Small Craft Advisory 947, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
450 AM AKST Fri Nov 30 2018

.ANALYSIS AND UPPER LEVELS...

An upper level low centered over the Kamchatka Peninsula extends
across most of the Bering while a short wave ridge is centered
over Southcentral. A strong 120kt jet is pushing northeast across
the Aleutians and the southern Bering, undercutting the upper
level low. The upper jet is helping to support a surface low over
the Gulf of Anadyr with an associated gale force front extending
south from the low across the western mainland coast and Bristol
Bay. This front is pushing further inland bringing strong
southerly winds, unseasonably warm air, and a large swath of precipitation.
Precipitation has changed over to rain along the coast but colder
air inland is keeping precip mostly as snow.

Further east, high and mid level clouds can be seen advancing
into southcentral while the upper ridge is holding on over the
Copper River Basin, leading to cold temperatures and areas of
patchy fog and low stratus covering most of the inland basin.
Light offshore flow is leading to drier air and clear skies along
the North Gulf Coast this morning.

&&

.MODEL DISCUSSION...

Overall, models are in acceptable agreement at the synoptic level
this morning, though most of the challenges this forecast lie in
the details. Models are differing on how fast warm air moves up
the Cook Inlet this afternoon, impacting precipitation type
forecasts for the next 24 hours. The GFS is trying to keep colder
air around longer up the Cook Inlet corridor, which would lead to
more snow before switching over to rain. The NAM is a bit quicker
at eroding the cold air and transitioning snow to rain from south
to north, which aligns more with current observations and trends.

&&

.AVIATION...
PANC...Showers are in the vicinity of the terminal this morning as
a front approaches from the southwest. Challenges included timing
of wind shear and precip type with the approaching front this
evening. Southeasterly flow will allow a Turnagain Arm wind to
develop beginning Fri night, becoming strongest Sat morning.
Downsloping should help keep precipitation light over the
terminal and predominately VFR through the TAF period. With strong
southerly flow aloft, periods of wind shear were included in the
TAF.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Weak ridging remains over much of Southcentral Alaska which has
helped develop and keep a stratus deck over much of the area. As
the ridge axis pushes east today a front will ride up the western
side and bring precipitation to Kodiak Island, the Cook Inlet
corridor, and the Mat-Su Valleys. This evening it will spread
precipitation throughout the rest of Southcentral Alaska.

The biggest challenge with the forecast is the type of
precipitation the front will bring to areas around Cook Inlet.
Most areas should start off as snow, with low lying areas then
transitioning to rain. However, some areas, such as western Kenai
Peninsula, may see a brief transition to freezing rain before
warming up enough for rain. It does not look like areas that will
see the overrunning of warm air will be able to hold on to
colder temps at the surface for very long. On Saturday, another
wave will come through from the southwest bringing more rain to
Southcentral Alaska.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3/Friday
through Sunday)...
The first of two fronts impacting southwest AK over the next
couple days is currently pushing through the region. It is
producing light snow as it goes, with some locations gusting to
about 30 mph. This system is panning out as expected, which bodes
well for confidence with the next system, since this means our
forecast and the models handled the first one well.

The next system is expected to be more powerful than the front
currently over the area. This next system will cross over the
eastern Aleutians today, then move northeast with the low center
right over Nunivak Island tonight. The frontal system will be
pushing through Bristol Bay today ahead of the low center. As this
front moves in, it is still expected to bring warm air with it,
meaning we are still expecting a mostly rain event. Precipitation
will still start as snow but quickly shift to rain today, with the
possibility of some brief periods of freezing precip over inland
areas north of Bristol Bay. There is not enough of a threat to
issue an advisory though. There is a good fetch of wind out of the
southwest that looks to extend all the way into the lower
Kuskokwim Valley, so warmed temps there a bit tomorrow and Sunday.
As previously mentioned, this system will be stronger with storm
force winds pushing through Bristol Bay. These winds will mainly
occur on the backside of the low as it moves through tonight.
After the passage of this system, the area will see a brief break,
with the next low entering the area Saturday into Sunday.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3/
Friday through Sunday)...
The low that will pass over the eastern Aleutians into southwest
Alaska today is still on track. Gale force winds are expected for
many areas in the eastern Aleutians, with storm force winds
tonight on the south side of the eastern Aleutians. Out in the
central and west Aleutians, there is more zonal flow occurring
with winds steady at about 30 mph. This general pattern for the
central and western Aleutians will be in place through the
weekend.

The eastern Aleutians however, will see another low track over
them tomorrow night, in very similar fashion to the storm force
low expected through the area today and tonight. As of now, this
next low over the weekend will not be quite as strong, maxing out
with gale force winds.

&&

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

The forecast period commences with a longwave trough across the
Central Bering and portions of the North Pacific with a ridge
axis over the Yukon Territory and the Gulf of Alaska. Between
these longwave features southwest flow aloft will dominate much of
the mainland. There are numerous shortwaves embedded in the upper
level trough. At the onset of the forecast period, the
Canadian/GFS and ECMWF have a surface low near Dutch Harbor,
another surface low west of St. Matthew Island and a dome of high
pressure east of SE AK. Even at the onset of the forecast period
the global models struggle with the placement of the surface low
although they have these features. As the forecast period elapses,
the model divergence becomes more amplified. Generally speaking,
expecting a progressive and unsettled pattern for the Bering, the
Aleutians and SW AK. However, due to the lack consistency, this
forecast package reflects a blend of models.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Dense Fog Advisory 141.
MARINE...Storms 155 160 165 170 172 180. Gales 130 132 138 150 171
173 174 176 181 185 352 410.

&&

SYNOPSIS AND MODEL DISCUSSION...KVP/KO
SOUTHCENTRAL ALASKA...DK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...BB
LONG TERM...PS