NWS Anchorage, AK wrote its last area forecast discussion before the first event at Jan 16, 5:54 AM AKST, 7 hours 10 minutes earlier.
Area forecast discussion, Jan 16, 5:54 AM AKST, 7 hours 10 minutes before
Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
554 AM AKST Sat Jan 16 2021
.ANALYSIS AND UPPER LEVELS...
In the upper levels ridging continues to push through Southcentral
AK and into Southwest AK. This is the peak of a ridge that travels
all the up the West Coast of the continent from a high off the
coast of Southern California. Broad long wave troughing also
continues to exist in the Bering Sea. It is anchored by a 500 mb
low located south of the AKPen. It is this low that is providing
the upper level support for the large low at the surface in the
North Pacific.
The low at the surface is our major weather feature. This low is
a large occluded low with a center located just south of the AKPen
and moving northwest. Its frontal system is currently approaching
the Southcentral coast and is well observed on satellite and
RADAR. Meanwhile, precipitation along the front is also moving
across the AKPen and into Southwest AK. The backside of the low is
interacting with high pressure in the Western Bering to induce
northerly flow across much of the Aleutians. This is creating snow
showers for much of that area. So, quite the large system that is
determining the weather for all of our forecast area.
&&
.MODEL DISCUSSION...
Models continue their recent trend of good agreement in the short
term. They are handling the front currently moving through the
Gulf of ALaska well. Models are also doing a good job of handling
all features inland and in the Bering Sea, leading to a higher
confidence level in model data for the weekend. Sunday, models
begin to diverge on the current low as it moves north into
Southwest Alaska. However, they do not differ enough to make major
differences in the forecast. Monday shows another powerful low
moving into the Western Gulf. As with yesterday, even though
models differ on low track, there is enough certainty to begin to
hone the forecast.
&&
.AVIATION...
PANC...VFR conditions and light winds will persist. The possibility
exists of low-level wind shear developing at the airfield early
this morning. This is due to southeasterly winds coming out
Turnagain Arm. Winds will be strong enough above the surface to
induce directional shear. These winds are associated with a front
moving in from the Gulf of Alaska.
&&
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2: Today
through Monday)...
The very active weather pattern across Southcentral continues this
Martin Luther King Jr. Day holiday weekend. First up is a front
and associated low that are already in the Gulf and moving
towards the coast. The front is producing moderate to at times
heavy rain across Kodiak Island this morning, especially east
facing portions of the island, including Kodiak City. The front
will continue moving north into the coast this afternoon, then
move into the mountains tonight. Warm air associated with the
front has already moved into the Kenai Mountains, with rain and
very gusty winds to 65 mph already being reported through Portage
Valley this morning. All of the reporting stations along the
Seward Highway are at or above freezing, and will continue to warm
through the day. Thus, for most of the day today, the predominant
precipitation type along most of the Seward Highway will be rain.
However, an additional 1 to 2 feet of snow are likely at the
higher elevations of the adjacent Chugach and Kenai Mountains
today. Downslope drying winds are expected for Anchorage and much
of the rest of the Cook Inlet region, so little if any
precipitation is expected today in these areas. As with most
northward moving fronts, winds with the barrier jet will increase
substantially in response. For this particular front, winds will
increase to storm force today along the North Gulf Coast.
From tonight through Sunday, the weather pattern will be steady
state, with persistent onshore southeasterly flow keeping coastal
rain and mountain snow ongoing pretty much constantly, though at
much lighter rates than with today's front. This will keep
abundant cloud cover over much of the area, with occasional breaks
at times, well above normal temperatures, and generally dry
conditions in the downslope areas around Cook Inlet and the Copper
River Basin, and generally wet/snowy conditions at the coast and
in the coastal mountains.
On Sunday night, the next strong North Pacific low moves into the
Gulf which looks to cause high winds, heavy mountain snows, and
increased avalanche potential across portions of Southcentral into
Monday. High Wind Watches have been issued for Turnagain Arm and
higher elevations, Portage Valley, and the Cordova area from late
Sunday night through Monday afternoon. The low center will move to
near Seward Monday afternoon. Ahead of the low, the barrier jet
and strong easterly winds associated with the leading front and
the northern half of the low will increase with gusts to hurricane
force in the High Wind Watch areas. There is still significant
uncertainty with the eventual track and speed of the low, which in
turn will impact where the strongest winds will be, but the
strength of the low favors the high winds in those areas. For the
Cordova area, it will be a similar pattern as the low back on
December 21st and 22nd. The storm to potentially hurricane force
barrier jet will develop as the leading front approaches the
coast. Then as the low itself enhances the southerly winds behind
the front, the barrier jet will be pushed inland into the Cordova
area, causing the high winds during the day Monday.
