Storm Warning 173, NWS Anchorage, AK

824 
FZAK62 PAFC 230021
OFFALU

Offshore Waters Forecast
National Weather Service Anchorage AK
321 PM AKST Sun Dec 22 2019

Offshore Waters Forecast for the Bering Sea

Wind forecasts reflect the predominant speed and direction
expected. Sea forecasts represent an average of the highest
one-third of the combined wind wave and swell height.

PKZ499-231400-
321 PM AKST Sun Dec 22 2019
.SYNOPSIS FOR CENTRAL BERING SEA...

A 967 mb low at 56N 168W Sun afternoon moves to 58N 157W at 976 
mb Mon afternoon. A 989 mb low at 46N 179E Mon afternoon moves to
51N 169W at 973 mb Tue afternoon.

$$

PKZ411-231400-
Bering Sea Offshore West of 180 and East of the International
Dateline-
321 PM AKST Sun Dec 22 2019

...GALE WARNING THROUGH MONDAY...

.TONIGHT...NW wind 30 to 45 kt. Seas 16 to 23 ft. Freezing spray.
Snow.
.MON...NW wind 25 to 40 kt diminishing to 15 to 25 kt in the
afternoon. Seas 12 to 17 ft. Freezing spray.
.MON NIGHT...NW wind 10 to 25 kt. Seas 9 to 14 ft. Freezing spray.
.TUE AND TUE NIGHT...NW wind 10 to 25 kt. Seas 7 to 10 ft.
.WED THROUGH FRI...E wind 20 to 35 kt. Seas 9 to 14 ft.

$$

PKZ412-231400-
Bering Sea Offshore 171W to 180 and North of 56N-
321 PM AKST Sun Dec 22 2019

...GALE WARNING TONIGHT...

.TONIGHT...NE wind 20 to 35 kt. Seas 17 to 24 ft. Snow.
.MON...NE wind 10 to 25 kt. Seas 11 to 17 ft.
.MON NIGHT...NE wind 15 kt. Seas 10 to 13 ft. Freezing spray.
.TUE...NE wind up to 15 kt. Seas 8 ft.
.TUE NIGHT THROUGH WED...N wind up to 20 kt. Seas 7 ft.
.THU THROUGH FRI...NE wind 10 to 25 kt. Seas 6 to 11 ft.

$$

PKZ413-231400-
Bering Sea Offshore 171W to 180 and South of 56N-
321 PM AKST Sun Dec 22 2019

...STORM WARNING TONIGHT...

.TONIGHT...W wind 35 to 50 kt. Seas 20 to 34 ft. Rain with snow.
.MON...W wind 30 to 45 kt becoming NW 15 to 30 kt in the
afternoon. Seas 16 to 24 ft.
.MON NIGHT...N wind 10 to 25 kt. Seas 11 to 15 ft.
.TUE...NE wind 10 to 25 kt. Seas 9 ft.
.TUE NIGHT...N wind 10 to 25 kt. Seas 7 to 10 ft.
.WED...N wind up to 15 kt. Seas 7 ft.
.THU THROUGH FRI...E wind 10 to 25 kt. Seas 8 to 13 ft.

$$

PKZ414-231400-
Bering Sea Offshore East of 171W-
321 PM AKST Sun Dec 22 2019

...GALE WARNING TONIGHT...

.TONIGHT...N wind 30 to 45 kt. Seas 15 to 30 ft. Rain and snow
showers.
.MON...N wind 15 to 30 kt becoming 25 kt in the afternoon. 
Seas 12 to 20 ft. Snow and rain showers.
.MON NIGHT...NW wind 10 to 25 kt. Seas 9 to 14 ft. Freezing spray.
.TUE...N wind 10 to 25 kt. Seas 7 to 10 ft.
.TUE NIGHT...N wind 15 to 25 kt. Seas 6 to 9 ft.
.WED THROUGH FRI...NE wind 10 to 25 kt. Seas 5 to 10 ft.

$$

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
359 PM AKST Sat Dec 21 2019

.ANALYSIS AND UPPER LEVELS...
A vertically stacked storm force low continues to strengthen as it
lifts northward across the western Aleutian and into the southern
Bering Sea. The frontal system is tracking north and east from
the low center across the Bering Sea and Aleutian chain. Widespread
gale to storm force winds are being observed across the region
along with rain and snow. A trailing short-wave trough is tracking
eastward, leading to strong cold air advection across the far
western Bering Sea. This colder air is beginning to feed into the
back side of the storm system which is strengthening winds even
more. Expect hurricane force winds to develop along and near the
western Aleutians late this afternoon through this evening.
Precipitation will also change from rain to snow, though surface
temperatures near freezing will limit visibility reductions.

