Small Craft Advisory 997, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
500 PM AKST Tue Dec 22 2020

.ANALYSIS AND UPPER LEVELS...
The broad longwave pattern with a deep vertically stacked low over
the Central Bering with rapidly developing and fast moving lows in
the Gulf continues to dominate the weather pattern. The first of
these lows moved over Kodiak Island early this morning, bringing
storm force winds to the Gulf coast along with high winds from the
Kenai Peninsula into higher elevations of the Anchorage Bowl.
These high winds included some reports of wind gusts over 100 mph
along the Anchorage Hillside, with 60-70 mph observed in the
lower elevations, and reports of wind damage and power outages
throughout the Bowl. The next rapidly developing low can be seen
surging northward into the Gulf on a slightly more eastward track,
but with similar intensity to last nights Kodiak low.

Over the Bering Sea, the nearly stationary vertically stacked low
looks quite benign considering its central pressure analyzed at
nearly 950 mb this morning. It did however produce a broad area of
gale force winds through the Central-Eastern Aleutians today,
which haven't diminished much due to the steady state of the low
itself.

&&

.MODEL DISCUSSION...
The models are in very good synoptic scale agreement considering
the fast moving and rapidly developing nature of the 2 lows
progressing through the Gulf through Wednesday morning. As a
result forecast confidence in the general trends of the forecast
is quite good. The main forecast challenges being the details of
gap winds and precipitation type around Southcentral. This good
agreement and forecast confidence begins to break down late
Thursday as none of the guidance seems to have a good handle on
how quickly to the north the next North Pacific low will track
towards the AKPen. For now the forecast favors the middle ground
solution of the GFS, but the forecast into Friday will likely
need to be tweaked further with better agreement.

&&

.AVIATION...
PANC...Winds have dies down across the Anchorage area. This has
allowed temperatures to begin to cool. As a result, it now looks
favorable for rain and snow to switch to all snow. It should
remain quite light through around 09Z then pick up in intensity.
With higher confidence that precipitation will go to all snow and
remain snow, there could be a period of IFR conditions as heavier
precipitation moves through 09Z and 15Z. More unstable air will
move in behind the snow, which should allow ceilings/vis to
quickly recover to VFR tomorrow morning.

&&

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

The Blizzard Warning remains in effect until midnight tonight for
Turnagain Pass. Expect additional snowfall accumulations of 6
inches with wind gusts up to 50 mph reducing visibility and making
travel conditions very hazardous.

A Small Stream Flood Advisory remains for Moose Pass to Seward
until 12:30 p.m. Wednesday as it continues to rain in the area.
Heavy rainfall could lead to ponding of water on roads and
hazardous driving conditions as frozen culverts prevent adequate
drainage.

A High Wind Warning is in effect from 6 p.m. this evening until 5
a.m. Wednesday for Cordova as the hurricane force barrier jet
moves inland. Expect sustained easterly winds between 40 and 50
mph with gusts up to 75 mph. Be prepared for debris, property
damage, power outages and difficult travel conditions.

The Small Stream Flood Advisory for Anchor River remains in
effect through Wednesday morning/afternoon. Due to warm sea
surface temperatures, and temperatures not as cold aloft as
necessary for a transition to snow, we expect high winds with a
wintry mix... mostly rain for sea level locations. This could be
problematic for Anchor River, with ice jams and minor flooding
possible. For Homer Bluff, there is a greater chance for strong
winds with snow and blowing snow due to the higher elevations.

The Anchorage sounding indicates a warm nose, with a wet bulb
right around zero. This means that the warm air is not significant
enough to keep precipitation for the area as rain. Therefore,
there is more confidence that snow will easily replace any rain
across the area, as the backside of the low and associated cold
air move across the region. With this, snowfall is anticipated
tonight through Wednesday morning across Cook Inlet, with
potential snowfall accumulation of 1 to 3 inches expected in the
lowlands and up to 5 inches along the mountains (including the
Anchorage Hillside).

