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FXAK67 PAJK 202237

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
137 PM AKST Sat Jan 20 2018
.SHORT TERM...Large, vertically stacked upper low over the eastern
NPAC late this afternoon will weaken into more of an open trough 
as it drifts N then NW'ward into the NE Gulf by Sunday night. To
the west, strong, cold shortwave trough will dig into the central
Gulf by Sunday afternoon and then lift north across the Panhandle
Monday. At the surface, the pressure gradient across the northern
Panhandle will support continued N'ly outflow conditions. Expect
gales to develop in Lynn Canal tonight with SCA winds for southern
parts of Lynn Canal as well as Cross Sound, Stephens Passage and
Sumner Strait. These winds will gradually weaken on Sunday the 
gradient weakens.

Main problem in the short term period revolves around the
potential for accumulating snow for parts of the Panhandle,
beginning Sunday afternoon and continuing into Monday night.

Weak upper wave will impact the northern Panhandle on Sunday
afternoon into Sunday evening. NAM/EC are consistent in developing
some WAA between 850 and 700 mb over the N'ly outflow and this
will likely lead to some light snow accumulations from Juneau
northward, with amounts at this time expected to remain below
advisory criteria.  

As the second, stronger upper wave lifts north late Sunday night
into Monday, the state will be set for a more substantial precip
event. Mid level PVA combined with favorable orographics from low
level S'ly flow of 30 to 35kt will lead to about a 6 hr period of
moderate precipitation Monday. The question mark in all this are
temps. Onshore flow in the low levels will combine with a SE'ly
gradient to bring the boundary layer air in from off the Gulf. 
This air will be cold, but the rather long trajectory over the 
Gulf lends some caveats to an "all snow" forecast especially from 
the Petersburg/Wrangell area southward. However, can't ignore the 
fact that snow levels will be quite low and with steep lapse rates
of 7-8C in the 850-500 mb layer, some convective enhancement is 
likely. Bumped up snow totals over the interior and northern inner
channels Monday and left a rain/snow mix over the south. Think a 
Winter Weather Advisory may eventually be needed for parts of the 
northern Panhandle Monday. Precip will gradually taper off from 
the SE Monday night as the best forcing for ascent shifts W'ward.
.LONG TERM.../Tuesday through Saturday as of 10 pm Friday/ Still 
some important model differences to resolve with handling of 
energy in base of upper trof over the gulf and EPAC for next week.
Ended up using WPC for the most part as it was based on ensemble 
means and operational model blend. This shifted some of the 
systems a bit further S especially for around Thu. Does appear 
that a couple of fronts will move NE across the area for Mon-Wed, 
so kept in higher POPs for those days. POPs were nudged down a bit
for Wed-Thu especially N area due to southward shift of main low 
S of the area. Another trof will dig into the W gulf for late week
and the weekend and this would help increase precip threat again 
as flow aloft becomes more S-SW.

Used GFS/EC blend to handle the 24 hr pre-WPC period for system
moving NW through the E gulf during Mon. Still looks like mainly
snow over the N half of the area, but ptype will be trickier over
the S as some milder air may come in ahead of the system. Will
likely be rain over the far S and outer coast from PASI 
southward for Mon. Once this system moves past, colder air
wrapping around the upper trof will move in, and much of the S area
will become cold enough for at least a rain/snow mix, with the N
still seeing snow. Still a decent amount of uncertainty on how
much snow would fall over the N as this will likely depend on how
much cold air is in place and the direction of the 850-700 mb 
flow. Too much of an E wind component in that layer would 
diminish precip on W side of Coast Mtns, but would enhance precip
along E side of Baranof and Chichagof Islands.

For Tue into late week, next front will likely bring more precip
to the area Tue, with mostly snow N and mix or snow S. Latest
models hint at a low moving into the far E gulf for Wed. This
system looks like it would try to warm things some especially over
the S so that precip there would trend more toward rain. After 
Wed the model differences increase so forecast confidence 

Did not change temps for long range. Still some model differences
on how much cold air moves in from the N and from the SW. Some
operational model MOS guidance suggested temps might be a few 
degrees warmer, but ensemble MOS spread remains on the high side
so felt staying with current forecast was best thing to do.

Still looks like some outflow winds will occur but not expecting
more than gale force winds due to lack of a strong high over the 
Yukon. The various fronts/lows moving in will probably have 25-35 
KT winds with them over the gulf as well.


.AJK Watches/Warnings/Advisories...
PUBLIC...Strong Wind through this evening for AKZ018.
MARINE...Gale Warning for PKZ012. 
     Small Craft Advisory for PKZ013-022-031-035-041>043. 
     Small Craft Advisory due to Seas for PKZ051-052. 




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