Small Craft Advisory 508, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
411 PM AKDT Sat Jun 27 2020

.ANALYSIS AND UPPER LEVELS...

Ridging over Southcentral is shifting eastward this afternoon
ahead of a trough that is over the central Bering. The trough
consists of two upper level lows. The stronger low is over the
Chukotsk Peninsula, with a weakening low just southwest of
Nikolski. As the weakening low gets absorbed into the stronger
low, it will tilt the trough negatively, which in addition to the
broad onshore southwesterly flow will be responsible for the rain
expected over all of southern Mainland Alaska on Sunday (more on
that below).

Clouds ahead of the trough/upper low complex have overspread much
of Southwest Alaska and Kodiak Island this afternoon. High clouds
that are simply filtering the sunshine have moved into the Cook
Inlet Region, and will continue to spread northeastward. Cloud
cover has held temperatures in Southwest Alaska in the 50s.
Meanwhile temperatures are in the 60s from Soldotna north and
east. The farther north and east, the more likely those
temperatures climb into the 70s late this afternoon due to less
cloud cover.

&&

.MODEL DISCUSSION...

The synoptic pattern is in very good agreement among all the
models. As is often the case, the challenges of the forecast are
in the minutiae and the local effects. The two streams of energy
(northern and southern) will interact tonight bringing a wide
swath of precipitation and cooler air across southern AK Sun-Mon.
The American Models are now advertising a bit more SE flow late
Sun morning/afternoon than previously depicted. This cross-barrier
flow could help to chip away at some of the QPF in places like
Anchorage and the interior of the Kenai Peninsula. However, it
still looks like a high PoP scenario. The cool and wet pattern
across the Southern Mainland will hold for Mon, but it does look
like the coldest air will remain confined over SW AK and north of
the AK Range. Finally, as westerly flow moves in earnestly on Tue,
this should dry things out quite a bit more quickly than originally
thought. This forecast package lowers the PoPs late Tue.

&&

.AVIATION...
PANC...VFR conditions will persist. The Turnagain Arm jet should
develop late this evening, though the timing could vary by +/- 2
hours. The core of the jet should move away from the terminal by
early Sun as a down-inlet gradient develops, though some southeast
wind will likely persist along the northern periphery of the jet.
Light rain will also develop Sun AM, but ceilings should hold in
the low end VFR range.

&&

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

A ridge of high pressure over Southcentral is currently shifting
eastward, allowing for a front to move in over the western Gulf.
Cloud cover will increase across Southcentral tonight as the front
nears the northern Gulf coast. Additionally, southeasterly winds
will increase Sunday morning through Turnagain Arm, Portage
Valley, Knik River Valley, and the Copper River Basin. Expect
gusty winds to continue through the day Sunday before tapering off
by the evening hours. In addition to winds, this system has ample
amounts of moisture, so expect plenty of rain across Southcentral
through the day Sunday with the greatest amounts of precipitation
along the northern Gulf coast. As midlevel winds shift to become
more westerly, the rain will eventually taper off Monday morning,
though by this time an upper level shortwave will push eastward,
bringing a continued chance for showers on Monday. Generally
these showers will remain over the northern half of Southcentral,
including the Susitna Valley and northern Copper River Basin. By
Tuesday, conditions are expected to improve as a ridge of high
pressure moves in.

&&

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

A closed low over the Bering Strait and St. Lawrence Island will
keep a chance of showers over coastal communities of Southwest
Alaska. The moisture from the northern system will merge with the
remnants from a weakening front that originated from the North
Pacific. The leading edge of the precipitation shield from the
North Pacific system was moving into the Alaska Peninsula this
afternoon. There is lots of onshore flow this forecast package.
The temperatures were bumped down a skosh this forecast package
for SW AK. High confidence that a gloomy and wet pattern will
impact most of the region.

&&

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

A mature low near Nikolski will push northeast and slowly weaken.
This system has a fetch of tropical moisture associated with it
that can be traced back to Midway Atoll. The leading edge of the
precipitation shield is already at the AKPEN. Expect widespread
precipitation, gusty winds and enhanced seas. This low is going
to move towards Sand Point, become an open wave and get absorbed
into the longwave pattern. Sensible weather wise, expect the
moisture to just keep moving eastward even though the front will
be getting sheared apart later this weekend. Models have locked
onto the next round of cyclogenesis in the NW Pacific and rapid
intensification takes place between Kamchatka and Attu Island.
Given the projected pressure gradient forces for Monday and
Tuesday...gales will materialize impacting Attu Island, Shemya
and the western Bering.

&&

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

A low will track into the western Bering Sea Monday night through
Tuesday night. The strongest winds will be east of the low center
across the heart of the Bering and Aleutians. While the forecast
mostly indicates small craft winds with this low, it is looking
more and more likely that we will see a fairly large swath of
southerly gales west of Atka on Tuesday. The gales could then
spread east to the eastern Bering by Wednesday morning, then
rapidly weaken by Wednesday evening. Forecast confidence for
the remainder of the forecast area is high.

&&

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

Models are in good agreement as we start the period on Wednesday,
but beginning Thursday the GFS seems to be the outlier as ECMWF,
Canadian and Ensembles are in good agreement with a ridge
building over the Gulf of Alaska extending north into most of
mainland Alaska. The operational GFS produces a series of troughs
with a weaker ridge over the area. The consensus is to follow a
blend of ECMWF and ensembles in the long term.

With that said, a strong ridge will build across Southcentral
with drier and warmer weather expected Wednesday through
Saturday. Some models have the ridge retrograding a bit to the
west so parts of Southwest Alaska may get in on the dry weather
for Friday and Saturday. Meanwhile, a series of low pressure
systems and fronts across the Bering Sea will move around the
ridge into the Arctic region and bring precipitation to the YK
Delta region and the Aleutian Islands for much of the long term.

&&

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

&&

SYNOPSIS AND MODEL DISCUSSION...JW/MSO
SOUTHCENTRAL ALASKA...KO
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PS
MARINE/LONG TERM...JW/RC