Blizzard Warning 19, NWS Anchorage, AK

942 
WWAK41 PAFC 181309
WSWAER

URGENT - WINTER WEATHER MESSAGE
National Weather Service Anchorage AK
409 AM AKST Fri Dec 18 2020

AKZ131-182100-
/O.CON.PAFC.BZ.W.0019.000000T0000Z-201218T2100Z/
Northeast Prince William Sound-
Including the cities of Valdez and Thompson Pass
409 AM AKST Fri Dec 18 2020

...BLIZZARD WARNING REMAINS IN EFFECT UNTIL NOON AKST TODAY...

* WHAT...Blizzard conditions occurring. Total snow accumulations
  of 4 to 8 inches. Winds gusting as high as 60 mph.

* WHERE...Thompson Pass.

* WHEN...Until noon AKST today.

* IMPACTS...Plan on slippery road conditions. Widespread blowing
  snow will reduce visibility to one quarter mile or less at
  times. 

PRECAUTIONARY/PREPAREDNESS ACTIONS...

A Blizzard Warning means severe winter weather conditions are
expected or occurring. Falling and blowing snow with strong winds
and poor visibilities are likely. This will lead to whiteout
conditions, making travel very hazardous. If you must travel,
have a winter survival kit with you. If you get stranded, stay
with your vehicle.

For the latest road conditions call 5 1 1 or visit
511.alaska.gov.

&&

$$

http://www.weather.gov/afc

230 
WWAK41 PAFC 182035
WSWAER

URGENT - WINTER WEATHER MESSAGE
National Weather Service Anchorage AK
1135 AM AKST Fri Dec 18 2020

AKZ131-182145-
/O.CAN.PAFC.BZ.W.0019.000000T0000Z-201218T2100Z/
Northeast Prince William Sound-
Including the cities of Valdez and Thompson Pass
1135 AM AKST Fri Dec 18 2020

...BLIZZARD WARNING IS CANCELLED...

Visibility across the pass continues to improve into the afternoon
and winds will slowly diminish. Blizzard-like conditions may
periodically occur over the next few hours, but the overall trend
is towards improvement. 

$$

http://www.weather.gov/afc

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
535 AM AKST Thu Dec 17 2020

.ANALYSIS AND UPPER LEVELS...

A diffused shortwave aloft could still be seen on water vapor
imagery as it moves northeast of Mat-Su Valley early this
morning. Meanwhile, another shortwave connected to an upper low
over southwestern Gulf extends eastward of Kodiak Island. Strong
winds and moisture aloft brings a wide spectrum of mid to high-
level clouds across Bristol Bay through the northern Gulf
including Prince William Sound. Over the Bering Sea, a stacked low
north of the western Aleutians treks eastward. Its associated
frontal boundary extends southeastward across the central Bering
then turns south past the eastern Aleutians. Strong upper levels
combined with heavy moisture along the frontal boundary will move
across the eastern Bering. On the west of Nunivak Island and
Bristol Bay, an upper level trough linked to an upper low over the
northern Bering will continue to push through Southwest Alaska
this morning. Low level stratus and patchy fog continues to make
their way across the eastern Bering and through the southern
AKPEN, and also over eastern Kodiak Island.

&&

.MODEL DISCUSSION...

GEM-Reg and ECM are making a strong pact with GFS and NAM for the
short term forecast. The current synoptic features over the
Bering and south of Kodiak Island are recognized by the models,
with GFS and NAM being the aggressive solutions by this evening
for the solo system over the southwestern Gulf. Both GFS and NAM
are ahead with the arrival time of the frontal boundary over
Prince William Sound (PWS) by tonight, while GEM and ECM arrives
much later tonight into early Friday morning across PWS.
Essentially, precipitation will be heavily focused over the
northern Gulf and portions of PWS tonight, then spreading inland
and over Cook Inlet Friday morning. Over the weekend, models
introduces another developing low over the southern Gulf by Friday
night, which could bring increased mix rain/snow across the the
Gulf and Southcentral through the weekend. However, there's quite
a bit of uncertainty for the models on this next system. Over the
Bering, the next frontal boundary and complex low arriving as
early as Saturday night are nicely handled by the models through
early next week.

&&

.AVIATION...

PANC...North winds will continue to help dry out the low levels
and eliminate any threat of fog or stratus. Winds will remain out
of the north through the day with VFR conditions expected.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 2/Thursday
through Friday)...

