Small Craft Advisory 922, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
502 PM AKST Thu Nov 22 2018

.ANALYSIS AND UPPER LEVELS...
The main upper level feature on today's weather map is the ridge
that extends from the Alaska Peninsula to near the Southwest coast
of the Alaska mainland and then into the northern Bering Sea. A
weak upper level trough extends out of the Arctic along the Alcan
border which is combining with the ridge to produce northwest flow
aloft over the mainland part of the state. The gradient is tight
enough over the eastern Alaska Peninsula and southwest Gulf to
produce a jet streak around 100 kt at the 250 mb level.
An upper level low is cutting off just south of the Central
Aleutians which is keeping the main 250 mb jet stream south of the
Aleutians. This low is also vertically stacked with the surface
low in the Aleutians which is why this low will remain nearly
stationary and weaken as another low to the southwest, which is
being fed by the jet stream, develops into the main low and
undercuts the original low Friday night.

&&

.MODEL DISCUSSION...
Important model differences creep up by Saturday afternoon in the
Gulf of Alaska where there is a 225 mile span in solutions in how
far to the northwest the low will be. This low will remain a
challenge with its track and intensity, but after analysis, the
GFS track looks the most reasonable overall. The concern is it
may be a little on the strong side as far as intensity. Basically
used a GFS track with a higher pressure for Saturday's forecast.
The next low moves into the southwest Gulf on Sunday, depending
on the model you look at. Models all agree on a large area of
low pressure in that general area but whether there will be one
consolidated low center or up to three in a complex low is a
point of disagreement. Therefore, using model ensemble data from
WPC for main guidance for Sunday and beyond.

&&

.AVIATION...
PANC...Foggy conditions and light winds will persist. Fog is the
problem around PANC again today with it developing around sunrise
and settling in dense. Winds will remain light and variable
through tonight and the low level inversion will persist. Overall
the thinking is that the predominate condition into tomorrow
afternoon is that there will be fog. There will also likely be
periods where it lessens, but the chances of it completely
dissipating and not returning are low.

&&

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

Happy Thanksgiving to all our loyal readers! The short-term
forecast period across Southcentral will get increasingly active
with time. Relatively quiet weather continues currently across
Southcentral with the primary weather hazard continuing to be the
dense freezing fog ongoing across much of Anchorage, Eagle River,
and along the Glenn Highway through Birchwood. Fog has also
returned to the Palmer area. With no change to any of the
atmospheric conditions expected (nearly calm winds), the freezing
fog is very likely to persist through the night and much of the
day Friday as well. Freezing fog is also ongoing across Glennallen
and the lower elevations of the Copper River Basin. For much the
same reason, the fog is likely to persist there as well. By Friday
night, an approaching front and area of low pressure will both
increase the mid level and upper level cloud cover and allow a
down-Inlet wind to develop which should finally end the freezing
fog threat for the foreseeable future after that.

On Saturday morning, a strong area of low pressure will move into
the southern Gulf. Since it will be tracking along a stalled
front, associated precipitation will begin in Kodiak overnight
Friday night, spreading north along much of the Gulf Coast through
Saturday morning. The winds will rapidly increase as the low
approaches, with widespread gales and localized storms along the
Gulf Coast east of Prince William Sound expected by Saturday
evening. The center of the low will track northwestward into the
southern Kenai Peninsula south of Seward Saturday evening. This
will be the first push of warm air, but the air mass currently
over Southcentral is cold enough that it is unlikely to warm
things up above freezing in the mid-atmosphere for inland
locations. Thus, as the associated upper level wave continues to
track northward Saturday evening, there is an increasing
likelihood for a widespread quick-hitting snow for the western
Kenai Peninsula, Anchorage, and the Mat-Su Valleys.

Behind the low for the second half of Saturday night through
Sunday, a much warmer air mass and associated atmospheric river
are expected to move into Southcentral. There remains some
disagreement on where it will set up and for how long, but there
is a chance for a period of heavy rain to move into the Seward
area during this event. Stronger winds associated with a second
front tracking northwestward across the Gulf will result in more
storm force winds with the barrier jet developing during the day
on Sunday. Due to the northwestward movement of the front, there
is a better likelihood for those storm force winds to get into
western Prince William Sound Sunday afternoon and evening. Those
strong winds are expected to impact the higher elevations of the
Anchorage Hillside and Turnagain Arm as well. The Matanuska Valley
wind will increase, though not to anywhere near storm force.
Temperatures will rapidly warm above freezing everywhere except
the Copper River Basin on Sunday as well. The warm air will linger
into the new work-week.

