Gale Warning 109, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
427 AM AKST Mon Feb 11 2019

.ANALYSIS AND UPPER LEVELS...

There is a surface low in the southern Gulf of Alaska and a deep
surface low near Attu Island with an approximate intensity of 958
mb. Additionally, there is a dome of high pressure over the central
Pacific with a ridge nosing its way towards southwestern Alaska. The
low in the western Bering has an occluded front north of the chain
with the warm front bisecting the Aleutians and the associated cold
front extends back towards Japan. This juggernaut is producing
hurricane and storm force winds and wave heights greater than 40
feet near the Western Aleutians. The core of the jet has curved into
the Bering and is steering this system. Meanwhile, the radars at
PAHG/Kenai, PAIH/Middleton Island, and PAKC/King Salmon are all in
"Clear Air Mode" early this morning. While the leading edge of the
precipitation shield from the next Bering system has moved into
Nightmute and Kipnuk and has been detected by radar at Bethel/PABC.

&&

.MODEL DISCUSSION...

The global models are in good agreement through 72 hrs with the
placement of the synoptic features. This pattern where the longwave
trough has dominated the western/central Bering and NW Pacific has
been going on for weeks now. The persistence of this pattern may be
impacting the performance of the global models. There are some minor
differences with the intensity of the jet core associated with the
current Bering storm and with the 1000-850 thickness values past 24
hrs which could be a red herring for p-type for the AKPEN and SW AK.
But otherwise the confidence level for this forecast package is
above average especially considering that it is February.

&&

.AVIATION...
PANC...Fog has again developed over the Anchorage Bowl this
morning. As of 1253Z or 3:53 AM local the visibility at PANC was
reduced to 1/4 SM freezing fog and the ceiling at PAMR was 100
feet. Looking at other spots around the Cook Inlet, fog was
observed at the metar for PAEN and fog was detected on the AVHRR
south of Anchorage. Generally expecting light winds and VFR
conditions at PANC for the ensuing 30hr forecast period.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2/Today
through Tuesday night)...
As a very weak short-wave trough exits this morning a ridge will
build right back overhead of Southcentral. Local areas of fog will
linger through the morning hours, especially from the western
Kenai Peninsula northward to Anchorage. A weakening surface front
will then approach from the west this afternoon, leading to
increasing mid to high clouds. With the parent low far away over
the northern Bering Sea, there will be very little in the way of
upper level forcing. Thus, the front will cross Southcentral tonight
with very little precipitation. Deep and strong low level southwest
flow will produce upslope along the west side of the Kenai, Chugach,
and Talkeetna Mountains. Thus, the best chance of some light snow
accumulation will be right along the mountains. Cold air advection
behind the front will kick up winds across the western Gulf as well
as the bays and passes of western Prince William Sound overnight
tonight into Tuesday.

A strong upper level short-wave tracking along the Aleutians
Tuesday will lead to cyclogenesis south of the Alaska Peninsula.
This low will track south of Kodiak Tuesday night then exit across
the southern Gulf Wednesday. This scenario is similar to what
occurred in this same area Saturday night and Sunday. A building
ridge upstream of the low will help tighten pressure gradients and
lead to pressure rises along the Alaska Peninsula. When combined
with cold air advection, this will lead to another round of strong
winds across the western Gulf and Kodiak Island on Wednesday,
with the typically much stronger gusts out of bays and passes of
the Alaska Peninsula.

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

A powerful hurricane force low over the western Bering Sea is
moving a strong warm front east into the coast of the Kuskokwim
Delta this morning. Snow and blowing snow have already developed
along the coast, and it will continue into the mid morning before
warmer air aloft begins to move in. This will introduce a wintry
mix of precipitation as snow, rain-snow, freezing rain, and sleet
will all be possible across the Delta and into the Bristol Bay
coast. However, the worst conditions will be along the Kuskokwim
Delta coast where visibility less than a half mile at times with
southerly winds as high as 55 MPH will continue through midday
before SW winds (up to 35-45 MPH...strongest along the Kusko Delta)
and the dryslot moves in. As the front moves inland, snow
accumulations will develop over the Lower Kuskokwim Valley and
northern interior Bristol Bay with a mix along the coastline. The
front will rapidly move over the Alaska Range by tonight and this
will end precipitation over most of the Mainland.

