Wind Chill Advisory 12, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
459 PM AKST Mon Mar 2 2020

.ANALYSIS AND UPPER LEVELS...
An upper level trough extending from the eastern Interior
southwest to the eastern Bering Sea is tracking southeast toward
Southcentral Alaska, the Gulf of Alaska, and the Alaska
Peninsula. This continues to produce areas of snow out ahead of it
across much of Southcentral Alaska. Meanwhile, cold air
advection has ensued across the region, with Arctic air moving
into Southcentral Alaska from the north and west. The arrival of
colder air has enhanced snowfall this morning in the Anchorage
area, but drier air will make its way in with the upper trough
during the afternoon and evening hours. The cold air advection is
combining with tight pressure gradients (between a surface low in
Prince William Sound and a ridge of high pressure in the Bering
Sea) to strengthen winds along the south side of the Alaska
Peninsula, the western Gulf and western Prince William Sound
(including Passage Canal and Resurrection Bay).

Out ahead of the tail end of the upper trough there are some snow
showers along the eastern Aleutians and southern Alaska
Peninsula. There are also some snow showers with a weak low near
Adak. Otherwise, weather across the remainder of the Bering Sea
and Aleutians and southwest Alaska is quiet.

&&

.MODEL DISCUSSION...
Model guidance is in very good agreement with handling of large
scale features over the next few days. The main forecast challenge
will be details of gap winds along the Alaska Peninsula, Gulf
coast, and Prince William Sound.

&&

.AVIATION...
PANC...Northwesterly low level flow will lead to dry air advection
this afternoon and evening, with ceilings lifting. Thus, VFR
conditions are expected to prevail. In this flow pattern, we often
do see lower clouds (and an occasional shower) bank up against the
mountains, so would expect lower clouds to the east of the
terminal. There is a small chance some of these could drift into
the terminal, but if so, it should be brief in nature.

&&

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

Snow showers across the region will diminish through this evening
as the upper level trough moves east, with a few showers
lingering over the Copper River Basin through early Tuesday.
Relatively dry conditions will persist across south central Alaska
beginning Wednesday through the end of the week.

A strong upper level trough moving across Southcentral Alaska on
Tuesday will bring significantly stronger winds to the region as
colder air moves in with it. Strong winds over Kamishak Gap and
through gaps in Shelikof Strait will increase through Tuesday
afternoon, when storm force winds and gusts are expected. The
tight pressure gradient and weak cold air advection will also
increase winds by Wednesday morning for the city of Valdez and
nearby marine areas, with possible wind gusts of 50 mph through
the city and port through Wednesday evening. These strong winds
accompanied by recently fallen snow for Valdez and Thompson Pass
will cause blowing snow conditions, reducing visibility at times.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3: Monday
night through Thursday)...

It will be cold and windy the next couple of days with northerly
flow in place. Windy conditions behind the passage of a shortwave
trough will drop wind chills to the -30s the next couple of nights
for areas around Bethel. Winds will then calm tomorrow night,
after which patchy fog will be possible for inland valleys.
Otherwise, quiet weather is expected for the next few days.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3:
Monday night through Thursday)...

Low-level cloud cover remains after the exit of low pressure.
Satellite shows cold weather stratocumulus forming off the edge of
the ice pack and moving south, indicating cold northerly flow has
developed over the area. This is a harbinger of things to come,
as we are expecting a strong push of colder air over the eastern
Aleutians late tomorrow into Wednesday. Therefore, added sustained
gale force winds and storm force gusts to the forecast for gaps
and passes in the eastern Aleutians, especially on the Pacific
side of the islands. These winds will taper late Wednesday into
Thursday. Forecast confidence is high with this event as models
have done a good job locking in on it and we have already seen the
northerly flow develop.

As winds die down in the eastern Aleutians, a gale force low will
be entering the western Aleutians out of the north Pacific Wed.
It will also bring moisture and warmer temperatures with it as it
pushes northeast through Thursday into the Bering Sea. We adjusted
the forecast today to account for gale force winds and increased
moisture. Forecast confidence is moderate to high with this system
as models generally agree, but some tweaks will surely need to
made as it develops.

&&

.MARINE (Days 3 through 5...Thursday through Saturday)...

A low along the southern half of the Alaska Panhandle will remain
nearly stationary through Friday before moving southeast. A ridge
of high pressure extending from the north Pacific across the
AKPEN and Southwest Alaska will shift east into the western Gulf.
Between the two, a strong pressure gradient will produce gales
(with perhaps some storm-force gusts) from the AKPEN across
portions of the northern Gulf through early Friday. Winds then
relax across the Gulf late Friday into Saturday as the ridge
settles overhead and the pressure gradient diminishes. For the
Bering, a gale-force low will transit from south of Shemya
Thursday to the central Bering by Friday as its front reaches the
Southwest coast. A triple-point low is then expected to spin up
near the AKPEN by late Friday and dive south. A ridge will quickly
track across the Aleutians behind this system with another gale-
force system moving over the western Aleutians late Friday,
reaching the AKPEN by late Saturday.

&&

.LONG TERM FORECAST (Days 4 through 7...Friday through Monday)...

The forecast begins with a broad upper-level ridge stretching
across the western half of the state. Farther west, an upper-
level low is moving east from the western Aleutians over the
southern Bering. Both features progress quickly eastward with the
ridge beginning to breakdown and slide south by late Friday as
the upper-level low elongates and shears apart as it enters the
western Gulf. At the surface, this translates to the continuation
of strong outflow winds across the Gulf coast for Friday, and a
surface low near the Pribilofs sweeping a front over the eastern
Bering. This reaches the Southwest coast and AKPEN by late
Friday. By Saturday, a flat ridge tries to establish itself over
the north Pacific and Aleutians but is quickly pushed eastward to
the Gulf as another upper-level low moves from the western
Aleutians late Saturday to the eastern Bering by Sunday. This
keeps the weather unsettled across much of southern Alaska as a
surface low near Shemya sweeps a front from the Aleutians late
Friday to Southwest coast by late Saturday. The surface low then
moves inland on Sunday with its front moving into the northern
Gulf with a new triple-point surface low developing. This low will
usher in a moist, southerly flow with another round of widespread
precipitation for interior Southwest and Southcentral Alaska. The
upper-level trough will remain over the state through Monday as
another polar low drops south into the eastern Bering, bringing
yet another shot of arctic air and snow showers across the Bering.
The pattern will be very reminiscent of what is currently
transpiring with the set-up going from snowy to cold.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Heavy Freezing Spray Warning 150 155 165 179 185 120-139
					  141.
		 Gale Warning 150 155 181 119 120 121 125 127 129 131 132
				      136 137 138.
		 Storm Warning 130.
FIRE WEATHER...NONE.

&&

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