Scientific Forecaster Discussion
fxak68 pafc 111758 cca
Southcentral and southwest Alaska forecast discussion...corrected
National Weather Service Anchorage Alaska
858 am akst Wed Dec 11 2019
Analysis and upper levels...
The upper flow is dominated by a cold upper trough over the
central Bering Sea, an upper level ridge axis which is building
across southern Mainland Alaska, and a large occluded low moving
north into the Gulf of Alaska. A deformation zone aloft is
sandwiched in between the North Pacific low and the cold upper
trough over the Bering Sea, and this feature is already
moving/spreading precipitation into Bristol Bay and the western
Alaska Range. At the surface, westerly cold air advection and near
gale force winds dominate most of the Bering Sea with a stronger
gale force front moving north through the Gulf of Alaska in
association with the npac low. This is promoting mostly offshore
flow across the southern Mainland coasts, which is also promoting
dry weather over southcentral and interior locations. Beyond some
patchy fog and patchy stratus (more extensive over the copper
basin), weather remains much quieter than recent days/weeks.
the numerical model guidance is in excellent agreement with the
large scale synoptic pattern as well as the sensible weather
impacts over land and marine through Saturday. Forecast confidence
panc...there remains a slight chance that fog or IFR stratus
reforms at the terminal this morning, but that threat seems to be
decreasing as higher cloud cover and weak northerly flow are
limiting cooling over Knik Arm and across Anchorage. Any fog would
likely be short-lived and it is more likely that VFR conditions
prevail, along with less than 12 kt northerly winds.
Short term forecast southcentral Alaska (days 1 and 2)...
a broad area of low pressure will slowly slide northward from the
North Pacific into the Gulf of Alaska. This system will dominate the
weather across the area for the next couple of days. It will
create cyclonic (counter-clockwise) flow across the entire Gulf
and into the Gulf Coast. These winds over the marine zones will
be the strongest today (40-50 mph) before slowly diminishing Thu and
into Fri. They will remain rather strong along the north Gulf Coast
(barrier jet region) and down through Cook Inlet. The general
pressure gradient will continue to switch to offshore through the
day today. This should help any fog/low stratus to dissipate as
drier air feeds into the area. Several waves of energy will spin
around this system as well. Each one will bring more rain from
Kodiak Island up through western Prince William Sound. Nearly all
inland locations will remain downsloped thanks to cross-barrier
flow, so expect mostly dry conditions to persist.
By late Thu-Fri, the vertically stacked low will draw closer and
closer to the coast. As it does so, some models are hinting at a
slightly stronger wave of energy working up from Cordova through
the Copper River basin. This looks to be the next best chance for
some precipitation to move inland. Even that system still looks
very weak with only minimal snow accumulations at best in the
Copper River basin.
Short term forecast southwest Alaska (days 1 and 2)...
The main weather phenomena across southwest Alaska today is a
deformation band that is setting up over the Alaska Peninsula and
into the southwest coast. Most areas in interior Bristol Bay will
see rain as this warm system moves in, with heaviest
precipitation amounts along the coast west of Dillingham.
Meanwhile, areas farther north in the Kuskokwim Delta will see
mostly snow or a wintry mix, with possible freezing rain,
extending from coastal areas inland towards Bethel. This band will
shift slightly westward throughout the day over the eastern
Bering. A showery pattern will remain on Thursday through Friday
as the deformation band sticks around, but it is less certain if
it will be positioned over inland locations or the water.
Short term forecast Bering Sea/Aleutians (days 1 and 2)...
Cold air advection over the western Bering will help keep Small
Craft Advisory winds in the forecast through Thursday morning.
Looking towards the east, a strong North Pacific low will help
keep strong winds across the eastern Bering. The area of highest winds
will be over the Alaska Peninsula which could cause typical
northerly gap winds. Synoptic scale gale force winds south of the
Alaska Peninsula are also expected through Thursday morning when the low
begins to exit this region. While the eastern Bering starts to see
winds diminishing by Thursday afternoon, the next system begins to
enter the western Bering and Aleutians.
Marine (days 3 through 5)...
..Bering Sea and Aleutian coastal waters...
A rapidly deepening area of low pressure will move from the North
Pacific on Friday to near Atka by Saturday morning. This feature
will send a frontal boundary into the south central Bering Sea
during Saturday morning, with the low bottoming out near 960 mb.
Some sustained gales will likely accompany these features, with
gusts just short of storm force likely, especially out of bays and
passes. This system then moves off to the east southeast by
Given a long fetch length, look for seas to build into the 20 to 25
foot range south of the Aleutian chain during the forecast period.
For the remainder of the region, winds look to remain below gales
with seas less than 15 feet expected.
..Gulf of Alaska...
An area of low pressure will move into the southwestern Gulf by
Friday morning, with sustained gales developing for the eastern
Kenai Peninsula coast and eastward towards the Copper River
Delta. Sustained gales are also likely across the southern Gulf.
This low then slowly reaches Kodiak Island Saturday while
weakening, before moving into southwest Alaska on Sunday and
dissipating. As a result, area wide winds will decrease to small
craft criteria near the coast on Saturday, and remain below
headlines for Sunday.
As for your waves, they look to be highest across the eastern
half of the outer waters, where they'll peak in the 20 to 25 foot
range Friday, dropping further for the weekend.
Long term forecast (days 3 through 7)...
Models have come into better, though not perfect, agreement for
the long term. Expect a continued active pattern over the
Aleutians through the end of the weekend, while quieter weather
will persist over southcentral. Friday, a low in the western Gulf
of Alaska will continue to swing a series of weak fronts toward the
north Gulf Coast, bringing periods of precipitation through the
end of the weekend. Precipitation will be concentrated on
southeast facing slopes, particularly on Kodiak Island, the Kenai
Peninsula and the Alaska/Aleutian Range. Meanwhile, surface high
pressure over the Yukon and interior Alaska will strengthen, and
conditions will trend colder over inland areas of the southern
Out west, a strong low south of the central Aleutians will swing
an occluded front across the chain and generate a healthy round of
precipitation over the Aleutians and likely into the southwest
Mainland Saturday night into Sunday. Precipitation chances look
highest in the Bristol Bay area, with the Kuskokwim valley/Delta
likely staying drier in offshore flow. However, it's hard to have
much certainty this far out and with run-to-run model
inconsistencies. Another low will approach the western Aleutians
Monday, bringing another round of precipitation and active
In short, expect temperatures to trend colder through the long
term, especially for interior areas. With each system,
precipitation will progress from the Aleutians toward
Marine...gales 119 120 130 131 132 136-139 150 155.