Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 201325
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
525 AM AKDT Sat Oct 20 2018

.ANALYSIS AND UPPER LEVELS...
The fall storm season has officially commenced for the Alaska
region and the North Pacific. The upper air pattern is starting to
look more late fall-like with a broad and powerful North Pacific
polar jet north of a moderately strong subtropical ridge. ASCAT
passes and satellite imagery are depicting the rapid development
of an intense hurricane force low along the left exit of the
aforementioned upper jet, which is currently south of Kodiak
Island and moving northeast. Over Alaska, the remnants of the once
powerful warm seclusion have moved inland, with the upper trough
associated with that low rapidly weakening and deamplifying into a
shortwave trough as it tracks east into the Yukon. The low level
flow remains onshore over Southcentral, so instability enhanced
precipitation is still ongoing along the coast and into interior
Southcentral, along with higher elevation snow through Thompson
Pass. Out west, there is a weak and shearing shortwave trough
which is located north of the upper jet, along with a set of non-
distinct embedded waves rotating over the Bering Sea. But
overall, the Bering Sea pressure gradient is rather weak as the
Pacific storm track remains to the south. The aforementioned
hurricane force low will eventually track north as a gale/storm
force low into the Gulf of Alaska, which will bring another round
of moderate to heavy coastal rain.

&&

.MODEL DISCUSSION...
The numerical models are in fantastic agreement with the next sub-
970 hpa low moving over Kodiak Island. The associated front will
move into the North Gulf Coast and bring more heavy rain to all of
the Gulf. Model discrepancy does not really develop until Monday
night and Tuesday as a series of lows retrogrades into the Gulf of
Alaska from the southeast (see extended forecast discussion for
more details).

&&

.AVIATION...
PANC...The main challenge with this TAF package will be winds as
they will be gusty from the southeast through this afternoon, then
quickly switch to northerly by this evening as the next low moves
into the Gulf. Over top the northerly winds will be southeast
winds, so expect LLWS at the terminal from this evening through at
least Sun morning.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Today through Monday)...
This very active and fall-like weather pattern continues as two
separate low pressure systems will impact the area today. The
first is the remnants of the storm force Gulf low from yesterday
that made landfall near Cordova overnight, and is still bringing
rainfall from Prince William Sound into the Copper River Basin.
Temperatures still remain cold enough through the higher
elevations of Thompson Pass and Western Copper River Basin to
bring accumulating snowfall to those areas, however with
temperatures hovering above freezing and much weaker dynamics than
yesterday this is only expected to bring a few inches of wet
accumulation.

After this first low dissipates over the Copper River Basin later
this morning, the focus will quickly shift to the hurricane force
low currently spinning south of Kodiak Island. While this low is
expected to weaken to storm force as it reaches the waters of the
Gulf, deep southwesterly fetch will allow seas as high as 35 ft to
spread into the offshore waters of the Gulf by late tonight. The
other impact of this low will be from the atmospheric river that
is still expected to bring 3-4 inches of rainfall to the Eastern
Kenai Peninsula beginning this evening into Sunday. There may
even be some lightning strikes reaching the Gulf Coast by Sun
afternoon as cold air aloft tracks through the Gulf, supporting
enough instability for thunder development with the seasonably
warm waters of the Gulf. This low will then track across the AKpen
into Bristol Bay on Sunday. This will bring gusty winds out of
the north-northeasterly winds out of Matanuska Valley into Cook
Inlet, which along should provide enough low level drying along
with downsloping from the Chugach the limit rainfall from this
system over interior Southcentral to less than one-tenth of an
inch.

Monday then finally looks to be a respite of the active
weather most of the area has experienced this week as the Gulf low
weakens over the Southwest coast and weak high pressure approaches
from the Yukon. This will allow winds and showers to diminish
throughout the area, with temperatures remaining above normal.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Today through Mon)...
SW AK will undergo fairly dynamic changes in the 24 hours. First,
the low that is moving through Southcentral is still wrapping
some light precipitation along the Alaska Range. Some of this is
likely falling as light snow based off of temperatures in the
upper 20s and low 30s in that region. Otherwise, much of the
mainland has seen clear skies overnight allowing for plenty of
radiational cooling. The result has been some patchy dense fog in
the YK-Delta. This should persist through the morning hours
before offshore flow ushers in some drier air. Near Bristol Bay,
some instability is allowing for some very light showers to clip
the King Salmon area. This instability also seems to be keeping
the fog at bay in that area.

The main weather maker today will be the storm developing south of
the Alaska Peninsula. This is a very healthy low that will
continue to deepen into the mid 960 mb range as it tracks over
Kodiak Island later today. The net effect across the area will be
increasing and gusty offshore flow. For the most part, the area
will remain dry (and somewhat sunny) through the daylight hours.
Not until this evening will precipitation spread from SE to NW.
Places in the lee of the mountains (King Salmon, Iliamna,etc) will
see less accumulation as they will be "shadowed" by cross-barrier
flow off the AK and Aleutian Ranges. On Sunday, the low will move
in and start to fall apart. This will leave the entire area
vulnerable to showers. A similar story will be true on Mon as
well, except there will be some enhancement in the precipitation
with another shortwave rotating in from the east.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Today through Mon)...
A few small circulations will move through the Central Aleutians
today before the entire falls under the influence of a broad
trough into next week. 2 tight circulations are visible on
satellite this morning. The primary one is slipping just south of
Atka and Adak. There is enough instability with this system to
keep a slight chance of thunderstorms over the marine zones
through the morning.

The aforementioned low south of the Alaska Peninsula will be the
primary wind driver. As it moves north today, expect winds to
increase over the Eastern Bering. Widespread small-craft north-
northeast winds are likely with some gale-force sustained winds
across Bristol Bay. By Sunday, the speeds will diminish, but the
direction will remain northerly across most of the region. The
northerly winds will help to usher in some colder air with 850 mb
(5,000`) temperatures dropping to near -8C Mon. This will help to
create widespread showers due to instability with SSTs near their
climatological max.

&&

.LONG TERM FORECAST (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 its likely another barrier jet forms 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 once again 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 as it does so. 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 130 131 138
         Gale 119 120 125 132 136 137 139 140 150 155 160.
HYDROLOGY...Flood Watch 125

&&

$$

SYNOPSIS AND MODEL DISCUSSION...JA
SOUTHCENTRAL ALASKA...DEK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MSO
LONG TERM...JW


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