Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 220049
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
449 PM AKDT Sat Jul 21 2018

.ANALYSIS AND UPPER LEVELS...
An upper level ridge stretching from the eastern Pacific, north
across the Gulf and over Mainland Alaska is the dominant feature
influencing today`s weather. For Southcentral Alaska, this ridge
is primarily bringing warm and dry weather with marine stratus
along the coast. For southwest Alaska, a diffuse negatively tilted
upper level trough stretched between the upper low in the Bering
and another smaller upper level low south of the Alaska Peninsula,
is spreading clouds, some light rain and breezy conditions to the
Southwest Alaska coast. Further inland, partly cloudy skies and a
weak influence from the interior thermal are combining to
increase instability this afternoon and bring the potential for
isolated thunderstorms. Further to the west, cold air aloft to the
south of the upper low tracking east through the Bering has been
bringing a few thunderstorms. Elsewhere across the Bering,
Aleutians and Alaska Peninsula, a cloudy mix of stratus and
strato-cumulus prevail with areas of rain and showers that are
associated with the various small troughs rotating around the
upper low.

&&

.MODEL DISCUSSION...
Models remain in good synoptic agreement through early next
week. Forecast uncertainty at the moment hinges primarily on
stratus location, movement and growth, the finer scale details of
which none of the models have been resolving particularly well.

&&

.AVIATION...
PANC...With the upper level ridge axis continuing to shift
gradually east, the chances for low stratus to impact PANC
continue to diminish but there is a small possibility of brief
LIFR stratus or patchy fog Sunday morning. Winds will remain light
through the TAF period with afternoon sea breezes.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3...Sun
through Tue)...
Sunday will provide one more sunny and warm day of this summer
stretch for all areas expect Kodiak Island. The ridge of high
pressure that has been keeping weather quite quiet for nearly the
past week is slowly shifting off to the east. This is due in part
to a weak front approaching from the North Pacific/Eastern
Aleutians. Low stratus and patchy fog will likely lurk around Cook
Inlet and the North Gulf again tonight and through Sun morning.
It is extremely difficult to pinpoint exactly which areas will see
the worst conditions, but for now, it looks like Anchorage will
stay out of it.

The front will make its way east through the day Sun. Clouds will
overspread Kodiak through the day. Onshore flow will increase
at Kodiak City and this should help rain to start by mid-day. High
clouds will then move over the Kenai Peninsula Sun afternoon and
into the Anchorage Bowl late Sun night. Models have shown some
signs today of trending slower with the ridge breaking down. They
have also shown that when it does break down, it tilts back to the
west limiting the front`s northward extent. So while rain should
reach the Gulf Coast of the Kenai by Sun night, it will struggle
to move up the Cook Inlet. Southeast winds will also play a role
here as a tighter gradient will bring some moderate downsloping
winds off the Kenai and Chugach Mountains. Thus, have backed off
the PoPs a bit for Mon over many of the population centers.

By Tue, the vertically stacked low pressure center will be right
along the Alaska Peninsula. Various weak pieces of energy will
continue to make for rain along Kodiak and the Gulf Coast. A
southeasterly gradient will remain in place across much of
Southcentral. This will lead to breezy conditions, especially
through the usual gaps, and could aid in keeping many inland
locations drier longer.

&&

.FIRE WEATHER...The Copper River Basin will see temperatures,
humidities, and winds approach very near critical levels for fire
concerns on Sun afternoon and evening. The hot and dry trend
across the area will continue for just one more day as the ridge
holds in place. As the aforementioned front draws nearer, it will
allow southeast winds to develop over the Copper River Valley Sun
afternoon. The combination of these elements will raise fire
weather concerns Sun until cooler and moister air arrives on Mon.
While we expect conditions to remain just above warning criteria,
caution should be exercised across the area.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 3)...
An upper level low complex extending across the Bering is running
into a persistent upper ridge to the east leading to a tightening
pressure gradient and gusty southeasterly winds across the AKPEN
and Bristol Bay coast. This system will bring mostly cloudy skies
and a few rain showers to coastal areas tonight, while inland
locations will remain less cloudy and warmer. Tomorrow, the main
upper-level shortwave begins to drive north into southwest Alaska.
This will bring a better chance of wetting rains to the area,
especially for the AKPEN and west coast. The upper-level ridge
upstream, however, will become negatively tilted with the ridge
axis remaining in place across the central portion of the state.
This will keep the shortwave closer to the coast and make it quite
difficult for any precipitation to develop east of a Bethel-
Iliamna line. The interaction between the shortwave and ridge will
also increase southeasterly winds through the Kamishak Gap and
down across Lake Iliamna for Sunday.

