Gale Warning 172, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
456 PM AKST Wed Feb 22 2023

.ANALYSIS AND UPPER LEVELS...

An amplified pattern is in place with the jet stream moving north
out of deep trough across the north Pacific and towards western
Alaska. It then transverses over a ridge across western Alaska
and then dips southward towards Southeast Alaska. A complex pair
of lows are moving across the western Aleutians and northern
Bering Sea. These lows are pushing an occluded warm front into
Southwest Alaska with gusty winds of 30 to 40 knots and a mix of
rain and snow. Across the Aleutians gusty winds up to 40 knots are
occurring with periods of rain as the trailing cold front moves
east. The ridge across western Alaska is bringing quiet conditions
across Southcentral with an increase in clouds with light winds
and dry conditions this afternoon.

&&

.MODEL DISCUSSION...

Models are in good agreement with the synoptic pattern and are
coming into better agreement with some of details. The main
challenge in the near term is temperatures and precipitation types
driven by the southeast gap winds along Turnagain Arm and the
Knik river valley. All models now agree, even the GFS, that
southeast winds will develop across parts of the Anchorage bowl
and the Matanuska valley which should warm temperatures into
upper 30s to near 40 with snow changing to rain. Across the
western Kenai Peninsula models continue to hint at a period of
light freezing rain Thursday morning as the snow transitions to
rain. Out west models continue to agree on gusty winds and a rain
snow mix across Southwest Alaska.

&&

.AVIATION...

PANC...VFR conditions and light northerly winds will persist today.
A low pressure system approaches from the west tonight and is
expected to bring light snow to the airport with MVFR and IFR
conditions possible. By the early morning hours southeasterly
Turnagain winds are expected to develop and change the snow to
rain. These winds may be intermittent at times tomorrow. Rain will
then change to snow sometime Thursday evening. Snowfall totals
tomorrow are expected to be 2 inches or less depending on how
quickly it changes to rain.

&&

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

A complex and active weather pattern is in store for Southcentral
over the next couple of days as the next low pressure system moves
in. Snow will begin across Southcentral Thursday morning. The snow
will mix with rain across the lower elevations in the Anchorage
Bowl and Matanuska Valley Thursday afternoon. The snow will change
to all rain along the southern Kenai Peninsula late Thursday
morning as warmer air makes its way in. Colder air will then work
its way in during the afternoon hours and allow for the rain/snow
mix across the Anchorage Bowl to change back to all snow during
the late afternoon Thursday. The highest snowfall accumulations
will be across Hatcher Pass and the Upper Hillside in Anchorage.
Winter Weather Advisories have been issued for the Anchorage Bowl,
Matanuska Valley, western Kenai Peninsula and Susitna Valley for
Thursday morning through Thursday afternoon/evening. Please see
weather.gov/afc for the latest advisory information.

The other big story will be a Turnagain Arm wind bending into far
southern Anchorage and across the airport. There is lower
confidence in how much this wind will bend into Anchorage due to
high resolution guidance not being very representative of the
broad synoptic pattern. This wind may affect the precipitation
type across Anchorage, the more the wind bends into Anchorage, the
more the temperature rises and more likely the snow changes to
rain.

&&

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

Southerly winds across Southwest Alaska will keep temperatures
above freezing for most locations tonight as a front pushes
inland. Rain, which has mostly cleared out this afternoon, will
pick up again early this evening as a cold front sweeps from west
to east across the region, covering most of Southwest Alaska in
precipitation by midnight. Rain falling onto snow covered
surfaces will continue to present an ice hazard, particularly on
roadways or trails that are compacted. However, this ice is not
expected to accumulate and build on surfaces as surface
temperatures are expected to remain above freezing. The heaviest
precipitation clears out from west to east starting early Thursday
morning as the back edge of the front crosses the area. As the
precipitation comes to an end, there is a chance for snow to
begin mixing in with rain as cold air moves in on westerly flow
aloft behind the front. Snow amounts will be very low, and there
should be no accumulation with surface temperatures above
freezing Thursday morning. Temperatures cool below freezing by
Thursday afternoon, and weak disturbances moving in behind the
front should be enough to produce intermittent snow showers within
this relatively unstable environment. Any accumulations from these
showers are expected to be minimal.

A stronger shortwave trough pushes across Southwest Alaska on
Friday, bringing more widespread snow showers. These will bring a
higher chance of snow, though accumulations remain less than an
inch and will be highest in the Kuskokwim Delta and Kuskokwim
Valley. Behind this disturbance, a weak transient ridge moves in,
leading to clearing skies and colder temperatures Friday night.
For Saturday, a weak low looks to track from the Bering Sea across
the Alaska Peninsula. There is good agreement on the track of the
low and it looks likely that its front spares the majority of
Southwest Alaska. North of King Salmon, little more than another
round of scattered snow showers is expected for the weekend.

&&

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

Rain pushes east of the eastern Aleutians this evening as a cold
front pushes into Southwest Alaska tonight. Scattered snow
showers move in across the Bering Sea in the wake of the front.
A weak low in the western Bering Sea will track eastward, bringing
a swath of gale force south to southwesterly winds from the
central Aleutians to the Pribilof Islands by Thursday morning as
the low opens into a trough. With a colder airmass moving in from
the west, slightly below average temperatures and light westerly
winds sweep across the area. A stronger low enters from the North
Pacific on Friday, bringing gale force winds to the western
Aleutians. The low tracks eastward across the Bering through
Saturday, pushing its swath of westerly gales along the Aleutian
Chain. Along with the low's front, light to moderate rain sweeps
eastward along the islands through Saturday. Heavier rain begins
to move in for the Chain on Saturday as a tap of deeper tropical
moisture takes aim along the western/central Aleutians for the
weekend.

&&

.LONG TERM FORECAST (Days 4 through 7: Sunday through Wednesday)...

Deterministic guidance along with ensemble support continue to
show agreement with the large scale pattern evolution throughout
the long-term period. A broad ridge looks to build northward from
the North Pacific through the central Bering Sea and slowly
progress eastward through time. There are some minor differences
in how fast the ridge axis shifts eastward and if there will be
weak shortwaves traversing the periphery of the ridge. Even with
the broad ridge, the odds tilt toward the Bering Sea, especially
the western Bering, remaining active during the long-term with
overall continued lower heights and troughing. Mainland Southwest
Alaska will see a break from activity on Sunday with northwesterly
upper flow downstream of the broad ridge. Moist southwesterly
flow and active weather returns Monday for Southwest as the ridge
shifts eastward; this allows Bering Sea low pressure systems and
fronts to plow into the coast.

Meanwhile, Southcentral Alaska also looks to be in for a quiet
weather period Sunday and through most of Monday while under the
favorable side of the ridge. As the ridge axis continues to shift
eastward Tuesday, Southcentral also looks to return to deep and
moist southwesterly flow, not unlike the current setup and pattern
for the short-term. While the exact track and details of where
surface features develop and eventually track are fuzzy at this
time, confidence is relatively high that the deep southwesterly
flow and northward pull of Pacific moisture will result in heavy
precipitation for coastal Southcentral and possibly interior
locations of Southcentral. Inevitably, precipitation-type will be
a forecast challenge for Southcentral as well as Southwest due to
the possibility of warm air intrusion from the southwesterly
flow. Continue to follow the forecast as details hopefully become
clearer for the evolution of Monday through Wednesday's pattern.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 101 111 121 145.
MARINE...Gale Warning 160 165 170 173 175-177 179-181 185
					  411-414.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...RC
SOUTHCENTRAL ALASKA...ED
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CQ
LONG TERM...DAN