NWS Anchorage, AK wrote its last area forecast discussion before the first event at Dec 31, 5:09 PM AKST, 6 hours 51 minutes earlier.
Area forecast discussion, Dec 31, 5:09 PM AKST, 6 hours 51 minutes before
Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
509 PM AKST Sat Dec 31 2022
.ANALYSIS AND UPPER LEVELS...
A broad, longwave trough extends from the southern Bering and
northern Pacific east to the western Gulf of Alaska. Embedded
within the trough, are a series of shortwaves rotating around the
anchoring upper-level low situated well south of the eastern
Aleutians. The first of these shortwaves is stretched out from the
AKPen east across the northern Gulf. This feature is associated
with a weak surface trough that is bringing gusty easterly winds
across Price William Sound and a rain/snow mix to portions of the
Kenai Peninsula, Kodiak Island, and the Bristol bay region.
The main feature, however, is the aforementioned upper-level low,
vertically stacked with an occluding surface low. This surface low
is extending an occluded front from the low center to south of
Kodiak Island. A new surface low is beginning to spin up at the
triple point of the occlusion, easily depicted on satellite
imagery. This feature is aided by a strengthening shortwave
rounding the base of the upper-level low and a stout cyclonic jet
streak curving up toward the southern Gulf. A well-defined
moisture plume originating east of Midway Island is also lifting
northward out ahead of the shortwave trough and surface cold
front. This low and associated frontal system are the features
that will drive northward, along with the developing atmospheric
river, bringing the strong winds and wintry precipitation to
Southcentral.
Farther west, conditions remain generally unsettled, especially
over the Bering and Aleutians as multiple waves rotate around the
longwave trough.
&&
.MODEL DISCUSSION...
Model agreement continues to improve through the latest runs (12z
and 18z), especially with the larger, synoptic scale features.
There is still some issues between various models, mainly the hi-
res and NAM versus GFS and GEM, with regard to the timing of the
front and transition of snow to rain. It appears that the peak of
the winds, especially across Portage Valley and Turnagain Arm may
end up, a hybrid of all models, with the hi-res capturing the
stronger winds initially (06z Sunday) and the GFS capturing the
continuation of strong winds for these areas through 18z Sunday.
The latest NAM (18z) has also trended more toward the GFS and GEM
in terms of precipitation and winds for Monday. The 12z NAM and
NAMnest was hinting at a bending to the Turnagain Arm wind into
Anchorage for Monday. However, this now look less likely.
The other main issue is the track of the parent low as it crosses
the AKPen for Sunday into Monday. However, the resultant wind and
precipitation fields look similar, even with slight discrepancies
in the storm track.
&&
.AVIATION...
PANC...VFR conditions will persist with scattered very light snow
showers / flurries this afternoon and evening. Northerly winds
will gradually increase through the afternoon hours, becoming
gusty by this evening. Precipitation will begin to fill in over
the terminal around midnight, but will likely remain virga for a
couple of hours due to the drier air and northerly winds near the
surface. Periods of light freezing rain are then expected by early
morning Sunday. Any freezing rain will likely be very light in
intensity.
&&
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3: Today
through Tuesday)...
The winds of change are brewing on the eve of the new year as a
large low pressure system swirling in the North Pacific takes aim
at Southcentral tonight. A strong southerly jet will quickly
push a warm front to the north Gulf coast late this evening,
intensifying the currently ongoing shower activity along the
coast, as well as increasing the winds out of north-south
oriented gaps as the pressure decreases in the Gulf of Alaska.
These winds will be fairly run of the mill for a strong front,
however, one particular area can expect notably high winds with
this system. The winds through Passage Canal, the Portage Valley,
and eastern Turnagain Arm will increase to at least gale force
sustained, with a wide gust spread allowing for gusts as high as
75 mph during the overnight hours with the arrival of the front. A
storm force barrier jet will also accompany the front along the
north Gulf coast and into Prince William Sound.
