Gale Warning 123, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
535 AM AKST Sat Feb 23 2019

.ANALYSIS AND UPPER LEVELS...

A broad ridge encompasses mainland Alaska and the eastern Bering,
while a negatively tilted trough digs over the western Aleutians.
Around the trough, cold air is advecting over the western Bering
and Aleutians. This cold air is butting up against warm air
advecting over the eastern Aleutians and southwestern Gulf from
the North Pacific on the western edge of the ridge. On the
leeward side of the ridge, northerly flow continues to push cold
air over Southcentral and the Gulf of Alaska. This cold air aloft
is creating a very dry vertical profile through Southcentral which
is observed by the Anchorage 00Z upper-air sounding. At the
surface, an elongated low is present over the southern Bering.
Ahead of this low, a cold front is advancing eastward bringing
gusty winds to the Bering, and precipitation to the central and
western Aleutians. Satellite imagery depicts the movement of the
front as a swath of clouds that extends from the northern Bering
through the central Bering, and into the North Pacific. In
opposition, the area under the ridge and surface high pressure
remains clear and dry.

&&

.MODEL DISCUSSION...

Models are in generally good agreement with a building ridge over
the mainland and Gulf, and with strong low pressure in the
Bering. Wind speeds around the low remain a struggle as the
American models depict stronger winds than the ECMWF and Canadian
in the Bering through tonight. Sunday morning is when the next
storm moves into the area, and there is a lot of variation in the
solutions. The GFS is much more robust than the NAM, ECMWF and
Canadian as the low moves through the Bering on Monday. This is
the only real discrepancy as the models generally agree with the
track of the system. With the GFS being the outlier, the NAM was
the general model of choice.

&&

.AVIATION...

PANC...High pressure over Southcentral will keep skies clear and
winds light through Sunday. There is a potential for very patchy fog
early this morning, but no prolonged reductions in visibility are
expected. Clear skies combined with northerly flow and dropping
temperatures at the surface will increase the chance for more
significant fog to form in the area Saturday night/Sunday morning,
thus reducing visibility. The fog threat will persist for the
next several nights.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2: Today and
Sunday)...

The offshore winds funneling through the gaps in Valdez, Seward,
and Whittier continue to diminish this morning. A giant upper
level dome of high pressure is building into Southcentral and is
expected to be overhead by late tonight. This will allow winds to
become nearly calm area-wide.

Meanwhile, a feature of the high pressure system will be the
gradual warming of the air mass in the upper but especially mid-
levels of the atmosphere. As it warms and the resulting inversion
strengthens, the injection of moisture into the lowest levels of
the atmosphere from snowmelt and the unfrozen water bodies around
Southcentral, will make fog development increasingly likely each
night. That being said, upper level cloud cover, like we have
presently may still be present and could work to keep the fog from
becoming very widespread. Nevertheless, Anchorage, the Knik
Valley, Western Kenai Peninsula, and interior valleys should plan
on areas of fog to develop each night and persist until the sun
can burn it off in the afternoon. Areas with thicker and denser
fog may possibly see it persist throughout the day as the sun's
heating ability is still far from its annual maximum.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2...Saturday
through Sunday night)...

A ridge over the southern mainland is bringing mostly clear and
dry conditions across the Bristol Bay area and lower Kuskokwim
Valley. A stream of moisture advecting northward into the Bering
will eventually drift eastward this evening, bringing a surge of
warm moist air into Southwest. Precipitation will fall as rain
across the AKPEN and Southwest coast Sunday morning while areas
further inland will see less of a chance for precip as the ridge
over the mainland maintains its influence keeping things drier.
As this front pushes up against the coast, strong southeasterly
winds will deliver gusty conditions along the northern Kuskokwim
Delta coast through Sunday morning. Southerly winds along the
Southwest coast will ramp up again Sunday evening into Monday
morning as the next storm force low moves into the western Bering
and an enhanced pressure gradient develops.

&&

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

A long stream of moisture is being advected into the central
Bering as an area of low pressure positioned along a baroclinic
zone drifts northward through the afternoon. Temperatures will
warm over the eastern Bering and Pribilofs through the day as
southerly gales persist. However on the backside of the low, cold
air and westerly gales will move in over the western Aleutians.

