Gale Warning 456, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
518 AM AKDT Thu Sep 10 2020

.ANALYSIS AND UPPER LEVELS...

An inverted ridge stretched from its coastal washington position,
through the tip of the Alaska Peninsula, to Saint Matthews
Island. to the west of this coastal Washington ridge was a cutoff
low pressure system (almost in a hybrid Rex Block type pattern),
with another low centered east of the Kamchatka Peninsula with its
attendant trough bisecting the western Aleutians.

Another ejecting shortwave trough was located east of the Bering
ridge, as it stretched from Fairbanks to the northern Susitna
Valley. This feature was helping kick off a few sprinkles/light
showers across the Mat-Su Valleys and Anchorage Bowl.

&&

.MODEL DISCUSSION...

The models remain in good agreement through the next few days. The
main discrepancies is a low that the NAM tries to form near Attu
in the 72 to 84 hours. That said, the low is a fairly weak low,
with the Canadian hinting at it in the pressure field, while the
GFS and ECMWF show no semblance of it.

The other change today is for a stronger ridge to be anchored
across the eastern Bering and Aleutians. This will aid in cold
advection and a strengthening offshore flow across the southern
mainland.

&&

.AVIATION...

PANC...VFR conditions and light winds will persist through the
period.

&&

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

An upper level wave has been moving across Southcentral from the
northwest to the southeast this morning. The wave had a bit more
strength to it than originally expected, resulting in some shower
activity from Anchorage north, and associated cloud cover reaching
well south across the Kenai Peninsula. The back edge of the cloud
cover is quickly following behind it, so skies will continue
clearing across the Cook Inlet Region early this morning, taking
a bit longer to clear across the Copper River Basin and Prince
William Sound.

The cloud cover and showers have definitely kept temperatures much
warmer across much of the Cook Inlet region. Anchorage remains
near the 50 degree mark early this morning, with most other areas
of Southcentral staying in the 40s. The coldest spot is east of
Homer, where the cloud cover and light winds have allowed Bluff
locations to fall into the 30s.

Behind the wave, a repeat gorgeous day is expected for much of
Southcentral with plenty of sunshine and highs in the mid to upper
50s today. The main issue today will be gusty gap winds through
Seward, Whittier, Thompson Pass, and the Valdez Narrows. Winds
could gust to 45 mph at times near Whittier and Thompson Pass,
with up to 35 mph gusts expected in Seward and through the
Narrows. The gusty winds are expected to persist into Friday,
though today will feature the strongest winds of the next few days
as the high over Southwest Alaska continues to build over the
area.

Then tonight, forecast temperatures are expected to fall below
freezing for most of Southcentral, except portions of West
Anchorage, around Homer, and in the aforementioned gap wind areas.
Mainly clear skies, light winds, and a cooler fall-like air mass
moving in will all contribute to the first freeze of the fall
tonight. A Freeze Warning remains in effect for Anchorage and the
Mat Valley. There is one concern that may throw a wrench into the
cold temperature plans. The models are in good agreement that a
second upper level disturbance will traverse the area tonight.
While it is not expected to be as strong as the one going by right
now, i.e. much less in the way of shower activity, it may have
enough strength to both kick up the winds around the coast and
increase the cloud cover tonight. The presence of either of those
would keep temperatures much warmer, though for now continuing to
stick to the forecast that it will not be strong enough to do much
to prevent the temperatures from plummeting tonight.

Most of the area will repeat another day of plentiful sunshine on
Friday, though Kodiak may see more cloud cover with a disturbance
stalling out south of the island. Friday night will be another
cold night, though maybe a couple degrees warmer than tonight for
most areas. Without any wind, any areas not freezing tonight will
get another shot at it Friday night. No significant upper level
disturbances are expected Friday night that could complicate the
temperature forecast.

&&

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

Upper level high pressure is expected to remain in control over
Southwest Alaska for the forecast period. As a result, expect
a mostly sunny to partly cloudy sky for the next 3 days. 850 mb
temperatures will be in the 0 to -5 degrees Celsius range each
morning through Sunday morning across most of Southwest. The
coldest morning is expected to be Friday morning as surface high
pressure will be almost directly overhead. The Kuskokwim Valley
has the best chance of falling into the mid to upper 20s with
elsewhere in the low to mid 30s. Saturday morning the surface
high continues to lift northward with the pressure gradient
beginning to tighten some which may cause the atmosphere to
remain mixed enough Saturday morning to prevent temperatures from
falling as low as Friday morning. A slow moderating trend is
expected heading into Sunday as an upper level shortwave pushes
through Southwest. This is expected to bring increasing clouds to
Southwest for Saturday night into Sunday especially the Kuskokwim
Delta. The interior areas of Bristol Bay and the Kuskokwim Valley
may squeeze out one more chilly morning.

&&

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

Upper level high pressure is expected to remain across the eastern
Bering Sea, eastern Aleutians, and AKPEN today leading to mostly
sunny conditions. A piece of upper level energy is expected to
nose its way northward for Friday that is expected to bring some
rain and possibly some low end small crafts to the marine areas
near Sand Point to Chignik. Another upper level ridge will build
in behind this upper level wave that will stretch the energy apart
(weakening it) Friday night.

A vertically stacked low and associated surface occluded front
over the western and central Bering and Aleutians continues to
push up against the west side of the aforementioned ridge this
morning. The front will remain stationary through today keeping
the western and central Bering unsettled.

A second low will develop and intensify over the northwest Pacific
Thursday night and absorb the first low out ahead of it. This low
will reinvigorate the previous frontal system that is currently
stationary across the area. As a result, gale force conditions are
expected with periods of rain beginning tonight through Saturday
morning. After the front passes through, the rain will become more
showery in nature. The front will only progress eastward as it
continues to push up against a rather strong ridge out ahead of
it continuing the same general pattern as of recent.

&&

.MARINE (Days 3 through 5, Saturday through Monday)...

A Western Bering low remains nearly stationary while its front
slowly moves across the Bering through Mon. The track and
development of the low vary, but confidence is good for gale force
winds and waves to wrap the low and extend along the front
through the central Bering on Sat, then diminish to small craft
winds with the front across the eastern Bering for Sun and Mon.

High pressure spreads across the Gulf through Mon with mostly
Northerly winds over the Gulf. Weak troughs ride over the high
through Mon.

&&

.LONG TERM FORECAST (Days 4 through 7, Saturday through Monday)...

The pattern is expected to remain amplified through the weekend
with a ridge over mainland Alaska and a trough situated over the
western half of the Bering. This will keep conditions cool,
clear, and dry across Southwest and Southcentral Alaska through
Monday. By Sunday night, an occluded front associated with a ~982
mb low northwest of Attu will sweep across the eastern Bering and
approach the Southwest coast with a triple point low potentially
developing over the eastern Bering. By Monday, the front is
expected to move onshore with rain and gusty southerly winds for
Southwest Alaska as the low lifts north. The front and the
associated upper-level shortwave then continues east, as the ridge
retreats, toward Southcentral with increasing chances of stronger
winds and widespread rain by Tuesday. The entire longwave trough
over the Bering moves east as well, resulting in a more pronounced
south to southwesterly flow extending from the eastern half of
the Aleutians to the Gulf of Alaska. this looks to usher in a
period of unsettled weather across much of the region by mid-week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...FREEZE WARNING 101 111
MARINE...GALE WARNING 177 178 411
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...PD
SOUTHCENTRAL ALASKA...JW
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MV
MARINE/LONG TERM...MK/TM