Gale Warning 195, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
527 AM AKST Thu Feb 20 2020

.ANALYSIS AND UPPER LEVELS...

A vertically stacked low over the Bristol Bay area is lifting to
the northeast and moving inland. Northerly flow over the western
Bering is pulling down cold Arctic air which is wrapping around
the backside of the low. The associated front over the Gulf is
lifting inland over Southcentral. While areas along the northern
Gulf coast have seen additional snowfall, midlevel
southeasterly winds have largely kept the Anchorage Bowl dry with
downsloping occurring overnight.

&&

.MODEL DISCUSSION...

Model solutions continue to remain in good agreement with synoptic
features through Friday night. The biggest forecast challenge will
be figuring out details including precipitation types and the timing
of arrival of colder air over the southern mainland Thursday
night into Friday. By Friday night, models show a low developing
south of the western Aleutians and tracking eastward for Saturday,
though there is vast disagreement on the strength and track of
this low. Overall, forecast confidence is near normal.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist through
the morning. Southeasterly winds aloft will produce wind shear
this morning but these midlevel winds are expected to taper by midday.
A front lifting inland over Southcentral will bring MVFR
conditions in snow this afternoon through midday Friday.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
(Today through Friday evening)

Not too much has changed from the previous forecast package, with
just a few minor changes made this morning. The first change was
to drop the high wind warning for the Turnagain Arm and Upper
Hillside areas. McHugh Creek did see peak gusts of 77 mph, with
the Upper Hillside having topped out in the mid 60 mph range. The
pressure gradient will continue to weaken over the next few hours,
with several observation sites in the previously warned area
showing a continued downward trend in the wind speeds.

Our attention now turns to getting a better estimate for the
Anchorage Bowl and western Kenai Peninsula regarding snowfall
timing of increased coverage and intensity. Early this morning,
the Kenai Doppler Radar shows a well pronounced boundary
stretching from near Clam Gulch, to near Soldotna, to east of
Kenai. This is likely an artifact of the down Inlet flow
producing a surface pressure trough/convergence axis, which has
allowed some localized heavier bursts of snow with some
intermittent visibility reductions. However, the latest guidance
continues to show the "precipitation shadow" in the lee of the
Chugach Mountains continuing for a few more hours.

However, with the winds having decreased earlier than originally
thought, one may think a quicker onset of snowfall will commence
as the winds aloft also weaken. That is in fact what the latest
high resolution models now show, with some of the longer range
models also converging on this trend. This is in part due to a
series of disturbances in the southwest flow aloft moving in
faster, with the first such disturbance located over Kodiak Island
now. With winds aloft weakening and the disturbance providing an
earlier onset of ascent/lift above the surface, the radar imagery
should be increasing with precipitation coverage by late morning.
As such, precipitation chances were increased to categorical
(above 80%) for the Anchorage Bowl/western Kenai northward into
the Matanuska Valley.

For the remainder of the area, the forecast remains on track with
the highest snow totals expected over the northern and western
Susitna Valley (roughly from Skwentna west and Broad Pass north),
where around 6 inches of accumulation is anticipated. This is just
below advisory criteria, and will continue to be monitored. For
Thompson Pass, we still expect perhaps a brief reduction in
visibilities to near 1/2 mile, but this should be limited
sufficiently enough in duration to preclude any headline issuance.
Finally, for the western Kenai Peninsula through the Matanuska
Valley, some warm air may still allow for a brief mix, as forecast
soundings do show a shallow/narrow area of above freezing
temperatures. That said, the previously mentioned increasing
ascent should compensate enough for wet bulb-cooling effects to
keep that layer isothermal with better snowflake production, with
2 to 4 inches of snowfall expected.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...

The winter weather advisory for Bristol Bay has expired this
morning as the heaviest snowfall has passed, allowing low
visibility to improve over the region in lingering snow showers.
Later this afternoon, another band of snowfall sets up across the
Alaska Range.  This could bring another round of light snowfall
accumulations over interior Bristol Bay on the backside of the low
that has departed into Southcentral. A second band will also set
up over the Kuskokwim Mountains this afternoon, giving areas from
Aniak to Bethel a brief period of light snowfall through late
tonight.

As the active pattern moves into Southcentral, snow showers begin
to taper off with dry conditions and colder temperatures moving
back into Southwest Alaska for the weekend. Wind chills of 40
below zero for the Kuskokwim Delta coast tonight into Friday has
led to the issuance of a Wind Chill Advisory as gusty winds move
back into the area. The main areas affected will be along the
coast from Hooper Bay to Toksook Bay, including Mekoryuk.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...

A low over Bristol Bay this morning will continue its path
eastward out of the region early today. On the backside of this
low, expect strong winds and gusts until late tonight along the
eastern Aleutians and Alaska Peninsula, especially through the
gaps, as colder air moves in across the Bering Sea. The gusty
winds and widespread snow showers could create blowing snow
conditions along these areas, but should not have any significant
visibility reductions.

Over the western Bering, quiet conditions persists through Friday
morning when a small low tries to spin up over the central Bering.
Models are still struggling on the strength and placement of this
feature and whether a second one tries to develop shortly after.
The next significant system moves over the western and central
Aleutians on Saturday. Stay tuned for additional forecast updates.

&&

.MARINE (Days 3 through 5 Sat through Mon)...
Westerly gale force winds are expected on Saturday in the Gulf,
which will weaken on Sunday. A low will move from the western and
central Aleutians on Saturday eastward across the eastern
Aleutians Sunday and into the Gulf on Monday. Gale force winds are
expected with this low. Confidence on the timing of this low is
fairly high.

&&

.LONG TERM FORECAST (Days 4 through 7 Sun through Wed)...
On Sunday morning the pattern will consist of an upper level
trough over southcentral Alaska extending south into the Gulf of
Alaska, an upper level ridge over the central and eastern Bering
Sea, and an upper low over the Aleutians. The trough over
Southcentral will slowly slide east, moving into Canada Sunday
night. The Bering ridge will then build over southern Alaska
Sunday night, sliding slowly eastward through Wednesday when the
axis should be in northwest Canada. The Aleutian low will move
east and a bit north through Wednesday, undercutting the ridge
over southern Alaska.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Wind Chill Advisory 155.
MARINE...Storms 119 125.
         Gales 128 132 150 155 160 352.
         Heavy Freezing Spray 150 160 165 179 180 185 411 412 414.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...KO
SOUTHCENTRAL ALASKA...PD
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AH
MARINE/LONG TERM...BL