Gale Warning 254, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
525 AM AKDT Fri Mar 15 2019

.ANALYSIS AND UPPER LEVELS...

An upper level low in the northern Bering is creating a broad
trough that encompasses much of the Bering. Cyclonic flow is
advecting cold air over the western and central Bering. A
vertically stacked low is advancing toward the southwestern Gulf
of Alaska, however it still remains well south of Kodiak Island.
General southerly flow aloft is ushering in warm and moist air to
the Gulf of Alaska.

In the Gulf, a front is draped along the northern and western
Gulf coastline. The alignment of this front is creating an
easterly wind through Southcentral and the northern Gulf, while a
more northerly flow is present over the western Gulf and Kodiak
Island. The easterly wind is creating gusty conditions through the
typical favored passes, Turnagain Arm and Anchorage area. The
Friday 12Z sounding depicts an unstable atmosphere which is
allowing the stronger winds from aloft to mix down to the surface
near Anchorage. Coastal areas like Seward and Cordova are
receiving the bulk of the precipitation, with warm temperatures
keeping it mostly as rain. In the southwest, weather conditions
remain fairly calm.

&&

.MODEL DISCUSSION...
Models remain in relatively good agreement lending to above
normal forecast confidence. The NAMNest was generally preferred
for short term updates in both the west and the east. The biggest
areas of discrepancy are the exact location of the next closed low
moving into the Western Gulf Sun afternoon and a shortwave diving
through the Northern Bering on Sat afternoon. The most critical
aspect of the forecast that models notoriously have a hard time
resolving are the snow levels across the Western Prince William
Sound and through Turnagain Pass. The overnight package generally
kept snow levels much the same for today's system and then used
manual edits to raise them into the weekend for that area.
Finally, models are coming into better agreement with a
deformation axis over the interior portions of Bristol Bay late
Sun. Right now, it looks as precipitation will be all snow with
this feature and we will need to keep an eye on it.

&&

.AVIATION...
PANC...Southeast winds are hanging on this morning, but should
sharply turn out of the north to northeast right around 15z, maybe
a bit earlier. This will set the stage for wind shear over the
Turnagain Arm and Chugach Mountains as southeast winds increase in
response to our next storm. Conditions should remain VFR with
showers along the mountains. It will be possible for a brief
shower to move over the terminal, especially during the afternoon.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Today and Saturday)...

Yet another storm system heads north across the western Gulf with
the center moving just south of Kodiak Island Saturday morning
and to over the Barren Islands Saturday evening. The associated
frontal trough moves to the North Gulf Coast late tonight then
weakens as it moves inland Saturday. Gale to Storm force winds are
expected along the leading edge of the front as it heads north.
Marine winds should peak along the North Gulf overnight tonight
then diminish Saturday. Strong and gusty gap winds are also
expected From Portage Valley to western Turnagain Arm, Matanuska
Valley, and mountain passes and channeled terrain. Pressure rises
on the backside of the front as it moves inland Saturday will
produce gusty southerly winds across the Copper River Basin as
well as the Anchorage Bowl and Knik Valley.

The brunt of the precipitation will be along the Gulf Coast (generally
the eastern Kenai Peninsula) and should fall mainly as rain at
the lower elevations. Snow levels will generally be 800 to 1000 ft
range through tonight then rising some on Saturday. Areas inland
of the coastal mountains will be largely downsloped and see very
little precipitation other than a few showers.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3...Fri
through Sun)...

The longwave pattern features an elongated trough from the Gulf Of
Alaska to NE Russia. Looking at 500 mb, there is a pair of lows
at 500 mb that are moving across the Bering and interacting with
each other resembling the "Fujiwhara Effect." At times, these
upper level shortwaves clip the Kipnuk and Hooper Bay area. At the
surface, there really isn't an organized frontal boundary, just a
few troughs which is seen in the latest ASCAT/Advanced
scatterometer pass and in the isobars at the surface of the
models. There will be enough dynamics in place to keep snow
showers in portions of west Alaska. Meanwhile, a front in the
North Pacific/GOA region will push towards the AKPEN later today.
A deformation band is expected to set up over Southwest along with
wrap around moisture which will bring accumulating snow to the
southern portion of Southwest Alaska starting tonight and
persisting through Sat.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3...Fri
through Sun)... There is a low southwest of Adak that will get
shunted southeast today taking the bulk of the stratiform
precipitation with it. Meanwhile, over the northern Bering there
is a surface low between St. Matthew and St. Lawrence Island and
the models show it continuing to decay and being absorbed into the
longwave pattern. Looking at the temperatures aloft, cold air
will continue to advect from NE Russia across much of the Bering
and into Western Alaska, resulting in a showery regime. Cold air
over warm water is inherently unstable, so expect more
convection/snow showers over the Bering.

&&

.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 island 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. The ridge eventually builds 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...Storm Warning 119 130.
         Gale Warning 120 125 129 131 138-140 351 352. FIRE
WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...RA/MSO
SOUTHCENTRAL ALASKA...RMC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PJS
MARINE/LONG TERM...KVP