Gale Warning 174, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
514 AM AKST Thu Feb 23 2023

.ANALYSIS AND UPPER LEVELS...

An amplified pattern continues as a deep trough supported by a
strong jet begins to swing over the Mainland. Radar is showing
returns over the Kenai Peninsula, Anchorage Bowl, and the Copper
River Basin with snow being reported to start off this morning.
Temperatures are ranging in the mid to upper 20s with southern
portions of the region, like Homer, reporting temperatures in the
upper 30s. A warm front that is associated with an occluded front
originating in the northern Bering, is draped across central
Alaska and stretches into Southcentral across the southern Kenai
Peninsula. Its cold front is noticed traversing the Kuskokwim
Delta stretching southwestward into the eastern Aleutians.

&&

.MODEL DISCUSSION...

Models continue to be in overall good synoptic agreement and are
resolving the features for today slightly better. However, the
main challenge will be temperatures and precipitation types that
are driven by the southeast gap winds along Turnagain Arm and the
Knik river valley. Models agree that a southeasterly wind will
develop across portions of the Anchorage Bowl and the Mat Valley.
However, they are struggling with the exact timing on when these
winds will develop and then switch back to more of a northerly
component; This will effect snow totals and precipitation types
for today. Another thing to note is the potential for light
freezing drizzle/rain for the western Kenai Peninsula and
Anchorage Bowl as rain may transition back to snow; models are
struggling to resolve this and confidence is low in regards to
this occurring.

&&

.AVIATION...
PANC...Light snow will continue with MVFR and IFR conditions
possible. By 16z gusty southeasterly Turnagain winds are expected
to develop and change the snow to rain or a rain/snow mix. These
winds may be intermittent at times and will be the determining
factor on precipitation type. Precipitation is expected to change
back to snow by 4z and remain snow as more northerly component
moves back over the terminal.

&&

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

A front featuring a deep fetch of moisture is moving over
Southcentral today bringing snow and rain into the region. A weak
triple-point low will develops along the front near Kodiak Island
today, and that will usher the warmest air over the Gulf of
Alaska with limited warmer air making it inland. However, gusty
Southeasterly winds are expected through Channeled terrain from
Turnagain Arm northward including the Knik River valley and along
the Copper River. This is due to the strong pressure gradient
between the large low in the northern Bering Sea and high pressure
in the Yukon Territory. These southeasterly winds remain the real
challenge for precipitation-type today as the overall atmosphere
is certainly cold enough for snow over all areas except near the
Gulf coast (including Kachemak Bay). However these southeast winds
can warm up the surface enough to turn precipitation to rain so
Anchorage, the Matanuska Valley and Western Kenai Peninsula
remain under the threat for some rain today from this system.
Since the warm air intrusion will be mainly at the surface, it is
not really a freezing rain situation with warm air aloft, but a
rain or snow situation as temperatures should warm above freezing
wherever the southeast winds are with the exception of the Copper
River Basin which should remain below freezing. All that to say
that there remains a lot of uncertainty to precipitation type
today from the Western Kenai northward through the Matanuska
Valley. What should be noted is that in spite of the southeast
winds, overall temperatures are looking cooler that they did
yesterday which could keep the predominant precipitation type snow
for the areas under the winter weather advisories.

The Susitna Valley will remain all snow and with the southwest
flow aloft, should see a bit of it. A winter weather advisory for
the snow is in effect until this evening as 6 to 12 inches are
expected today into this evening.

After this evening, all precipitation that remains over
Southcentral Alaska will be snow aside from one possibility. There
is a slight chance for freezing drizzle over Anchorage and the
Matanuska Valley overnight Thursday night due to low-level clouds
remaining over the areas and clearing at all levels above these
low clouds. This is, in general, a good freezing drizzle setup as
it allows supercooled water to precipitate out of the low clouds.
This is not a sure thing or even deemed likely at this point, but
a possibility that should be mentioned.

For Friday into Saturday, the cold upper level trough moves back
over Southcentral which will increase northerly winds through
channeled terrain. It will also clear out skies and bring back
below normal temperatures by Saturday night over most of
Southcentral.

&&

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

The back edge of the precipitation over Southwest, which fell
predominately as rain, can be seen on the Bethel and King Salmon
radars this morning moving east into the Alaska Range. Expect one
more day of above freezing temperatures as southerly wind persist
before a gradual cool down takes place on Friday as westerly
winds usher in a more seasonal airmass. Generally scattered snow
showers will take hold of the region starting Friday morning, with
the mostly likely areas to see measurable snow being western
facing coastlines and mountain ranges. A few more focused periods
of showers are expected early Friday morning and again Friday
afternoon as two shortwaves move in from the Bering. An additional
front will clip the Y-K delta Saturday morning ahead of an
approaching front that will clip Bristol Bay Saturday afternoon.

&&

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

Cool and quiet weather is expected across the Bering through
Friday morning. A few weak lows and shortwaves are expected to
bring localized gales, but for the most part winds will be light
with scattered snow showers. A stronger system will move over the
Western Aleutians Friday morning, bringing high end gales to
storms as the system moves east across the Bering and into Bristol
Bay by Saturday afternoon.

&&

.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: 131, 177-179, 181, 185, 411-413.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...NSC
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CJ
LONG TERM...DN