Meanwhile, an upper level short-wave trough is tracking eastward
across Southwest Alaska and headed for Southcentral. With a cold
and dry air mass in place, this trough is coming through mostly
dry, with just a few areas of flurries or light snow over the
Kuskokwim Valley. There are some mid to high level clouds out
ahead of the trough which are helping to keep temperatures a bit
warmer than they otherwise would be. Still, temperatures are in
the single digits to teens across most of the southern mainland
of Alaska.

Low level cold air advection accompanies the upper trough,
particularly along the south end of the trough from Bristol Bay
to the Gulf. This is leading to strong westerly winds across the
western Gulf and Kodiak Island coastal waters. This is also
helping maintain offshore gap winds along the north Gulf coast
and Prince William Sound.

&&

.MODEL DISCUSSION...
Models are in fair agreement with key large scale features. There
is a split in solutions with track of the Bering low on Monday,
with the NAM/GFS stalling the low in Bristol Bay and the
ECMWF/Canadian tracking it across the Alaska Peninsula into the
western Gulf. This will greatly affect the precipitation and wind
forecast for the Bristol Bay and Alaska Peninsula area. For now,
will stick with current forecast which favors the NAM/GFS idea
of bringing another round of precipitation and wind to Bristol
Bay.

Solutions also diverge on Tuesday with whether a short-wave moving
out of Southwest Alaska phases with a trough over the Gulf. The
12Z NAM is the current outlier with development of a closed low
over the Gulf and maintaining two completely separate troughs.
The rest of the solutions indicate some aspect of phasing as the
troughs lift northward toward Southcentral. This latter idea would
lead to greater potential for a snowstorm in the Anchorage area
sometime in the Tuesday to Wednesday time frame.

&&

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

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Offshore flow will continue bring cold temperatures across much
of Southcentral. Although an upper level trough will track across
the region Sunday morning, it should depart eastward fast enough
for skies to clear before sunrise, thus dropping low temperatures
to near zero in Anchorage. Clouds will again increase through the
day Sunday with light snow developing in the afternoon over the
Kenai Peninsula and Prince William Sound. Though accumulations are
not expected to be significant (1 to 3 inches possible along the
Seward highway between Girdwood and Seward), brief periods of
blowing snow with reduced visibility are likely early Monday
morning in Turnagain Pass.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Cold temperatures will persist through Sunday night across all of
southwest Alaska. The main item of interest will be an
approaching front that's attached to a strong Bering Sea low.
This low approaches the coast for Sunday. Increasing wind speeds
and snow developing will proceed the front, with the strongest
winds expected along the northern portion of the Bristol Bay
coast from Cape Newenham eastward to Dillingham. Here, winds
gusting up to 35 mph combined with snowfall will likely reduce the
visibility at times. However, the numerical models still disagree
with the track of the Bering low, as the NAM, GFS, and GFS
ensemble models take this low across northern portions of the bay,
while the Canadian global and ECMWF take the low into the Alaska
Peninsula. The latter solution would decrease the snowfall amount
and blowing snow potential. Stay tuned.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
A strong Bering Sea low continues to slowly deepen and passes to
the northwest of Amchitka this afternoon. The latest satellite
loops show this nicely, as the infra-red shows cooling cloud tops
on the backside of the low, as additional shortwave energy dives
into the base of the associated trough from Siberia. Visible
imagery shows the destabilizing effects of the colder air wrapping
into the system as the upper-level disturbance gets ingested,
with a few TCU (towering cumulus)-like features noted in the
latest loops. This is of no surprise though, given ocean delta-Ts
approaching 15 deg. C in the surface to 850 mb (~4400 feet), with
model estimates on soundings of MUCAPEs of 100-150 J/kg.

Thus, sufficient instability exists for good vertical growth in
the backside of the low, with perhaps a flash or two of lightning
not out of the question. The more important concern is with this
type of instability, given the amount of lift/upward motion
associated with this low, as colder (heavier) air invades the
region you enhance the potential of mixing the stronger winds
aloft down to the surface. With that in mind, we increased the
winds for much of the Bering Sea and Aleutians for tonight and
Sunday, before the storm slowly weakens by Sunday night as it
becomes more vertically stacked.

At this time, the current high wind warning headlines look good.
We'll need to keep an eye on the observational and model trends
though, as the Nikolski and Dutch Harbor/Unalaska areas will be
right near warning level thresholds.

