Small Craft Advisory 9, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
419 AM AKST Wed Jan 3 2018

.ANALYSIS AND UPPER LEVELS...

The upper level trough that has been over the Bering for the past
couple days is finally moving eastward into mainland Alaska this
morning. As a result, the entire vertical column of the atmosphere
is gradually cooling and saturating as it approaches. The more
dramatic cooling of the upper levels is increasing instability, as
evidenced by the cumulus field that has developed along the
waterways around Kodiak Island and the Pacific coast of the Alaska
Peninsula. The increased instability and diminishing winds at all
levels of the atmosphere are keeping the moisture that is in
place across Southcentral from going anywhere.

Take the increasing instability, saturated atmospheric column,
and cooling temperatures and combine it with a negatively tilted
shortwave in the atmosphere, whose approach and passage results in
increasing lift and southwest winds coming up Cook Inlet and you
have a recipe for some snowfall potential in Southcentral today.
More on that below.

Southwesterly onshore flow across Bristol Bay is pumping Bering
moisture into Southwest Alaska this morning, creating areas of
localized fog with low ceilings. Temperatures across the area are
slowly coming down, with most locations at or just below freezing.
Temperatures are considerably colder along the Kuskokwim Delta.
Across the Bering, the next strong low and associated occluded
front are moving into the western and Central Aleutians, while the
remainder of the Bering is under showery, westerly flow. The winds
and precipitation will continue increasing through the Aleutians
today.

&&

.MODEL DISCUSSION...

The surface low over the Gulf continues to be well-resolved by the
models, moving into Southcentral this morning and spreading some
light snow to the area. A small area of high pressure over the
Gulf is also well resolved, weakening into a surface ridge
Thursday night as it gets squeezed out between a low over the
eastern Gulf, and a very strong low over the Bering. The eastern
Gulf low will have only minor impacts on Southern Alaska outside
of troughing extending along the north Gulf Coast. The low over
the Bering becomes an increasingly dominant aspect of Southern
Alaska weather going into late week. Unfortunately with that
feature, the models have numerous disagreements, lowering overall
forecast confidence. All the models agree on a series of triple
point lows developing east of the occluded parent low, inevitably
becoming the dominant low later. This process repeats at least
twice over the next couple days. The models disagree on the track
of these new lows and how quickly they take over becoming the
dominant low, however the agreement is improving over previous
forecasts. The NAM is generally the middle- ground model on both
east and west sides and was the model of choice for this forecast
package.

&&

.AVIATION...
PANC...A weak, slow moving low moving into Prince William Sound
this morning is causing numerous rain and snow showers to develop.
Colder air is slowly working in which will gradually change over
the rain to snow. Slower moving upper level forcing now looks to
focus the heaviest snow for this afternoon, when occasionally IFR
conditions are possible. The snow ends this evening with VFR
conditions returning.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Precipitation is expected over much of the area today as an upper
level short-wave pushes through southern Alaska. Many sea level
sites that are receiving rain early this morning will transition
to snow as precipitation rates increase, and then cold air
advection pushes in this afternoon. The surface low will move
into Prince William Sound late this morning, which is a favorable
location for precipitation in Anchorage and the Matanuska valley.
This weather system will push inland later today, and snow will
change to showers and diminish overnight. Conditions will be dry
Thursday and Thursday night as an upper level high builds into
the area. Surface high pressure will build inland, and offshore
flow will increase at the lower levels. Some fog is possible
Thursday night over areas with lighter winds, such as the Copper
River basin and the Anchorage area.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2...Wed and Thu)...
Snow (and some rain) showers intermixed with patchy dense fog
will be the rule for today. Cold air aloft is moving into SW AK
from the west as the remnant low pressure center pulls off to the
northeast. It is bringing some instability along with it. With
abundant moisture lingering across the area, snow showers have
been quite widespread overnight. There will still be a few pockets
of rain showers early (King Salmon to Iliamna), but this should
change to all snow through the day. We are not expecting much at
all in the way of accumulation with the highest amounts being
around an inch. However, some of the heavier showers could produce
a quick burst of snow reducing visibilities briefly. In between
snow showers, the lingering moisture and general onshore flow will
leave the entire area across the Delta and Bristol Bay vulnerable
to fog. When it does move in, it could be dense for a period.
However, we do not expect it to last very long.

