Small Craft Advisory 1020, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
427 PM AKST Fri Dec 23 2016

.ANALYSIS AND UPPER LEVELS...
Major shift in the longwave pattern from yesterday as the upper
level trough has shifted eastward and the longwave ridge is
building ahead of the low moving into the Central Aleutians. Upper
level jet shows a strong southerly connection, which is resulting
in a lot of warm air flowing through the Aleutians into the Bering
Sea. In the lower levels, the upper level low in the Aleutians is
well reflected at the surface, and the strong gradient between the
surface low and the building ridge to the east is bringing 70mph
plus winds to the Eastern Aleutians. The mainland is still under
the influencing of ridging at the moment, which means that
freezing fog along Cook Inlet and Bristol Bay has been quite
pervasive this morning.

&&

.MODEL DISCUSSION...
Models have been in fairly good agreement on the position and
strength of the low pressure system moving into the Central
Aleutians. Winds with the warm front through the Aleutians were a
bit weak with the GFS, although the NAM seems to have a better
handle on these. In the east, timing on the front moving into the
area over the weekend has been very consistent, however there is
variability between the models on how quickly the air warms with
the front. This could mean the difference between snow, rain, or a
mix for the eastern Kenai peninsula and Portage Valley. Current
thinking is that the models are underdoing the amount of warm air
moving in and that snowfall will be heavy and wet before switching
to rain at lower elevations.

&&

.AVIATION...
PANC...Fog continues to persist this afternoon with high pressure
overhead and little wind. A brief break is expected this evening
but fog will most likely return after the sun goes down. The next
front moving in should mix the air enough to dissipate the fog but
this is not expected until early tomorrow morning.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Ridging over Southcentral Alaska will push off to the east
overnight and behind it a strong front will move into the area
from the southwest. It will first make it to Kodiak Island
Saturday morning bringing a rain and snow mix that will quickly
transition over to all rain. The front will then make it to the
Kenai Peninsula by Saturday afternoon initially bringing snow.

The majority of the snow will be for the eastern Kenai Peninsula
and will then spread to other locations around Prince William
Sound. The front will also bring strong winds with it, especially
for channeled areas of the eastern Kenai Peninsula like Portage
Valley and Turnagain Pass. These areas will see blowing snow
conditions late Saturday with the combination of heavy snow and
increasing winds. The challenge comes in determining how much warm
air will move in, how fast it will move in, and how quickly that
will mix in rain with the snow.

In this type of pattern temperatures will likely rise throughout
Saturday night with the coldest temperatures in the evening. It is
also likely that lower areas like Portage Valley will see rain
mixing in earlier, while snow continues at higher elevations such
as Turnagain Pass along with wind-sheltered valleys. However, even
when areas around Portage Valley transition to rain, the rain
falling on snow covered roads can still make for difficult driving
conditions. The winds will then taper down on Sunday, but the
precipitation will continue.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
An impressive Bering Sea front is currently bringing strong gusts
across both the eastern Aleutians and the Alaska Peninsula. Low
level southwesterly flow at low to mid levels is acting as a
trapping layer and contributing to particularly strong gusts.
Strong winds are next expected to develop along the Kuskokwim
Delta coast and western portions of Bristol Bay as the front
pushes eastward overnight and tomorrow. While some blowing snow
is possible along the Kuskokwim Delta, enough warm air advection
is associated with the front to generally keep visibilities above
blizzard criteria. However, at higher elevations, enough cold air
is in place for several new inches of new snow especially along
the Ahklun Mountains.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
As the potent east Bering front continues to push eastward, a
more mellow pattern will develop across the Bering over the next
few days. Enough instability will be in place to support scattered
showers across much the region, as broad low pressure dominates.
Meanwhile, a fairly weak front will skirt across the Central
Aleutians this evening and support more widespread showers for
the Aleutians.

&&

.LONG TERM FORECAST (Monday evening through Friday)...
Clear and cold conditions will dominate the extended period. On
Monday evening, the previous front will be all but dissipated over
the northern Gulf of Alaska. The responsible low pressure system
will still be spinning overhead with mostly cloudy conditions and
snow showers, mainly along the north Gulf coast. On Tuesday, the
system pulls away to the east as another weakening upper system
slides into the southern Gulf. The pattern will change, brought
about by a rapidly deepening Kamchatka low that will amplify the
upper pattern. The system will bring another frontal boundary with
strong winds across the Bering Sea. On the larger scale, the
system will build a large ridge over the eastern Bering and
western mainland. The result for Southcentral Alaska will be a
return to outflow conditions for much of the area. Expect clear
skies and gusty winds along the coast for favored gap locations,
with light winds elsewhere, as well as frigid arctic air.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...High Wind Warning 185.
         Winter Weather Advisory 125.
MARINE...Heavy Freezing Spray Warning...140.
 		 Storm Warning 119 130 131 155 165 170 172 180.
		 Gale Warning 120 125 132 136 137 138 139 140 141 150
					  160 171 173 174 175 176 177 179 181 185.
&&

SYNOPSIS AND MODEL DISCUSSION...LF
SOUTHCENTRAL ALASKA...DK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...EN
LONG TERM...MTL