The ridge begins to lift north Sunday night. This will allow yet
another shortwave and associated surface low to track toward the
Alaska Peninsula. This will help bring more clouds and widespread
rainfall to the coast and across the majority of the southwest
interior. The low lifts further north and east on Tuesday focusing
precip over higher terrain along the coast and the AKPEN.
Southeasterly winds will ramp up again through the Lake Iliamna
area, AKPEN, and the Bristol Bay coast, with gusts reaching over
40 mph in some favored locations.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
A large upper low will slide southeast through the forecast period
bringing plenty of cloud cover and widespread rain showers across
most of the region. A shortwave moving east across the Central
Bering, including the Pribilofs, and Central Aleutians will bring
periods of heavy rain to the area tonight and tomorrow morning.
At the same time, a strong upper level jet will move over the
western Aleutians supporting good dynamics for a narrow band of
heavy rain to develop over the island chain through Monday. Total
rain fall over the next 2-3 days could be over 2" over the western
part of the chain. The upper low continues to track east through
Tuesday which will bring cooler NW flow on the backside of the
system across the Central and Eastern Aleutians, though the drier
air will also provide a break from precipitation. A ridge will
move in over the far western Aleutians by Tuesday bringing mid-
level warm air advection which should lead to the development of
low clouds and fog.

&&

.LONG TERM FORECAST (Days 3 through 7: Tuesday through Saturday)...

Tuesday will feature a wave of moisture moving northwestward
around the upper level low centered near Sand Point moving into
mainland Southern Alaska. The heaviest rains will be along the
coast of the Kenai Peninsula, primarily staying south and west of
Prince William Sound until later in the day, and Kodiak. With
ongoing downsloping continuing through Turnagain Arm and the Knik
River Valley, most communities from Anchorage north should remain
dry until the evening hours. Tuesday night into Wednesday morning
will feature better chances for the precipitation to get into
Anchorage and the Matanuska Valley as a reinforcing wave of
moisture in the upper levels moves into Southcentral. Scattered
showers will be prevalent Tuesday and Wednesday across Southwest
Alaska, with the most favored locations for precipitation being
the east facing mountains. Precipitation chances will diminish the
farther north and west one goes as well. From midday onwards on
Wednesday, Anchorage and the Matanuska Valley should be drier
behind the wave as the downsloping winds continue.

Another round of rain moves into Southcentral, including Anchorage
and the Matanuska Valley on Wednesday Night. This wave moves north
into the Susitna Valley during the day Thursday, with scattered
showers continuing for much of the rest of the area. The passage
of the front will allow Kodiak to dry out relatively speaking as
the rain becomes much more intermittent there on Thursday.
Meanwhile that moisture will also spread westward into Southwest
Alaska during the day Thursday. Outside of some isolated showers,
Friday should be a generally drier day, though unfortunately still
mainly cloudy for most areas, as the upper level low and its
associated moisture retreats south and westward.

The models diverge greatly late Friday into Saturday area-wide,
due to different handling of a strong low and its associated
fronts moving into the western Bering on Thursday then pushing
eastward through the rest of the Bering and potentially Southwest
Alaska by Saturday. The models are generally agreeing on the
ridging over the Panhandle and western Canada slowly building back
to the west into the Copper River Basin late in the week. That
should help keep that area warmer and sunnier than the rest of
Southern Alaska this week with the exception of the front and
associated rain moving through there as well on Thursday. Breezy
conditions and relatively warmer temperatures may be cause for the
continuation of fire weather concerns Tuesday in the Copper River
Basin.

&&


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

&&

$$

SYNOPSIS AND MODEL DISCUSSION...JR
SOUTHCENTRAL ALASKA...MO
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KVP
LONG TERM...JW


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