&&

.MARINE (Days 3 through 5: Tuesday through Thursday)...
Tuesday will be the quietest of the extended period days for
Southern Alaska waters as a whole, with no strong features to
speak of. A weak spinoff triple-point low will be over the western
Gulf, the last remnants of the giant hurricane force low
currently out over the Bering. Thus, save for some small craft
winds south of this low the Gulf will be quiet on Tuesday.

Meanwhile a developing North Pacific low will begin producing gale
force winds north of the low center on the Pacific side of the
Alaska Peninsula and eastern Aleutians Tuesday afternoon. The low
will strengthen as it moves into the southwestern Gulf Wednesday
morning, producing storm force winds on the west side of the
center. It strengthens even further as it moves to between Kodiak
and Middleton Island Wednesday evening. There are some model
differences, but barrier jet winds to hurricane force are not
outside the realm of possibility at this time. Regardless of local
hurricane force winds, widespread storm force winds are likely
over large swatch of the Gulf starting Wednesday afternoon and
continuing into Thursday with the return flow behind the low.
Storm force winds persist through the day Thursday through the
Barren Islands, with widespread gales across the central and
southern Gulf.

A separate gale force front will move into the western Aleutians
on Wednesday. While it will maintain its strength through
Thursday, it will stall out its movement over the central Bering,
and thus, impacts from the front should be minimal.

&&

.LONG TERM FORECAST (Days 3 through 7: Tuesday through Saturday)...
A very interesting, active weather pattern is in store across the
Southern mainland through the long term. Uncertainty begins right
from the get-go. A deep polar upper level trough will be over
Southwest Alaska Tuesday morning. An associated shortwave trough
extending southeastward from the low will attempt to phase with a
second upper level low over the far southern Gulf. The models are
in poor agreement as to how much phasing occurs. The better the
phasing, the more likely accumulating snow across Southcentral,
including Anchorage, will be. Snow lovers will like the EC, which
begins steady snow as early as Monday evening. The rest of the
models aren't quite as bullish with the moisture until Tuesday,
when the main trough axis approaches. Even with the driest models,
all of them show some snow during the day, a very fitting pattern
for Christmas Eve. The GFS holds off on the heaviest snow until
overnight Christmas night and into Christmas Day. For now that is
an outlier, as the general model consensus has the snow stopping
overnight.

As mentioned in the marine discussion, on Christmas Day, all eyes
will turn to a strong low tracking northeastward through the Gulf
towards Prince William Sound. The center is decently agreed upon
in the models to track to near Cordova late Wednesday night into
Thursday morning. This would normally be a dud for Anchorage
snow-wise. However, there is good agreement that the supporting
500 mb trough axis remains west of Anchorage, which would support
moisture being drawn northwestward from the low and into Anchorage
and the Mat-Su. Thus, potential for another snowfall event on
Thursday is certainly there. Elsewhere, blizzard conditions are a
significant threat with the low, including for Kodiak, Seward,
Thompson Pass, and potentially Cordova if they can stay cold
enough. All of Southcentral will want to stay particularly attuned
to the forecast over the coming days. One other thing that can be
said for certain is that everywhere away from the immediate coast
will stay plenty cold enough that precipitation type will not be a
problem, snow will be the name of the game.

Model agreement fades quickly on the details beyond the low on
Friday and Saturday, but, it seems likely that an even colder air
mass, colder than the one we have in place presently, will move
into Southcentral, likely causing some areas first sub-zero
temperatures of the winter.

Outside of Anchorage-proper, interesting weather will also be
occurring. For Southwest Alaska, the forecast is actually simple.
The storm track will stay out over the Gulf, with a very
persistent strong upper level trough or low that sits over
Southwest Alaska for much of the long-term period. This means dry,
windy, but most importantly, bitterly cold temperatures. Many
interior areas won't even get to zero for highs, and lows will
routinely flirt with 20 below across the Kuskokwim Valley.
Bitterly cold wind chills will be a significant threat for several
days next week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...High Wind Warning 187.
 Winter Weather Advisory 161.
MARINE...Hurricane Force Wind Warning 173 175 177 178.
 Storm Warning 155 165 170 172 174 176 179 185.
 Gale Warning 119>121 130 131 138 139 150 160 171 180 181.
 Heavy Freezing Spray 129 140 180 181.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...SB
SOUTHCENTRAL ALASKA...AP
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD
MARINE/LONG TERM...JW