Small Craft Advisory 25, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
504 PM AKST Sun Jan 7 2018

.ANALYSIS AND UPPER LEVELS...
An upper level high over the mainland part of the state will build
in a little stronger over the next day. The jet stream is south of
the Aleutians but fueling the strong surface low and front that
is approaching them. This strong low will begin to weaken by
tomorrow as the main upper level low moves from the near the
Kamchatka Peninsula to the central Aleutians tomorrow making the
system vertically stacked.

&&

.MODEL DISCUSSION...
Model agreement remains good through the middle of the week. The
big uncertainty for the next day or so in Southcentral AK will be
whether stratus or fog develops around Cook Inlet. The NAM is
hinting at this but is the only one. However, the NAM's higher
resolution is often the most accurate one for stratus around Cook
Inlet. The Arctic airmass moving into the region midweek will
likely bring in gusty gap winds and the NAM is indicating these
winds will persist for a number of days.

&&

.AVIATION...
PANC...Light winds will persist into Monday afternoon but should
start to increase from the north late Monday afternoon. With a
strong inversion in place over the area, there will be a risk of
low stratus (and possibly fog) developing after midnight tonight
as the mid level clouds dissipate. Stratus is more likely of the
two due to the inversion strength and the dry air.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
An area of high pressure over Northern Alaska will shift south
tonight phasing out a weakening low over the Gulf of Alaska. This
will mean continued clearing from north to south across the region
this evening as drier air filters in. Clearing skies will promote
fog formation over the Cook Inlet/Anchorage region tonight and
tomorrow morning with some stratus already showing up on webcams
over South Anchorage this afternoon.

As high pressure builds in over Southcentral Monday and Tuesday,
an upper level trough will dig down from NW Canada across the
ALCAN border bringing strong northerly flow. The trough will draw
down frigid polar air from interior NW Canada into western parts
of Alaska, which will bleed into the Copper River Basin and
interior sections of Southcentral. This looks to be the coldest
air of the season so far with temperatures bottoming out well
below zero for the Mat-Su Valley and parts of Anchorage by
Wednesday morning. The coldest air will be located over the Copper
River Basin where temperatures will likely plummet into the -40's
F. The large thermal gradient between cold dry air in the
interior and relatively mild moist air near the coast will drive
strong northerly gap winds across the region, including through
the Matanuska Valley, Valdez, and out of the Copper River Delta,
where winds in these locations could gust upwards of 50 mph by
Tuesday afternoon.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
High pressure over mainland Alaska along with increasing low
pressure over the Bering with the next system approaching the
southern Bering, will generate some drier offshore flow over the
southwest Mainland tonight. With no real increase in 850 mb temps
and increased offshore flow, fog is not likely persist or
redevelop again overnight over much of the southwest Mainland.
With the clearing skies and a tightening pressure gradient across
the northern portions of the lower Kuskokwim Delta, colder air
will drain into the area from the north, helping to cool
temperatures for the Monday into Tuesday time frame. Our focus
then turns to the front pushing inland over the southwest Mainland
early Tuesday morning with precipitation developing along and
immediately ahead of it.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
A broad area of low pressure begins to retreat from the Bering, as
the next north Pacific low and its associated leading front begins
pushing through the central/eastern Aleutians and into the
southern Bering by tonight. This system is pretty similar to the
storm that impacted the region over the last few days. Overall,
look for a strongly occluded front to sweep eastward across the
chain, while the parent low stalls. Widespread precipitation, some
of which will be moderate to heavy in intensity will accompany
the front, with more showery type precipitation behind it and
closer to the area of low pressure.

Winds will rapidly increase along and ahead of the front. For the
land zones, current indications are these winds will remain below
headline criteria. For the coastal and offshore waters, storm
force winds are likely for west central portions of the Aleutian
waters and into the southern Bering Sea, with winds increasing to
gales elsewhere along the front and near the low.

&&

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

The long term forecast period begins Tuesday night with a high
latitude blocking high over the interior and a broad upper trough
extending across the Pacific Ocean to the eastern Bering Sea. The
main jet energy will remain well to the south of the AOR with the
blocking high acting as the main dominant weather feature for
Alaska. Expect a return to colder than average mainland
temperatures, especially in prone low lying areas and basins. Any
precipitation will be relegated to coastal regions like Kodiak
and the North Gulf Coast through Wednesday night.

Thereafter, the North Pacific jet will begin to shift the overall
trough position farther eastward. This will eventually result in
increasing cyclogenesis and more southerly flow Thursday through
the weekend. There will be a noticeable uptick in frontal
progression northward with increasing chances of coastal
precipitation (probably turning to predominantly rain as it warms
up by the weekend). Inland areas will gradually warm up as well as
clouds increase and mid level temperatures warm dramatically.
Precipitation looks to remain low, however, as cross-barrier flow
will likely play a strong role with this pattern. This means
stormy weather is likely for the immediate coast Thursday and
beyond.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Storms 173-175 178. Gales 150 160 165 170-172 177 179 180
                                   181 185 411-414.

&&

SYNOPSIS AND MODEL DISCUSSION...EZ
SOUTHCENTRAL ALASKA...KVP
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...TP
LONG TERM...JA