Gale Warning 267, NWS Juneau, AK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
157 PM AKST Sun Nov 25 2018

.SHORT TERM...Main forecast concerns in the short term revolve
around high marine and land wind potential tonight and early
Monday.

Vigorous shortwave trough embedded on the nose of a 150kt upper
jet seen on WV imagery early this afternoon near 49N 138W. This
wave is interacting with an existing low level frontal boundary
stretching across the eastern Gulf of Alaska and down into the
NPAC. Rapid surface wave development is currently taking place
over the far SE Gulf where forcing for ascent attendant to the
shortwave trough is intersecting the frontal zone. Guidance has
come in to a bit better agreement on the track and intensity of
this low as it tracks generally northward and then northwestward,
through the eastern and northern Gulf waters into Monday morning.
Went towards the stronger NAM/EC guidance as they seemed to better
depict the more compact structure of the wave based on satellite.

Winds over the Gulf will rapidly strengthen to storm force tonight
ahead of the compact surface wave. While the wave will pass fairly
far offshore the southern Panhandle, the effect of the strong jet
aloft and forcing for ascent over the existing frontal zone over
the southern panhandle will tighten the surface gradient this
evening. Thus gales will likely spread into the southern inside
waters such as Clarence and Chatham Strait this evening. 25 to 30
kt winds will spread northward even into Lynn Canal by Monday
afternoon as the surface wave passes. For the land areas of the
southern Panhandle, low level flow increases to between 60 and
75kt in the lowest 5000 ft as the surface gradient become oriented
a bit more favorably this evening. Stability will be an issue for
high winds, as forecast soundings show at least some low level
frontal inversion through early Monday morning. Think that the
surface gradient will be enough, coupled with the strong flow just
off the surface, for gusts to 60-65 mph as the front passes
tonight. Thus issued high wind warnings for POW Island and the
Ketchikan areas where channeled flow near Hydaburg and the
Tongass Narrows will be enhanced. As the compact low goes inland
west of Yakutat Bay Monday morning, there will be a short window
of time for the Yakutat area to experience some strong winds.
Small differences in the placement of the low lead to a bit lower
confidence in the strength of the winds there, so opted to go just
short of warning criteria at this time.

Heavy precipitation will also occur through early Monday as
integrated vapor transport values increase to around 500 kg/m*s
over the southern Panhandle. Heaviest rainfall will be over the
south through Monday closer to the greatest moisture transport and
jet energy. No immediate wintery precip concerns as even
temperatures in the Chilkat valley will likely be able to mix
above freezing by this evening, ending any freezing precip threat
there. Only the immediate White Pass area will see snowfall with 2
to 4 inches possible through early Monday.

.LONG TERM.../Tuesday through Sunday night/...Extended range
forecast begins with a large negatively tilted upper trough over
the western Gulf of Alaska and a weakening southerly jet situated
over Southeast Alaska. Upper level jet strength will diminish to
trivial levels by Tuesday evening, but numerous shortwave pulses
rotating around the base of the upper trough will maintain that
feature over the Gulf until Friday afternoon. At that time, a
weak ridge and equally weak upper level zonal flow will displace
the trough off to the east. The upper ridge will not last long, as
an upper trough will depress both it and the strong upper jet
energy to the south by Saturday afternoon. This situation will
remain dynamic, as a highly amplified upper ridge will be moving
east from Bristol Bay and the Bering Sea late in the weekend.
Model agreement on this was significantly better today, so
forecast confidence is decidedly lower than yesterday.

Weak frontal signatures through Wednesday night will keep
precipitation likely through the first half of extended range
forecast. At this point, the second half of the week is looking a
little drier, with PoP values in the chance range through the
weekend. Best chance for dry weather looks like Thursday night
through Friday night.

A weak low level warm pool of air over the Gulf and Southeast
Alaska at the start of this long range forecast period will shift
east through the week. Based on the prevailing deep layer flow and
forecast temperatures at 850 mb, snow at the lower elevations is
not likely. Nonetheless, temperatures for late next week going
into the weekend were lowered by a few degrees.

At the surface, southeasterly winds of 15 to 25 kts over the
eastern gulf with lower values over the inner channels will
prevail for much of the coming week.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...Strong Wind from 6 PM AKST this evening through late tonight for
     AKZ023-026.
     High Wind Warning from 6 PM this evening to 5 AM AKST Monday for
     AKZ027-028.
     Strong Wind from midnight AKST tonight through Monday morning
     for AKZ017.
MARINE...Storm Warning for PKZ041>043-051-052.
     Gale Warning for PKZ022-033-035-036-053.
     Small Craft Advisory for PKZ011>013-021-031-032-034.

&&

DEL/Fritsch

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