Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 190158
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
458 PM AKST Sat Jan 18 2020

.ANALYSIS AND UPPER LEVELS...
The pattern that was previously dominated by a strong, closed
Arctic high is transitioning to more and more influence from the
North Pacific. The net effect is more moisture crossing from east
to west across the Gulf of AK and slowly warming temperatures.
Some of the first evidence of this is across Kodiak Island where
snow has been falling most of the night. However, temperatures at
the airport have warmed into the mid 30s and allowed some brief
bouts of rain to mix in. Some of these "easterly waves" have
enough energy to bring some light snow to portions of Bristol Bay
as well. Meanwhile across the Bering, things remain very quiet. A
weak deformation band stretches from north to south across the
entire Bering Sea. It continues to make for brief bursts of
heavier snow from St. Matthew Island, down through the Pribilofs,
and into Nikolski.

The primary storm track remains well south of the mainland. It is
moving quickly from the Western Aleutians southeast into the North
Pacific and then turns sharply into the Panhandle of AK. Wind
speeds at 300 mb (30,000`) top out in the North Pacific at about
150 kts. Thus, expect most of the stronger storms and warm air to
stay well south of the mainland at least for the next few days.

&&

.MODEL DISCUSSION...Models are in good agreement with each other
regarding synoptics. However, they are struggling to depict some
of the finer, key details. In general, they painted Kodiak last
night with nearly an inch of QPF. However so far, they have only
been able to muster about 1/3 of an inch. As these "easterly
waves" transit over the AK Range into SW AK later tonight, there
is still high uncertainty on exactly where some of the heavier
bands of snow will set up. Some of the recent hi-res models want
to park some of the bands near Dillingham, but forecast thinking
is that these bands will be transient and move off into the Bering
after a few hours.

&&

.AVIATION...
PANC...VFR conditions and light north winds will persist.

&&

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

A weakening trough is moving across the AKPEN this evening,
keeping some scattered snow showers and gusty easterly winds
continuing across Kodiak Island this evening. Elsewhere, gusty
outflow winds will continue through the overnight hours through
bays and passes near and along the Gulf coast as a strong
pressure gradient remains between high pressure inland and lower
pressure over the Gulf.

A second surface low is spinning up in the north Pacific and will
swing a front across the Southcentral coast from east to west by
Sunday morning as the surface low lifts north into the Gulf. This
frontal system will usher in snow for much of coastal Southcentral
with the potential for a changeover to mixed rain/snow or even
all rain along the immediate coast for places like Cordova and
coastal Prince William Sound by Sunday evening. For more protected
locations along the coast (e.g. Seward, Whittier, Valdez) enough
cold air should remain in place for the precipitation to stay as
snow through the duration of the event. The heaviest snow amounts
look to be in the Chugach and Kenai Mountains, with as much as 5
to 10 inches possible through Turnagain Pass and Portage Valley
from Sunday evening through Monday evening.

Also on Monday, a shortwave trough digging south across the AKPEN
will undercut the surface trough over coastal Southcentral and
begin to lift it north, guiding it over interior Southcentral.
As it does, the flow will turn more southerly and allow some
moisture to work up Cook Inlet. This will allow for some light
snow to develop over western Kenai Peninsula north across the
Anchorage Bowl and into the Susitna Valley. Right now, it looks
like a quick shot of light snow, with perhaps a coating to up to
2 inches over this area.

Another aspect of the surface frontal system will be the influx of
warmer air across the region. The east-southeasterly flow will
help to scour out much of the arctic air currently in place with
temperature climbing above 0F in the Copper River Basin, into the
teens and twenties from the Kenai Peninsula north across the Mat-Su,
and around the freezing marks along the coast by Monday.

Some clouds and scattered snow showers will linger, mainly across
the mountains, into Tuesday as temperatures begin to cool back
down.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Through Tuesday)...

A weak frontal boundary situated over western Alaska will bring
snow to areas in and around Bristol Bay Saturday night and Sunday
with a few isolated snow showers making it into the Kuskokwim
Delta. Otherwise, expecting cold and dry conditions to persist
through the weekend and first part of the week. Temperatures are
expected to warm a bit towards Tuesday with the introduction of
some clouds into the region thanks to an upper level disturbance
tracking through on Monday.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Through Tuesday)...

Snow showers will continue to track through the Bering along a
weakening frontal boundary. Expecting some shift of this pattern
northward of the Pribilofs on Tuesday morning as the disturbance
responsible for it also lifts north. Consequently, colder
temperatures will move in Tuesday behind the frontal boundary
with a weak area of high pressure.

&&

.MARINE (Days 3 through 5...Tue through Thu)...
The primary areas of concern during this time period will be
along the Aleutian Chain. One system will just skirt along the
south side of the chain Tue into Wed. This could bring easterly
winds as high as high end gale force, but for now it looks like
the strongest winds will stay south of the marine zones. The next
system will push towards the Western Aleutians (Zones 177-178)
late Wed into Thu. This system looks to bring a healthy swath of
gale force winds from the Central to the Western ALeutians.

&&

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

On Tuesday, zonal flow is expected to exist in the Bering Sea and
a pretty diffuse pattern (not much going on) will exist in
Southcentral AK. This will change Wednesday as long range guidance
continues to show an upper level blocking ridge building from the
north Pacific across the AKPEN and into the Bering Sea. An
anchoring high will be present by Friday, securing the position of
this ridge for the remainder of next week.

This means high pressure for the Bering Sea and cold air over
mainland AK. It is possible for deep cold to return to the Copper
River Basin late next week with this pattern. Offshore gusty winds
will be possible in southwest AK and the western Gulf. Low
pressure will be present somewhere in the Gulf of Alaska, but this
is one feature guidance is having trouble with. We know a trough
will dig south through eastern Alaska into a low in the gulf.
However, there is disagreement where this will set up. For now it
looks like the eastern Gulf will be impacted by any low pressure
setup, but confidence is low with this aspect of the forecast.
This pattern shift is something we will be paying close attention
to, so expect modifications with the forecast in coming days.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Wind Chill Advisory 131.
MARINE...Heavy Freezing Spray Warning 127 139.
         Gale Warning 119 120 127 130.
FIRE WEATHER...NONE.

&&

$$

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



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