After this, snow will move north with drier air moving in to
replace what was previously a moisture laden Southcentral. Expect
continued snowfall for Thompson Pass and Cordova into Wednesday
night. Beyond this, temperatures look to remain above average as
the active storm pattern provides a break as we approach the
holiday weekend.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2...Tue evening
through Thu)...The pattern will be relatively quiet and cooler
heading into the middle of the week. The strong storm system that
brought winds and warm temperatures is pushing inland as it slowly
slides to the east. Steady snow is lingering across the Alaska
Range. However, for most of the area, we expect only some spotty
snow showers through the evening. Southerly flow overnight into
Wed will keep temperatures on the mild side, but not as warm as
they were today. They should be cold enough to support all snow
with any showers that do form. By midday Wed, a much smaller low
will start to track towards the YK-Delta Coast. This will bring a
brief shot of some snow back to much of the coastal areas of SW
AK heading into Wed night into early Thu. This system will bring
some steady onshore/southwest flow with it which could allow some
rain to mix in along the coast, but this would be very isolated.
The system falls apart quickly as it tries to move inland into the
Lower Kuskokwim Valley. Broad onshore flow with cold air advection
will follow this system on Thu. As high pressure starts to dry
things out late Thu, onshore flow could lead to some patchy fog.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2...Tue
evening through Thu)...A rather strong low is moving right through
the Central Bering this evening. The surface pressure associated
with this is in the mid 950 mb range. However, it is mostly away
from any landmass, so the impacts associated with it should remain
minimal. It will keep a fairly widespread swath of gales across
the Central and Southern Bering Sea and sliding into the Central
Aleutians by late tonight. It will also bring some snow along with
it, possibly mixed with some rain at times, to the Central
and Easter Aleutians tonight into early Wed. Meanwhile, over the
Western Aleutians, another front will slide through early Wed.
This front will quickly be replaced by a broad low complex with
several smaller circulations by early Thu. These circulations
will be tough to nail down due to their compact nature. However,
there looks to be enough cold air aloft to support a predominantly
snow precipitation type. Wherever these smaller lows land will
have the potential for gusty (gale-force) winds and snow showers.
With sea surface temperatures still as warm as 40F, blowing snow
should not be a big concern with these systems.

&&

.MARINE (Days 3 through 5: Friday through Sunday)...
Gulf of Alaska: Models are in relatively good agreement that a
low will track into the eastern Gulf, keeping quiet conditions
over the western and central Gulf through Saturday morning. By
Saturday afternoon, models all agree that a North Pacific low will
track into the northern Gulf. However, there is still some
uncertainty in the strength of this system, which will likely
remain below gale force before moving inland by Sunday morning.
Behind this low, a strong front enters the western Gulf through
Sunday afternoon, which could bring a round of gale force winds to
the region.

Bering Sea: Models are struggling with the placement of a low over
the eastern Bering/AKPEN region on Friday afternoon, which could
see an area of gale force winds, before quickly weakening Friday
night. A much stronger, strengthening system farther west will be
the main feature to focus on starting Friday afternoon. The
associated gale force front will move over the central Aleutians
through Saturday morning. However, there is still model
uncertainty as to how strong this system will get as it moves
eastward through Saturday afternoon. Models agree that there will
be high end gale force winds, with the potential to even bring
storm force winds into the forecast area.

&&

.LONG TERM FORECAST (Days 3 through 7: Friday through Sunday)...
The long term forecast starting Friday looks to start with
continued persistence with a deep vertically stacked low over most
of the Bering, with weaker but quickly developing lows in the
Gulf. With deep southerly flow over most of the Mainland and
continued active weather, confidence is good that temperatures
should remain above normal with an increased risk for storms in
the Gulf through the upcoming weekend.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Blizzard Warning 125.
         High Wind Warning 135.
         Flood Advisory 121 125.
MARINE...Hurricane Force Wind Warning 119 125.
         Storm Warning 120.
         Gale Warning 128 129 130 131 132 139 140
                  155 165 170 172 173 174 175 177.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...DEK
SOUTHCENTRAL ALASKA...CK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MO
MARINE/LONG TERM...AH