Small Craft Advisory 20, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
421 PM AKST Sat Jan 6 2018

.ANALYSIS AND UPPER LEVELS...

The pattern consists of a broad long wave trough over the North
Pacific and Bering Sea. There is a closed blocking high over the
interior of Alaska. This is causing a triple point low/front over
the North Gulf Coast this afternoon to rapidly weaken and
elongate as it tries to move inland (aka frontolysis). Over
Southwest Alaska, the remnants of the first low has moved into
inland Bristol Bay and weakened. However, rain and snow is still
impacting a large part of Southwest Alaska as the front itself
stalls and weakens in place. Over Southcentral, the effect of the
upper blocking high is more pronounced with dry air and the
easterly flow component in the mid levels keeping most
precipitation relegated to the immediate coastline.

The next major weather feature, expected to move into the Bering
Sea Sunday and Monday, is already forming on satellite imagery
near 40N latitude south of Kamchatka. This low will undergo over 1
Bergeron of rapid deepening (defined as 24 mb in 24 hours at
latitude 60N) as it moves into the Bering as a storm force low.

&&

.MODEL DISCUSSION...

The numerical weather models are in excellent agreement with the
large scale synoptic pattern for the next 5 days. They are also in
excellent agreement with a deep cyclone moving into the Bering Sea
Sunday and Monday, and are in great agreement with the overall
sensible weather pattern for mainland Alaska through Tuesday.
Forecast confidence is extremely high.

&&

.AVIATION...

PANC...VFR conditions and light winds will persist through the
forecast period.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A weak weather front currently bringing precipitation to areas
along the Gulf Coast will stall out this evening as it weakens
which should slowly taper off the rain and snow along the coast
tomorrow. A surface low behind the front will continue to move
north through the Gulf and weaken through the day tomorrow. This
will bring continued cloudy weather to most of Southcentral but
inland locations will remain dry. This should help to keep
temperatures mild and slightly above normal through tonight and
tomorrow. By Monday, an upper level ridge to the north will begin
to drift south and take over as the predominate weather maker
across the region as clearing skies and offshore flow will lead to
much colder and drier conditions through at least the first half
of next week.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Weak troughing and onshore flow will keep showers along the
Kuskokwim mountains and areas south through most of tonight. A
weak, but transient ridge will slide in from the west and allow
the region to dry out through Monday night. Colder air that is
being brought south with a weakening low in the Bering will
invade the southwest into the short term and beyond. 850mb
temperatures will be in the -6 to -8 range on Monday and Monday
night. With clearing skies, highs in the Kuskokwim Valley will be
falling from the mid to lower 20s to the teens on Monday.
Overnight temperatures are expected to dip below zero. For the
Kuskokwim Delta and Bristol Bay the same downward trend in
temperatures are expect, but they are not going to be nearly as
extreme.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
Low pressure currently in the Western Bering will continue to
weaken and lift north through Sunday afternoon. A new low will be
lifting across the Western Bering late Sunday evening and into
Monday. At this stage, it is looking nearly identical to the
system that just impacted the region these last few days. A strong
front will sweep from west to east, bringing storm force wind
gusts across the island and marine areas. High pressure across
the interior will again impede the movement of the front deep
into Southwest Monday night. With this system, modest rainfall can
be expected with the front as it moves along the chain. Weather
behind the front see the typically expected windy and showery
conditions.

&&

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

The long range forecast beginning on Monday night through the end
of the work week generally keeps drier and colder conditions
across the southern mainland with an active windy and
precipitation pattern along the southern Bering and Gulf of
Alaska. On Monday night, an Arctic ridge shifts south across the
interior advecting a cold dense air mass toward the northern
Gulf. Dry offshore flow increases late Monday night through
Tuesday bringing gusty winds across typical gap locations.
Meanwhile, a powerful low pressure system slowly weakens over the
Bering.

Wednesday through the end of the week...a slow transition in the
pattern begins as the subtropical jet flattens out with strong
zonal flow setting up just south of the Aleutian Chain. The jet
sends the next North Pacific low further to the east, than
previous systems, before taking a northward turn. This low works
against the ridge to break it down. There is still a bit of
uncertainty on how quickly the ridge influence will be overridden
by low pressure. A nearly equal blend of operational models
including their ensembles was used for updating the forecast mid
week, then trends toward ensemble solutions for the latter part
of the week. Temperatures should moderate slightly warmer at the
end of the week for the southern mainland with continued active
weather across area waters and coastal communities.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Storms 173-177 413. Gales 150 155 165 170-172 178 179 411
                                   412 414.
&&

SYNOPSIS AND MODEL DISCUSSION...JA
SOUTHCENTRAL ALASKA...KVP
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...SS
LONG TERM...KH