Heavy Freezing Spray Warning 71, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
514 AM AKST Wed Feb 15 2017

.ANALYSIS AND UPPER LEVELS...
The flow pattern aloft is anchored by an upper level low pressure
system spinning over the Alaska Peninsula. Several shortwave
trough disturbances are rotating around the low; several along the
north Gulf coast, another over the southwestern mainland, as well
as several lobes of vorticity associated with the upper low
itself. A frontal band is stretched from the Alaska Peninsula
through the Pribilof Islands and the western coast of the
Kuskokwim Delta. Over the northern Gulf, a surface low pressure
associated with the embedded shortwave trough aloft is riding a
cold front. Arctic air has wrapped all the way around from the
Bering Sea within the larger scale surface low, and looks to make
the morning forecast an interesting one.

&&

.MODEL DISCUSSION...
On the synoptic scale, there are not many differences to speak of
in this stagnant pattern. Most of the differences between guidance
in this pattern are on the small scale concerning precipitation
rates around the Anchorage area. Higher resolution guidance seems
to really be ramping up the moisture along the mountains with the
overnight runs. While this makes little difference for west
Anchorage, the increased amounts along higher terrain make sense
given the mesoscale setup with an arctic front moving up the Inlet
against the terrain and antecedent southeast flow. High-res
guidance was favored for this forecast.

&&

.AVIATION...
A cold front moving up the Inlet will bring a fairly quick but
heavy shot of snow this morning. Confidence is high in a period of
low ceilings and visibility with this morning snow, followed by
improving condition in the afternoon.

&&

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

Our nonstop train of interesting weather systems continues over
Southcentral the next couple days. Working chronologically, here
we go...

The Kenai radar has filled in with snowfall across much of the
Kenai Peninsula and southwestern Susitna Valley once again this
morning. This precipitation is associated with an area of low
pressure rapidly pushing northwestward across the northern Gulf
towards Southcentral. This low pressure system is the result of a
phenomenon that has been fairly atypical, the advance of much
colder air (in the mid- levels, relatively speaking) from the
south. The cold air did not originate south of here, but rather it
is associated with the nearly stationary upper-level low over the
Alaska Peninsula.

The positive factors to getting several inches of snow this
morning are spelled out in Anchorage's atmospheric sounding. The
change to the sounding over the past two has been quite
remarkable. The lower levels near the surface have rapidly warmed,
while the midlevels (700 mb and above) have rapidly cooled. The
result is a very unstable sounding. Our still very deep snow pack
has helped temperatures near the ground return to near freezing,
but the instability above that remains intact. That instability
will play a key role in this morning's snow, as it will allow
nearly free lift of the increasing atmospheric moisture through
the dendritic snow growth layer. The dendritic snow growth layer
is the layer of the atmosphere between -12 and -18 Celsius, which
is essentially 0 to 10 above Fahrenheit. This is the optimal
temperature range by which liquid cloud droplets will freeze into
snow crystals. Our forecast of periods of heavy snow this morning
requires a saturated, unstable atmosphere through this layer,
which is now present. Growing snowflakes moving through this layer
then have the best chance of rising and falling through this layer
multiple times, maximizing the time they have to grow very large.

Also, the track of the Gulf surface low essentially right over
Anchorage and into the Susitna Valley will effectively shut off
any and all downsloping along the track of the low, so all areas
should see some snow, though since north of the low the winds will
remain out of the southeast, that will dampen snow totals up north
in the Susitna Valley while enhancing them to the south across the
Kenai Peninsula.

The primary negative factors inhibiting large snowfall totals is
all about high forward speed of the system. The low and the
attendant upper level wave are moving very quickly. Simply put, it
seems likely there will be a window where atmospheric conditions
favorable for heavy snow will be nearly ideal, but it will be
brief. Dry air behind the wave should quickly end the snow from
south to north for all areas as early as late morning. The winds
through the lower atmosphere will return to out of a southeasterly
direction this afternoon, returning the drying effects of
downsloping.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
The surface low that had been in Bristol Bay is currently
crossing the Alaska Peninsula in the vicinity of Cold Bay early
this morning. The low will then turn back to the east tracking
along the south side of the Alaska Peninsula today and tonight to
reach southern Shelikof Strait early Thursday morning and cross
the Alaska Range and move over the greater Bristol Bay area
Thursday.

