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FXAK67 PAJK 202309

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
209 PM AKST Tue Feb 20 2018

.SHORT TERM...Currently, weak shortwave aloft is beginning to
track SE`ward along the spine of the coast range and will continue
S tonight. A weak push of moisture in the low levels has allowed a
lower cloud deck to develop along the Gulf coast. Some light snow
or flurries are possible this evening across the Panhandle
associated with the weak onshore flow and the minor perturbation
aloft but overall moisture will be lacking and little to no
accumulation expected. Winds will become northerly behind the
departing wave tonight and some slightly cooler air will be drawn
into the Panhandle, especially northern areas. Skies will begin to
clear from N to S by early Wednesday morning although some clouds
may linger over the southern Panhandle into the morning hours.

Main forecast concern will revolve around the next significant
mid/upper trough forecast to move from the Bering Sea into the
northern Gulf by Thursday night. Models are generally in good
agreement with the overall pattern through Thursday night,
although unfortunately the agreement is in the fact that snow
levels and boundary layer temperatures will be marginal for
significant snowfall for parts of SE Alaska. Subtle variances in
precipitation rates and/or boundary layer winds may mean the
difference between minor slushy accumulations of snow and a
significant heavy, wet snowfall especially in interior areas of
the Panhandle such as Juneau, Petersburg/Wrangell, and Hyder.

As the mid/upper shortwave approaches from the west Thursday
afternoon, height falls will overspread the northern Gulf.
Divergence aloft will also increase, especially over the northern
Panhandle and into the Yukon. The response will be a deepening
surface trough over the northern Gulf along with increasing lee
troughing over the Yukon. This will tighten the low level
gradients significantly by late Thursday afternoon especially over
the northern inner channels. Expect gale force southerly winds to
develop in Lynn Canal with south winds becoming quite gusty into
Skagway as well. Strong cold advection underneath the upper trough
over the Gulf will lead to a period of strong gale W to NW winds
by thursday night and these may briefly spread into Cross Sound
Thursday evening.

Strong moisture advection is expected to develop by Thursday
evening across the Panhandle, Moist WSW flow aloft will lead to
favorable orographics along the Coast Mountains where snow levels
will already be 500 ft or less. This could allow for the precip
type to remain snow a bit longer especially due to the lack of
deep SW fetch off the Pacific. Forecast soundings indicate that
heavy, wet snow is likely from Juneau to Petersburg and also
eventually into the Hyder area. Rain or rain/snow mix will likely
predominate eventually to the west along the outer coast and
southern Panhandle. Think snow ratios will be rather low, and with
surface temps in the low to mid 30s, it may be hard to get
significant accumulations unless the precip rates are heavy
enough. For now, held accumulations below warning levels but still
think several inches of wet snow are likely for parts of the
interior Panhandle by late Thursday night. Later shifts can
monitor the low level thermal structure of this system and make
the determination regarding any winter headlines.

.LONG TERM.../Friday through Monday as of 9 PM Monday/ The
persistent area of high pressure over the Gulf is looking to break
down by mid to late week, as a strong low will have enough energy
to override the ridge. Most of our focus for tonight were
adjustments made to Thursday`s system. Saturday`s shortwave has
pretty much become non-existent at this point. But, another
shortwave will pass through the Gulf Sunday into Monday and the
ridge will once again set up over the Gulf.

Overall, the GFS and ECMWF were once again similar in pattern
placement. The GFS has been fairly consistent in amplifying the
low crossing the Gulf, in which the timing has now shifted in time
later on Thursday into Friday. After Friday, model consistency
greatly decreases and used mostly WPC for the long range.

With a focus on Thursday`s system, winds were increased throughout
the inner channels with an earlier shift to southerly winds ahead
of the low on Thursday morning. Then many locations will see a
wind shift to WSW with the main front later in the day, including
gale force winds over the outside waters. Then as the low center
passes south over the area during Friday winds will shift back out
of the north. Friday has the potential to be a strong outflow
event as the wave exits.

Thursday has some precip concerns, as snow could potentially
change to rain. There is the possibility of warning-level snow
amounts for Juneau and Petersburg, but this will be highly dependent
on temperatures and whether any mixing occurs. The minimum
temperature for Thursday night was raised to near 30`s. The GFS
from BUFKIT was showing most precipitation as rain for Thursday
night, however. Most location will see at least some snow, but the
outer coast is most likely to see a change to rain. Stay tuned
to forecast updates on this developing system. A blend was used
to increase POP`s for Thursday and NAM/GFS for QPF.

Confidence has increased somewhat, as models are having better
grasp of this progressive pattern even if some timing differences
still exist.


.AJK Watches/Warnings/Advisories...
MARINE...Small Craft Advisory for PKZ012-041-042.




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