Gale Warning 225, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion...CORRECTED
National Weather Service Anchorage AK
513 AM AKST Fri Mar 1 2019

.ANALYSIS AND UPPER LEVELS...

The upper level pattern of strong ridging and upper level high
pressure over much of mainland Alaska paired with a deep
stationary trough over the Bering is beginning to break down this
morning. The trough over the Bering is finally beginning to push
east into western Alaska. Thus, the ridging over mainland Alaska
is beginning to move east into the Yukon. This will not be a clean
eastward translation however, so Southcentral remains dry for now.
Such will not be the case in a few days, which is described in
more detail below.

The synoptic situation currently is nearly identical to what it
was 24 hours ago. A dying front moving into the coast of Southwest
Alaska is inching its way eastward, with associated cloud cover
present from west of Iliamna across the rest of Southwest Alaska.
The typical low stratus/fog continues over much of the Gulf,
though none of that cloud cover as yet exists anywhere from Homer
north. Thus, nearly all of Southcentral except Kodiak is enjoying
another night of clear skies. Thus, temperatures are once again
falling into the single digits for much of Southcentral, with
Anchorage recording its seventh consecutive day of below average
temperatures to conclude the final day of February. The Anchorage
sounding continues showing a strong inversion over the area at
about 3,000 feet above sea level with temperatures around 40
degrees. This very stable profile along with a very dry atmosphere
continues to suggest the sunny conditions will persist for at
least a while longer.

&&

.MODEL DISCUSSION...

Agreement between the models remains excellent over the next
couple days. The approach of a dying front from the west with high
pressure out over the Gulf is expected to cause a weak barrier jet
to develop on Saturday which will increase the southeasterly
onshore flow into Southcentral. While nothing impactful is
expected, the models differ a bit on the extent of any shower
activity that may develop over the mountains as a result. The
models all agree that precipitation associated with the front
itself over Bristol Bay and the Lower Kuskokwim Valley will remain
west of the Alaska Range. A large, strong low moves across the
western Bering this weekend, but the models are all in very good
agreement on the track, intensity, and speed of the leading
warm/occluded front tracking through the Pribilofs Sunday morning
as a gale-force front. With excellent agreement on the synoptic
features and only minor differences in the models elsewhere,
forecast confidence is also very high, with no major changes in
the current forecast expected through the weekend.

&&

.AVIATION...
PANC...Occasional LIFR conditions in fog are likely this morning.
The fog should clear by noon, after which VFR conditions and
light winds are expected.

&&

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

The upper level ridge will continue to weaken through Saturday
night. This will allow some low to mid-level moisture to sneak
into the north Gulf Coast from the southeast later tonight
through Saturday night, bringing chances of precipitation to that
area. An upper level front will also approach from the west tonight
and Saturday. This front is quite weak, wih increased cloud cover
over the area as the main impact. Chances of precipitation will
mainly be limited to Kodiak Island and the southern portion of the
Kenai Peninsula.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
(Today through Sunday night)

Wet weather looks to return to portions of southwest Alaska over
the next 24 hours or so, as a trough and associated surface front,
connected to a low in eastern Siberia, crosses the region. This
front will bump into a ridge of high pressure to its east, causing
the front to weaken some. However, disturbances in the flow aloft
should allow it to maintain some integrity as it pushes eastward
towards the Alaska Range. Overall we expect light precipitation
amounts, with not too significant of a drop in temperatures, as
the 850 mb temps only cool about 4 degrees Celsius. By Sunday,
warm advection ahead of the next incoming front will switch winds
back to a southerly direction, allowing temperatures to warm again
and perhaps causing some morning fog to develop.

&&

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

A general westerly to southwesterly flow just above the surface
will become southerly by later tonight, as the next system moves
into the western Aleutians and Bering, As these winds switch,
there could be some patchy areas of fog for the western portions
of the area. However, confidence in temporal and areal coverage
was insignificant to include in the forecast at this time.
Otherwise, look for winds to increase to storm force along and
just ahead of the front, which should reach Shemya and the
western Bering between midnight and sunrise. Given the intensity
of the storm (sub 960 mb), and good fetch length, seas look to
climb into the 25 to 35 feet range. As the parent low lifts north
towards eastern Siberia, the associated front should reach the
central Bering and Aleutians by Sunday, with winds still at least
in the high-end sustained gale range.

&&

.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 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 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...Storm: 175 176 177 178.
Gale: 173 174 185.

FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...JPW
SOUTHCENTRAL ALASKA...BL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD
MARINE/LONG TERM...AP/KVP