The frontal band currently stretched across the northwest corner
of the Kuskokwim Delta coast and Nunivak Island bringing snow and
blowing snow will rotate offshore this morning while snow
accompanying the surface low spreads across the Alaska Peninsula.
Over Southwest Alaska, another frontal wave swinging in from the
east will push across the Alaska Range into the middle Kuskokwim
Valley and northern Bristol Bay area this morning and Kuskokwim
Delta this afternoon bringing snow with it. Snow will taper off
again tonight then increase again over the Bristol Bay area as the
surface low pushes in from the south Thursday. Snow showers will
spread north into the Kuskokwim Valley Thursday night and taper
off Friday.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The surface low and cold upper level low will continue through
one more orbit spiraling around each other over the Alaska
Peninsula today and tonight to become vertically stacked over
Southwest Alaska by Thursday afternoon. A trailing trough will
extend west across the Aleutians with cold northerly flow and snow
showers continuing to stream across the Bering through Friday.

&&

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

Beginning the long term forecast Thursday evening, a closed low
over southwest Alaska will be in the process of deamplifying and
and merging with another arctic trough over northwest Alaska. This
will represent itself as snow threats from inland southwest
Alaska to Southcentral, including coastal locations. The snow
threat will continue through Friday and move east as the shortwave
trough moves toward the ALCAN and will continue into Sunday before
drier air filters in across southern Alaska Monday.

Thereafter, a deep low over the Western Bering Sea will track into
southern Alaska Tuesday and Wednesday, bringing widespread
moisture along with it. It is too far out to get too concerned
with exact timing, precipitation types, or amounts, but it does
look like it may be cold enough for all snow for inland locations.
We will have to wait and see, but the potential is for increased
activity once again middle of next week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory for Snow...101 121. Winter
Weather Advisory for Blowing Snow...195

MARINE...Gale Warning...119 130 131 132 181 185
Heavy Freezing Spray...179.

&&

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

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE ANCHORAGE AK
936 AM AKST WED FEB 15 2017

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0935 AM     SNOW             1 E RABBIT CREEK        61.08N 149.78W
02/15/2017  M6.0 INCH                           AK   PUBLIC

            SNOWFALL SO FAR TODAY.

&&

BLUDWIG

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE ANCHORAGE AK
957 AM AKST WED FEB 15 2017

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0955 AM     SNOW             2 SE ANCHORAGE          61.19N 149.83W
02/15/2017  E4.5 INCH                           AK   PUBLIC

            SNOWFALL SO FAR TODAY.

&&

BLUDWIG

PRELIMINARY LOCAL STORM REPORT...SUMMARY
NATIONAL WEATHER SERVICE ANCHORAGE AK
101 PM AKST WED FEB 15 2017

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0702 AM     SNOW             3 SSE ANCHORAGE         61.18N 149.85W
02/15/2017  M2.0 INCH                           AK   PUBLIC

            SNOWFALL SO FAR THIS MORNING.

0730 AM     SNOW             2 N RABBIT CREEK        61.11N 149.83W
02/15/2017  M3.0 INCH                           AK   PUBLIC

0734 AM     SNOW             2 N RABBIT CREEK        61.11N 149.83W
02/15/2017  M4.5 INCH                           AK   NWS EMPLOYEE

            STORM TOTAL ACCUMULATION, SNOWING HEAVILY AT OBSERVATION.

0830 AM     SNOW             2 SE ANCHORAGE          61.19N 149.82W
02/15/2017  M3.7 INCH                           AK   PUBLIC

0830 AM     SNOW             3 ENE WASILLA           61.59N 149.38W
02/15/2017  M2.0 INCH                           AK   PUBLIC

            2 INCHES OVER 3 HOURS

0845 AM     SNOW             5 SW ANCHORAGE          61.16N 149.98W
02/15/2017  M2.8 INCH                           AK   OFFICIAL NWS OBS

            SNOW DEPTH 22 INCHES

0846 AM     SNOW             3 ENE WASILLA           61.59N 149.38W
02/15/2017  E2.0 INCH                           AK   PUBLIC

            SNOWFALL SO FAR TODAY.

0857 AM     SNOW             4 S ANCHORAGE           61.16N 149.89W
02/15/2017  M3.0 INCH                           AK   PUBLIC

            3 INCHES OVER PAST 8 HOURS

0915 AM     SNOW             1 E RABBIT CREEK        61.08N 149.78W
02/15/2017  M6.0 INCH                           AK   PUBLIC

            6 INCHES OVER 6 HOURS WITH STEADY RATE OF FALL

0935 AM     SNOW             1 E RABBIT CREEK        61.08N 149.78W
02/15/2017  M6.0 INCH                           AK   PUBLIC

            SNOWFALL SO FAR TODAY.

0945 AM     SNOW             2 SE ANCHORAGE          61.19N 149.83W
02/15/2017  M5.0 INCH                           AK   PUBLIC

            5 INCHES OVER 3 HOURS

0955 AM     SNOW             2 SE ANCHORAGE          61.19N 149.83W
02/15/2017  E4.5 INCH                           AK   PUBLIC

            SNOWFALL SO FAR TODAY.

