Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 200028
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
428 PM AKDT Thu Apr 19 2018

.ANALYSIS AND UPPER LEVELS...
The dominant feature on the map is the low in the Southern Bering
Sea inside a well-defined trough over the North Pacific and Gulf
of Alaska. The current surface wave has occluded over the
eastern Gulf. Wrap-around clouds and rain were filling in along
the outer coast of Prince William Sound while only high clouds
were found over parts of the Kenai Peninsula as deeper moisture
held east.

In southwest Alaska a cold, light rain was falling over parts of
the lower Kuskokwim Valley with a steady easterly flow responding
to Arctic high pressure pushing southward and a 984mb surface low
north of Unalaska. A large patch of dry air was limiting the
extent of any rainfall in the region. Enough dry air was working
off the western mainland that even the Pribilofs stood a chance
to defeat the Bering Sea stratus machine for a time being this
afternoon. Showers surrounded the surface low from the Eastern
Aleutians to the Alaska Peninsula.

&&

.MODEL DISCUSSION...
Models were in good agreement at initialization through 72 hours
(00z Sunday) for position and intensity of the low. A blend of the
NAM/GFS was used for Southcentral and Southwest Alaska`s forecast
today.

&&

.AVIATION...
PANC...VFR conditions will persist. Fairly strong southerly winds
are expected to develop Friday afternoon, along with a chance of
some showers.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2: Friday and
Saturday)...

Tonight and Friday: After a beautiful day today across much of
Southcentral not on the immediate Gulf Coast, clouds will slowly
return towards sunrise and into the morning Friday as an area of
low pressure and its associated Gulf moisture move westward into
Prince William Sound. Meanwhile, an upper level trough will
further increase lift across the area as it moves to the west side
of the Alaska Range. The combination will likely override the
ongoing downsloping southeast winds through Turnagain Arm and the
Knik River Valley to some extent. Thus, some shower activity is
expected Friday afternoon and evening through Anchorage and the
Mat-Su Valleys. The timing of the precipitation around peak
daytime heating should keep the precipitation mostly rain, but
anywhere it can get heavy enough in any given shower that there
may locally be brief heavy snow that may lead to a minor
accumulation on grassy surfaces. West Anchorage and those impacted
by the Knik River wind will see less precipitation as the wind
acts to dry out the atmosphere a little more. However, the added
lift from the trough should be enough to overcome the downsloping
at times.

Further north in the Susitna Valley, precipitation will also break
out due to the stronger lift from the upper level trough by
midday. However, unlike areas further south, the longer duration
of the lift and the broad south to southeasterly flow causing
upslope conditions will lead to a prolonged period of mountain
snow and mainly valley rain during the day, with some snow mixing
in from Talkeetna north overnight. The snow should be a wet type
with temperatures near freezing, which should inhibit advisory
criteria accumulations in any of the valleys and Broad Pass. For
those on the south side of Denali National Park however, over a
foot of snow is possible through the mountains.

On Friday night, the trough will move further westward, bringing
the strongest lift with it. Thus, a diminishing trend in the
precipitation rates is expected overnight. However, the easterly
winds with a developing barrier jet will continue the upslope
precipitation through western Prince William Sound. Thus, the
pattern will return to the typical downsloping one with the
Matanuska Valley, Anchorage, and the western Kenai returning to
mainly dry conditions. This pattern will continue into Saturday.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Southeast upper flow remains over the region, and with daytime
heating, is supporting the development of more afternoon
convective showers, the most prevalent downwind of the large
mountain ranges. Otherwise, weather is rather serene as the low
which has been over the Bering shifts east and weakens. The next
system of note is an arctic trough sagging south over southwest
Alaska Friday through Saturday. This system will be cold enough
for a mix of rain and snow, with the best chances over the Lower
Kuskokwim Valley and Kuskokwim Delta

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
Very quiet weather is in store for the Bering Sea as high
pressure builds in. A weak low will skirt the far western and
Central Aleutian Chain bringing some 25-30 kt easterly winds and
rain/snow. Otherwise, low stratus and areas of fog will be the
dominant mode along with some showers over the Eastern Aleutians.

&&

.LONG TERM FORECAST (Days 3 through 7)...
A low will head out into the Eastern Bering on Sunday, bringing
precipitation to the western portions of the Bristol Bay and
Kuskokwim Delta zones. Most Bristol Bay locations should be warm
enough for rain, with the areas from Dillingham north and west
seeing some light snow. The Kuskokwim Delta coast will stay cold
enough for a few inches of snow accumulation.

Meanwhile a progressive pattern yielding gusty, cloudy, and cool
conditions across southcentral, and will remain in place from
Sunday through late next week. Models are showing a series of
fronts entering the Gulf region originating from a north Pacific
low south of Kodiak (Sunday into Monday) and eventually a large
low over the Bering (Monday through Thursday). This will continue
the pattern of southeast flow downsloping the areas west of the
Chugach and Alaska/Aleutian ranges, bringing cloudy, cool, but
drier conditions to these areas. Whereas along the Gulf coast,
steadier precipitation is expected with each frontal passage,
along with gusty winds along the northern Gulf and through favored
channeled terrain. By Friday, there is a chance that precipitation
over the area could begin to dissipate as high pressure starts
moving back over the southern Mainland. However, there is very
little model agreement on this solution, lowering confidence in
the forecast for the later part of the week.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...None.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...DS
SOUTHCENTRAL ALASKA...JW
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JA
LONG TERM...PEPE



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