Gale Warning 454, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
503 PM AKDT Sat Sep 5 2020

.ANALYSIS AND UPPER LEVELS...

A strong, compact low is approaching the Alaska Peninsula this
afternoon. The low is producing gale force winds, with gusts to
storm force, based on data from a buoy in the area. The small
nature of the low means the worst effects are confined to the
local area. A second, much bigger low is developing well south and
west of the first, but this second low will be the one to control
the weather across all of southern Alaska over the next few days,
as it absorbs the first low. Elsewhere, the weather is very quiet,
with typical Bering stratus across the Western Aleutians, and high
pressure allowing for ample sunshine across all of Southern
mainland Alaska north of Dillingham and nearly all of
Southcentral, except in the Copper River Basin, where a cumulus
field associated with instability and moisture in the area have
developed.

Temperature-wise, for almost the entirety of southern mainland
Alaska, last night was the coldest night of the fall so far, with
a few sites like Skwentna and Kenai dropping below freezing. Parts
of East Anchorage also hit the freezing mark last night.
The majority of other stations around the area dipped into the
30s. Anchorage International dropped into the 30s for the first
time yesterday morning, only 4 days later than average. This was
the earliest first low in the 30s at the airport since 2015. Just
2 years ago, it took until October 1st for the first low in the
30s to occur.

In the upper levels, a large upper level low associated with the
aforementioned second surface low is located well south of
Adak. An upper level trough, associated with the first low, is
rapidly weakening over the Eastern Aleutians and Alaska Peninsula.
A diffuse, weak upper level ridge is centered over the Alaska
Range, near Cantwell, which is supporting the gorgeous fall
weather ongoing over much of the mainland.

&&

.MODEL DISCUSSION...

The models remain in good agreement about the evolution of the
upcoming storm system that will be making its way into Alaska this
holiday weekend. The low still looks to track near or over the
Pribilofs Sunday evening, then turn more northeastward over
Nunivak Island late Sunday night and up the west coast of Alaska
during the day Monday. Despite this agreement, local forecast
challenges are abundant this afternoon. For the Anchorage area,
the strength and magnitude of the upcoming winds remains the
biggest forecast concern, especially as the highest gusts could
approach warning levels. Out west, another forecast challenge is
the potential for high surf and minor coastal flooding across
Kuskokwim Bay on Monday, with the severity of the coastal flooding
being another challenge.

&&

.AVIATION...

PANC...VFR conditions and light winds will persist. A storm moving
in just beyond the TAF period will cause the 2,000 foot winds to
increase Sunday afternoon, so wind shear is expected and will
increase into Sunday evening.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2: Saturday
afternoon through Monday evening)...

A strong frontal system is moving towards the area with clouds
increasing tonight and tomorrow. This will bring rain and windy
conditions to Kodiak island tonight and tomorrow, and storm force
winds expected in the Barren Islands tomorrow evening. This front
will weaken by Sunday morning and is replaced by yet another low
center which moves into the eastern Bering Sea Sunday night. This
low will bring gale to storm force winds over the Gulf waters as
its front sweeps north Sunday and Sunday night. Additionally,
pressure gradients will increase across the mainland and give rise
to strong gap flows: Turnagain Arm, Portage Valley, Knik River
Valley, and along the Copper River to name a few. Expect strong
Turnagain winds to spill over into the hillside tomorrow night
with gusty winds into Anchorage of 25 to 35 mph as we head into
Monday. Rain will also spread north with the front Sunday night.
With broad southerly flow setting up aloft Sunday night, areas
which would be typically downsloped west of the Kenai, Chugach,
and Talkeetna Mountains will receive measurable rain.

&&

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

An upper level ridge of high pressure remains in place across
Southwest Alaska this afternoon bringing generally sunny and dry
conditions. However, the pattern is about to change across the
region beginning tonight and continuing through the forecast
period. High cirrus clouds currently moving into Bristol Bay will
continue moving northward overspreading Southwest Alaska through
this evening. This is all a result of a front moving north in
association with a compact storm force low pressure system
currently south of Cold Bay. As the front continues to lift north,
it will become sheared (stretched apart). The main area of rain
shower activity with this first round will be mainly across the
Alaska Range and the Kuskokwim Mountains. The flow is expected to
be southeasterly through the lowest portion of the atmosphere
leading to a downslope component for areas to the west of the
Alaska Range and west of the Kuskokwim Mountains. A shower or two
still cannot be completely ruled out in the lower valley areas
however. Additionally, there will be an upslope component for the
eastern Kuskokwim Mountains leading to a better chance of rain.

Next, a secondary and broader area of low pressure currently over
the North Pacific will continue moving northward crossing over the
Pribilof Islands. This is the front that will bring a more
widespread rain to much of Southwest Alaska on Sunday. The flow
will turn more southerly through the day dissipating the downslope
component causing many areas to get rain Sunday morning through
Sunday evening. Precipitation will become more showery in nature
Sunday night through Tuesday night as multiple upper level waves
of energy rotate through the area and as the surface low continues
to slowly move northward towards Nome by Tuesday evening.

Thirdly, as the low is passing the Pribilof Islands, the flow will
turn more southerly into Kuskokwim Bay. As a result, sustained
winds are expected between 25-35 kts and gusts to 40 kts. These
winds will remain fairly steady state for a period of 12-16 hours
Monday covering two high tide cycles. Due to this a High Surf
Advisory has been issued for the Kipnuk to Quinhagak region from 2
am to 6 pm Monday. There is more certainty for the Kuskokwim Bay
area, therefore, the reason for the advisory. However, the winds
are expected to be southwesterly further south into Bristol Bay
during the same time that may allow for high surf into these areas
as well, though, this is more uncertain. This will be fine tuned
and looked at closer in future updates.

