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
000
FXAK67 PAJK 160015
AFDAJK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
315 PM AKST Fri Dec 15 2017

.SHORT TERM.../Through Sunday/A gale force frontal system will
move into the eastern gulf tonight, then weaken as it moves across
SE AK Sat. Several shortwaves aloft and associated sfc trofs will
move across the gulf Sat into Sun. Kept pressure field as is as it
looked reasonable. Did make some other changes which will be
described below.

Main forecast concerns will be precip potential and amounts, ptype,
winds, and chances for TSTMS. As of 23z...there were some showers
still occurring over mainly the inner channels, but areal
coverage was scattered or less. The coverage may increase slightly
as front approaches, but the main stratiform precip band with the
front will rapidly overtake any showers late this evening into the
overnight hours. Looking like precip amounts will be in the one
half to one inch range with the front, with locally higher
amounts along S facing slopes. Not enough rainfall to cause any
hydrologic concerns for area rivers/streams though. After the
front moves through, precip will become showery again. Shortwaves
moving through with associated sfc trofs Sat-Sun will keep fairly
high POPs over the E gulf and much of SE AK.

Have raised snow levels a bit over the N tonight and Sat. Looking
like enough low level SE flow to keep low level temps in the mid
to upper 30s near sea level tonight into Sat, so have taken snow
mention out of the Haines area itself. Kept the WSW warnings going
for zones 18 and 19 tonight into Sat, but mentioned W of the
Chilkat River bridge for zone 19 and went with above 1000 feet in
zone 18. Thinking 5 to 7 inches near the Canadian border in zone
19, with 6 to 8 inches in zone 18, with highest amounts near White
Pass. There should be enough wind and temps cool enough for
blowing snow near White Pass as well.

Gale force winds are expected ahead of the front over the gulf
tonight. May see winds approach gale force on some of the central
and southern inner channels, with Clarence Strait being most
likely channel to reach gale force. Have strong wind mentioned
along the central and southern coast tonight, and in the Juneau
area late tonight and Sat morning. Winds should decrease after
frontal passage in most areas, but Lynn Canal will see winds
increase Sat afternoon to near gale force. Wind will pick up some
again over much of the gulf as several sfc trofs move across the
area, but they should not get above SCA levels Sat-Sun.

Models show 500 mb temps falling to the -36 to -38C range Sat into
Sun, and with several shortwaves moving across the area, think
some TSTMS may occur, especially over the NW half of the E gulf.
Have them staying over the gulf for now, but it is possible the
immediate outer coast from PASI NW could see some as well during
this period if boundary layer temps are warm enough along the
coast.

.LONG TERM.../Sunday night through Thursday/ As of 10pm Thursday
Main highlight of the long term period is the potential for a
break, albeit short, in the seemingly endless progression of warm
and wet frontal systems which have made the first half of December
feel more like early fall than meteorological winter lately.

Prior to this above-mentioned break, cool upper trough will drop
into the northern Gulf on Sunday with onshore flow and a showery
regime continuing into at least the first half of Monday. Low
levels will cool as chilly air from the Bering Sea gets advected
around the trough. This air will be cool but not cold as it will
likely be modified by a moderate trajectory over the Gulf. May
just get cool enough over the northern inner channels for the
showers to be in the form of snow with a mix down towards the
Juneau area. Due to the showery nature of the precip and
borderline temperatures, not expecting significant accumulations
at this time.

The trough will drop quickly southeast on Monday as yet another
strong upper ridge re-asserts itself over the Gulf. The difference
between this ridge and the one that has parked itself over the
west coast of North America over the past 2 weeks is that the mean
ridge position looks to set up more over the central Gulf. This
will provide a period of offshore flow starting Monday night and
lasting through Tuesday night. This will likely promote clearing
skies and diminishing precip Monday night with the break in the
precip lasting until at least early Wednesday. Operational models
then disagree on the amount of Pacific energy that is able to make
it over the ridge and into the Panhandle for mid week while
ensembles indicate more ridging maintaining or re-asserting itself
by late week.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...Strong Wind from late tonight through Saturday morning for
     AKZ023-027.
     Strong Wind from late tonight through Saturday morning for
     AKZ025.
     Winter Storm Warning from 9 PM this evening to 9 AM AKST
     Saturday for AKZ019.
     Winter Storm Warning from midnight tonight to 3 PM AKST Saturday
     for AKZ018.
     Strong Wind Saturday afternoon for AKZ018.
MARINE...Gale Warning for PKZ036-041>043-051-052.
     Small Craft Advisory for PKZ011>013-021-022-031>035-053.

&&

$$

RWT/DEL

Visit us at http://www.weather.gov/Juneau



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