Small Craft Advisory 276, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
426 AM AKDT Wed Mar 25 2020

.ANALYSIS AND UPPER LEVELS...
A broad upper level trough is over the Bering early this morning
with a closed low center near the Pribilof Islands. Nearly zonal
flow is noted over the southern mainland and Gulf of Alaska with
abundant moisture streaming overhead from the Bering. At the
surface, a remnant frontal boundary is draped over the Southwest.
Rain and snow continues to develop across this region as embedded
disturbances aloft propagate away from the Bering low. Otherwise,
relatively benign conditions across the southern mainland this
morning under weak surface flow. Temperatures range from the 20s
to mid 30s across the forecast area with slightly warmer
temperatures between 30 to 45 degrees for marine areas.

&&

.MODEL DISCUSSION...
Models are in relatively good agreement through the end of the
work week. High resolution models were used to refine stronger
gap flow Thursday and Friday across Southcentral locations. A
blend of the NAM and GFS were used for updates across the
Southwest and Bering which handles a fast moving front across the
Northern Bering on Thursday. There are some timing differences as
this system moves across the Alaska Peninsula and Southwest
mainland on Friday, so refining is anticipated.

&&

.AVIATION...
PANC...VFR conditions are expected through the next TAF package;
however, a few weak upper level disturbances may briefly bring
ceilings down today. A mention of vicinity fog was included for
Thursday morning as skies scatter out with a building ridge from
the interior. Winds will be relatively light through late Thursday
morning.

&&

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

A moist trough moves east across Southcentral today. The 850 to
700 mb flow will be generally from the west today. So it looks
like the best shot at accumulating snow will be along the
Talkeetna Mountains and upslope areas along the Chugach Mountains.
Temperatures at lower elevations will generally be in the 30s so
expect mixed precipitation types from the Kenai Peninsula, across
the Anchorage Bowl/Matanuska Valley, to the southern Susitna Valley.
Drying occurs overnight as the trough clears the area leading to
gusty outflow conditions across the north Gulf Coastal locations
and Barren islands through Thursday. Patchy fog is also expect
tonight from the Kenai Peninsula to the MAT-SU Valleys.

&&

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

An upper level low moving across the Bering this morning will open
up to an open wave as it continues east. As a result, rain showers
will continue to impact Southwest Alaska with temperatures
remaining above freezing due to cloud cover in place and not much
in the way of any temperature advection across the area.

As the upper trough continues moving to the south and east, it
will take the precipitation with it causing the region to dry out
except for the Kilbuck Mountains, Alaska Range, and Aleutian Range
where enough orographic lift will be occurring to keep rain/snow
showers in the forecast. Weak cold advection is expected
Wednesday night into Thursday allowing temperatures to fall back
down to the upper 20s and near freezing.

Zonal flow aloft is expected for Thursday keeping the area quiet.
Attention then turns to the next weather system that will move
into the region Thursday night into Friday. A system moving north
through the North Pacific will cross the Northern Bering and
strengthen as it passes over St. Matthew Island which will
bring snow to much of Southwest Alaska. As of now it appears the
snowfall will remain below advisory criteria for all areas but
this will continue to be monitored.

Behind this low, an upper level trough will dive southeast
bringing strong cold air advection throughout Southwest Alaska
which will bring colder temperatures and strong gusty winds for
Saturday.

&&

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

An upper level low swirling over the Bering will continue moving
east becoming an open wave Wednesday morning. A weak upper level
ridge will build over the Bering during the day Wednesday bringing
quieter weather and light winds.

A second low tracking well west of Attu Island will bring a front
into the western Aleutians and the western Bering Wednesday night.
This front will continue moving across the Aleutian Chain through
the day on Thursday. The aforementioned low will move into the
northern Bering Thursday night into Friday and strengthen. Strong
high pressure will move in behind the low on Friday night ushering
in a polar air mass with strong winds expected out of the AKPEN
gaps and passes.

&&

.MARINE (Days 3 through 5)...
Gale force southerly winds are expected across the western
Bering/Aleutians on Thursday. Gale force northwesterly gap winds
are also expected in the western Gulf on Thursday. Forecast
confidence for Thursday remains high. Gale force west-
southwesterly winds are expected across the central/eastern Bering
and Pribilof Islands Friday morning. Only moderate confidence
exists on Friday and Saturday for funneling westerly winds through
Unimak Pass. High confidence exists for gale force winds in the
western Gulf on Saturday with low confidence in storm force gap
winds along the AKPEN. There is also the potential for 20-25ft
long period northwesterly seas in the western Gulf on Saturday,
though confidence is low.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Strong confidence exists in upper level ridging persisting across
the Alaskan mainland on Thursday. The upper level ridging will be
strongest across the southwestern portion of the mainland and
thus temperatures are expected to be warmer than normal. However,
this upper level ridging will be of short duration as an upper
trough will move into the western mainland Friday afternoon ahead
of a surface cold front. As a result, temperatures across the
southern Alaskan mainland will fall as a much colder airmass from
Siberia moves into the region. Forecast confidence decreases
into the weekend as significant differences arise between
guidance. The only area of congruence between guidance is the
jetstream remaining south of the Alaskan mainland which will allow
for colder temperatures. Confidence remains very low Sunday
onward as there is a significant amount of disagreement between
guidance with respect to the place of upper level synoptic
features which leads to a strong disagreement in the placement of
surface features.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...NONE.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...KH
SOUTHCENTRAL ALASKA...BC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MV
MARINE/LONG TERM...ED