Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 100139
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
439 PM AKST Sat Dec 9 2017

.ANALYSIS AND UPPER LEVELS...
There is an upper level low centered south of the Alaska Peninsula
with a trough extending northward across the eastern Bering Sea
and another extending northeast into the Gulf. Associated with the
Gulf trough is a surface low centered southeast of Kodiak Island
moving to the northwest. An occluded front extends northeast from
the low and is approaching the northern Gulf coast, with a decent
slug of moisture and Gale Force winds developing. An upper level
ridge is over southern Alaska, which is and extension of the
Massive western North America High. This ridge is retreating
northward as the Gulf low approaches. Further west, there is a
strong low west of the Aleutians, with a Storm Force front over
the western Aleutians. A weak negatively tilted ridge is over the
central Aleutians stretching northwest across the Bering Sea.

&&

.MODEL DISCUSSION...
The numerical models are in reasonably good agreement through
Monday afternoon. There are some differences in the exact speed
and path of the strong low that moves from the southwest Gulf into
southwest Alaska Sunday night, but the basic impactful weather
associated with this low is similar. The precipitation type
continues to be a forecast challenge, and this lowers the forecast
confidence to near normal.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist through
tonight. Some rain showers are possible late this evening into
Sunday morning with a weak front moving through, but ceilings and
visibilities should stay VFR. There may be some freezing rain as
this rain starts, but temperatures should be slowly rising through
the night and any freezing rain is not expected to be around long
before turning to rain. After the front moves through, some
breezy southeast winds are expected to develop Sunday morning.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
An active pattern continues across Southcentral through Monday as
a pair of systems tracks into the Gulf. The first of these systems
will move through tonight as a low tracks over Kodiak Island and
the associated front moves over the Gulf coast. Precipitation will
mainly be rain at sea level, with another few inches of snow
through Thompson Pass as snow levels generally hover around 2000
ft. A brief round of mixed precipitation is possible tonight for
inland locations including the Anchorage Bowl before downslope
flow takes hold and dries these areas out. A brief period of
strong winds will move through favored gaps including Turnagain
Arm as the front pushes inland. The next system arriving Sunday
night into Monday will be stronger than the first and also a
little farther to the south, but will otherwise will behave much
the same as the first. The bulk of the precipitation will once
again fall along the coast, with primarily rain at sea level and
snow mixing in at higher elevations. Gap winds will be stronger
with this system, with a potential Chinook setup for Anchorage on
Monday as the low moves into Southwest AK and allows gusty winds
to spread into the Bowl. With these winds, temperatures are
expected to be quite warm throughout Southcentral, with temps
topping out in the low to mid 40s across Anchorage.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
The synoptic flow will be primarily northeasterly, which is
offshore and more of a drying flow. Although last night saw dense
fog in the Kuskokwim Delta, fog should be less intense and for a
shorter duration tonight, as an approaching trough from the
southeast will bring some instability to the area after about
midnight. This trough is associated with a low that is approaching
the southwest from the Gulf of Alaska, which will bring another
slug of warm air through Southwest tomorrow. Mixed precipitation
is expected in the Bristol Bay region beginning tomorrow morning.
Right now there is a slight potential for freezing rain with this
trough, but temperatures should warm quickly, making for a fast
switch to rain, particularly in the lower elevations.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
A strong low pressure system currently positioned west of the
Aleutian islands is bringing storm force winds into the western
Aleutians tonight. This storm is not expected to move much in the
next few days, so the front will linger in the Central and Western
Aleutians. Between this storm and the mainland, northerly winds
will bring snow showers to most of the Bering Sea. As the low in
the Gulf moves northward and tightens the gradient, winds will
pick up to Gale force over the Central Bering, which is fairly
seasonable for this time of year.

&&

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

The subtropical jet continues to keep an active pattern over
southern Alaska through the next work week. However the upper
level pattern shifts from high amplitude to a zonal flow which
sets up just south of the Aleutian chain heading into the middle
of next week. This upper level change is significant as it will
lead to a cooler trend Wednesday through Friday.

Models are struggling with the pattern shift mid week as they vary
on how quickly they break down the high amplitude ridge over
Southeast Alaska. Kept forecast updates conservative as models
tend to break ridging down too quickly. The most notable change
arrives with cold air advection across the Bering early Monday
with a front plunging south across the Aleutian Chain. The front
gains eastward momentum on Wednesday allowing colder air to wrap
into the following lows moving toward the southern mainland during
the second half of the week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 127-129 136 137 140 141 160 177 179 181 185.
 Storm 119 120 125 130-132 138 139 150 178.

&&
$$

SYNOPSIS AND MODEL DISCUSSION...BL
SOUTHCENTRAL ALASKA...CB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...LF
LONG TERM...KH



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