Area Forecast Discussion
Issued by NWS Anchorage, AK

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FXAK68 PAFC 120236 CCA
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion...CORRECTED
National Weather Service Anchorage AK
536 PM AKST Tue Dec 11 2018

.ANALYSIS AND UPPER LEVELS...
A persistent, negatively tilted long wave trough encompasses the
Bering and is pushing into the Gulf. Northerly flow persists over
much of southwest Alaska and the Bering as cold, Arctic air is
being advected behind the trough. A weak low has formed over the
western Aleutians. Weather in the west is fairly benign with only
light precipitation falling over the western mainland and weak
winds through the Bering. Within the flow around the trough, a
shortwave extends over the eastern Aleutians with a North Pacific
low anchored at the base. This low is well south of the Aleutian
chain and is tracking eastward. A second short wave is spread over
the northern Gulf which is driving much of the weather for the
area. A strong band of increased vorticity is extending from
Kodiak Island through the Gulf, coinciding with the shortwave.
Throughout Southcentral, winter weather has returned. At the
surface, a closed low pressure system sits over eastern Prince
William Sound which is causing strong advection of cold air over
Southcentral and the western Gulf. Winds are increasing and
southwesterly flow is dominant through the Cook Inlet and western
Gulf. A surface shortwave over the northern Gulf coast is aiding
precipitation formation in Southcentral mainly in the form of snow
showers.

&&

.MODEL DISCUSSION...
The numerical models are not in very good agreement and are
showing poor run-to-run consistency. However, when it comes to
the sensible weather, they come to a similar conclusion on many of
the important features. The low pressure in Prince William Sound
has decent agreement as it remains nearly stationary through
Wednesday morning. As the North Pacific low traverses eastward,
models quickly diverge. As it reaches the southern Gulf on
Wednesday morning, the ECMWF is a far faster solution while the
GFS and NAM lag behind. The GFS is the strongest of the solutions
when it comes to wind speeds while the NAM is weaker. A blend was
used to find a middle ground. Models return to a similar track on
Thursday as this low moves into the eastern Gulf.

In the west, models again are out of sync. Starting early
Wednesday morning, the once closed low over the western Aleutians
breaks down and becomes generalized low pressure over the area.
Models, while they agree on this pattern, do not have a consensus
on the organization of the multiple lows. On Thursday, as a low
originating in Norton Sound moves into the eastern Bering, models
are in good agreement.

&&

.AVIATION...

PANC...MVFR to IFR conditions in snow are expected through the TAF
period. A few VFR breaks are possible, but these will only last a
few hours at most. Brief gusty south winds are possible this
evening, but in general winds will be light.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2...
Wednesday and Thursday)...

The pattern will remain favorable for continued light snow. Snow
across Southcentral today will likely continue throughout this
evening until Thursday morning and evening as the surface low
remains in place. The intensity may ebb and flow, but it should
remain more or less continuous. As a result, POPs were increased
from the morning forecast package from the Kenai Peninsula through
Prince William Sound, to include the Anchorage Bowl. While light
snow will be persistent, low QPF will limit accumulations.
Generally a few inches are expected over the next 36 hours.

Below freezing temperatures stretch across the region and will
continue to sit below freezing through the remainder of the week,
as a low over the northern Bering pushes cold air advection across
Southcentral. Most places will see relatively calm winds. However,
places along the western Kenai Peninsula will continue to see
gusty conditions. The wind combined with the cold will make for
seasonable wind chills. Some blowing snow along the Homer Bluff is
possible through the evening hours, however, visibilities should
remain above advisory criteria.

Uncertainty remains in today`s forecast with the low south of the
Chain that will move into the eastern Gulf for Wednesday
afternoon. Models continue to struggle with the placement and
intensity of this low, which in turn will impact the wind speed
and direction across the Gulf into the north Gulf coast. Thus,
little changes were made to this part of the forecast today.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3/Tuesday
night through Friday)...
Low pressure to the north, centered in Norton Sound will be the
weather influencer for the remainder of the work week. It is
expected to continue to wrap cold air into southwest AK behind it,
bringing the coldest temperatures so far this winter to the area.

The big question here will be onshore flow also induced by wind
direction. So, cloud cover will likely prevent temperatures from
falling too much, especially along the coast. However, how far
inland the cloud cover will push will be the main factor in the
forecast. Right now, thinking cloud cover will push inland except
for eastern areas of zones 161 and 152, so have the coldest temps
forecast in those areas. If cloud cover does clear and winds
drop, temperatures will drop quick, so this bears watching the
next couple of days.

The low will then push south into Bristol Bay Thursday, changing
the wind direction and creating offshore flow. This will almost
certainly clear out the cloud cover for southwest AK. Meanwhile,
high pressure will build over the interior, so high confidence
that cold temps from north of the Alaska Range will be working
their way into southwest AK later this week.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through
3/Tuesday night through Friday)...

Northwest flow is the name of the game for the next few days. This
means cooling temperatures and cold weather cumulus with snow
showers will persist over much of the area. While the broad
scale conditions are predictable, timing out snow showers at
airfields throughout the Bering has made some interesting
aviation forecasting. Snow showers have been intense and fast
moving, quickly reducing visibility in snow. Winds are also gusty
with these showers as they are able to mix down winds aloft. This
type of weather can be expected to continue.

This pattern is resultant from a low pressure system generally
located in Norton Sound, wrapping cold air all the way from the
arctic behind it. In fact, this airmass originated in the high
arctic. Cold air is being funneled southwest around a high
pressure system in the Arctic Ocean and along the north Slope.
This air then enters the influence of the low and pushes south
through the Chukchi Sea, through the Bering Strait and all the way
into the Bering Sea. Quite the journey and this shows how
interconnected our weather is through the whole of the upper
latitudes. This low pressure will meander south along the coast
the next couple of days, but will maintain its influence through
Friday.

Outside of this cold air push, the Aleutians will see a couple of
compact lows push west to east across the chain tomorrow. This
will bring some spotty precipitation and gusty winds to the chain.

&&

.LONG TERM FORECAST (Days 4 through 7)...

The forecast period commences with the longwave trough extending
from Western Alaska to the Gulf Of Alaska, meanwhile the ridge
will be over the Central Aleutians. On the 15th of December at 12z
the GFS at 500 mb, has a closed low near the Brooks Range,
another in the Gulf Of Alaska and a third one southwest of Attu
Island. The ECMWF is pretty similar although the feature south of
the Western most Aleutians is depicted as more of an open wave.
Each upper level disturbance has multiple shortwaves embedded
with the cyclonic flow. At the surface, the GFS and ECMWF have
rendered some pretty significant differences with the placement of
the surface lows early in the forecast period. Generally
speaking, expecting a progressive pattern this forecast period.
However, given the vagaries of the models hedged towards NCEP WPC
guidance for the pops.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gales...119 120 130 131 132 138 139 141 150 155 165 170
 177 178 179.
 Heavy Freezing Spray...185.

&&

$$

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


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