Gale Warning 493, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
521 AM AKDT Sun Sep 20 2020

.ANALYSIS AND UPPER LEVELS...

On synoptic scale, GOES-17 Water Vapor imagery shows the jetstream
in a zonal pattern with the core of the jetstream between 45N and
50N across the North Pacific. There are two areas of low pressure
in the region. The first low is in the central Gulf of Alaska and
it is moving northeastward. The second low is in the western
Bering Sea and it is moving northeastward. A warm front extending
across the eastern Bering/Aleutians is moving northeastward. A
cold front is currently moving eastward in the central
Bering/Aleutians.

On the mesoscale, radar imagery shows showers moving northwestward
across the northern Gulf of Alaska into Prince William Sound and
over the Kenai Peninsula. Observations and radar show a
convergence zone in the eastern Susitna Valley which is producing
a band of rain from Willow to the base of the Alaska Range in the
northern Susitna Valley. Radar imagery also shows areas of
showers moving northeastward across the Kuskokwim Delta.

&&

.MODEL DISCUSSION...

Guidance is in good agreement with the placement and intensity of
synoptic scale features over the next 3 days. The consistent
solution is the energy from the aforementioned low in the central
Gulf of Alaska will become absorbed into the low in the Bering,
creating a stronger low that will move into the Gulf Monday
afternoon. Longer range, deterministic models are consistent with
their ensembles as the Arctic Oscillation weakens later this week
while the Eastern Pacific Oscillation becomes stronger in the
positive phase, this will allow for a more meridional pattern with
a upper level trough over the Alaskan mainland and the Gulf of
Alaska. Due to the good agreement between deterministic models and
the deterministic models having consistency with their ensembles,
forecast confidence remains above average.

&&

.AVIATION...
PANC...Conditions will be VFR before 18z today. After 18z,
conditions will become MVFR as ceilings fall to near 1500ft.
Ceilings will then rise back to VFR at 00z Monday. Winds will
remain light throughout this TAF package.

&&

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

An upper level short wave is slowly moving eastward across the
Susitna Valley and Kenai Peninsula early this morning which has
been the focus of some rainfall overnight. The heaviest rainfall
has bee from just west of Anchorage northward to Talkeetna due to
a convergence zone due to southwest wind traveling up Cook Inlet
meeting Southeast wind coming out of Turnagain Arm. These winds
are diminishing as the wave moves eastward so this enhancement
should end this morning.

The vertically stacked low in the Gulf will continue to weaken
today and then drift northward to Prince William SOund late
Sunday night. The interaction of this sort wave and Gulf low will
keep some showers over the Chugach and Talkeetna Mountains
through the day and then the region of rain showers should move
over the Copper River Basin and all of Prince William Sound for
early Monday.

A break in between systems is clearing out skies from Cook INlet
to Kodiak early this morning allowing some areas of fog to
develop in the saturated lower atmosphere. By tonight, the next
front will be over Kodiak Island and then to the north GUlf coast
by Monday night. A barrier jet with this front will develop and
bring in at least Gale force winds, though there is a chance for
some storm force winds to develop east of Montague Island Monday
night, but enough uncertainty is still there for that front to
keep the forecast at Gale force for now.

&&

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

Quiet weather conditions continue for Southwest today as a weak
upper level ridge remains in place. A front associated with a low
pressure system over the central Bering Sea will move up to the
coastal regions of Bristol Bay west to Kipnuk. There has been a
shift south with the front for this forecast package. Some rain
will still make it to the Capes and Goodnews Bay, though its
progression any further inland will be hard to come by. As a
result of the front being slightly further south this will create
mainly cross barrier flow for much of Southwest. King Salmon and
areas to the west of the Kuskokwim Mountains may just see a brief
shower but there is the chance they remain dry. The main story
will be the winds increasing as the front approaches and pivots
over the coastal areas of Bristol Bay. Breezy conditions are
expected for Southwest beginning later this afternoon and
continuing through the forecast package.

