Gale Warning 139, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
500 PM AKST Wed Feb 6 2019

.ANALYSIS AND UPPER LEVELS...
The upper level ridge continues to slowly move over Southcentral
Alaska this afternoon. Southerly flow and a few rain showers have
been observed over the AKPEN as the frontal boundary, extending
from Southwest to south of the Chain, progresses eastward.
Localized flurries have been seen through the Anchorage Bowl
earlier today. Other parts of Southcentral remain quiet and dry
this afternoon. To the west of the mainland ridge, a deep trough
is situated over the western Aleutians and Bering. Circulation
from a closed low, along with a few showers, can be spotted on
water vapor imagery moving over the western Aleutians.

&&

.MODEL DISCUSSION...
Models continue to be in good synoptic agreement as the amplified
ridge sits over Mainland through Saturday. Models are also in
agreement with the placement of the low that moves into the
western Bering Fri afternoon. However, the models differ with the
intensity of this storm.

&&

.AVIATION...
PANC...Fog and low stratus remain the big concern for overnight.
Guidance is indicating little to no fog, but that does not look
reasonable. Expect periods of fog and stratus through Thursday
morning.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2/Wednesday
night through Friday)...
High pressure has begun to move into southcentral Alaska. A
change in weather has taken place with this pattern shift. We have
moved from being concerned about precipitation and winds the past
couple days to quiet weather on the large scale, with fog and low
stratus being our focus tonight and over the next couple of days.
This is due to the ridge of high pressure that is trapping
moisture underneath it. The moisture layer is shallow over the
MatSu and on down through Anchorage and to the Kenai, so fog is
expected there. A low level stratus cloud deck exists in the
Copper River Basin due to a more robust moisture layer, so stratus
is expected to continue for that region. Dense fog is also
present along the coast around Cordova and is expected to remain
in place through the morning tomorrow.

To elaborate more on Anchorage area fog, we currently have a
dense fog advisory out until midnight for the Palmer-Wasilla area
and Knik Arm. We will evaluate further this evening to see if we
need to extend this advisory through the morning. We are expecting
fog to develop in Anchorage tonight, but due to the unpredictable
nature of fog we will have to watch and see how dense the fog
gets around town. So stay tuned to the NWS forecasts for
developments tonight.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
A low level jet of warm, moist air ahead of a trough digging into
the North Pacific will continue to be the main player in the
Southwest's weather through Thursday. Strong south winds and
cloudy skies with areas of precipitation have persisted through
much of the day in the Kuskokwim Delta and Bristol Bay areas. Of
note, cross- barrier flow has kept conditions dry on the north
side of the AKPEN with temperatures in the 40s, and expect
Thursday to be even warmer potentially reaching 50 degrees.
Elsewhere, with temperatures in the mid 30s, precipitation types
continue to be a challenge, but many observing sites have changed
over to rain this afternoon and expect the changeover to continue
into this evening. Rain could switch back to snow tonight in
colder areas as temperatures drop, but by tomorrow afternoon low
lying areas will switch back to rain as temperatures approach the
upper 30s.

The lower Kuskokwim Valley has been slow to warm up with
temperatures still below freezing. As temperatures in the lower
levels of the atmosphere increase, stagnant cold air under an
inversion could facilitate brief freezing rain Thursday night...
however considering models show temperatures aloft right around
freezing, or just barely above, there is enough uncertainty to
preclude this from the forecast.

The trough will push eastward Thursday into Friday, and cold air
on the backside of the trough will veer to southwest or west, and
Southwest will see a brief one day break from warm wet weather.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
Warm air advection ahead of a trough digging into the North
Pacific continues to bring precipitation to the eastern Aleutians,
while farther west the cold pool associated with this trough will
continue to trigger showers tonight into Thursday. A compact gale
force low will approach the western Aleutians from the southwest
Thursday afternoon, but is expected to skirt far enough south of
the Aleutians that gales are unlikely in the coastal waters.
Models continue to project a hurricane force low developing in the
Pacific Friday, which will bring storm force winds to the western
Aleutians and Bering...possibly even areas of hurricane force
winds.

&&

.MARINE (Days 3 through 5)...
There remains high confidence of a strong low tracking northward
across the western Bering which will bring diminishing storm force
winds to the western Aleutians/Bering Saturday. These will
generally persist throughout the day, weakening through the
evening into Sunday morning as a weak ridge builds in behind the
low. There is good agreement that winds and seas will rapidly
increase back to southeasterly gales Sunday afternoon and further
increase and veer to southwesterly storm force overnight through
Monday afternoon as an intense ~960mb low tracks to the west of
Attu. The most intense winds will occur over the western
Aleutians, gradually diminishing away from the low center, though
gales are still possible as far east as Cold Bay. Winds will
overall diminish across the entire Aleutians Monday evening as the
low departs to the north.

Light winds and calm seas are expected to be over the Gulf of
Alaska Saturday under high pressure. A low is expected to develop
over Bristol Bay Sunday morning and track east into the Gulf of
Alaska by the afternoon, increasing waves and seas over the
western Gulf. There is the potential for gale force offshore winds
to develop over Shelikof Strait and the Barren Islands Sunday
afternoon/evening, and may continue in this region into Monday,
however there is significant disagreement in the position of the
low, thus confidence remains minimal at this time.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Beginning Saturday, a high amplitude upper ridge will be located
over the eastern half of the mainland with a deep upper low
spinning over most of the Bering Sea. The ridge, which had
previously deflected eastward propagating energy to the north over
the ridge keeping Southcentral dry, will begin to tilt to the
northeast allowing the upper low to shift further east while
sending a series of disturbances into Southcentral. Initially, a
closed surface low south of the AKPEN will become absorbed into
the main flow of the upper low and push east across Southcentral
by Saturday night. Next, a cold front will rapidly move across
the southwest mainland on Sunday bringing precipitation, changing
over to snow during the day on Sunday, to the area before pushing
into Southcentral later in the day on Sunday. There is still a
fair amount of uncertainty regarding how far north this system
tracks, which will determine snow potential across inland areas
over Southcentral.

The overall progressive pattern will continue with another strong
North Pacific low approaching the western Aleutians Sunday night,
with increasing confidence that the system will strengthen to at
least storm force as it crosses into the Bering. The low will
induce a ridge downstream over the mainland, leading to a break in
the weather on Monday. A front associated with the strong low
will plow across the western mainland Monday evening before
weakening some as it pushes east into Southcentral. The
progressive trough ridge couplet pattern looks to continue through
the rest of next week with the North Pacific storm track spinning
up a low across the Bering and into the mainland every 2-3 days.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Dense Fog Advisory 111.
MARINE...Gale 155 160 165 172 178 180.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...SS
SOUTHCENTRAL ALASKA...BB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MM
MARINE/LONG TERM...KVP