Heavy Freezing Spray Warning 154, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
516 PM AKDT Fri Apr 9 2021

.ANALYSIS AND UPPER LEVELS...

Low pressure that traversed our forecast area yesterday has moved
to the Northeast Gulf of Alaska and become nearly stationary. This
has provided a sustained northerly pull of cold air behind the low
through much of Alaska, including our forecast area. Record cold
temperatures for many locations overnight are a result. Strong
winds associated with this influx of cold air are starting to
subside, leading to the expiration of the high wind warnings this
afternoon. However, surface observations and ASCAT imagery show
continued gusty winds through gaps and passes along the coast.

Out in the Bering, the next large low pressure system is pushing
ashore in Southwest AK, after moving through the Pribilofs and
producing near blizzard conditions there. While the front from
this system is impacting Southwest AK, it has become quite
detached from the low center which is located just west of Attu
Island - all the way in the Western Bering. Thus, this system
encompasses the entirety of the Bering.

The above surface conditions are supported in the upper levels
with a busy synoptic pattern. A longwave trough and anchoring 500
MB low are located along the ALCAN border with the anchoring low
in the Fortymile Country. A shortwave ridge is located over
Southwest AK. This couplet is providing strong northwesterly flow,
supporting the movement of the arctic air mass into Southern AK.
Another trough exists in the Bering Sea, sandwiching the ridge
and supporting the broad surface low there.

&&

.MODEL DISCUSSION...

Models perform well until Monday. They are in good agreement on
developing a long wave trough over the Bering and upper level
ridge over mainland AK going into Monday. However, on Monday
models begin to diverge on placement of the upper level embedded
within the trough over the Bering. This will affect timing and
placement of frontal features at the surface that this low will be
supporting. Models have better agreement over Southcentral AK at
least. So, moderate confidence to start next week and expect
adjustments in the forecast, especially over the Bering and in
Southwest AK.

&&

.AVIATION...
PANC...Northerly winds will continue to slacken over the airfield
through the evening. VFR conditions will persist with skies clear
and unrestricted visibility. Some snow may move into the vicinity
midday Saturday, but conditions are expected to remain VFR.

&&

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

This morning at the National Weather Service Office in Anchorage
the temperature bottomed out at 2.5 degrees Fahrenheit. Expect
one more day of brutally cold air from the arctic intrusion.
Sunday Anchorage will get into the 30s and Monday Anchorage will
reach the 40s. Also noteworthy, Valdez had a peak wind of 85 MPH
this morning and Thompson Pass had a peak wind of 96 MPH.

In satellite imagery, a front pushing across the Bering has a
tropical connection entrained in it. This system will reach
the western Gulf and Kodiak Island Saturday morning. A lobe of
vorticity will get ejected from the parent upper low and move over
portions of Southcentral. The possibility of scattered light snow
is actively being monitored for Southcentral on Saturday. A second
round of upper level energy will get kicked out of the same parent
upper low in the Bering and move towards Southcentral late
Saturday and Sunday. The expectation is that this event will bring
the aforementioned modified tropical air to portions of Southcentral.
Heavy wet snow is expected for the mountains of Prince William
Sound/Eastern Kenai. Unless the models change, downslope flow will
keep Anchorage and the Matanuska Valley dry on Sunday and Monday
morning. But by Monday afternoon, its 50/50 as to whether
Anchorage and the Mat Valley will get precipitation.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Today through Sunday)...

An Arctic air mass will continue to retreat across the region as a
front moving onshore brings another round of blizzard conditions
to the Kuskokwim Delta, mainly along coastal locations from
Kipnuk to Hooper Bay. Falling snow of up to 8 inches will
accompany the front across the same area, with lesser amounts of 3
to 5 inches expected between Dillingham and Goodnews Bay. Winter
weather headlines are in effect beginning this evening into
Saturday morning, with a blizzard warning for the Y-K Delta and a
Winter Weather Advisory for north and west of Bristol Bay.

