High Wind Watch 6, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
450 PM AKST Sat Dec 7 2019

.ANALYSIS AND UPPER LEVELS...

A high amplitude pattern has set up with a ridge axis over eastern
Alaska and longwave trough axis bisecting the central Aleutians.
There is a potent jet core south of the Aleutians with an
intensity of 170+ knots. A fetch of warm air is pushing up the
Cook Inlet. This morning the observed temperature at Seldovia was
52 degrees Fahrenheit. On the latest satellite imagery there is a
low near Kodiak and another low near Nunivak Island. The
precipitation shield spans from the central Bering across
Southwest Alaska into portions of Southcentral and back into the
north Pacific. Meanwhile farther west, a low continues to spin
near Attu Island.

&&

.MODEL DISCUSSION...

Models continue to move towards a consensus with the primary
surface lows that will impact AK over the next couple of days.
They have all converged on a solution that carries tonight's
system up through the Kenai Peninsula and Northern Cook Inlet.
Then, shifting to the storm on Sun night, there is some marginal
discrepancy between the NAM and some of the other models on the
exact strength and track of the low as it hugs the Western AK
coast. In general though, most of the impacts associated with this
would be much the same. However, we will continue to refine the
track to assess the need for any High Wind Warnings or Winter
Weather products. The higher resolution guidance was used this
afternoon to hone in on a deformation band of snow setting up
across the Lower Kuskokwim Valley this evening.

&&

.AVIATION...
PANC...Wind shear remains the main concern today as winds at the
terminal will remain out of the north with stronger southeasterly
flow aloft and over the Turnagain Arm. As a frontal boundary
approaches tonight, it will bring southeast winds into the
airport. The timing of these winds could vary by an hour or two.
There will be a brief shot of precipitation as the front passes
through that could briefly lower the ceilings.

&&

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

An active weather pattern will continue through early next week.
Strong southerly flow aloft is advecting a stream of subtropical
moisture into Southcentral this evening. Rain, heavy at times, is
occurring over the southern Kenai Peninsula and Prince William
Sound. Snow is seen farther north in the western Susitna Valley as
well as wind gusts up to 55 mph over Turnagain Arm. All of this
will continue though Sunday morning as the associated low tracks
northward. Precipitation will remain all rain over the Kenai
Peninsula. Surface temperatures are expected to remain at or just
above freezing overnight across the Anchorage Bowl and much of the
Mat-Su. However, some models are hinting at slightly cooler air
briefly building in behind the aforementioned low. As such,
precipitation type north of and including Anchorage remains
somewhat uncertain. Given the current southeasterly surface winds,
much areas from approximately from the city of Kenai northward
will be downsloped for the majority of the event. Thus, any
wintery mix that does occur early Sunday morning will be light and
likely short-lived.

After a break midday Sunday, a second similar system will
approach Southcentral in the evening. This will again bring
strong winds along Turnagain Arm and the Anchorage Hillside by
early Monday morning. With the exception of higher elevations, an
all rain event is expected, with several inches possible over the
southern Kenai Peninsula.

&&

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

Active weather continues across Southwest as the relentless North
Pacific wave-train of lows continues to barrage the region. One
departing strong gale force low is now lifting into the Bering
Strait, bringing strong southerly winds to the Kusko Delta while
another deformation zone tracks north into the Bristol Bay and
Kusko Valley. This second feature will bring rain/snow mix
turning to snow over the eastern Bristol Bay area and Alaska
Range, with heavier snow over the Kusko Valley. The bulk of snow
ends tonight with a brief respite in weather Sunday. Very quickly
Sunday evening and Sunday night, the next storm force low barrels
into the Bristol Bay region with the front plowing across
Southwest, bringing snow which will then transition to rain
(mainly over Bristol Bay). The bigger impact will be snow and
blowing snow once again over the Kusko Delta, however, this time,
there will likely be limited amounts of snow before the front
clears into the Bering Sea. However, with 35+ MPH winds, there
will likely be reduced visibility in brief heavy snow Sunday night
and Monday morning. Powerful E-SE winds will also blast through
parts of the region with this low, especially across Bristol Bay
and the Alaska Range where wind gusts will reach 60+ MPH.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...

One storm force low departs the Bering into the Strait, another is
right on its heels Sunday evening as it moves into Bristol Bay
and slowly tracks northward through Monday. This will bring more
rain/snow quickly turning to all rain to the Eastern Aleutians.
Broad cyclonic flow will remain over the Bering Sea as the low
tracks north, with cold air advection rain/snow showers spreading
over the remainder of the Chain and Bering Sea.

&&

.MARINE (Days 3 through 5)...

...Bering Sea and Aleutian Coastal Waters...

An area of low pressure near Nunivak Island early Tuesday morning
will bring high sustained gale force winds to the eastern Bering
Sea and Bristol Bay region, with perhaps some low-end storm force
winds just offshore from the Kuskokwim Delta. This low will then
retrograde (move west) and slowly weaken as it reaches Saint
Matthew Island by evening, with the winds having subsided to less
than 30 knots by then. For the remainder of the region, winds and
waves will generally be less than 25 knots with waves generally
less than 15 feet. This will continue into Wednesday and Thursday
for all of the forecast area.

...Gulf of Alaska...

Sustained storm force winds ahead of a surface front will plague
the northern Gulf early Tuesday morning. A favorable dynamic fetch
ahead of this feature will allow seas to build into the 25 to 30
feet range. These winds will rapidly drop to the small craft
variety by the afternoon, as the best dynamics associated with the
front pushes inland. The wave heights will also drop due to the
decrease in wind energy.

This reprieve will be short- lived though, as yet another front,
associated with a north Pacific low, moves into the region late
Tuesday night/Wednesday morning. Sustained winds will increase to
gale force, with the long fetch allowing waves to once again
increase into the 20 to 25 feet range.

Heading into Thursday, both the waves and winds will subside as a
weakening area of low pressure enters the southwestern Gulf.

&&

.LONG TERM FORECAST (Days 3 through 7)...

The forecast remains fraught with uncertainty due to model
discrepancy, but some items remain consistent. Synoptically, a
progressive pattern will persist over the Bering and Alaska.
Starting Tuesday, a vertically stacked low will drift to the
northern Bering, steering modified Arctic air over the Aleutians
and Bering. The low will gradually weaken and shift towards
Siberia, but in the meantime expect scattered showers over much of
the Bering/Aleutians through Thursday. Meanwhile, a North Pacific
low will drift northward and push a warm occluded front toward the
Southern Mainland. Models are consistent bringing decent
precipitation to the AKPEN/eastern Aleutians, but how far north
and east precipitation reaches is tough to discern, given the lack
of consistency with track of the steering low and potential for a
triple-point low to develop. But, right now it looks likely that
the North Gulf Coast will see precipitation as a front pushes
north in the Wednesday to Friday timeframe. By Friday, a strong
low will approach the western Aleutians and bring a good swath of
precipitation and gusty winds. The front will quickly traverse the
chain Friday, possibly turning the Southwest Mainland wet
Saturday.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...WSW 155.
MARINE...Storms 120 130-132 138 352.
         Gales 119 125 139-141 150 155 160 165 180 181 185.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...PS
SOUTHCENTRAL ALASKA...AP
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JA
MARINE/LONG TERM...MM/PD