Area Forecast Discussion
Issued by NWS Anchorage, AK

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FXAK68 PAFC 191340

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

A powerful trough is currently deamplifying and rapidly
intensifying over the Gulf of Alaska this morning. The culprit is
a tropopause fold which has intersected a rather moist warm front
originating from the Central Pacific. The upper and lower
potential vorticity anomalies are now "phase locked" and are in
the process of mutual self-development. Water vapor satellite
imagery this morning looks, without a better word available,
"sick", as the broad baroclinic leaf to the east of the PV
anomaly (over the ALCAN) is splitting into a well defined warm
conveyor belt and TROWAL (Trough Of Warm Air ALoft) to the west of
the PV anomaly. A rapidly descending core of air (drying as it
descends) can be seen on water vapor satellite imagery, and ASCAT
confirms this sting jet is reaching storm force (50+ knots). As a
result of all these physical meteorological processes, the overall
trough is taking on a severe negative tilt as broad and strong
diffluence aloft overspreads Southcentral Alaska. Middleton radar
(PAMD) is lighting up as the warm front approaches with increasing
offshore winds along the coast, in response to the rapidly
intensifying pressure gradient.

With the powerful low over the Gulf, the weather over the Bering
looks rather mundane in comparison. Over the eastern Bering Sea,
the remnants of the weakening trough is shearing apart, leaving a
rapidly weakening surface low which had remained in place for at
least 4 days. Over the western Bering Sea, there is a split upper
level trough with an associated occluded front moving east towards
the Central Aleutians. The basal southern stream shortwave is now
digging into the Central Pacific, and has intersected a robust
subtropical warm front. This will eventually bring the next round
of rain and storm weather to the Gulf Coast this weekend.



The numerical models are in excellent agreement with the placement
of the current powerful beast of a low in the Gulf of Alaska as it
is expected to track north into Prince William Sound. They are
also in agreement with the second low (round two) which is
expected to take a track over Kodiak Island and then west of the
Alaska Range by Sunday afternoon. Thereafter, confidence
diminishes as the global models quickly diverge with a potential
third low retrograding northwestward into the Gulf of Alaska
Tuesday, with the ECMWF and Canadian GDPS favoring that more
aggressive track while the GFS currently does not support such a
scenario. For this forecast, a blend of the GFS and NAM was


PANC...Light rain will move into the terminal this morning, but
CIGS should remain above 5000 feet with gusty northerly winds.
Eventually the winds will shift to S-SE and CIGS may periodically
drop to MVFR with the wind shift during the overnight. However,
confidence is not high.


Strong winds and rain will continue spreading throughout the Gulf
this morning as the storm force low moves northward through the
Gulf. As is often the case with rapidly intensifying lows such as
this, there were once again subtle but important changes in the
guidance overnight with the track of this low that resulted in
changes to the gap wind forecast around Prince William Sound. This
low, which 2 days ago was expected to track through the Kenai
Peninsula, is now moving much further east and weaker towards
Prince William Sound and Valdez. As a result many of the gap winds
from Resurrection Bay as well as Turnagain Arm into Passage Canal
were both weakened and swung around to offshore. This low will
also bring a brief round of widespread rain to the Gulf Coast, as
well as into Southcentral as an inverted trough slings overhead
this morning. There is also enough cold air in the higher
elevations of the Western Copper River Basin as well as in
Thompson Pass to bring a quick inch or two of fresh snowfall to
the road level. Temperatures throughout the AOR will however
quickly warm enough by this afternoon as warm air advection from
the Gulf low pushes trough to lift the snow levels well above the
road system for the remainder of the short term forecast.

While the current Gulf low will dissipate over the South Mainland
early Sat, this active and very fall-like pattern will continue
into this weekend as yet another intensifying North Pacific low
will track into the Gulf for late Sat-Sun. This system will bring
deep and moist southerly flow and a further west track towards
Kodiak Island, allowing for an atmospheric river to setup over
the Gulf Coast for Sunday. This system still looks to bring an
additional 2-4" to much of the Eastern Kenai Peninsula this
weekend, causing additional rises to the already swollen waterways
in the area. Of particular concern is the Seward area which
experience much more favorable southerly flow for heavy rain than
todays system, causing additional flooding concerns so soon after
the flooding experienced earlier this week. As a result a Flood
Watch was issued with the morning package to cover this concern
through Sunday afternoon.


