Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 211302
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
502 AM AKDT Sun Oct 21 2018

.ANALYSIS AND UPPER LEVELS...
IR Satellite shows the mature storm system, that moved up from
the North Pacific on Saturday, still centered along the southern
end of Kodiak Island early this morning. The system is beginning
to slowly move northwest and fill with storm force winds tapering
down in response. The occluded frontal band stretches across
Bristol Bay to the Kenai and along to the northeast Gulf Coast.
Strong low level easterly flow is causing downsloping of
precipitation, limiting rainfall amounts along west facing slopes.
Outside of the front, showers are developing as the dominant low
spans across the southern mainland and surrounding waters.

&&

.MODEL DISCUSSION...

Models are in relatively good agreement with the synoptic pattern
through Monday night, then diverge on the track of the next
surface low that races up from the eastern Gulf early Tuesday. The
NAM data was favored for this package with updates through late
Tuesday.

&&

.AVIATION...
PANC...VFR conditions are expected to prevail through the next TAF
package. LLWS was kept through this morning as surface winds
remain from the northwest until the surface low near Kodiak lifts
northwest into Bristol Bay. Gusty southerly winds mix down by noon
today then taper down late evening.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The third large low of the past week for the Gulf of Alaska region
passed over the southern part of Kodiak Island early this morning
and is headed toward the Bristol Bay region. This low is an
occluded low with the associated front wrapped around it a number
of times. The first frontal band has lifted north of the Alaska
Range and brought increased instability to the region behind it.
This brought some strong showers and a few thunderstorms to the
north Gulf coast near Seward overnight. the next frontal band is
moving over the Kenai Peninsula early this morning with a third
over Kodiak Island. This pattern will continue to bring rain to
coastal regions today with inland areas seeing a bit of downslope.
The Seward are will continue to see periods of heavy rain through
the day and another thunderstorm is possible in the unstable
atmosphere over the region.

Monday will see generally calmer weather with just some scattered
showers and lighter winds over the region as this large low has
moved out of the area. The next low on tap will move across the
northern Gulf of Alaska from east-to-west Tuesday night and
Wednesday bringing in the next round of wind and rain at that
time.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3...Sun
through Tue)...
Unsettled weather will remain over the area to start the week as
a weakening low drifts into Bristol Bay and wobbles around.
Satellite shows the low center is just south of the Alaska
Peninsula as of 5 am this morning. To start the day, light rain
has moved as far north as Aniak already. However, cross-barrier
flow is keeping many of the population centers (Lake Iliamna, King
Salmon, etc) in the lee of the terrain mostly dry. These
conditions will change through as the low tracks overhead this
afternoon-evening and the cross barrier flow shuts off. This will
allow steady precipitation to move into these locations as well
as the rest of the Bristol Bay area. Besides precipitation, gusty
winds are the other big story with this storm. East to northeast
winds should be peaking this morning before slowly diminishing
through the day as the low starts to fill. Finally, this system
is also bringing a healthy slug of warm air. Some inland valleys
experienced some light snow on Sat morning. But with 850 mb
(5000`) temperatures climbing above freezing (nearly 3C on the 12z
PAKN sounding), all precipitation will fall as rain.

Mon and Tue will see a similar pattern. Winds will be much
quieter, but all areas will be vulnerable to showers as the low
sits just off shore. By late Tue night, the next stout system
moving into Southcentral will start to spread a new precipitation
shield over the AK Range.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3...Sun
through Tue)...
The low moving into Bristol Bay will be the primary weather maker
for this time period. Small-craft to near gale-force NE winds will
continue for the Eastern Bering through today. The low will serve
to reorient the longwave trough just a bit further east over the
area. The net change will be to bring primarily northerly winds
across the Bering. This will in turn open the door to somewhat
colder air dropping down from the Bering Strait. 850 mb (5000`)
temperatures this morning at St Paul were nearly 3C. These are
progged to drop to -5C on Mon. Anytime you bring cold air over
warmer air (or warmer waters), it creates an unstable environment.
So the entire Bering can expect showers with some gusty winds in
this pattern. The best chance for these conditions will be from a
corridor north of the Pribilof Islands to the north side of the
Alaska Peninsula. The rest of the area should expect winds to
remain primarily below small craft level with just some embedded
shortwaves bringing slightly better chances for showers.

&&

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

Two lows remain in the forecast headed for the Gulf during the
long-term forecast, and colder, more seasonable temperatures may
finally be on the way to Southwest Alaska by the end of the
workweek.

The low on Tuesday continues to be a struggle for the forecast
models regarding the low`s eventual strength and intensity.
Compared to yesterday, the models are trending weaker with the low
intensity, with the NAM and EC showing a weaker low that barely
produces gales along the north Gulf Coast, with the low center tracking
into Prince William Sound Tuesday afternoon. The Canadian has
widespread gales and takes the low inland near Seward Tuesday
evening, and the strongest solution, the GFS, has storm force
winds (55-70 mph) with the barrier jet and brings the low center
inland near Seward Tuesday evening. Thus, considerable uncertainty
still remains on the impacts. The NAM is the middle-ground model
for track, but in deference to previous runs having stronger
winds, gales will be kept along the Gulf Coast for this forecast
package. This also is a positive trend for rainfall along the Gulf
Coast, especially in Seward, where the NAM and EC solutions both
suggest under an inch of rain falling there from this low. Even
with the stronger and possibly wetter solutions, the track likely
keeps Seward relatively dry than if the storm passed west of
there. None of the guidance suggests prolonged heavy rain due to
the fast movement of the low.

The next low moves into the Central Gulf on Thursday morning. The
models remain unanimous that this one will be by far the stronger
of the two lows. Thus, more rain is likely with this second low at
all points along the coast. Uncertainty remains high as to how
fast the low moves as well, with the GFS the fastest and EC the
slowest. The difference between the two is a full 12 hours.
Regarding track, the GFS and EC both take the center of the low
over the far southern Kenai Peninsula, tracking west, but the GFS
has it there Thursday evening while the EC waits until Friday
morning. The Canadian makes landfall over Afognak Island Thursday
evening. On the low`s approach, easterly winds will mean
relatively little rain in Seward. Most of the rain there will
occur after the center passes their longitude and the winds turn
out of the south. The east to west track across the central Gulf
should keep winds along the coast lower, but widespread gales
(35-50 mph winds) are likely, especially closer to the low center.
The model solutions diverge greatly over the weather pattern
behind the low Friday and Saturday, but none of them as yet shows
any more strong lows in that timeframe.

Well above normal temperatures are likely to persist area-wide
through Thursday as each low brings in reinforcing warm air to
mainland Alaska. However, the lows will help bring colder, far
more seasonable temperatures to the western Bering on Wednesday.
The cold air slowly tracks east to the central Bering late on
Thursday, then perhaps tries to encroach on the Kuskokwim Delta
late Friday into Saturday. However, since by late Friday into
Saturday the models diverge greatly, there is no consensus on how
or even if the cold air makes it that far east. The very warm
October continues.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Flood Advisory 125.
MARINE...Gales 120 130 131 132 136 150 351 352.

&&

$$

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


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