Behind the front, an atmospheric river bringing a conveyer belt
of warm air and moisture will allow for a short-duration heavy
precipitation event along the coast. The strong dynamics of the
storm system combined with abundant moisture will allow for some
precipitation to overrun the coastal mountains into inland
portions of Southcentral. At first, precipitation will begin as
snow for most inland locations, while rain continues along the
coast. Then, early Sunday morning, warmer temperatures aloft will
overspread the region, resulting in a change in precipitation from
snow to a wintry mix of snow and freezing rain, eventually
transitioning to all freezing rain across large portions of
Southcentral from the Susitna Valley south of Talkeetna to the
western Kenai Peninsula, including Anchorage and the Matanuska
Valley. The southern Copper River Basin is expected to see
freezing rain beginning Sunday morning as well. Due to the nature
of the storm dynamics, precipitation will be intermittent, and
periods of light freezing rain with breaks in between will lead to
the accretion of up to one tenth of an inch of ice, with higher
amounts possible in the southern Copper Basin.
The exact timing of precipitation is hard to predict with this
type of storm, though the most likely periods of precipitation are
expected to be during the day on Sunday as the front sweeps
northward up Cook Inlet, and then in a series of waves on Sunday
night into Monday as several smaller- scale disturbances rotate
around the strong low and initiate rounds of precipitation as they
pass overhead. By Monday morning, the front pushes fully inland,
and winds diminish along the coast, with precipitation intensity
decreasing as well to a more showery, upslope regime.
While northerly winds are expected to remain dominant at this
time, there is a possibility that winds shift easterly on Monday,
with a Turnagain Arm wind grazing or even pushing into Anchorage.
This would be likely to raise temperatures above freezing. Monday
night into Tuesday, the low weakens and elongates over Bristol
Bay, allowing for most inland precipitation to end and
temperatures to cool as the warm air pushes north with the
decaying front into interior Alaska.
A plethora of products have been issued in anticipation of this
storm, with widespread potential for freezing rain and localized
areas of high winds. See the relevant Winter Weather Advisory or
High Wind Warning for your area for more details.
&&
.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3: This
afternoon through Tuesday afternoon)...
A broad upper level low lingering near Southwest Alaska will keep
unsettled weather in the forecast through Tuesday. A Winter
Weather Advisory is in effect for Bristol Bay (from Aleknagik and
south to the Bristol Bay coast) as a strong surface low approaches
the Alaska Peninsula. A tight surface pressure gradient will lead
to gusty northerly to northeasterly winds across Southwest
Alaska. Strong winds, in combination with a band of snow rotating
around the low, could lead to blowing snow and low visibility.
While there is high confidence in gusty winds, model difficulties
in pinpointing the exact low track means that there remains some
uncertainty with snow amounts. This solution leans on high
resolution models, which suggest greater snow amounts than the
global models. However, if global models verify and lower snow
amounts occur, we'll see better visibility than forecast.
As we head into Sunday, we should see winds diminish as the
surface low weakens. With the low lingering near Chignik, areas
along the Alaska Peninsula will see persistent showers due to
onshore flow. There is a slight chance that we could see freezing
precipitation on the Alaska Peninsula and in interior Bristol Bay
Sunday as there is the potential for warm air aloft to move over
colder air at the surface. This will ultimately depend on the low
track, which will affect downsloping potential and whether warm
air can push in at the surface through gaps in the terrain. We'll
continue to monitor this, but for now, freezing precipitation
looks unlikely.
Otherwise, conditions become more benign as we head into Monday
and Tuesday, with light winds and occasional snow showers
expected.
&&
.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3: This
afternoon through Tuesday afternoon)...
