Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 191405
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
505 AM AKST Wed Feb 19 2020

.ANALYSIS AND UPPER LEVELS...

A 980 mb vertically stacked low in the Gulf is lifting northward
over Prince William Sound. Ample moisture continues to spread in
over Southcentral. Satellite and radar imagery show convective
signatures, indicating cold air has wrapped around the backside of
this low and it lifts inland over Southcentral. Out west,
another low centered south of the western Aleutians is being
steered eastward by a 160 knot upper level jet. This upper level
jet is forcing the leading edge of the front to elongate as it
moves eastward.

&&

.MODEL DISCUSSION...

Model solutions are in good synoptic agreement through Friday.
There are some differences regarding strength and timing of the
low in the Bering as it moves eastward this week. While all models
seem to agree the surface low will arrive over Bristol Bay Wed
night, the NAM/EC shows a faster solution of the low moving inland while
the GFS/Canadian keeps the center slightly farther west.

&&

.AVIATION...

PANC...IFR conditions in snow are expected into the early morning
hours Wednesday. Conditions should improve to MVFR during the
late morning, and then to VFR in the early afternoon as the
snowfall diminishes then ends. LLWS will become a concern again by
early Thursday morning as midlevel southeasterly flow increases.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2...Wed and
Thu)...
One storm system will move out and another one will immediately
replace it. Snow has continued overnight from the Kenai Peninsula
up through the Susitna Valley along a deformation boundary.
Meanwhile a closed low is moving up into western Prince William
Sound and pushing more moisture and instability over the Chugach
and Kenai Mountains. This low pushed some warmer air into parts
of the Prince William Sound and Mat Valley allowing for a brief
period of rain this morning. However, we are now expecting a
colder airmass to surge up the Cook Inlet and turn all of the
remaining precipitation back to snow this morning. This will be
the parting shot of snow for much of the area as this system winds
down. A suite of Winter Weather products remain in effect this
morning and we have extended a few into the afternoon to account
for this last burst of accumulation.

Another strong system will move towards the area tonight.
The most notable differences will be antecedent cold air and
strong downsloping for much of the Anchorage Bowl through the Mat
Valley. The cold air that settles in today should create some
blowing snow issues by late tonight in parts of the Portage Valley
and Turnagain Pass area as snow starts to fall. Another couple of
feet of snow will be possible for the higher terrain over Western
Prince William Sound with this system. Blowing snow will also be
a concern once again through Thompson Pass as heavy snow develops.
Southeast winds will again crank up across the Turnagain Arm.
However, due to the track of the low, the gradient should remain
strong enough down Cook Inlet to keep the strongest gusts away
from Anchorage and the Hillside. These winds and cross barrier
flow will also keep the area mostly dry initially. Moderate to
heavy snow is expected for the Western Susitna Valley once again,
but most of that looks to remain west of the Parks Highway.

As the low passes over the area, winds will die down and all
locations will become vulnerable to snow by late Thu into Fri.
The system will drift slowly off to the east and pull down another
blast of Arctic Air. This will create very gusty westerly winds
across portions of Shelikof Strait and eventually Southern Cook
Inlet. It is also a favorable pattern for long duration light snow
over many of the population centers.

&&

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

Southwesterly onshore flow and lingering snow showers continue
across Southwest Alaska this morning, which will taper off by late
morning. The next low approaches the region this afternoon, which
will help gusty winds and a shot of snow return to the greater
Bristol Bay area. A Winter Weather Advisory has been issued for
blowing snow, mainly to areas north of King Salmon along the
coast. The greatest threat is to Dillingham and surrounding areas,
which could see visibility reduced to a half mile and a few
inches of new snow accumulation. The track of this low should
keep snow out of the Kuskokwim Delta, but will track northward
into the Kuskokwim Valley overnight and into Thursday morning.
Snow showers will continue over this region and over Bristol Bay
much of the day Thursday and even into Friday.

Temperatures will be on the downward trend over the next few days
as Arctic air moves into Southwest Alaska starting on Thursday.
This will bring temperatures below zero for the Kuskokwim Delta by
Thursday night, and temperatures into the teens for the rest of
Southwest by Friday.

&&

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

A strong gale-force low positioned near Adak this morning will be
the key weather maker across the Bering and Aleutians for the next
few days. As this low moves into Bristol Bay by Thursday morning,
winds switch to a north to northwest flow as strong cold air
advection quickly follows. There is better model agreement that
this low will bring strong winds along the eastern Aleutians and
Alaska Peninsula on Thursday, especially through gap areas.
Expect widespread snow showers and heavy freezing spray to persist
over the eastern Bering and Aleutians through this time. This
pattern could also bring potential blowing snow conditions for
Dutch Harbor and along the AKPEN for much of the day Thursday
before winds taper down Friday as the low exits the region.

By Friday morning, models are trying to spin up a compact low
over the western Aleutians, which could bring an area of small
craft winds. However, there is still a lot of uncertainty in this
feature.

&&

.MARINE (Days 3 through 5 Fri through Sun)...
Friday night through Saturday night, Gale force west to northwest
winds are expected over the Gulf and south of the Alaska
Peninsula. A storm system moving into the western and central
Aleutians on Saturday will produce gale force winds, which will
slide into the eastern Aleutians on Sunday.

&&

.LONG TERM FORECAST (Days 4 through 7 Sat through Tue)...
On Saturday morning the pattern will consist of an upper level
trough over southwest Alaska extending southeastward into the Gulf
of Alaska. The trough will very slowly slide northeast across
southern Alaska through Sunday then exit into Canada Sunday night
as the main upper low retreats northward. A weak ridge will then
build over southern Alaska Sunday night, sliding eastward on
Monday. An upper level low just south of the western Aleutians on
Saturday will move eastward through Sunday, moving north into the
central Aleutians on Monday, then into the southern Bering Tuesday.
The exact path and timing of this feature is somewhat uncertain
at this time.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Storm Warning 101 145.
 Winter Weather Advisory 111 121 161.
MARINE...Gales 119 120 125 130 131 132 150 155 160 165 170 172-177 351 352 413 414.
         Heavy freezing spray 160 179 180 185 411 412 414.
FIRE WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...KO
SOUTHCENTRAL ALASKA...MSO
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AH
MARINE/LONG TERM...BL


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