Small Craft Advisory 154, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
433 PM AKST Fri Feb 16 2018

.ANALYSIS AND UPPER LEVELS...
Satellite imagery today is indicating a broad area of cloudcover
over much of the Southern Mainland, however this has primarily
consisted of upper level clouds, as much of the lower level
clouds and precipitation is centered around a low moving through
the Southern Gulf. This low is far enough south that outflow winds
along the North Gulf Coast have generally been relatively minor
(below gale force) and flow down Cook Inlet weak enough for fog
and low stratus to persist across Northern Cook Inlet/Anchorage
and on up to the Matanuska Valley. Over the Southwest Mainland,
snowfall from the upper level trough that brought a few inches of
snow to Bristol Bay and the Kuskokwim Delta has moved off to the
east, with rapid clearing behind it as weak high pressure moves
in to replace it. This high pressure is quickly beginning to move
over the Mainland in advance of a gale force low that is tracking
toward the Eastern Aleutians/AKpen.

&&

.MODEL DISCUSSION...
The models remain in generally good agreement until Sunday when
they begin to diverge significantly with the track of a frontal
system moving from the Southwest Mainland into the Gulf. The 12z
NAM and GFS were in significantly better agreement in pinching
this low off in the Gulf, and keeping the bulk of the
precipitation south of the North Gulf Coast. Since then the 18z
NAM has come in significantly farther north than any other model
run today, bringing snowfall into Southcentral for later in the
day on Sunday. Since this is such a strong outlier and a large
difference from the previous run, this solution was treated as
an outlier in favor of the much drier GFS which would keep
Southcentral dry and sunny.

&&

.AVIATION...
PANC...The forecast for tonight is challenging, with a threat for
redevelopment of fog in the area. The 00Z Anchorage sounding shows
a shallow, but steep inversion with increasing northerly winds
above the inversion. The big question is whether there is enough
low level wind to mix out the top of this inversion and disturb
the processes which lead to fog formation. This should become
apparent by early this evening because if fog is going to form
there should be some signs of it within a few hours after the sun
sets. For now, have maintained the existing forecast which calls
for LIFR ceilings, but will modify this if don't see development
by around 03-04 UTC. If fog does form, it should hold through mid
morning Saturday.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Dry conditions are expected across Southcentral through Sunday
afternoon as a ridge moves overhead from the Southwest. High
pressure building in from the Alaska Peninsula centers over the
southern mainland late Saturday bringing mostly clear skies
through much of the weekend. Gusty offshore flow increases through
typical gap coastal locations this evening in response to the
building ridge, however gusts taper off by late Saturday afternoon
as the ridge center shifts directly overhead.

On Sunday, a front moves across the southern Gulf increasing
chances for precipitation for Kodiak Island and the southern
Kenai Peninsula. Precipitation is expected to be light along the
weak front as the associated upper level wave shears apart when
it runs into the amplified ridge across the southern mainland. The
low driving the front will have little impact as it tracks along
the southern Gulf through late Sunday bringing wind shifts and
small craft gusts.

&&

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

Skies will continue to clear across the lower Kuskokwim Delta,
with the remainder of the region already seeing clear skies. This
trend will be courtesy of a short-wave ridge building overhead
tonight, allowing for ideal radiational cooling to set up. The
increasing temps aloft will strengthen the nocturnal inversion,
and given the recent snowfall which had a higher water content,
look for areas of patchy fog to develop. This fog may be locally
dense at times along the Kuskokwim River.

As the ridge shifts eastward for the weekend, temperatures will
moderate as a southeast offshore flow develops on the backside
of the high and ahead of another incoming front. This front looks
to reach the coast Saturday night, reaching the Alaska Range by
Sunday night. Widespread precipitation will accompany this
feature, with snow being the initial precipitation type. However,
as warmer air mixes down to the surface in a continued warm
advection pattern, the snow will change over to rain for most
locations, even inland, before cold air filters into the area
immediately behind the front where a brief change back to snow is
expected. This front will also bring strong winds gusts to the
coast and higher elevations. Drier conditions will return for
Monday.

&&

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

The parade of storms much of the Bering and Aleutians have seen
will continue through early next week. Today's system spinning
across the northwest Bering will continue to lift away from the
region. However, another weak disturbance will approach the chain
from the south, as it rides along a surface trough extending
southward from the Bering low. As this next low lifts northward
through the next 48 hours, the surface trough will push eastward
with widespread precipitation and increasing winds accompanying it
for eastern portions of the Bering Sea/Aleutians and along the
AKpen.

Further west, showers will increase in coverage as a strengthening
low moves across the western portions of the chain into the
western Bering Sea. This system will also send a front eastward
with near storm force sustained winds and moderate to heavy
precipitation (mostly rain) developing along and immediately ahead
of it, with the waveheights increasing as well. The front should
reach the east central potions of the Aleutians by Monday, with
winds subsiding and precipitation becoming more showery in nature
behind the frontal passage.

&&

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

Long term period starts off with increased uncertainty due to
models suddenly diverging with the movement of the low across
Southcentral Alaska and the Gulf of Alaska. Most reasonable
approach to Sunday night through Monday is to nudge precipitation
chances down over most of Southcentral Alaska and increase them
some in the Gulf. The Bering Sea storm at that same time frame
seems to still be on track with a deep low in the western Bering
Sea and general south to southwest flow over most of the Bering
Sea into Southwest mainland Alaska. After Monday, there is still
a high likelihood that there will be a general progressive
pattern over that region with a high amplitude upper level ridge
building over the mainland part of the state which will usher in
warmer temperatures. This ridge looks to get displaced later in
the week by a trough and hence the progressive pattern. The big
uncertainty remains the timing of these features, as models have
not been consistent with one another or even with themselves on
a run-to-run basis. So what does all this mean for the sensible
weather next week?: A warm-up for the middle of the week with a
chance of precipitation. Precipitation type may be mixed for
inland areas in both Southwest and Southcentral Alaska mid-week
so that will be something to keep an eye on.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale Warning 150 155 160 165 170 171 172 173 175 176 178
179 180 181 185.

&&

SYNOPSIS AND MODEL DISCUSSION...DEK
SOUTHCENTRAL ALASKA...KH
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD
LONG TERM...EZ