Small Craft Advisory 810, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion...CORRECTED
National Weather Service Anchorage AK
758 AM AKDT Sun Oct 25 2020

.ANALYSIS AND UPPER LEVELS...

On satellite imagery, there is a broad shield of clouds from
Cordova to Anchorage and to the AKPEN into the North Pacific. A
new low circulation is churning up southwest of Kodiak while the
latest ASCAT (Advanced Scatterometer) data has detected a discernible
weak low near Nunivak Island. Also on ASCAT, there is an area of
gale force winds south of Attu Island. There is a 150 knot jet
core over Bristol Bay. The radars at King Salmon (PAKC), Kenai
(PAHG) and Middleton Island (PAIH) have been active overnight and
early this morning. Also on a different channel of satellite
imagery, there are two fetches of deep moisture. The first plume
extends from Hawaii to Kodiak, while the second one is placed near
the Mariana Islands to southern Kamchatka and is moving towards
Shemya.

&&

.MODEL DISCUSSION...

The run to run continuity of the operational models has been poor
over the past 48 hrs. However, the general consensus is that a
moisture laden system will move across Southcentral today. The
thermodynamics of the models related to precipitation type
continue to be tricky. However, the confidence was strong enough
to issue a Winter Weather Advisory for this evening for portions
of Southcentral including Anchorage, Eagle River, Palmer, Wasilla,
and Talkeetna.

&&

.AVIATION...

PANC...LLWS will continue to be monitored. The TAF period will
commence with VFR conditions. However, a new system will move
across Southcentral today. The cigs will decrease. There is
potential for accumulating snow and IFR conditions this evening
through early Monday. Amendments will be made as needed.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2/Today
through Monday night)...

The first major winter storm of the season will bring widespread
snow to Southcentral Alaska tonight through Monday. All the
ingredients are in place for moderate to heavy precipitation:

1. A strong meridional jet extending from the tropics northward to
mainland Alaska will transport abundant moisture to Southcentral.
2. A potent upper level wave embedded within the southerly flow
will track right overhead of Southcentral tonight.
3. A surface low will develop along the leading warm front and
intensify over the western Gulf this afternoon and evening in
response to the upper short-wave.
4. The larger upper trough over the Bering Sea will swing
eastward, feeding colder air into the low and causing additional
deepening tonight.
5. The low will track to western Prince William Sound, which is
a favorable track to produce heavy precipitation for Anchorage
and the Matanuska Valley.
6. The Bering trough will move through Southcentral Monday,
keeping the precipitation going.

The most challenging part of the forecast is precipitation type.
The Gulf coast is quite warm and will remain all rain. As you head
away from the coast, much of the western Kenai Peninsula,
Anchorage, and the Matanuska Valley are currently warm enough for
rain at the moment. However, the warm air is shallow. The 12Z
Anchorage sounding shows a wet-bulb zero height of 970 feet. If
you drop enough precipitation into the column, the wet-bulb will
drop to zero and result in snow at sea level. There has been a
very consequential westward shift in the model forecast track of
this storm over the past 24 hours which will result in much
heavier precipitation. Based on this, have shifted heavily toward
a snow forecast as precipitation rates increase late this afternoon
through evening. Any change to the newly expected storm track
could affect the delicate balance, as most model soundings show an
isothermal near-freezing layer for Anchorage and the Matanuska
Valley. As a result, forecast confidence in storm total snow
totals from the western Kenai to Anchorage and the Mat-Su is low.
Nonetheless, there was enough evidence to support issuing Winter
Weather Advisories for Anchorage, the Matanuska Valley, and the
Susitna Valley. Left the western Kenai out for now, as warmer air
will remain in place with the low passing just south of the
peninsula tonight.

The Winter Weather Advisories issued yesterday for Thompson Pass
and the Copper River Basin remain largely on track. However, with
the more westerly low track, do expect warmer air to move into the
southern Copper River Basin. Could see a mix with rain or even a
period of all rain late tonight through Monday for Chitina
eastward to McCarthy. Also, have shifted the heaviest snow totals
from the Glennallen area northward to Paxson and areas along the
Alaska Range.

A remnant low will linger over the northern Gulf through Monday
night, but precipitation will come to an end for most of the
region. Cold air advection and tight pressure gradients will
maintain strong gusty winds across the Gulf waters, along with
moderate winds through Resurrection Bay and Passage Canal.

&&

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

A weakening system over the eastern Bering will keep precipitation
across much of Southwest Alaska through Monday. Rain will be the
dominant precipitation type today, which will transition to snow
overnight for the Kuskokwim Valley and Delta with colder
temperatures moving in. Any new snowfall should see little
accumulation. Temperatures across interior Bristol Bay should
remain warm enough to keep any new precipitation as rain through
Monday as a low over the North Pacific continues to lift northward
into the Gulf. There is still some uncertainty in the track of
this low and how much precipitation it will bring to the region.
This active pattern should weaken overnight Monday keeping a few
lingering showers through Tuesday morning.

&&

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

A low in the eastern Bering will finally dissipate by this
afternoon, keeping relatively quiet weather across much of the
Bering today. Farther west, a weak front approaches the western
Bering and Aleutians this morning, but quickly weakens as it
progresses eastward through Monday morning.

A much more active weather pattern is setting up across the
eastern Aleutians and Alaska Peninsula this morning as a front
associated with a North Pacific low lifts northward. Widespread
precipitation is expected, along with a brief period of gale force
winds south of the AKPEN late afternoon through early Monday
morning. Winds associated with this system will continue to weaken
through Tuesday morning for this area as the aforementioned low
continues to trek into the Gulf.

&&

.MARINE (Days 3 through 5: Tuesday through Thursday)...

The weather to watch on Tuesday will be an area of low pressure
located south of Prince William Sound. Uncertainty remains with
the strength and track of this system, however, models agree on
gale force winds across the central Gulf before the low moves
inland on Wednesday. Through Wednesday, calmer conditions will
provide for light winds across much of the Gulf. Over the Bering
Sea and Aleutians, winds and waves look to remain relatively calm
for the forecast period. There is potential for an area of low
pressure to skirt the Chain beginning Wednesday afternoon,
however, model confidence is low at this time.

&&

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

A weak surface low lingers over the northern Gulf on Tuesday and
tracks slowly to the east. Meanwhile, an upper-level ridge sets up
over the interior, beginning the flow of cold air towards
southern Alaska, which will be quickly pushed out by a series of
arctic troughs moving across the state mid-week. While the
greatest impacts will remain well to the north, temperatures for
the Southwest and Southcentral regions will drop as the colder air
is advected in.

Late next week, there is better agreement between the Canadian and
ECMWF models across all parameters for a North Pacific surface
low that will pass just south of the Aleutians. The GFS seems too
slow and is quite a bit deeper in comparison, so less reliability
is placed on this solution. This system can potentially bring
strong winds to the coastal waters.

By Saturday, another system approaches the Aleutians from the
west. There is decent model agreement with this surface low. The
ECMWF seems fast and travels with an easterly path but
approximates a similar depth to the Canadian. The GFS, while much
deeper than the other two, shares a similar location and path
(southeasterly) with the Canadian.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 101 111 145 131 141.
MARINE...Storm 119 131 351. Gale 150 120 130 132 136 137 138 139
141 352.

&&

SYNOPSIS AND MODEL DISCUSSION...PJS
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...ALH
MARINE/LONG TERM...CK/KM