Heavy Freezing Spray Warning 148, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
459 PM AKDT Tue Apr 6 2021

.ANALYSIS AND UPPER LEVELS...

A fairly active weather pattern exists over our forecast area
today. Weak low pressure exists in the Gulf of Alaska, while high
pressure exists inland. Cold air at the surface is well observed
on IR satellite this morning, in association with the high. This
is setting up off shore gap winds through the typical locations on
the coast of Southcentral AK. Northwesterly winds trailing the
low are moving across the Alaska and Aleutian Ranges, bringing
winds through Kamishak Bay. The high is also the culprit of below
normal temperatures for Southcentral AK.

Out west, on shore flow is producing low-level stratus and light
snow in coastal areas, while high pressure is keeping the interior
clear, calm and cold. Over the Bering Sea, zonal flow is quickly
moving a developing low pressure system through the Western and
Central Bering. It can currently be observed on satellite as an
open wave moving over the top of ridge that is pushing north from
the North Pacific. It is this system that has prompted the issuance
of weather hazards for Southwest Alaska, in anticipation for its
arrival into Southwest AK.

&&

.MODEL DISCUSSION...

Models are in good agreement through the end of the week. They are
handling the expected cold weather pattern well, both in the upper
levels and at the surface. This leads to high forecast confidence
for the short term. Model agreement does diverge quite a bit starting
Saturday, but this is best left to the long term discussion.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3:
Tonight through Friday Night)...

Generally quiet weather persists across Southcentral to start off
the short term forecast this afternoon, with high pressure
having eliminated almost all of the cloud cover except over Kodiak
Island, and in Broad Pass. Gusty winds through Seward, Whittier,
and Valdez remain the only weather concern of note going on right
now. Temperatures remain well below average with most areas from
5 to 15 degrees below the normal highs for this time of year.

On Wednesday, a low moving into Southwest Alaska from the Bering
will transfer its energy to a developing low over the Gulf on
Wednesday night. As this occurs, a period of light snow is
expected over much of the area from Wednesday evening through the
day on Thursday. This will be a minor snow event for everyone, as
the low will track far enough south in the Gulf that only coastal
areas and the mountains get much more than a few inches of
accumulation. For most interior areas such as Anchorage, up to an
inch of accumulation is expected, with a general 1 to 3 inches for
the Kenai Peninsula and the Seward and Sterling Highways.

The real active weather occurs in the wake of this low, as it
brings down an air mass cold enough that it would make January
blush. Temperatures that could approach 25 degrees below normal
are possible on Friday night. In the meantime, expect temperatures
to begin falling on Thursday as the frigid cold air mass arrives.
The air mass will be so cold that it will be able to overcome 14
hours of daylight and a 36 degree solar noon sun angle, resulting
in falling temperatures during peak heating Thursday afternoon. An
air mass this cold doesn't arrive unannounced. Gusty winds are
expected to develop across much of Southcentral Thursday
afternoon, with the gap winds potentially causing 50 to 60 mph
gusts in Seward and Whittier as well. These winds will not spare
the Anchorage Bowl, as northwesterly gusts up to 45 mph are
possible. Precautions including tying down loose items or bringing
them inside should be taken by Thursday. Further, the winds and
cold will team up to produce extreme levels of heavy freezing
spray across lower Cook Inlet and Shelikof Strait Thursday night.
See the Marine Weather Statement for more details.

The gusty winds will have one benefit, keeping temperatures
warmer than they otherwise would be. As such, raised overnight
lows quite a bit area-wide to account for the warming effect of
the winds overcoming otherwise bitterly cold air. The winds will
diminish during the day on Friday, such that they will not be of
much help by Friday night. If record lows occur, which remains in
the forecast, it appears more likely to happen Friday night. The
only caveat to the record lows will be the potential for high
clouds in advance of the next storm system potentially stopping
the radiational cooling process in the hours just before sunrise
Saturday morning. Stay tuned!

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Tonight through Friday
afternoon)...

Weak upper level ridging will continue building into Southwest
Alaska this afternoon that will bring partly cloudy skies and
light and variable winds overnight. Some lingering snow showers
are expected to continue across the Kuskokwim Mountains and Aniak
as well as Sparrevohn and Lime Village through the overnight
hours. Little to no accumulation is expected for these areas.
There may be some patchy fog for the coastal communities of
Bristol Bay overnight into early Wednesday morning due to onshore
flow in the low levels.

