Small Craft Advisory 211, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
434 PM AKST Tue Mar 3 2020

.ANALYSIS AND UPPER LEVELS...
There is a high amplitude upper level ridge over the central and
western Bering Sea. A sharp north to south oriented trough is
draped across the middle of the state, slowly moving eastward.
Strong northerly flow aloft is between these features from
southwest Alaska through the central Bering and Aleutians. At the
surface there is a low centered near Montague Island with a high
over the central and western Bering. Strong gap winds are
occurring from the northern Gulf through the Alaska Peninsula.
Some showers associated with the Montague low are occurring around
Prince William Sound and into the Copper River valley. There are
also some showers over the Alaska Peninsula and eastern Aleutians
associated with the unstable northerly flow.

&&

.MODEL DISCUSSION...
The numerical guidance is in rather good agreement through
Thursday afternoon. The main forecast challenge will be details
of cold advection and gap winds along the eastern Aleutians,
Alaska Peninsula, Gulf coast and Prince William Sound. Forecast
confidence is higher than normal.

&&

.AVIATION...
PANC...VFR conditions are most likely. A stray snow shower or two
are possible through this evening which could briefly drop
conditions to MVFR. Northerly winds could be gusty at times.

&&

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

Southcentral Alaska will transition into a much drier pattern
through the the end of the week as snow showers across the Kenai
Peninsula and the Copper River Basin taper off this evening.
Strong winds and gusty conditions will continue through Thursday
as the offshore flow pattern continues across Southcentral, with
colder air moving in across the region. Gale to storm force winds
will persist over the western Gulf through Wednesday evening,
with an area of hurricane force gusts near the Barren Islands. As
high pressure builds over interior mainland into Wednesday, the
pressure gradient will increase along the North Gulf Coast due to
lee side troughing over the Gulf.

Strong winds and cold temperatures this evening from Broad Pass
north will bring wind chills as low as 45 below zero, keeping a
Wind Chill Advisory in effect through Thursday afternoon. A
Blizzard Warning for Thompson Pass remains in effect through
Wednesday evening. A few inches of snow accumulation is possible,
with strong winds persisting after the snow has ended. These winds
will help transport snow on the ground, causing reduced
visibility to one quarter mile or less at times. The increased
gradient mixed with weak cold air advection will help to
strengthen winds for Thompson Pass, the city of Valdez, and the
Copper River Delta beginning Wednesday afternoon. These areas
could likely see wind gusts peaking around 80 mph through Thursday
afternoon. Thus, a High Wind Warning for Thompson Pass and Valdez
has been issued during this time period.

&&

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

A tight pressure gradient between a trough centered over the
northern Gulf and a ridge anchored over the western Bering
continues to produce strong northwesterly winds and occasional
snow showers across the eastern Bering and the AKPEN this evening.
These winds will be further enhanced beginning later tonight as a
shortwave trough currently over the northern Bering dives quickly
south toward the AKPEN. Arctic air will advect in behind this
system, with temperatures at 850mb (approx. 4,500 ft AGL) dropping
to -27C. The cold air advection, along with the increase in
upper-level energy, will produce storm-force winds, with
occasional hurricane-force gusts through the bays and passes of
the Pacific Side of the AKPEN through Wednesday night. Scattered
snow showers will remain likely across the AKPEN and eastern
Aleutians through Wednesday given the deep northerly flow. Snow
showers are also possible this evening across the Kuskokwim Delta
as the shortwave passes overhead. Winds will relax beginning
Thursday morning as the ridge of high pressure slides east. Clear
and very cold conditions will also persist across much of
Southwest Alaska through Friday morning as the ridge dominates the
weather pattern.

Clouds return on Friday as the next frontal system approaches
from the west. Guidance is indicating that much of the support
for this system starts to fall apart as the front crosses the
eastern Bering. Thus, there is still some uncertainty as to how
much precipitation makes it over Southwest Alaska. Right now, it
looks like the best chance for light snow will stay along the
coast as the bulk of the energy slides south toward the AKPEN.

&&

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

The main weather concern through late Wednesday will be the
persistent strong northwesterly winds across the eastern
Aleutians and AKPEN. The strongest winds will occur overnight and
linger through late Wednesday, with storm-force gusts possible
over the eastern Aleutians and hurricane-force gusts south of the
AKPEN. Snow showers will also remain likely as the cold,
northwesterly flow increases instability over the eastern half of
the Bering and Aleutians.

Farther west, a gale-force, north Pacific low will sweep a front
over the western Aleutians beginning Wednesday night. This system
will quickly track east, with the low over the central Bering by
Thursday night with its front draped from the Pribilofs south to
the central and eastern Aleutians. As the low tracks across the
Bering, areas of snow and blowing snow are likely, first across
Shemya Wednesday afternoon and evening then over the Pribilofs on
Thursday. Temperatures across the Chain will climb into the mid to
upper 30s ahead of the low, changing any snow during the onset of
precipitation to rain. This system then falls apart as it reaches
the eastern Bering on Friday. Another gale-force low approaches
the western Aleutians by Friday afternoon.

&&

.MARINE (Days 3 through 5)...
The most active pattern will be over the Bering on Friday as a
995 mb low, just west of Saint Paul, slowly weakens on a
southeasterly track. This is followed by the next North Pacific
low Friday night which continues on a northeasterly track nearing
the Pribilof Islands late Sunday. Both of these systems are
expected to bring small craft winds with rain along the southern
waters and snow from the Pribilofs north. There is some
uncertainty on wind speeds with the second system; some
adjustments may be needed to increase winds around the core of
low.

&&

.LONG TERM FORECAST (Days 3 through 7)...

Saturday through the beginning of next week, a wetter and warmer pattern
is expected as two low pressure systems move from west to east from
the Bering. The first low pressure system over the central Bering
on Saturday morning tracks quickly across the southern mainland
bringing the first round of snow into Sunday. Temperatures
moderate into the 20s to lowers 30s with the frontal passage,
keeping precipitation snow for the southern mainland. Southern
Bering and Gulf locations, including the Alaska Peninsula, Kodiak
Island and southern tip of the Kenai are expected to warm enough
to change snow over to rain. The second system follows quickly
behind the Saturday storm, tracking across the Bering Saturday
night. This storm will likely pull a warmer maritime air mass
further north Sunday into Monday across Bristol Bay. There is high
uncertainty on the evolution of the second storm with models
varying on the track and strength of this system from run to run.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Blizzard Warning 131.
         High Wind Warning 131.
         Wind Chill Advisory 145.

MARINE...Storm Warning 127 130 131 132 150.
         Gale Warning 119 120 121 125 126 128 129 136 137 138
                      155 165 174 175 176 177 178.
         Heavy Freezing Spray Warning 119 120 121 125 126 127
                      128 129 130 131 132 136 137 138 139 141
                      150 155 160 165 179 180 185.
FIRE WEATHER...NONE.

&&

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