1000 AM     SNOW             1 E RABBIT CREEK        61.08N 149.78W
02/15/2017  M7.0 INCH                           AK   PUBLIC

            7 INCHES OVER 9 HOURS

1000 AM     SNOW             4 N SEWARD              60.17N 149.40W
02/15/2017  M3.0 INCH                           AK   PUBLIC

            3 INCHES OVER 12 HOURS

1000 AM     SNOW             1 NE KENAI              60.57N 151.18W
02/15/2017  M5.0 INCH                           AK   TRAINED SPOTTER

1015 AM     SNOW             4 N KNIK                61.51N 149.72W
02/15/2017  M2.0 INCH                           AK   PUBLIC

            2 INCHES OVER 3 HOURS

1015 AM     SNOW             1 NE GLEN ALPS          61.11N 149.67W
02/15/2017  M7.0 INCH                           AK   PUBLIC

            7 INCHES OVER 8 HOURS

1015 AM     SNOW             5 ESE ANCHORAGE         61.18N 149.73W
02/15/2017  M4.3 INCH                           AK   PUBLIC

            4.3 INCHES TUDOR-MULDOON CURVE

1045 AM     SNOW             5 SSE SPENARD           61.13N 149.83W
02/15/2017  M6.0 INCH                           AK   PUBLIC

            6 INCHES OVER 6 HOURS

1050 AM     SNOW             2 W GLEN ALPS           61.10N 149.75W
02/15/2017  M6.5 INCH                           AK   PUBLIC

            6.5 INCHES UPPER DEARMOUN

1055 AM     SNOW             1 W GLEN ALPS           61.10N 149.72W
02/15/2017  M9.0 INCH                           AK   TRAINED SPOTTER

            9 INCHES OVERNIGHT. PLOWED TWICE ALREADY. VIS 1/2SM HEAVY
            SNOW.

1100 AM     SNOW             1 NW GLEN ALPS          61.12N 149.71W
02/15/2017  M8.0 INCH                           AK   TRAINED SPOTTER

            8 INCHES OVER 5 HOURS. 24F. LGT EAST WINDS. VIS LESS THAN
            1/2SM

1115 AM     SNOW             7 ESE ANCHOR POINT      59.74N 151.63W
02/15/2017  M1.5 INCH                           AK   CO-OP OBSERVER

            1.5 INCHES OVERNIGHT. 29F. S BREEZES.

1115 AM     SNOW             1 WNW HOMER             59.64N 151.55W
02/15/2017  M2.0 INCH                           AK   LAW ENFORCEMENT

            1/2 INCHES IN TOWN. 2 INCHES NEAR DIAMOND RIDGE.

1115 AM     SNOW             5 NNW BUTTE             61.60N 149.12W
02/15/2017  M2.0 INCH                           AK   PUBLIC

            2 INCHES OVER 3 HOURS

&&

KUTZ

SPECIAL WEATHER STATEMENT
National Weather Service ANCHORAGE AK
252 PM AKST Wed Feb 15 2017

AKZ101-111-121-125-131-145-161815-
Anchorage-Matanuska Valley-Western Kenai Peninsula-
Western Prince William Sound-Northeast Prince William Sound-
Susitna Valley-
Including the cities of...Anchorage...Eagle River...Indian...
Eklutna...Palmer...Wasilla...Sutton...Chickaloon...Kenai...
Soldotna...Homer...Cooper Landing...Whittier...Seward...
Girdwood...Moose Pass...Valdez...Thompson Pass...Talkeetna...
Willow...Cantwell
252 PM AKST Wed Feb 15 2017

...More snow expected for Southcentral Alaska Thursday and Friday...

Another round of widespread snowfall is expected to impact
Southcentral Alaska beginning on Thursday and lasting through the
Friday morning commute. A weather system will slowly lift northward
from the Alaska Peninsula. Snow will primarily impact the gulf
coast and the Kenai Peninsula during the day on Thursday before
spreading into Anchorage and the Mat-Su regions late Thursday
evening into Friday morning. The greatest snowfall amounts are
expected from Seward northward to Turnagain Pass. Significant
accumulations are possible along the western Kenai Peninsula,
including the Homer area and Kenai. Farther north, several inches
of snow will also be possible from Anchorage into the Matanuska
and Susitna Valleys, with snow potentially impacting the Friday
morning commute and leading to difficult travel conditions in
these areas. The heaviest snowfall will then spread eastward
towards Valdez and Thompson Pass Friday morning into Friday
afternoon.

Although there is high confidence that much of Southcentral Alaska
will see accumulating snow from this system, there is still lower
confidence on the exact timing and amounts of snowfall. Please
visit www.weather.gov/anchorage for your latest forecast and
updates on this developing winter weather situation.