Gale Warning 353, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
517 AM AKDT Fri May 12 2023

.ANALYSIS AND UPPER LEVELS...

A large upper-level low continues to rule the roost in the North
Pacific this morning. A weak shortwave pivots northwestward from
the eastern Gulf of Alaska towards the northern Gulf Coast. As
such, Middleton Island radar shows a band of rain mostly out of
the Gulf, but clipping extreme southern portions of the northern
Gulf coast as well as Prince William Sound. Another stronger
shortwave, behind the aforementioned one and on its heels, lifts
northwestward towards the north-central Gulf. This system will be
the focus for coastal portions of Southcentral Alaska later this
afternoon and into the evening hours.

Southwest Alaska remains mostly dry as an upper-level ridge
builds in from the central and eastern Bering Sea. The Pribilof
Islands are directly under this ridge and see low stratus clouds,
mist, and reduced visibilities as a result. The eastern Aleutians
are in between the clockwise around the Bering ridge and counter-
clockwise flow around the North Pacific low to the east. The
result is comparable with the Pribilofs with witnessing lower
cloud decks this morning. Meanwhile, the Central Aleutians, being
centered under the upper and surface ridges, observe high clouds
to mostly clear conditions. A front coming off a Kamchatka low
sweeps the western Aleutians/Bering and moves eastward towards the
central Aleutian Chain.

&&

.MODEL DISCUSSION...

While guidance, for the most part, continues to be in reasonably
good synoptic agreement, there are some evolutionary questions
that exist; especially later into the short-term period for Sunday
and Monday. The main point of disagreement is the timing of a
Bering Sea frontal system and the speed at which it progresses
across the Bering/Aleutian Chain and eventually to mainland
Southwest Alaska. The 00z NAM has the quickest progression of the
guidance suites with enhanced precipitation already in interior
portions of Bristol Bay and the Kuskokwim Delta by early Sunday
morning. The 00z GFS does not have precipitation in Bristol Bay by
then, while only having light precipitation mostly confined to the
Kuskokwim Delta coast. The 00z ECMWF and 00z Canadian are in
between the NAM and GFS solutions. Another challenge remains the
duration and strength of the Turnagain Arm wind and how much it
bends into Anchorage, as well as timing of weak mesoscale
shortwaves coinciding with upper-level cold pools to potentially
set off convection for portions of interior Southcentral and
Southwest Alaska.

&&

.AVIATION...

PANC...VFR conditions will persist. Light winds, mostly out of the
north, will continue through this morning. Pressure gradients then
realign and become favorable for southeasterly Turnagain Arm winds
to bend over the terminal through the afternoon and continuing
through the TAF period. There is still some uncertainty in the
strength of the winds, particularly overnight with the loss of
instability and daytime heating. However, the surface pressure
gradient, in conjunction with middle and upper-level easterly to
southeasterly flow, will provide a continuation to the southeast
winds overnight as opposed to a lightening and turn back to the
north. Southeast winds will become gusty again tomorrow morning
and into the afternoon hours as daytime heating ensues and
instability builds. Winds could gust as high as 30 kt with 35 kt
not out of the question; especially Saturday mid-day and
afternoon.

&&

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

Two distinct upper-level shortwaves will move across the northern
half of the Gulf of Alaska today. Each feature will have an
attendant inverted surface trough that will sweep across the
northern Gulf from east to west through Friday evening. The first
of these troughs has an axis south of Middleton Island this
morning and is pushing an area of widespread precipitation across
the northern Gulf. Much of this precipitation will remain just
offshore through the morning hours before diminishing in areal
coverage as both the shortwave and surface trough weaken. A second
wave, trailing the first, will bring a stronger inverted trough
into the Gulf by this afternoon, moving toward Kodiak Island by
Friday night. A southeasterly flow in the wake of the trough axis
will push the widespread rain and high elevation snow along the
coast. Moisture advecting over interior Southcentral will also
result in the development of scattered showers, mainly across the
Copper River Basin and Susitna Valley through Saturday morning.

The bigger story is, perhaps, the strong southeasterly gap winds
developing for Turnagain Arm, the Knik River Valley, and Copper
River Basin as the inverted trough moves toward the Kenai
Peninsula and Kodiak Island. As the trough moves west, it will
induce a coastal ridge along the mountains. This ridge, coupled
with a deepening thermal trough over the interior, will help
produce gusty winds, drawing the strongest winds across the
Anchorage Bowl, Palmer, and Glennallen. These winds may diminish
slightly overnight as the airmass stabilizes somewhat. However, a
second round of strong wind gusts is likely by late Saturday
morning as the pressure gradient again tightens and the airmass
destabilizes due to daytime heating. These winds will persist into
Sunday as a southerly synoptic flow develops both at the surface
and aloft. Showers will also continue along the coast through the
weekend.

Snow levels today are expected to approach 4,500 feet, but will
likely drop back to around 2,500 feet tonight as slightly colder
air aloft moves over the region in the wake of the trough.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA AND BERING SEA/ALEUTIANS
(Days 1 through 3: Today through Sunday)...

The axis of an upper-level ridge is currently nudging its way
eastward into Southwest while high pressure at the surface
spreads across Southwest and much of the Bering and Aleutians. As
a result, dry and clear conditions with light winds persist,
although some low stratus lingers near the Pribilof Islands and
eastern Aleutians. Near the western Aleutians and Bering, a
weakening front will be revived today as an incoming shortwave
tracks from the North Pacific. The front will lift east to the
Pribilof Islands and eastern Aleutians on Saturday morning, then
to the Kuskokwim Delta coast and Alaska Peninsula by Saturday
evening/night, and finally across the Southwest mainland through
Sunday. There are still some differences between the models
regarding the speed and timing of the front as it reaches
Southwest, however generally expect cloud cover, gusty southerly
winds, and precipitation to return to the region for the weekend.
Precipitation with this front should fall as rain, and southerly
winds across Southwest are expected to be the strongest from
around Saturday evening into Sunday morning.

&&

.LONG TERM FORECAST (Days 4 through 7/Monday through Thursday)...

There is a large amount of model spread in the extended forecast
which will affect the forecast for southern Alaska. The basic
upper level pattern is a high amplitude trough centered over the
eastern Bering Sea and western Alaska, with a series of strong
short-waves moving through the flow. Models disagree on handling
of specific short-waves, location of surface low(s) and resultant
distribution of precipitation and wind over southern Alaska. The
one trend noted in model guidance over the past few days is to
extend the upper trough eastward across all of mainland Alaska,
keeping ridging off to the east over Canada. This general pattern
looks unseasonably wet and windy, especially early in the week.
Colder air aloft with the trough combined with clouds and
precipitation will lead to near to slightly below normal
temperatures for southern Alaska.

Models also disagree on how the pattern evolves later in the week,
with solutions varying from keeping the trough over mainland
Alaska, progressing it eastward, and even retrograding it back
westward into the Bering Sea. Thus, forecast confidence takes a
nosedive as we head through the work week. The weather does look
fairly quiet upstream of the trough across the western to central
Bering and Aleutians.

&&

SYNOPSIS AND MODEL DISCUSSION...DAN
SOUTHCENTRAL ALASKA...TM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AF
LONG TERM...SEB