Small Craft Advisory 345, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
501 PM AKDT Fri Apr 27 2018

.ANALYSIS AND UPPER LEVELS...
The fairly active weather pattern continues over southern Alaska
and the Bering Sea. There is a negatively tilted upper level
trough stretching from the northern Gulf to Norton Sound. This is
bringing precipitation to much of southern Alaska, mainly in the
higher terrain and areas near the coast. There is an upper level
low over the west central Bering Sea that is rapidly moving east.
The associated front is roughly situated from the Pribilofs
through the eastern Aleutians. Some ridging is evident over the
southwest Bering and western Aleutians. Another weather system is
spinning up well to the southwest of the western Aleutians, moving
rapidly to the northeast.

&&

.MODEL DISCUSSION...
The numerical models are in reasonable agreement for the short
term portion of the forecast (Sunday afternoon). The notable
exception is the ECMWF on Sunday. Most models take a surface low
across the Bering from southwest to northeast, bringing it near
the Yukon Delta Sunday afternoon. The ECMWF, however, takes it
almost strait north, with it centered near 60 degrees north
latitude and 180 degrees longitude. For now a solution more in
line with the GFS and Nam was preferred. Forecast confidence is
thus a little below normal.

&&

.AVIATION...
PANC...Generally VFR conditions will continue into Saturday with
some occasional MVFR conditions possible this evening and then
again Saturday afternoon through evening. This should be brief if
it does occur. Breezy southerly winds will diminish late this
evening, then increase again Saturday afternoon.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Rinse and repeat, or should I say wind and repeat? The weekend
looks very familiar with several systems that will bring breezy
conditions across town. The unstable air mass aloft, responsible
for showers the past couple days will be moving out this evening
as the upper trough pushes to the east ending the shower activity
across the Cook Inlet region first, lingering a bit longer along
the north Gulf coast. Some of the more intense showers may mix
with or change to snow. Saturday brings a more stable air mass, so
no chance for the convective snow showers that we've been seeing.
The main concern will be continued gusty winds. Winds will peak
both Saturday afternoon and Sunday afternoon, however, nothing
like our previous wind storm. Gusts should peak into the 40s at
the higher elevations on Saturday and 50s on Sunday as well. Down
at lower elevations, gusts aren't expected to get out of the 30s.
Light rain will accompany each system, with Sunday being a bit
more moist that Saturday. Snow snow is possible during the
overnight hours, but daytime temperatures will be warm enough to
support rain. The progressive pattern continues into early next
week. Expect breezy afternoon conditions, mostly cloudy skies, and
high temperatures in the upper 40's.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2: Saturday and
Sunday)...
A series of three disturbances will move across the area through
the short-term portion of the forecast. The first disturbance has
mostly cleared the area already, but is still setting off
scattered to numerous rain and snow showers across the Lower
Kuskokwim Valley and the Alaska Range mountains. This disturbance
will clear the area to the north tonight.

The second disturbance is following very close on the heels of the
first, with high clouds already overspreading the Kuskokwim Delta
and Bristol Bay regions. The disturbance is being aided by a
pocket of colder air aloft. That along with the associated
precipitation falling during the coldest time of day should mean
most areas away from the coast see mostly snow overnight as it
moves through. Fortunately, the disturbance is also fast-moving,
and will be past the area by Saturday afternoon. Most locations
can expect anywhere from a trace (near the coast where a mix with
rain is expected), to 2 inches of new accumulation (higher
elevations of the Lower Kuskokwim Valley). The westerly winds
behind the disturbance may keep mountain snow showers continuing
into the late afternoon on Saturday.

Finally, disturbance number three begins moving into Southwest
Alaska from the Bering after midnight Saturday night. Unlike the
previous two, this disturbance will have much more warm air with
it, as the cold air stays north and west over the Bering. Thus,
precipitation-type issues should only be a factor over the
mountains. The heaviest rain with this disturbance are expected
Sunday morning. It too will quickly slide across the area, with
most of the associated precipitation over with in the eastern
parts of the area before nightfall. Sunday night should be
relatively quiet with the exception that continued south to
southwest winds will keep mountain snow showers ongoing right
through Monday.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2: Saturday
and Sunday)...
The Bering forecast is following the same disturbances mentioned
in the Southwest forecast. Disturbance two is currently moving
into the eastern Bering, and will move into Southwest Alaska
tonight. The third disturbance quickly moves into the western
Bering and Aleutians Saturday morning, reaches the eastern Bering
Saturday night, then moves into Southwest Alaska. South and east
of the parent low, which tracks from the western Bering to the
northern Bering by Sunday morning, sustained winds well into gale
force, with storm force gusts are expected. This very progressive
pattern should result in highly changeable weather through
Saturday night. On Sunday behind disturbance three, another upper
level low moves into the western Bering on Sunday afternoon. That
will keep the strong westerly winds ongoing over much of the
Bering into Sunday night.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Beginning Tuesday and Wednesday, a slow pattern shift begins to
evolve in the upper level jet stream, initially a ridge amplifies
across the Alcan border with troughing across the north Pacific
and Aleutian Chain. This jet stream shift cuts off the mid level
low over the northwest mainland, which weakens in response as it
slowly spins along the Brooks Range. Models are all in descent
agreement with a slow progressive change of the jet stream through
the end of the week; however, there are significant differences
between the low and mid level dynamics which lead to less
confidence in the extended forecast through the end of next week.
Overall, the storm tracks are expected to move at a slower pace
the the current pattern as amplified ridging separates each
system. Also, storm systems are expected to track along or just
south of the Aleutian Chain as the jet stream migrates further
south.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 150 155 165 170 172-179.

&&

SYNOPSIS AND MODEL DISCUSSION...BL
SOUTHCENTRAL ALASKA...MTL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JW
LONG TERM...KH