Winter Storm Watch 18, NWS Juneau, AK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
546 AM AKST Sun Dec 19 2021

.SHORT TERM.../Sunday and Sunday night/ A rather weak upper level
disturbance is moving through the panhandle at the moment. The
only effects so far have been some mid to high level cloud cover
and some light snow at Yakutat overnight. There are some radar
returns of some light precip just off the outer coast this
morning. That should remain offshore though there is a slight
chance that Sitka could see some light snow this morning. The
cloud cover has also not allowed temperatures to get as cold as
they got Sat morning. Northerly outflow is still ongoing in the
northern panhandle. Winds were 10 to 20 kt overnight, but they are
just starting to increase again as of 5 am.

Decreasing clouds and cold temperatures are the rule of the next
24 hours. The upper level disturbance is expected to clear the
panhandle by this afternoon allowing skies to clear out once again
from NW to SE by this evening. Lows tonight are expected to drop
into the teens and single digits again under the clear skies,
especially in wind sheltered areas.

Northerly outflow winds also remain in play as the trough over the
outer coast and the strong Yukon high persist. N winds of 20 to 30
kt are expected out of the passes east of Yakutat, down Lynn
Canal, out of Taku Inlet, and down Stephens Passage today. The
Yukon high is then expected to weaken and slip SE into northern
British Columbia by later tonight. This will weaken northerly
outflow in the northern panhandle, but winds coming out of the
Stikine River valley, Portland Canal and other interior passes in
the southern part of the panhandle will persist.

Forecast changes were minor with mostly local effects for winds
and temperatures being the main changes.

.LONG TERM.../Monday through Thursday night/...the mid range
forecast begins with clear, cold, quiet weather around the
panhandle. But Monday is looking like a transition day to a quick
hit of snow on Tuesday.

Clouds are expected to increase late Monday. And the pressure
gradient over the panhandle should lighten up so winds should
decrease as well. High temps look to be in the 20s for most of the
area, 30s are possible along the outer coast.

An area of low pressure is projected to move southeast through
the gulf through Tuesday so the snow would track southeast through
the panhandle. It would begin in the Yakutat area late Monday and
track southeast, exiting through the southern panhandle Tuesday
night. Just ahead of the low, 850mb temps are projected to get
near -4C in the central and southern panhandle, so rain/snow
mixing was allowed to stay in the forecast but confidence is low
due to surface temps only get a few degrees above 32 before
cooling back to the 20s Tuesday night.

The 24 hour QPF amounts for Tuesday through Tuesday night range
from near a tenth of an inch in the northern inner channels to
about a half inch in the central inner channels. This idea is
backed up by the model ensemble outputs. 24 hour QPF probabilities
of more than 0.10” was greater than 70 to 80% and probabilities
of more than 0.50” rose to around 30-60%. So if typical SE AK
snow ratios play out, overall snowfall amounts shouldn’t be too
high. Went with a comfortable one to four inch range, but locally
higher amounts are still possible. Once this event gets closer,
forecaster confidence will increase with use of hi-res models in
addition to ensemble probabilities.

Inner channel wind forecast was, overall, left to ride due to
expected strong outflow winds. Outside waters were touched up to
try to incorporate the details in speed and direction as the low
passes. Overall, still expecting gale forecast winds in outside
waters as low passes on Tuesday.

Models solutions late in the week in general agreement in deep Nly
flow through the atmosphere with strong signals of below normal
temps expected heading into next weekend.

&&

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Small Craft Advisory for PKZ012-013-031-041.

&&

EAL/GFS

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