Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 180151
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
451 PM AKST Fri Jan 17 2020

.ANALYSIS AND UPPER LEVELS...
The upper level ridge is closed off and centered over the Arctic
coast, but stretching southward over most of Southcentral and
Southwest Alaska. The upper level low is elongated south of the
Gulf of Alaska to the Alaska Peninsula. This pattern is keeping
mostly clear skies over the mainland part of the forecast area
with persistent easterly flow over the Gulf of Alaska. The
easterly flow over the Gulf is continuing to bring snow showers
to Kodiak Island. In the southeast Gulf of Alaska, satellite shows
a small but strong low developing which will move to Kodiak
Island after midnight tonight. This low is the source of the
Winter Storm Warning that is in effect for Kodiak Island overnight
tonight into Saturday.

&&

.MODEL DISCUSSION...
As was the case yesterday, models remain in good agreement
overall for the next few days with the western Gulf of Alaska the
only region to really discuss. The easterly flow across the Gulf
is ripe for the development of small, but strong lows to cross it
and reach Kodiak Island. The one that is expected to reach Kodiak
overnight into tomorrow is stronger on the past few NAM runs than
GFS, but the satellite imagery showing it in the southeast Gulf
late this morning shows a stronger low. However, the slightly
farther north track of the GFS is better aligned with the
satellite data than the NAM. Therefore neither model is the
`model of choice`; rather, prefer a blend of the solutions
showing the farther north track with a stronger core.

&&

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

&&

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

A large upper-level ridge across the northern half of the state
will elongate as it lifts to the north and east tonight. As it
does, the flow aloft will become more east-southeasterly,
allowing for a westerly propagation of short-waves across the
northern Gulf. The first of these is currently lifting northwest
along the immediate Southcentral coast. At the surface, this is
reflected by an inverted trough stretching from Prince William
Sound to north of Kodiak Island. This feature is producing bands
of snow showers from south of Cordova across portions of the
eastern Kenai Peninsula and south to Kodiak City, with some heavy
in intensity as showers push west through the overnight hours.

The second short-wave is situated over the eastern Gulf helping
to spin up a surface low. This low is quickly deepening as seen by
the expanse of cooler cloud tops on the most recent IR satellite
imagery. The surface low maintains its strength as it tracks west
tonight, moving toward the southern half of Kodiak Island by
Saturday morning. Snow and gusty northeasterly winds will ramp up
in intensity over the Island around midnight with periods of
heavy snow expected for both the island and Kodiak City through
Saturday morning. Gusts around 40 mph coinciding with the falling
snow will act to significantly reduce visibility to near white-
out conditions at times. It is also possible that enough warmer,
marine air works in along the coast to provide for a brief
rain/snow mix before the precipitation ends Saturday afternoon.

Elsewhere across Southcentral, it will continue to remain mainly
clear and very cold across much of the region through Sunday. By
late Sunday, clouds will be on the increase and temperatures will
begin to moderate as another low in the eastern Gulf lifts north
and west. This system will track closer to the coast, with snow
likely developing across the Southcentral coast by Sunday,
continuing through Monday. Right now, there is still quite a bit
of uncertainty as to how much, if any, snow makes it over the
Chugach Mountains to fall over the Anchorage Bowl and western
Kenai Peninsula.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Through Monday Morning)...

An upper level high will remain over interior Alaska keeping
temperatures cold and conditions mostly dry for much of southwest
Alaska. A series of easterly upper level waves will move ashore
early Saturday between Shelikof Strait and the western AKPEN
bringing some clouds and light snow to the area with minimal
accumulations. Temperatures are expected to moderate some across
the area with the introduction of precipitation and cloud cover.
A line of precipitation from the western AKPEN to the Kilbucks
will linger into Sunday, again bringing minimal snow accumulations
to the affected areas.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Through Monday Morning)...

A weakening front stretching from the northwestern Bering into
the central Aleutians will bring scattered snow showers to the
Pribilof Islands and the Aleutian chain through Monday morning.
As the front decays, expect more cold air to filter in behind
accompanied by more snow showers.

&&

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

The Gulf of Alaska continues to look fairly quiet into next week.
For Sunday into Monday a weak low pressure will create small
craft advisory winds along the north Gulf Coast with a barrier jet
there. Beyond that, weather calms. We are confident in this
forecast for the Gulf.

For the Bering Sea, guidance now indicates a compact low
traversing the Central Bering northwest to southeast. This low is
reflected in the forecast. Gale force winds are forecast on the
backside of this low. High pressure and quieting weather are
expected to move in behind it. There is moderate confidence in
this forecast as the low was not previously depicted by model
guidance. This means it will likely need to be adjusted in coming
days as model guidance gets a better handle on it.

&&

.LONG TERM FORECAST (Days 4 through 7: Monday through Thursday)...

The long term forecast starts with weak low pressure in the Gulf
of Alaska, which will likely be producing snow in the Chugach.
Snowfall amounts and specific locations will be honed in the next
couple of days. This system decays Tuesday and becomes a nominal
factor in the weather. Out in the Bering, a compact low has been
identified by model guidance. This low will cross the Bering Sea
quickly and dissipate as it reaches the Southwest Coast on
Wednesday.

High pressure then builds in behind general low pressure for our
forecast area. A prominent upper level ridge looks to be oriented
southeast to northwest across the Alaska Peninsula into the
Bering. It will reflect as a surface high in the Bering Sea and
likely usher in more cold air for Southcentral AK. Right now the
Mat-Su is on the eastern (downstream and cold side) of the ridge,
so more cold air can be expected, but it will really depend on
where that border sets up and how much cloud cover we get. The
Copper River Basin has a shot to get some really cold air again,
as they will be on the eastern side of the ridge where cold air
will be pulled south from the arctic. Models are in agreement of
the ridge setting up, but differ on placement slightly. So cold
temperatures will depend on what resolves over the next several
days. With this not occurring until late next week, there is time
to get this figured out.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Storm Warning 171.
MARINE...Heavy Freezing Spray Warning 139 160 180.
         Gale Warning 120 130 131 132 137 138 139.
FIRE WEATHER...NONE.

&&

$$

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


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