Gale Warning 193, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
448 PM AKST Thu Feb 28 2019

.ANALYSIS AND UPPER LEVELS...
The upper level ridge pattern that has been holding in place over
mainland Alaska for the past week is finally starting to weaken
and shift slowly east. A large upper level trough supplied with
cold air from Siberia continues over the Bering and Aleutians,
supporting a vertically stacked storm force low tracking towards
the northern Bering. The associated front is progressing eastward
from the eastern Aleutians/Bering towards the Alaska Peninsula
and Southwest Alaska coast this afternoon with a long fetch of
moisture being brought up from the North Pacific. Dry, sunny
weather persists over Southcentral Alaska with offshore gap winds
diminishing in response to the movement and weakening of the
ridge.

&&

.MODEL DISCUSSION...
Models continue to be in good agreement in the short term,
however, they start to diverge on Saturday over the Gulf. The
NAM/Canadian keep a stronger ridge/trough pattern with a surface
low forming near Kodiak, which the GFS/EC keep a weaker solution
with a broad trough of low pressure over the Gulf. With either
solution, cloud cover and increased chances of precip will return
to Kodiak and the Southcentral coast. Over the Bering, models
agree that a storm force low will be the dominate pattern moving
into Saturday. A NAM/GFS blend was used for the forecast for both
the east and west domains.

&&

.AVIATION...
PANC...Light winds will continue as ridging persists over the
area. Patchy fog will be possible again around sunrise Friday,
however, with the dry air mass in place, any fog that does form
should be shallow and only last for an hour or two.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The upper level ridge over the area began to split apart today
with one portion of the ridge moving into the Arctic Ocean on
Friday. The other portion will push south into the southern Gulf
by Friday night. This continues to leave Southcentral with mostly
clear skies and some local patchy late night and morning fog. By
late Friday night an upper level trough will begin to approach the
area from the west, bringing increasing clouds to Kodiak Island.
The clouds should begin to filter into the Cook Inlet region on
Saturday but any potential precipitation will be relegated to
Kodiak Island and the north Gulf coast.

&&

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

The remnants of a front have pushed eastward, lifting over
Southwest Alaska. This has brought gusty southerly flow along the
coast. Precipitation will eventually spread in this evening
falling mostly as rain over the AKPEN while further north over the
Kuskokwim River Valley, a rain/snow mix is likely. As the front
continues to weaken and move inland, gusty winds along the coast
will eventually diminish by early Friday morning. Similarly,
precipitation will gradually taper off from west to east through
Saturday morning. Fairly calm and quiet weather conditions are in
store for Southwest through Saturday evening, before another front
associated with a storm force Bering low approaches from the west
for early next week.

&&

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

Although the synoptic pattern remains the same as earlier in the
week, calmer conditions are in store for the Bering for Friday. A
departing low to the north will exit the Bering late this evening,
leaving behind westerly flow with generally colder air moving in.
This will allow for a few snow showers over the Bering ahead of a
weak ridge that attempts to build Friday afternoon. Meanwhile
further west, a warm front associated with a storm force low
approaches the western Aleutians Friday evening, packing a swath
of southerly gales with embedded storm force winds. Although the
low center is positioned well west of Attu Island, its front will
progress eastward, reaching the central Aleutians by Sat
afternoon. Strong gusty winds are expected with this front, and
not unlike the previous storms, ample moisture will accompany this
system. This will allow for precipitation to fall mostly as rain,
however with colder air in place ahead of the front, a rain/snow
mix is possible across the western and central Aleutians for
Saturday.

&&

.MARINE (Days 3 through 5)...

A weakening front will bring southeasterly gales and higher seas
to the central Aleutians (generally west of Cold Bay) while upper
end gale to storm force southwesterly winds will move over the
far western Bering near Attu on Sunday. Winds will gradually
diminish to sub gales for the entire region Monday through Tuesday
morning as the associated low weakens and departs over Siberia.
Confidence is high that a broad, strong low will approach the
western Aleutians Tuesday evening, likely leading to potential
storm force winds and high seas by Wednesday.

The ridge that has been in place for several days over the Gulf is
expected to break down as a trough moves over the region Sunday.
However, sub gales are still expected to persist through Tuesday.

&&

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

Moderate precipitation will track across the central and eastern
Aleutians Sunday into Monday ahead of a weak cold front. The front
will reach southwest Alaska by at least Monday afternoon and has
the potential to bring significant accumulating snowfall to the
region. GFS is currently the most aggressive model and we are
leaning towards this solution given recent run-to-run consistency
compared against the ECMWF. Also of note is that the moisture
associated with this system is initially being advected from
nearly 2000 miles south in the Pacific and thus would also merit
higher snowfall totals. A broad, strong low will move into the
western Bering on Wednesday with moderate to locally heavy rain
tracking across the Aleutians through Thursday. Model agreement
timing is poor beyond then, however the system could bring more
snow to southwest Alaska by late-week.

As mentioned above, the fair weather that has been in place over
Southcentral the past week will give way to a more active pattern
next week. A trough will track over the region Sunday likely
bringing overcast conditions for much of the day. At this point,
confidence is moderate to high that given the dry conditions, snow
will be unlikely for most locations, with the exception of light
snow over the mountains and the North Gulf Coast. A weak cold
front associated with a low south of Siberia will amplify a ridge
over Southcentral on Monday and bring fair skies throughout the
day. This front will move into the region by Tuesday into
Wednesday with the GFS and Canadian models most aggressive with
potential accumulating snowfall through Wednesday. We expect ECMWF
to follow suit in the coming days. The pattern is expected to
repeat itself with a ridge bringing generally fair skies Thursday
and a trough bringing possible precipitation to the area by late
week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale 185.
		 Storm 177 178.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...JR
SOUTHCENTRAL ALASKA...TM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KO/DK
MARINE/LONG TERM...AP/KVP