Gale Warning 208, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
524 PM AKDT Thu Mar 14 2019

.ANALYSIS AND UPPER LEVELS...
There is an upper level low centered over the northern Bering Sea.
A front associated with this low is over the Kuskokwim Delta and
Bristol Bay, moving slowly towards the northeast. There is a low
south of the western Aleutians, with a front draped across the
central and western Aleutians that is nearly stationary, while the
low itself is moving eastward. Southerly flow aloft is over
Southcentral, with a dissipating front and its associated showers
moving northward through the area. A strong low is situated well
south of the Alaska Peninsula, moving towards the north. A well
developed front associated with this low is over the Gulf, moving
rapidly northward towards the coast.

&&

.MODEL DISCUSSION...
The numerical models are in rather good agreement through
Saturday afternoon. There begin to be some minor differences on
the exact location of some surface low locations by Saturday, but
with the GFS and ECMWF in good agreement, the forecast confidence
is a bit higher than normal. The one model exception is the 12z
NAM, which begins to differ on the location of several surface
lows starting Friday night.

&&

.AVIATION...
PANC...VFR conditions will persist. Gusty southerly winds will
diminish late tonight as another system approaches from the south.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
(Tonight through Saturday evening)

A rather wet and unsettled period is in store for coastal areas,
with locations in the favored down-slope areas seeing relatively
mild and dry conditions. Our next storm looks to move into the
region around mid-morning Friday, when a warm front reaches
Prince William Sound and the Kenai coast extending southwestward
through Shelikof Straight. This occurs as a ~976 mb low heads for
the western tip of Kodiak Island, sending a warm front northward.
Strong 850 mb theta advection is very pronounced in the model
cross sections, with 700 mb RH progs pushing 100%, and 850 mb
(just under 4800 feet) temperatures climbing to 0 deg. Celsius.
This front will continue to push northward through the day, with
southeast winds aloft increasing. This then in turn allows for a
rain shadow effect to develop (like usual) for the western flank
of the Chugach and Kenai Mountains, and Copper Basin north of
Thompson Pass. Meanwhile, further inland across the western
Susitna Valley, look for fairly widespread precipitation as
orographic lifting lends a big hand.

The other story with this storm is the winds, as once again we
expect winds to increase into the 45 to 60 mph range for the
higher elevations, including the Upper Hillside in Anchorage.
Some of the models indicate gusts near 75 mph possible and this
may need to be included with later forecasts. For now, we kept the
gusts under 70 mph as there's some question as to the strength of
the downward mixing potential, as the bulk of the 500 mb vorticity
and potential slantwise instability looks to be juxtaposed to our
west across Southwest Alaska. Still, this is another component to
be watched in later model runs.

Four our mariners, sustained storm force winds look to develop
south of the Copper River Delta. Sustained Gales are also a solid
bet for much of the Prince William Sound Coast, lower to middle
Cook Inlet, and Shelikof straight. Here, seas will peak in the 15
to 25 feet range. Conditions will briefly improve for Saturday
evening, before our next storm quickly moves in on its heels for
Sunday.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2...Tonight
through Saturday afternoon)...

An upper level low positioned north of the Pribilofs is in the
process of lifting northward. This has brought some snow to the
Kuskokwim Delta this afternoon, which will eventually taper off
from west to east as it spreads inland through the evening. The
next key features include a North Pacific low moving into the Gulf
of Alaska and a second upper level low moving into the Bering
from Kamchatka. As the front associated with the North Pacific low
lifts over the AKPEN from the east beginning Friday afternoon,
precipitation will initially start as snow. As the upper level
Kamchatka low reaches the central Bering and interacts with the
front, a deformation band is expected to set up over Southwest,
oriented from north to south from Dillingham to Aniak. This will
be where the heaviest precipitation is observed. However, knowing
that the front associated with the low in the Gulf will generate
east to southeast flow (which favors a downsloping pattern),
areas on the west side of both the Aleutian and Alaskan Ranges may
see little to no precipitation at all.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2...Tonight
through Saturday afternoon)...

An overall convective and showery regime continues over the
Bering. An upper level low centered over Saint Matthew Island will
lift northward Friday afternoon allowing for a second upper level
low to arrive into the Bering from Kamchatka by Saturday morning.
As this low extends further south than the first, it will allow
for continued cold air advection and an unstable airmass to
persist. Meanwhile, a front associated with a North Pacific low
extends northward over Adak for Friday. Temperatures will be warm
enough for a rain/snow mix over the central Aleutians, though the
low will quickly exit to the south by Friday evening.

&&

.MARINE (Days 3 through 5/Sunday through Tuesday)...

The Gulf of Alaska will continue to see an active storm track
with several systems moving into the area from south to north
between Sunday and Tuesday. There decent model agreement with a
system on Sunday with good confidence that an easterly gale force
barrier jet will develop along the North Gulf Coast in the AM with
southerly gales developing across the open Gulf later in the day.
Winds will diminish below gale force on Monday ahead of the next
system which is expected sometime between Monday night and
Tuesday. Model agreement falls apart with this storm, thus
forecast confidence diminishes past Monday.

The Bering Sea is expected to remain generally quiet with continued
weak northerly flow as high pressure builds in. A low is expected
to push east just south of the Aleutians, potentially bringing low
end gale force winds to the coastal waters on the south side of
the central and eastern portions of the Aleutian chain Monday
into Tuesday.

&&

.LONG TERM FORECAST (Days 3 through 7/Sunday through
Thursday)...

The long term outlook continues to show a highly amplified and
active pattern with persistent mean upper-level troughing over the
Bering and an upper-level ridge axis building across western
Canada and the US west coast, eventually building north over the
Alaskan Panhandle. This setup will keep the storm track south of
the Bering with persistent southerly flow directing energy north
into the Gulf of Alaska and the southern mainland. The active
storm track will be supported by a strong southerly jet which will
pump warm moist air north from the sub-tropics. This will lead to
periods of heavy precipitation, especially along the coast,
above average temperatures, and rising snow levels approaching
2000+ ft by early next week.

Though confidence is high regarding the longwave pattern, models
are still struggling quite a bit with specific features embedded
within the overall flow, especially after Monday. Thus, confidence
in timing and location of individual systems is still low at this
point.

Looking past Thursday, ensemble solutions add extra confidence to
the pattern persisting well into next week, with day 8 maximum
positive 500 mb height anomalies centered over west central Canada
and minimum negative anomalies centered between the Aleutians and
Hawaii. This will generally keep southerly flow directing storms
north into the Gulf of Alaska.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning 120 125 129 130 131 138 139 351.
         Storm Warning 119.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...BL
SOUTHCENTRAL ALASKA...PD
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KO/SEB
MARINE/LONG TERM...KV