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Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
440 am akdt Fri Sep 22 2017

Analysis and upper levels...

Two upper level lows, one centered just inland of Norton Sound,
and a secondary weaker upper level low south of Dutch Harbor are
the primary drivers of this morning's weather pattern across
southern Alaska. Cooler air is slowly filtering in, especially in
southwest Alaska. East of the upper level lows, much warmer, moist
air is being replaced by the cooler air mass north of it. This
clash of air masses, along with some upper level forcing, is the
driver for the ongoing precipitation occurring over a good portion
of the area. The upper low, while the dominant feature in the
upper level pattern, is not associated with particularly cold air
in the lower levels of the atmosphere, and thus, precipitation has
broadly remained light as baroclinicity is unimpressive.

&&

Model discussion...

The models are tightly clustered through much of the next two days
as a nearly stationary front draped across Bristol Bay and
extending northeast into southcentral, and a low working its way
eastward moves into the western Gulf by Saturday. All of the
models agree on this scenario well. By Sunday however, there is
more divergence between the models as to how the low moves
northward into southcentral. Thus, by Sunday the model differences
especially in precipitation start to appear, and the primary
question for southcentral becomes how much precipitation gets over
the Kenai Mountains into the western Kenai Peninsula, Anchorage
bowl, and the mat-su valleys. The NAM and ec had the best
agreement in the short-term so those were the models used going
into Sunday. Forecast confidence is high.

&&

Aviation...
panc...off-and-on rain will be present throughout the day,
occasionally lowering visibilities and ceilings to MVFR. Light
winds will persist until this evening, then building surface
ridging along the coast and troughing along the Alaska Range cause
the Turnagain Arm jet to potentially move into west Anchorage and
panc.

&&

Short term forecast southcentral Alaska (days 1 and 2)...
a frontal boundary extending along The Spine of the Alaska
Peninsula and up to Cook Inlet will remain nearly stationary
through this evening. Surface waves riding up the front combined
with a steady stream of upper level waves moving through the
same general area will continue to produce steady rain from the
western Kenai Peninsula up to Anchorage and the mat-su.
Precipitation will be more showery east of the main forcing,
across the Gulf and on up to Prince William Sound.

These steady state conditions will begin to change tonight as the
remnants of a low south of the Aleutians tracks to the western
Gulf out ahead of a digging short-wave over the eastern Bering
Sea. This will cause flow to back toward the south and southeast,
which in turn will push the northern part of the stationary front
westward. The strengthening low level southeast flow will cause a
downslope hole to open up from the western Kenai to Anchorage and
the mat valley be early Saturday, bringing an end to the rain in
these areas.

Expect strong upward vertical motion to then develop ahead of
the digging trough on Saturday as it rotates into the Gulf and
becomes negatively tilted. This will lead to heavy rain over
western Prince William Sound as well as the west side of Cook
Inlet and western Susitna Valley. This will also help re-invigorate
a weak surface low which moves into the Gulf, leading to a gale
force front along the north Gulf Coast Saturday afternoon. The
low will ultimately track to the vicinity of the Kenai Peninsula
Saturday night into Sunday, which will cause downslope flow to
weaken and likely lead to rain moving back into the population
centers.

The heavy rain over the weekend will add to high water levels on
Kenai Lake and Kenai River produced by the release of water from
the snow Glacier dammed lake. Thus, a Flood Warning remains in
effect all the way through Monday.

&&

Short term forecast southwest Alaska (days 1 and 2)...
a stalled front draped along the western Alaska Range, to eastern
areas of Bristol Bay, to the Alaska Peninsula remains through
Saturday then lifts slowly north. Another low pressure system
develops within the trough and brings higher chances for rain
across the Kuskokwim Delta on Saturday and then dives to the
peninsula on Sunday. The general flow across the southwest
Mainland and peninsula will be out of the north through Saturday
then switches to a southerly direction as the surface low
associated with the front lifts through the Kuskokwim valley.

&&

Short term forecast Bering Sea/Aleutians (days 1 and 2)...
a cooler weather regime is on tap through the upcoming weekend as
a cold air mass advects from the northern Bering along the
backside of a trough along the West Coast of the Mainland. The
pattern stays on the drier side as a ridge across the western
Bering slides south. Overall, winds are expected to be low impact
with small craft northerly winds along the the central and
eastern waters.

&&

Long term forecast (days 3 through 7)...
beginning the extended period forecast Saturday evening, with an
upper level low tracking southward over the eastern Bering and
bringing waves of North Pacific moisture northward toward the Gulf
of Alaska and southern Mainland through the early part of next
week. This pattern will support wet weather across the region into
Sunday afternoon, as a front will move toward the Gulf Coast.
Gusty winds are also expected to increase along the Gulf Coast
Saturday afternoon through Sunday morning, as the front pushes up
along the northern Gulf Coast.

Showers will begin to taper off over much of the southern
Mainland by Monday morning, and remain slightly drier through
Tuesday morning when the next system begins tracking into the
Gulf. Precipitation will begin to spread northward back into the
southwest Mainland and northern Gulf Coast Tuesday afternoon, as
a North Pacific low tracks northward into the western Gulf. There
is some disagreement between the models with regards to the track
and timing of this system, which impacts when the precipitation
spreads further inland. Currently the forecast is trending toward
precipitation moving further inland overnight Wednesday into
Thursday morning, with the low tracking inland over the Kenai
Peninsula.

&&



Afc watches/warnings/advisories...
public...flood warning: 121 125.
Marine...none.
Fire weather...none.

&&

$$

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