Gale Warning 303, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
512 PM AKDT Sat Apr 16 2022

.ANALYSIS AND UPPER LEVELS...

An upper-level ridge remains in place from the eastern Gulf of
Alaska north across Southcentral. A wide expanse of marine stratus
and stratocumulus is trapped underneath the ridge and extends
along the entirety of the Southcentral coast this afternoon. Rain
and low clouds also continue to stream toward Kodiak Island as a
shortwave moves northwestward, embedded in the flow around the
upper-level ridge. A weak shortwave is also dropping south from
the Yukon; however, there is little to define this feature other
than visible satellite imagery depicting the southward advancement
of mid and high level clouds across the Alaska Range.

Out west, the shortwave trough over Kodiak Island extends into
Bristol Bay and across the Southwest coast. Clouds and showers
linger for Bristol Bay, but diminish quickly over the Kuskokwim
Delta due to a drier, downsloping flow over the Kilbuck Mountains.

A decaying low continues to fill and weaken between Saint Paul and
Saint Matthew Islands. Scattered gales and rain showers persist
over the central and western Bering and Aleutians with this
system. A second extra-tropical low well south of Attu and Shemya
is the remnant of Typhoon Malakas. This feature is supported by a
160 kt jet streak and is continuing to tap into plenty of tropical
moisture as it begins to move toward the western Aleutians.

&&

.MODEL DISCUSSION...

Model guidance remains in good agreement with decent run to run
consistency through 12z Monday. After that time, models begin to
diverge with regard to the forward movement of a storm-force low
over the central and eastern Aleutians. The NAM/EC/GEM all stall
the surface low over Unimak Pass, while the GFS begins to track
the center of circulation to the southeast. By Tue 12z, all models
do indicate that this feature will begin to weaken and elongate
as it runs into a ridge downstream. Thus, even with the differences
in the position of the low, differences in the forecasted conditions
are not significant. All models hold onto some semblance of a
surface ridge over the northern Gulf and Southcentral Alaska.
However, weak upper-level waves will likely allow for clouds and
showers across these areas through the short-term.

&&

.AVIATION...
PANC...VFR and light winds will persist. There is the potential
for ceilings to drop below 5,000 ft, but remain VFR, by Sunday
morning as a weak upper-level trough moves south across the
region. Have opted against including a lower ceiling in the taf
for now because the upper level trough is so weak and not sure
there will be much impact on the low levels of the atmosphere.

&&

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

The main synoptic feature over Southcentral is associated with an
Arctic shortwave trough pushing southward from the AK-Yukon
Territory. Pressure gradient driven northerly winds will funnel
through the gaps and passes, including the Alaska Range/northern
Copper River Basin and Thompson Pass, with higher gusts at times.
Given the limited instability and snowpack in the mountains, only
shallow cumulus clouds are forming this afternoon over lower
elevations of the Copper River Basin. Would expect little or no
precipitation as the trough crosses inland areas of Southcentral.
However, as the shortwave moves further south into the coastal
waters, some showers are possible along the Kenai Peninsula and
Kodiak Island from Sunday through Monday. Otherwise, a ridge of
pressure will become reestablished over the mainland, with mostly
clear skies and seasonable temperatures through early next week.

&&

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

A fairly quiet weather pattern is expected to continue for
Southwest Alaska as upper level ridging gradually extends farther
southwestward across the area. This will allow for lingering rain
and snow showers to taper off Sunday night. Strong southeasterly
winds are expected across the coastal Kuskokwim Delta, Bristol Bay
and Alaska Peninsula Monday morning through Monday night as a low
pressure system passes to the distant west.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3/Tonight
through Tuesday)...

An active pattern continues across the Bering/Aleutians over the
next couple of days as a strong low pressure system moves across
the area. This low will bring widespread storm force winds, both
ahead of the low (southeasterlies) and behind the low
(northwesterlies). The low will be vertically stacked with surface
pressure holding steady on Sunday. However, low level winds ahead
of the leading occluded front will continue to strengthen as it
pushes into a downstream ridge. The strongest core of winds will
cross the eastern Aleutians (including Dutch Harbor) during the
day Sunday and may reach the far southern Alaska Peninsula Sunday
evening before winds begin to diminish. Looking at some forecast
soundings for Dutch Harbor, warming up above the surface will lead
to formation of a temperature inversion between 3000 and 6000 ft,
while the sub-3000 ft layer looks weakly unstable. Based on this
profile, there is potential for mountain waves and trapping of
stronger winds, some of which could mix down to the surface.
Therefore, have decided to issue a High Wind Watch for the Eastern
Aleutians for gusts up to 75 mph. Have also included gusts up to
65 kt for bordering marine zones.