Lastly, as the aforementioned front continues moving northward it
will create a tight pressure gradient across Kamishak Gap. This
will result in gale to storm force winds with high end storm force
gusts possible across the Gap especially from Sunday afternoon
into Sunday night. Winds will be sustained out of the east between
30-50 kts with gusts upwards of 50-64kts.

&&

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

A 989 mb storm force low continues to move slowly northward
towards Cold Bay this afternoon. This low will gradually weaken
this evening into the overnight hours as it moves up against a
strong area of upper level high pressure to its north. This will
cause the energy with it to be stretched or sheared apart,
therefore, weakening it. A much broader area of low pressure is
developing and strengthening currently over the North Pacific
southwest of the aforementioned low. This low will absorb the
energy of the first one and become the bigger impact low that will
affect much of the Bering for forecast period. Storm force winds
are expected to continue with this low north and south of the
AKPEN with storm force gusts being observed by a buoy southwest of
Sand Point this afternoon.

This bigger low will bring widespread gales to much of the Bering
Sea over the next couple of days with widespread periods of rain.
Along with gale force winds will be increased wave heights as well
with much of the central and northern Bering seeing waves of
15-20ft beginning early Sunday morning. This will continue
through Monday afternoon before conditions begin to subside as the
low moves northward towards Nome while weakening.

Another possible gale force frontal system will move into the
western Aleutians Monday morning bringing rain and wind.

&&

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

An Eastern Bering low and front move into Northwest Alaska for
Thu. Confidence is good for areas of small craft winds and waves
wrapped around the low to diminish Wed. A North Pacific low and
front moves over the Western Aleutians and Bering through Thu.
While the model track and development of the low are mixed,
confidence is good for widespread gales with this system across
the Western Bering through Wed and diminishing to small craft Thu.

For the Gulf of Alaska waters, a front moves across the Gulf
through Thu. Confidence is good for small craft winds and waves
ahead of the front diminishing Wed. High pressure moves into the
Western Gulf through Thu, with small craft winds and waves from
Kamishak Bay into the Gulf developing Wed and diminishing Thu.

&&

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

A broad and gradually weakening low center will likely move
inland near the Seward Peninsula on Tuesday as strong southwest
flow continues across the southeast and over the Gulf. While
showery/cloudy conditions should persist in Southwest Alaska,
onshore winds will be dying off enough to diminish any minor high
water issues for the Kuskokwim and Bristol Bays by Tuesday
morning. The other main concern through Wednesday will be the
threat for heavy rainfall in the eastern Prince William Sound
where a series of lows or impulses move along an "atmospheric
river" directed at the northeastern Gulf coast. It remains a bit
unclear exactly where the heaviest totals will end up, but river
rises along the Prince William Sound and southern Wrangells are
certainly possible through Wednesday night with several inches of
rain possible.

Looking farther ahead, a pattern shift to drier and calmer
conditions remains on track for Thursday and Friday behind a
shortwave that should swiftly track across the central interior
from west to east Wednesday-Thursday. There is good agreement
that a ridge will develop immediately upstream near the Bering
coast by Thursday morning. Model solutions diverge by Friday,
though there is low to moderate confidence that another system
could approach the AKPEN region around Friday night. Otherwise,
the overall trend heading into the start of the weekend remains
uncertain.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...High Surf Advisory 155.
MARINE...Gale 119 120 129 131-138 150 155 165 170 172 173 175
179-185.
Storm 130 160
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...JPW
SOUTHCENTRAL ALASKA...RC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MV
MARINE/LONG TERM...MK/AS

SPECIAL WEATHER STATEMENT
National Weather Service ANCHORAGE AK
241 AM AKDT Sun Sep 6 2020

AKZ101-061400-
Anchorage-
Including the cities of Anchorage, Eagle River, Indian,
and Eklutna
241 AM AKDT Sun Sep 6 2020

...STRONG WINDS SUNDAY AFTERNOON THROUGH MONDAY MORNING ALONG THE
HILLSIDE, TURNAGAIN ARM, AND HIGHER ELEVATIONS...

Another Fall storm moving into Southcentral will cause strong
winds along the Hillside, Turnagain Arm, and higher elevations. At
times, gusty winds will impact lower elevations as well.

On the Upper Hillside, Turnagain Arm and higher elevations, winds
will rapidly increase during the day Sunday, gusting as strong as
70 mph from Sunday afternoon through late Sunday night. The winds
will diminish the most Monday morning, but may continue to gust
up to 50 mph along Turnagain Arm into Monday afternoon.

For the Lower Hillside, gusts up to 60 mph are possible late
Sunday afternoon into Sunday evening down to 1,500 ft elevation.
The winds will diminish Sunday night, as they turn northeasterly
for much of the overnight hours. Then on Monday morning when the
winds return to the southeast, gusts of 35 to 45 mph are possible
for much of the rest of the day.

For most of the event, Anchorage Bowl, Eagle River, and lower
elevation winds will be relatively light and variable in
direction. A stray gust can't be ruled out. Gusty winds are most
likely Monday morning into Monday afternoon, when gusts of 25 to
35 mph are expected.