Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 120459
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
759 PM AKST Tue Feb 11 2020

.ANALYSIS AND UPPER LEVELS...

A large upper low is over the western Bering and Western Aleutians
with a front that extends southeast to forming upper level low
near Adak. This supports the strong surface low west of Shemya and
the surface low southwest of Unalaska. The front from the low off
Unalaska extends southeast to a triple point in the Gulf of
Alaska. The moisture advection into this storm extends south to
25N and the total precipitable water is greater than 200% of
normal. Weak low pressure is along the North Gulf Coast that is
moving east as it dissipates and high pressure is over the
mainland.

&&

.MODEL DISCUSSION...

The models are in good agreement over the short term, then
continue to struggle in the long term. Ample moisture is showing
up over Kodiak Island and along the North Gulf Coast and 850
temperatures continue to support snow for the coastal regions.
Kodiak Island is the big question, as the models have the southern
third of the Island warming to rain while the remainder of the
Island stays snow or a mix of rain and snow. Snow amounts range
from5 to 12 inches depending on the model used.

In the longer range the latest model runs have converged on a
strong low in the central Bering Sea, but have about 10 mb
differences in the intensity of the storm. There is also
significant differences as we approach the end of the week with a
low or trough south of the Aleutians.

The models all agree on the arctic air over the interior through
the latter part of the week.

&&

.AVIATION...

PANC...VFR conditions are expected into tonight then we will
likely see lowering ceilings and visibilities as snow should move
over the area toward the morning around 15Z. The lower conditions
should persist until around 21Z, but may last until 00Z if the low
slows and stalls south of Prince William Sound, and if the low
tracks further north. At this point, it looks like the low should
stay far enough south to limit the snow and end it tomorrow
afternoon.

&&

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

The main forecast challenge is for the Kodiak Island areas with
the front moving into the area tonight into tomorrow morning.
While the main low remains south of Kodiak as it drifts eastward,
the front will move just up to the eastern coast of Kodiak and
have a triple-point low develop there overnight. This correlates
with the front occluding and is the location where the warmer air
is cut-off from progressing westward. It also puts Kodiak right on
the dividing line of the snow-rain line. A winter weather advisory
has been issued for Kodiak Island overnight through tomorrow
morning for snowfall up to 10 inches and strong winds reducing
visibility to around one-half mile in blowing snow. There is a
good chance for a period of rain from Kodiak City southward in the
early morning hours, if it occurs, it should be brief. Warmer air
will, of course, limit the snowfall amounts, while just slightly
colder air could increase that which is in the forecast, so this
is a situation that should be monitored for the Kodiak region.

Elsewhere, it looks like a better chance for snowfall over the
Kenai Peninsula northward to Anchorage tonight into tomorrow. The
Copper river Basin will also see a better chance for snowfall and
could see some areas with close-to-advisory snow accumulation levels
there tomorrow.

Expect a trend toward colder conditions with diminished cloud
cover Thursday as the low exits the Gulf. Offshore will develop;
expect increased out of the usual gaps and passes.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 2)...

A low moving in from the North Pacific this evening and tonight
will bring snow and blowing snow to the Alaska Peninsula and
parts of interior Bristol Bay, including King Salmon and
Dillingham. The low quickly exits into the Gulf of Alaska
Wednesday morning, with the winds turning to more northwesterly,
which will quickly result in drying and colder temperatures and
clear skies for Wednesday night. WInd chills tonight will once
again drop across the Kuskokwim Valley as northeast winds
increase. However, temps will not be as cold as recent events with
wind chill values expected to not drop into advisory level
criteria which is -40 F (we are expecting them to not drop below
-35 F).

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 2)...

A front stretching from the western Bering Sea to the North
Pacific is bringing a mix of rain and snow to the entire Aleutian
Chain, especially from Dutch Harbor to the far western Alaska
Peninsula. The front will then strengthen into a low as it moves
into the Bristol Bay, bringing mostly snow as well as some blowing
snow as northeast winds increase. The low exits Wednesday morning
as gusty northwest winds increase which will also become quite
gusty through favored gaps in the AKPEN and through the Eastern
Aleutians.

Attention quickly turns to a rapidly developing bomb cyclone which
moves into the Western Bering Sea and Aleutians Tue night and
Wednesday before wrapping up into a strong occluded low on
Thursday. This low will reach high end storm force for sustained
winds across the Central Aleutians with strong south/southwest
winds lasting all the way through Friday. Some gusts will likely
reach hurricane force across Atka and adak, but they will not be
frequent or much above high wind criteria (74 MPH).


&&

.MARINE (Days 3 through 5)...

Bering/Aleutians: A gale force low in the central Bering will
weaken Friday into Saturday. Another strong low will approach the
western Aleutians late Sunday, likely bringing another round of
gales.

Gulf of Alaska: Winds will generally remain below gale force
through Sunday. However, model agreement is poor so confidence in
the forecast is below normal.

&&

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

Beginning the extended period forecast on Thursday afternoon,
there will be a large and deep upper level cyclone with a stacked
surface low over the Western Aleutians with the occluded front
moving into the eastern Bering Sea. Cold northwest offshore flow
will be keeping the mainland dry at this time. As the front shifts
east, the zonal flow will result in a developing triple point low
around Kodiak Island. Snow will move into the AKPEN and parts of
interior Bristol Bay, but this will not make much northward
progress as the arctic high will be in place, so much of the
energy will move into the Gulf of Alaska and fall apart as it
moves east Friday. This pattern means most of Southern Alaska will
remain drier and colder, although Southwest Ak will be open to
brief bouts of clouds and snow through the weekend as the Alaska
Range will keep Southcentral much drier given the fast westerly
flow aloft.

The upper level flow re-amplifies sometime Sunday night, and this
will likely bring in more stormy weather for the Southern
Mainland, with a myriad of impacts including inland snow and
stormy seas. It looks to get colder thereafter as another arctic
trough digs in.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...WSW 171 and 125.
MARINE...Storms 174-177 411 413. Gales 150 155 160 170 172 173 178 179 185
                                       351 352 412 414.
FIRE WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...SA
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JA/LB
MARINE/LONG TERM...MM



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