Low stratus has by and large cleared out of the Anchorage Bowl
this morning with the increasing northerly low level winds, and
conditions should remain quiet and gradually colder over the MatSu
valleys from now into Friday. Areas of lower cloud cover will
persist for longer over the Copper River Basin where an arctic
airmass will continue to gradually filter into place, and
temperatures may drop well below zero Fahrenheit where breaking or
clearing of in the stratus deck occurs over the next day or so.

Attention continues to hone in on a significant outflow wind
event that is anticipated to cause blowing snow through Thomson
Pass and across the Copper River Delta. A Gulf low approaching
from south of Kodiak will reach the coastal areas to the
southeast of PWS by early Friday morning, and winds are expected
to peak in intensity through mountain gaps across the central and
eastern Chugach Mountains as the low approaches the northern Gulf
coastline and then moves inland by Friday Afternoon. Additionally,
while model disagreement persists on the exact placement, a
deformation band of moderate to heavy snow may form to the
northwest of the low center, or over and northeast of PWS. With a
foot of transportable snow in place, and with the potential for
several additional inches of snow to fall in sync with winds
gusting to near 50 mph through Thompson Pass, blizzard conditions
remain possible by this evening and possibly into much of Friday.
A winter storm watch remains in effect for Thompson Pass due to
the potential for significantly reduced visibilities to a 1/4 mile
or less during the height of the wind and blowing (and possibly
falling) snow event.

Models continue to struggle beyond Friday into early Saturday with
another system entering the Gulf from the north pacific with
varying degrees of phasing or a lack thereof depicted between a
shortwave ejecting quickly into the northern gulf and a second
wave moving up the AKPEN by early Saturday morning. Another round
of coastal precipitation appears likely with the second low by
early Saturday, but the details remain difficult to resolve at
this time with the significant model divergence by this point

&&

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

Today, moisture will wrap around the back side of a low near
Kodiak and into the AKPEN. The moisture combined with weak
shortwave energy will bring bands of light snow to portions of SW
Alaska and the AKPEN. Friday, a second low will move from the
Central Bering and the associated shortwave will become negative-
tilt as it reaches Bristol Bay and SW Alaska. Another round of
light snow is expected. The models are not showing significant QPF
nor accumulations with either system.

&&

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

There is a surface low west of Attu island and another low north
of Adak this morning. The system north of Adak is already
occluded and getting jet support. While the low west of Attu is
weak and will wobble around the western Bering. Looking at
satellite imagery, the PWAT signatures are relatively weak. The
models have light precipitation coincident with the front as it
pushes across the chain. At this time, expecting mostly a
rain/snow mix with the front. There are light bands of rain ahead
of the frontal boundary. The second system near Attu has more cold
air entrained in it. That system will produce snow showers. The
models bring the next major system towards Shemya Sunday morning,
but the model agreement with this system is lackluster. Stay tuned
for more details!

&&

.MARINE (Days 3 through 5/Saturday through Monday)...

Gulf of Alaska:

There is moderate confidence that low pressure will persist over
the Prince William Sound through the forecast period. Model
discrepancies remain in regards to the strength of the westerly
winds to the south of the low center. Small craft to low end gales
are possible. There is high confidence a front will reach the
western Gulf on Monday with small craft to gale force winds
likely.

Bering Sea:

Generally benign conditions are expected throughout the Bering on
Saturday. On Sunday a broad, strong and complex low with multiple
centers is expected to move into the western Bering. Areas of gale
force winds are possible near the individual low centers Sunday
and Monday; however, confidence in the exact location of the
stronger winds is low at this time.

&&

.LONG TERM FORECAST (Days 4 through 7/Sunday through Wednesday)...

A west to east pattern change of warmer and wetter weather appears
to take place on Sunday as a broad and strong low pressure system
moves into the western Bering. Ridging along the storms front is
expected to bring much warmer temperatures and rain to areas of
the Aleutian Chain and Southwest Alaska through Monday. The finer
details of the forecast are messy due to the complex and broad
nature of the front and the low itself. Guidance indicates the
presence of many small low pressure systems and which one will
become dominant over time will be a forecast challenge. For
Southcentral, weather appears to be quiet through Monday morning
other than a few areas of snow near the coast. Guidance then
agrees that broad southerly flow will take hold of the region
through Wednesday. Generally speaking this pattern would mean rain
for lower elevations of coastal areas with snow likely inland and
over high terrain. The details of the rain/snow line is highly
uncertain at this time.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Storm Watch 131
MARINE...Storm Warning: 175
         Gales: 119-120 127 130-131 138-139 155 170-174 176-177
         Freezing Spray Warning: 181
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...CMB
SOUTHCENTRAL ALASKA...AS/MO
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PZ
MARINE/LONG TERM...CJ