By Saturday evening, a ridge briefly builds over the central
Aleutians but quickly dissipates as another storm force low
approaches the western Aleutians Sunday morning. The main
forecast challenge with this system is the track it will take and
uncertainty still remains. The leading front associated with this
low packs a large area of southeasterly storm force winds that
will lift over the central Aleutians Sunday afternoon.
Temperatures within this southerly flow will be warm enough for
precipitation to fall as rain. On the backside of the low, another
round of storm force winds (this time from the northwest) will
move over the western Aleutians for Sunday evening. Temperatures
will be cold enough at this point to allow for snow, resulting in
the likelihood of blowing snow for the western Aleutians Sunday
evening and possibly into Monday morning.

&&

.MARINE (Days 3 through 5)...

There is high confidence in a deep storm-force low crossing the
Western Aleutians late Sunday into Monday. A widespread area of
southeasterly gales, with the potential for smaller core of
storm- force winds, are expected across the central and western
Bering ahead of this system through late Monday. Westerly storm-
force winds will wrap underneath the low, impacting the coastal
water surrounding the western and central Aleutians. Waves with
this system will rise to 30 to 40 feet on the Pacific side of the
western and central Aleutians and to 20 to 30 feet across the
western and central Bering. Winds and waves will slowly decrease
in intensity and height as the low lifts north Tuesday. Confidence
is also above average with a second deepening storm-force low
moving across the central Aleutians and into the central and
eastern Bering late Tuesday into Wednesday with widespread
southerly gales, areas of storms, and wave heights building to
over 20 feet.

High pressure anchored over the Gulf of Alaska will allow
generally westerly, sub small-craft winds along the coast from
Kodiak Island to Cordova through midweek.

&&

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

An established omega block stretching from the western Aleutians
east to the Canadian Interior will anchor a ridge directly over
Southcentral through much of next week. The vertically stacked
ridge will result in a rather tranquil weather pattern, with the
biggest forecast challenge through the period being fog, due to
the weak flow in the lower levels and warmer air advecting
northward in the mid-levels. Drier conditions and light winds will
also allow for larger diurnal temperature swings, with daytime
highs across Southcentral running slightly above average and
nighttime lows below average. A few areas of scattered cloud cover
may develop late Monday into Tuesday from Kodiak north through the
Cook inlet and again late Wednesday into Thursday across western
areas of Southcentral as two shortwaves rotate around the ridge.
Both of these systems, however, are expected to weaken and shear
apart as they lift northward.

For the southwest mainland, conditions will remain more unsettled
with the region sitting in the transition zone between the main
trough to the west and the ridge to the east. Here, a series of
fronts will track toward the coast, move inland and fall apart as
they run into the ridge. The first will arrive late Sunday and
linger through Monday before a second reinforces the southerly
flow and precipitation (especially along the coast) late Monday
through Tuesday. These systems then fall apart as the ridge to
the east shifts a bit to the west. A third front is then expected
to impact the southwest late Thursday into Friday. The challenge
here will be precipitation type, especially as a persistent
southerly flow continues to bring warmer air into the region. The
first two systems look to be mostly a rain event along the coast
(with snow aloft and over the interior), but slightly colder air
in place over the region prior to the third frontal system may
bring a wintry mix to all locations during the onset of
precipitation before the warmer air arrives.

Farther west, the weather pattern is expected to remain quite
active with a deep trough anchored over the Bering. we have a
very active pattern on tap for next week. The week starts with a
strong storm-force low crossing the western Aleutians into the
western Bering. This low lifts north, a second gale-force low
follows behind the first and tracks north into the western Bering
beginning early Wednesday. Broad, cyclonic flow and areas
of showers continue across the Bering as this second storm lifts
north. A newly developing North Pacific low is then expected to
advance toward the western Aleutians by the end of next week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gales: 150 155 160 165 170 172-174 178-181 185 412 414.
         Storms: 175-177 411 413.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...RJA
SOUTHCENTRAL ALASKA...RC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KO/DK
MARINE/LONG TERM...TM