Gale Warning 189, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
407 AM AKST Mon Feb 25 2019

.ANALYSIS AND UPPER LEVELS...
The two most recent ASCAT/Advanced Scatterometer passes detected
both gale and storm force winds north and south of the Aleutians
in association with the deep low northwest of Adak. The tight
pressure gradient spans from Kamchatka to the Pribilof Islands.
The front is already occluded and the baroclinic zone is over the
Western Aleutians and extends southwest into the NW Pacific.
Behind the cold front, there is an expansive area of tufts of
stratocumulus clouds that go all the way back to Kamchatka. Over
the Eastern portion of Alaska, the ridge retains its stronghold
and widespread subsidence brought clear skies to many parts of
Southcentral today. With those clear skies in place, the
temperatures began to plummet once the sun set. Numerous locations
are in single digit or below zero territory. For a second night
in a row, the temperatures at Ted Steven's International Airport
were colder than Deadhorse. There is an area of low stratus/fog
over the Cook Inlet and Knik Arm.

&&

.MODEL DISCUSSION...
With the longwave trough taking up residence over the NW Pacific
and the Bering, the initialization tool of choice is the
ASCAT/Advanced Scatterometer pass. Both the GFS and NAM
initialized well when comparing the placement of the surface low
north of Attu Island. But the standout feature was the inverted
trough that extends northwestward out of the surface low towards
NE Russia and how the ASCAT pinged right into that wind shift.
Kudos to both NCEP models. The global models are in good agreement
with the major synoptic features through Wednesday morning. This
is true with the domes of high pressure over the Gulf of Alaska
and Central Pacific, the lows impacting the Aleutians and the
super typhoon near the Mariana Islands. The only loose cannon is
the pesky fog over southcentral. The models simple struggle with
the microscale features over Cook Inlet, Knik Arm and Turnagain
Arm.

&&

.AVIATION...
PANC...As of 13z or 4 AM AST the sky conditions at PANC were
clear and the VSBY was unrestricted. However, once again the Baldy
Webcam has detected an area of fog/low stratus near Knik Arm and
PAMR reported FEW001 at 1253z. The potential for fog or low
stratus moving over PANC is actively being monitored. The
potential for low cigs or intermittent low cigs and light winds
will persist this morning. If fog does form over PANC, the current
thinking is that conditions would improve early afternoon as the
temperature begins to climb.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
(Tonight through Tuesday night)

The upper ridge over the Gulf and Southcentral slowly drifts east
over the next couple of days. The flow in the low to mid levels
will become increasing southerly and push moisture in the form of
clouds up Cook Inlet tonight and Tuesday. Increased cloud cover
should weaken surface inversions and lessen the aerial extent of fog
across the Anchorage Bowl Tuesday night. In the interim, areas of
fog are expected across the north and western Kenai and to greater
degree across the Anchorage Bowl this morning and then again
tonight. Interior valleys may also experience some patchy fog.
Southerly flow across the western Gulf will keep Kodiak Island
generally in the clouds with a smattering of precipitation.

Cold air entrenched over the Copper River Basin aided by
favorable pressure gradients (high inland and weak inverted trough
over the North Gulf) will sustain periods of gusty outflow winds
into Tuesday night.

&&

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

A surface front near the coast of southwest Alaska will make slow
progress eastward through tomorrow, before dissipating once it
reaches the interior, as it encounters a downstream ridge of high
pressure. A few disturbances in the flow aloft along with modest
surface convergence along the front will maintain a chance of
precipitation as it makes its eastward journey. Another front will
approach the coast on Thursday, bringing a renewed chance for
precipitation. With a warm southerly to southeasterly flow in place,
look for unseasonably warm temperatures to continue.

&&

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

A repetitive pattern looks to continue across the Bering and
Aleutians through the period. This mornings low across the
western Bering continues to produce sustain storm force winds,
with the latest ASCAT pass showing 55 to 60 knot winds. It's
associated surface front arcs northeastward before straddling the
coast of southwest Alaska, with showery type precipitation noted
along it from here through the AKPEN. As the low continues to lift
north today, storm force sustained winds should continue.

Our attention then turns to the next storm moving from the western
Pacific into the west central Bering. The models were in decent
agreement through the 00z run with the features in the mid levels of
the atmosphere and at the surface, then showed minimal disagreement
(about 3-4 mb in surface pressure strength). However, the train has
gone off the tracks with the latest 6z run, as the GFS model is now
slower and up to 11 mb different in the surface pressure of the low.
This will likely continue (models becoming somewhat erratic in their
continuity) as a complex scenario of tropical moisture streaming
north from Super Typhoon Wutip (a category 4 storm as of this
morning per the Joint Typhoon Warning Center), while interacting
with troughs moving into the area from eastern Russia southward
through Japan.

Typically this type of scenario allows for very strong lows to
develop, as you increase the vertical instability due to warm and
moist advection (the horizontal movement of an airmass) in the lower
levels of the atmosphere, with colder air juxtaposed aloft. It'll be
interesting to see how things play out in the later model runs.

&&

.MARINE (Days 3 through 5)...

A good amount of uncertainty exists for the Bering in the long
term. The GFS model is currently trying to bring a large, storm
force low through the Bering. Meanwhile, the European model has a
solution of two smaller lows that would only produce small craft
advisories in the Bering. So quite the difference between the
two. Right now we are leaning towards keeping a GFS solution and
stronger winds. However, we will have to see how model runs unfold
in the next day or so before we can increase our confidence level in
the Bering Sea long term.

The Gulf of Alaska looks to remain quiet, with forecast models
agreeing that high pressure will hold over the area. This means
generally light winds and seas in the long term.

&&

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

The long term picture actually doesn't look so bad for our
forecast area when it comes to model agreement and the general
outlook for the weather. High pressure is expected hold over
southcentral Alaska and the Gulf, keeping things pretty quiet.
Models each have their slight differences on placement of the
high, but all agree it will stay in place.

The Bering Sea will generally remain under a broad longwave trough,
keeping a more active weather pattern. Confidence in specific low
pressure systems moving through the Bering is low, with reasoning
detailed in the marine section. A shortwave ridge does appear to
work its way through the Bering on Friday providing a brief break,
but this is short lived. Southwest Alaska will be stuck in the
middle, with some chance of precipitation and mostly cloudy skies
mainly close to the coast, but not much going on.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Storm 177 178 185 413 412 411. Gale 160 170 172 173
174 175 176 179 180 414.

&&

SYNOPSIS AND MODEL DISCUSSION...PJS
SOUTHCENTRAL ALASKA...BC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD
MARINE/LONG TERM...BJB