Area Forecast Discussion
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FXAK67 PAJK 232349

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
249 PM AKST Thu Jan 23 2020

.SHORT TERM...Currently, an upper trof is amplifying over the
western gulf this evening, and is expected drift into the central
gulf tonight into Fri night. A series of shortwave trofs are
expected to move out of the southern stream into the far eastern
gulf and SE panhandle. At the sfc, a low located in the far
eastern gulf has come onshore and will move across the northern
panhandle before weakening tonight. Another low will move N into
the area late tonight and Fri followed by a sharp cold front
swinging in from the east.

Ended up issuing a Winter Weather Advisory starting at midnight
tonight for the Juneau area, mainly for the Mendenhall Valley
with 3 to 5 inches possible. The Winter Storm Warnings for zones
18 and 19 were kept as is. For this evening, as the band of precip
moves across the panhandle into areas of favorable frontogenesis
and ample lift, precip rates will begin to increase. With this,
snowmelt cooling should help to change from rain to snow. The
forecast track of the approaching low was not changed significantly
with this update. Based on this, areas including Sitka to
Gustavus will also see snow accumulations but under advisory
criteria. One area of concern is that any change in track of this
system will effect snowfall amounts. Also, there are still some
timing differences on the lifting strength associated with the
cold side of the system, which would affect QPF/snowfall forecast.
As of now, advisory criteria only for Juneau with snow amounts
widely varying, but as timing issues align, an update may be
warranted as confidence is moderate.

After frontal passage and colder air begins to take over, precip
will become convective causing more snow showers. For the far SE
panhandle, the precip is expected to remain mixed as the deeper
cold airmass will be further N. The snow showers will be most
frequent/heavy across the NW half of the area, especially along
the NE gulf coast. By Saturday, our next weather maker approaches
from the south.

.LONG TERM.../Saturday through Thursday/As of 10 PM Wednesday/Complex
and quite stormy pattern will dominate the weather over the
Panhandle through the long term.

Upper ridging will be in place over the Bering Sea this weekend
and will slowly migrate westward through mid week. The downstream
result will be deep troughing over mainland AK and the Gulf of
Alaska. Very cold airmass will be in place over much of the state
but will generally be centered to the north and west of the
Panhandle through the period. To the south, shortwave energy
riding an active Pacific jet will periodically interact with the
deep northern stream trough and arctic airmass to generate several
potent storms. These storms will track towards the Panhandle in
the SW to S flow aloft. As usual with complex stream interactions,
operational guidance is in complete disarray in regards to tracks
of frontal waves along the tight baroclinic zone in the eastern
Gulf through mid next week. As stated yesterday, precip type may
be a huge issue as the pattern favors bouts of quite significant
precipitation along with shots of modified arctic air ejecting
across the Gulf and into SE Alaska. Will likely see precipitation
across the northern half of the Panhandle alternate between rain
and snow several times through the long term while the southern
Panhandle remains warm enough for mainly rain.

Given the strong baroclinic zone in place over the eastern
Gulf,periods of strong winds are also possible especially if
phasing between northern and southern streams can occur.

All in all, way too early to nail down details of any particular
system so left the forecast generally unchanged. The potential is
there for one or two significant winter storms with strong winds
and heavy precipitation (potentially heavy snow for the north)
from the weekend onward which is hard to reflect in a
deterministic forecast at this time.


.AJK Watches/Warnings/Advisories...
PUBLIC...Winter Weather Advisory from midnight tonight to 3 PM AKST
     Friday for AKZ025.
     Winter Storm Warning until 6 PM AKST Friday for AKZ018-019.
MARINE...Small Craft Advisory for PKZ012-013-031-033-034-036-041>043-051-




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