Wind Chill Advisory 5, NWS Anchorage, AK

327 
WWAK42 PAFC 180035
WSWALU

URGENT - WINTER WEATHER MESSAGE
National Weather Service Anchorage AK
335 PM AKST Fri Feb 17 2017

AKZ155-181800-
/O.EXT.PAFC.WC.Y.0005.170218T0600Z-170218T1800Z/
Kuskokwim Delta-
Including the cities of...Bethel...Hooper Bay...Nunivak Island
335 PM AKST Fri Feb 17 2017

...WIND CHILL ADVISORY NOW IN EFFECT FROM 9 PM THIS EVENING TO
9 AM AKST SATURDAY...

* WIND...North 10 to 20 mph.

* WIND CHILL VALUES...35 below to 45 below.

* TIMING...Wind chills will drop to advisory levels by late this
  evening and continue through late Saturday morning.

* IMPACTS...Increased risk of hypothermia and frostbite.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

A wind chill advisory means that very cold air and strong winds
will combine to generate low wind chills. This will result in
frostbite and lead to hypothermia if precautions are not taken.

&&

$$

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
457 AM AKST Fri Feb 17 2017

.ANALYSIS AND UPPER LEVELS...
An upper level trough extends from the central Bering to Southeast
AK this morning. A closed low near Bristol Bay is in the process
of lifting north and being absorbed by a trough diving down the
west coast of AK. A smaller low is south of Nikolski. At the
surface, a low pressure center with areas of snow is moving inland
from Bristol Bay, while an associated trough is crossing the north
Gulf coast. Snow showers are moving north over the Kenai peninsula
and Prince William Sound, and across Anchorage and the Mat-Su. A
surface low is also located south of the central Aleutians, as
high pressure builds into the western half of the Bering.

&&

.MODEL DISCUSSION...
Models are in agreement that the Bristol Bay low will move inland
today and tonight, and bring more snow showers to Southcentral AK.
High pressure will build into the Bering behind this system and
remain until the next north Pacific low approaches the western
Aleutians with a front Sun into Mon. No particular model has a
great handle on location of ongoing snow showers, so have largely
based short-term forecast on satellite and radar trends.

&&

.AVIATION...
PANC...MVFR conditions with some fog and snow showers will
transition to VFR later this morning. Snow showers will return
this evening, likely bringing a return to marginal ceilings/vis.
There is some potential for some heavier embedded showers, so
there may be brief periods of IFR conditions.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Bands of snow showers have set up along a 500 mb jet streak on the
leading edge of an upper level short-wave, from the Kenai Peninsula
to Anchorage and the Mat-Su. Speed divergence is also noted from
Seward to Anchorage, indicative of stronger upward vertical motion
which is being manifested as a narrow band of heavier snow. The
banded nature of the snow leads to a challenging snow forecast,
with large differences in snow intensity and accumulation over
small distances. In any case, the 500 mb jet will lift quickly
northward and weaken through the morning hours. Therefore, do not
expect more than an inch or two of accumulation for most locations.

Attention will then shift to the closed upper level low sitting
over Bristol Bay. It will open into a trough as it lifts
northeastward toward Southcentral today then track overhead
tonight. This short-wave will be much more conducive to a larger
area of snow out ahead of it, so expect marginally higher snow
totals across the region. Instability will increase as colder
aloft moves overhead, so still expect localized areas of heavier
snow with continued variability over small distances. Have
generally upped amounts in the latest forecast.

Low level cold advection behind will ensue behind the short-wave,
which will lead to a few lingering showers on Saturday. Another
upper level short-wave will then lift north from the Pacific on
Saturday/Saturday night, so more (light) snow is possible.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
The vertically stacked low centered over the Greater Bristol Bay
area will continue to weaken through the day and then shifts east
across the Alaska/Aleutian Range this afternoon and evening. Snow
showers will diminish to scattered coverage this afternoon and
then become progressively more limited to areas along the
mountains through the weekend. A cold upper level arctic low will
dig south today to be centered over the Seward Peninsula where it
will remain through the weekend. A trough swinging around the
arctic low will push into Southwest Alaska on Saturday dropping
temperatures from west to east. With brisk north winds continuing
along the coast, wind chills will be an increasing concern along
the Kuskokwim Delta coast as temperatures fall. A colder, deeper
trough will follow Sunday night further reinforcing the very cold
air mass over Southwest Alaska.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
An upper level ridge currently building over the western Aleutians
will progress east tonight and Saturday, pushing the upper level
low south of the Aleutians east as well. A relatively weak frontal
system will reach the western Aleutians Saturday evening followed
by a significantly stronger frontal system Sunday night. The
associated surface low looks to deepen to around 950 mb with storm
force winds likely.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Beginning the long term forecast on Sunday, there will be a large
closed arctic low over Northwest Alaska with a weakening and
shearing frontal zone moving into the Western Bering Sea. The cold
air associated with the upper trough will be filtering into
Southwest Alaska with temperatures trending well below average.
Some of this cold air will filter into Southcentral by early week
which will push most locations slightly below average. Snow
threats will mainly be over Southcentral where more favorable
dynamics will exist as the aforementioned upper trough digs
southeast. Light snow threats look to linger through Monday
evening before finally ending as the trough moves east and
weakens.

A deep and strong storm force bent-back occlusion is still on
track for the Western Bering Sea Sunday night and Monday. This
will shift the pattern to a much more progressive southwest flow
pattern for Southern Alaska midweek and beyond. With large scale
amplification of the Northeast Pacific ridge, this will also
warm things up by the end of the week which will create some
precipitation type issues for inland locales as thicknesses warm
sufficiently to support some rain mixing in. However, the first
low on Wednesday looks potentially favorable for all snow for
inland locales, but some model runs such as the 18z GFS take this
feature so far south that it completely misses the mainland. For
now will favor a WPC/ECMWF approach which still shows increasing
precipitation chances by late Tuesday into Wednesday.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Heavy Freezing Spray Warning 179 185.

&&

SYNOPSIS AND MODEL DISCUSSION...DS
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JR
LONG TERM...JA