The other aspect of this storm will be the heavy precipitation
rates. For most areas, this storm will be just as warm as the
current front, meaning most of the road network, including through
Portage Valley, most of the Seward Highway, and the Seward area
will see mainly rain, which when added to melting snow will cause
rising river levels. There is some uncertainty, related to the
storm track, as to how high the freezing levels get, so rain is
possible even up to Turnagain Pass, but it's equally probable that
it remains a heavy wet snow. As such, blowing snow could be a
concern in Turnagain Pass that may require forecast updates over
the next couple days. The other area of concern for heavy snow
with gusty winds is through Thompson Pass and along the Richardson
Highway south of the Edgerton Highway. A Special Weather Statement
has been issued for those areas. It currently looks as though a
heavy wet snow will fall on gusty southeast winds through these
areas. Southeasterly winds are perpendicular to the Pass, and as
such should keep the wind speeds down. This combined with the
heavy, wet nature of the snow should preclude widespread
blizzard/blowing snow conditions in this area. However, localized
visibility reductions in the windier areas to near whiteout are
very possible Monday into Monday night. Another 1 to 2 feet of
heavy, wet snow are expected in the mountains around Turnagain
Pass, Thompson Pass, and along the Richardson Highway through
Monday. This will likely elevate the avalanche potential in those
areas. Refer to the local avalanche forecast centers for more
details.
&&
.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)..
Precipitation across parts of Bristol Bay has begun as a front
pushes into the AKPEN and Southwest Mainland. Currently this is
all snow, but will transition to a rain/snow mix or all rain for
Southern Bristol Bay and the AKPEN later today. Temperatures will
be warmer for Iliamna and King Salmon, as well as surrounding
areas, today and tomorrow before beginning to cool off again.
Areas along the coast of Bristol Bay will teeter on the edge of
freezing today and drop back below freezing tomorrow. A band of
snow will extend up into the Kuskokwim Delta and Western Kuskokwim
Valley by Sunday. Winds are also expected to become gusty,
especially along the coast, thus, blowing snow is likely to be an
issue at times. Advisory criteria is not expected, however,
visibilities may be reduced during periods of heavy snowfall.
Conditions across the Mainland should begin to calm down again by
Sunday night.
&&
.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
A front off a North Pacific low approaching Bristol Bay is
bringing rain and snow showers to the Southern AKPEN and Eastern
Aleutians, as well as some gusty winds today through Sunday.
Widespread small craft winds stretch across the Bering and gales
persist along the southern portions of the Bering and along the
coastal waters from the Central Aleutians to the Southern AKPEN.
Winds will be strongest through the bays and passes through Sunday
afternoon before starting to diminish Sunday evening. Periods of
blowing snow is likely across Dutch Harbor and Cold Bay, which
could result in significantly reduced visibilities. Each passing
snow band looks to be narrow, so these conditions are expected to
be brief. By Sunday afternoon the pattern shifts and a second
system approaches the Western Bering. This system will bring gale
force winds and a band of precipitation to the Western Aleutians.
&&
.MARINE (Days 3 through 5): Monday through Wednesday...
Gulf of Alaska: A strong North Pacific low moves over Kodiak
Island into the Alaska Peninsula on Monday with an estimated
intensity of 970 mb. Model tracks and development vary somewhat,
but forecast confidence is good. Widespread high end gale force
winds and storm force winds move northward over the Gulf. Monday
night the surface low will move over the Kenai Peninsula although
the cyclonic flow will persisted over the Gulf Of Alaska until
Tuesday. Brief ridging will follow in the wake of the low.
Aleutians/Bering: A strong Kamchatka low and front moves east
through Tuesday. Widespread high end gale force winds anticipated
with this system as the front moves to the Central Aleutians and
across the Bering. Enhanced seas expected with this system as wave
heights are forecast to reach 19 feet for the Bering side, and
the Pacific side. A new low will develop in the NW Pacific on
Monday and will stay south of the chain.
&&
.LONG TERM FORECAST (Days 3 through 7): Monday through
Thursday...
A progressive pattern is on tap for this forecast period. The
longwave trough extends from NE Russia to Southcentral Alaska and
the Gulf Of Alaska. There are numerous embedded shortwaves.
Expect a series of lows for the Bering, North Pacific and Gulf Of
Alaska. Monday there will be surface low near Kodiak and a second
low northeast of Attu Island. The jet stream is highly amplified.
There is a jet core south of Kodiak with a magnitude of 155 knots
and a second jet core with a magnitude of 170 knots south of
Amchitka/Adak. Sensible weather-wise, expect snow in portions of
southcentral Alaska. The Chugach Mountains, Kenai Mountains and
Cordova Mountains will receive 2+ feet of new snow in a 24 hr
window. As this low complex moves north additional snow
accumulations anticipated. Right now the snow level is hovering
between 800-1500 feet above sea level. Also on Tuesday the front
out west will be near Cold Bay. Past Wednesday, divergence has
emerged in the models but they are all showing a continuation of
the wet pattern.
&&
.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...High Wind Watch: 101 125 135.
MARINE...Storms: 119 120 131 351.
Gales: 130 132 136-139 150 155 160 165 170-178 180 352
411 413 414
Heavy Freezing Spray: 160 180 185 411 414.
FIRE WEATHER...NONE.
&&
SYNOPSIS AND MODEL DISCUSSION...BJB
SOUTHCENTRAL ALASKA...JW
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KM
MARINE/LONG TERM...PS