Gale Warning 638, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion...CORRECTED
National Weather Service Anchorage AK
434 PM AKST Fri Nov 6 2020

.ANALYSIS AND UPPER LEVELS...
Upper level low pressure along the Arctic Coast and a large upper
level high south of the Gulf of Alaska is bringing the 250 mb jet
stream and strong zonal westerly flow over the southern mainland
Alaska today. This is really the break between systems as dry air
aloft has trapped the low stratus over Cook Inlet with flurries
and freezing drizzle at times falling from the stratus.

Strong southwesterly winds in the Kuskokwim Delta and Bristol Bay
areas are diminishing this afternoon and the threat of coastal
flooding with this system has now abated for those areas.

The next storm which will move into southern Alaska this weekend
can be seen on satellite near 40N 170W this afternoon and starting
to make it move toward the northeast.

&&

.MODEL DISCUSSION...
The main issue with the models continues to be with the
development and track of a triple point low in the lower Cook
Inlet or North Gulf coast area on Sunday. While the solutions are
in better agreement overall than a few days ago, the track of this
triple point will be the main feature that determines whether the
Kenai Peninsula and Anchorage and Matanuska Valley gets all snow
or a mix of rain and snow on Sunday. The 18Z runs of the NAM and
GFS have edged slightly to the northwest putting the triple- point
over Kachemak Bay. Every little bit to the north or west this low
tracks increases the rain potential for the areas mentioned.

&&

.AVIATION...
PANC...The lower ceilings around the area look like they will
persist much of the night and then improve late tonight as the
atmosphere will start to become better vertically mixed with the
approach of the next weather system. This next system will bring
snow to the terminal area by late Saturday afternoon.

&&

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

The next 12-18 hours across Southcentral will remain relatively
quiet before the next big system arrives late Saturday morning
and continuing through Sunday. Expect the stratus deck that has
been across much of the Mat-Su Valleys Anchorage Bowl and the
Kenai Peninsula to remain in place through the overnight hours as
there is nothing to push these clouds out. Underneath this stratus
deck there may be some areas that experience some flurries from
time to time. Attention is now turning to the big storm system
expected to affect Southcentral late Saturday morning through
early Monday. This storm system is currently getting its act
together to the northwest of Hawaii. As this system comes
northward, it will be bringing a great deal of warm air with it.
This has been making the forecast for this system extremely
difficult. Storm track continues to be an issue with this system
as the ECMWF continues to bring this system northward through
Bristol Bay and just floods Southcentral with warm air. If this
solution pans out, even areas across the Susitna Valley may have
precipitation type issues but this looks unlikely. The NAM and GFS
show the same low the EC does moving northward into the Bristol
Bay region but it develops a triple point low or secondary low to
the east of Seward which then moves onward to Prince William
Sound. If the GFS and NAM solution pan out, thinking the Susitna
Valley has a great chance of seeing significant snowfall
accumulations from this system beginning Saturday afternoon
through Sunday afternoon.

As a result, a Winter Storm Watch remains in effect for the
Susitna Valley for the aforementioned time period. Due to the
uncertainty still on the track of this system, we will keep the
watch valid for at least a model cycle run or two before making a
decision on upgrading it to a warning or even advisory. Also,
places like Alyeska and Turnagain Pass may see upwards of 2 feet
of snow from this system, Hatcher Pass may receive 1.5 feet of
snow from it, while Thompson Pass will receive the highest totals
of 2.5 feet possibly from this system.

While the forecast is more certain in those areas, the forecast
continues to remain very difficult from Kenai northeastward
through Anchorage to Palmer in the Mat Valley. Of course, this is
through where much of the population is which is one area of the
forecast you would like to have that certainty but unfortunately
the certainty isn't there. Current thinking is precipitation will
start as snow area wide with the first places to changeover being
from north of Homer to Kenai east to Sterling Saturday evening.
For now, opted to keep Anchorage all snow with just a chance of
rain for Sunday morning. Track and how the new low develops will
be everything for what happens across much of Southcentral. Either
way this low is bringing so much warm air up that mixing with or
changing to all rain can't be ruled out. The threat still exists
for possibly advisory level snow across the Mat Valley, Anchorage
Bowl and the Kenai Peninsula but the risk is also there for this
to be all rain as well if the warm air gets in.

