NWS Anchorage, AK wrote its last area forecast discussion before the first event at Dec 21, 4:53 PM AKST, 4 hours 7 minutes earlier.
Area forecast discussion, Dec 21, 4:53 PM AKST, 4 hours 7 minutes before
Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
453 PM AKST Mon Dec 21 2020
.ANALYSIS AND UPPER LEVELS...
Aloft, a broad trough of low pressure continues to exist in the
Bering. A closed 500MB low exists in the center of the trough,
with shortwave troughs moving through the flow around the base of
this trough. A high amplitude ridge is pushing north through the
Gulf of Alaska and into mainland AK. One of the shortwaves
currently rounding the base of the trough in the North Pacific is
helping to develop a strong surface low there.
Transitioning to the surface, the 500MB low aloft is reflected on
the surface as a large occluded 946 MB low in the Central Bering
that is largely stationary. A frontal system extends from this low
that is currently pushing into Southwest Alaska. A post frontal
trough is also working its way through the Central Aleutians.
Lastly, but not least significantly, a low is developing off of
the southern end of this system's front in the North Pacific. It
is this system that is developing into a strong surface low
mentioned in the previous paragraph. This low is moving northeast
towards the Western Gulf of Alaska. All of this is creating very
active weather. Please see the individual sections of the AFD for
more details about the forecast in your area.
&&
.MODEL DISCUSSION...
Synoptically, models are in good agreement in capturing the long
wave pattern. They are all depicting a broad area of low pressure
over the Bering, with a ridge working its way north through the
Gulf of Alaska. Models are also in agreement on upper level
shortwave troughs moving through this flow, more specifically the
trough that is working its way towards Southcentral AK from the
North Pacific, which is responsible for the incoming weather.
Models have come into better agreement with this system, which is
giving us a higher confidence level with all aspects of the
incoming weather.
Models have also come into better agreement with a secondary low
moving up the Cook Inlet tomorrow night. Initially, the GFS was
the only model to predict the low, but all major models are now
showing this low, so we are now anticipating its arrival. One
major difference is timing, with the GFS being considerably slower
than the NAM, ECMWF and Canadian. With the GFS being such an
outlier, we are leaning towards the other three as they are in
agreement with each other. This does lead to a drop in confidence
in weather associated with this secondary low as the GFS cannot be
completely discounted despite the timing difference.
&&
.AVIATION...
PANC...VFR conditions will be present with overcast cloud cover
staying above VFR minimums. The main impact to the airfield will
be low-level windshear, as northerlies dominate at the surface and
southeasterly winds rapidly increase aloft. It had looked like
there may be a frontal passage on Tuesday, with southeasterly
winds coming into the terminal. However, the front sort of stalls
as a new low over the Gulf approaches. Thus, there is a chance
winds could stay northwesterly at the terminal all day Tuesday,
with very strong low level wind shear during the morning hours.
Anchorage-Homer pressure gradients look to be very strong in the
morning (~10 mb), which will maintain that northerly surface wind.
They might weaken just enough during the afternoon to bring in
the southeasterly wind. Either way, the Anchorage- Homer pressure
gradient then strengthens again late in the afternoon, so any
southeasterlies would be brief.
&&
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
An incredibly strong and active weather pattern continues through
Wednesday for nearly all of Southcentral.
The worst weather is anticipated north of Seward across Turnagain
Pass, Moose Pass and Portage Valley. Snowfall totals of 3 to 5
feet are expected, with highest amounts through Turnagain Pass and
on up into the nearby mountains. When combined with strong gusty
winds (as high as 90 mph through Portage Valley), this will cause
blizzard conditions and make travel very difficult, if not
impossible. A Blizzard Warning remains in effect from 9 p.m. this
evening through noon Tuesday. Additionally, a Small Stream Flood
Advisory has been issued for Moose Pass to Seward until 12:30 p.m.
Wednesday as snow transitions to rain in the area. Heavy rainfall
could lead to ponding of water on roads and hazardous driving
conditions as frozen culverts prevent adequate drainage.
A High Wind Warning remains in effect for Anchorage. Peak wind
gusts of 115 mph are expected for higher elevations of Turnagain
Arm with 100 mph wind gusts anticipated for the Hillside. Gusts
are anticipated to reach 60 mph for the Anchorage Bowl, the Knik
River Valley and the Matanuska Valley through this time. All in
all, expect a windy day beginning tonight and through the morning
commute tomorrow, when winds should be at their strongest, before
tapering off the rest of the day.
These strong winds will create a rapid rise in temperatures across
Southcentral. It is possible that temperatures will reach the 40s
on Tuesday as these strong winds bring warm air into our area as
an area of low pressure moves across Kenai/Cook Inlet. Following
this, on the back side of the low, temperatures will cool as cold
air aloft is brought back to the region. This will cause what has
melted on the ground to refreeze, creating dangerous driving
conditions. Based on the expected low track, there is a chance of
snow in Anchorage tomorrow morning as a short-wave approaches,
despite the very strong winds coming across the mountains. If snow
does fall, blizzard conditions are a strong possibility. Will
continue to evaluate this potential this evening and update
forecasts if it looks likely. Chances of snow increase on
Wednesday, as cold air settles in and the upper level trough
moves through.
