Gale Warning 639, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
445 AM AKST Wed Nov 14 2018

.ANALYSIS AND UPPER LEVELS...
There is a surface low south of Kodiak with an occlusion wrapped
into it with a second decaying low situated south of False Pass.
A third low south of the Central Aleutians is undergoing rapid
cyclogenesis, which is also where the jet core is located. There
is a ridge axis building over the Western Aleutians. On the
satellite which has been verified in multiple surface based
observation platforms, there has been fog over portions of
Anchorage Bowl, Knik Arm and the Kenai Peninsula.

&&

.MODEL DISCUSSION...
The models are decent agreement with the synoptic features through
Thursday, after which the solutions begin to diverge. The area
that is the biggest eye sore is the new low in the North Pacific,
which is on a course for the AKPEN. With the ridge in place over
Southcentral, fog will remain the main forecast challenge.

&&

.AVIATION...
PANC...Patchy fog from yesterday evening quickly turned into
dense fog overnight. Fog often ebbs and flows resulting in
fluctuations in visibility which was observed in the early hours
of this morning. The current thinking is that the atmosphere is
stable enough to hold onto the fog until as late at 10 am.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2/Today
through Thursday night)...
High pressure will build over Southcentral Alaska the next couple
days as the current low over the Gulf exits off to Southeast
Alaska. Surface inversions already in place across much of the
region will only strengthen as the ridge builds in. Thus, expect
areas of dense fog to persist through at least Thursday, especially
in the Cook Inlet region. This includes the western Kenai
Peninsula, Anchorage and portions of the Mat-Su. The Copper River
Basin would be the other area favored for persistent fog. Areas
not in the fog can expect abundant sunshine during the daytime.
Colder air filtering down from the north will support northerly
flow and localized gap winds along the coast, including Seward,
Whittier, and Thompson Pass areas. As far as gap wind events go,
this one will be quite weak.

As the ridge begins to slide eastward Thursday night, a storm
system will lift northward from the Pacific, with a gale force
front bringing rain and wind to Kodiak Island Thursday night. If
there is any fog remaining over Southcentral (which is a distinct
possibility), increasing clouds (and eventually precipitation)
should clear it out Thursday night or Friday.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3...Wed
through Fri)...The relative "lull" in the pattern will continue
through Thu. The only weather features of substance are the snow
showers along the YK-Delta coast and the low stratus across much
of the Kuskokwim Valley and Delta. The snow showers are associated
with a baroclinic zone that is nearly stationary over the Hooper
Bay area. This feature should begin a slide to the west over the
open ocean later today. The other problem is the low stratus
currently stuck across much of the northern interior of the
region. The offshore surface winds have been enough to keep any
fog from forming thus far. However, a fairly dense shield of
stratus is in place. As high pressure settles over the area today
and winds begin to relax, it could be difficult to break this
stratus up. If the stratus does break and some radiational cooling
can take place tonight into Thu, there should be a better chance
for fog Thu morning.

Yet another warm system starts to press up into Bristol Bay by
late Thu into early Fri. This system will bring another round
of east to southeast winds, especially through the usual gaps.
There should be enough cold air back in the valleys of SW AK to
allow precipitation to at least start as snow. However, as warm
air moves overhead and southeast winds mixing down to the surface,
expect precipitation to change over to rain rather quickly. There
is an outside shot for some freezing rain once again in this
scenario, but at this time the surface temperatures look to warm
too quickly to add it into the forecast.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3...Wed
through Fri)...
The majority of the Bering will be dominated by colder northerly
flow and rain (turning to snow) showers today before a few
systems move in/take shape on Thu. First, what were once strong
storm- force northerlies have diminished all the way down to just
barely small- craft level winds over the Central Bering. This flow
will continue to abate today. However, the cold air over the
Central and Western Bering will work in tandem with some warmer
air over the Eastern Bering. This will create an environment ripe
for continued showers and even for a surface low to develop near
St Matthew Island late Thu. Moisture and warm air will move back
into Bristol Bay as another warm front slides into the area from
the N Pacific late Thu. This will bring an increase in easterly
winds and widespread stratiform rain and snow.

The other system is a weakening warm front that will push into the
Western Aleutians early Thu. This system will bring gale-force
easterly winds and steady precipitation. The precipitation will
start as a mix of rain and snow before quickly transitioning to
all rain as warm air plows in from the N Pacific.

&&

.LONG TERM FORECAST (Days 3 through 7)...

The long-term forecast begins Thursday night with an amplifying
upper-level ridge moving from the Gulf over the Alaska Peninsula.
Upstream, a negatively-tilted trough persists from the eastern
Bering south to the western Gulf while a second trough deepens
over the western Aleutians as a strengthening jet rounds its base.
Through the first half of the weekend, the longwave pattern
remains fairly progressive, with the leading ridge moving well
inland over Canada as the main trough axis settles along the west
coast of Alaska. Moving into early next week the eastward
progression of the trough slows as it anchors over the eastern
half of the state. Model agreement breaks down significantly after
Monday, especially with their respective solutions upstream of
the trough axis. The GFS is the more amplified solution, trying to
build a ridge over the Bering ahead of the next trough exiting
eastern Russia. The EC, on the other hand, keeps the pattern zonal
with an active jet south of the Aleutians and a broad low over
the Bering. The Canadian takes the middle ground with weak ridging
over the Bering quickly replaced by a trough. What this means is
that guidance in the medium and near long-term range (through the
weekend) remains in good agreement with a wet and active pattern
across much of the forecast area. Beyond that, there is some
agreement of a trough/ridge/trough setup from the western Bering
to the eastern Bering to the Gulf. But confidence on placement and
intensity of these features is slim to none.

At the surface, this translates to a low approaching Kodiak Island
late Thursday with a trough extending across the Bering keeping
things unsettled over Southwest Alaska. The low moves into the
northern Gulf with its occluded front draped across the
southcentral coast by early Friday. Farther west, a second surface
low slides across the Aleutians Thursday into Friday with its
occluded front nearing the southwest coast late Friday. A triple
point low is also expected to form south of the AKPEN late Friday,
moving into the western and northern Gulf through the weekend.
This will keep conditions quite wet an unsettled over much of the
mainland through the weekend with some drying likely for southwest
Alaska by late Monday into Tuesday as a weak surface ridge tries
to develop.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Dense Fog Advisory 101 121.
MARINE...Gale Warning 150 177 178.

&&

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