Gale Warning 125, NWS Anchorage, AK

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

.ANALYSIS AND UPPER LEVELS...

A mature storm-force low is situated over the western Bering this
afternoon and lifting northward. Its front is moving slowly eastward
towards the AKPEN. Gusty winds and rain have been observed as
this fetch of moisture moves from the North Pacific across the
central Aleutians to north of the Pribilofs. A front remains over
southwest Alaska, weakening as it moves towards the interior.
Southcentral remains under the amplified ridge, once again
seeing another day of generally clear and calm conditions. This
high amplitude ridge has progressed a bit more eastwards towards
the Alcan border, but overall the classic "Omega block" pattern
remains the same across the mainland with high pressure building
over the Gulf.

&&

.MODEL DISCUSSION...

The overall synoptic pattern is relatively stable, especially with
the high amplitude omega block over the state. The uncertainty
lies over the Bering Sea with smaller scale perturbations as a
series of lows will move through the Bering Sea toward the Bering
Strait Tuesday through Thursday. There had been quite a bit of
uncertainty with these strong lows with the global models shifting
back and forth between one consolidated strong low or two separate
but weaker lows. The ECMWF led the way with that development, and
the GFS and NAM have now shifted to that scenario now as well.
With that said, the general pattern of stormy and warm weather
over the Bering Sea will continue with no end in sight. The
favored model was the ECMWF with the NAM nest early on for
resolution.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist through late
this evening. Slightly better chances for fog are expected
tonight, but high uncertainty still exists exactly where the fog
will setup. Last night it formed over the Inlet but remained
there and never moved into the terminal...that is once again a
possibility tonight. For now, will hedge for some fog developing
overnight.

&&

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

The upper-level ridge will continue to drift east tonight,
setting up a more pronounced southerly flow in the lower and mid-
levels across the western Gulf. This fetch will help push a band
of low stratus north through Cook Inlet tonight and keep it in
place through midweek. Farther north, strong surface inversions
will again set up due to a combination of radiational cooling,
weakening northerly surface winds, and warmer air advecting in
aloft. The challenge in all this remains the spatial extent of the
fog overnight, specifically in and around the Anchorage Bowl
south to Kenai, as this will be dependent on how far north the low
stratus advances. Currently, events looks similar to last night,
with the low stratus having a hard time advancing any farther than
Anchor Point and Kenai. So the expectation is for fog to develop
north of Kenai and south of the MAT-SU valleys. For areas north of
Palmer and Wasilla, there should be just enough of a northerly
flow to keep the surface air mixed. Fog will be less of a concern
Tuesday night into Wednesday as cloud cover increases from Kodiak
Island north to the lower MAT-SU valleys in response to a passing
weak shortwave. Patchy fog, however, will remain a possibility in
interior valleys of the Copper River Basin the next two nights.

Elsewhere, the persistent southerly flow will keep clouds and
scattered mixed rain/snow showers over Kodiak Island. Colder air
in the Copper River Basin aided by favorable pressure gradients
(high inland and weak inverted trough over the North Gulf) will
keep outflow winds going along favored coastal locations into
Wednesday.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2/Monday night
through Wednesday)...

Southwest Alaska continues to be sandwiched between a high to its
east over southcentral AK and a low to its west in the Bering. As
a result, the front that has been sitting over southwest Alaska will
continue to do so. It will finally decay Wednesday. Expect a showery
rain/snow mix mainly along the coast until then. Inland areas
will stay east of the front so weather will remain quiet there.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2/Monday
night through Wednesday)...

A storm force low currently in the western Bering will continue to
lift north tonight and move out of our forecast area. This well
wrapped, mature system is well defined on satellite. Tomorrow
will bring a brief reprieve for the Bering and Aleutians. Tomorrow
night, low pressure will enter the central Bering from the south
bringing gale force winds with it. This will be the first of two
lows, with the second following the same path and arriving
Wednesday.

Significant changes to the forecast were made today regarding
this weather pattern. Initially, one large and stronger low was
forecast to move through the next couple of days. The GFS was
initially showing this one system, with other models indicating
the two low solution now in the forecast. In the past day, the GFS
has merged with other models, indicating the two low solution is
the way to go. This increases our confidence in this solution as
models now generally agree. Looking at satellite far upstream in
Japan and China, the beginnings of these systems are forming with
the looks of two separate lows to also increase confidence.
Therefore, made changes to the wind and precipitation pattern to
indicate a large area of gale force winds and precipitation. These
systems will be in quick succession, so gale force winds will be
sustained all the way through Wednesday as there is little break
between each system. The same goes for precipitation. This
forecast will certainly be tweaked over the next day to hone in on
winds...so stay tuned for updates.

&&

.MARINE (Days 3 through 5)...

We begin Wednesday with a strong pressure gradient in the Bering
Sea east of 175W. There remains a good amount of uncertainty
as we progress into Wednesday night and Thursday with the strength
and location of the low centers. This translates to some wind
direction and intensity uncertainty. At this point expect gale to
storm force wind over much of the Bering east of 175W most of
Wednesday and Thursday. Following this storm is a break in the
action as the next strong storm is not expected to impact the
western Aleutians and Bering Sea until late Friday. There is a
significant difference in model solutions for this storm, though
there is agreement that we will likely see gales, and possibly
storm force winds, over the western Bering Sea and western
Aleutians late Friday.

Persistent high pressure over mainland Alaska into the Gulf of
Alaska equates to generally benign conditions, sub gales Wednesday
through Friday.

&&

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

The strong upper high will continue to be the dominant feature
over mainland Alaska into the Gulf of Alaska while the Bering Sea
and Aleutians remain very active. Models and ensembles are in
good general agreement with this overall pattern.

Beginning Wednesday, the ridge will persist over eastern Alaska
as the broad deep upper trough is entrenched over the Bering Sea
Thursday through Saturday. A strong surface low treks north
through the central Bering Sea Wednesday leaving a broad weak
trough of low pressure across the Bering Thursday and Friday. This
is followed by a weak ridge poking into the eastern Bering Sea
late Friday into Saturday while yet another strong low moves into
the picture over the western Aleutians. Here the models diverge on
the location and intensity of this storm, but do agree on a tight
pressure gradient moving into the central to western Bering Sea.
This prefrontal pressure gradient then moves to the eastern Bering
Sea through the weekend, bringing with it gale to storm force
wind along with warmer low level air.
&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale 165 170 172-176 178-181.
		 Storm 185.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...AH/JA
SOUTHCENTRAL ALASKA...TM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...BB
MARINE/LONG TERM...SA