Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 130144
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
444 PM AKST Sun Jan 12 2020

.ANALYSIS AND UPPER LEVELS...
An upper level trough extends from Norton Sound southeast through
the middle Kuskokwim valley, across the southern Kenai into the
Gulf of Alaska. Precipitation is occurring along this trough,
which is generally snow over the land and rain over the Gulf. A
surface low along this trough is over the northwest Gulf and
moving rapidly eastward, with gusty westerly winds behind it. A
ridge of high pressure is sitting over eastern Alaska, including
the Copper River basin. Temperatures have moderated quite a bit
over the southern mainland, warming into the 20s to low 30s over
southwest Alaska and in the teens to 20s over the Cook Inlet area.
Broad westerly flow aloft is over the Bering sea with an upper low
over the Bering Strait and a ridge over the north Pacific south of
the Aleutians.

&&

.MODEL DISCUSSION...
The numerical models are in good agreement for the short term
portion of the forecast. There are the usual minor differences in
precipitation with the trough over the area through tonight, but
this has only a small impact on the forecast. There is also some
discrepancy with the exact placement of the developing low east of
Kodiak. This feature seems to be moving faster than models can
keep up with. Forecast confidence is a little above normal today.

&&

.AVIATION...
PANC...MVFR to IFR conditions in snow are expected through
tonight. MVFR ceilings may be slow to lift to VFR Monday morning,
but should reach that criteria by noon. Light winds are expected.

&&

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

A shortwave associated with an upper trough over the western
Bering Sea continues to progress eastward tonight. A surface low
that developed near Kodiak Island has made its way to the central
Gulf, with gales around the backside. It will continue
southeastward, while the parent upper trough becomes centered over
the Alaska mainland. As the wave tracks through the Anchorage
Bowl tonight, we still expect 1 to 2 inches of snow. Overnight
lows tonight will be moderated by cloud cover, and should be above
zero for most of Southcentral. Exceptions include the Copper
River Valley and parts of the Susitna Valley. Precipitation will
diminish by midday Monday, initiating a return to a dry pattern.
Temperatures will be similar to today... lingering cloud cover
will limit diurnal heating. By Monday night, cold air advection
will set back in. Arctic air in tandem with low pressure in the
Gulf will enhance the offshore pressure gradient, and winds out of
the usual gaps and passes (Valdez waters, Resurrection Bay,
Whittier and the Copper River Delta) will strengthen. Clearing
skies, healthy available moisture from recent snowfall and surface
high pressure will facilitate patchy valley fog development
Monday night into Tuesday morning. As for temperatures, Tuesday
will be noticeably colder than Monday.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
(Tonight through Wednesday)

A weakening front continues to make progress this afternoon as it just
reached east of a Sleetmute to Iliamna line. Widespread snowfall
continues in advance of this feature, with precipitation becoming
much more isolated and showery in nature following the frontal
passage. However, a disturbance aloft will cross the Kuskokwim
Valley tonight, allowing for perhaps a brief uptick in snow
showers coverage, with minimal accumulations.

For Monday afternoon through Wednesday, a ridge of high pressure
builds across the region. The ridge axis will remain west of the
area, allowing for a dry northerly flow to develop, cooling
temperatures aloft and at the surface. This high then becomes
centered over the central Kuskokwim River by Wednesday, allowing
the dry weather to continue, with southerly winds along the coast
on the backside of the high helping to increase the temperatures
some.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
(Tonight through Wednesday)

Cyclonic flow around a Chukchi low will allow for scattered snow
showers to persist across most of the Bering over the next 24
hours, aided in part by a disturbance ejecting from near Shemya
towards Saint Paul Island. This disturbance is riding the
periphery of a building ridge, and it`ll make a right turn on
Monday, as it dives towards the Alaska Peninsula by evening.
Building high pressure will put the kibosh on any precipitation
Monday night, before it gets shunted east by a meridional ~993 mb
low lifting across the western Aleutians. This low will continue
to deepen to near ~970 mb Monday night across the western Bering,
sending a front eastward. In response to this, winds will rapidly
increase to storm force as the pressure gradient tightens with the
high to the east.

A secondary low will also develop across the North Pacific
Tuesday afternoon, and it too will rapidly deepen as it heads
towards Dutch Harbor by Wednesday morning. The net affect from
these features will be widespread gales, with periods of storm
force winds near the low centers and immediately ahead of the
initial front, with hurricane force gusts a distinct possibility.

After a brief reduction in wave heights for tonight into Monday,
look for seas of 20 to 30 feet to become common by Tuesday into
Wednesday for the western half of the area.

&&

.MARINE (Days 3 through 5)...
On Thursday, a storm force low over the northern Bering exits
north overnight while another North Pacific low develops south of
the Central Aleutians. There is high uncertainty on the track and
strength of this low on Friday. A second system we are monitoring
is expected to develop across the eastern Gulf Friday night. Once
again, there is high uncertainty on the development of this storm
which could have impacts across the Gulf through the weekend.

&&

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

The forecast beginning on Tuesday night across Southcentral will
see a shift toward colder temperatures as a cold front across the
northwest tracks to the Gulf. This front and northwesterly flow
setting up aloft will increase offshore flow especially through
north to south oriented gaps Tuesday night through Friday. There
will be several locations that experience strong winds Tuesday
through Friday. The largest focus for winds in the extended
forecast included Broad Pass, Matanuska Valley, Seward, Whittier,
Thompson Pass, Valdez and the Copper River Delta.

Active precipitation is expected to mainly impact the Bering and
the Gulf Tuesday through the weekend. Tuesday night a front along
the Bering is re-enforced with another developing low along the
base of the trough. This strengthens the front to storm force with
the upper level wave lifting the system to northern waters Thursday
night.

There is high uncertainty on the forecast Thursday through the
weekend as models diverge greatly on the development to two new
storms...one along the western Aleutians and the other over the
eastern Gulf. The extended forecast during this time was updated
with ensemble blends of the GFS and ECMWF which leads to a very
weak pattern advertising higher chances for precipitation across
the Bering and Gulf with a colder and drier pattern over the
southern mainland.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Flood Advisory 121.
MARINE...Heavy Freezing Spray Warning 119 127 129 139
          Storm Warning 178
      Gale Warning 175-177 185 411 413 352 130-132 138
FIRE WEATHER...NONE.

&&

$$

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



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