Small Craft Advisory 25, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
522 PM AKST Mon Jan 7 2019

.ANALYSIS AND UPPER LEVELS...

An upper level ridge remains in place extending along an axis
from Southwest Alaska to the Gulf of Alaska. Out west, an upper
level Kamchatka low has retreated from the Bering to the
northwest, but its corresponding fronts continue to move eastward
over the central and eastern Aleutians. Cold air on the backside
of this Kamchatka low is filling in over the western Aleutians,
producing gusty westerly gales.

Cold air continues to pour in over Mainland Alaska as well. An
Arctic high centered north of the state has generated plenty of
cold air advection and brought temperatures down well below zero
across the Interior and also across Southcentral, especially in
and around the Copper River Basin. Fog and low level stratus
persist across Southwest and Southcentral again today in response
to subsidence associated with the ridge of high pressure. Radar
imagery shows some showers over Bethel as the approaching, albeit
weakened, eastern Bering front begins to move inland.

&&

.MODEL DISCUSSION...

Models remain in generally good synoptic agreement through the
next 48 hours. Localized challenges include fog and low level
stratus that persists around Cook Inlet and both Turnagain and
Knik Arms. By Wed evening, models indicate discrepancies with the
placement and strength of an incoming North Pacific low over the
western Aleutians. Models are also struggling with the timing of
an upper level low moving in from the north over the Mainland by
the end of the week. At this time, higher resolution models were
preferred to help resolve local effects such as wind and fog.

&&

.AVIATION...
PANC...Expect at least some improvement in ceiling/vis late this
afternoon through early evening as some mid level clouds move
overhead. However, don't think this will be enough to completely
erode all of the fog and stratus in the area, so it could expand
later tonight once mid clouds pass by. Forecast confidence in
details of location of fog (and specific conditions at the
terminal) remains low.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2: Tonight
through Wednesday)...

Relatively quiet weather is expected to continue across nearly all
of Southcentral for the next couple days. The fog that has stayed
across the lower elevations of the Anchorage Bowl may be disrupted
during the evening hours by some passing higher stratus, but it
appears unlikely at this point that the stratus will persist long
enough to entirely dissipate the fog. Thus, it has been added to
the forecast into Tuesday morning, when more extensive cloud
cover is expected.

The winds are slowly dying down across Thompson Pass this evening.
This should allow the wind chills to rise generally into the -30s
for the rest of the night, and the Wind Chill Advisory there will
expire at 5 pm.

The latest model runs have trended much further west with a system
that had potential to bring a light snowfall to Anchorage and the
surrounding areas Tuesday evening. Now, all of the models have
shifted west, leaving the area dry for that time frame. That wave
eventually does swing across Southcentral, but not until Wednesday
night. Now it is expected to move through from the northwest,
which allows the Alaska Range to keep nearly all of the associated
moisture. Thus, other than potential snow showers from any remnant
moisture, Southcentral remains dry.

Finally, temperatures are expected to moderate just a bit the next
couple days, as highs get into the low to mid teens in Anchorage,
and lows stay above zero courtesy of the additional cloud cover.
This still will keep Anchorage below average temperature-wise. On
Wednesday night after the aforementioned front goes through,
temperatures drop once again to readings at or even a few degrees
colder than last night's temperatures. Bundle up!

&&

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

An occluded front will move inland across Southwest Alaska tonight
then move inland Tuesday. This will bring accumulating snowfall
to most of Southwest, with the highest amounts east of the
Kilbuck Mountains. Blowing snow is likely and may briefly reduce
visibilities across the Kuskokwim Delta to a half mile with
northeast winds in the 30 MPH range. A winter weather advisory
for blowing snow has been issued for this threat, as well as the
fact that wind chills may fall as low as -20 to -30 F along with
blowing snow. As the front moves inland, it will stall across the
interior and drop more accumulating snow, possibly as high as 5",
over parts of northern Bristol Bay and the Lower Kuskokwim Valley.
However, confidence in the placement of this snow band is not
very high, but the threat does exist for brief heavier snowfall
Tuesday into Wednesday morning. Snow will end Wednesday afternoon
as northerly offshore flow commences.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
An occluded front associated with a low near Kamchatka is pushing
through Cold Bay, bringing precipitation and increased south
winds. This front will continue pushing eastward tonight and
slowly dissipating, with weak ridging in its wake. A series of
shortwaves rotating around that low over Kamchatka can be seen on
satellite. One impulse is currently bringing gale force winds to
Shemya, and expect that front to bring gales and precipitation to
the central Aleutians and into the central Bering into Tuesday
afternoon. Showers will continue over much of the area into
Wednesday, but expect general benign conditions until the next
storm force low approaches Thursday.

&&

.MARINE (Days 3 through 5)...

Beginning Thursday, forecast confidence remains high as a strong
945 mb North Pacific low approaches the Western Aleutians. This
system will spread gale force winds across most of the Western
Aleutians and north into the Western Bering Sea. Models have
actually been strengthening the low over the past couple of days,
so there is now increasing confidence that storm force winds will
impact locations between Adak and Shemya on Thursday. Areas of
storm force winds could also redevelop on Friday over zones 413
and 414 as the low pushes north then exits to the west of Shemya,
though models are still working out the details for this time
period. Regardless, there is rather high confidence that most of
the Western Bering (west of the Pribilofs) will see sustained
east to northeast gale force winds through the weekend with wave
heights increasing to 20-30 ft. Additionally, the potential for
heavy freezing spray will increase by Saturday over the Northern
Bering as frigid air spills south from mainland Alaska.

The Gulf of Alaska is expected to see rather benign conditions
with high pressure maintaining through Saturday.

&&

.LONG TERM FORECAST (Days 4 through 7)...

The long term forecast begins Friday as a large dome of cold
arctic air spills south across mainland Alaska under a descending
polar vortex. The coldest air will stay north of the Alaska Range
but cold air will still seep south across mainland Alaska. This
will likely lead to the coldest temperatures seen over the region
in the past couple of years, with lows in interior locations
across southern Alaska, especially southwest Alaska, dropping
below -30F Friday morning. Luckily, the cold outbreak will be
short-lived as a strong North Pacific low south of the Aleutians
will help to re-establish a broad upper level low over the Bering
as we head into the weekend. As the low deepens, a downstream
ridge over the Pacific West Coast will amplify causing a portion
of the jet stream to push north up into the Gulf of Alaska. This
will lead to a return of strong southerly flow with much warmer
air spreading north by the end of the weekend. There is still
quite a bit of model uncertainty regarding the specific timing and
track of multiple lows and short-waves pushing across the Gulf,
but there is increasing confidence of warmer, wetter, and overall
more unsettled weather over the southern mainland as we head into
next week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 155.
MARINE...Gale Warning 175 178 185.
         Heavy Freezing Spray Warning 127 139 140 185.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...DK
SOUTHCENTRAL ALASKA...JW
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MM/JA
MARINE/LONG TERM...KVP