Small Craft Advisory 19, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
458 AM AKST Sat Jan 6 2018

.ANALYSIS AND UPPER LEVELS...

A front moving across Southwest Alaska and the Gulf is quickly
weakening and falling apart over the area this morning. Thus, the
associated precipitation is becoming more scattered and losing
intensity with time as it moves north. The most telltale sign of
the frontolysis is the rapidly warming cloud top temperatures over
the region as soon by infrared satellite. The 00Z Anchorage
sounding shows a strong inversion over the Bowl, and that is
reflected in temperatures in the upper 10s to lower 20s through
the Bowl, but readings are closer to 30 at higher elevations such
as Glen Alps. The inversion is a stable weather pattern
preventing atmospheric mixing, which in turn is keeping the low
level moisture trapped near the surface, resulting in areas of fog
and low stratus.

In the upper levels, a negatively tilted upper level trough
extends well eastward from a sprawling upper level low over the
far western Bering. The trough is coincident with the dying
surface front, and it too is weakening with time. However, it's
still supporting the surface front enough to keep a pressure
gradient ongoing, which may help to advect some of the fog out of
Southcentral later today. More on the forecast below. The final
upper level feature of note is a second shortwave trough
extending south from the upper level low over the western Bering.
The shortwave is ushering in more ridging and warmer air aloft
behind the wave, briefly switching up the upper level weather
pattern.

&&

.MODEL DISCUSSION...

The models are all in excellent agreement over the next two days
regarding all of the major weather features expected to traverse
the area. Thus, there really was no preferred model over any of
the others. Any large scale forecast changes only reflected
differences that all of the models have locked on to. The forecast
challenges therefore will revolve around small-scale features that
none of the models have a particularly good record tracking, such
as where and when fog will develop, the strength of the gap winds,
and other local weather phenomena. Therefore, forecast confidence
is high.

&&

.AVIATION...

PANC...VFR conditions and light winds will persist. Fog will be in
the vicinity of the airport through the morning, but will remain
patchy. Northerly winds will advect any remaining fog away by mid
morning as a dying front approaches.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The east-to-west oriented weather front in the northern Gulf will
slowly move north today, then stall and weaken tonight along the
north Gulf coast. Almost all of the precipitation with this system
will fall near the coast, over the Kenai Peninsula, and over the
eastern Copper River Basin. Most other areas will not see any
precipitation, though abundant moisture will keep things rather
cloudy. The upper level trough responsible for the front will
"close off" into an upper low near Kodiak Island on Sunday, moving
east into the Gulf Sunday night. At the same time an upper level
high to the north will build south into Southcentral Alaska,
drying out the atmosphere and leading to clearing skies and an
end to the showers.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
The King Salmon radar has detected rain near Pilot Point extending
north of Dillingham and Levelock. The models have the remnants of
the moisture pushing north as the trough lifts and changing over
to snow. One to four inches of new snow will blanket portions of
southwest. Sunday precipitable water values transition from half
an inch to roughly a quarter of an inch across the region. At the
surface, expect the precipitation to taper off with only isolated
snow showers Sunday night. Monday there will be a lull before the
next storm moves into the area.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The broad cold core low centered over the western Bering will
continue to weaken as it ambles northward. A rapidly intensifying
storm will track towards Adak. The system's surface low will
drop roughly 24 mb in 24 hrs from 12z today to 12z Sunday. Both
the GFS and the ECMWF have the surface low at 950 mb and
virtually on top of Adak on Monday. At its apex, the pressure
gradient span from Attu to the AK Pen. This low will bring some
fierce seas and prolonged gusty winds to the chain beginning
Sunday and into Monday.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Beginning Sunday night, a deep storm force low will be making its
way northward across the Aleutian Chain and into the southern
Bering Sea with an occluded front spreading north and east across
the Bering Sea. Strong height rises out ahead of this system will
re-enforce ridging over northern Alaska, though it will re-center
farther west toward Siberia. The GFS is a big outlier in
maintaining two separate upper level high centers which results
in a fundamentally different solution as we head through the week.
For the moment, will stick with the idea that ridging will
persist over mainland Alaska, keeping storminess confined to the
Bering Sea and Gulf. Thus, most of Southcentral and Southwest
Alaska will be seasonably cool and dry, with coastal areas seeing
occasional clouds and a chance of precipitation. The Bering Sea,
Aleutians, Alaska Peninsula, Kodiak Island, and the Gulf will see
continued active weather, which means warm, windy, and wet for
much of this region.

One feature to keep an eye on is an arctic trough dropping down
the east side of the upper ridge on Monday, southward across the
Yukon Territory. Some models, led by the ECMWF, dig this
southwestward into Southcentral Alaska, around the south side of
the upper ridge. This would result in colder temperatures and
perhaps an offshore wind event along the Gulf coast. For now,
trended slightly in that direction, but with so much uncertainty
in position of morphing upper level ridge, will have to wait until
models get a better lock before considering further changes to
the forecast.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Storm 176 177.
         Gale 119 130 131 173 174 175 178 411 412 413.

&&

SYNOPSIS AND MODEL DISCUSSION...JPW
SOUTHCENTRAL ALASKA...BL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PS
LONG TERM...SEB