Small Craft Advisory 298, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
458 PM AKDT Wed Apr 1 2020

.ANALYSIS AND UPPER LEVELS...
A short-wave trough is tracking eastward across the Bering Sea,
with a slightly out of phase short-wave digging over the North
Pacific. Water Vapor shows a tap of deeper moisture from the
North Pacific up to the Alaska Peninsula. At the surface, a
weak low is pushing onshore near Cape Newenham, with a warm
front lifting slowly northward across the Bristol Bay region,
and a trailing stationary front down across the Alaska Peninsula.
Widespread light overrunning precipitation is ongoing over
Southwest Alaska, with some of this also spreading to portions
of Southcentral. Precipitation is primarily in the form of snow,
with the exception of Bristol Bay where snow is in the process of
changing over to rain from south to north. Conditions across the
remainder of the Bering Sea and Aleutians are quiet thanks largely
to a flat ridge moving in behind the short-wave trough.

&&

.MODEL DISCUSSION...
Models are in good agreement with large scale features through
Saturday. There have been some common trends over the past few
model cycles and forecasts will be updated to reflect these.
First, all solutions indicate a good surge of deep moisture
(and heavier qpf/snow amounts) across the Alaska Peninsula into
portions of Bristol Bay tonight (Port Heiden to Iliamna) and all
the way up to the Kuskokwim Valley. Models have also been trending
higher with qpf/snow amounts in the Anchorage area Thursday night
through Friday morning. The strong short-wave moving in from the
west at that time does not dig quiet as aggressively as previous
model runs indicated. As a result, Anchorage and vicinity will
catch more of the brunt of vertical motion out ahead of this wave.
Lastly, models have all trended stronger with system crossing the
Bering Sea Thursday night through Saturday and latest forecasts
will reflect that.

&&

.AVIATION...
PANC...Light overrunning snow will continue overhead through
tonight. However, with dry low levels in place and a shallow
layer of low level southeasterly flow across the Kenai and
Chugach Mountains, expect most of this snow will not reach the
ground and VFR conditions will prevail. As an upper level short-
wave approaches Thursday morning, do expect light snow to develop
at the terminal. Low level flow will shift to south-southwest
during the day, so while snow will remain light, ceilings will
likely drop. Generally expect MVFR conditions. If conditions do
drop to IFR, it should be brief in nature.

&&

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

The forecast remains largely on track, but there's been a few
changes made today. The first of which was to increase snowfall
amounts, mainly for late Thursday night into Friday morning.
Today's model runs continue to advertise a fairly stout shortwave
trough crossing the region, with a mid-level low developing in
its base. The change today though is for the trough to not dig as
far south, with a slower onset of the cyclogenesis (low pressure
development). This in turn brings the base right over the
Anchorage Bowl and Kenai Peninsula early Friday morning, as it
dives southeast, with the low not forming perhaps until it
reaches the eastern Gulf. The models have also slowed down the
speed at which it crosses.

In short, with a slower trough progression and the base of the
trough (where the air will rise the most i.e. vertical ascent)
moving overhead or very near it, a longer duration of lift and in
turn precipitation is expected. Additionally, with the slower
timing it'll mainly be crossing late Thursday through late
Friday morning time frame. This is a more favorable time for
snowfall to accumulate given the cooler nocturnal (nighttime)
temperatures, versus during mid to late afternoon with a higher
sun angle helping to warm temperatures/mitigate accumulation
potential.

However, there will be precipitation occurring Thursday and
Friday afternoon as well. Thus, most areas should see a general
rain snow mix, then snow, back to a mix type of scenario. The
exception will be the immediate coastal areas along the Gulf,
where a higher probability of a rain snow mix or all rain exists.

A drying trend will commence for Friday night into Saturday, as a
shortwave ridge builds across the area ahead of the next system.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3/Wednesday
night through Saturday)...

