Small Craft Advisory 249, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
439 AM AKDT Mon Mar 16 2020

.ANALYSIS AND UPPER LEVELS...

A ridge of high pressure stretched from the Gulf, across
Southcentral, then to the Seward Peninsula. To the west of this
ridge axis was a weakening area of low pressure to the southeast
of the Pribilof Islands, and another area of low pressure was
located well south of Dutch Harbor. Finally, a weakening front was
moving through the western Aleutians and Bering Sea.

&&

.MODEL DISCUSSION...

The numerical models have come into better agreement with how the
handle the phasing of a shortwave trough ejecting northeast from
the Pacific towards our neck of the woods, and another shortwave
trough diving southeast from Siberia into our region. A similar
improvement is seen closer to the surface as they all are in good
agreement that weak cyclogenesis (low pressure development) will
occur, with just some minor temporal and location differences.

&&

.AVIATION...

PANC...Light winds will prevail through the forecast period. The
night microphysics imagery is showing some low stratus located
over the western Kenai Peninsula and northern Cook Inlet, with
Kenai itself having ceilings in the 3000 TO 3500 feet range. This
will likely affect the terminal for a brief period of time this
morning.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3/Today
through Wednesday night)...

A weak upper level short-wave moving overtop of the ridge which
has been in place will lead to some clouds across Southcentral
this morning. However, a separate ridge over northwestern Alaska
this morning, will then connect with the ridge over the Gulf and
build back over Southcentral this afternoon and evening, leading
to decreasing clouds. The passage of the short-wave will cause
gap winds along the coast to kick up once again, primarily
Seward/Resurrection Bay and Whittier/Passage Canal.

Meanwhile, a weak storm system tracking up the west side of the
upper ridge into the eastern Bering Sea will push a front toward
Kodiak Island, with light rain spreading into the southern to
eastern portions of the island today. An upper level short-wave
associated with this system will lift northward today, but then
take a more northeasterly trajectory beginning late tonight as
the upper ridge over the Gulf and Southcentral begins to shift
eastward. Precipitation will spread northward to the southern
Kenai Peninsula by late tonight, then across much of Southcentral
during the day Tuesday.

A stronger short-wave will rapidly track eastward across the
Bering Sea tonight through Tuesday, then move into Southcentral
right on the heals of the first short-wave. This will enhance
precipitation rates across Southcentral Tuesday afternoon through
Tuesday night. Precipitation type will be primarily snow across
Southcentral, though the Gulf coast and portions of the southern
to eastern Kenai Peninsula look like they will warm enough to lead
to a period of rain or a rain/snow mix Tuesday afternoon and
evening. Marginally cooler air will move in Tuesday night, helping
to change any remaining precipitation back to snow. The upper
waves will move through Southcentral relatively quickly, so total
snow accumulation will be light, with approximately 2 to 4 inches
across the region. Dry conditions will return Wednesday through
Wednesday night as the trough exits eastward and a ridge builds
back in from the west.

&&

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

A ridge over Southwest Alaska this morning will slide east this
afternoon as an upper-level low currently south of the eastern
Aleutians lifts toward Bristol Bay. This feature is associated
with a surface low that will also move north over the eastern
Bering later this evening. As it tracks north, its warm front will
advance toward the region. A moist, southerly flow accompanying
the frontal system will usher in warmer temperatures and
widespread precipitation from the AKPEN north across Bristol Bay
and the Kuskokwim Delta through the day. The challenge with this
system continues to be precipitation type, as the higher degree
of uncertainty remains with both the surface temperatures and the
degree (no pun intended) of how much warm air works in aloft. The
NAM is the favored model for the evolution of this system, but
also remains the warmest with the GFS the coldest. Thus, the NAM
is trying to erode the cold air away quickly, changing any snow
during the onset of precipitation to rain, especially near the
coast (King Salmon and Dillingham north to Bethel). The GFS, on
the other hand, is colder and keeps any frozen (and freezing
precipitation) persisting for a longer duration. Given, the
antecedent conditions and data from nearby 00z and 12z soundings,
it was felt that the NAM may be too warm and GFS too cold.
Therefore, a blend of the two was used for precipitation types and
amounts through tomorrow night before the upper-level trough
moves through and colder air returns, transitioning the wintry mix
back to snow showers before tapering off by Wednesday morning.

