Heavy Freezing Spray Warning 98, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
443 AM AKST Wed Dec 14 2016

.ANALYSIS AND UPPER LEVELS...
A highly amplified pattern aloft exists over the Bering Sea, and
is beginning to encroach onto the mainland. The driver of the
pattern is a large, sprawling, and stacked low pressure system
east of Kamchatka. The cyclonic flow stretches across the whole
Bering and north Pacific. A narrow ridge sits to the east from
Kodiak Island (and south) up into the arctic. The remainder of the
mainland is still under the influence of weak cyclonic flow aloft
for the time being. Between these two features lies the front
associated with the Kamchatka low. The front is essentially the
dividing line between the continental arctic air mass and the
maritime polar air mass.

Sensible weather-wise: the arctic air mass still has a hold on
the eastern mainland with temperatures approaching -30F in the
Copper River Basin. Over in the Cook Inlet region, temperatures
have moderated a little bit with some mid and high level clouds to
keep a little bit of heat in the low levels. The front is pretty
washed out and lacks upper level support. The main low is filling
in the northwest Bering with gale-force winds on the southern
side.

&&

.MODEL DISCUSSION...
Guidance is in good synoptic agreement with larger scale features.
Agreement breaks down slightly when the next disturbance in a
developing progressive flow pushes the Bering trough toward the
mainland. Placement, timing, and strength are not well agreed upon
with models, however all models do have a trough passage on
Thursday evening with weak cold advection.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist. As ridging
builds back overhead tonight, and clouds clear. Patchy fog will be
around Cook Inlet.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Another mostly dry day is on tap for Southcentral as a ridge
builds overhead. A jet streak on the leading edge of the ridge
combined with strong subsidence will lead to an increase in
gap winds along Prince William Sound and the north Gulf coast,
strongest in the Valdez/Thompson Pass area as well as through
the Copper River delta. Mid-level clouds in the Cook Inlet to
Susitna valley corridor will break up some by this afternoon.
As the surface begins to radiate out again later today and
tonight expect some patchy fog to develop. As the upper ridge
slides eastward tonight into Thursday clouds will quickly
increase which should keep fog from becoming more widespread.

Attention then shifts to a short-wave trough moving up from
the North Pacific to Southcentral Thursday through Thursday
night. All indications are this short-wave will head right
for Southcentral and be accompanied by moist south to southwest
flow in the low levels. Thus, this is shaping up to be a
widespread snow event. There is still some uncertainty in the
amplitude of the short-wave and the exact track across
Southcentral, which will affect snow amounts. For now, have gone
with a 2 to 3 inches most places, but there is potential for
higher amounts, particularly along south to west facing slopes.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
The "warm-up" is underway across much of the area with 850mb
temperatures increasing under southeasterly flow, with areas along
the Bristol Bay and Kuskokwim Delta coast observing temperatures up
to or above freezing this morning. Some of this warm air will push
north and slightly east today as the front pushes inland this
morning. The underlying problem with this scenario is that with
this warmer air aloft and some precipitation associated with the
front, there remains a chance for some mixed and freezing
precipitation across parts of the area. Right now, it looks like
the best bet for any freezing rain will be in the colder, deeper
valleys across the Kuskokwim Valley this morning. The warm air
should mix in at the surface with some gusty winds at places like
Bethel and Dillingham, so the threat of freezing precipitation is
much lower there. The pattern will remain quite active with
another, stronger front approaching the coast by late Thu. Gusty
southwest winds will keep temperatures from cooling much in
advance of this next front, so expect a rain/snow mix along the
coast with wet snow falling inland.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
Showery conditions will linger over the western and central
Aleutians/Bering today, then taper off by this evening as the low
over the far western Bering weakens and tracks northward. This
showery regime will be quickly replaced by the next frontal system
tracking into the Western Aleutians tonight. This system will be a
bit stronger than its predecessor as it holds in a favorable spot
along a 160kt jet. The primary concern will be some hurricane-
force gusts wrapping around the system on Thursday. As of right
now, it looks like these stronger gusts will stay west of Adak and
east of Shemya. However, we will continue to monitor the system as
it takes shape to see if a high wind watch would be warranted.

&&

.LONG TERM FORECAST (Days 3 through 7...Fri through Tue)...

A pattern change remains on track for late this week into next
week as a stacked low pressure system becomes stationary over the
western and central Bering Thursday night into Friday. This low
will help to usher a train of fast-moving systems in a strong jet
stream toward the general direction of the southern mainland and
Gulf of Alaska through the remainder of the long-term period. This
will bring a moderating trend to temperatures to all of our area
of responsibility, as well as increased precipitation chances,
especially along the coastlines. Trends suggest there may be
ample snow opportunities inland by early next week as well.

For the Bering, the main impacts will be strong winds given the
decent jet energy with the associated low pressure systems and
their associated fronts.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Hurricane 178.
         Storm 170 173 174 175 176 177 179.
         Gale 155 165 171 172 180 181 185.

&&

SYNOPSIS AND MODEL DISCUSSION...MTL
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...TP
LONG TERM...CC