A weak low pressure system is currently moving through Southwest
Alaska. With spring in the air, a mix of rain and snow is falling
depending on location. Coastal areas around Bristol Bay have
switched to rain today, while interior areas and the Y-K Delta
have remained snow. As a result, determining temperatures and
precipitation type were tricky today. We did adjust both to
account for the rain, which is expected to continue overnight
along the coast of Bristol Bay until switching to snow for all
areas tomorrow morning. This system will clear from the coast
tonight, with low stratus and some possible fog lingering behind
it. Snow will continue to fall for interior areas, mainly along
the Alaska Range. Therefore, increased snow amounts to around 4
inches areas around Iliamna before this system exits late
tomorrow.

Southwest Alaska will see a short break until the next low
pressure moves in from the Bering late Friday into Saturday. This
next system will be more powerful and bring a good push of warm
air with it. So, expect some warming temperatures going into the
weekend and increased winds in coastal areas. Whether or not this
low will bring rain or snow is something we will watch in the
coming days. For now, we can be confident in inclement weather
over the weekend with precipitation type needing to be ironed out.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 3/Wednesday
night through Saturday)...

Weak low pressure lingers in over the Eastern Aleutians while a
short wave ridge extends from the North Pacific into the Central
Bering. The weak low pressure exits the eastern Bering tomorrow
and high pressure shifts east, filling in behind it.The cirrus
shield from the next low pressure system is entering the Western
Bering. The leading edge of precipitation and winds associated with
this system should be entering the Western Bering tomorrow night.

This next system looks to be a powerful one, especially for this
time of year. Models have been trending towards increasing winds
with this system and are now forecasting some areas of storm force
winds in the boundary layer, which we use as they translate well
to the surface. Models are in agreement on storm track, which
indicates growing confidence in the forecast. Therefore, we have
increased wind speeds with this system to include large swaths of
gale force winds. Did not go full tilt on the storm force winds
just yet as we'd like to see the strengthening trend continue for
a couple more models runs (to increase confidence) before it goes
in the forecast. We are hedging that direction though. We expect
this storm to track northeast with its front to be located in the
Eastern Bering on Friday, with the front weakening as it goes.

&&

.MARINE (Days 3 through 5/Saturday through Monday)...

Bering Sea/Aleutians...
There is a high degree of confidence that widespread gale force
winds will occur across the Bering on Saturday and continue into
Sunday morning. Confidence is much lower with a weak low
developing in the western Bering on Sunday, then moving into the
eastern Bering on Monday, as guidance shows major discrepancies
with the placement and track of this low. As of now, the winds
associated with this weak low are expected to remain below gale
force and the compact nature of the low means the core of
strongest winds would affect a small area.

Gulf of Alaska...
The most significant weather for the Gulf will be strong cold air
advection across the Alaska Peninsula and into the Gulf. The best
estimate of timing is beginning sometime Saturday night and
continuing through the day Sunday. While the current forecast
advertises small craft advisory winds, there is good potential
that these winds could reach gale force, with storm force gusts
out of bays and passes along the Alaska Peninsula. These strong
northwesterly gap winds will also increase the chance of freezing
spray.

&&

.LONG TERM FORECAST (Days 4 through 7/Sunday through Wednesday)...

There is consistency with the general synoptic pattern across the
region on Sunday with an upper low over the Seward Peninsula and
an upper ridge over the eastern portion of the Alaskan mainland
near the AlCan border. This means temperatures will be generally
cooler to the west and warmer to the east. Forecast confidence
decreases into Monday as guidance shows a higher degree of
variation in the placement of the upper low as well as the
placement of the upper trough associated with this low. This is
significant because this means our confidence is lower in where
precipitation will occur and how intense the precipitation will
be across the Alaskan mainland. One area of consistency is that
due to the strong cold air advection expected with this trough,
the precipitation type will most likely be snow. Guidance shows
a more consistent synoptic set up across the area on Tuesday
with an upper trough over the Alaskan mainland while an upper
ridge is present across the Bering/Aleutians. Due to this, the
coldest temperatures are expected across Southcentral on Tuesday.
Confidence then becomes very low on Wednesday as guidance shows
a very high degree of variation between the upper level synoptic
pattern which will then influence the surface synoptic pattern.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning 173 175 176 177 178 411-413 130.
FIRE WEATHER...NONE.

&&

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