Small Craft Advisory 876, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
511 PM AKST Wed Nov 7 2018

.ANALYSIS AND UPPER LEVELS...

The upper level air pattern consists of an arctic trough centered
over Northwest Alaska with an amplified upper level ridge
extending from the Haida Gwaii northward through the Yukon
Territory. There is another low in the Gulf of Alaska lifting
northward toward the Gulf Coast. Water vapor satellite imagery
depicts a plume of warm and moist air lifting northward over the
entire state, out ahead of the eastward moving arctic trough. Over
the Bering Sea, there is a weak shortwave ridge axis out ahead of
a strong mobile shortwave ejecting off the Kamchatka Peninsula.
The associated warm front is moving into the western Bering Sea
and will quickly occlude this evening as the overall system moves
eastward.

Locally, the aforementioned front moving north toward the Gulf
Coast and Kodiak is already picking up heavy precipitation returns
on the Middleton Island radar. Some of the warm air
advection/isentropic lift is also moving into the interior west
of the Chugach Mountains and is spreading light rain over parts of
the Kenai Peninsula. With the low in the Gulf of Alaska, this is
promoting a pretty strong down Cook Inlet flow pattern. This
general pattern will have an impact on the upcoming snow event
which will move into Southcentral tonight and Thursday (see
Southcentral discussion).

&&

.MODEL DISCUSSION...

The numerical weather models are in excellent agreement with the
general large scale features which make-up the synoptic weather
regime. However, as usual in these types of patterns, they are in
lesser agreement with the details regarding all the disparate
"moving pieces" which are coming together as we speak. Currently
the main differences revolve around how much precipitation falls
across parts of the interior as dry air will be in place, and with
weak northerly down inlet gradient, that dry air will generally
be reinforced through the event. With that said, the models are
close enough with the general pattern that high enough confidence
is in place with this forecast that at least some light
accumulating snow is likely from the Kenai Peninsula to parts of
the Matanuska Valley as well as the Copper Basin, where the
highest amounts are expected.

&&

.AVIATION...
PANC...VFR conditions will persist for today. Offshore winds will
steadily increase as a low approaches from the south. Light snow
and low ceilings will cause MFVR conditions tonight into Thursday
midday before a rapid drying trend with a return to VFR likely.
Snow should be rather light and is not expected to be heavy at any
times.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
(Tonight through Friday)

The main forecast challenge continues to be snow amounts. Overall
the models have come into excellent agreement. They all show
deformation type precipitation developing from east to west
through tomorrow, for the western Kenai through Anchorage Bowl
through eastern Mat-Su Valleys. This afternoon's upper air
sounding for Anchorage also shows a continued moistening of the
airmass, with the dry layer having been reduced to the lowest 4500
feet, and much less of a dewpoint depression there compared to
the morning sounding. As has been the case today, Doppler radar
continues to show echos aloft that are falling into this dry layer
and evaporating while moistening continues from the top down.

The snowfall total looks to be pretty tight in the deformation
zone, with locations along the western edge described above
likely seeing up to 1 inch, while the Hillside to east of Palmer
may be closer to 2 to 3 inches. Further east across the Copper
River Basin it looks like a general 2 to 6 inches will fall, with
the highest totals seen across the higher passes, and between Glen
Allen and Valdez. Closer to the coast, enough warm air will be
pulled northward for this to be mainly a rain event. Precipitation
should quickly diminish in coverage by late morning for the Kenai
through Palmer area, while continuing into the evening for the
Copper Basin. Otherwise, look for seasonal temperatures to
continue.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...

