Gale Warning 95, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
501 PM AKST Tue Jan 29 2019

.ANALYSIS AND UPPER LEVELS...
There is a Rex block to the west of mainland Alaska, with the
high centered just to the west of the Bering Strait and the low
centered just south of the Rat Islands. There is a front in
between these features stretching from the eastern Aleutians and
western Alaska Peninsula westward across the southern Bering. A
trough extends from the low northeastward into Southwest Alaska.
This trough is producing some precipitation over much of
Southcentral, and over the lower Kuskokwim river and the Delta.
There is a mix of precipitation types from the Kenai Peninsula
through Cook Inlet into the Matanuska Valley and southern Susitna
Valley as low level temperatures are just on the line between
rain and snow. At the surface, there is a low centered just south
of the Kenai Peninsula, a low vertically stacked with the Rat
Island upper low, and a low forming south of the Alaska Peninsula
on the eastern end of the front. Surface high pressure is over the
northern Bering.

&&

.MODEL DISCUSSION...
The numerical models are in good agreement for the short-term
portion of the forecast (Thursday afternoon). The forecast is
still quite challenging. This is mainly a result of precipitation
type through tonight for many areas in Southcentral. The 850 mb
temperatures and 850mb-1000mb thicknesses are marginal for snow,
but slowly cooling through Wednesday morning.

&&

.AVIATION...
PANC...MVFR ceilings and visibilities are most likely through
Wednesday afternoon, with IFR conditions at times, especially
when the precipitation type is snow. Precipitation type is
expected to be a mix of rain and snow this evening, eventually
changing over to all snow overnight. Conditions will begin a
slow improvement Wednesday afternoon.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2: Tonight
through Thursday)...

A weak surface low currently sitting south of Seward continues to
spread precipitation across Southcentral this evening. A mix of
rain and snow are currently being reported across Anchorage, the
Kenai Peninsula and the mix likely continues into the southern
Susitna Valley. A "back edge" to the precipitation has moved north
of the Sterling Highway and continues to press northward. Any
stoppage in the precipitation will be short-lived as a series of
upper level waves move northward, which will enhance the lift and
any precipitation later this evening. With the loss of diurnal
heating and temperatures currently hover a degree or two above
freezing in most areas, that should be enough to allow some
cooling back to 30 or 31 degrees and switch any rain back over to
snow sometime this evening. Since the forcing with these waves
will otherwise be pretty weak, most areas of Southcentral should
hold additional snow accumulations tonight into Wednesday in the
1 to 3 inch range. The snow will slowly diminish through the day
Wednesday as temperatures once again get a degree or two above
freezing.

On Wednesday night a strong upper level shortwave associated with
the western edge of an Arctic air mass will dig southward along
the Alaska/Yukon border. This will allow a much colder air mass to
quickly advect southward into Southcentral. First to be hit will
be the Copper River Basin Wednesday night. While current indications
are that the core of the coldest air remains well east of the
area over the Yukon, the cold air will eventually drop into
Southcentral and return the area to seasonably cold conditions for
the start of February on Friday.

The more significant hazard associated with this feature will be
strong, gusty winds expected to impact most of the gaps through
Southcentral. The strongest winds are likely to be through
Thompson Pass and the Copper River Delta Thursday afternoon into
Friday. However, the Matanuska Valley wind, Broad Pass, and the
Seward area are all expected to be gusty from Thursday into
Friday. There remains a fair bit of uncertainty with this
however, which is essentially due to the models trending eastward
with the upper level wave. The further east it goes, the lesser
the impact to Southcentral.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Areas of snow continue in the lower Kuskokwim Valley/Delta under
a weak deformation zone associated with a trough over the Bering.
As that feature lifts north tonight, expect patchy fog to develop
in the Kuskokwim Delta...though low level clouds could preclude
fog development. Lingering low level moisture in the Bristol Bay
area from showers earlier today has resulted in patchy fog, which
will probably persist at least into mid-morning Wednesday. A low
south of the AKPEN will shift northeastward tonight, and warm air
advection ahead of the low will bring snow to the southern Alaska
Range tonight through Wednesday. Conditions will trend colder and
drier with offshore flow Wednesday into Thursday as an Arctic high
sets up over northeast Alaska and the Yukon.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
A vertically stacked low roughly 100 miles south of Attu is
gradually lifting northward and weakening. Winds in the periphery
have dropped to below gale force per recent ASCAT pass, other than
a swath of gales south of 50N. Winds will mostly diminish to sub
advisory level through Wednesday morning for the Bering, Aleutians,
Pribilofs and AKPEN. An occluded front wrapping around the low
will bring precip to the western, then central Aleutians into
tomorrow, then the area will trend drier until the next system
approaches. That next system is a low currently roughly 1000 miles
southwest of Attu and will shift northward into Thursday bringing
storm force winds to the western and central Aleutians and gales
to much of the Bering late Wednesday into Thursday.

&&

.MARINE (Days 3 through 5)...

The operational models all ping into back to back cyclones
developing south of the Aleutians, jumping the chain and then
bringing unsettled weather into the Bering. There are some minor
timing issues but any way you slice it, very confident that the
active weather pattern will continue for the western/central
Aleutians and Bering from Thursday through Sunday. Confidence is
high for a minimum of gale force winds, rough seas and more
precipitation with both storms. The surface low anchoring the
first front will be just south of Attu Island 12z Thursday. The
second surface low will be west-southwest of Adak 00z Saturday and
this low will rapidly accelerate north being west of the Pribilof
Islands by 12z Saturday.

&&

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

The longwave pattern features a ridge from the Hawaiian Islands
extending into Southcentral Alaska with the longwave trough over
Central Siberia, Kamchatka and the Aleutians which is an atypical
orientation for a trough axis. There is a secondary dome of high
pressure aloft north of the Bering which is keeping things
interesting in the Aleutians/Bering. As time elapses, the dome of
high pressure to the north moves to the west and the upper level
ridge remains steadfast from the Gulf Of Alaska and into the
interior of Alaska. All the while, to the east of the ridge, an
arctic trough pushing south through the Yukon Territory will
advect a brutally cold airmass into eastern Alaska Wednesday
and Thursday. With a tight pressure gradient in place and the
placement of the ridge axis expect a surge of strong northerly
winds to Southcentral Alaska in many locations. Offshore flow is
expected from Seward to Cordova. The communities of Palmer and
Wasilla can expect the "Matanuska Wind" on Friday and possibly
Saturday. By Sunday, the ridge axis moves eastward enough to
change the wind pattern. For the remainder of the forecast period,
this ridge is going to have some impact over the eastern half of
the state of Alaska.

Out to the west, there will be a series of storms. That longwave
trough axis is going to spin up storm after storm. This pattern
doesn't show any chance of changing for the Bering/Aleutians
through Day 7 and arguably through Day 10.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Storm Warning 175 176 177 178.
	     Gale Warning 173 174.
         Heavy Freezing Spray Warning 185.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...BL
SOUTHCENTRAL ALASKA...JW
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MM
MARINE/LONG TERM...PS