Area Forecast Discussion
Issued by NWS Anchorage, AK

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726
FXAK68 PAFC 190036
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
436 PM AKDT Mon Sep 18 2017

.ANALYSIS AND UPPER LEVELS...
A long wave upper trough stretches southeast from the Chukchi
Sea, across Southwest Alaska, to over the western Gulf of Alaska.
This feature is producing widespread cloud cover across much of
Southwest Alaska and Southcentral. Radar and surface observations
show areas of rain across Southwest Alaska, western/northern
Gulf, and inland areas of Southcentral west of the Copper River
Basin. Brisk and gusty west to northwest winds, associated with
weak cold advection on the backside of a southeast moving cold
front, were evident across coastal Southwest Alaska/Bristol Bay
to over the Barren Islands/western Gulf.

&&

.MODEL DISCUSSION...
Models overall are in good agreement of features affecting the
Mainland and Gulf of Alaska. The GFS/ECMWF have a better handle
on a developing low heading north toward the Aleutians Tuesday
and Wednesday.

&&

.AVIATION...
PANC...Light winds should prevail through tonight. Abundant low
level moisture will give rise to low VFR to high MVFR ceilings
this afternoon into Tuesday morning.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
An upper level low currently centered over Kodiak will slowly move
SE across the Gulf tonight. A weak deformation zone near the top
of low will continue to support areas of precipitation across
southcentral through the rest of the evening. Rain will taper off
west to east across the area tonight as an upper level ridge
builds in quickly behind it. The building ridge from the west will
tighten the pressure gradient across the AKPEN leading to gusty
NW winds tonight and tomorrow, especially through channeled
terrain including Kamishak Bay and along the Southern Kodiak
Coast. Generally pleasant weather can be expected across
Southcentral tomorrow and into Wednesday as high pressure
dominates the region. A weak system moving across Western AK will
clip the northern portions of the area on Wednesday, with light
rain expected as far south as the Mat-Su, though downsloping off
the leeward side of the AK Range should limit the amount of
precipitation. On Wednesday night, skies should temporarily clear
enough to produce the coldest temperatures of the season.
Temperatures are expected to cool down into the upper 30`s to low
40`s across Anchorage and the Mat-Su, while low 30`s to possible
sub-freezing temps are expected in the Copper River Valley as well
as Talkeetna. The next system looks to move into the area on
Thursday spreading steady rain from south to north across most of
the region.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Precipitation will be on a downward trend across the southwest
mainland this evening as the upper level trough over western
Alaska is replaced by weak transient ridging. This will bring a
period of drier conditions to the greater Bristol Bay region
through Wednesday, however the break in precipitation will be
more short lived across the northern part of the area as a fast
moving system tracks from the northern Bering into the Yukon
Delta tonight. This will bring a trailing front across the
Kuskokwim Delta and the lower Kuskokwim Valley beginning early
Tuesday morning, with showers continuing into Wednesday. More
robust precipitation will return to the Bristol Bay region by late
Wednesday as a front extending from a North Pacific low begins to
push inland.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
Benign cloudy conditions will prevail across most of the Bering in
the presence of surface high pressure and dry northeast flow
through Tuesday afternoon before the front arrives over the
Central and Eastern Aleutians. This will be accompanied by rain
and gale force winds, with these conditions spreading east to the
Alaska Peninsula Tuesday night and then north to the Pribilof
Islands on Wednesday. The parent North Pacific low will keep rainy
conditions over the central part of the chain through midweek as
it lingers south of Adak.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Beginning the extended period forecast Wednesday night, there will
be an elongated arctic trough over the northern part of Alaska
with a secondary North Pacific trough centered over the Eastern
Aleutians. A very strong Northeast Pacific subtropical ridge will
be south of the Gulf of Alaska. This will set in motion a broad
region of southwesterly flow aloft over Southern Alaska with rain
impacting the entire region. This pattern will continue through
Saturday with wet and cloudy weather, although no significant
heavy rainfall. By Saturday evening, the pattern becomes a bit
more complicated as the most recent simulations of the GFS and
ECMWF deterministic models have trended toward a strong secondary
subtropical warm front merging with the arctic trough. The
associated low would track into the Gulf of Alaska and toward
Southcentral sometime Saturday evening into Sunday, bringing a
swatch of possibly gale force winds to the coast and another batch
of rain, locally heavy along the coast. Either way, even with that
uncertainty, there is high confidence that the overall pattern
will keep a trough aloft and moist flow directed at Southern
Alaska. This will keep wetter conditions in place into the next
week.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning 130 174 175 176 177.
FIRE WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...RC
SOUTHCENTRAL ALASKA...KVP
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CB
LONG TERM...JA



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