Area Forecast Discussion
Issued by NWS

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
FXAK67 PAJK 192341

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
241 PM AKST Fri Jan 19 2018

.SHORT TERM...The last time this writer prepared vichyssoise, he
broke a blender. In this chilled potato atmospheric soup variety
in Juneau this morning, one may have broken a leg from the
freezing drizzle that coated roads, streets, sidewalks, and trails
outside Juneau. As a low churchs to our south, a weak surge of
very low-level moisture settled into the north-central Inner
Channels, and given a relatively strong inversion, we suffered
through yet another freezing liquid event in the Mendenhall
Valley. As deeper moisture is now migrating northward across the
region, drizzle has transitioned into showers of mixed rain/snow.
With more organized dynamics it is hoped we can put this to bed as
temperatures fall back below freezing during the evening.

Meanwhile, the forecast changes little through Sunday. One low in
the southeast gulf will sputter northwestward through Saturday
evening allowing a stronger low now rushing east along 50 N to
assert its dominion over our weather. As evidenced by the great
region of cellular convection across the outer gulf, this low
carries with it a modified maritime cool air mass with lapse rates
approaching 8C/km. We have added a slight chance of thunderstorms
for marine zone 41 as well as the far southern Panhandle Saturday
night and Sunday as the low replaces the former low in the
southeast gulf. With increasingly onshore flow at mid levels the
Panhandle will see rising chances of snow/rain going to Sunday

Mainly northerly winds through the Inner Channels and arguably
easterly through Cross Sound and Sumner Strait will occasionally
accelerate to 25 kt or higher as low near from the south. The
stronger low may enhance the gradient such that northern Lynn
Canal reaches gale Saturday night/Sunday morning. For this, we
carry freezing spray, but temperatures may need to be warmed
alleviating the threat. Winds though will remain relatively light
across the gulf.

Herein lies some potentially impactful uncertainty. There appears
to be an axis of vorticity riding above the colder air mass that
has settled across Southeast. But strength and timing are at
issue. However, confidence has increased that there will be a band
of enhanced snow shower activity passing northward across the
greater Panhandle Monday, and therefore, we have raised pops to
likely. If the ECMWF is more correct, we could see an earlier
development, but this model is certainly in the minority
currently. This event has potential to affect the entire
Panhandle region with snow, but it is more likely a mix along the
central coastal and far southern regions. We stand now with a
forecast of around an inch where snowfall will be marginal, and
up to around 2 inches farther north and inland including Juneau,
Petersburg, and Wrangell. This forecast may be ramped higher
should the system look stronger in the coming couple of days.

We used a blend of NAM, ECMWF, and GFS for changes. Forecast
confidence good after this evening. This evening, we still will
watch any threat of freezing liquid precipitation in the north
central Inner Channels, but we think a deeper moisture layer
should have killed it.

.LONG TERM.../Monday night through Friday as of 10 pm Thursday/
Models in a little better agreement on the larger scale features
through early next week. Upper trof will dig S into the W gulf
over the weekend then base of the upper trof will shift E into the
central gulf early next week as another shortwave digs S into the
W side of the upper trof. After Tue however, the models differ on
where the various shortwaves within the trof go. Does appear that
the mean trof axis will reform further W late in the week
however. Blended in latest WPC for most parameters from Monday
night onward.

Did warm temps some especially Tue. Initial colder airmass coming
in from the N will stay over the far N area. Colder air will
slowly wrap around sfc low over the central gulf into the area
early next week, but this cool down will be slower. Later in the
week, as upper trof reforms to the W, the cooling trend will
likely end and some moderation in temps is likely by late week.

Did keep in likely POPs over much of the central and S area early
next week as a couple of systems move in from the SW. Some
decrease in POP is possible for latter part of the week as main
low level flow may become more offshore, but still a lot of model
differences to resolve that far out.

Looking like the most wind will be with any lows over the gulf.
There may be some outflow as well over the N third of the area,
but it does not look particularly strong due to lack of a
strong sfc high over the Yukon. However, any stronger lows that
would move into the E gulf could increase outflow more than
current forecast has it.


.AJK Watches/Warnings/Advisories...
MARINE...Small Craft Advisory for PKZ012-022-041.
     Small Craft Advisory due to Seas for PKZ042-043-051-052.




Visit us at is the U.S. government's official web portal to all federal, state and local government web resources and services.