Small Craft Advisory 588, NWS Juneau, AK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
1005 PM AKST Wed Nov 22 2017

.UPDATE...Precipitation type has again given us issues from Sitka
to Wrangell to Petersburg. Temperatures are way colder than
guidance for this region but guidance is indicating some warmer
air aloft that potentially could give us fits with freezing
liquid. However, calls to those communities including Kake reveal
none have reported a glaze tonight, just periodic wet snow, or
rain in the case of Sitka. Aside from the south central Panhandle,
we also made a quick early update to re- bottom temperatures a
bit lower given clearing skies near Haines and Skagway.

A new system will approach us on Thanksgiving Day. Initial lift
across the cold air dome will efficiently give us light snow
across much of the northern Panhandle, light rain for the south,
and a potential mix for the south-central areas as many feast with
friends, neighbors, and friends. We think snowfall amounts could
be around a couple of inches for much of the north-central
Panhandle including Juneau and Gustavus with lighter amounts due
to warming to the south and due to drier air to the north.

Thursday night, uncertainty rises among guidance solutions with
the amount of precipitation. Currently we favor the GFS and ECMWF
solutions, as both unite in positioning the low. Meanwhile, the
NAM seems to march too quickly toward shore. But of greatest
impact, is the NAM's enhancement of precipitation around Juneau
during frontal passage late Thursday night (wee hours Friday
morning). While we acknowledge this could occur and therefore
necessitate greater snow amounts in our forecast than our 3
inches, many variables like warming and the angle of frontal
advancement--as it aligns almost east-west at that time--may
inhibit both QPF as well as the fluffy factor (ie, how dry the
snow is and how much it compacts, or does not, when it
accumulates). For these reasons, we feel the lighter QPF of the
ECMWF and GFS equating to around 3 inches of wetter snow
particularly toward morning is our highest confidence forecast at
this time. Please stay tuned. We have issued a special weather
statement to alert the public for this uncertainty, as confidence
is not there for a winter storm watch. We will continue to
monitor trends that can hopefully supply more answers rather than
raise more questions.

&&

.PREV DISCUSSION......ISSUED AT ...226 PM AKST WEDNESDAY

SHORT TERM.../Through Thursday night/ A weakening occluded front
over the central panhandle will dissipate by late tonight. Another
occluded front will move E into the central gulf Thu, then move NE
across the outer coast of SE AK Thu night. Other than some early
adjustments for this evening, did not make much change to the
overall forecast through Thu night.

One adjustment was to cancel the WWA for zone 25. The snow with
the occluded front has been on the meager side so far. While some
snow is still expected around the Juneau area this evening,
amounts will be less than 2 inches. Elsewhere, allowing the WWA
for zone 26 to expire at 4 PM this afternoon, as most of the
freezing rain threat should have ended by then.

Otherwise, precip should move out of the area tonight, but more
will move in for Thu and Thu night as next occluded front moves
in. There is also a shortwave trof preceding the front that will
bring precip to the area as well. Looking like it will be mostly
snow over the N third of the area, rain and snow over the central
third...and mainly rain over the S third. At this point, best
threat for WWA headlines will be in the N third of the area, but
confidence is not high enough to issue any attm. Currently keeping
snowfall amounts below advy levels for the N Thu and Thu night.

Northerly winds will pick up a bit over the N inner channels early
tonight, then drop off late tonight and Thu morning. They should
increase later The afternoon and evening. Looking like SCA level
winds at most for these areas. Also, SCA to gale force winds are
likely over the gulf Thu into Thu night ahead of the next occluded
front. Some of these stronger winds will get to the S inner
channels Thu night...with SCA level SE winds likely by then...as
occluded front moves onshore.

LONG TERM /Friday through Wednesday/ A weather front lying
across the Panhandle will support widespread precipitation Friday.
Temperatures across the central Panhandle will be marginally cold
enough for snow but confidence is only moderate. Even as warm air
lifts northward along the front, we think wet- bulb temperatures
should be lower than freezing. But given frontal orientation, we
are not considerate significant accumulations. Confidence is
higher for rain in the south. Very little changes were made in
winds during this time, as agreement continues to remain strong,
even as precipitation type is not as sure.

As the low pulls south, we are increasingly confident in a dry,
colder forecast. We lowered temperatures yet again for Saturday
and Sunday as outflow revs up again through the northern and
central Inner Channels. While currently we are only forecasting
gale force winds for northern Lynn Canal, it is conceivable these
may need to be expanded in coverage. We held back some of the
coldest air from some solutions as sky cover may continue to be
mainly cloudy due to a low to our southeast.

Leading into next week, we see some form of deeper onshore flow
returning along with a moderating temperature trend. This will
reintroduce precipitation type issues, but it is safe to say the
southern Panhandle and east and southeast gulf coast will return
to rain into the middle of the week while we forecasters continue
to debate and chase the ever impactful rain/snow line. As most
American students (and seasoned forecasters) will tell you, "When
in doubt, guess 'C.'" Our "C" stands for Wrangell to Peril
Strait, and this supplies a reasonable first guess. Forecast
during this time period continues to more strongly rely on
ensembles as evolution back to a wetter pattern highlights the
creative differences of our solutions. For all of these reasons,
forecast confidence drops next week.

RWT/JWA

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...None.
MARINE...Small Craft Advisory for PKZ012-013.
     Small Craft Advisory due to Seas for PKZ041>043.

&&

JWA

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