Heavy Snow in Alaska, March 8, 2020

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
302 PM AKST Sat Mar 7 2020

.SHORT TERM...A trough crossing eastward over the northern
panhandle and a weak surface low over the northern gulf coast
caused snow showers over Yakutat today. These will continue
through the night under onshore flow and will see some snow
showers over the northern panhandle as the upper level trough
continues east this evening. The clouds assoc with the trough
should keep minimum temperatures from getting as cold as Saturday
morning, but still about normal for this time of year. The
southern panhandle will remain dry and mostly clear, so expect
another cold, below normal morning.

The jet gradually moves into the gulf tonight, then increases out
of the west Sunday. Shower potential increases Sunday with the
increasing onshore flow and bands of vorticity create more
organized bands of showers. Most of these will be snow showers to
start, mixing with rain through the afternoon for some places. A
stronger vort max moves into the the northern gulf coast late
Sunday night assoc with a front. Southerly winds will increase
with the front as well, causing winds of 25-30kt through the
inner channels. Precip rates will pick up with 2-4 inches of snow
falling over parts of the inner channels (Haines- Juneau-
Petersburg). The wind will cause other locations to be mixed with
or change to all rain and limit accumulations.

Another wave or new low center moves up from the SW on Monday,
warming temperatures further and causing a change to rain for more
locations. Forecast confidence in this new development is slightly
lower as it causes a quick switch in wind directions, so timing of
that may change.

.LONG TERM...After a parade of successive quick moving but weak
systems passing across Southeast Alaska this weekend, a stronger
shortwave looks to amplify the long-wave pattern into next week.
Rather than light snow across the north and a mix over the south,
we are looking at a southerly jet with sufficient warm air
advection to push the rain/snow line farther north. The better
organization looks to be a change over yesterday when thicknesses
were held low enough to keep snow in the forecast for the north
through the day and limit rain mixing until Monday night or
Tuesday. Adding more confidence is that MOS numbers suggest a
breezier pattern in spots associated with the low-level jet and
tighter pressure gradient. That said, we still cling to the
possibility of significant accumulation late Sunday night into
Monday morning for Yakutat, the Icy Strait communities of Gustavus
and Hoonah as well as Juneau and Haines. The most confident area
of stronger winds lie in the central and southern Panhandle where
it is possible we could see some gusts of 40 or 50 mph. But
complicating matters is a guidance consistency issue in that it
now appears that another system rides into the eastern gulf late
Monday following the first. This one appears to ride in with more
baroclinicity and perhaps looks stronger than the one earlier
Monday. Resulting from the second system, we then expect milder
temperatures into Tuesday with widespread rain and snow showers
for the north and widespread rain showers for the south.

Behind Tuesday's onshore showers, cold air advects from the Gulf
of Alaska. With 850 temperatures approaching -10 C Wednesday we
feel strongly that most of these showers will be snowy with maybe
a mix towards the south. There are still hints that split flow
could yield a juicy southerly system to bomb out from the south,
therefore focusing a late season snow event for somewhere in the
Panhandle mid-week. But the mid-week transition to cold and dry
remains murky and could be less eventful.

Growing support for a significant late season dry cold wave has
become harder to merely hint at. We lowered temperatures and pops
Wednesday into Thursday and leading into next weekend. We also
strengthened outflow winds through the Inner Channels. While WPC
hints at a ridge breaking down for next weekend, the strong ridge
begins to form an omega block that could last into next weekend.
Sunshine lovers may have something shine about, even if it will be
chilly and quite windy for this time of year.

Forecast confidence is average early next week and falls-mid week.
Then rises for the dry cold late next week.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...None.
MARINE...None.

&&

Ferrin/JWA

Visit us at http://www.weather.gov/Juneau