Small Craft Advisory 263, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion...CORRECTED
National Weather Service Anchorage AK
703 PM AKDT Fri Mar 20 2020

.ANALYSIS AND UPPER LEVELS...
An upper level short wave trough which brought snowfall to
Southcentral overnight has moved into western Canada. Subsidence
on the backside of the feature has brought some clearing to
Southcentral. Broad moist southwesterly flow in the lower levels
produced widespread stratus and some fog to much of Southwest
Alaska today. Abundant moisture associated with a broad frontal
zone is evident over the north Pacific. Satellite imagery shows a
well defined upper jet couplet with a band of cirrus extending
from over the central Aleutians, to the Kuskokwim Delta.

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.MODEL DISCUSSION...
Models overall are in good synoptic agreement on the phasing of
the north pacific moisture feed and eastward moving upper short
wave across the Bering Sea/Mainland Alaska tonight and Saturday.
The challenge will be on snow amounts and precipitation types and
timing of the cold air across Southcentral Saturday.

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.AVIATION...
PANC...VFR conditions are expected tonight. MVFR to IFR
conditions in snow will likely develop Saturday morning and
continue into the evening hours. Light winds are expected.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
After a mild spring like day, a front is on tap to bring
widespread snow across Southcentral for Saturday. A long fetch of
subtropical moisture spreads clouds from the North Pacific in
advance of an approaching Bering front. The frontal boundary
should move across to the western Gulf and Cook Inlet early
Saturday morning spreading snow across the much of the region by
mid day. Snow develops a little later, Saturday evening, for the
Copper River Valley as the front focuses on a developing low over
the Central Gulf Saturday afternoon. Snow tapers off Saturday
night followed by dry conditions Sunday as a ridge builds from the
West. An offshore flow regime sets up Saturday night as we
transition from an exiting low to a building ridge. Gusty winds
through channeled terrain along the coasts are expected in
response with blowing snow possible in the Thompson Pass location.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
Snow will develop across Southwest Alaska late this evening and
persist through much of Saturday as a front tracks across the
region. Highest totals are expected over the Bristol Bay and
Middle Kuskokwim Valley areas near the base of a deep upper
through. However, these totals will likely not exceed 4 inches in
any populated areas. Skies will clear and temperatures will
slightly fall Sunday through Monday under strong high pressure.
Scattered snow showers will continue across the Middle Kuskokwim
Valley through Monday under a series of upper level shortwave
troughs.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
Rain will continue this evening across the eastern Aleutians as a
front quickly moves over the area. Strong high pressure will
develop Saturday afternoon and persist into Monday bringing mostly
clear and dry conditions. Showers will redevelop Monday afternoon
ahead of a low. Partly cloudy skies will continue across the
western Bering/Aleutians, with precipitation beginning Sunday
evening on the north side of the aforementioned North Pacific
low.

&&

.MARINE (Days 3 through 5)...
Models remain in good agreement on a ridge of high pressure
remaining in place over the eastern Bering through the early part
of next week keeping the weather quiet. A weakening North Pacific
low continues moving northward over the central Aleutians Sunday
into Monday. This will create some elevated southeasterly winds
through gaps and passes along the central and eastern Aleutians
through Monday due to the tight pressure gradient with the strong
high over the eastern Bering. As this system weakens significantly
and the high moves eastward, generally quiet conditions area
expected across the Bering for Tuesday.

Meanwhile for the Gulf, a building high over the eastern Bering
and a departing low centered over the southeastern Gulf will
generate elevated winds through Sunday in usual gaps and passes,
including Kamishak Bay and the Barren Islands. Winds will diminish
throughout the day on Monday as high pressure moves in, leading
to generally quiet conditions precipitation wise but a medium
pressure gradient will be in place Tuesday and Tuesday night
leading to some areas of small craft advisory winds.

&&

.LONG TERM FORECAST (Days 3 through 7)...
There remains strong confidence that a strong upper level ridge
will be in place over the southern half of the state early next
week. An upper level low is expected to move north towards the
central Aleutians from the North Pacific weakening as it does so.
This upper level low will eventually force the ridge to weaken
and move southward by midweek, opening up a zonal flow pattern
aloft. By the end of the week, models continue to suggest a
strong low will approach the Bering from the west with a track
just west of the Aleutian Chain. There still remains uncertainty
with the exact placement of the low but models continue to show
better agreement on this feature leading to moderate confidence in
the forecast.

Further east across South Central and Southwest Alaska, models
are in disagreement on what the upper level pattern may look like
across the Mainland. Some guidance shows an upper level trough
diving southward while other guidance shows an upper level ridge
leading to a tough temperature forecast. Also of note, each model
differs on the strength of this upper level trough leading to low
confidence in the long term.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning 127 130>132 136 150.
 Heavy Freezing Spray 139 185.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...RC
SOUTHCENTRAL ALASKA...KH
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AP
MARINE/LONG TERM...MV