Small Craft Advisory 272, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
355 PM AKDT Sat Mar 31 2018

.ANALYSIS AND UPPER LEVELS...
A large and strengthening high pressure center over the eastern
Bering is the most dominant feature on the morning weather map.
This high is producing northerly/offshore winds and plenty of
sunshine across the majority of southern Alaska. There is a
shortwave coming down the front/east side of this high pressure
center that is on the verge of bringing some mid-level clouds to
the Susitna Valley and Cook Inlet areas later this afternoon, but
all the thicker clouds and snowfall are being confined to the
north side of the Alaska range. Meanwhile, light onshore flow over
the northern half of the Kuskokwim Delta has allowed some low
clouds and fog to squeeze into the area from the Bering Sea, but
as flow becomes offshore this afternoon these conditions should
improve.

Way out west, a storm in the North Pacific has an associated front
stalled over the central and western Aleutians. This front is
bringing generally 20-30 kt winds with rain slowly moving into the
area. Due to the dominant aforementioned high pressure center this
front is basically not going to make any addition progress
northward after this afternoon, so while locations from Dutch
Harbor westward will stay fairly cloudy and showery for the next
few days, locations along the AKPEN will remain considerably nicer.

&&

.MODEL DISCUSSION...
The models are in very good agreement over the next few days.
Thus, the higher resolution models were utilized to best capture
the locally enhanced winds moving through the complex terrain of
Southern Alaska.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The primary concern with the short term forecast through Tuesday
is the outflow winds through the coastal mountains. The strong
high and ridge over western Alaska coupled with an upper level
trough dropping south through the eastern interior brings the
potential for strong gusty winds through the Chugach Mountains
and Alaska Range. Once this upper trough drops into the Gulf of
Alaska an inverted surface trough begins to form along the North
Gulf Coast. The missing ingredient for this to elevate to warning
level is deep cold air over the interior that typically enhances
the downward vertical motion. Thus we will shy away from any
advisories or warnings.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Following the passage of an extremely weak shortwave trough
bringing some clouds and the chance for a few snow flurries to
northern areas tonight through early Sunday, upper level ridging
will strengthen along the coast and build north through Monday.
Offshore flow will begin strengthening through the day Sunday with
breezy northeasterly winds Sunday night through Monday keeping
skies clear and humidities low. Offshore flow will weaken somewhat
on Tuesday as the upper level ridge moves further away to the
northwest and cold upper level troughing builds into interior
Alaska.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
A west to east oriented frontal system over the north Pacific is
lifting northward and will reach the central and eastern
Aleutians this evening. The front will start shearing apart as it
continues slowly north into the southern Bering Sunday and Sunday
night. Despite the parent low to the south not being particularly
strong the compression of the pressure gradient up against the
strong ridge over the Bering will continue to bring Gales to the
Aleutians and southern Bering through Sunday night. The very large
upper level ridge with its axis currently stretching over the
eastern Bering will slowly move north to be centered over Russia
by Tuesday.

&&

.LONG TERM FORECAST (Days 4 through 7...Wed through Sat)...
This timeframe will feature a transition from a ridge of high
pressure near the Bering Strait giving way to a trough of lower
pressure developing over Southern AK. This will open the door for
some energy to slide across the mainland from east to west. While
there still remains uncertainty in exactly where the energy
consolidates and reflects at the surface, there is moderate
confidence that there will be some noticeable changes. Even though
we will be in the first week of April, 850 mb (5,000') temperatures
will hover between -10C and -20C as the airmass comes from the
Yukon Territory. This could help push our morning lows below
normal, especially if skies clear. Outflow winds will also cease
by this timeframe as the ridge of high pressure moves away. This
will bring a much welcome break from the wind for places like
Whittier, Seward, and Valdez.

Exactly where the east to west energy establishes itself will be
key for precipitation and cloud cover. The GFS is a bit more
aggressive forming a closed low over the Alaska Peninsula Wed.
This would result in better precipitation (mostly snow) chances
for SW AK. Meanwhile, the EC and GEM models keep surface features
a bit weaker and further south. So precipitation in that scenario
would favor mainly coastal areas.

As we approach next weekend, all models show a potent (mid 960
mb) low tracking quickly from west to east along the Aleutian
chain. This would bring a healthy swath of precipitation and
gale-force winds to the chain. It should also then bring a better
chance for warming temperatures and precipitation to at least
Southwest AK. The closed high over the Arctic still remains in
play though, so things remain less certain for Southcentral AK.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale Warning 170 172 173 174 175 176.

&&

SYNOPSIS AND MODEL DISCUSSION...AMD
SOUTHCENTRAL ALASKA...SA
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JER
LONG TERM...MSO