Small Craft Advisory 167, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
457 AM AKST Wed Feb 21 2018

.ANALYSIS AND UPPER LEVELS...
The pattern, in the short-term and long-term, remains much
unchanged. Yet another transient shortwave ridge is passing
through the southern half of AK. The ridge has worked in tandem
with lingering low-level moisture to produce another night of
widespread fog and stratus. Meanwhile, the leading warm front
associated with the next shortwave is pushing into Southwest AK.
The leading edge of the warm front brought some very light snow
that is now steadily turning over to rain. It is also ushering in
yet another round of very moist air. Precipitable water values
over the North Pacific range from 0.8" to 1.0". This airmass is
posed to spread further east across all of southern AK. At the
20,000' level, the jet continues to support this transient
pattern. It comes into SW AK at about 130 kt before riding up and
over the ridge at 110 kt. This will serve to not only keep the
pattern moving, but also to keep plenty of moisture and mild air
across the state.

&&

.MODEL DISCUSSION...
Models continue to be in rather good agreement regarding the
synoptic pattern through the short-term (48hrs). The result is
moderate forecast confidence in the main weather features.
However, the fog and stratus continues to be very challenging to
nail down as it ebbs and flows with each passing upper-level wave.
The models have reached more of a consensus with the snow event
across Southcentral tonight into Thu. The EC continues to be the
most aggressive pushing the upper-level trough into a negative
tilt. However, the other models all concur that it will more
likely be neutrally tilted (due north to south). We continued to
fine-tune the forecast in that neutral tilt direction. This will
impact the Turnagain winds and the precipitation type for the
Anchorage Bowl (see Southcentral portion below for details). It
also means the pattern will continue to move quickly from west to
east, thus limiting snow totals. All models agree that by early
Fri a decent offshore cold air advection pattern will set up. This
will lead to rapidly clearing skies and gusty winds through many
of the gaps.

&&

.AVIATION...
PANC...VFR conditions are very precarious this morning as fog and
low stratus lurk across the area. With minimal change to the
synoptic pattern, could still see the potential of the terminal
dropping to LIFR at any time this morning. By this afternoon,
that changes as southeast winds (and mixing) start to increase in
advance of the next system. The Turnagain Arm winds will start to
blow over Fire Island late this evening. At exactly what time and
how strong they make it into the airport remains somewhat
uncertain, but it looks like after midnight. This should allow
precipitation to become a rain-snow mix. Then, as the winds die
off in the early morning and precipitation turns to all snow,
except cigs and vis to rapidly deteriorate.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A rather "dirty" ridge (considerably cloudy) will be over the area
today. Some patchy fog remains near Cook inlet this morning, but
should dissipate by afternoon. A system approaching from the west
will enter Southcentral Alaska tonight. This will bring
precipitation from west to east across the area tonight and
Thursday. The precipitation will generally be in the form of
snow north of Anchorage. For the Kenai peninsula, the snow will
mix with or change to rain fairly early around Kachemak Bay, and
then some rain may mix in by late morning further north. For
Anchorage, some southeast winds are possible off Turnagain arm.
This will raise the temperature and could limit the precipitation
potential as well. However, there is enough moisture and dynamics
that some snowfall is expected, with the current forecast
indicating amounts somewhere in the 2 to 4 inch range. Higher
amounts are expected further north in the Matanuska and Susitna
Valleys. The trough will push through the area in the late
afternoon and evening. Strong offshore winds will then develop as
a low forms in the Gulf and high pressure builds in from the
west.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...

A warm front is moving through Southwest Alaska this morning with
a mix of rain and snow. The rain transition has moved through the
Kuskokwim Delta with all snow currently falling in the Lower
Kuskokwim Valley. Eventually the warmer air will also make it into
the Kuskokwim Valley by afternoon where snow will mix with rain.
Any snow accumulations will be in the morning and will generally
be less than 1-2". The trailing cold front will move east tonight,
but most precipitation will be pre-frontal, so most of this will
fall as rain with a brief period of potential back-end snow, but
accumulations will be minor although the western slopes of the
Alaska Range will see 5-10" of snow. Cold air advection will be
rather intense behind the front, so temperatures Thursday night
will drop well below freezing again as high pressure settles in.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...

A gale force low over the western Bering Sea/Aleutians will slowly
move east through tonight as it moves inland along the west coast.
Immediately trailing it will be another front moving into the
western Bering Sea which will bring another swath of gale force
winds to most of the Bering Sea Thursday and Friday as the surface
low moves toward the Bering Strait. This will bring widespread
rain to the entire chain and the Pribilof Islands.

&&

.LONG TERM FORECAST (Fri through Tuesday) (from Tue afternoon
discussion)... Expect the active pattern to continue with near to
above normal temperatures, periodic clouds/snow with dry and
colder periods between.

These conditions will hold on Friday, slowly weakening, as the
next system pushes into the area from the southwest overnight
Friday into Saturday. Guidance is struggling with how to track the
triple point low as the front moves into the Gulf, and hence how
far north to track the frontal boundary. The front should make it
to the coast and most of the Cook Inlet region, bringing another
few inches of snow to these areas. A return to short- lived
outflow conditions comes late weekend when that system exits.

Into the extended forecast, the upper-level southwesterly flow
remains. The progressive nature of the pattern will continue
through next week, with front and lows rolling into the region
every 1-2 days. At this point, the low amplitude pattern should
stop any big warmups from happening, any warmer air associated
with the fronts should slide to the east with the triple-point
lows.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale Warning 119 120 130 131 165 170 173-179.

&&

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