Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 240016
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
416 PM AKDT Mon Mar 23 2020

.ANALYSIS AND UPPER LEVELS...
Satellite today shows a widespread cirrus shield moving over the
Bering Sea and spreading over the mainland as well. In spite of
the high clouds, onshore flow contributed to fog along the West
Coast, which persists even into early afternoon. Fog in the
Bristol Bay area has risen slightly and formed a low cloud deck
across the region. Rain is moving in to the Central Aleutians
ahead of the main low, with some gusty but not excessively strong
winds.

East of the Alaska Range, high pressure continues to cause
offshore flow, although winds have diminished slightly as the
ridge axis moves overhead.

&&

.MODEL DISCUSSION...
Models are in fairly good agreement in the short term, in
particular on the timing of the front moving shore along the
West Coast. The main question surrounds when the warmer air will
move in at the surface. Southwest flow at the surface indicates
this may happen quicker than the models indicate, which would
reduce the threat of freezing rain. This will need to be watched
as the event unfolds.

&&

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

&&

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

Outflow winds along the coast have been on the decrease today and
will continue to trend downward through the evening and overnight
hours. This is in response to an upper-level ridge over the
western Gulf and a trough over western Alaska sliding eastward and
shifting the upper-level winds to a more westerly direction.

The aforementioned shortwave will track across Southcentral during
the overnight hours, dropping toward the Alaska Panhandle Tuesday
morning. Given the west-northwesterly flow and limited available
moisture, most of the precipitation with this system will stay
north of the Alaska Range, with areas south mainly seeing only an
increase in mid and high level clouds. On Wednesday, two additional
upper-level shortwaves, one over northwest Alaska and a second
near the Southwest Alaska coast, will move in tandem southeast,
crossing the Copper River Basin and Kodiak Island, respectively.
These features will have better upper-level support to produce
areas of light snow across interior Southcentral with a mix of
rain and snow along the Southcentral coast. With a general west-
northwesterly flow aloft, however, there will not be a lot of
moisture and, hence, precipitation, to speak of. The highest
precipitation amounts will mainly be confined to the higher
terrain were upslope flow is prevalent. For valley locations (Mat-
Su, Copper River Basin, Anchorage Bowl), a couple tenths of an
inch of snow could accumulate Wednesday morning before warmer
afternoon temperatures limit the potential for additional
accumulation from the falling snow.

Both shortwaves will continue to track southeast, departing the
region on Thursday with only a few isolated showers lingering
over the mountains. As the trough enters the eastern Gulf and a
ridge builds back across the interior, outflow wind will again
trend higher beginning Thursday afternoon.

&&

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

The large area of high pressure that`s been in place the past
couple of days is shifting off to the southeast. The departure of
high pressure will usher in a low that is lifting across the
western Aleutians tonight. The first impacts to southwest Alaska
will be precipitation coming ashore in the Kusko Delta late
tonight. A generous push of warm air over the colder air at the
surface will see some chances for freezing rain through Tuesday
morning. Areas in and around Bristol Bay will see a similar
pattern unfold Tuesday night into Wednesday as the low approaches
Nunivak Island. Forecast confidence in freezing rain is not high
due to the precise alignment of warm air over colder air, but the
structures will certainly be in place during this event to make it
a possibility. The bulk of sensible weather will push through late
Wednesday night and Thursday looks to be fairly quiet ahead of a
developing system coming in on Friday.

&&

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

As high pressure in the eastern Bering pushes off this afternoon,
a frontal boundary affiliated with a north Pacific low has lifted
into the western Aleutians pushing temperatures into the 40`s and
bringing some light rain. The parent low is expected to push
across the Aleutian chain west of Adak sometime early Tuesday
morning. As this system moves northeastward, rain can be expected
across the Bering and Aleutians, as well as the Pribilofs. This
warmer and wetter pattern will persist into Thursday, with the
Pribilofs seeing some snow mixed in Wednesday night and Thursday
morning thanks to cooler air behind the low. Thursday will see
fairly quiet conditions across much of the Bering. However,
another north Pacific low will bring a front into the western
Aleutians early Thursday and continue the active pattern.

&&

.MARINE (Days 3 through 5)...
In the Gulf, winds should not exceed small craft advisory through
Friday. The Aleutians and the Bering Sea will also remain at or
below small craft criteria through Friday morning. After this, a
low pressure system will bring the potential for gale force winds
across the central to eastern Bering and Aleutians Friday night.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Wednesday, an upper level low centered in the Bering Sea will
weaken to an upper level trough. Upper level ridges upstream and
downstream will propagate the trough southeast across the eastern
Aleutians and Alaska Peninsula into the North Pacific by Thursday.
Afterwards, an upper level trough looks to drop down from the
north and merge with an upper level trough located in the western
Bering, strengthening the low pressure system at the surface.
This merger will bring precipitation to the central Aleutians and
Bering and across into the southwest through Saturday. Confidence
has increased that the upper level trough will spin up a closed
off low as it moves across Southcentral and enters the northern
Gulf Saturday. This feature will increase precipitation chances
across Southcentral through Sunday. The models diverge on how they
handle the parent low as it moves across southwest, which places
the main band of precipitation slightly further northward and
westward for the EC solution versus the GFS/Canadian. This will
have the greatest effects in Southwest Alaska, so the forecast
bears watching for changes at the end of the week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 155.
MARINE...None.
FIRE WEATHER...None.

&&

$$

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



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