Area Forecast Discussion
Issued by NWS Anchorage, AK

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FXAK68 PAFC 051240

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
440 AM AKDT Wed Aug 5 2020


A vertically stacked low positioned over the southeastern Gulf
continues to fill and weaken as it drifts toward the Alaska
Peninsula. Radar returns from the Middleton Island radar show
an increase in shower activity over the northern Gulf in
association with an upper-level trough extending from the low.
Over Southwest Alaska, a transient upper-level ridge stretching
from the Kuskokwim Valley south to Bristol Bay is resulting in
drier and more stable airmass with only an area of low stratus
and fog settling along the Bristol Bay coast overnight. An upper-
level wave continues to drop across Southcentral overnight, caught
up in the northeasterly flow between the ridge and the low over
the Gulf. This is the feature responsible for the scattered
showers and isolated thunderstorms across parts of the Susitna
Valley and Copper River Basin this afternoon and evening.

Farther west, a 990 mb low is situated approximately 200 nm north
of Adak. This system continues to deepen as it swings a warm front
from the Pribilofs to the eastern Aleutians and a trailing cold
front across the western Aleutians, bringing widespread clouds,
gusty winds (including areas of gales), and rain to the
region. The upper-level dynamics are favorable for further
strengthening as the surface feature sits in the left exit region
of a 130 kt jet streak. This system is still on track to enter the
eastern Bering by late Wednesday with its front pushing onshore
along the west coast.



Models are in very good agreement regarding the departure of the
Gulf low to the southeast and strength and movement of the Bering
low as it tracks to the Southwest Alaska coast. There are some
minor discrepancies in the track of the upper-level trough as it
moves into the northern Gulf on Friday. However, any noted
differences have little impact on the expected sensible weather.
Given this, the morning forecast package will continue to trend
towards the NAM/GFS solutions with the use of hi-res models for
local effects.


PANC...VFR conditions will persist through Thursday morning.
Southeasterly winds will develop around 00z and are expected to
persist through early Thursday morning. Gusts of 25 to 30 kts are
possible from mid-afternoon through late evening Wednesday.
Shower activity in the vicinity of the terminal is also possible
this afternoon and evening, but is expected to remain mainly over
the Chugach Mountains east of the airport.


through Friday)...

An upper level trough extending up into eastern Southcentral from
a large low over the eastern Gulf will bring numerous showers to
the southeastern Copper River Basin and eastern Prince William
Sound regions today. Elsewhere, the thermal trough will strengthen
a bit from Tuesday as a weak upper ridge moves into western
portions of Southcentral. Surface heating in combination with
unstable air will lead to another round of diurnal showers and
thunderstorms. As was the case yesterday, convection will
initiate along the mountains. However, today they will track in
the opposite direction as southerly winds develop this afternoon.

The continued instability will also bring locally strong and gusty
gap winds to Turnagain Arm/Anchorage and the Knik River Valley
into Palmer late this afternoon through evening as a surface ridge
builds along the north Gulf coast. A front associated with a
strong vertically stacked low currently residing in the Bering
will then make its way into the Gulf tonight, leading to
increasing low level southeasterly flow as well as strengthening
of the coastal surface ridge and pressure gradients. As a result,
strong and gusty winds are again expected to push through
Turnagain Arm and spill into Anchorage (mainly the upper hillside)
and the Matanuska Valley on Thursday.

The details of the wind forecast in the Anchorage area are a bit
tricky. As the front approaches Southcentral, pressure gradients
between Anchorage and Homer favor the Turnagain wind bending
southward and staying out of Anchorage. This also usually results
in a diminishing of winds along the Anchorage hillside. However,
surface-based instability and a well-mixed boundary layer along
with the aforementioned increase in low level flow (at mountain
top level) will favor some winds mixing down to the surface during
the afternoon hours. In particular, this seems like a scenario
where Eagle River and the S Curves along the Glenn Highway may
see some gusty winds.

This frontal system will bring moderate to heavy rainfall to
Kodiak Island tonight then to the Eastern Kenai Peninsula and
Prince William Sound areas on Thursday. Southeasterly flow will
also enhance rainfall on the west side of Cook Inlet and the
western Susitna Valley (near the Alaska Range). The remainder
of Southcentral will experience downslope flow and will thus
remain mostly dry, with just a chance of passing light showers.

