Heavy Freezing Spray Warning 145, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
524 AM AKDT Sun Apr 4 2021

.ANALYSIS AND UPPER LEVELS...

The upper levels gradually become less amplified and transition
to a more zonal flow over the next couple days. The main system
pushes west to east across the Interior through Monday, while its
trough moves across Southern Alaska. Moisture riding over top of
the now flattened ridge is being pulled into Southcentral,
bringing warmer air both aloft and as the surface, as well as a
fair amount of precipitation across the Susitna Valley and
northern most portion of the Matanuska Valley, as well as to areas
along the North Gulf Coast, such as Cordova and Valdez. A tight
pressure gradient exist where the trough and ridge meet, which
means gusty, westerly to southwesterly winds across Southcentral.

&&

.MODEL DISCUSSION...

Models are in decent overall synoptic agreement for the next
couple days. In the upper levels, the models are struggling to
account for individual low centers within a complex over/near the
Kamchatka Peninsula Sunday and Monday. These differences shouldn't
cause any issues with the forecast. A weak low pressure system at
the surface moves towards and along the Southwest Coast Tuesday
morning with subtle differences on the low's center, with further
discrepancies as the low pushes onshore and dissipates. There is
large model spread Tuesday night into Wednesday morning with a
weak surface low moving across the Bering. The Canadian model is
the biggest outlier, being the strongest and showing reflection in
the upper levels with a closed low at 500 mb.

&&

.AVIATION...
PANC...MVFR conditions are expected to persist through the
afternoon with low cloud ceilings and a rain/snow mix. Ceiling
should being to lift gradually through the afternoon into the
early evening. Gusty winds will prevail through much of the day
before decreasing later in the evening.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2:
Sunday and Tuesday)...

A deep moisture fetch from the tropics riding on top of a
weakening ridge is causing widespread high elevation snow and
a wintry mix in the low lying areas over much of the Southern
Mainland. A mid-level southwesterly winds coupled with a surface
low situated underneath the left exit region of a 140+ knot jet
streak, and upslope flow will keep the snow ongoing throughout
today. Snowfall will intensify over the northern portions of the
Mat-Su Valleys and most areas of the west- southwest facing
mountain slopes before subsiding during this afternoon (please see
the Winter Weather Advisory on our web page for more details).
Some subtle differences exist with the latest model solutions on
how much precipitation, if any, will fall over Anchorage Bowl.
Current thinking is leave isolated chances with any snow
accumulations concentrated in the aforementioned areas. Meanwhile,
low-level Turnagain Arm winds has allowed overnight temperatures
to increase from the mid-30s to low 40s with a southerly, warm
surge making its way in the surrounding high elevations as well.
With temperatures at or above freezing, some areas could expect to
see a rather wet, snow event.

A shortwave trough is moving toward Norton Sound and is expected
to traverse the interior state today. As it does, a surface low
will develop near the North Gulf Coast and moisture will shift
further east where precipitation will be confined to the coastal
areas of Prince William Sound and the Copper River Basin as well.
By Monday morning, precipitation begins tapering off with
lingering mountain snow showers as the surface low weakens and
moves over the eastern Gulf. Northerly winds will commence
thereafter in the wake of the low and funnel through the usual
gaps and passes over the Southern Mainland, including Seward and
Whittier. Additionally, with cold air advection, expect
northwesterly gale force winds to flow through the usual gaps and
passes with the potential of freezing spray over Kamishak Bay and
Shelikof Strait late Sunday into Monday. The aforementioned
Turnagain Arm winds are expected to shift back to the northwest
later today, allowing a cold, arctic airmass to return with
temperatures dropping back down below normal for this time of
season.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2: Sunday
through Tuesday morning)...

