Winter Storm Warning 3, NWS Anchorage, AK

941 
WWAK41 PAFC 201833
WSWAER

URGENT - WINTER WEATHER MESSAGE
National Weather Service Anchorage AK
933 AM AKST Thu Dec 20 2018

AKZ125-201945-
/O.CAN.PAFC.WS.W.0003.000000T0000Z-181220T2100Z/
Western Prince William Sound-
Including the cities of Whittier, Seward, Girdwood, 
and Moose Pass
933 AM AKST Thu Dec 20 2018

...WINTER STORM WARNING IS CANCELLED NORTH OF MOOSE PASS...

The heavier snow has started to taper off across much of the area.
Periods of snow showers will continue through this afternoon.

$$

http://www.weather.gov/afc

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
431 PM AKST Wed Dec 19 2018

.ANALYSIS AND UPPER LEVELS...

Since yesterday, the trough over Southwest Alaska extending over
Kodiak has now become a newly formed closed low over the western
Gulf east of Kodiak City. The remaining trough still persists
across Bristol Bay and extending northwestward over the Kuskokwim
Delta. This has made the entire upper level trough become very
negatively tilted, forming a boomerang shape. Meanwhile across the
Bering, a very pronounced deformation band has developed, which
extends southeast to northwest across the Cold Bay area and out
into the western Bering. This is a classic boundary signature of
cold air to the north and east of the boundary deflecting the
incoming warmth and moisture from the south into a line
perpendicular to the incoming moisture. In the upper levels, this
ridging is rather weak. To the west, heights actually drop due to
an incoming trough associated with an even deeper upper level low
centered over the southern Sea of Okhotsk. The ridge in between
the incoming trough and the trough over mainland Alaska is
essentially being squeezed out and elongated with time, a feature
common with distinct deformation zones.

The upper low over the Gulf is slowly tracking northward with
time and enhancing lift and upper level divergence ahead of it.
In this case, that means along the Gulf Coast and the Kenai
Peninsula. Thus, those areas are continuing to see steady light
snow. Even Cordova has cooled enough to get all snow. The key is
the strongest lift, and thus, heaviest snow are concentrated along
the Gulf Coast and Prince William Sound, with a rather distinct
northern edge, which is approximately along an Anchorage to Valdez
line. The upper flow out of the east is helping keep Anchorage
and points north relatively drier due to cross-barrier flow across
the Chugach. Conversely, snow extends further north across the
southern Susitna Valley due to upslope.

Temperature-wise across Southwest, we are seeing conditions
broadly below zero at most inland locations. That said, those
sites with a significant wind, such as Iliamna are staying above
zero. A fair bit of stratus is moving southward across the
Kuskokwim Delta south of Bethel. The cold spots this morning are
Aniak and King Salmon, both of which are well colder than negative
20 with calm winds and mainly clear skies.

&&

.MODEL DISCUSSION...

As has been the case the past several days, the models are in
generally good agreement on the placement of the large scale
features, but how those translate to the surface is very much in
disagreement. Light steering level winds ultimately will make any
surface lows that form in the Gulf move very slowly. Thus, any
changes in the weather would be slow. Additional disagreements in
the models appear by Thursday regarding a front moving into the
western Bering and where any associated surface lows develop. In
addition, the models are having a difficult time deciding even by
this afternoon how quickly a stationary surface low south of
Seward begins to move eastward and how quickly any secondary lows
form. Confidence even with as early as today's forecast regarding
timing and location of snow and any associated wind shifts
remains unusually low for how short the time frame involved is.
Quieter weather returning Thursday and beyond to Southcentral will
help improve forecast confidence as concerns turn to gap winds and
how cold the temperatures get.

&&

.AVIATION...
PANC...Scattered snow showers are producing brief periods of MVFR
conditions, but conditions are expected to steadily improve over
the next few hours as the snow tapers off. A few more snow showers
may get as close to the Hillside, but otherwise offshore winds
will dry out the area with predominately VFR conditions expected
for the rest of the TAF period.

&&

.CLIMATE...
Anchorage International Airport tied its record consecutive days
with measurable snowfall this morning at 9 days. This is the first
time in the 21st century this has happened. Below are the other
streaks of 9 consecutive days of measurable snow and the total
snow that fell during those 9 days:

1) December 11-19, 2018: Total Snowfall: 21.7 inches
2) December 4-12, 1991: Total Snowfall: 13.0 inches
3) November 26- December 4, 1987: Total Snowfall: 19.3 inches
4) February 16-24, 1968: Total Snowfall: 23.4 inches
5) March 31- April 8, 1956: Total Snowfall: 10.6 inches

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)...

An upper-level low stretching across the western and northern
Gulf will drift northward overnight tonight. This feature is
associated with a weakening trough and meso-low at the surface,
with the trough over Prince William Sound and the meso-low in the
northern Gulf moving northwest over Middleton Island. Areas of
snow have been falling along the trough from Whittier and
Turnagain Pass west through Turnagain Arm with widespread moderate
snow showers throughout Prince William Sound. This will continue
through the evening and overnight hours as the trough stalls and
the surface low tracks farther west. For locations like Whitter
and Portage, an additional 6 to 10 inches of snow is expected
overnight into tomorrow. The upper-level trough lifts north
tomorrow, but persistent moist, easterly fetch through the sound
will keep snow showers going along the upslope areas of the
eastern Kenai and Chugach Mountains through Thursday. Another
trough moves north late Thursday into Friday with another round of
snow and perhaps a rain/snow mix along the east coast of Kodiak
Island. For the Southcentral interior, clear and dry conditions
will persist through the end of the week.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
The southwest mainland remains under a dry cold air advection
pattern through the end of the work week. Expect temperatures to
begin to moderate slightly warmer on Friday as the low pressure
from the west begins to lift toward the Southwest, shunting the
dominant ridge north of the Bering Strait.

&&

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

A developing low pressure system slowly tracks east south of of
the Aleutians tonight spreading a mix of rain and snow along with
gusty gale force winds through Thursday night, then tracks south
of the Gulf. Another weak surface low which developed this
afternoon just west of the Pribilof Islands, which was not
previously expected, will keep winds more easterly resulting in
colder temperatures. Issued a Winter Weather Advisory for Blowing
Snow with the afternoon package as the freezing line remained just
south of St. Paul Island. This new surface low drifts west early
Thursday morning ending blowing snow conditions as the
precipitation bands shift. The next front quickly moves to the
western Aleutians on Thursday night bringing another gale force
front.

&&

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

Saturday starts off by bringing a weakening low into the southern
Gulf with gusty conditions along the north Gulf coast. The exact
position of the low is still a bit different between models, but
no matter where it is, offshore flow and precipitation along
coastal areas of Southcentral should will be the main focus for
the weekend. Early next week (Mon/Tues) should remain relatively
dry for Southcentral.

Models have come into better agreement with a surface low moving
over the western Aleutians on Saturday, which previous models
showed as a front. The models tend to show this low weakening by
Sunday as another deep low sitting south of Adak lifts northward
over the Aleutian Chain. A front associated with this low will
move into the Gulf sometime between Tuesday and Thursday. Exact
movement and intensity of this low looks to be the biggest
forecast challenge as it should weaken and move into the Bering
through the middle of next week. Updating these low features were
the focus of today's forecast.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 195.
MARINE...Gale 130 155 172-179 185.
         Heavy Freezing Spray 130 160 181 185.

&&

SYNOPSIS AND MODEL DISCUSSION...RA/BC
SOUTHCENTRAL ALASKA...TM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KH
LONG TERM...AH