Gale Warning 130, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
453 AM AKST Fri Feb 1 2019

.ANALYSIS AND UPPER LEVELS...

An upper-level ridge extending from the western Gulf north to the
Brooks Range is continuing to keep conditions clear and dry
for the most part across much of Southcentral. The exception to
this is persistent low stratus and fog, as evident on the
nighttime microphysics satellite product, hanging around the
northern half of Cook Inlet as well as through portions of the
Anchorage Bowl and the southern half of the MAT-SU valleys. These
areas of fog are not nearly as widespread and dense as the
previous night, as the drier air slowly works in from the
interior.

Farther east, an area of stratus is sliding south along the ALCAN
border. This is in association with a vigorous upper-level trough
approaching eastern portions of the Copper River Basin. This
feature is responsible for plenty of cold air advection in the mid
and upper-levels (850 mb temperature falling to -20C) and is
increasing the thermal gradient across the Copper River Basin
tonight. At the surface, a strong dome of high pressure is
anchored over the Yukon while an inverted trough lingers from the
AK Peninsula northwest into the northern Gulf. Between the two, is
a strong north/south oriented pressure gradient. The combination
of the strong thermal and pressure gradients is driving gusty
outflow winds along coastal Prince William Sound and the Copper
River Delta. Gusts of 35 to 45 mph have been common throughout
the overnight hours in Thompson Pass, Valdez, and through the
Valdez Narrows.

Out west, a decaying occluded front positioned from the eastern
Aleutians north over the Bering continues to lift northeast. A
warm front south of the eastern Aleutians is lifting toward the
AKPEN and southwest coast. Strong southerly flow, with weakening
gales over the central Bering as depicted on the latest ASCAT
scatterometer pass, is bringing rain to the Pribilofs and eastern
Aleutians as temperatures climb into the upper 30s to lower 40s.
The occlusion is associated with a 949 mb low well west of Shemya
this is continuing to slowly drift northward. A cold front,
separated from the low, is advancing east behind the first
occluded front and is spreading clouds across the central
Aleutians.

&&

.MODEL DISCUSSION...

The numerical models remain good agreement through the short term
with only minor differences in the position of frontal features
moving from southwest Alaska into the western and northern Gulf.
These minor differences, however, are expected to have minimal
impacts to the overall forecast. By Sunday, there is some more
significant divergence in guidance. Both the Canadian and EC
models attempt eject a shortwave west to east across central
Alaska, breaking down the ridge upstream. The NAM and GFS runs
hold onto the ridge through the weekend before shifting the ridge
axis across the Yukon. These differences in the upper levels
translate to differences at the surface, specifically in the
movement of the timing of the frontal system across Southcentral
for the weekend. Here, the NAM and GFS slows the system down and
keeps precipitation and southerly gap winds mainly confined to
areas west of the Copper River Basin while the EC and Canadian
move the system farther east. Given the strength of the ridge
bookended by deep troughs (a quasi omega block pattern), the
slower GFS and NAM solutions were favored.

&&

.AVIATION...

PANC...Intermittent fog will persist this morning as low stratus
over Cook Inlet and Knik Arm move in and out over the airport
complex under light and variable winds. VFR conditions are
expected to dominate the TAF package overall; however, periods of
IFR and possibly LIFR cannot be ruled out through mid-morning. Any
lingering fog/mist should completely clear by late morning as
higher cloud cover increases in advance of a frontal system moving
in from the west.

&&

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

Cold interior air combined with packed pressure gradients along
the Gulf coast continue gusty outflow winds from the eastern
Kenai Peninsula to the Copper River Delta. Winds will slowly
diminish through today as upper support wanes. Although greatly
diminished, gusty outflow conditions will continue into Saturday
at some locales from Prince William Sound to the Copper River
Delta. A weakening weather front brings increased chances of rain
and snow to Kodiak today. The front dissipates tonight and will
likely not make much headway into Southcentral. Could see some low
chances of some light snow tonight in the Cook Inlet Region and
perhaps the Kenai Peninsula/north Gulf. The remainder of
Southcentral will mainly see some increased cloud cover.

