Small Craft Advisory 22, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
447 AM AKST Sun Jan 5 2020

.ANALYSIS AND UPPER LEVELS...

The upper air pattern is a bit convoluted this morning which is
often the case in weather patterns detached from the upper level
jet stream. That upper jet is shunted well to the south over the
Central Pacific as cold arctic air dominates the low levels. This
air mass continues to be one of the coldest in a couple years as
temps at the office here in Sand Lake never warmed beyond -5 F. Aloft,
there is a well defined arctic trough positioned squarely over
Southcentral and the Alaska Range. There is a departing mobile
shortwave trough moving into the Panhandle. There is a weak cold
frontal boundary/trough axis associated with the aforementioned
arctic trough over the Gulf of Alaska extending into Prince
William Sound. With very cold air over warm water, this is forcing
a convective snow band which is spreading snow from Valdez into
the Copper Basin.

Out west, cold arctic air remains entrenched, but lower clouds
from the Bering are actually helping to slightly warm the coldest
areas, so temps are not as cold tonight as the previous. Even
farther west, a series of subtle shortwave impulses aloft are
embedded within the shallower cold air over the Bering. This
pattern is very tricky as the low pressure systems are relatively
compact and mesoscale in nature, and are also somewhat
convectively forced. The most significant of these features is a
developing low north of Dutch Harbor with another weaker leading
low moving into Bristol Bay.

&&

.MODEL DISCUSSION...

The numerical weather models are in good agreement with the
general synoptic pattern, but they still remain in terrible
agreement with a low skirting the Aleutian Chain today, then
moving across Kodiak Island into the Gulf tonight and Monday. They
have come into better agreement that the low will now take a more
southward track, skirting the southern AKPEN and then moving
across Kodiak Island. Thereafter, confidence drops precipitously
as the hi-res NMM and ARW, NAM, ECMWF, Canadian, and GFS all take
difference tracks with this compact low. The Canadian regional
went from being the farthest north to farthest south in 24 hours,
while the GFS is in between the far north NAM and the southern
Canadian. The hi- res models also track much farther north, which
impacts snow distribution along the coast and local winds as this
low skirts by. This forecast favors a GFS blend which is a middle
ground, but continues the general trend south, which will likely
keep all of Cook Inlet area dry including Anchorage, with snow
impacting the coast from Prince William Sound east, and possible
light snow into the Copper Basin.

&&

.AVIATION...
PANC...Areas of fog have set up in Anchorage and there is nothing
in store to get rid of it. The challenge will be in determining
when and for how long fog will sit over the terminal. Will rely
heavily on trends in observations across the city to aid in the
forecast process. Sufficed to say, confidence in the visibility
at any specific time is low, especially beyond the first few hours
of the TAF.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2...Sun and
Mon)...
The main sources of notable sensible weather remain the various
compact lows tracking from west to east across the Gulf of AK and
the frigid temperatures inland. First, the lows tracking across
the Gulf continue to give the forecast models fits (see Model
Discussion). One of these lows is currently moving just south of
Prince William Sound. It continues to bring snow to the Northern
Gulf Coast up through the Eastern Copper River Basin. This system
should track quickly east today as it wraps up leaving only
lingering light snow over the Edgerton Highway area. The next
system is currently near Dutch Harbor and will track quickly to
the east through the day. It will move over top Kodiak Island late
this evening into early Mon bringing another round of snow and
gusty winds to much of the island. Due to the various tracks on
different models, we have taken a consensus approach and brought
around an inch of snow to Kodiak City. However, if the track
changes slightly, some of the population centers could see several
more inches fall in a short period of time, especially over the
higher terrain.

This airmass is one of the coldest in a couple years. Ted Stevens
International Airport touched down to -9 F on Saturday. The last
time the airport was that cold was back in January of 2017 when it
reached -15 F. Campbell Creek Science Center is down to -21 F this
morning and could fall a few degrees more before the sun rises.
As we head into the work week, this cold airmass will not
relinquish its grip over inland areas anytime soon either. The
storm track will remain over the northern Gulf of AK which will
serve to reinforce an offshore pressure gradient. This will also
serve to bring gusty winds to the typical channeled locations such
as Whittier and Thompson Pass. The only real variation in
temperatures will come from the patchy/fog stratus floating around
some of the open waters sources. So for most of the population
centers, expect high temperatures to continue to run about 15
degrees below normal.

&&

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

Clouds and light snow have moved over coastal Bristol Bay this
morning in advance of a weak low currently spinning north of Port
Heiden. As this system moves inland later this morning and
weakens, a second stronger area of low pressure will move along
the northern AKPEN coast on the heels of the first. This second
system will bring light snow accumulations from Cold Bay to south
of King Salmon Sunday afternoon through Monday morning before
continuing east over Kodiak Island. Northwesterly flow behind this
second low will help keep scattered snow showers over the
southern AKPEN through Monday evening.

Elsewhere across Southwest Alaska, clouds and light snow over the
Kuskokwim Delta coast this morning will diminish through the day.
By Sunday night, the ridge over the interior strengthens, keeping
the arctic air firmly entrenched over the region.

&&

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

A low over Dutch Harbor this morning will continue to track east
today bringing a mix of rain and snow showers to the eastern
Aleutians through Sunday afternoon. Behind this feature, a second
trough of low pressure diving south will enhance northerly flow
and scattered snow showers over the eastern Bering from the
Pribilofs to the eastern Aleutians through late Monday.

Farther west, a storm-force low will lift a front across the
western Aleutians Sunday night through Monday morning. This front
slowly weakens as it pushes to the central and eastern Aleutians
by Tuesday morning. Precipitation along the front will initially
be snow, but will change to a mix of rain and snow as
southeasterly winds advect warmer air across the Chain. By late
Tuesday, a North Pacific low will lift a second front north over
the western Aleutians with another round of mixed precipitation
and southeasterly gales.

&&

.MARINE (Days 3 through 5)...
The main impacts Tuesday through Thursday will be across the
Bering as an upper level low sends several fronts northward. The
first front pushing north across the western Bering on Monday is
followed by the next system Tuesday morning. This second system
will be a gale force system that tracks slowly north, with the low
center crossing to the Bering just west of Shemya Wednesday
evening. There is still high uncertainty for the following storm
developing over the North Pacific on Thursday which could bring a
gale to storm force front to the western half of the Aleutians by
Thursday afternoon.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Monday night through Wednesday afternoon is expected stay mainly
dry and cold for the southern mainland as a surface ridge remains
dominant across the interior. Although coastal areas could see
light snow at times with fronts diminishing just offshore. Meanwhile,
the active pattern continues across the Bering with a parent low
just south of the western Aleutian Chain late Monday, lifting to
the Northern Bering waters Thursday night. This system is followed
by yet another storm system moving toward the southern Bering
Thursday night. The extended forecast this afternoon will be
updated with a blend of the operational models and ensemble
solutions which are in fairly good agreement with storm momentum.
Winds are going to mainly be gale force with the next several
systems, with the next gale system for the Gulf expected next
Thursday and Friday.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 131.
MARINE...Gale 177.
         Storm 178.
         Heavy Freezing Spray 121 127 129 130 139 160 180 185.
 FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...Ahsenmacher
SOUTHCENTRAL ALASKA...MSO
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...TM
MARINE/LONG TERM...KH