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FXAK67 PAJK 192232
AFDAJK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
232 PM AKDT Thu Oct 19 2017

.SHORT TERM.../Thursday night through Friday night/...Rain showers
continue over much of the panhandle this afternoon with more
expected tonight, tomorrow, and Friday night. Overnight lows over
the far northern inner channels and northeast gulf coast are
forecast to be low enough that an overnight rain/snow mix is
possible during this short term forecast period, particularly for
Friday night. 1000-850 and 1000-500 thickness numbers as well as
850mb temps are all indicating this possibility as well.
Nonetheless, so low elevations expected yet. Not a lot of QPF in
the forecast for the Haines or Klondike Highways, so accumulations
there are less than an inch for any given 12-hour period.
Eaglecrest may see as much as an inch tonight and Friday night.

Satellite imagery is very clearly convective and this is borne out
in model data. Convective parameters were really quite high for
the coastal waters and the Dixon Entrance. Even so, most observed
lightning today was confined to Hecate Strait and coastal British
Columbia. As low pressure over the northern and eastern gulf
weakens, a ridge will build in first from the west and then from
the south, causing the atmosphere to begin stabilizing by Friday
afternoon. A high end gale force front, will then approach from
the west late Friday night to finish the stabilization process,
but will bring its own complications in the form of strong marine
winds and windy conditions along the southern outer coast. At
present, am expecting strong wind gusts to have begun by late
Friday night.

Used a blend of GFS and ECMWF for updates to the pressure field
through Friday night. Model divergence beyond that time was judged
too severe to make any changes to the existing forecast with any
confidence. used GFS as a foundation for winds through Friday
night. Also used GFS for PoP and QPF. Very little change to temps
with some minor adjustments to snow levels. Overall forecast
confidence is average.


.LONG TERM.../Saturday through Thursday/Model agreement is fair
for the weekend, then they differ quite a bit next week. Main
issue is timing of systems moving across the NPAC, potential for
at least 1 tropical system getting into the westerlies, and
interaction between these systems and an upper trof over W AK.
Decided to use the 12z/18 ECMWF to handle Sat, then went with
mainly WPC for Sun onward.

First main system will be a complex low moving into the gulf by
Sat. Looking like an occluded front will move N across the area
Sat but will weaken as it moves across the N half of the area.
This front will most likely bring at least gale force winds with
it over the gulf, with at least SCA level and possibly gale force
winds to mainly the more E-W inner channels. Should see some rain
with the front Sat, although as front weakens over the N, precip
may become more spotty as it encounters offshore low level flow.
The main low appears like it will move to the far N-central gulf
by late Sat night, then weaken there Sun. As this happens, looking
more showery for the area especially later Sun.

While model agreement is not good next week, they suggest at least
1 strong system will affect the area. This is when some effect
from what will be remnant of Typhoon Lan will come into play. The
model timing on how fast the typhoon moves into the westerlies
varies somewhat, with GFS/GEM faster than the EC by about 1 day.
(Of interest...the Joint Typhoon Warning Center timing is right
in the middle of the 2 extremes) It is this feature that the
GFS/GEM spins up into a powerful storm S of the Aleutians early
next week, then moves it into the gulf by late week. EC also shows
a fairly strong system but it does not have remnants of Lan in
it, and is somewhat weaker than the GFS/GEM. Still plenty of time
to monitor this situation. In between this larger system and the
one over the weekend, models suggest another low may try to move
NE into the gulf early next week but vast model differences exist
on this, so later forecasts may need to be updated to reflect this
system more than current forecast has it.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...None.
MARINE...Small Craft Advisory for PKZ041>043.
     Small Craft Advisory due to Seas for PKZ022-051-052.

&&

$$

Fritsch/RWT

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