Area Forecast Discussion
Issued by NWS Anchorage, AK

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988
FXAK68 PAFC 161409
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
509 AM AKST Fri Nov 16 2018

.ANALYSIS AND UPPER LEVELS...
A ridge axis is spanning from Bettles to to Valdez. There are two
occluded fronts at each end of the Aleutians with a decaying
surface low over St Matthew Island. A precipitation shield in
association with the first occluded low has brought a wintry mix
to portions of Southwest Alaska, with observations of rain at
Dillingham and freezing rain at Bethel. Meanwhile the second low
near Attu is bringing rain and gale force winds to the region.

&&

.MODEL DISCUSSION...
The models are having some run to run consistency issues.
Generally speaking, there is decent agreement with the synoptic
features through the weekend. But there are some differences with
the placement of the QPF and with mode of precipitation. The
models significantly diverge over the Bering by Monday, with the
GFS keeping a ridge over the region and the ECMWF bringing a full
blown cyclone into the Western and Central Bering.

&&

.AVIATION...
PANC...VFR conditions and light winds expected at the airport for
the forecast period.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2/Today
through Saturday night)...
Southcentral will be right on the periphery of an upper level
ridge to the east and a trough to the south and west today. Most
of the energy lifting northward through the trough this morning
will head to Southwest Alaska. The southern Kenai Peninsula, right
on the edge of this, has the best chance of seeing some very light
precipitation. One of the big limiting factors for precipitation
is the dry airmass in place, as is evident in the 12Z Anchorage
sounding.

Meanwhile, a weakening surface low over the North Pacific is
headed toward Kodiak Island. Much more impressive is the gale
force occluded front, which is lifting northward across the
western Gulf, pushing up against the much higher surface pressure
to the north and east. This front, which is bringing strong winds
and moderate rain to Kodiak Island this morning will lift to the
northern Gulf this evening. At the same time, the upper trough
will become negatively tilted and lift toward Southcentral. The
strongest forcing will head for Prince William Sound tonight and
then across the Copper River Basin on Saturday. Therefore, the
bulk of precipitation with this storm will follow this same path.

With the low headed for Kodiak, surface flow over the northwest
Gulf and Kenai Peninsula will be northerly to easterly. This is
important for two reasons; the heaviest precipitation will fall
over the western Prince William Sound mountains, with much lighter
precipitation inland for the Seward Highway corridor and
Turnagain Pass and the northerly flow will help maintain dry low
levels from the western Kenai Peninsula to Anchorage and the Mat-
Su. The combination of some downslope flow across the Kenai and
Chugach Mountains along with the dry low levels means precipitation
will likely not reach the ground in the aforementioned areas. If
it does, it will be very light and brief. This is important because
warm air riding over the sub-freezing surface air would likely
lead to freezing rain.

Issued a Winter Weather Advisory for the Copper River Basin
beginning this evening and continuing through midday Saturday
from Glennallen southward. With the trough headed right for them,
it looks favorable for a good period of snow. Warm advection south
and east of Glennallen will then lead to a mix or changeover to
freezing rain Saturday morning for some areas.

As the upper trough exits Saturday night, a large high amplitude
trough will move in from the Bering Sea. With a complex series of
lows and short-waves moving through the trough, models are
struggling with the details. In any case, active weather looks to
continue.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
(Today through Saturday)

Cyclonic flow around both a northern Bering low and another
system approaching the AKPEN is advecting warmer air with
increasing amounts of moisture in the middle and upper layers of
the atmosphere this morning. However, area soundings show the
low-level dry air holding strong near the Bristol Bay region, as
dry air on the backside of the Bering low advected in behind a
weak surface trough. The net result is easily seen in this
mornings sounding from King Salmon, with large dewpoint
depressions seen below 6000 feet while being moist aloft. This
explains why some 30 to 40 dbz returns has produced very light
precipitation at Dillingham for instance, as the precipitation
largely evaporated during its descent into this dry wedge.

Further north and east, the dry air never made it this far north
with the Bethel soundings showing much more low-level moisture and
a sharp inversion just above the surface, with above freezing
temperatures in that layer. Meanwhile, the latest surface
observations indicate sub-freezing temperatures with areas of
freezing rain reported. This will allow for some light
accumulations of ice to develop across the lower Kuskokwim Valley
and points northward. However, temporal considerations and light
precipitation amounts preclude any headline issuance at this time.

Precipitation may become more scattered in nature later this
evening and overnight as the Bering low and surface trough wash
out. This looks to be short-lived though, as a low moving north
towards the AKPEN phases with another developing low moving from
the central Aleutians towards Bristol Bay. Increased southeast to
southerly winds will allow another surge of warmth and moisture to
move into the region. This time around, any wintry type
precipitation to the Kuskokwim Mountains region and interior
valleys of the upper Kuskokwim River. Depending on the track of
the low, precipitation amounts, and how quickly the cold air is
scoured out, some ice accumulation concerns may develop. The
models are not in good agreement with regard to all of these
factors so stay tuned.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
(Today through Saturday)

A weakening front will continue to progress from the western
Aleutians towards central portions of the Chain over the next 234
hours, with a series of lows rotating around each other (Fujiwhara
effect) near Shemya. This will maintain sustained gales across the
western Bering, with small craft conditions further east as the
complex of lows slowly treks eastward. This combined with the
remnants of a low further north will maintain a showery regime
behind the frontal passage, with some wintry precipitation
possible.

Further east, the series of lows mentioned above will consolidate
into a single low that`ll deepen as it heads east northeast
towards Bristol Bay. This new low will absorb another system
moving into the region from the north Pacific. This will allow for
more widespread precipitation and small craft conditions to
develop on Saturday for the Bristol Bay region and for coastal
waters south of the peninsula.

&&

.LONG TERM FORECAST (Days 3 through 7/Sunday Night through
Thursday)...

An upper level trough currently working its way east from the
Bering will be in place over southern Alaska by the start of the
longterm. This will keep weather unsettled for southern AK through
the weekend and into early next week. A series of shortwave
troughs embedded in the longwave system will impact southcentral
AK, with the first trough moving into the area Sunday and the
second Monday. Timing and placement are not quite dialed in yet,
so continuing to refine the resultant forecast from these shortwaves.

Going into the middle of next week the GFS and ECMWF look to have
come into decent synoptic agreement, with a series of troughs
working their way across the Aleutians and through southern AK.
The long term forecast period ends with troughing over southern AK
and the eastern Aleutians, ridging over the central Aleutians,
and troughing over the western Aleutians. All of this means an
active weather pattern for the foreseeable future. With model
consistency being not great, also wouldn`t be surprised to see
this pattern shift. This also means a messy forecast pattern with
low confidence after Monday. At the end of the forecast period on
Thursday, model agreement begins to completely fall apart, so
very little confidence for the tail end of the forecast period.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 141.
MARINE...Gale Warning 119 120 130 131 132 136 137 138 139
178 351 352 411.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...PJS
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD
LONG TERM...BB



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