The storm will weaken over the region on Friday. With weakening
winds (and downslope flow) will transition to a showery pattern
as the upper trough sets up shop over the Gulf and Southcentral.


(Today through Friday)

Rainfall potential will rapidly increase over the next 24 hours.
For this afternoon and evening, a few thunderstorms may develop
along the Alaska Range from Iliamna north, but given the lack of
thunderstorm development the last two days, which featured
sufficient instability but a lack of appreciable lapse rates,
hopes for today are fading fast. Incoming high cloud cover from
the approaching front won`t help matters either.

The real story will be a deepening low moving towards the eastern
Bering Sea, sending a front ashore over the next 24 hours. As the
low deepens, surface convergence along the front and ascent aloft
will combine with increasing moisture though the column to foster
widespread rainfall, some of which will be of decent intensity at
times. Also, with the strengthening front, a tightening pressure
gradient will allow for some gusty winds to develop, especially
near coastal areas. The steady rainfall should reach the coast by
late this afternoon/early evening...reaching the far interior
locations by Thursday morning.

Then for Thursday afternoon into Friday, the low slowly fills and
weakens as it slides across the Alaska Peninsula (AKPEN) into the
Gulf. However, cyclonic flow on the backside of the low will be
accompanied by disturbances rotating around the parent
low...helping to maintain at least scattered (if not numerous)
showers through the period.


(Today through Friday)

The proverbial "calm before the storm" best sums up the short
term weather for the eastern 2/3rds of the Bering and Aleutians,
as a deepening low over the west central Bering will trek eastward
while deepening, reaching the Bristol Bay region by Thursday
afternoon, before sliding off to the southeast into the Gulf for
Friday. The associated front is already approaching Dutch Harbor,
and it`ll reach the coast of southwest Alaska and the AKPEN by
late this afternoon. Preceding its arrival, rainfall will be
widespread, and at times somewhat intense rainfall given a semi-
tropical connection.

The real story will be the the winds and waves for the eastern
Aleutian and peninsula coastal waters. As the low deepens, dry
advection behind the front will allow for rapid pressure rises.
Cold advection aloft, aided by modest tropospheric anomalies (as
seen on the 1.5 mb PV anomaly forecasts) reaching 400 mb will
help augment downward momentum transfer to some extent.

The net result will be sustained gales developing east of Adak
across the coastal waters and southern Bering Sea. Here, seas will
build into the 10 to 15 feet range north of the chain, and 15 to
20 feet south of it (due to a longer fetch length). For the
western bering and Aleutians, short-wave ridging will build behind
the storm, allowing for drying conditions to commence.


.MARINE (Days 3 through 5: Friday through Sunday)...

Bering Sea/Aleutians

A Southwest Alaska low moves into the Gulf of Alaska by Sat.
Confidence is good for rear flank small craft winds and waves over
the Eastern Bering and Bristol Bay diminishing Sat. A low exiting
Anadyr moves across the Bering through Sun. Although model tracks
are mixed, confidence is good for Westerly small craft winds to
move across the Bering and diminish Sun. A North Pacific low
approaches the Western Aleutians and Bering for Sun. Confidence is
good for Southerly small craft winds with local gales over the
Western Aleutians through Sun.

Gulf of Alaska

A low and its front to the East of Kodiak Island tracks into the
Northeastern Gulf for Sun. With consistent model tracking,
confidence is good for Easterly small craft winds and waves
through Sun with local gales near capes ahead of the front for
coastal zones diminishing Sat, and widespread Westerly small
craft winds through Sun with local gales diminishing Sat to move
across the Gulf.


.LONG TERM FORECAST (Days 4 through 7: Saturday through

A fairly active and progressive longwave pattern looks to setup
this weekend as a series of systems move eastward from the Bering
Sea/Aleutians into the middle of next week. The first of these
will be a developing low diving from Siberia into the Gulf,
bringing cloudcover to much of the South Mainland and rain
primarily to the Gulf Coast. The next low will quickly follow the
Siberian Low, and will be the first in a series of lows moving
into the Bering from Kamchatka. This pattern will allow for
generally cloudy, cool conditions for much of the area, with
storminess for the Bering Sea and Gulf of Alaska.


MARINE...Gale Warning 155, 165, 170-177.



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