 AM to noon AKST Monday for AKZ017.
MARINE...Gale Warning for PKZ041>043-051-052.
     Small Craft Advisory for PKZ012-013-021-022-031-033>036-053.

&&

PRB/GJS/JWA

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