&&

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

High pressure settling over the region has helped scour away any
remaining fog as offshore flow develops. Under the ridge, temperatures
will fall to below normal as clearer and drier conditions prevail
through the first half of Friday. A front stretching across the
Bering from a strong area of low pressure situated near Adak will
bring clouds and increasing chances for snow along the southwest
coast beginning tomorrow night and for locations in the southwest
interior by early Saturday. As warmer air rides along the boundary
aloft, snow will change to rain across much of the AKPEN and into
Bristol Bay. Enough cold air, however, should remain in place
along the north coast of Bristol Bay north through the rest of
southwest Alaska to keep the precipitation all snow. The snow is
expected to linger into early Saturday morning along the coast
and through the rest of day from the Kilbuck Mountains north and
west through the Kuskokwim River Valley. Accumulations between
4 to 8 inches are likely from Togiak to Dillingham along with
areas of blowing snow at times, so have gone ahead and issued a
Winter Weather Advisory for this area. Lighter amounts, 1 to 2
inches, are possible in the Kuskokwim Delta.

A second developing North Pacific low then races toward the Gulf
on Saturday, bringing with it a re-enforcing push of moisture
along with a warmer air mass advecting across the Southwest.
Precipitation type continues to be a challenge with this next
system with the stronger push of warmer air aloft. Right now,
it looks like precipitation will start as snow, but quick.
Precipitation with this next system will be limited as winds
aloft will have a southeasterly downsloping component while
easterly winds at the surface keep a moist, onshore flow at bay.
With the stronger push of warmer air aloft, however, precipitation
type with this system remains a challenge. Right now, it looks
like precipitation will start as snow Saturday night, but quickly
change to a brief period of freezing rain/drizzle then rain for
the AKPEN and the southwest coast (Bristol Bay region) by Sunday
morning.

&&

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

The active pattern over the Bering and Aleutians will remain
through the forecast period as a low anchored over the central
Aleutians ushers in a series of fronts and newly developing
surface lows toward the eastern Aleutians and southwest Alaska.
The first of the fronts will continue its eastward trek this
evening with widespread gales as it lifts toward the mainland
tomorrow night. Rain is expected for locations along the southern
Bering waters with snow for locations along the Western Capes
over to the Pribilof Islands. A second North Pacific low then
races toward the eastern Aleutians for early Saturday with a third
low developing from the triple point of the second low's frontal
boundaries. This third low moves north toward the AKPEN late
Saturday swinging a warm front over the southwest mainland. There
is still some uncertainty with the overall position and intensity
of this complex system as the second low gets absorbed into the
third. However, given the placement of the low south of the
eastern Aleutians, expect strong southerly winds along the gulf
side of the AKPEN for the weekend and colder northerly winds
upstream across the Bering from the Pribilofs west.

&&

.LONG TERM FORECAST (Days 4 through 7: Monday Nov 26th through
Thursday Nov 29th)...

The latest runs of the GFS and ECMWF continue to be in relative
good agreement depicting the major synoptic features in the early
portion of this forecast period. This consensus is apparent at
the surface and echoed at 500 mb with the baroclinic features
aloft. A strong low south of Unalaska will bring another round of
gale force winds to the Central Aleutians. Both models show a
second low south of the chain that will rapidly intensify and head
toward Kodiak. The GFS has this low bottoming out at 951 MB while
the ECMWF only has it bottoming out at 964 mb. Also noteworthy is
the emergence of the "Atmospheric River" in the Gulf Of Alaska.
The current thinking is that the bulk of the "Atmospheric River"
will move into Southeast Alaska, and Southcentral will only get a
sliver of the "Atmospheric River". The pops were bumped up this
forecast package for the Prince William Sound Region. In addition
to the moisture, this system has a robust pressure gradient
associated with it, gusty winds expected in some inland locations
as well as over the water.

All the while, the Bering and the Sea of Okhotsk is brewing up a
series of lows which will affect the Aleutians through the
extended period.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Dense Fog Advisory 101 111.
		 Winter Weather Advisory 161.
MARINE...Gale Warning 170 171 172 173 175 177 178 179.

&&

SYNOPSIS AND MODEL DISCUSSION...EZ
SOUTHCENTRAL ALASKA...JW
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...TM
LONG TERM...PS