Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 200049
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
449 PM AKDT Fri Oct 19 2018

.ANALYSIS AND UPPER LEVELS...
The trough that once encompassed much of southern Alaska has
dissipated as a strong low has taken hold in the northern Gulf of
Alaska. The low, stemming from the Central Pacific, is advecting
warm and moist air into Southcentral which can be seen with water
vapor satellite imagery. The rotation of the low is beautifully
displayed in visible satellite with the outer edges encompassing
most of the Gulf. The vertically stacked low is intensifying as it
moves north over Prince William Sound increasing instability in
the area. Several embedded shortwaves in the flow are amplifying
vorticity both at the low center but also ahead of the associated
front where it is draped over the Susitna Valley. Isobaric
alignment is in a favorable position for creating southerly flow
aloft through the MATSU. A second band of instability and the
associated front extend over the central Aleutians. They are
associated with an upper-level Kamchatka low that has entered the
western Bering.

The front associated with the Gulf of Alaska low brought storm
force winds to the northern Gulf Coast overnight and significant
precipitation throughout Southcentral. Snow has fallen throughout
Gulkana and freezing levels have dropped to a level at which
Thompson Pass is experiencing heavy snow fall. Rain is amping up
through Prince William Sound as the front floods the area with
warm, moist air. While lighter, rainfall totals will be on the
rise throughout the coastal areas. Winds however, are a different
story. Southerly flow at the surface has formed a slight MATSU
wind and gusty conditions have been reported. In addition to this
front is a second front which has entered the southwestern Gulf.
This front is associated with a strong low, again stemming from
the Central Pacific. This upcoming storm holds the potential for
yet more significant rain. While the east is quite active, the
west is in a more mundane pattern with very little cloud cover and
precipitation, except with the front over the central Aleutians.

&&

.MODEL DISCUSSION...
Models are in excellent agreement regarding the track and
intensity of the low in the Gulf. While the models once struggled
with this system on pin-pointing the exact landfall location,
they have come to consensus of it setting up over Prince William
Sound. Synoptically, the models are also handling the Kamchatka
low that has newly entered the western Bering. However, this
agreement doesn`t last long as the low begins to weaken Saturday.
The EC and NAM are in step with one another while the GFS jumps
ahead for a short period. By Sunday models come back into
agreement as the low is absorbed by a second deepening low coming
from the Central Pacific. At this point, models diverge and they
struggle to track this low. The EC in general is the much faster
solution brining the low over Kodiak Sun Morning while the the NAM
barely brings it into the Gulf by that point. The GFS was chosen
as the model of choice for this system given it`s middle ground
approach to the low. This pattern persists through Sunday with the
EC and GFS moving the low inland further west Sunday afternoon
while the NAM takes longer bringing it to a similar on-land
position Sunday night.

&&

.AVIATION...
PANC...Light rain persist on and off this evening, but CIGS
should remain above 5000 feet. Around midnight the winds will
shift to the south, and CIGS are expected to drop to MVFR. The
system will move through Saturday morning, and conditions should
then improve to VFR.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)...
Strong winds and rain continue across the northern Gulf this
evening as a now gale-force low situated over Middleton Island
continues to track north and east. As has been the pattern with
this system, guidance continues to track the low slightly farther
east then previous runs. Therefore, the downward trend in
precipitation amounts and wind speeds west of the low continues
for the overnight hours. For example, Anchorage has yet to see
anything more than a few sprinkles of rain today; however, a band
of light to moderate rain does extend from the western Susitna
Valley south to the western Kenai Peninsula, and this will slide
over the Anchorage Bowl tonight as the low moves closer to shore.
Colder air aloft has also meant snow for the higher elevations
such as Thompson Pass, where 10 inches of snow has fallen so far
today. Another 4 to 6 inches are expected before the low moves
inland and warmer air arrives. Along with the influx of warmer air
southerly gap winds will also increase beginning overnight
tonight through the passes, along Turnagain Arm, and in the Copper
River Basin.

As the first system dissipates over the South Mainland early Sat,
a second intensifying North Pacific low will track into the Gulf
bringing more rain with strong gales and storm-force winds along
the coast Saturday afternoon through Sunday. This system will
bring deep and moist southerly flow and with a track west towards
Kodiak Island, allowing for an atmospheric river to setup over
the Gulf Coast for Sunday. Right now, it appears the precipitation
bull`s-eye will once again be focused over western Prince William
Sound and the Eastern Kenai Peninsula, where an additional 3 to 4
inches of rain is expected. This will likely exacerbate flood
concerns as rivers and streams in the area are already swollen and
grounds saturated. Of particular concern is the Seward area which
will experience a prolonged southerly flow conducive for periods
of heavy rain. As a result, a Flood Watch is in effect through
Sunday afternoon to cover this concern.

The precipitation will begin to wind down late Sunday into early
Monday as low moves through Southwest Alaska and a weak offshore
flow develops. The respite from the widespread rain is expected
to be short-lived, however, as a third area of low pressure
develops in the eastern Gulf late Monday and tracks northward.

