Small Craft Advisory 1034, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
401 PM AKST Tue Dec 25 2018

.ANALYSIS AND UPPER LEVELS...
A stacked low south of the AKPEN has brought light snow, rain,
and a mix to some areas of the western Aleutians and Southwest
mainland earlier today. A surge of moisture along the frontal
boundary of a low in the eastern Gulf has lifted northward,
bringing wet conditions around the coastal regions of
Southcentral this afternoon. South of the western Aleutians sits a
third low, which has brought primarily snow showers and gusty
conditions to Shemya. Quiet conditions remain out west north of
the Aleutian Chain as high pressure sits over the Bering.

&&

.MODEL DISCUSSION...
The main difference in the models across the Gulf and Southcentral
concerns a frontal wave developing under a left exit jet position
that heads north into the western Gulf Wednesday night. The GFS
continues to be the further west position and the ECMWF the
eastern. The NAM and Canadian NH/Regional are in general agreement
and a good middle ground. So will be leaning toward the
NAM/Canadian for the afternoon package through Friday afternoon.
Out west, the main concern there will be the position of the front
approaching the western Aleutians Friday. For now will go with a
NAM/GFS solution.

&&

.AVIATION...
PANC...Generally VFR conditions and light winds should prevail
through tonight.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The front along the north Gulf coast and Kenai Peninsula will
weaken tonight, with little movement. On Wednesday this system
will move north through the Cook Inlet area and into the Susitna
valley. Some snow showers are possible with this, but PoPs are
low as the system will have limited dynamics by this time. On
Wednesday night another system will develop in the southwest Gulf
and then rapidly move northward through Thursday. The exact track
of the surface low is still up for debate, but there is some
confidence that this will be a fairly strong system with gale
force winds in many of the marine areas, and a decent amount of
precipitation along the coast and coastal mountains. There is a
reasonable chance of some precipitation making it inland as well
on Thursday afternoon. Precipitation will likely be a mixture of
rain and snow along the coast, with all snow expected for inland
areas north of the central Kenai Peninsula. There is still some
chance the low will drift further west, which could introduce
mixed precipitation...but for now that looks unlikely.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2/Tuesday night
through Thursday night)...
The anticipated push of moisture across southwest Alaska is currently
moving through Dillingham as well as areas inland and is approaching
Bethel. The RADAR at King Salmon and surface observations are indicating
that precipitation is falling as areas of snow. Satellite is also
indicating a possible area of enhanced precipitation due to upsloping
over the Kuskokwim Mountains. This was anticipated in the forecast,
but we cannot be certain how much is falling due to a lack of
observations in the region.

The upper level, closed low currently spinning over the Alaska
Peninsula is expected to diminish to a wave and push north
tomorrow afternoon as it gets flushed into the northeasterly flow
over the area. This wave will produce an area of snowfall in the
lower levels along the eastern Alaska Range, mainly north of
Iliamna. About 5 inches of snow are expected in areas receiving
the most precipitation, so an advisory will not be needed. It is a
fairly remote area as well, so just something to keep an eye on
with the main impact being to aviation forecasting.

Lastly, some colder temperatures are still hanging on in some
inland areas, mainly in low lying areas like river valleys. It got
down to -21F at Sleetmute last night, where remnants of the cold
airmass was still stubbornly entrenched. The cold air will try
and hang on tonight, so went for minimum temps in the minus single
digits, since there will be some warming as these pockets of cold
air are degraded from the weak occluded boundary moving through
the area. The push of moisture from the south tomorrow should
finally kick the last of this air out. However, this will be short
lived and last only a few days.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2/Tuesday
night through Thursday night)...
Remnants of the low that pushed into the eastern Aleutians
yesterday continues to spin over the Alaska Peninsula. The
backside of this low is producing low cloud cover, snow showers
and gusty winds for locations in the eastern Aleutians. It will
continue to do so for the next days or so, until the low lifts to
the northeast.

Widespread cloud cover persists over much of the Bering Sea
today. It is present in the form of a stratus deck that is being
fed by northerly flow from the Arctic. The northerly direction of
flow is expected to continue at least through Thursday, so cloud
cover will remain in place for the next couple of days. In
addition, a low currently south of the western Aleutians will work
its way east, staying south of the Aleutian Chain through
Thursday and pulling cold air in on its backside, which will
enhance this northerly flow. This low will produce gale force
winds across the central Aleutians tomorrow into tomorrow night,
after which it weakens as it goes.

&&

.LONG TERM FORECAST (Days 3 through 7...Fri Dec 28 through Tue Jan
1)...
The extended forecast still features a very active weather pattern
across the entire area. Models continue to advertise a fast-moving
SW-NE storm track from the North Pacific into SW AK. The major
shift from yesterday is that the ECMWF has backed off on how
strong it builds the ridge over the ALCAN Border by early next
week. Yesterday, the EC and GEM (Canadian) were advertising a
strong high-amplitude ridge building along the ALCAN Border. This
would have set up a very meridional pattern going in to next week.
However, the EC has now trended towards the GFS which would
yield a colder forecast with several modest storms moving quickly
through the pattern.

With that much uncertainty between models and even from run to run
within a model, the forecast for next week is highly uncertain at
this time. The few things we can lock onto though are late this
week and into the weekend. The deep, cold trough digging down from
the Bering Strait into the North Pacific will be sure to push some
snow showers over the Alaska Peninsula. These will favor the north
side of the chain. For the marine zones south of the peninsula,
expect gusty winds through bays and passes into very early Sat
morning. There is also high certainty with a deep (possible
mid-940 mb range) low developing over the Western Aleutians Sat
morning. The associated front will bring a healthy area of gale to
storm force south to southeast winds. It will also push a good
amount of warm air up into the chain. So while precipitation could
start as heavy snow, it will quickly transition into heavy rain.

Along this front will be the primary forecast challenge. There
will be triple point lows forming along this occluded boundary.
But trying to nail down exact timing, placement, or strength of
these lows is a fool's errand at this time. Each one of these
systems will have the potential to bring a surge of warm air into
the mainland with it. However, we will not start investigating any
specific system until the models have come into better agreement
regarding the upper levels (ALCAN ridge) first.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 130, 139, 173, 174, 175, 176
 Heavy Freezing Spray 180, 181, 185

&&

SYNOPSIS AND MODEL DISCUSSION...AH/BC
SOUTHCENTRAL ALASKA...BL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...BB
LONG TERM...MO