For Thu, conditions will be mostly dry with the exception of some
snow showers lingering along the AK Range. The next front will
approach the area from the Eastern Bering as we go through the
day. This will bring a change from the cooler southwest flow to
once again the warm southeast winds (like we saw over the weekend).
Wind gusts will develop first along the coast by late morning
before slowly creeping inland by Thu eve. Precipitation associated
with the front will stay offshore until Fri.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2...Wed and
Thu)...
The Bering Sea will have the most active weather in the state for
this time period. The first in a series of warm fronts is already
knocking on the doorstep of Central Aleutians. Northerly flow
across this area the past few days has allowed for colder air to
settle firmly in place. This will allow the initial precipitation
associated with the front to start as snow. With increasing
easterly winds, there could be a brief period of some blowing
snow. However, the snow will not last long as 850 mb (5k ft)
temperatures surge quickly from -10C to near 0C by this evening.
This will turn the snow over to rain as winds shift out of the
south. A secondary area of low pressure will develop along the
front south of Atka this afternoon. This system will then track
north as well bringing very similar conditions as mentioned above
to Dutch Harbor and eventually the AK Peninsula.

Then on Thu, another stronger storm will move towards the
Central/Eastern Aleutians. This system will deepen all the way
down to near 950 mb as it moves towards the Central Bering early
Fri. As it does so, it will push yet another warm front through
Dutch Harbor and Cold Bay. The biggest impact will be sustained storm-
force winds along the front itself. Moderate to heavy rain is also
expected with this rather warm system for early January.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The main item of interest in the long term will be a pair of
strong low pressure systems moving through the Aleutian chain and
western Bering Sea. The first of which will continue its
northward journey across this region, as a pair of lows rotate
around themselves in a Fujiwhara-like fashion. An attendant
surface front will move through the eastern Aleutians into
portions of southwest Alaska on Friday. Widespread precipitation
and strong gusty winds will develop along and ahead of the front,
with precipitation becoming more showery behind it and near the
areas of low pressure. This front will weaken during the day
Saturday as it moves into South Central Alaska, with eastward-
facing slopes and adjacent valleys seeing the best chance of
precipitation, as the typical southeast downslope flow dries the
air out on the western sides.

Another storm will approach western portions of the Aleutian
chain Sunday night, moving into the western Bering by Monday,
sending another surface front eastward. Similar to its predecessor,
strong wind gusts and widespread precipitation will accompany the
front.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale: 155 165 170 171 172 173 174 175 1776 177 178 179
180 181 185 411 412 413 414.

&&

SYNOPSIS AND MODEL DISCUSSION...JPW
SOUTHCENTRAL ALASKA...BL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MSO
LONG TERM...PD/DEK

SPECIAL WEATHER STATEMENT
National Weather Service ANCHORAGE AK
1159 AM AKST Wed Jan 3 2018

AKZ101-111-040600-
Anchorage-Matanuska Valley-
Including the cities of Anchorage, Eagle River, Indian, Eklutna,
Palmer, Wasilla, Sutton, and Chickaloon
1159 AM AKST Wed Jan 3 2018

...Snow will continue through this evening in Anchorage and the
Matanuska Valley...

Steady snow will continue in Anchorage and the Matanuska Valley
through this evening. While the snow will generally be light, there
will be an uptick in intensity this afternoon through early this
evening. Expect total snow accumulation of 3 to 5 inches, with
localized amounts up to 7 inches by the time snow tapers off
overnight tonight.

Drivers should be prepared for snow covered roads and low
visibilities. Parking lots and neighborhood roads are
particularly slick thanks to the freezing rain which fell this
morning. Leave extra time to get to your destination.