Model guidance then shows the cold air rushing back in Sunday
afternoon through early Monday as the upper trough swings through
which may cause a deformation band to sit over Anchorage for a
couple to several hours which is when the heaviest snowfall would
occur.

&&

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

Mostly dry conditions will continue through early Saturday
morning. Scattered light rain/snow showers are possible north of
Bethel this evening under a weak upper shortwave.

A low will approach from the North Pacific on Saturday morning.
There is better model consensus today, with the GFS/NAM taking
the low center slightly farther north and other models trending
slightly south. As such, there is good confidence now that the
center will move along the north coast of the Alaska Peninsula.
Consequently, more warm air will move into interior Bristol Bay.
All rain is expected for much of this zone, with any snow confined
to higher elevations. This will likewise be the case for much of
the Kuskokwim Delta. Higher sea surface temperatures will lead to
all rain along the coast. Areas east of and including Bethel will
likely see a mix changing to all snow by Saturday night, however
accumulations will be minimal. The Middle Kuskokwim Valley will
see mostly all snow. Although temperatures in the snow growth
layer will be favorable, the associated upper trough is not deep
enough to produce much lift and temperatures gradually rise to
near freezing at the surface. Thus, the accumulating snowfall will
remain below advisory criteria in populated areas and lessen
farther west. Precipitation will end from west to east through
Sunday as the low departs over Southcentral. After a brief lull
overnight Sunday, another front will quickly move onshore Monday
morning. Given strong steering winds behind it along with strong
southwest flow, accumulating snow appears to be minimal at this
time.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 Through 3)...

Scattered showers will continue generally east of Unalaska this
evening. A low will approach from the North Pacific early Saturday
morning, bringing steady rain to the same aforementioned area
through Saturday night. A series of lows will bring periods of
rain to the western Bering/Aleutians through Monday. Gusty
southwesterly winds tracking west to east are likely with the
system on Sunday, eventually reaching the Pribilofs by the
evening.

&&

.MARINE (Days 3 through 5: Monday through Wednesday)...

Gulf of Alaska:

Starting Sunday evening into early Monday morning, storm force
winds will emerge to the east and northeast of Kodiak, near
Barren Islands, and offshore marine zones. These winds are well
associated with a frontal system inland, but as the frontal zone
weakens Sunday night, the storm force winds will also diminish to
gales by Monday morning. Essentially, gales will subside to below
34 kt Monday afternoon, and so will hazardous wave heights by
Monday evening.

Bering Sea:

On Sunday evening, the potential for storm force winds are
expected to extend up to 320 nm southeast of the low over the
Bering, and up to 150 nm south of the same system. Wave heights
will peak to 30 feet near the center of the low Sunday night. The
areal coverage of developing storm force winds will gradually
reduce Sunday night along the southeast quadrant of the low, then
essentially taper off to gales by Monday mid-morning. As the low
weakens while tracking northeastward toward Yukon Bay, so will
the winds and seas.

&&

.LONG TERM FORECAST (Days 4 through 7: Tuesday through Friday)...

There is high confidence that the pattern will remain active
through the forecast period. Synoptically on Monday there will be
ridging over the Gulf and southern Mainland with a trough over the
Bering Sea. At this time a moderately strong surface low is
expected to be over the northern Bering. For Southwest Alaska
this should mean snow transitioning to rain as the warm front
moves north throughout the day on Monday. On Tuesday a complex
upper level trough moves east into the Bering from the Kamchatka
Peninsula. Models are in poor agreement in regards to finer
details of the resulting surface low, or low complex, but impacts
indicate widespread rain across Southwest Alaska on Tuesday and
early Wednesday. There is moderate agreement that a warm front
will push into Southcentral on Wednesday with models in
disagreement in regards to precipitation type. Most models
indicate widespread warm air and therefore rain but a transition
to snow on Thursday is possible if a triple point low forms in the
Gulf. On Friday the low complex is expected to be southeast of the
AOR, which will result in the return of cold air to most of the
region.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Storm Watch 145.
MARINE...Storm Warning 119.
 		 Gale Warning 120, 125, 130-132, 139, 150, 155 178, 351, 352.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...EZ
SOUTHCENTRAL ALASKA...MV
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AP
MARINE/LONG TERM...CB/CJ