FXAK68 PAFC 190051
Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
451 PM AKDT Sun Mar 18 2018
.ANALYSIS AND UPPER LEVELS...
Strong southwesterly flow aloft is over southern Alaska and most
of the Bering Sea this afternoon. This is the result of a large
upper level low centered just northeast of the Kamchatka
Peninsula, and a positively tilted upper level ridge over the
northeast Pacific nudging into the Gulf of Alaska. There is a
front exiting the area along the Canadian Border, with some
lingering showers over Southcentral Alaska. There is a front
stretching from the eastern Aleutians through Bristol Bay to the
northwestern Alaska Range. An upper level shortwave trough is over
the central Bering Sea and moving toward the east.
The numerical models are in good agreement through the short term
portion of the forecast (Tuesday afternoon). There are some
differences in how they handle precipitation amounts and types
(rain vs. snow) with the passage of the front and upper level
trough tonight through Monday, making for a bit of a tricky
forecast. Therefore the forecast confidence is near normal.
PANC...Precipitation returns tonight, with the type still not
certain. However, with southeast winds expected, rain is most
likely. MVFR to possibly briefly IFR conditions are expected. The
rain will probably change to snow briefly on Monday morning as
colder air moves in for a period of lower visibilities.
Conditions will improve late in the morning to early afternoon as
colder and drier air moves in.
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A strong upper level trough will sweep down from the Bering
tonight and an embedded shortwave trough with associated vorticity
maximum will promote strong dynamics for an intensifying frontal
system moving across southcentral tonight through tomorrow.
Precipitation will spread across the region overnight as warmer
air is pulled up from the south out ahead of the front. This will
lead to rain and mixed precipitation for most locations with the
exception of the Northern Susitna Valley, where temperatures
should remain cold enough to stay all snow. Colder temps combined
with good southwest flow will lead to total snowfall amounts of
5-8 inches for Talkeetna northward along the Parks Highway making
for difficult travel conditions in the morning. Therefore, a
winter weather advisory has been issued through late tomorrow
morning for that area. Snowfall looks to be heaviest between 3 and
9 am tomorrow morning before a vigorous cold front passes
through. This should help to change precipitation over to all snow
for the Matanuska Valley south through the Anchorage Bowl
tomorrow morning, with 2-4" possible in the Mat Valley and less
than 1" in the Anchorage Bowl. Higher amounts are possible along
the Anchorage Hillside and Chugach Front Range.
Starting late Monday morning most areas should see an abrupt
change in the weather as strong cold air advection begins to
spread in behind the departing Gulf low. This not only will allow
for any snow that does develop in Southcentral to end quickly as
drier air moves into the area, but more importantly for the
development of gusty gap winds throughout the Gulf Coast. There
remains good confidence that this low will bring widespread gale
force winds to much of the Gulf and North Gulf Coast beginning
late Monday. This will also bring strong outflow winds to the
Seward, Whittier, and Valdez areas that will persist into Tuesday.
Maximum wind gusts should stay below warning levels due to the
lack of antecedent cold air in the Copper River Valley, but may
still gust up to 60-70 mph for places like Thompson Pass Tuesday
afternoon and Wednesday.
.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
As the trough currently moving through Southwest progresses
eastward today, rain and snow will continue to move into the
Kuskokwim Valley. Behind the trough, warm air is bringing
advection fog into Bristol Bay. This fog should end tonight as the
cold front approaching from the west increases instability. Precip
will begin as a mix, or all rain for Bristol Bay, but switch over
to snow as the colder air moves in aloft. Since the precip is
expected to shut off fairly rapidly before the bulk of the cold
air moves in, snow amounts should be minimal. Behind the front,
clearing skies are expected across Southwest tomorrow and tomorrow
night, but gusty northwest winds should keep the fog at bay.
.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
Gale force winds continue for the Western Aleutians today on the
southern side of the low near Russia. This low will slide off to
the east on Monday, bringing a temporary respite in winds for the
Aleutians and Bering Sea as high pressure moves in. Fog is
expected near the high center, however the air should be unstable
enough aloft to prevent it from being extensive. Monday, another
low pressure system will move up along the Kamchatka Peninsula,
bringing gale force winds and rain once again to the western
Aleutians beginning Tuesday morning. This pattern is expected to
slide eastward, causing winds to increase over the Bering Sea on
.LONG TERM FORECAST (Days 3 through 7)...
The long term begins Tuesday night with ridging dominating the
southern mainland with the axis along the west coast. This setup
leads to outflow winds along the north Gulf coast which is
supported by a shortwave moving through the area. Behind the
ridging is a front over the western Bering. Both features will
track east through Saturday when the front finally makes it over
Southcentral Alaska. The pattern will become more active following
the departure of the ridging with a new system moving into the
Bering Saturday. The models are in decent agreement through early
portions of the long term but struggle further out in time leading
to ensembles being preferred later on in the forecast period over
the deterministic solutions.
PUBLIC...Winter Weather Advisory 145.
MARINE...Gale 119 120 127 130 131 178 185.
SYNOPSIS AND MODEL DISCUSSION...BL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...LF
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