Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 100126
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
426 PM AKST Sun Dec 9 2018

.ANALYSIS AND UPPER LEVELS...
A large upper level trough is currently encompassing the Bering
Sea. Just south of this trough, a jet streak with max values near
170 knots is generating a wide swath of zonal flow at the upper
levels across the North Pacific. Over the last few days, this
North Pacific jet has generated multiple lows along the left exit
region, leaving a trail of cyclonic flow over the central
Aleutians.

At the surface, cold air advection persists across Southcentral
ahead of an inbound warm occluded front currently draped across
the AKPEN. This front is associated with a 964 mb parent low
centered near Saint Paul Island. In response, snow showers are
observed on radar this afternoon across the northern AKPEN while a
few residual showers are also spreading in along the north Gulf
coast.

&&

.MODEL DISCUSSION...
The numerical models are in good agreement on the synoptic
pattern. However, there are some differences in the timing and
strength of various individual features. This lowers the
confidence mainly for precipitation probabilities, where
widespread "chance" Pops are common over the southern mainland.

&&

.AVIATION...
PANC...VFR conditions and light winds will continue. Some showers
are possible beginning around noon Monday, but probabilities are
low enough that VCSH was used.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)...

Long story short, a wintery pattern is in store for Alaska as we
finally have driven the Pacific jet stream to the south, allowing
for colder moderated arctic air to infiltrate Southcentral. A
series of fronts and lows will move up along the coast beginning
tonight, with each low bringing coastal rain and snow and
increasing inland snow threats. The first gale force front tonight
will move along the coast then stall, with gale force winds moving
into Prince William Sound. There will be a few isolated snow
showers that will move inland, but most of the interior will
remain downsloped.

The next low will move toward the coast Monday night and Tuesday
and eventually move toward the vicinity or inside of Prince
William Sound. There is a lot of uncertainty at this juncture,
with Gulf moisture in place, cold air infiltrating across the
Alaska Range in conjunction with an arctic trough, and low level
instability, this pattern will become increasingly favorable for
snow threats inland. These chances increase Tuesday and will
continue through Wednesday.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3/Sunday
night through Wednesday)...
The warm front associated with the elongated low in the Bering
Sea is now pushing inland. As expected, it is weakening as it
goes. It is producing some areas of light snow, mainly along the
coast. Dillingham and King Salmon are seeing the "heaviest" snow
out of this system, though RADAR and surface observations show
even this snow is still light.

The current low supporting this weak front moves southeast
tonight, causing this front to really fall apart. However, models
have honed in on a second low spinning up and heading into Bristol
Bay early tomorrow morning. Both the GFS and the NAM are in
agreement on this compact feature, so decided to put it in the
forecast. There is a second push of snow forecasted to move
through zone 161, which right now looks like it will provide up to
another 3 inches of snow along the coast. Up to 5 inches are
possible in the Kuskokwim Mountains due to enhanced lift there.

After this second push, the broad low supporting these smaller
systems will move east Tuesday morning to be located over Southcentral
AK. This will induce northwesterly flow over southwest AK and
establish a cooling trend through the middle of this week.
Temperatures will actually get legitimately cold, with lows below
zero inland and in the single digits along the coast.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3/Sunday
night through Wednesday)...
The low in the Bering that has been the subject of much discussion
is still slowly working its way east, with its elongated warm
front now pushing into southwest AK. GOES satellite imagery shows
that several smaller circulations within this low have indeed
spun up, as models indicated. There are at least four different
smaller low centers currently viewable on satellite. These small
lows are creating a forecast challenge to predict specific
locations and timing of precipitation. Areas of heavier snow
showers are embedded within this system, reducing visibility when
they do strike a location with an observation site. This is making
our TAFs a challenge in the Bering today.

This low will continue to push east through Tuesday, with smaller
circulations continuing to spin within it. Right now models are
locking onto these smaller lows about a day out it seems, so there
is some predictability of areas of enhanced precipitation. Wide
spread areas of gale force winds are also still present with this
low and will continue to be present as the low pushes east, which
is a bit stronger than initially expected. The low completely
exits Tuesday morning, with northwest flow and high pressure
building in behind it. This will creating a cool trend with likely
some convective snow showers in the Bering later this week.

&&

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

The general trends for the long range forecast is for the most active
weather to continue across area waters with temperatures trending
colder Wednesday night through the weekend. A long wave trough
Wednesday night is projected to have a parent low just south of
the Bering Strait and another low over the southeastern Gulf of
Alaska. Models are generally in good agreement with the closed
center over the Bering Strait remaining the dominant system. They
are struggling with the northward progression and organization of
the Gulf and system and the progressive wave pattern over the
Bering. The main changes for the long range forecast were to
decrease precipitation chances across the southern mainland and to
cool temperatures through the second half of the week.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gales...119 120 130 131 132 138 139 150 155 160 165 179
 180.
 Heavy Freezing Spray...185.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...KO/BL
SOUTHCENTRAL ALASKA...JA
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...BB
LONG TERM...KH



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