Small Craft Advisory 671, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
450 PM AKDT Sun Sep 6 2020

.ANALYSIS AND UPPER LEVELS...

A surface low that is south of Unalaska continues to push
northeastward. The latest ASCAT (Advanced Scatterometer) data has
detected gale force winds north of False Pass and the Alaska
Peninsula. The longwave pattern is highly amplified, with the
longwave trough axis extending from the Central Bering southward
to 30 degrees north latitude. On satellite imagery there is a
deep fetch of moisture from the Central Pacific (west of Hawaii)
to the North Pacific just south of Cold Bay. There are 2 distinct
cyclonic curls in the water vapor imagery.

&&

.MODEL DISCUSSION...

The models are in good agreement with the placement of the major
synoptic features through 48 hrs. However, there will be some
forecast challenges regarding timing and magnitude of the winds
for the Anchorage Metro area. Additionally, for portions of
Southwest Alaska, equally vexing will be the process of fleshing
out the scope of the coastal issues along portions Kuskokwim Bay
on Monday.

&&

.AVIATION...

PANC...The core of the upper level jet stream will move up Cook
Inlet and directly over PANC with an estimated magnitude of 135
knots by 12z Monday. At the onset of the TAF package expect VFR
conditions and light winds. There will likely be a period of SE
winds this evening, but they should shift back to the NW at the
surface after a few hours. Expect persistent wind shear through
the night until the Southeast surface winds kick in.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 3...Sunday
afternoon through Wednesday morning)

A frontal system associated with a strong low pressure system over
the Bering Sea will bring gusty winds and rain to Southcentral
through Monday and Tuesday. Winds will increase across Turnagain
Arm tonight with gusts to 70 mph possible over the Arm and higher
terrain. The upper hillside will see gusts to 60 mph, while in
the Anchorage Bowl winds will be light overnight. The front will
move across the Kenai Peninsula tonight and then is expected to
pass over the Anchorage area tomorrow morning. When the front
passes through winds will increase from the southeast across the
Anchorage Bowl from the 25 to 35 mph range. These winds will
continue through Tuesday although they will be in the 15 to 30 mph
range on Tuesday. Additionally, gusty east to southeast winds
around 20 to 35 mph are expected in the Knik Valley into Palmer
and Wasilla tonight through Monday and then weakening on Tuesday.
The winds will also blow in the Copper River Valley from tonight
through Tuesday. Otherwise rain will spread northward from the
Kenai Peninsula into the Anchorage area and Prince William Sound
area overnight and through the interior by Monday morning. The
heaviest rain is expected in coastal areas along the eastern Kenai
to Prince William Sound area with 1 to 3 inches of rain possible.
In the Anchorage area and the MATSU valleys the best chance of
rain is late tonight through tomorrow morning with a tenth to a
quarter of inch of rain possible. Once this initial band of rain
moves through it will become drier across most valley locations
Monday afternoon through Tuesday with a few passing showers
possible, but the bulk of the rain will remain over higher terrain
and south and southwest facing hills and slopes Monday afternoon
through Tuesday with upslope enhancing precipitation in these
areas. Tuesday night and Wednesday a shortwave begins to approach
from the west which will increase chances of rain by Wednesday.
Finally, another frontal system moves into the Gulf late Tuesday
into Wednesday which will bring increasing winds and rain to areas
as far north as the eastern Kenai Peninsula and Prince William
Sound, but winds area expected to remain below Gales for now.

&&

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

The main weather feature is a 984 mb low situated just south of
Saint George Island. An occluded front attached to this low is
draped over Bristol Bay and the AKPEN this afternoon and is
producing widespread rain across much of Southwest Alaska. This
feature is also producing a widespread area of southerly to
southeasterly gales along the coast in conjunction with the
northward advancing front and through the gaps and and channels of
the AKPEN. Wind gusts are approaching, and even exceeding, 45 to
55 mph for many locations along the AKPEN, southwest coast, and
interior Bristol Bay (close to Kamishak Gap) These gusts will
continue through the late evening before diminishing slightly
overnight as the front lifts farther inland and the low tracks to
Nunivak Island.