As the front pulls away Tuesday afternoon, an upper level
shortwave will move southward across the Kuskokwim Delta that will
bring a colder air mass and the chance for showers especially for
the Kuskokwim Delta coastal areas. Temperatures may not get out of
the 40s for much of the Kuskokwim Delta Tuesday afternoon.

&&

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

A 980 mb low pressure system continues moving eastward across the
Bering this morning. This is bringing gale force winds to much of
the Bering Sea especially areas to the south of the center. As
cold air advection strengthens behind the low today, storm force
winds and gusts are possible for the marine zones near Adak. When
the cold air advection along with the tight pressure gradient
increases behind the low and for Adak, this may cause some gusts
upwards of 60-63kts this afternoon. There is the potential the
winds can be higher than forecast with gusts potentially greater
than 64 kts (75 mph) around noontime to 4-5 PM today when the
cold air advection is moving through. As the low continues across
the Bering Sea it will be accompanied with periods of rain as well
for many areas. Wave heights will be 20-30 ft for areas mainly to
the south of the Aleutian Chain with 10-20 feet in the Bering on
the northern side of the Chain.

Starting Monday afternoon through the remainder of the forecast
package, an even colder air mass will move southward through the
Bering. This will create an unstable atmospheric profile that will
produce showers across the Bering during this time period along
with breezy conditions. Between Saint Matthew Island north to
Saint Lawrence Island, there may even be some snow that mixes in
with these showers Monday night into early Tuesday.

&&

.MARINE (Days 3 through 5/Tuesday through Thursday)...

Gulf of Alaska: A broad and strong storm system is expected to
remain nearly stationary and weaken over the southern Gulf through
Thursday. There is high confidence in Gale force winds over the
northern Gulf coast with potential for storm force gusts if a
secondary low is to develop in the lee of Kodiak Island on Tue. By
late Wednesday night the low is expected to weaken, which will
bring winds down to at or below small craft criteria.

Bering Sea/Aleutian Islands: There is high confidence that the
broad low pressure system over the Gulf will bring moderate to
strong northerly winds over the eastern Bering. The winds will be
strongest (gale force) through the terrain gaps from Nikolski to
Sand Point through Thursday morning. By this time winds should
calm down to well below gale criteria as the low to the east
breaks down. Over the western Bering, there is high confidence
that high pressure and calm winds are expected to persist through
Wed night. On Thursday confidence decreases significantly in
regards to the next system, with models showing a 20 mb spread.
If the stronger solutions comes to fruition, gale force winds are
possible moving east across the Aleutian Chain Thursday night into
Friday.

&&

.LONG TERM FORECAST (Days 4 through 7/Wednesday through
Saturday)...

A broad and strong low pressure system is expected to be centered
over the Gulf at the start of the forecast period. The main impact
from this system will be for the Gulf coast and Kodiak Island,
where strong winds and rain are expected to persist through
Wednesday afternoon. For Southcentral it should be mostly dry,
with seasonal temperatures and the occasional shower as weak
shortwaves move around the low center. On the backside of the
low, strong winds and increasingly cool, northerly wind are
expected to persist over the eastern Bering and coastal areas of
Southwest through Thursday night. By Friday morning, an arctic
trough is expected to move in from the Bering Strait, which will
cool off the Bering considerably through Saturday.

The main forecast question is in regards to the next system
forecast to move over/near the central Aleutians Thursday night.
Forecast confidence decreases considerably due to a large model
spread in both the location and strength of this system. The GFS
and Canadian models have a storm force low clipping the central
Aleutians on Friday as it rapidly moves east into the Gulf by
Saturday morning. The primary difference between these solutions
and the EC, which has a considerably weaker system, is whether or
not jet remains zonal or becomes slightly meridional. If the jet
is to become meridional, then the storm will have a better chance
of tapping into the strong temperature gradient N/S of the
Aleutian Chain to produce a stronger storm. Regardless, model
agreement and forecast confidence increases once again Saturday, with
a strong system moving into the Gulf.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning 138 351 352 150 160 165 170 172 173 177 178
179 180 411 412 413 414.
Storm Warning 174 175 176.
FIRE WEATHER...NONE.

&&

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