The area will be caught between the low just south of Nunivak
Island and the much stronger low moving through the Gulf of AK.
Both features will have some influence on the sensible weather.
The low that is offshore is continuing to bring some very light
showers to the YK-Delta. But its influence is waning as it fills
and slides further out to sea. The low in the Gulf will only be
able to sling some rain and high clouds over the Alaska and
Aleutian Ranges before it moves east. So in between these two
systems this morning, if skies can clear, some patchy fog could

As the Gulf low slides away, it will allow some colder air to wrap
into SW AK. While not much precipitation will be able to make it
over the AK Range, what does make it could produce a light
dusting of snow in parts of the Lower Kuskokwim Valley early Sat.
For locations closer to the coast, onshore flow could actually
lead to a rather foggy start to Sat.

By Sunday, focus shifts to yet another moisture-laden low moving
into the Gulf. This low will take a very similar track to the
first low. It will allow some more rain to spill over the Aleutian
Range. It will also bring more cool, offshore, NE flow to the
area helping to keep temperatures more seasonal.


The remnants of an occluded front will slowly decay as it tracks
through the Aleutians before broad cyclonic flow takes over for
the weekend. The front is between Atka and Adak this morning
bringing small-craft winds and moderate rain along with it. The
front will continue to elongate and weaken as it slowly slides
east today. Behind it, the low center itself will swing through
the Western Aleutians. With enough upper level support, there
should still be a slight chance for thunderstorms for the far
western marine zones later this morning and afternoon.

The rest of the weekend looks relatively quiet. Broad, cyclonic
(counter-clockwise) flow will establish itself across the Bering.
A few embedded shortwaves could bring more persistent showers and
pockets of small-craft winds, but we do not expect any strong
systems across the domain.


.LONG TERM FORECAST (Sunday Night through Thursday)...

Sunday night starts off with the remnants of the low slowly
diminishing overnight. Onshore southeasterly flow is likely along
the entirety of the Gulf Coast as the low center moves into
Southwest Alaska and slows its northwestward movement
significantly. Unfortunately for areas such as Seward and the
eastern Kenai Peninsula, this will allow the associated rainfall
to continue through much of the overnight and into at least the
first part of the day on Monday. However, rainfall rates will be
diminishing throughout the night as the parent low quickly
weakens. The low`s westward track will bring in a renewed slug of
warm air causing wintry precipitation to remain confined to
higher elevations. Rain will persist with diminishing rates
across portions of Bristol Bay and the Lower Kuskokwim Valley on
Monday. The low will also help to draw colder air southward
across the Bering on Tuesday, which may threaten the Pribilofs
with some snow showers.

Turning back to the east on Tuesday, the models are coming into
much better agreement compared with yesterday`s runs concerning
the development of another strong low in the Gulf. As expected,
there is still plenty of disagreement on the details, but all are
showing at least a gale force low, with perhaps storm force winds
in localized areas. It looks to develop over the southeastern
Gulf and track northwestward, reaching the central Gulf Tuesday
afternoon, then moving into the eastern Kenai Peninsula Tuesday
evening. Along this track, moderate to heavy rain at times will
overspread much of the Gulf Coast starting Tuesday morning and
then diminish Tuesday night. At the time of this forecast, this
storm does not look to have the same moisture feed as the Sunday
storm. Thus, while excessive rainfall amounts are not expected
anywhere, being the latest in this train of storms will not help
any ongoing flooding concerns in places like Seward.

In similar fashion to Sunday night behind the storm Tuesday night,
onshore flow and continued upslope rain will persist through the
day along much of the Gulf Coast. The low will leave nearly all
of its moisture along the coast, with relatively little expected
to make it inland. What the low will do out west however, is help
to draw increasingly colder air southward out of Russia into the
Bering. As a result, snow showers become increasingly likely as
far south as the Aleutians Wednesday and Wednesday night. By
Thursday, the models hint at...guess what...ANOTHER strong low
tracking northward out of the Pacific into the Gulf, but by that
time uncertainties are very high. There is also growing consensus
that some of the cold air over the Bering getting into Southwest
Alaska, particularly the Delta on Thursday, may produce some
below average temperatures somewhere in the area for a change.


MARINE...Storm 119 120 131 132.
         Gale 125 130 136-141 150.
HYDROLOGY...Flood Watch 125



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