Northerly small craft winds, rain/snow showers, and heavy freezing
spray are the main concerns for the Bering Sea and Aleutian
Islands during the short-term. A broad upper level low sits near
the Eastern Aleutians and Alaska Peninsula, inducing northerly
flow across much of the region. Cold air advection in this
northerly flow is leading to freezing spray and heavy freezing
spray in the Northern Bering Sea and along the Southwest Alaska
coast. As shortwaves rotate around this upper level low, we'll see
occasional showers across much of the region, with more
persistent showers along the Eastern Aleutians. Temperatures will
gradually drop, with rain or rain/snow showers transitioning to
snow as we head towards Tuesday.
On Monday afternoon, a front will push eastward into the Western
Bering Sea and Western Aleutians. Expect southerly to
southeasterly small craft winds and snow transitioning to rain
along the front. Several mesolows embedded in this front could
lead to localized gales. These compact features are difficult for
models to accurately represent this far in the future, so it will
be difficult to pinpoint where exactly gales will occur until we
get closer to the event.
&&
.LONG TERM FORECAST (Days 4 through 7: Wednesday through Saturday)...
Looking into the extended forecast, the active weather pattern
that both the Bering Sea/Aleutians, Southwest, and Southcentral
Alaska has experienced in the near to short-term looks to continue
through the long-term as well. Overall ensemble guidance and
deterministic guidance is in decent agreement that the entire
domain remains withing the influence of the upper trough pattern
that has been the status-quo of late. While there is general
agreement on the upper synoptic long-term pattern, surface
features and their evolutions (timing, track,
precipitation/precipitation type) remain up for debate.
The pattern for the Bering and Aleutians features an upper level
low complex that remains nearly stationary as it spins in the
western Bering Sea Wednesday through Saturday. Multiple shortwaves
rotate cyclonically around the complex and draw up Pacific
moisture from the south and southwest. The forecast challenge will
be nailing down where surface low pressures might spin up in
response to the various shortwaves as well as cold air
availability. The synoptic pattern supports an active and wet
western to central Bering/Aleutian Chain with not quite as active
of a pattern for Southwest. Although, there will be shortwave
troughs that move in from the Bering; especially for Thursday and
Friday. The North Pacific side of the Alaska Peninsula looks to
stay on the active side mostly due to moist southerly to
southeasterly upslope flow. Odds tilt that temperatures as well
as precipitation will be around normal for Southwest and the Bering
Sea/Aleutians, regardless of the continued active weather pattern.
The pattern for Southcentral features continued southerly to
southeasterly moist flow which will keep coastal sections of
Southcentral unsettled. A notable potential surface feature that
will need to be watched will be a Gulf of Alaska low near Kodiak
Island that has the potential to bring a storm-force barrier jet
with it to the northern Gulf Coast. Another feature of note which
has decent guidance agreement is a negatively tilted trough which
extends from an upper level Bering low that will enter the Gulf
during the day on Saturday. Kodiak Island and the eastern Kenai
Peninsula will need to follow the progression of the forecast
regarding this feature for possible impacts down the line.
Ultimately, the general long-term synoptic pattern favors an
active coastal Southcentral; especially western Prince William
Sound and eastern Kenai Peninsula with odds leaning slightly
towards above average precipitation. Meanwhile, interior areas
look to have normal precipitation with not quite as much activity
as the coast due to potential cross-barrier downslope flow.
Temperatures look to be around normal for Kodiak Island, the
Kenai Peninsula, Mat-Su Valleys, and Anchorage, while odds tilt to
slightly above normal temperatures for Prince William Sound
communities and the Copper River Basin.
&&
.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...HIGH WIND WARNING: 125
WINTER WEATHER ADVISORY: 101 111 121 125 131 141 145 161
MARINE...STORM WARNING: 119 120 130 131 138 139
GALE WARNING: 125 127 129 132 136 137 140 150 160 351 352
HEAVY FREEZING SPRAY WARNING: 180 185
FIRE WEATHER...NONE.
&&
SYNOPSIS AND MODEL DISCUSSION...TM
SOUTHCENTRAL ALASKA...CQ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KC
LONG TERM...DAN