&&

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

The main forecast challenge in the immediate future is fog
potential tonight, as skies have cleared out and winds are light.
The caveat is forecast soundings today indicated substantial
drying from the top down in the column, so it`ll be interesting to
see how much fog materializes. At this point the safest bet would
be any river valleys or areas near low-lying lakes as the light
winds and moisture flux from those water sources should be enough
for at least some shallow but locally dense ground fog. Many of
the lower elevations across the interior will see some below
freezing temperatures overnight, and with the low in the gulf
spreading some deformation-type precipitation back into (possible
over) the Alaska Range, minor snowfall accumulations wouldn`t be
totally out of the question.

Our attention then turns to the next storm quickly moving as it
strengthens. This looks to enter our picture by mid afternoon
Saturday, as the model consensus is for a ~968 mb low to reach the
western end of Kodiak Island by Sunday morning, before stalling
near King Salmon later in the evening as additional short-wave
energy dives into the backside of the storm. This then allows
several smaller-scale surface lows to develop along and just south
of the AKPEN as they rotate around each other in a Fujiwhara-like
fashion through Monday. With this type of synoptic setup,
widespread light to moderate intensity rains will develop and
persist for the peninsula and northeastward along the Alaska
Range. Farther north and west, precipitation will be lighter. But
here too, periods of steadier rainfall will develop east of the
Kilbuck Mountains, with just showers expected closer to the coast.

&&

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

A rather showery pattern looks to persist across most of the
Bering Sea and Aleutians, as a weakening area of low pressure near
Nunivak Island retrogrades to the southwest as it gets absorbed
into a 990 mb western Aleutians low. Both of these systems will
then get absorbed by a deepening low moving towards the AKPEN on
Saturday. The net affect here is for showers and perhaps periods
of steadier rainfall to persist across most of the area, aided by
vort maxes rotating around the weakening and western lows. Closer
to the chain, rainfall coverage will be somewhat greater as
orographic lift helps improve parcel lift and moisture
condensation.

The exception to this scenario will be along the peninsula and
adjoining coastal waters, where steady light to moderate rainfall
and sustained gale force winds are expected. Seas will build into
the 15 to 20 feet range, especially south and east of the
Shumagin Islands.

&&

.LONG TERM FORECAST (Days 4 through 7: Monday Night through Friday)...

The pattern of one powerful low after another making their way
into the North Gulf Coast will continue throughout the extended
forecast period. The next low in the series will be gathering
strength in the southeastern Gulf on Monday night. The models are
slowly coming into better agreement on the track, with the low
center expected to track northwestward into the central Gulf to
the vicinity of Middleton Island Tuesday morning, then move into
the Kenai Peninsula around Seward on Tuesday afternoon. Along this
track it is likely another barrier jet will form between the low
center and the coast during the day on Tuesday. There is some
uncertainty since the low will be paralleling that section of
coast as to how strong the barrier jet gets, but gales along with
storm force gusts appear likely. The low would also kick up strong
southeast winds through the Copper River Basin as well along that
track. It will also bring up another reinforcing shot of warm
air, so any winter precipitation will likely be confined to the
mountain passes with this storm.

The break between storms is very brief Wednesday morning as yet
another low brews in the Northeast Pacific Wednesday afternoon.
Understandably, there is considerably more uncertainty with the
track and timing of the low. However, all of the models show the
minimum central pressure starting off significantly lower than the
Tuesday storm, entering the Gulf at pressures down in the 950s
Wednesday evening. As it tracks northwestward towards the Kenai
Peninsula, it will be slowly weakening. This should help to keep
winds from getting to warning levels, but high end gales to storms
is a distinct possibility through much of the Gulf once again
along with another round of rain. The low tracks into the central
Gulf Thursday morning, then makes landfall somewhere between
Prince William Sound and the southern tip of the Kenai Peninsula
Thursday evening. Once again, impacts and rainfall amounts for any
one location are highly uncertain at this point, so stay tuned to
future forecasts. Another brief break in the action looks on
track for Friday.

For Southwest Alaska and the Bering, conditions will be much
quieter throughout the long-term than locations further east. As
each Gulf low moves inland and dissipates, there may be brief
periods of light rain at times across Southwest Alaska with each
one. Each low will also help build the upper level ridge resulting
in a continuation of well above normal temperatures for
Southcentral and parts of Southwest Alaska, while also supporting
moving cold air out of Russia and into the Bering. Right now the
cold air over eastern Russia takes until Thursday to make a
significant move southeastward into the Bering and perhaps even
Southwest Alaska by Friday. A vigorous warm front moves into the
western Aleutians on Friday.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Storm Warning 130 131 138.
 Gale Warning 119 120 125 132 139 140 150 155 160.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...RA
SOUTHCENTRAL ALASKA...TM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD
LONG TERM...JW


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