Gale Warning 673, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
516 PM AKST Thu Dec 7 2017

.ANALYSIS AND UPPER LEVELS...
There is a northward moving upper low centered over Bristol Bay,
with the surface low centered just southeast of Sand Point. Upper
level "troughiness" extends south-southwest from this low. There
is a very strong upper level ridge along the west coast of British
Columbia and the west coast of the lower 48 states. A plume of
moisture is pushing northward into southern Alaska between the
Bristol Bay low and the west coast ridge, riding fairly fast flow
aloft. This is bringing precipitation to much of the area. The
air mass moving northward is fairly warm, and precipitation in
many areas is falling as rain or even freezing rain. There is a
nearly vertically stacked low over the west-central Bering Sea,
with weak ridging over the eastern Bering.

&&

.MODEL DISCUSSION...
The numerical models are in fairly good agreement through the
short term portion of the forecast. There are some differences in
the timing of fast moving surface lows with the fast southerly
flow over the Gulf. There are also some differences with how the
models handle the moisture and precipitation. Forecast confidence
is therefore near to a little below normal.

&&

.AVIATION...
PANC...Intermittent freezing rain should turn to rain late this
afternoon. Some MVFR ceilings are expected late this afternoon and
evening with rain. Some LLWS is also expected during the afternoon
to early evening. By midnight, conditions should improve to VFR
as the front responsible moves north of the area.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Wintry precipitation will continue to impact a large portion of
Southcentral into Friday morning as a disturbance tracks north
across the area aided by favorable jet support in the upper
levels. Extended the Winter Weather Advisories in effect from the
Kenai Peninsula up through Anchorage and the Mat-Su for another
few hours based on current radar and model trends, as it appears
precipitation will continue a little longer than previously
expected. Freezing rain will continue to fall intermittently over
these areas this evening as temperatures hover near or just below
freezing. Precipitation rates have been great enough to
occasionally change back over to snow near Kenai and Soldotna,
with additional minor accumulations possible through late evening.
A few additional inches are also expected between Willow and
Talkeetna, with localized higher amounts west of the Parks and
north of Talkeetna. Precipitation will taper off from south to
north after midnight as the disturbance exits the area, yielding
drier conditions through Saturday.

Meanwhile, heavy snow will also continue to impact Thompson Pass
and portions of the Copper River Basin south of the Edgerton
Highway through Friday morning. Cut a few hours off of the
Advisory here as models now suggest the heaviest snow should
taper off by morning. Precipitation type will be challenging going
north along the Richardson Highway to Glennallen, as warming
aloft may support some areas of freezing rain along with periods
of snow and even rain if temperatures can creep up above freezing
in lower elevations. After a brief break on Friday, yet another
wave coming up from the Gulf will bring more snow to the
Richardson Highway corridor on Friday night, though amounts are
not expected to be as impressive as in recent days. Precipitation
with this wave will stay all rain along the coast as warm
temperatures continue in southerly flow.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
A weakening frontal system tracking across the southwest Mainland
this afternoon will continue to bring light snowfall over the
area from southeast to northwest through tonight. For tonight,
localized areas can see an additional 3 to 5 inches out of a
deformation band forming to the north of the surface low tracking
north into Bristol Bay. Another frontal system will rotate up over
the Alaska Peninsula Friday afternoon and evening, but looks to
stay far enough south to mostly miss mainland Southwest Alaska.
The next front following a similar track will push over the Alaska
and Aleutian ranges Saturday afternoon, and across the southwest
Mainland through Sunday.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
Low pressure over much of the Bering will slowly erode from west
to east as ridging builds over the western Aleutians and Bering
Friday morning through Saturday morning. Showery conditions will
persist over much of the western and central Bering/Aleutians
through Friday night. Gusty northwest winds area expected over
the central and eastern Bering/Aleutians Friday morning through
Saturday morning on the eastern periphery of the transient ridge.
Ridging over the southern part of the Bering and along the Western
and Central Aleutians will give way to a deepening north Pacific
low and associated strong frontal system moving into the area by
Saturday. This system is expected to bring another round of
precipitation to the western and central Aleutians/Bering, along
with strong gusty winds over the Western Bering and Aleutians.
However, ridging over the northwestern Bering will persist through
Saturday, keeping the system from progressing any further eastward
than the western and central Aleutians/Bering.

&&

.LONG TERM FORECAST (Days 3 through 7)...

The long-range forecast through the middle of next week continues
to depict an essentially unchanged overall weather pattern. The
very strong ridge of high pressure over western North America
looks like it will generally hold strong between now and then.
This will continue to steer numerous weather systems from south to
north through the Gulf of Alaska, which will result in additional
multiple rounds of heavy rain along the Gulf coast (and heavy snow
in the mountains) for the next week. The storm moving through this
pattern on Sunday night/Monday continues to look like it will be
pretty strong, with a storm-force weather front slamming into the
coast. Once again there will be a deep subtropical connection with
this storm, and impacts from heavy rain and snow will need to be
monitored.

Meanwhile, the Bering Sea and Aleutians will remain unsettled but
mostly benign during the forecast period, as the deep longwave
trough keeps most of the storms to the east (in the Gulf of
Alaska). By the end of next week in the Thursday or Friday
timeframe there is starting to be some indication that more active
weather will shift back into the Bering Sea, which potentially
could be the first sign of a change from this pattern featuring a
highly amplified ridge in the eastern Gulf/Panhandle.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 101 111 121 131 141 145.
MARINE...Gale Warning 119 125 173 174 175 176.

&&

SYNOPSIS AND MODEL DISCUSSION...BL
SOUTHCENTRAL ALASKA...CB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...TP
LONG TERM...AMD