Small Craft Advisory 302, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
450 PM AKDT Fri Apr 3 2020

.ANALYSIS AND UPPER LEVELS...

Low pressure is pushing off to the east of southcentral AK,
clearing weather for that region. Clearing in Southcentral is
supported at 500MB by a shortwave ridge moving over southcentral
AK. There is also drying occurring in the midlevels, which is
aiding the clearing of cloud cover. Visible satellite also shows
off-shore flow developing through some gaps along the Southcentral
coast, with even some transverse banding along the AK
Range/Aleutian Ranges. This indicates strong winds and movement
of cooler air into the area.

Meanwhile, another low pressure system is moving through the
Bering Sea and approaching southwest AK. Satellite and surface
observations show the leading edge of the front has reached the
coast, with rain and snow falling at observation sites. The
frontal system and low pressure are supported by a longwave trough
in the upper levels that is digging into the Bering Sea.

&&

.MODEL DISCUSSION...

Models continue to be in sync with each other for the short term.
This means forecast confidence continues to be high for our
forecast area. Some finer points in the forecast will need to be
honed, but overall numerical guidance is solid.

&&

.AVIATION...

PANC...IFR ceilings that lingered over the aerodrome for morning
are scattering out Satellite shows cloud cover breaking up over
much of Anchorage Bowl. Moisture in the area from recent snow is
expected to cause ceilings to form overnight. We expect conditions
to remain VFR, but MVFR ceilings are not out of the question.

&&

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

The active pattern continues for the next few days as a series of
disturbances round the top of a flattening ridge (area of high
pressure) to the south. The first of which looks to bring most of
southcentral a brief bout of snow tomorrow morning before possibly
mixing with rain as it ends by early afternoon. Then the next
disturbance tracks further north and west, giving the mainly the
northern Susitna Valley and perhaps the northwestern Copper valley
a chance for more snow Saturday night into Sunday. There's an
outside change some of this may develop from the Talkeetna
Mountains down to the Chugach Mountains/east side of town, but I
wouldn't hang my hat on it.

Otherwise the main stories will be some patchy fog that may
develop tonight as light winds, clearing skies, and a fresh
snowfall all combine for this potential. For the weekend, looks
for warming temperatures and decreasing clouds with time.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3: Tonight
through Monday)...

The leading front associated with a powerful low is moving into
Southwest Alaska this afternoon. This front is associated with a
push of warm, springlike air. Thus, the rain/snow line is expected
to move well inland with this particular round of precipitation.
It is raining at many coastal sites around the area, and that rain
should move inland through tonight, despite the loss of solar
heating. As a bit of cooler air tries to work in late tonight, the
rain may mix with snow again across the Kuskokwim Valley. On
Saturday, any steady precipitation will diminish to mostly rain
showers for much of the area, with the exception of the west
facing slopes of the mountains, where steady precipitation will
continue. Temperatures aloft will try to cool a bit during the day
Saturday across the Kuskokwim Delta, so a mix with snow in any
shower activity is possible. No accumulations are expected during
the day.

On Saturday night, the cold front following behind the low will
sweep across Southwest Alaska from west to east. This will return
the area to well-below-average temperatures. Once the front moves
through, the winds will turn out of the west, and become quite
strong, especially along the Kuskokwim Delta coast. With the
center of the low tracking well to the north of the area (to
around Nome), the big forecast question will be how far south do
snow bands wrapping around the low extend. The models do not do
well with ocean-effect snow wrapping around a powerful low, but
the pattern will be in place for widely scattered bands of snow to
impact the Kuskokwim Delta coast both Saturday night and Sunday.
Thus, added blowing snow to the forecast. Time will tell whether
the blowing snow will be widespread enough to require an advisory.
The winds will die down on Monday, though mostly cloudy skies with
widely scattered snow showers will remain possible. Temperatures
Sunday night will drop into the teens across much of the area,
with even colder temperatures in any areas where skies manage to
clear out.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3: Tonight
through Monday)...

A strong front over the central Bering this afternoon will move
into Southwest Alaska Saturday evening. Until then, the front will
be largely stationary, extending from the central Aleutians into
Southwest Alaska. Thus, the eastern Aleutians and Alaska Peninsula
should be largely spared from most of its effects. Such will not
be the case with the strong cold front following behind the low,
which remains over the far western and northern Bering through
Sunday. The front is moving into the western Aleutians now, and
will sweep across all of the Bering by late Saturday night. The
cold air behind it will return the entire Bering to a snow showery
regime with strong westerly winds. A much weaker Arctic low will
track eastward across the Bering, but without much warm air to
work with, any precipitation it develops will be in the form of
snow. The track remains in question, as the models are having a
hard time resolving it, but most have it near the Pribilofs by
late Monday. Behind the low, a large, strong area of high pressure
builds northward across the western Bering, which will then
gradually shift eastward with time as another strong front
approaches the western Aleutians late Monday night.

&&

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

Bering Sea/Aleutians:

There is low confidence in the wind forecast on Monday as
guidance shows significant discrepancies in the placement and
intensity of a low in the central Bering. However, the winds are
expected to remain below gale force. These discrepancies continue
into Tuesday as guidance shows different solutions in the low
placement. Due to this, there is low confidence that strong
northwesterly gap winds will occur in Unimak Pass, though these
gap winds cannot be completely ruled out. As of now, it appears
that these winds will remain below gale force. There is also a low
confidence in the wind forecast across the western and central
Bering on Tuesday and Wednesday as a front moves through as
solutions vary significantly on the speed and intensity of this
front.

Gulf of Alaska:

Above average confidence exists in Monday's wind forecast as a
weak low is expected to develop in the northern Gulf. This low
will cause gale force northwesterly gap winds and increase the
chance of freezing spray as cold air advection will occur.
Confidence decreases significantly on Tuesday as an area of low
pressure moves into the western Gulf. Guidance shows different
solutions in the placement and intensity of this low. For now
winds appear to remain below gale force, however gale force winds
cannot be completely ruled out. Guidance is consistent with this
low deepening Tuesday into Wednesday and thus increases the
likelihood of gale force winds in the Gulf. However, there are
different solutions on the placement of the low center and thus
confidence is low with respect to the extent that these gale force
winds will occur.

&&

.LONG TERM FORECAST (Days 3 through 7: Tuesday - Friday)...

Guidance has a decent handle on the overall upper level synoptic
pattern on Tuesday with an upper trough moving eastward over
Southwest Alaska and an upper ridge over the western Bering. Due
to this pattern, colder temperatures are expected over the Alaskan
mainland with warmer temperatures being expected in the western
Bering. There is consistency that the pattern will amplify on
Wednesday but there are significant differences with respect to
the extent this amplification will occur. There is a consistent
synoptic pattern that the upper trough will move over the eastern
Alaskan mainland while the upper ridge moves over the central
Aleutians. Temperatures will be coldest over the eastern mainland
and warmest in the central Aleutians, but there is a high degree
of uncertainty in the magnitude of these temperatures. The same theme
is expected for Thursday as the upper ridge moves over the southwestern
mainland, though significant amplitude differences are expected.
Generally, this trend means there is high confidence that a
gradual warming pattern is in store for the Alaskan mainland but
low confidence in the magnitude of this anticipated warm up.
Confidence becomes very low on Friday as guidance shows
significant differences in the upper level synoptic pattern and
thus result in significant differences in the synoptic pattern at
the surface.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale 170 173 175-181 185.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...BJB
SOUTHCENTRAL ALASKA...PD
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JPW
MARINE/LONG TERM...EB