Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 220116
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
516 PM AKDT Sat Sep 21 2019

.ANALYSIS AND UPPER LEVELS...
A deep Siberian low sits just off the northwestern coast of
Alaska. From this low extends a large upper level trough across
much of the state, reaching deep into the northeastern Pacific.
Ahead of this trough low pressure is moving from the Gulf of
Alaska, northeast and inland. This complex was responsible for the
rain across the region, as well as fresh termination dust across
the Chugach and Talkeetna mountains. Snow was noted around 3500 to
4000 ft in many locations, though select non-front-range valleys
saw the snow level dip to close to 2800 feet. West of this first
system and across western Alaska is a transient and dry ridge.
Beyond the ridge is the strong 150 kt north Pacific jet rounding
the base of a second trough that is extending south from the
Siberian low. This jet is responsible for deepening low pressure
southwest of Adak this morning. The jet is helping to drive
a moisture rich front eastward.

&&

.MODEL DISCUSSION...
All of the standard global models initialized relatively well
today. In the short-term, all guidance handles features similarly.
Differences begin to show with the low pressure that is near Adak
this afternoon, and how quickly it skirts the AKPEN before moving
into the Gulf of Alaska. While the overall picture is similar,
the biggest differences are in the timing and depth. The biggest,
and slowest, outlier is the GFS. The EC, NAM, Canadian and even
the GFS ensembles support a quicker, slightly weaker solution.
Beyond this system and moving into the longer range, models align
again and all indicate a weak ridge moving across Southcentral
Alaska as another deep arctic trough moves over the Bering.

&&

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

&&

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

A quieter period of weather is expected for much of interior
Southcentral through Tuesday. Cloud cover over most of the area
currently is expected to gradually thin out and dissipate this
evening. Meanwhile, a cooler, drier air mass is filtering through
the Alaska Range. This should allow some locations to see their
first sub-freezing temperatures of the season tonight. For more
information, see the Special Weather Statement. The challenging
aspect to the forecast tonight continues to be how much the cloud
cover dissipates tonight and how long that takes, which will have
a large effect on how low the temperatures drop. For Palmer,
Prince William Sound, and the Copper River Basin, the clouds are
expected to hold tough all night, keeping temperatures safely
above freezing.

The period of clear skies will be short-lived, as clouds return to
most areas Sunday morning. This is in advance of the next low that
will be moving into the Gulf, with the jet stream bringing the
cloud cover in well in advance. The low is expected to track
eastward across the southern and central Gulf. Thus, associated
rain will be confined primarily to Kodiak Island and the north
Gulf Coast, with little if any rain making it very far inland.
Precipitation chances in the forecast through Monday were reduced
for interior areas accordingly. Weak upper level ridging will
build into Southcentral by Monday afternoon. The persistent cloud
cover Sunday and Monday nights will keep lows held into the upper
30s and 40s.

By Tuesday, the next series of waves along with a cold air push
will approach Southcentral from the west. With the first wave
Tuesday afternoon, an unstable atmosphere will support scattered
rain shower development over much of Southcentral. With cooler air
moving in aloft, any showers over elevated areas in the mountains
could very well mix with or briefly change over to snow. An even
more interesting wave moves in late Tuesday night into Wednesday
morning, which will bring in even cooler air aloft. The timing and
strength of this wave will be critical as to what it can produce.
However, with freezing levels falling to between 1,000 and 2,000
feet, along with the unstable atmosphere keeping the precipitation
showery in nature, any heavier showers may result in non-accumulating
snow reaching the ground even to sea level. Stay tuned.

&&

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

The front associated with an occluded low just north of Nunivak
Island is progressing inland, bringing increased cloud cover,
gusty winds and precipitation to the Southwest mainland. Winds
will diminish tonight into Sunday, as the low shifts toward the
Seward Peninsula. A deformation band can easily be seen on
satellite stretching from the col of the Nunivak Island low and
a low over the central Aleutians towards the Kobuk Mountains.
Precipitation will likely be enhanced under this band, but
elsewhere precipitation rates/coverage should gradually diminish
tonight.

As the low over the Aleutians progresses eastward Sunday into Monday,
Southwest will see another round of rain and elevated winds primarily
in the Bristol Bay/Greater Bristol Bay area. Along the north side
of the AKPEN, downslope winds in the eastern periphery of the low
will bring temperatures to the mid to upper 50s. The Kuskokwim
Valley/Delta will be on the northern fringe of this system and
precipitation should be minimal, though chances are higher over
the Kuskokwim Mountains.

A ridge will build over the Bering Tuesday, and ahead of the ridge
a shortwave will dive south along the coast from the Bering
Strait...bringing possible showers and cooler conditions as
modified Arctic air pushes inland.

&&

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

A low centered just south of the western Aleutians will continue
its trek along the south side of the chain tonight into tomorrow,
bringing precipitation and small craft advisory conditions across
the Aleutians...increasing to gales in the eastern Aleutians and
Bristol Bay Sunday afternoon. Winds will diminish during the day
Monday and broad westerly flow will prevail over the Bering. Late
Tuesday, a deep low (post-tropical storm Tapah) will approach the
western Bering/Aleutians and bring gales, possibly storm force
winds to the western Aleutians/Bering. Models are struggling with
the timing/strength of this feature, so be sure to monitor the
forecast over the coming days.

&&

.MARINE (Days 3 through 5)...

A tight pressure gradient will exist with an area of low pressure
in Prince William Sound butted against high pressure west of the
AKPEN. Confidence remains high that the Barren Islands area will
see gale force northwesterly winds Tuesday afternoon and into the
evening hours. Across the Bering, the remnants of typhoon Ta Pah
will be sliding north and near the Kamchatka Peninsula. A strong
front will accompany this low and sweep across the western coastal
and offshore zones. Early indications are that 12 to 15 mb
pressure drops within a 3 hour period are possible. There is high
confidence that southerly storm force winds will accompany this
front (moving across Shemya Tuesday afternoon) through Attu,
become high end gales as it slides east. There is a lower chance
that winds could meet or exceed low end hurricane force. Resultant
seas with the front are expected to exceed 25 to 30 feet at the
peak.

&&

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

There is high confidence moving into the end of the week of high
pressure building in over mainland Alaska. This pattern change
will help to clear out the rain that`s been over the southern
mainland. However, as the ridge axis tracks east through the
southern mainland, the western side will see southwest flow and
rain returning. There is some disagreement amongst the models on
the timing of the movement of the ridge axis.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning 119 120 130 131 132 136 137 138 150 155 160
165 170 171 172 174.
FIRE WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...SS
SOUTHCENTRAL ALASKA...JW
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MM
MARINE/LONG TERM...SS/DK


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