A trough over the northern Bering will push southeast and over the
region through the day today. This will end precipitation, falling
as predominately snow, from west to east this morning. Eastern
Bristol Bay will be the last area to switch from rain to snow before
the front moves out of the area. The trough will also bring in
clearer skies and a much colder air mass, so low temperatures
tomorrow may fall below zero over the interior with single digits
to teens expected along the coast. Weather will then become
active again Monday morning as the next frontal system reaches the
Y-K Delta coast. At this time winds are expected to remain at or
below 25 mph, but will be monitored as the ageostrophic component
may help to enhance winds as the front first reaches the coast.
Blowing snow is expected across the delta, but will likely remain
above the one half mile visibility criteria needed for an
advisory. Some uncertainty remains in the timing of snow for
Bristol Bay, but models at the moment look to have snow starting
in the afternoon. Storm total snow by Tuesday morning looks to be
around 2-4 inches for the Y-K Delta coast and 2-6 inches for
Bristol Bay, with the higher accumulations between Togiak and
Platinum.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2: Sunday
through Tuesday morning)...

A front extending from Unalaska to the AKPen this morning will be
pushed out of the region today by an advancing trough to the
north. Meanwhile, the next frontal system has reached the western
Aleutians with gale force winds and rain. High pressure over the
north Pacific should keep the front to the north of the Aleutians
Monday as it progresses east. Snow is likely over the Pribilofs
early Monday morning, transitioning to rain in the afternoon.
Current thinking is that the snow will be too wet due to warmer
temperatures to cause significant visibility reductions in
conjunction with the brisk southwesterly winds. By Tuesday
morning precipitation will then reach the Central and Eastern
Aleutians as the high retreats to the west.

&&

.MARINE (Days 3 through 5: Tuesday through Thursday)...

The big story for this next week will be the southward intrusion
of Arctic air across mainland Alaska and the eastern Bering Sea.
This will lead to periods of strong gap winds along the Alaska
Peninsula, Kodiak Island waters, and the Gulf coast. Colder air
pouring southward will also lead to an increase in freezing spray
region-wide.

Forecast confidence is high in the general pattern, but much
lower in the details of timing and strength of winds. A series of
lows will track from west to east across the Bering and the Gulf.
Generally expect the strongest winds on the back side of these
lows along the Alaska Peninsula and into the western Gulf, with
gale force gap winds likely. However, there is potential for storm
force winds and heavy freezing spray with a deeper surface low in
the Gulf. Stay tuned as we continue to track these systems and
the unseasonably cold air.

&&

.LONG TERM FORECAST (Days 4 through 7: Wednesday through Saturday)...
The upper level flow, which starts out zonal on Tuesday, will
amplify mid to late in response to a deep low tracking from the
northwest Pacific to the Aleutians. While the numerical models
agree that the pattern will amplify, there is a large spread in
location of individual waves which results in low confidence in
the details of the forecast. However, there is enough agreement to
discuss generalities. As a ridge builds ahead of the Pacific low,
short-waves will dive southeastward across the eastern bering and
Mainland Alaska. This will help drive Arctic air southward across
the eastern Bering and mainland Alaska. As a result, expect
continued unseasonably cold temperatures across southern Alaska
along with periods of strong gap winds along the Alaska Peninsula
and Gulf coast. The most amplified (and extreme) solution is the
GFS, which advects 850mb temperatures of -20 to -30 degrees
Celsius into the heart of southern Alaska. There isn't a whole
lot of support for this at the moment, so will generally stick
with a more moderate solution for now.

The precipitation forecast is very much in question. A series of
lows will track from the Bering to the Gulf, but there is a huge
spread in model solutions with track and depth of these lows. Best
chance for precipitation over southern Alaska with lows transiting
the region looks to be Wednesday/Thursday. Attention will then
shift to the low tracking to the Aleutians. This will send a
strong frontal system north and east across the Bering Sea
Thursday/Friday. The front will likely reach southern Alaska
sometime Friday or Saturday. While temperatures will quickly warm,
the preceding airmass is so cold that precipitation will likely
fall primarily as snow for most areas. With such an amplified
pattern and strong features, large changes to the forecast are
possible - so stay tuned. Bottom line, no matter how you slice
it, winter looks to maintain a from grip on Alaska.

&&
.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 111 145
MARINE...Gale Warning 130 131 139 140 178 411
  		 Heavy Freezing Spray Warning 160 180 181
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...KM
SOUTHCENTRAL ALASKA...MF
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CJ
MARINE/LONG TERM...SB/CK