We are closely monitoring the situation in Homer as we expect
strong winds and heavy precipitation on the front and back side
of the low. A Small Stream Flood Advisory has been issued for
Anchor River, as snow transitions to heavy rainfall throughout the
day Tuesday that may result in ice jams and minor flooding. The
remaining question is currently how much cold air moves in on the
back side of the low to switch rain over to snow. If this comes to
fruition, increased winds out of the southwest and snowfall can
lead to lowered visibility for Tuesday night into Wednesday.
Models were all over the place with this solution, and with warm
sea surface temperatures, the Special Weather Statement continues
at this time with no additional changes.
The High Wind Watch for Cordova has been upgraded to a High Wind
Warning from 6 p.m. Tuesday to 3 a.m. Wednesday. A hurricane force
barrier jet, supported by a strong jet streak aloft, looks to
move inland during this time. The strong winds may move loose
debris, damage property, cause power outages and lead to difficult
travel conditions.
&&
.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Multiple shortwaves extending off a nearly stationary vertically
stacked low in the Bering track south to north across the
Southwest Mainland over the next couple days. Thus, expected
persistent snow and rain showers across the AKPEN, Bristol Bay,
and western Kuskokwim Delta through tomorrow. Winds are expected
to be strong and gusty through Areas around Bethel and Aniak will
likely be downsloped out of most of the precipitation and will
only see spotty light showers if anything. The AKPEN and coastal
Bristol Bay will start tonight as rain or rain/snow mix before
transitioning to all snow tomorrow. Additionally, predominately
southerly flow will raise temperatures tomorrow and Wednesday
briefly across the Southwest Mainland and the AKPEN and will begin
dropping again by Wednesday evening. Blowing snow advisory
remains in effect through late tonight for the Kuskokwim Delta
coast with strong gusty winds and accumulations around 3-7 inches
possible. Upslope areas along the eastern Kuskokwim Mountains may
see snow accumulations of 2-5 inches through tomorrow.
&&
.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The strong low in the Bering remains almost stationary through
tomorrow then moves northeast towards the Y-K Delta. Winds are
strongest on the backside of the low, mainly gales and possibly a
brief period of storms, along the Northern and Western Bering
where significantly colder air is being advected in. Thus, expect
the transition back to snow along the Chain and Southern AKPEN by
tonight as the colder air begins to wrap around the low. Blowing
snow may be a concern for areas along the Central and Eastern
Aleutians tomorrow night.
&&
.Marine (Days 3 through 5: Thursday through Saturday)...
Gulf of Alaska
Models have come into better agreement on the track of low moving
out of the North Pacific and then towards the AKPEN on Thursday,
but there continues to be disagreement on the strength. There
remains upside risk to the southerly winds currently in the
forecast, with gales looking possible across the western Bering.
On Friday/Saturday fairly quiet conditions are expected with good
forecast confidence.
Bering Sea:
On Thursday models continue to struggle with the placement of a
compact storm force low, with the GFS bringing this low through
the central Aleutians, while other models track this system mostly
south of the Aleutians. Our forecast sides with the further south
track, keeping lighter winds in across the Aleutians. On Friday a
front pushes into the western Bering/Aleutians, and a few areas of
Gales appear possible. On Saturday a deepening storm system looks
to set up across the western Aleutians, but there are significant
model differences as to how quickly this storm strengthens. There
may be significant upside risk to winds across the eastern Bering
on Saturday if the stronger solutions work out.
&&
.Long Term (Days 4 through 7/: Friday through Monday)...
On Friday models have come into better agreement on the track of a
North Pacific low, with good agreement on this next storm system
heading towards the AKPEN, bringing another wintry mix to
southwest Alaska and Kodiak. A gale force front looks to push into
the western Aleutians at the same time. On Saturday the AKPEN low
mostly dissipates. Meanwhile, a storm system begins to rapidly
intensify across the western Aleutians with the potential for some
storm force winds setting up across the central Aleutians along
with plentiful rain. On Sunday most models show this Bering/Aleutians
storm becoming fully mature and retrograding north and west
towards the Kamchatka Peninsula while unsettled weather continues
across the Bering and Southwest Alaska.
Model consistency disintegrates heading into Monday, but the
general idea of an active pattern across the Bering and relatively
quiet weather across Southcentral should hold.
&&
.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...High Wind Warning 101 125 135.
Blizzard Warning 125.
Winter Weather Advisory 155.
Flood Advisory 121 125.
MARINE...Hurricane Force Wind Warning 119 125.
Storm Warning 120 130 131 132.
Gale Warning 128 129 136 137 138 139 140 141
150 155 160 165 170 171 172 173
174 175 180 185.
Heavy Freezing Spray Warning 180 185.
FIRE WEATHER...NONE.
&&
SYNOPSIS AND MODEL DISCUSSION...BB
SOUTHCENTRAL ALASKA...CK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KM
MARINE/LONG TERM...BG