Areas of light rain and snow are coming to end this afternoon as
the deformation band over the Alaska/Aleutian Ranges begins to
break down as the surface low in the western Gulf moves away from
the area and the upper-level support diminishes. High pressure
currently situated over the eastern Bering will move over the
southwest tonight bringing clearer and drier conditions to the
region. As the ridge settles overhead tonight, lingering moisture
at the surface combined with partial clearing will pave the way
for areas of fog to develop. The next frontal system will reach
the Kuskokwim Delta coast by midday Thursday, quickly pushing
through the interior by Thursday evening. Behind the main frontal
boundary, cold air aloft and a couple of shortwave troughs
following will bring scattered snow showers and strong westerly
winds to the southwest late Thursday night through Friday. The
combination of persistent west-southwesterly near gale-force winds
from Kuskokwim Bay north to Hooper Bay will produce a one to
three foot surge leading to water levels above MHHW early Friday
morning. This could lead to some minor coastal erosion and
flooding. Precipitation will taper off Friday as weak ridging
develops. By late Friday into early Saturday, the next low lifts
across the AKPEN and into the eastern Bering bringing strong
southeasterly winds and a mix of rain and snow along the southwest
coast.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...

High pressure over the eastern Bering is moving toward the west
coast in advance of a front draped from west of Adak northwest to
eastern Russia continues to swing east. Southerly gales and
widespread rain will accompany the front as it approaches Dutch
Harbor tonight and the AKPEN and west coast of Alaska tomorrow.
Behind the front, the parent low will track into the central
Bering. As it does, a wide area of gales will encompass much of
the western two-thirds of the Bering with a core of storm-force
winds riding underneath the center of circulation. The low begins
to weaken late tomorrow as it lifts closer to Saint Lawrence
Island. The long westerly fetch across the bering will build seas
across the central Bering to 25 to 35 feet across the offshore
waters. A second upper level low supported by abundant cold air
aloft will track from the Kamchatka Peninsula into the western
Bering Thursday night and then swing east into the central Bering
Friday. In response, a developing north Pacific low will track
north to the Alaska Peninsula Friday Night while a series of
shortwaves ejecting from the back side of the main low develops
two surface lows will impact the western and central Aleutians
with areas rain/snow showers and northwesterly gales Friday and
Saturday.

&&

.LONG TERM FORECAST (Days 4 through 7/Saturday night through Wednesday)...
A very stormy pattern is shaping up for mainland Alaska beginning
Saturday into Sunday and continuing through much of next week. A
strong polar jet digging out of Russia across the Bering Sea Friday
will continue to dig southward across the North Pacific Saturday
through Saturday night. This will cause a ridge to pump up downstream
across the east Pacific. The end result will be a high amplitude
pattern with a trough and deep closed low centered near and to the
south of the Alaska Peninsula and a strong southerly jet centered
along 150 degrees W longitude. Models indicate a tropical connection
with the trough digging as far south as 20 to 30 degrees N
latitude. Thus, expect development of an atmospheric river which
will transport abundant moisture and warmer air northward to the
Gulf of Alaska along with parts of Southcentral and Southwest
Alaska.

Conditions will be extremely favorable for cyclogenesis out ahead
of the digging trough. Expect a series of lows to develop over
the Pacific and track up into the Gulf. Over time, the strength of
these systems will help push the upper ridge eastward, shifting
the anchor trough/low into the Gulf and shifting the atmospheric
river toward the eastern Gulf and Southeast Alaska. As would be
expected in such a high amplitude/dynamic pattern, models differ
with handling of individual storm systems during this period.
However, based on the overall pattern described here, there is
high confidence in periods of heavy precipitation and high winds
along the Alaska Peninsula, Kodiak Island and the Gulf of Alaska
on up to Prince William Sound and the Gulf coast. With the
presence of the atmospheric river would expect precipitation to be
all rain near sea level, but heavy snow is a likelihood in
coastal mountain ranges. If the atmospheric river remains pointed
at the same general area for any length of time, flooding would
also be something to watch out for. The forecast for interior
portions of Southcentral and Southwest is a little trickier as
precipitation will depend heavily on track of storms, but would
generally expect periods of snow.

Meanwhile, back on the west side of the trough deep Arctic air
will pour southward across the Bering Sea and Aleutians Sunday
night through Tuesday. This will produce persistent strong and
gusty northerly winds along with showers. The air mass looks cold
enough that much of the precipitation will fall in the form of
snow. As the low along the Alaska Peninsula shifts into the Gulf
late next week this will allow a trough to move in from the
northwest Pacific, bringing storminess back to the Aleutian chain.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Storms 119 120 411 412. Gales 130-132 138 139 175 178-181
185 351 352 413 414.

&&

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