The initial front will quickly degrade as it moves inland Saturday
morning, ending the first round of snow and blowing snow. However,
another front will be right on the heels of the first, reaching
the Southwest coast by Saturday evening. Persistent southwest flow
and warm air advection will follow the initial front, leading to
questions as to how much precipitation with the second frontal
passage will fall in the form of snow. Hence, mixing with rain is
possible for the lowest elevations and coastal areas by late
Saturday night into Sunday. Expect a heavy, wet snowfall for the
inland areas of the Y-K Delta and Bristol Bay that remain cold
enough for snow through Sunday as temperatures warm to near or
even above freezing during the daytime hours. This milder air
would also act to limit the potential for blowing snow near the
coast compared to this evening's frontal passage. Stay tuned to
the forecast as this potential for a second round for snow/mix
becomes more defined.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Today through Sunday)...

A front will track east to west across the central Bering today
bringing blizzard conditions to the Pribilofs this morning. Light
snow is likely across the Aleutians as well. Precipitation will
change to rain in southwesterly winds behind the front. A broad
low will slowly track across the western Bering through the
weekend, continuing the unsettled pattern and leading to
widespread rain/snow showers.

A front will move onshore over Southwest Alaska by late tonight,
southeast gales becoming southwest small craft or lower over the
eastern Bering by early Saturday morning. Meanwhile, a low over
the western Bering will bring southwesterly gales to low end
storms near and south of the Western Aleutians through this
evening. This low should quickly weaken to gale force as it drifts
towards Nunivak Island over the course of the weekend, with the
strongest winds remaining north of St Matthew Island through
Sunday. The low will also send a second, fast-moving front across
the southern and eastern Bering, bringing another round or rain
and snow as it moves swiftly towards Southwest Alaska by Sunday
morning.

&&

.MARINE (Days 3 through 5: Monday through Wednesday)...

Bering Sea/Aleutians

A broad area of low pressure will remain centered over the northern
Bering Sea on Monday. Winds are expected to remain below gale
force with no high seas expected.

Gulf of Alaska

A front over the northern Gulf of Alaska will result in gale
force southeasterly winds across the coastal waters of the north central
Gulf of Alaska. A front associated with a low Tue will bring
another round of easterly gale force winds throughout the coastal
waters.

&&

.Long Term Forecast (Days 4 through 7: Tuesday through Friday)...

The pattern will be amplified as an upper level low moves over the
Bering Sea on Tuesday while upper level ridging extends across the
eastern Alaskan mainland near the AlCan border. This low will then
move southeastward into the North Pacific on Friday. There is good
ensemble agreement that onshore flow and higher precipitable water
values are expected Tuesday and Wednesday as a front associated with
this low moves across the northern Gulf of Alaska into Southcentral
Alaska. This will allow for an increased chance of rain along the
coast and snow in more inland locations such as Turnagain Pass Tuesday
night through Wednesday afternoon. Due to the prevailing flow being
southeasterly, it is likely the Anchorage Bowl will have cloudy skies,
though little or no precipitation due to downsloping off the Chugach
Mountains. The precipitation associated with this front will taper
off across Southcentral Alaska Wednesday afternoon and evening.
Teleconnections support broad upper level ridging across the
Bering Sea and Alaska mainland Thursday afternoon onward. Due to
the dynamics of the aforementioned upper level low, there is good
ensemble agreement of a Rex Block forming over the Alaska
mainland and Bering Sea being under the upper level ridge Thursday
and Friday. The significance of the Rex Block is the upper ridge
will become more pronounced and is likely to persist for a longer
period of time. This upper level ridge will allow for a moderation
of the cold airmass and allow for more sunshine than clouds across
the Alaska mainland.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Blizzard Warning 155.
		 Winter Weather Advisory 141  161.
MARINE...Gale 119 120 125 127 128 130 131 150-170 172 173 175-177
180-185
Freezing Spray 119 121 130 139-141 160 180 185.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...BB
SOUTHCENTRAL ALASKA...PJS
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AS
MARINE/LONG TERM...ED