Gale Warning 388, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
441 PM AKDT Mon Aug 12 2019

.ANALYSIS AND UPPER LEVELS...
A strong upper level ridge remains over Southcentral Alaska today
with Southwest mainland Alaska under southwest flow aloft on the
other side of the ridge axis. This pattern is bringing in the
areas of rain to Southwest mainland Alaska with just high clouds
moving over the ridge in Southcentral. This ridge is strong enough
to suppress convection across Southcentral Alaska.

At the surface the ridge is over the Gulf of Alaska and
stretching to the south of the Alaska Peninsula. The main low in
Eastern Russia has a front stretching across the Bering Strait and
then southward over the Bering Sea. A wave along this front can
be prominently seen on IR satellite near Adak.

&&

.MODEL DISCUSSION...
Models remain in good agreement through the middle of the week and
not much needs to be mentioned for that time period. What is quite
notable is for the end of the week where there has been a good
amount of uncertainty in the models. Overall we have been leaning
toward the ECMWF solution which maintains the strong ridge over
the mainland part of the state over the GFS solution which shows a
strong trough digging southward near the Alcan border on Friday
and Saturday. Part of the reasoning has been the stability of the
ECMWF and the way the GFS has tied to break down the ridges too
fast this summer. With all that said, the 12Z ECMWF run did a
major jump toward the GFS solution and shows a 500 mb low over
Northwest Alaska Thursday night sweeping to the northeast Gulf of
Alaska by Friday night. This would drop upper level temperatures
significantly and surface temperatures will follow. This is the
first run of late where the models are showing similar solutions
and more runs will be needed to confirm this forecast shift.

&&

.AVIATION...
PANC...VFR conditions will persist. Winds are expected increase as
they shift to the south-southeast late this evening and persist
through tomorrow.

&&

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

The main forecast challenges were afternoon high temperatures and
our potential for some much needed rainfall. Speaking of those
temperatures, Ted Stevens International Airport has climbed to 77
degrees this afternoon, tying the record from 2005. We may climb
another degree or two before leveling off and beginning our
descent. These warm temperatures look to continue, as the models
have decreased the 700 mb RH for the next few days, especially for
Tuesday night and wednesday. This will allow for more breaks in
the cloud cover to develop, boosting afternoon highs. Thursday is
where we may have to really boost them some more, as 850 mb temps
are progged to be 12 deg. C, which is a degree C above today, when
we hit 77. At that time, projected RH fields indicate scattered
cloud cover at best. Overnight lows look to remain relatively
unchanged, compared to what has been observed the last few days.

As for the precipitation, a series of disturbances will pass by to
our north over the next 48 hours. The first of which will send a
weak front southward, reaching the north central portions of the
Matanuska Valley and Copper Basin, before frontolysis commences
(the weakening/dissolving of a front). This combined with
southwest winds just above the surface should provide enough
surface convergence for some showers to develop, reaching as far
south as a Kenai to coastal Gulf line for Tuesday into Wednesday.
These rainfall chances fall by the wayside for Thursday, with dry
weather expected for most locations, with the exception being
near the Alaska Range.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3/Monday
evening through Thursday)...

A frontal system continues its slow push across Southwest
Alaska. It will continue to move slowly and drag warm, moist air
behind it. So expect it to bring rain, cloud cover, and cooler
temperatures to the area for the next couple days. Closely following
this system is another low that will skirt the southwest coast
Wednesday into Thursday. This low will take a more northerly track
so it will only provide a chance of precipitation for all except
the Yukon Delta. The more northerly track will also provide a
chance for interior areas of southwest AK to warm and dry some.

&&

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

A frontal system is entering the central Bering Sea from the south,
just as the frontal system in the eastern Bering Sea moves over
Southwest AK. This quick succession of systems that are tied together
means continued southerly winds, low level clouds, fog, and rain
for the Bering Sea and Aleutians. Tuesday night into Wednesday a
strong system enters the Bering, again from the south. Gale force
winds will also accompany this system as well as more rain. This
system takes a more northerly track and enters through the western
Aleutians, so the western and central Bering will see most of the
action later in the week.

&&

.MARINE (Days 3 through 5)...

With upper-level ridging remaining over the Gulf through midweek,
expect west-southwesterly winds in the 20 to 25 knot range for
Shelikof Strait, southern Cook Inlet and across the Barren
Islands. Offshore flow then increases Thursday and lingers into
Saturday with areas of northwesterly gales likely across the
Barren Islands, out of terrain gaps, and in other favored
offshore flow locations.

For the Bering and Aleutians, a ridge building across the eastern
Aleutians and Bering will produce gusty offshore winds across the
channeled bays of the southern AKPEN Thursday evening through
Saturday. Low pressure south of Shemya will produce southeasterly
gales along a stalled front draped across the western and central
Aleutians Thursday and Friday. Winds will weaken late Friday into
Saturday but remain at small-craft advisory levels.

&&

.LONG TERM FORECAST (Days 3 through 7)...

By Thursday evening, a broad upper-level ridge stretching from
the eastern Aleutians to the Alaska Panhandle begins to weaken
with the axis shifting to the south and west. This is in response
to a vigorous upper-level shortwave dropping southeast from the
Chukchi Sea toward the Yukon. This shortwave elongates as it moves
into the Yukon on Saturday, introducing a northerly cyclonic flow
across the state as the ridge retreats over the Bering. At the
surface, the upper-level dynamics will result in the movement of a
frontal system across the state with the redevelopment of a
surface low over the eastern Gulf sometime late Friday or
Saturday. The bulk of the moisture with this system will remain
north of Southcentral due to upslope flow along the Alaska Range;
however, areas of light rain showers remain possible over the
mountains and within the Copper River Basin through late Friday as
the system passes. Another area of focus will be the likely
development of stronger offshore winds through the favored
channeled areas across coastal Southcentral in response to the
development of low pressure in the Gulf. There is still quite a
bit of uncertainty with timing and intensity with this feature
(see aforementioned model discussion), but these winds are
expected to increase late Friday and linger through the weekend.
Finally, temperatures are also expected trend cooler, becoming
more seasonal through the weekend, as the northerly flow advects
in colder air, dropping temperatures aloft by around 5C.

Farther west, the retreating ridge will keep conditions rather
quiet and slightly cooler over Southwest Alaska and the eastern
half of the Bering through the weekend as a stalled frontal
boundary allows southerly winds and precipitation to persist
across the central and western Aleutians.

&&

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

&&

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