As the low lifts north, the winds will shift from southeast to
southwest and ramp up again along the coast as colder air wraps
underneath the system. This will not only bring subsequent bands
of showers into Southwest Alaska for Monday, but the long
southwesterly fetch across the eastern Bering and Bristol Bay will
drive up the surf along the coast, especially from Kipnuk to
Naknek beginning early Monday morning. Persistent southwesterly
winds between 25 and 45 mph is expected to push water levels 1 to
2 feet above high astronomical tide through late Monday.

By Monday night, the low is expected to move over the Seward
Peninsula and weak considerably. As it does, winds will continue
to trend downward. However, the onshore flow will allow clouds and
showers to linger across the region on Tuesday. The flow turns
northwesterly by Wednesday as high pressure drift in from the
west. This flow will usher in clearer and drier conditions, with
any shower activity confined to the upslope regions of the
Kilbuck Mountains and Alaska Range.

&&

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

A broad 984 mb low is south of Saint George Island this
afternoon. This low is producing southeasterly gales and storm-
force gusts along the western Alaska coast and channels of the
eastern Aleutians with northerly gales wrapping around the back
side over the central Bering. The southeasterly gales will turn
southwesterly by Monday morning as the low tracks to Nunivak
Island overnight then into Norton Sound by Monday afternoon. The
strongest gales will occur from north of Cold Bay to Bristol Bay
and Kuskokwim Bay Monday, with seas building over 15 feet. Seas in
the central Bering could approach 20 feet in conjunction with the
northerly gales.

The winds and waves diminish late Monday as the low moves over the
Seward Peninsula and weakens considerably. Upstream, a ridge of
high pressure will slide across the southern Bering and central
Aleutians resulting in calmer winds and waves. This feature slowly
advances east by Wednesday. Farther west, a Kamchatka low will
sweep a front over the western Bering and Aleutians beginning late
Monday. Southeasterly gales will develop along the front by
Tuesday morning. The front moves slowly east, remaining west of
Adak on Wednesday.

&&

.MARINE (Days 3 through 5/Wednesday through Friday)...

High pressure exits the Eastern Bering by Fri. A Western Bering low
and front move to the Central Bering for Fri. While track and
development of the low varies slightly between models, confidence
is good for widespread Southerly gale force winds and waves with
the front diminishing to Easterly small craft winds in the Central
Bering by Fri.

For the Gulf of Alaska waters, a front moves into Southeast Alaska
for Fri. Confidence is good for Southerly small craft winds and
waves with the front diminishing by Thu. High pressure moves
across the Gulf through Fri. Confidence is good for Westerly gale
force winds through the Barren Islands dissipating Thu. A new
front approaches the Southwest Gulf Fri.

.LONG TERM FORECAST (Days 4 through 7/Wednesday through
Saturday)...

The recently discussed "atmospheric river" event will be in the
process of closing out by Wednesday as the jet axis shifts east
and as a sizable shortwave traverses the state from west to east.
Areas of rain are possible for particularly western slopes of the
Alaska, Talkeetna and Wrangell ranges with this wave as the flow
becomes more westerly and then northwesterly by Thursday. Given
any higher rainfall totals from the previous days for the
southeast region, any additional precip on Wednesday ahead of the
main shortwave near the eastern Chugach Mountains or Copper River
Basin could extend or aggravate higher levels on area waterways.

By Thursday-Friday, agreement remains good for a window of drier
weather for the mainland with a shift to northwest flow and
ridging forming over the eastern Bering. A brief period of enhanced winds
through the Kamishak Gap into the Barrens is possible late
Wednesday to early Thursday, but otherwise calmer low level winds
can be anticipated for the latter half of this week. Some global
solutions show additional midlevel waves riding in with the
synoptic flow through parts of the interior, but there will be
considerably less moisture for these perturbations to work with by
this point. Therefore, both Thursday and Friday should be calmer
and drier all in all.

Model discrepancies become larger going into the weekend,
especially for a small low lifting up from the North Pacific. The
GFS shows this feature making it into the AK Gulf where minor
coastal and marine impacts would be possible with the
ECMWF/Canadian instead showing the low weaker and stalling out
well to the southwest. There is much better agreement for a
deeper/closed low to track through most of the Aleutians between
Thursday and Saturday with the usual impacts (fog, low stratus and
showers) expected around this time frame.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...HIGH SURF ADVISORY 155 161
MARINE...GALE 119 120 138 139 150-165 172 180-185.
		 STORM 130 131.
FIRE WEATHER...NONE.

&&

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