Winter Storm and Heavy Snow in Alaska, March 20 to 21, 2021

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
247 PM AKDT Sat Mar 20 2021

.SHORT TERM...The next snow producer has begun to work its way
into the panhandle. Cloud cover has rebuilt across SE Alaska and
snow has already begun to fall in parts of the area. The track of
the system's associated low has shifted slightly northward, but
the northern panhandle should still stay on the cold side of the
low. This will help keep a majority of precipitation from the Icy
Strait Corridor northward snow through the night. Further south,
the situation becomes more complex. The shift in the low track
could result in more warm S flow in the central panhandle. While
the advisory was kept for Petersburg and Kake, these areas could
receive lower snow totals than anticipated if they get an early
changeover to rain. The Juneau area still has the best chance to
get higher snowfall rates and stay cold enough with N-NE outflow
from Taku Inlet and Lynn Canal, and so continue to have the
warning out for 5-8" of accumulation. In addition, winter weather
advisories are out for the Icy Strait Corridor, Petersburg,
Wrangell, and Kake. Gusty winds will also arrive with the front
causing blowing snow as well as breezy conditions across much of
the area. Small craft advisories are posted for many of the inner
channels, with gale force winds expected in the outer coastal
waters.

How the system weakens through Sunday has some uncertainty due to
model spread. But most of the frontal moisture shifts off to the
SE and expect a transition to showers with breaks developing
through the late afternoon. A weak remnant low over the Mt.
Fairweather region will likely keep showers going longer across
the north Sunday night. Skagway and the Klondike Highway could see
some appreciable snow accumulations from these showers late
Sunday and Sunday night.

.LONG TERM.../Monday through Saturday as of 10 PM Friday/ The
extended range forecast begins with a weak upper low situated over
the eastern Gulf and tracking east. To the south, a 130 kt jet
will push the low along and keep southeast Alaska on the cold side
of the jet. As the week progresses, upper flow remains relatively
zonal until Tuesday morning. At that time, a more amplified
pattern moves into the forecast area with an upper ridge over the
Panhandle and a sharp, positively tilted trough upstream over
Kodiak and the far western Gulf. The trough passes over the
Panhandle Tuesday night, being pushed along by a broad
northwesterly jet. Another upper ridge moves over us by Thursday
morning, leaving Southeast Alaska on the warm side of the jet at
least for a little while. By the weekend, yet another trough moves
through the gulf and over the panhandle.

At the surface, the long range forecast begins with a weak area of
low pressure over the northeast gulf coast keeping showers over
the northern panhandle. By Tuesday morning, the next well
organized front will arrive over Southeast Alaska from the west.
This system will be just in the process of occluding as it arrives
over the Panhandle and the triple point will be located over the
far southern Panhandle, meaning that this will be a true warm
frontal passage over the inner channels. Model guidance has came
into better agreement that the parent low will track through the
central panhandle Tuesday afternoon. This system will bring deep
southern moisture into the panhandle causing moderate to heavy
rain rates at times across the southern panhandle. The northern
panhandle and areas along the Icy Strait corridor will be on the
north side of low keeping chances of snow as the system moves
through. By Wednesday the surface low will head south out of the
forecast area as a surface high builds in the central gulf. This
looks to offer a good break in precipitation Wednesday as weak
offshore flow takes over. Some areas across the panhandle could
even see some sunshine, best chances across the northern panhandle
where the strongest northerly flow will be.

Beyond this time, the surface high will steer midlevel
disturbances over the far northern Gulf, and then southeast over
the Panhandle. Thursday model guidance begins to degrade with the
GFS keeping the panhandle dry and the ECMWF showing a deepening
low in the northeast gulf coast. By Saturday, confidence increases
while models come back into alignment on the next significant
feature. Once again another surface low develops in the northern
gulf bringing widespread precipitation across the panhandle.

&&

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory until 10 AM AKDT Sunday for AKZ020>022-
     026.
     Strong Wind from 7 PM AKDT this evening through late tonight for
     AKZ027-028.
     Winter Storm Warning until 10 AM AKDT Sunday for AKZ025.
     Strong Wind from 7 PM this evening to 1 AM AKDT Sunday for
     AKZ023.
MARINE...Gale Warning for PKZ035-036-043-051-052.
     Small Craft Advisory for PKZ012-013-021-022-031>034-041-042-053.

&&

GFS/CC

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