Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 181322
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
522 AM AKDT Thu Oct 18 2018

.ANALYSIS AND UPPER LEVELS...

A high amplitude longwave upper trough encompasses most of the
Bering Sea and the state of Alaska this morning. Embedded within
the trough is a shortwave trough which is digging south on the
back (west) flank of the larger trough across the eastern Bering
Sea. However, the excitement exists at the base of the longwave
trough. There is a basal shortwave trough which is intercepting a
subtropical warm front around 44N 158W. Water vapor satellite
imagery is depicting the development of a dual baroclinic leaf as
warm air advection aloft ensues. Ahead of the basal shortwave
trough, broad onshore flow is keeping instability showers over
Southern Alaska more active along the immediate coast. Another
batch of organized shower activity exists over the Kuskokwim
Delta where a mid level deformation zone has formed in conjunction
with the aforementioned digging shortwave trough over the eastern
Bering Sea. Finally, there is a split upper trough moving east
towards the far western Aleutians. The main subtropical warm front
is over the NPAC with a secondary low forming at the base of the
leading upper shortwave trough which is currently moving east of
the Kamchatka.

&&

.MODEL DISCUSSION...

The numerical models are now generally locked into the powerful
storm force low moving into the Gulf of Alaska. However, due to
the rapid development of the low, even tiny track differences and
changes in the upper level shortwave have rather large differences
in wind fields, local winds, and precipitation distribution. The
largest discrepancy is how far west and how quickly the low
stalls out, with the NAM favoring a farther west track and a
quicker stalling while the ECMWF and Canadian RDPS favor an
eastward track. For this forecast the GFS was favored as that
lands right in the middle of of the NAM and ECMWF.

Another deep 970 hpa (or so) low is on track to move into the
AKPEN with the front moving into the Gulf of Alaska, bringing
another round of heavy precipitation to the North Gulf late
Saturday into Sunday. The models have largely been locked into
this low for a few days and confidence is high with this low. With
that said, the NAM and GFS were favored for precipitation amounts
and winds.

&&

.AVIATION...
PANC...VFR conditions will persist in spite of some showers moving
through the area this morning. The gusty southeast winds will
end by mid morning and become northerly this afternoon, increasing
overnight. Rain will move back into the terminal Thursday morning,
but CIGS will remain above 5000 feet.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Today through Saturday)...
Periods of clouds and showers but otherwise relatively benign
weather will generally describe the weather today. This will then
quickly change by this evening as a developing North Pacific low
rapidly intensities to storm-force as it tracks into the Gulf
late tonight. One important trend in the models in this system
since yesterday has been for a slightly slower low position over
the Western Gulf as it briefly stalls to the east of Kodiak Island
for making landfall near Prince William Sound late Fri evening.
This trend has resulted in slightly less rainfall than initially
expected for the Seward area, with heavier rain and stronger winds
stretching from the Southern Cook Inlet into Kodiak Island. The
guidance is also in very good agreement now that the low will
track into Prince William Sound, which is further east than was
initially expected through the Kenai Peninsula. As a result the
gap winds that were expected through Turnagain Arm (including into
the Anchorage area) have been weakened significantly, and in some
cases switched to westerly as the low tracks to the east.
Instead, widespread rains look more likely throughout Southcentral
on Friday as an inverted trough pushes through the western edge
of this low with no downsloping expected.

This low will slowly begin to dissipate over Southcentral
Saturday morning. However, yet another North Pacific low will
quickly move into the replace it. This system will bring rain and
gale force winds throughout the Gulf on Saturday, including
another round of heavy rains to the Gulf Coast for the remainder
of the weekend. A Special Weather Statement has been issued to
address this.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3...Thu
through Sat)...
The low pressure center over the outer portions of Bristol Bay
will continue to drift to the east as it fills today. It will keep
pushing showers onshore through most of the day, but those will
slowly diminish by this evening. By late evening, a strong low
will be developing and driving north over the Gulf of AK. It will
throw an inverted trough into SW AK. This trough will bring steady
rain to the Bristol Bay area, favoring the Aleutian Range, late
tonight through Fri morning. By Fri afternoon, SW AK will be
caught between the remnants of the system from the Eastern Bering
and the strong low over Southcentral AK. This combination will
actually bring a dry  day to much of the region. The primary
exception to this looks to be the interior Lower Kuskokwim Valley
where some light rain will move in from the east. By early Sat,
the flow will shift to more of an onshore direction. This could be
a good fog set-up for Sat morning, but we did not introduce that
into the forecast just yet.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3...Thu
through Sat)...
One low will weaken over the Eastern Bering, while a new one moves
into the Western Aleutians. First, the Eastern Bering system will
continue to wrap around some colder air and showers through early
Fri. 850 mb (5,000`) temperatures near places like Cold Bay could
drop as much as 7C over the next 24 hours. This cold air moving
over the warmer waters helps to create stronger showers and some
gusty winds. The showers and winds should slowly dissipate late
Fri as the system falls apart.

Over the Western Aleutians, a broad low complex is preparing to
enter the region late tonight into Fri. On the leading edge of
this system, the occluded front will bring a swath of gale-force
winds later today. After that front moves east, it will open the
door for another shortwave to round the base of the trough. This
shortwave looks to have enough energy and instability that we have
elected to introduce a slight chance of thunderstorms over the
marine zones for Fri afternoon. On Sat, this system will slide
south of the chain and leave the area in a very broad, ill-
defined trough. General unstable, cyclonic flow will keep showers
in the forecast, but there will not be a definitive feature to
bring strong winds or heavy rain into the weekend.

&&

.LONG TERM FORECAST (Days 3 through 7: Friday Night through
Wednesday)...From Wednesday`s afternoon discussion...

The primary impactful weather across Southern Alaska through the
long-term will be associated with yet another strong North Pacific
low. There is still some disagreement between each of the models
about how strong the low becomes, where it tracks, and the timing.
While all of the models show a strong low tracking near the
western Gulf Saturday and Sunday, the aforementioned disagreements
make it imprudent to specify impacts for any one particular area
at this time. However, mariners and those planning recreational
activities outdoors this weekend should keep a close eye on the
forecast.

Here are the aspects of the storm that can be said with some
confidence: The initial front brings the start of the rainfall
into Kodiak Island on Saturday, spreading northward through the
afternoon. The low looks strong enough at this point that gales
are likely, especially south and east of the low center. Storm
force wind gusts are a possibility. The low will once again be
able to tap into a feed of subtropical moisture, and take that
rainfall into the North Gulf Coast once again. Fortunately, unlike
this last event which caused localized flooding in spots such as
Seward, this time that moisture feed and associated heavy rain
looks to steadily track eastward, so no one area should pick up
excessive amounts of rain for a long time frame. The track of the
center of the low, while uncertain, looks to be further west than
previous lows, so the impacts to Prince William Sound with a
possible barrier jet look reduced with this event.

Behind the low during the Monday through Wednesday time frame,
uncertainty remains high, but the center of the low and its
steering upper low look to anchor themselves near Southwest
Alaska, which should generally keep onshore flow and moist
conditions through Southcentral. Some of the models suggest a
North Pacific low tracks into the Gulf during the time frame, but
there is zero consensus on that. There is somewhat better
agreement that colder air moves into the Bering by mid-week and
some of that sneaks into mainland Alaska for late next week. This
would allow Southern Alaska to experience more seasonable
temperatures for a change.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Storm Warning 119 120 130-132 139 351 352.
         Gale Warning 178 125 127 136-138 140 141.


&&

$$

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



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