Heavy Freezing Spray Warning 49, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
405 PM AKST Sat Feb 3 2018

.ANALYSIS AND UPPER LEVELS...
The upper level high over northwest Alaska with a ridge extending
into the Tanana Valley is preventing the northward push of clouds
associated with the surface lows and upper level troughs south of
the Alaska Peninsula and Kodiak Island. This has resulted in a
strong inversion over much of the Copper River Basin and portions
of the Susitna Valley. Frigid air is in place under the inversion
with marked temperature difference over short distances of 30 to
40 degrees due to the inversion and cold air trapped beneath it.
This dome of high pressure and cold air, coupled with low pressure
to the south is keeping the strong outflow gap winds going along
the Alaska Range and Chugach Mountains.

An upper level and surface low south of the Aleutians has a front
extending east into the Gulf of Alaska. A tight gradient in
advance of the front is resulting in gale force winds along the
Alaska Peninsula. This has also held the warm air across the
Aleutians and southern half of the Bering Sea to the Alaska
Peninsula.

&&

.MODEL DISCUSSION...
The models all remain in generally good agreement through the
short term. The models differ in the initialization of a weak low
south of Kodiak Island that does result in different QPF solutions
for Kodiak Island tonight. If the Canadian solution comes to
fruition, more rain and snow will occur than if the NAM solution
wins the day. At this point we are sticking with the current
forecast solutions that takes the middle ground between the two
solutions. The main concerns over the next day or two is the
extent of the precipitation and visibility reduction issues from
Kodiak west along the Aleutians. The outflow gap winds across the
southern mainland are expected to slowly diminish over the next
couple of days.

As for the next front approaching the Gulf of Alaska coast Monday,
the models have come into better general agreement bringing the
bulk of the energy toward southeast Alaska and holding the
precipitation along the immediate coast while allowing increased
cloud cover and an increased chance of precipitation for inland
portions of Southcentral Alaska.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
An upper trough and a couple surface lows will lift northward into
the southern Gulf tonight, with precipitation and wind spreading
northward. Strong riding over the Alaska mainland will keep these
storms confined to the Gulf and Kodiak Island. The bulk of forcing
is out over the Gulf, but Kodiak will see periods of light
precipitation through Sunday. Although there is still quite a bit
of cold air aloft over Kodiak Island, with most of the warm air
advection remaining offshore, the persistent strong northeasterly
winds at the surface have helped temperatures climb into the upper
30s. The afternoon sounding shows the freezing level has risen to
around 1000 feet. Thus, expect mostly rain near sea level, perhaps
mixed with snow at times.

Cold and dry conditions will persist over Southcentral for the
remainder of the weekend. Gap winds will continue to very slowly
diminish in the Matanuska Valley and Valdez/Thompson Pass area
through Sunday night as the air mass begins to moderate and
surface pressure gradients slacken. Temperature in the
Palmer/Wasilla area have remained quite warm in recent days
compared with nearby areas which are out of the wind. As winds
finally begin to lay down a bit, temperatures will quickly fall -
with widespread sub-zero temperatures expected Sunday night.

The beginning of a warm-up is headed to Southcentral Monday as
ridging weakens, allowing a front extending from a low along the
Alaska Peninsula to make more northward progress. May see
precipitation as far north as the southern tip of the Kenai
Peninsula, with mid to high clouds extending northward across
much of Southcentral. While most of the significantly warmer air
will remain down over the Gulf, the presence of clouds will help
limit radiational cooling and start a slow upward trend in
temperatures.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2...Sun and Mon)...
The end of the dry and cold weather is in sight. First things
first, the warm occlusion that is moving through the Eastern
Bering is just clipping SW AK. While little to no precipitation is
expected from this feature, it has been enough to turn winds to
more of an east-southeast direction. These winds have allowed
significantly warmer air to push into much of the area. King
Salmon and Iliamna are both 21F degrees warmer than yesterday
afternoon, and Bethel is 11F degrees warmer. However, these winds
and warmer air have not been able to penetrate through the cold
dome over the Lower Kuskokwim Valley where temperatures remain at
or just above 0 degrees. The big question for tonight is whether
or not the winds will continue and keep the inversion from setting
up. The forecast thinking right now is that the winds will
slacken quite a bit, so we have dropped temperatures for many
locations for Sunday morning. Sunday will be the last dry and
mostly sunny day across the entire area.

By Monday, a new low pressure center will push yet another warm
front towards SW AK. The difference this time will be that the
cold dome of high pressure that has been in place will not be
there to deflect it. Thus, we expect snow to return part of
Bristol Bay, especially from Dillingham north. This system will
also bring another round of gusty east-southeast winds. These
winds could help to blow the snow around a bit, but with more warm
air pushing it, we think the threat of this is low and would be
short-lived.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2...Sun and
Mon)...
The deep trough sitting over the Aleutians will make for a warmer
and wetter weather pattern. Embedded within the trough are various
areas of low pressure spinning within the broad cyclonic gyre.
Because of the depth of the trough, each system has a fair amount
of moisture to work with from the North Pacific. The first front
(early Sunday) will slowly decay as it stretches from the Eastern
Bering through the Western Aleutians. The next disturbance will be
close on its heels bringing more rain to Dutch Harbor Sunday
afternoon and evening. Then the next stronger area of low pressure
will press through the Alaska Peninsula Monday. Most of the
precipitation associated with this will be rain, but could mix
with snow at times, especially over the higher terrain of the
peninsula. Gale-force winds will also accompany this low for
marine zones south of the chain/peninsula.

&&

.LONG TERM FORECAST (Monday evening through Saturday night)...
Expect slightly moderated temperatures from the current regime
with more chances for snow in most areas. The extended forecast
begins with the blocking high pressure far enough north to begin
to allow some of the cyclonic flow and embedded shortwave troughs
northward toward the southern mainland. The result is two weak low
centers, one moving over Bristol Bay toward Nunivak Island, and
the other toward the Panhandle in the northeastern Gulf of Alaska.
A stretching/decaying front stretches between the two and will be
moving ashore southern Alaska, bringing a chance of snow to most
areas, with the best chances along the north Gulf coast.

In to the longer term forecast, the upper level pattern will be
anchored by a longwave trough centered near Kamchatka. The jet
stream and storm track will ride south of the Aleutian chain into
the Gulf of Alaska, a potentially more active snow pattern for
most areas. However, this pattern looks to be more progressive,
pushing waves off to the east. To get a real good snow pattern for
southcentral Alaska, we'd like to see the longwave trough centered
over southwest Alaska.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale Warning 127 130>132 138 150 155 180.
 Heavy Freezing Spray 127 180.

&&

SYNOPSIS AND MODEL DISCUSSION...SA
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MSO
LONG TERM...ML