Gale Warning 319, NWS Juneau, AK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
238 PM AKST Wed Dec 27 2017

.SHORT TERM...The main story of the week is going to be the
coldest temperatures. Overnight Friday looks to be the coldest
day, but with the greatest model differences. Just how cold will
it be is the question. Most of the panhandle will remain in the
teens and single digits Friday night.

As a ridge weakens over the the contiguous U.S. and western
Canadian border late Thursday, winds will lessen from gale to
small craft status and less cold air advection. A system of lows
pushing through the Gulf is on track for early Friday, with the
easternmost low affecting mainly the southern panhandle. With
this associated shortwave energy comes another shot at
northeasterly flow and even colder air.

Some adjustments were made to temperatures, generally a
decreasing trend. Maximum temperatures for tomorrow were actually
raised by a degree or two, while min's were lowered for tonight to
follow the trend in the persisting pattern.

Modifications made to pressure followed the GFS and Canadian early
on and quickened the progression of Friday's upcoming pair of
lows. Local edits were made to wind, especially over the
Skagway/Haines/White Pass area, corresponding to the tightening
pressure gradient over the northern panhandle. Combined with the
colder temperatures, the White Pass area has the potential to see
another wind chill advisory in place for Thursday night. While
wind speeds aren't particularly strong for the Douglas and
downtown Juneau area, mountain wave guidance is considered
favorable for Thursday evening.

Changes were made to the early part of POP's using a SREF/NBM/SREF
blend. Most changes to QPF were made to the tonight and Thursday
night period, which backed off precipitation in the southern
panhandle. Any precipitation for the early period was kept at
convective with CAPE values approaching 300 J/kg and unstable LI
indices.

Models still have somewhat good agreement at least through early
Friday.

.LONG TERM.../Friday through Wednesday as of 10 PM Tuesday/
Large scale pattern looks like it will be in transition during the
long term. High latitude blocking appears like it will break down
by late in the weekend. Meanwhile, active southern stream will
continue as systems move across the NPAC into the gulf or SE
Bering Sea area starting this weekend. Models continue to struggle
with the individual systems with timing, track, and intensity
differences noted. Larger scale pattern will evolve to a broad
upper trof over the N-central PAC into the gulf, while ridging
builds over western NOAM by early next week. Looking like after a
fairly cold first half of the long term, warmer air will move in
with the building ridge early next week. There will be an
increased threat for precip from latter part of the weekend into
next week as well. For the most part, did not make much change to
longer range other than to deal with system for late in the week.

Main change was to limit precip threat to the southern third of
the area Fri and Fri night. Right now, only the GFS brings precip
into the southern third, while other models keep vast majority, if
not all, of their precip S of the area. Took out precip threat
over about the N 2/3rds of the area for Fri-Fri night. POPs were
capped at chance levels over the S during this time as well. With
the lowered POPs also reduced cloud cover over the N with
offshore flow going on. Temps were left alone, with still some
significant differences in how much cold air gets in late this
week. Current forecast represents a middle ground compromise on
this.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...Strong Wind from midnight AKST tonight through Thursday
afternoon for AKZ018.
MARINE...Gale Warning for PKZ012. Small
Craft Advisory for PKZ013-022-031-035-041. Small Craft Advisory
due to Seas for PKZ042.

&&

SS/RWT

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