Meanwhile, there will be a core of storm force winds in cold air
advection on the back side of the low, crossing the central
Aleutians Sunday night through Monday. An upper level short-wave
will then drop into the back side of the low MOnday night as an
upstream ridge amplifies. This will cause low level northwesterlies
to strengthen Monday afternoon through Monday night. Thus, have
issued a high wind watch for the central Aleutians for winds up
to 75 mph.

Winds will diminish across the region Tuesday through Tuesday
night as ridging building in from both the east and the west
forces the low southward into the Pacific.

&&

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

Gulf of Alaska: Light winds and seas less than 8 ft will persist
through Wednesday for most of the Gulf. Areas south of Kodiak
Island will likely see small craft winds and seas approaching
14 ft from Tuesday morning through Wednesday afternoon as a front
grazes the Western Gulf. Another front pushes eastward across
the Gulf beginning Thursday, bringing a swath of gales and seas
as high as 20 ft.

Bering Sea: Elevated winds and seas along the central to eastern
Aleutians and Alaska Peninsula will diminish Tuesday as the
responsible storm exits across the North Pacific. Confidence is
increasing in the merging of two new storms along the Aleutians on
Wednesday, leading to one single very large and intense storm
Wednesday night. While confidence in the track of this storm and
location of strongest winds/highest seas is low, it looks
favorable for widespread gales with potential for smaller swaths
of storm force winds. This storm will progress eastward Wednesday
night and Thursday, so expect impacts along the Southwest Alaska
coastal waters.

&&

.Long Term Discussion (Days 4 through 7: Wednesday through
Saturday)...

With the blocking high that has brought mostly calm weather to
Southcentral gets minimized to a ridge over the northern portion
of Alaska, it is clear that a more active pattern is in store for
the extended. At this point, the extended looks to be dominated by
an anomalously deep upper low that will traverse Alaska from west
to east, first arriving along the western Aleutians Wednesday
morning, moving eastward towards the Alaska Peninsula by Saturday.
Looking to ensemble data for guidance as deterministic runs
remain variable in details, have noted that model ensembles
support the development of this storm low mid-week, but then
diverge in whether to bring the low straight towards the southwest
coastline and the Alaska Peninsula, or drop it southeast into the
western Gulf. This all seems to depend on whether the high
remains over northern Alaska, steering flow south- and east-ward,
or whether the strength on the transient ridge ahead of this low
strengthens as it moves over the Canadian mainland.

What’s clear is that Wednesday through Thursday will be active
for the Aleutians and Alaska Peninsula as the anomalously strong
low moves west to east, bringing the potential for moderate to
heavy precipitation, very strong winds, and high seas for areas
across the Bering Sea, the Aleutians, and Alaska Peninsula. To the
east, cool upper level temperatures drawn out of the interior and
weak lift around a weak low situated along the northern Gulf will
continue to support isolated to scattered showers across Southcentral
and interior portions of the Y-K Delta and Bristol Bay, especially
along valley locations where snow cover is lacking and afternoon
heating will be most effective. This weaker low will not stick
around long, however, as the deeper low over the Bering progresses
east, absorbing the weaker low, and reinforcing in strength
Thursday.

By Friday and Saturday, models divergence is pronounced enough
that it is unclear whether Southcentral and the Gulf or the
Southwest coastline will become affected by this deep low, with
portions of the forecast area becoming battered by its winds, rain
and high seas, depending on how/where the system tracks. Keeping
pattern recognition in play, it is more likely that the low tracks
into Southcentral and the Gulf, with more than one model ensemble
indicating this progression. Check back for forecast updates as
details of this stronger system become more apparent.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...High Wind Watch 185 187.
MARINE...Storm Warning 170 171 172 173 174 175 176 177.
         Gale Warning 155 160 165 178 179 180 181.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...TM
SOUTHCENTRAL ALASKA...MF
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...ED/SB
MARINE/LONG TERM...KC/AB