Heavy Snow in Alaska, March 4, 2020

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
412 PM AKST Tue Mar 3 2020

.SHORT TERM...Westerly flow across the gulf has lower level
clouds moving into the area formed from the remains of cold air
cloud streets streaming off of southwest Alaska into the western
gulf. This will continue the showers along the coastline, when the
clouds reach the enhancement area from being orographic lifted,
resulting in a mix of snow and rain showers. A low at surface and
one aloft moving into area near Prince William Sounds will shift
flow over the northeast gulf more directly onshore over night and
Wednesday. Accumulations will increase during the day with better
snow rates and total accumulations around 5 to 7 inches for the
Yakutat area, so winter storm warning going out with the afternoon
package.

High pressure over northwest Canada will combine with a developing
storm system headed to the southern panhandle. Wind switching
around and increasing through Wednesday night will push an arctic
front southward. While the front moves southward Wednesday it
should cause some overrunning snow as the colder air undercuts the
warmer and moister air. Looks like 3 to 5 inches for the Haines
and Haines Highway ( so posting Advisory statement ). Skagway
expecting less, but the White Pass area should see some blowing
snow, that will lower visibilities a little.

The low over the southeast gulf will pull in moderate to strong
low near 980 mb. The track is still being fine-tuned by the by
model solutions but looks to be west of Dixon Entrance Thursday
morning. Precipitation with the system looks like it may be
limited to the panhandle primarily south of Frederick Sound,
although the weakening frontal band Thursday night maybe
spreading into the north central panhandle.

The winds to be a significant feature again Wednesday to Thursday.
Strong gales over Lynn Canal, and those might increase more in
later issuance's but have 40 kt for now. Outflow from interior
passes for the Northeast Gulf Coast might have gusts to 50 kt from
passes and bays. 25 to 35 kt for the southern marine waters.
Gusts through the panhandle on later Wednesday and Thursday will
likely support Strong wind Headlines (gust 40 mph or higher)

.LONG TERM.../Thursday through Tuesday night/Thursday starts us
out in the extended period under a trough that will undergo
tremendous deepening in the following days. This will be reflected
at the surface by an arctic front that will be collapsing
southward through the region before stalling out over the region
by Saturday. All the while this is occurring, a low in the Gulf
will be churning along the coastal areas, delivering a decent
amount of precip before eventually getting shoved south by the
aforementioned front. The timing for how fast or slow all of this
happens is still up for debate between the models, but as we get
closer this will be critical first for the temperature forecast,
but second the where and how much snow forecast. For the time
being, have opted for a ECMWF/GFS blend to begin the forecast
period, then slowly phased into the NBM and WPC guidance for any
changes later on. This solution yields a fair enough compromise
where most high temperatures for Thursday remain in the 30s,
becoming colder the further north you go from a line roughly from
Juneau to Pelican.

After the front pushes the low away, skies will begin to open up
across the region, helping us to get to some noticeably colder
temperatures, mainly in the northern half of the panhandle. Skies
should remain fair through most of Saturday before they begin to
advect in again from the west ahead of another low. If you're not
a fan of the cold this should be welcome as the cloud cover and
warm front associated with the low should bring most areas back
into the 40s for highs and 30s for lows by Monday. However this
next system will also bring back the chance of precip. While far
too early to determine where or if any significant snow can be
expected from this system, the early guess is for Yakutat as they
will be seeing the precip first and are just far enough north
that they likely don't get too far above freezing before the onset
Sunday. This is not to say no one else will see snow; quite the
contrary, snow could be fairly widespread with this system, it is
simply too far out to conjecture with any certainty about the
where, much less the amounts. As the forecast stands right now,
temperatures on Monday and through the end of the forecast for
nearly all locations near sea level suggest that rain will be the
most widely seen p-type.

Extended forecast begins during a period of unsettled weather as
a low to the west will continue to send a series of weak
disturbances toward SEAK. Continued onshore flow will keep up an
ample amount of moisture over the region through Wednesday. This
low will be absorbed into the overall flow as a second low begins
tracking toward the region from the southwest Wednesday night into
Thursday. Models are beginning to come into agreement that this
low will deepen quite a bit and could throw a significant slug of
moisture at the panhandle.

Exact timing and location of any particular feature is still
uncertain at this time due to discrepancies within the guidance.
What is becoming more certain though is that some locations stand
a good chance of seeing a significant amount of precip Wednesday
night through Thursday which could translate into a good amount of
snow. At this time, opted not to advertise significant snow for
any particular place, but should be noted that it does look like
it is probable to hit somewhere.

Thursday, as the aforementioned low continues it's assault on the
region, an arctic front also begins to descend from the north
before stalling out over the middle of the region Saturday. The
previous "slight" cooling trend from previously in the week will
be replaced by a surge of much colder, drier, continental air,
plunging night time lows into the teens for most of the north-
central and northern panhandle by Friday night. Daytime highs in
the southern part of the region will be for the most part
unaffected, however north of the boundary, highs for many will
struggle to get out of the 20s. These chillier temperatures for
next weekend will also be accompanied by our first real
opportunity for some clear skies in what seems like forever. By
the very end of the forecast period, temperatures will begin to
relent slightly as the flow begins to shift back around to the
south. This will begin to bring clouds back in from the west come
Sunday ahead of our next potential rain/snow maker.

Winds will be mostly benign aside from the coastal waters at the
start of the forecast period, but as the pressure gradient around
the low passing to the south begins to strengthen, should begin to
see an uptick, mainly in the south. The arctic front coming in on
Thursday will be accompanied by it's own strong pressure gradient
which could be rather problematic through Cross Sound and Lynn
Canal. These conditions should begin to relax Sunday as the
gradient starts to weaken.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...Winter Weather Advisory from 6 AM to 9 PM AKST Wednesday for
     AKZ019.
     Winter Storm Warning from 3 AM to 3 PM AKST Wednesday for AKZ017.
MARINE...Small Craft Advisory for PKZ012-013-022-031-033-034-036-041>043-
     051>053.

&&

Bezenek/JDR

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