The next big weather story will be a frontal system moving into
Southwest Wednesday morning. Snow will overspread the Kuskokwim
Delta Wednesday morning with increasing winds leading to blowing
snow and reduced visibilities to one half and one quarter mile.
Snow will continue spreading eastward bringing snow to all of
Bristol Bay as well as the Middle Kuskokwim Valley throughout the
day on Wednesday. Snowfall amounts of 3 to 5 inches are expected
across the Kuskokwim Delta and the Middle Kuskokwim Valley with
4 to 8 inches for interior Bristol Bay (i.e. Iliamna to Port
Alsworth where the higher snowfall amounts are expected). Snowfall
is expected to end west to east across Southwest Wednesday night
through Thursday morning. Strong northerly winds are expected to
persist even after the snowfall ends across the Kuskokwim Delta as
the Arctic air moves rapidly into the region. Temperatures at 850
mb are expected to drop from around -6 degrees Celsius in Bethel
to around -23 degrees Celsius Thursday afternoon. With the
recently fallen snow from the aforementioned system, there is the
potential for ground blizzard conditions for the Kuskokwim Delta
through Thursday morning. Therefore, the Blizzard Warning has been
extended in time and is also out for the entire area now
(including Bethel). A Winter Weather Advisory is in effect for
Bristol Bay from Wednesday morning to Wednesday night. See
advisory for more details.

Heavy freezing spray will also be a major concern for Bristol Bay
and for north of the Alaska Peninsula from Wednesday evening
through Friday morning and south of the Alaska Peninsula Thursday
evening into Friday morning when the deeper Arctic air moves
through.

Lastly, temperatures will be very cold Thursday morning through
Friday with temperatures below zero across interior locations
both mornings and single digits to low teens along coastal
locations. Friday morning is expected to be the coldest morning
with many places dipping below zero even to the coast.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Tonight through Friday
afternoon)...

An upper level low with its associated surface low are expected to
move across the north-central Bering tonight into Thursday
morning. Storm force winds are expected for the central Bering to
the Pribilof Islands Wednesday with widespread gale force winds.
Wave heights of 20 to 27 feet are expected especially on the south
side of the low Wednesday/Wednesday night. This system will
mainly bring rain to the Pribilofs and the Aleutian Island Chain
due to the warm air associated with it. Much colder air is
expected to move back in across much of the Bering Wednesday
afternoon through Friday with EXTREME heavy freezing spray
expected for the coastal waters of Southwest. See the marine
weather statement for more details regarding this.

Another gale force frontal system will move through the western
Aleutian Chain Wednesday night. This system will bring all rain
due to the amount of warm air that will be brought north with
this system. Colder air moving back in on the back side of the low
will bring rain/snow showers to the Western Aleutians Thursday
night and Friday. This front reaches the Pribilofs Thursday night
with enough cold air in place to remain snow Thursday night
through Friday morning before warming up enough to change to rain.

&&

.MARINE (Days 3 through 5: Friday through Sunday)...

A very active weather pattern is expected through the weekend with
high confidence to start. Friday, a low in the northeastern Gulf
will maintain gale to storm force winds, especially through bays
and passes across Shelikof Strait and towards Kodiak before
beginning to dissipate Friday afternoon. A low just to the west of
Shemya will keep gale to storm force winds along its front across
the central Bering. As it dissipates into Saturday and moves
north, winds and waves will die down.

After this, confidence breaks down with models in stark
disagreement on the placement of a front across the Gulf or the
formation of a low. At best expect small craft winds and waves
across the Gulf, at worst expect gale force winds and waves
through Sunday. For the Bering, there are also discrepancies on
the placement, strength and track of an area of low pressure. At
worst expect gale to storm force winds across the Aleutians and
at best expect small craft winds and waves through Sunday. How
long the increase in winds and waves associated with this low
will last has yet to be determined.

&&

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

Beginning Saturday, model solutions diverge with forecast confidence
being on the low end. There is some agreement with the location
of a front bringing precipitation and higher winds across the
eastern Bering and coastal locations of the Southwest beginning
Saturday morning. Precipitation is also anticipated across the
remainder of the Bering and the Aleutians with an area of low
pressure located somewhere in the western Bering. The amount of
precipitation and strength of the wind will vary depending on
which model solution plays out. This feature looks to propagate
into Southcentral on Sunday, with the same anticipated results of
precipitation and higher winds. The same issues continue with this
forecast as far as how much precipitation and how high the winds
will be, with further discontinuity in the location and strength
of this front. It does look as if precipitation could largely be
isolated to the coastal regions and higher elevations of the
northern Gulf through Monday, with downsloping possible for the
Anchorage Bowl. Precipitation seems to vary across the Bering and
the Southwest due to the low in the Bering and its track, strength
and placement from Saturday through Tuesday. All in all, expect
an active weather pattern for the long term, with the finer
details being worked out as models come into better alignment.

&&
.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Blizzard Warning 155. Winter Weather Advisory 161.
MARINE...Storm Warning 179.
         Gale Warning 119 130 139 150 155 160 165 170 171
                      172 175 177 178 180  181 185.
         Heavy Freezing Spray Warning 160 180 181 185.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...BB
SOUTHCENTRAL ALASKA...JW
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MV
MARINE/LONG TERM...CK