Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 240032
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
432 PM AKDT Sun Jun 23 2019

.ANALYSIS AND UPPER LEVELS...

Upper levels still show high pressure as the dominant feature over
the region, with the main axis of the ridge extending a lobe into
Interior Alaska. An upper level low centered over Nome extends a
weakening trough southward. This has provided enough moisture for
some lingering low level clouds across the Kuskokwim Delta today.
Elsewhere, satellite imagery showed clear skies this morning,
with the exception of fog and low stratus in the Susitna Valley,
which has since cleared.

As we progress into the afternoon, satellite imagery is showing
the beginnings of showers across the Copper River Basin and in the
interior portions of Southwest as daytime heating begins to
increase local instability. Right now the offshore flow isn`t
quite strong enough to dampen sea breezes around the area, so
coastal areas will see these continue for the afternoon, while
inland areas are still calm.

In the western Bering and western Aleutians, the weather is
primarily being driven by a stacked low, with a weakening front
draped across the Central Aleutians. This is bringing rain and
winds around 30 knots to the Aleutian Chain and Southern Bering,
which is not unusual, even for this time of year.

&&

.MODEL DISCUSSION...

Models start out in good agreement today, but by Monday afternoon
there are some differences in the way the low north of Nome is
handled. GFS continues to keep it a closed low, while the other
models are weakening it significantly. This does affect the
strength of associated upper level troughs moving through the
ridge. Consequently, by Tuesday, GFS has more northwesterly flow
over the mainland, while NAM, EC and Gem Regional have a more
westerly flow over the southern mainland, which may cool
temperatures slightly in the coastal areas.

&&

.AVIATION...

PANC...VFR conditions and light winds will persist. There is a
possibility of afternoon thunderstorms over the mountains but
these are expected to remain away from the airport.

&&

.FIRE WEATHER...

Overall pattern continues with an upper level ridge and a few
transient shortwaves. As we head into Monday afternoon, the ridge
axis moves further eastward, bringing more northwesterly flow in
the midlevels, which should dry out the air column. and bring
primarily offshore flow. With 850 temps expected to increase to
12 to 14 degrees C across Southcentral, temperatures in the mid
80s, especially in the inland areas, seem reasonable. Instability
parameters seem to indicate a gradual southward trend of
convection. Right now steering flow is primarily light, but
eastward, so any showers that form are mostly expected to remain
over the mountains and have little to no impact on the Swan Lake
Fire. By Wednesday the areas of greatest instability will be
confined mainly to the Copper River Basin.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3/Tonight
through Wednesday)...

The ridge in place over Southcentral continues to slowly drift
southward, allowing for generally warm and dry conditions with
diurnally driven afternoon thunderstorms over the next couple of
days.

Isolated afternoon thunderstorms across the Susitna Valley
and Copper River Basin have already begun to form this afternoon
and are expected to continue through the evening. The threat of
isolated thunderstorms extends as far south as the Kenai
Peninsula, where the most favorable conditions are over/near the
mountains. While isolated thunderstorms are most likely to form
over the Chugach Mountains this evening, a shower and possibly a
rumble of thunder over Anchorage cannot be ruled out.

For the beginning of the workweek, the upper level ridge will move
southward, allowing for more westerly flow. Convection will be
favored inland over the Susitna Valley and Copper River Basin.
More isolated convection is possible over the interior Kenai,
tracking eastward across the eastern peninsula.

On Tuesday night and Wednesday, the upper ridge will build
northward over the western Gulf and Southcentral. The ridge axis
will stay west of Southcentral which will lead to offshore flow
along the north Gulf coast and Prince William Sound. All of this
favors a hot day across the region, including most coastal
locations. In particular, localized offshore winds in Seward and
Whittier should keep sea breezes at bay for much of the day
leading to some of the warmest temperatures of the summer. There
will be some clouds rounding the top of the building ridge, so the
day could start out with mid to high clouds, but expect increasing
sunshine as the ridge strengthens. There will be less convection
on Wednesday do to more stable conditions under the ridge. The
best chance for a few showers/thunderstorms will be the northern
Susitna Valley eastward to the Copper River Basin.

&&

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

A ridge will gradually track eastward, leading to mostly sunny
skies across interior Southwest today and Monday. A front will
approach from the west Monday night, bringing increasing cloud
cover to all of the region Tuesday and Wednesday as well as a
chance of showers for the Kuskokwim Delta and coastal locations
along Bristol Bay.

&&

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

Showers will continue to track west to east across the Aleutians
through Monday along a front. Strong southerly flow between high
and low pressure will advect central Pacific moisture northward
over the central and eastern Aleutians, thus moderate rain and
gusty winds are likely for the area Tuesday and Wednesday.

&&

.MARINE (Days 3 through 5)...
(Wednesday through Friday)

...Bering Sea and Aleutian Coastal waters...

Fairly benign conditions will exist for the middle of the week
towards the weekend, as a weakening front stalls across the east
central Aleutians. There may be a brief window for some very low
end gales to develop near the eastern Aleutians into the tip of
the peninsula, but confidence in this materializing is mediocre at
best.

...Gulf of Alaska...

Minimal marine impacts are expected from the weather, as a ridge
of high pressure will encompass much of the region. This will help
promote offshore flow which may increased winds into the small
craft variety near the coast. Otherwise, look for this pattern to
hold into next weekend.

&&

.LONG TERM FORECAST (Days 4 through 7)...
(Thursday through Sunday)

Forecast uncertainty is higher than it was yesterday for the
middle of the week through the weekend, as the Omega Block that is
currently in place tries to break down as an area of low pressure
across the western Bering bumps into a shortwave ridge centered
from the north Pacific, through southcentral, peaking near
Fairbanks. This ridge is then flanked by a downstream low near the
Pacific Northwest.

As is typically the case, the model agreement on how fast this
ridge breaks down is not that good at all, with the GFS being the
biggest outlier compared to the ECMWF and Canadian Hemispheric
models. Given the strength and persistence of the ridge, prefer a
slower breakdown of this feature with the initial energy across
the Bering being deflected either to the north, as it rounds the
top of the ridge, or south into the central Pacific. This would
yield a continued warm and dry pattern for southwest and southcentral
Alaska, with seasonal temperatures and light winds from the
central Bering and Aleutians eastward. towards the mainland coast.
That in turn keeps the unsettled weather located across the
western half of the Aleutian chain and Bering Sea.

&&

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

&&
$$

SYNOPSIS AND MODEL DISCUSSION...LF
SOUTHCENTRAL ALASKA...KO/SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AP
MARINE/LONG TERM...PD



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