Small Craft Advisory 157, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
438 PM AKST Sun Feb 18 2018

.ANALYSIS AND UPPER LEVELS...
The remnants of the warm front that brought snow and rain to the
Southwest Mainland overnight into this afternoon is continuing to
spread eastward into the Gulf early this afternoon. This development
is becoming more and more apparent with each scan from the Kenai
radar, and this trend is expected to continue and spread northward
through Cook Inlet by this evening. Behind the front, the cold/dry
air is readily apparent on satellite and radar imagery with an
abrupt shift from widespread snow to clearing and showers. Over
the Western-Central Aleutians a mature storm force front brought
a brief round of hurricane force wind gusts to the Adak area
earlier this morning. Cold/dry air is wrapping in behind the low
and heading toward the Central Aleutians, which is now expected
to bring another round of high winds to the area this evening.

&&

.MODEL DISCUSSION...
The models remain in generally good synoptic agreement through
Monday with the general pattern, though there remains some
uncertainty in particular with how much precipitation will move
into Northern Cook Inlet and the North Gulf Coast this evening
from the weakening front begins to fall apart. Radar trends this
afternoon however are beginning to bring snowfall slightly faster
than many of the models, though based on this trend there is
still good confidence in deteriorating conditions throughout Cook
Inlet this afternoon into this evening. The other question is with
how much precipitation is expected, as there is a significant
spread between the GFS which is mostly dry in the Anchorage Bowl
and the rest of the models that bring in light amounts. With good
southwest flow and moisture advection ahead of the snow and no
downsloping present to limit precipitation from reaching the
ground this dry GFS solution was ignored for the afternoon package
in favor of the light amounts from the rest of the guidance.

&&

.AVIATION...
PANC...A brief period of snow will bring temporary IFR
visibilities, though ceilings are expected to remain MVFR. After
04Z more unstable air arrives and expect a period of VFR conditions.
As this air mass settles in overnight, an upper level ridge will
build overhead, which will bring a quick return to stratus potential.
Depending on how much snow falls, this could come in earlier than
the current taf indicates. While fog can't completely be ruled
out, the cooler and drier airmass will limit temperature
inversion, so it seems more favorable for stratus with an IFR
ceiling.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A weak front across the central Gulf to the Susitna Valley moves
east overnight bringing a brief period of snow across the southern
mainland. Ridging builds in behind this front with drier
conditions returning through Wednesday afternoon. Patchy fog
along Cook Inlet is expected to develop each night with a stable
air mass in place, and should diminish during the late morning
hours. Winds will generally be light through mid week with
offshore flow gusty at times through Whittier and Seward from
weak gradient forcing.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
(Tonight through Wednesday evening)
An unsettled pattern will continue over the next few days. This
afternoon a weak front is currently moving across the southwest
mainland, and this front looks to reach the Alaska Range in the
next few hours, with drying conditions following in its wake. The
real question tonight will be patchy fog coverage, mainly south of
a Cape Newenham to Port Heiden line. Here, decreased coverage of
mid to high-level cloud cover may allow enough breaks for
radiational fog to develop. As such, some patchy fog was included
in the forecast. Further north, the cloud shield looks more robust
in coverage, mitigating the fog potential enough not to include
any mention of it.

Another front looks to reach the coast tomorrow afternoon,
sweeping eastward through the evening and into the overnight
hours, with fairly widespread rain and snow accompanying it. This
system will clear the area by Tuesday morning, as another brief
ridge of high pressure brings clearing skies with drier conditions.
This will be short-lived however (sound familiar?), as a third
front approaches by Tuesday evening, with rain and snow potential
once again on the increase.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
(Tonight through Wednesday evening)
A familiar pattern looks to continue through the next few days,
as a series of low pressure systems and their associated fronts
cross the region. The first such storm is crossing the western
Aleutians right now, with ship observations and the latest
scatterometer pass showing storm force winds accompanying the
low/cold front across the western Bering, through the western
chain, into the north Pacific. Winds just below storm force are
also seen along a northward advancing warm front which looks to
continue affecting the central portions of the Bering Sea/Pribilof
Islands through this evening. Widespread precipitation will
accompany both fronts as well.

The real story will be the air mass/wind gust potential
immediately behind the cold front. For this evening into the
overnight hours, the low, currently near 960 mb, will generally
remain at about the same strength as it treks northward toward
the eastern Kamchatka Peninsula. As it does so, a 120 kt mid-level
jet will cross the front across the west-central portions of the
Bering Sea and Aleutian chain. At the same time, a strong vort
max will rotate through the region with strong dry and cold
advection continuing, aided by a decent PV anomaly (on the order
of 500mb). This anomaly will just compress the air more, and when
combined with the dry/cold advection, enhanced downward mixing
potential will be enhanced. With this in mind and based on the
observations that winds over-achieved earlier this morning at Atka
in the typically weaker warm advection pattern, the high wind
warming for this area which was issued earlier is being maintained.
Hurricane force gusts are expected, with winds topping out in the
80 mph range. Further east toward the eastern portions of the
chain, storm force wind gusts are likely, with even weaker winds
(gales) toward the AKpen.

As this system moves northward through much of the north central
Bering tomorrow, the disturbance aloft substantially weakens,
with just storm force winds expected. However, a favorable fetch
length will keep seas elevated in the 30 to 35 feet range. A pair
of systems looks to approach the region for Tuesday and Wednesday,
but they will be considerably weaker. Widespread precipitation
will accompany each system from tonight through Wednesday, mainly
along the associated fronts crossing the region from west to east
through the period.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The general trend for the long-term forecast beginning the middle
of this week will be for continued active weather moving from the
Bering Sea and across Mainland Alaska into next weekend. This
will be produced by a flattening of the ridge that has set-up
over the North Pacific, allowing for a broad westerly jet to
stretch from the Aleutians through the AK mainland and into
British Columbia. With the more progressive zonal flow, the
primary challenge in the extended will be timing of each storm
system. With lower amplitude waves, this is a favorable pattern
for snow over Southcentral Alaska, including Anchorage. Southwest
will be more exposed to a brief warm-up with each system, so will
likely see a mix of rain and snow.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Storm Warning 177 178 185.
 Gale Warning 165 170 171 172 173 174 175 176 179 180 181.

&&

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