Gale Warning 109, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
455 PM AKST Tue Feb 4 2020

.ANALYSIS AND UPPER LEVELS...
The general pattern consists of an upper level trough over the
Bering Sea and a weak upper ridge over northwest Canada. This puts
south to southwest flow over all of southern Alaska and most of
the far eastern Bering and eastern Aleutians. This is bringing
fairly mild and cloudy conditions to the area. There is a front
stretched basically west to east across the Gulf of Alaska. A
surface low along this front is located southeast of Kodiak. These
features are moving towards the north-northeast. There are a
couple of surface lows over the Bering. One of these is centered
just southeast of the Pribilofs and moving towards the northeast.
The other is in the central Bering near 180 longitude and nearly
stationary.

&&

.MODEL DISCUSSION...
The numerical models are in good agreement through Thursday
afternoon on the synoptic pattern, but differ in a couple
mesoscale details. There are some differences in the location of
the low over the northern Gulf tonight through Wednesday with an
east-west spread of the center by Wednesday afternoon from just
west of Montague Island to near Cape Suckling (about 150 miles).
There are also some differences in the handling of various surface
low centers associated with a "complex" low crossing the
Aleutians and Alaska Peninsula from Wednesday afternoon through
Thursday afternoon. The forecast confidence is thus near normal.

&&

.AVIATION...
PANC...VFR conditions are most likely through tonight. Occasional
IFR ceilings are possible with plenty of low level moisture in
the area. Conditions are then expected to drop to MVFR in light
snow Wednesday morning. Light winds are expected through Wednesday
afternoon.

&&

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

The weather pattern across Southcentral continues to favor
onshore, southerly flow. This will continue to transport warm air,
clouds, and precipitation into the area. With that being said,
amounts and precipitation type will be tricky. Overnight tonight,
after midnight, the remnants of a front along with an associated
weakening upper level trough will move across the area. For most,
this will result in about 24 hours of potential very light
precipitation. This should be in the form of snow for most away
from the coast. It will certainly not be steady, but instead
likely fall as an off-an-on type snow. Accumulations will be
minimal, staying at under an inch for most everyone. The mountains
may enhance the snowfall a bit along the mountains, particularly
in the Matanuska Valley, where the broad southwesterly wind flow
will upslope the moisture as it hits the mountains. On Wednesday
night, as the weak front passes Anchorage, winds will turn out of
the south, with some contribution from Turnagain Arm. While this
doesn't look to be a classic warm Chinook, it should succeed at
warming temperatures above freezing after the precipitation ends.
Thus, highs on Thursday may be in the morning (depending on timing
differences), and should be in the mid to upper 30s. A brief break
in the action is expected Thursday afternoon, and then another,
somewhat stronger front will move in on Thursday night, upping
precipitation chances once again.

Assuming there is enough space between these systems and some
clearing, temperatures may get to cool back below freezing in time
to keep any precipitation that does reach the ground away from the
coast in the form of snow. The best chances of sunshine look to be
Thursday morning, as the warmer winds are moving through town,
which should limit how much the temperatures will cool down
before more cloud cover rolls back in Thursday afternoon and
evening. Regardless, temperatures should be within seven degrees
of the freezing mark at all times over the next three days, and as
such precipitation type, despite the high confidence it will be
light, will remain a point of considerable uncertainty.

&&

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

A weakening low in the Bering Sea is slowly tracking northward and
maintaining south/southwest flow aloft over the Southwest Mainland.
The dynamics of this low combined with the moist synoptic flow is
continuing widespread snow over the Mainland with relatively mild
temperatures mostly in the 20s to 30s. Precipitation should
transition to more showery in nature Wednesday as the Bering low
fills and exits to the north and synoptic flow in the low levels
shifts to westerly. With that, expect a distinct cooling and drying
trend Wednesday into Thursday. In particular, north flow behind the
low and a strengthening Arctic trough will drop temperatures back to
the negatives in the Kuskokwim Delta. The Kilbuck/Ahklun/Russian
Mountains will provide some blocking for the middle Kuskokwim Valley
and Bristol Bay area, thus those areas shouldn't have as dramatic of
a cool down. A low will form near the AKPEN late Thursday into
Friday, bringing another round of precipitation in the Bristol Bay
area into the Alaska Range. Models are struggling with
strength/placement of this low, making the precipitation forecast
for the rest of Southwest AK a challenge. For now, went with
consensus of the 12z model runs, which was to keep precip south of
the Kuskokwim Mountains. Stay tuned for updates.

&&

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

A vertically stacked low roughly 100 miles northwest of Unalaska
is shifting northeastward, leaving prevalent small craft advisory
level winds over the eastern Aleutians and eastern Bering.
Meanwhile, a secondary low has developed in the northwest Bering,
and areas of gales are expected in its northern and western
periphery. An estimated 970 mb low currently roughly 400 miles
southwest of Shemya will track eastward through Thursday. While
storm force winds will likely develop in the periphery of this
low, it's expected to track far enough south that winds over the
Aleutians will peak at gale force. In any case, expect convective
precipitation over the Aleutians and Bering Wednesday into
Thursday with this system. Arctic troughing will strengthen
Thursday into Friday. Expect cold air advection showers over much
of the Bering Friday. A series of lows will track south of the
Aleutians during this time frame, enhancing geostrophic flow over
the central Bering. Expect widespread easterly gales and
heightened freezing spray concerns Friday.

&&

.MARINE (Days 3 through 5: Friday through Sunday)...

Gulf of Alaska:

Friday through Sunday remain modestly active. Friday will bring a
decaying front sweeping from Kodiak to Prince William Sound.
Winds are expected to be southeasterly small craft with the
occasional gale gust. On the heels of the first system, winds will
switch out of the southwest on Saturday to high end small craft
again. Confidence is gaining that winds will approach gale for
much of the Gulf, and confidence is high that winds will be strong
gales through Shelikof Strait, before abating come Sunday.

Bering Sea/Aleutians:

Benign weather will be coming to a close through the weekend.
Friday will open with increasing NE'rly winds, peaking well into
the gale range across the entire Bering. Into Saturday confidence
is gaining with a strong low lifting between St. Paul and Bristol
Bay. South and east of this low, SW'rly winds may reach high end
gales to low end storm gusts through Saturday. West of St. Paul,
winds will slowly turn more northerly and continue to diminish as
a much weaker low tracks along the same path through the end of
the weekend.

&&

.LONG TERM FORECAST (Days 3 through 7: Friday through Tuesday)...

Normal to above normal temperatures, unsettled conditions,
plentiful cloud cover, and a mix of rain and snow appears to be a
satisfactory summary of the long-term forecast at the end of this
week and into the next workweek. The mean longwave trough will
continue to support an active weather pattern for southwest Alaska
and then southcentral. The first of three systems will slide up
the AKPEN on Friday bringing a surge of warmer weather. Well above
normal temperatures will be the continued trend through the
weekend, with Sunday being the warmest. With the warm weather will
be the potential for strong Turnagain winds this weekend. Guidance
is still uncertain through that time, and more details will be
forthcoming.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...HEAVY FREEZING SPRAY: 160, 185, 412, 414
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...BL
SOUTHCENTRAL ALASKA...JW
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MM
MARINE/LONG TERM...SS