The upper ridge which builds in early Saturday shifts slowly
east Saturday night. As this happens, a broad upper trough and
associated surface front will encroach from the west late
Saturday and bring chances of light snow to Southcentral. There is
still some uncertainly as to how quickly the upper ridge will
move east, so stay tuned for further updates.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3/Friday
through Sunday)...
The anticipated front is approaching the coast of southwest
Alaska. It is weakening as expected, so gusty winds can be
expected along the coast, but not very far inland. The front
appears to have slowed just a touch, so anticipating precipitation
to start a few hours later this morning. After this front pushes
through, the next one will be quickly on its heals, arriving on
the coast early tomorrow morning. The overall pattern is very
similar, with this next front extending from a low center in the
western Bering. With the setup almost the same, expecting very
similar weather with the front tomorrow morning also weakening as
it approaches the coast. The main difference is this next round of
weather will bring another round of warm air, elevating
temperatures just enough to send rain and mixed precipitation into
coastal areas of southwest Alaska. This second front clears out
Sunday morning, with quiet weather behind it.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3/Friday
through Sunday)...

The first frontal system that entered the western Aleutians
yesterday is currently pushing through the eastern Aleutians and
approaching the coast of southwest Alaska. This front is weakening
as it goes, with small craft advisory winds and rain. Meanwhile,
the next round of weather is entering the western Aleutians. Storm
force winds are still anticipated with this low for the western
Aleutians. This next low will track north through the western
Bering and once again send a front east through the Aleutian Islands.
As with the previous low, the front will detach from the dynamics
of the low center, so it will weaken as it progresses east. After
this next low, a smaller and weaker low will approach the western
Aleutians Sunday, but the Bering will quiet a bit for the day.

&&

.MARINE (Days 3 through 5)...
Beginning on Sunday in the western Bering, southwest winds should
be ramping down as a low weakens and exits to the north with
generally sub gale force winds expected, though the long SW fetch
will keep seas rough, with waves heights expected to remain near
or above 20 ft. Models are showing another, weaker, low,
approaching from south of Adak by Sunday afternoon, though there
is still a fair bit of uncertainty regarding this system. As it
stands right now, this system would only bring a quick bump of
winds towards low end gales, though further intensification could
increase winds towards storm force Sunday night through Monday
night over the central Aleutians and Bering Sea. Another low
approaches the western Bering from the Sea of Okhotsk by Tuesday
afternoon, though models keep the low center over Attu with the
core of strongest winds to the south of the islands.

Conditions look to remain rather benign over the Gulf of Alaska
through Tuesday with general high pressure residing over the area
keeping the storm track just to the west.

.LONG TERM FORECAST (Days 3 through 7)...
Beginning Sunday, an upper level ridge previously entrenched over
most of the mainland will shift east with the ridge axis located over
the ALCAN border by the end of the day. Out west, an upper level
low will continue to support an active pattern with a series of
systems expected to push across the Bering into the western
mainland. The eastward shift of the ridge axis will allow these
systems to push into Southcentral with precipitation expected to
make it across the Alaskan Range by Sunday morning. Another
system pushes north across the Bering by Monday with its
associated front pushing precipitation across the southern
mainland by Monday night. The consistent southwesterly fetch will
mean these systems will be pulling up milder air with increasing
chances for rain along the coast and possibly a bit further
inland. The overall unsettled pattern looks to continue through at
least the end of next week with the upper ridge axis maintaining
its position east of Southcentral with the upper low over the
Bering.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning 127 170 172 173 174 178 179 181 185 411 412
413 414
		 Storm Warning 175 176 177.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...TM
SOUTHCENTRAL ALASKA...BC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...BB
MARINE/LONG TERM...KVP