Small Craft Advisory 529, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
409 PM AKDT Tue Jul 7 2020

.ANALYSIS AND UPPER LEVELS...

An upper-level spinning over the central Yukon Territory is
continuing to slowly drift southward this afternoon. Instability
associated with this system has already triggered a few isolated
thunderstorms along the Alaska Range and Talkeetna Mountains.
These storms are firing up along a weak boundary stretching along
the Alaska Highway.

Farther west, an upper-level ridge is building south of the AKPEN,
with the northern periphery pushing over Bristol Bay. Subsidence
along the lee (east) side of the Kilbuck and Kuskokwim Mountains
has helped scour out the residual marine layer; however, for areas
closer to the coast the low stratus and drizzle are hanging
tough. Farther north over the Kuskokwim Delta, an approaching
trough is reinforcing the low cloud cover and drizzle currently in
place.

Between these two features, a weak upper-level wave is tracking
south over the Kenai Peninsula. This feature, along with a
persistent southwesterly low-level flow, is keeping some low
stratus in place from Palmer and Wasilla south to Homer. The
stratus deck, however, is beginning to lift and break apart this
afternoon from north to south as this feature exits the region.

Farther west across the Aleutians, subsidence from the persistent
upper-level ridge is keeping low stratus and fog draped across the
Chain and the southern Bering.

&&

.MODEL DISCUSSION...

Models continue to perform rather well for what is shaping up to
be the development of a rather complex pattern across the state.
Guidance continues to track the upper-level low over the Yukon
south through Wednesday, but then diverge as to how quickly it
exits into the Gulf and get absorbed by a second low tracking from
Kodiak Island into the central Gulf. The NAM and Canadian are a
bit faster with this evolution, the GFS slower, and the ECMWF
pushing the first low back over Canada rather than phasing the
two together. Nonetheless, the track of the Yukon low will keep
the threat of thunderstorms over the Copper River Basin continuing
into Wednesday as the rest of Southcentral falls under the
influence of a more stable airmass with stratiform rain.

Models are also in good agreement with the movement of an upper-
level wave across Southwest tonight, transitioning into stronger,
more compact upper-level low tomorrow as it moves to the Kenai
Peninsula. Guidance is also in good agreement regarding the
strength and track of the resultant surface low as it develops
off the eastern Kenai Peninsula coast and moves into the northern
Gulf tomorrow evening. Given the good model agreement, forecast
confidence is above average moving into the second half of the
week.

&&

.AVIATION...
PANC...The low stratus deck this morning will give way to VFR
ceilings into the late evening to overnight hours. MVFR ceilings
are expected to return to the terminal by early morning Wednesday
as a storm system approaches. MVFR ceilings and visibility will
then continue into Wednesday afternoon. Guidance continues to
suggest a period of IFR ceilings around mid-morning Wednesday.
However, confidence is low so it is currently not included in the
TAF package.

&&

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

Lightning strikes were detected today between Tok and Glenn
Allen. Isolated convection will be possible this evening. A wet
pattern is on tap for this forecast period. Using a blend of the
GFS and NAM the rainfall amounts were significantly increased for
Southcentral. The longwave trough is directly over Southcentral
and the Gulf Of Alaska. This high amplitude trough actually extends
north of the Brooks Range, into the Yukon Territory and Northwest
Territories. There are a number of embedded shortwaves that will
impact the region. Portions of the Anchorage Bowl, Kenai Peninsula
and Prince William Sound will receive between .50 to over 1 inch
of rain from Wednesday to Thursday morning. Friday another slug
of energy will descend over the Parks Highway and this will be the
impetus for the next round of precipitation on Friday.

&&

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

The forecast remains on track, as the numerical models continue to
advertise a mid-level low with several associated vort lobes
crossing the region. As this occurs, a predominately northwest
flow develops tonight and continues through Friday. This allows
added low-level moisture to move inland from the marine layer of
the Bering. Meanwhile, increased forcing aloft from the assortment
of disturbances moving overhead and increased mid and upper-level
moisture will foster widespread rainfall developing overnight
while continuing on Wednesday, with this then transitioning to a
showery type pattern for at least the first half of Thursday.
Thereafter, drier air moves into Southwest Alaska from the north,
leading to decreased cloud cover, drying conditions, and warming
temperatures.

&&

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

The tale of two sides best sums up the Bering Sea/Aleutian
forecast. A shortwave ridge of high pressure at the surface and
aloft will develop overnight across the central Bering and eastern
Aleutians, peaking in strength and amplitude for Thursday into
Friday. To it's east, a cooler northwesterly flow will develop
across the eastern Bering and Alaska Peninsula (AKPEN), with a few
showers possible through late Wednesday night. After that, drying
and warming conditions will commence.

Further west, on the backside of the ridge, a series of fronts
will lift up from the west Pacific, reaching the Aleutian chain
before encountering the Bering ridge that has a slight negative
tilt. This causes the front to stall as Frontolysis (the decaying
of a front), only to be replaced by the next front 24 hours later.
Widespread precipitation with sustain small craft criteria winds
will accompany each feature. Otherwise, look for patchy fog to
affect all of the Bering Sea and coastal waters through the
period.

&&

.MARINE (Days 3 through 5: Friday through Sunday)...

An area of low pressure will continue over the southern Gulf,
which will weaken Friday evening and allow for ridging to build in
by Saturday morning. Over the northern Gulf and coasts, a trough
of low pressure moves in for the weekend. Winds will remain below
small craft advisory level through Sunday.

Over the western domain, a front associated with a North Pacific
low will move over the western and central Aleutians on Friday
afternoon. There is higher confidence today that small craft
advisory level winds will accompany this system, but less
confidence in the potential for gale force gusts embedded with
this system. By Sunday, this system should weaken as high
pressure tries to build in over the Bering.

&&

.LONG TERM FORECAST (Days 3 through 7: Saturday through Tuesday)...

A cloudy and showery pattern is expected across mainland Alaska
this weekend as cyclonic flow around a long-wave trough moves into
the region. This will be an active pattern as shortwaves continue
to move across the interior, before ridging starts to build over
the Gulf by the start of next week. Timing of these ways is still
a forecast challenge, but it's safe to say you will should keep an
umbrella handy with a chance of showers in the forecast through
Tuesday. Average temperatures for this time of year are expected
under this cyclonic flow pattern as there is no significant
ridging setting up.

&&

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

&&

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