Small Craft Advisory 931, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
520 PM AKST Sat Nov 24 2018

.ANALYSIS AND UPPER LEVELS...
Multiple lows are moving through the area, in between two areas
of ridging. The first ridge is over the western Aleutians and
Bering while the other extends into interior Alaska from Canada.
Of the lows, the most impactful one is in the Gulf approaching
Prince William Sound and is bringing storm force winds along the
north Gulf coast. It is also bringing strong winds to many of the
gaps around the north Gulf coast as well as precipitation to the
same areas. Two of the other lows are south of the Aleutians and
tracking east with one other weak low in the eastern Bering.

&&

.MODEL DISCUSSION...
The models continue to struggle in this pattern but seem to
finally be locking in on the track of the Gulf low and have it
moving through Prince William Sound tonight. Further west, there
is still a fair amount of disagreement with a triple point low
forming south of the Alaska Peninsula tonight. Models have not
been able to get a handle on this low and it gets worse as the
low starts moving east early in the week.

&&

.AVIATION...
PANC...Ceilings will lower this evening as bands of snow move over
the mountains from Prince William Sound. Northerly winds in the
low levels will maintain dry air, so snow will sublimate before
reaching the ground. At worst there may be a few flurries. There
is a likelihood of light snow after 09Z as an upper low crossing
the Gulf heads inland, with the upper level low opening into a
wave and passing overhead of Anchorage. For now have maintained
VFR conditions with very light snow, but there could be a brief
period of MVFR conditions between 09Z and 15Z. Ceilings will lift
back up after 15Z as cross barrier flow develops across the
Chugach Mountains helping to cut off any remaining precipitation.

&&

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

A "messy" weather pattern is in place as a storm force low is
currently tracking north toward the Gulf Coast. This low will move
into Prince William Sound tonight, bringing a mix of rain and snow
to the coast with light snow chances inland. The best chance for
inland accumulating snow will be over the western Copper Basin
and Susitna Valley with trace amounts for Anchorage. Light wet
snow will also be possible for Eastern Turnagain Arm and Portage
Valley. Heavy snow will occur tonight through Thompson Pass with
some blowing and drifting before the low moves inland.

Sunday morning the next storm force front moves toward the North
Gulf Coast. This will bring a second round of heavy coastal
precipitation, but with freezing levels moving up to as high as
2000-3000 feet, coastal precipitation will largely be rain with a
mix of rain, snow, and potential inland freezing rain. This part
of the forecast is much less confident given the poor model
consistency on frontal orientation and upper level trough
orientation and timing. Currently it seems that the best threat
for inland rain/freezing rain will be Sunday night into Monday
morning, and even if it is rain, with cold roads, expect the same
impact as freezing rain and temps around 30-32. As the trough
moves inland, expect a mix of accumulating snow and possibly some
spotty areas of valley freezing rain in the Copper Basin Monday
morning through the afternoon.

&&

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

The weather pattern will become more active over the next 24
hours, as a slow-moving North Pacific low heads towards Kodiak
Island. Southeast winds just above the surface (around 4000 feet)
will advect warm and moist air into Southwest Alaska, mainly for
locations east of the Kilbuck Mountains. At the same time, the
surface winds on the back side of this low will be from the
northeast, helping to draw cooler air down from central portions
of the state. The net result is a sub-freezing to freezing wedge
of temperatures near the surface, with warmer air aloft.

This is a classic freezing rain sounding with the latest
numerical output putting out 0.15 to 1/4 of an inch of ice
accumulation. Additionally, as the low draws closer and winds
become more easterly, normally the related down-sloping affect
would help surface temperatures warm more quickly. However, the
drier air will help delay this as the wet-bulb cooling process
delays the warm up. Locations closer to the Kilbuck Mountains and
south of the Nushagak Hills will see an even slower changeover
from snow, to freezing rain, to eventually rain. With this in
mind, a Winter Weather Advisory was issued to account for this.
It'll be interesting to see in later model runs if this potential
continues through Monday, as each numerical run has lowered the
initial temperatures while delaying the warm advection at the
surface.

&&

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

The main forecast challenge lies with the first storm affecting
the AKPEN and its coastal waters, then with a strong front pushing
into the western Bering/Aleutians. In either location, look for
sustained winds to increase to gales, with a few storm force gusts
possible for the coastal waters of the peninsula. Steady light to
moderate intensity rainfall will accompany both of these features,
with an increasing sea state as well. For the remainder of the
area in between these two storms, look for a northerly flow to
dominate the eastern two-thirds of the Bering, becoming
southeasterly towards the western part of the Bering/Aleutian
chain. Here precipitation will be of a more showery variety.

&&

.LONG TERM FORECAST (Days 4 through 7: Wednesday Nov 28th through
Saturday Dec 1st)...

The forecast period commences with a broad longwave trough across
the Bering, the North Pacific and the Gulf Of Alaska with a ridge
axis over the Western Bering. At the onset of the forecast
period, the models have a closed low at 500 mb near Atka and
another closed low at 500 mb in the western Gulf (near Kodiak).
There are numerous shortwaves in the base of the trough axis. As
the forecast period elapses, the model divergence becomes more
amplified. Generally, expecting an active pattern for the Bering
and the Gulf of Alaska. However, due to the degraded performance
of the models as of late, this forecast package reflects a blend
of models. Even with the blending and the hedging, categorically
there are confidence issues with the timing of the precipitation
shields for each storm, the timing of the gusty winds and the
tracks of the lows.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 161.
MARINE...Storm Warning 119 120.
		 Gale Warning 125 127 130 131 132 136
                      138 139 140 150 155 172.

&&

SYNOPSIS AND MODEL DISCUSSION...DK
SOUTHCENTRAL ALASKA...JA
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD
LONG TERM...PS