This forecast solution yields a wintry mix of snow, freezing rain,
and rain beginning later this afternoon for the Bristol Bay area
and toward the overnight hours for the Kuskokwim Delta and
interior locations, where a couple of inches of snow could fall
before warmer air works in. The best locations for prolonged
freezing rain looks to be from Dillingham to west of Iliamna, areas
west of Bethel, and the middle Kuskokwim Valley around Sleetmute.
A few hundredths of an inch of ice could accumulate before a
transition to rain by late tomorrow morning. No impacts from the
ice are expected, however, as it will fall on snow-covered
grounds. The rain then transitions back to snow by tomorrow
evening.

&&

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

Low pressure south of the eastern Aleutians will bring rain and
areas of small-craft advisory level winds to the eastern half of
the Chain through Tuesday. As this system lifts toward the
mainland, a ridge will build behind it. This system will slide
east from the central Aleutians Tuesday to the west coast by
Wednesday morning.

Farther west, a weakening front continues to track across the
western Aleutians today. This system will lift northeast and phase
with the low lifting into the eastern Bering by Tuesday. Also on
Tuesday, the next gale-force front approaches the western Bering
with another round of widespread rain. The front moves over the
Pribilofs by late Wednesday morning and to the west coast by
Thursday morning. The surface low associated with the front moves
to Attu by Wednesday and lifts north to the Anadyr Gulf by
Thursday. Gales and numerous mixed rain and snow showers will
persist around the center of circulation.

&&

.MARINE (Days 3 through 5 Wed through Fri)...

There is increasing confidence that a triple point low will form
near Kodiak Island Wednesday morning. Uncertainty remains with the
position of this low in the Gulf. If it skirts the north Gulf
coast as opposed to a more southerly track, gale force winds will
occur through channels including Shelikof Strait, the Barren
Islands and through gaps and passes on the Pacific side of the
Alaska Peninsula through Wednesday. Thursday morning, a front
associated with a low in the eastern Bering will enter the western
Gulf bringing with it small craft winds. There is low confidence
that another triple point low will form in the northern Gulf
Thursday night. If it does, expect gale force winds through the
same channels listed above through Friday.

Beginning Wednesday morning, a gale force front over the central
Aleutians will continue to push eastward across the Bering. As
the low center tracks northeastward, there is potential for storm
force winds on the backside of the low Wednesday over the
southwestern Bering. This low will track northward and exit the
Bering by Friday. There is moderate confidence that another front
will enter the western Bering sometime Friday. Depending on the
strength and track of this front, gale force winds will be
possible in the western Bering through Friday.

&&

.LONG TERM FORECAST (Days 3 through 7 Wed through Sun)...

Models continue to show good agreement that a triple point low
will form near Kodiak Island Wednesday morning. Upstream, an
upper level ridge quickly moves through Wednesday into Thursday,
ahead of a large Bering low that pushes a strong front eastward
across the Bering. This front will likely impact the southwest
coast midweek, bringing precipitation and strong southerly flow.
The track of this low still remains uncertain, though models are
showing the center departing the Bering to the north by Friday.
This will be followed by another upper level ridge moving across
the Bering and into southern Alaska through Sunday. Similar to
last week, warm temperatures and moist conditions will advect
into the Aleutians and Bering following the backside of the ridge.

&&

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

&&

SYNOPSIS AND MODEL DISCUSSION...PD
SOUTHCENTRAL ALASKA...SB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...TM
MARINE/LONG TERM...EZ