Behind this strong low, westerly cold air advection will switch to
drier northwest flow and cooling temperatures. Largely dry
conditions will remain in place as this colder air moves in, with
temperatures falling back below average by Wednesday night.

&&

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

A still powerful near hurricane force low over the western Bering
Sea will push strong storm force winds all the way up to the
northern Bering Sea by late this afternoon, along with seas in
excess of 40 ft. Large seas and high winds from the SW will also
impact the Pribilof Islands, and the High Wind Warning remains in
effect through this afternoon. Also expect the possibility of some
coastal flooding around the Saint Paul boat harbor this afternoon.
The strong low weakens then exits the Bering Sea by late tonight
with cold air advection westerly flow (in the 25 to 30 knot range)
setting up over most of the Bering through Tuesday. A low will
clip the Eastern Aleutians Tuesday and Tuesday night with snow and
possibly some blowing snow as gusty northerly winds develop. Stay
tuned to this low as some uncertainty remains with this feature.

The next major system of interest will move into the Western
Aleutians/Bering Sea Tuesday night through Wednesday night as
another powerful storm force low impacts that region. The
associated front will reach the Pribilof Islands, likely as gale
force, by late Wed night.

&&

.MARINE (Days 3 through 5)...

The period begins early Wednesday with a low around 960 mb southwest
of the Bering Sea and another strong low south of Kodiak Island. The
track and intensity of both of these lows is not changing much from
run-to-run on the computer models and the overall solution looks very
reasonable, so the forecast confidence is above normal as
compared to other lows recently.

The low south of Kodiak should pull southeastward and bring in
some north-to-northwesterly storm force winds south of The Alaska
Peninsula and near Kodiak Island Wednesday then move out of the
area by Thursday. The low approaching the Bering Sea and the
front associated with it will bring periods of gales to most of
the Bering/Aleutian region with a significant area of storms in
the western Bering/Aleutians on Wednesday.

&&

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

The long term forecast period starts early Wednesday with the
very active storm track over the western and northern Bering Sea
where it will generally remain through next weekend. A strong
frontal system will cross the west and central Aleutians on
Wednesday and then continue across the Bering Wednesday night and
Thursday to reach the southwest Alaska coast Thursday afternoon.
This front will then shear apart and dissipate Thursday night as it
tries to swing inland to be replaced by a high amplitude ridge.
The next strong front will move into the central Bering Thursday
night and continue east to push across southwest Alaska Friday
night. Another frontal wave will follow behind it, crossing the
Alaska Peninsula Saturday and then lifting northeast into southern
Alaska Saturday night through Sunday. Yet another strong front will
approach the western Aleutians late Saturday night. Its powerful
parent low will track into and across the Bering Sunday through
Sunday night with its associated frontal system swinging east across
the Aleutians and Alaska Peninsula to reach southwest Alaska Monday.

For southcentral Alaska, the long term forecast period will begin
with a low track from west to east across the southern Gulf
Wednesday with a cold upper level trough to the north digging into
southcentral Alaska behind it. The result will be a rapid
strengthening of offshore flow Wednesday afternoon through Wednesday
night, particularly through channeled terrain gaps. Upper level
ridging will build in from the west Thursday and Friday bringing a
gradual easing of the offshore flow and steady warming aloft. By
Friday night the axis off the upper level ridge will push east to be
centered over southcentral Alaska with a weakening frontal system
following behind on Saturday. A second, better organized frontal
wave will push into southcentral Alaska Saturday night and Sunday.
Ridging will briefly rebuild Sunday night ahead of another front
coming in from the southwest.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...High Wind Warning 191 195. Winter Weather Advisory 155.
MARINE...Heavy Freezing Spray 185. Storms 414 413 412 411 175 177
178 179 181 185. Gales 165 170 173 176 180 351 131 120.

&&

SYNOPSIS AND MODEL DISCUSSION...PJS
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JA
MARINE/LONG TERM...JR