Gale Warning 23, NWS Juneau, AK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
340 PM AKST Wed Jan 10 2018

.SHORT TERM...Main forecast issues through the weekend will be the
timing of the erosion of the arctic airmass currently solidly
entrenched across the Panhandle. Mid and upper level blocking
pattern, which has been responsible for our recent cold air and
outflow, will break down by Friday as a mid level wave and
associated front approach from the SW. Strong and cold surface
high over western BC will slide quickly to the SE as mid level
ridging builds over western North America Thursday night and
Friday.

In the immediate short term, outflow winds are on the decline as
the northern gales over the northern inner channels decrease
through tonight. Exception may be near Skagway where the favorable
position of the surface high may keep the stronger wind gusts
going a bit longer. Elsewhere, expect partly to mostly cloudy
skies with a few scattered light snow showers tonight as a weak
mid level wave lifts north. Cold temps will continue through
Friday.

Attention then turns to a potential snow to rain changeover event
starting Friday and continuing into early Saturday. Deep layer
southerly flow will develop on Friday overrunning the cold arctic
airmass in place. As the front moves north out of the Gulf,
precipitation will start as snow in most places Friday, but with
the surface high over BC weakening and sliding SE, there will be
nothing to hold the cold air in place. Anticyclonic flow aloft and
the initial stable cold air will probably delay the erosion of
the cold air a bit over the interior and northern channels, but
inevitably warm air will win out most places by Saturday morning.
Prior to the changeover, models indicate about 6 to 12 hour period
where forecast soundings show good saturation and lift within the
dendritic zone, which would support moderate snow accumulations.
Confidence is high enough in a period of accumulating snow for a
Winter Storm Watch to be issued in places where temperatures are
expected to remain colder longer. Snowfall amounts will likely be
fine tuned by later shifts but wanted to highlight the areas with
the best potential for significant snowfall.

.LONG TERM... /Sunday through Tuesday/ There will be several
transitions over the long-term period starting off with cold and
wet, then warm and wet, and finally warm and dry heading into
Tuesday. The pattern sets up with a ridge axis to the east of the
panhandle and a low tracking northeastward through the Gulf by
Friday. However, some uncertainty is brought into the mix between
individual model runs due to several wave features that will
propagate in between this synoptic scale pattern.

Minimal changes were made to the pressure field and edits to wind focused
mainly in Gulf and local edits due to wind shifts. Again, GFS was
the outlier in wanting to push these next frontal systems farther
north of other solutions.

There will be a large band of WAA late Thursday into early Friday
that will extend from the Aleutian Islands through the southern
panhandle. As a result, weekend temperatures will be quite warm,
or at least warmer than they have been. With the warming trend,
temperatures will peak on Sunday, before dropping down steadily
again. Maximum temperatures were nudged slightly using an EC
bias-corrected blend. Minimum temperatures were kept warmer based
on cloud cover and current WPC guidance. Even 500 mb temperatures
are showing ranges in the negative teens, which is anomalous for
this time of year.

As far as precipitation, there will be several changeovers from
rain to snow and back to rain during the Saturday timeframe. The
timing of this remains uncertain. Most models were wanting to go
categorical, but we kept POPs at likely at most for Day 5 onward
using a NBM/WPC blend and SREF early on. We are starting to see
increases in QPF amounts, especially during the late Saturday
timeframe.

Overall, there is not much confidence in model agreement. While
the large-scale pattern seems reasonable, confidence is lower for
individual waves in specific timing and tracking.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...Winter Storm Watch from Friday afternoon through late Friday
     night for AKZ019>022-025-026.
     Strong Wind through Thursday afternoon for AKZ018.
     Wind Chill Advisory until 9 AM AKST Thursday for AKZ018.
MARINE...Heavy Freezing Spray Warning for PKZ012.
     Gale Warning for PKZ012-022-031-052.
     Small Craft Advisory for PKZ013-021-032>035-041>043-051.
     Small Craft Advisory due to Seas for PKZ053.

&&

DEL/SS

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