Gale Warning 743, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
425 PM AKST Tue Dec 19 2017

.ANALYSIS AND UPPER LEVELS...
The dominant large-scale weather feature this afternoon remains a
once powerful but now weakening Bering Sea low with the main
circulation center moving towards the Bering Strait. A southwest
to northeast oriented upper level jet can be seen on water vapor
moving eastward as it tracks from Southwest Alaska to Southcentral.
Deep layer southwest flow aloft is bringing in a slug of moisture
to Cook Inlet and Prince William Sound with MIMIC TPW imagery
depicting a narrow but intact filament of enhanced water vapor
values. Over the remainder of the Bering Sea and Southwest Alaska,
broad southwest cold sector flow is supporting a convectively
enhanced showery environment. Finally, east of Kamchatka, two
separate shortwave troughs are forming along a baroclinic zone,
with the southern stream low eventually becoming a powerful storm
force front over the Bering Sea later tonight.

&&

.MODEL DISCUSSION...
The numerical weather models are in excellent agreement with the
general pattern heading into the weekend. However, they differ
with a series of lows moving through the Bering Sea Wednesday
through Friday, with each track of a low affecting the track of
the next low. For now, a general blended approach was favored
although the GFS model was given additional weighting, especially
by Friday as a powerful warm front/low moves through the Southwest
part of Alaska. For Southcentral, model discrepancy was low and
forecast confidence is high.

&&

.AVIATION...
PANC...Snow at the terminal is expected to taper off quickly
during the mid-evening timeframe. VFR CIGS and VSBYs are expected
through the remainder of the TAF period.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The front over Southcentral late this afternoon will pick up speed
this evening an upper level short-wave drives it eastward. Thus,
expect a rapid end to steady snow over the Cook Inlet-Susitna
Valley corridor early this evening, then over Prince William Sound
and the Copper Basin overnight tonight. An unstable air mass
behind the front combined with persistent low level southwest flow
will produce some showers or areas of snow in upslope areas of
the Susitna Valley, Talkeetna Mountains, and Alaska Range through
Wednesday. A strong ridge will then build over the region Wednesday,
leading to dry conditions with temperatures falling to more
seasonable norms. These falling temperatures will lead to development
of surface inversions which will be favorable for fog to develop,
especially in and along Cook Inlet.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Gusty southwesterly winds and widespread snow shower activity
will continue through Wednesday morning as unstable air continues
to move in from the Bering Sea. No significant accumulations are
expected, with temperatures hovering around freezing limiting the
ability for anywhere to get more than a dusting of snow. Weather
across the eastern half of the area will settle a bit Wednesday
afternoon through the overnight hours as weak ridging (high
pressure) moves into the area. The Kuskokwim Delta, and especially
the coastal villages along the Delta, will get clipped by a fast-
moving but pretty potent storm Wednesday evening/night. Another
round of 45-55 mph southwesterly winds should be expected as this
storm passes through. The heaviest precipitation should stay to
the north, but the winds will produce significant wave action and
higher water levels that may impact coastal locations. With
moderate tides, forecasted water levels should remain fairly close
to highest astronomical tide levels, but rough wave action on top
of the water could produce some localized impacts from erosion if
the strongest winds happen to coincide with the high tides.

By Thursday morning this storm will be moving away, and some
patchy fog to start the day is possible across much of the area.
This fog will quickly dissipate as a new storm track starts to
take hold which will bring increasingly strong east and southeast
winds to the Bristol Bay zone on Thursday night into the day
Friday. There is still uncertainty with where the strongest winds
will develop, so stay tuned. A high wind watch is in effect for
the area.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
Meanwhile, the Bering Sea and Aleutians will stay quick active
over the next few days and even as we transition into a new
overall weather pattern in the longer range periods. The quick-
hitting storm that will clip parts of Southwest Alaska will have a
more widespread impact on the Bering/Aleutians. The Adak area will
see winds continue to ramp up dramatically overnight tonight, and
should peak during the morning hours on Wednesday. There could be
a few gusts that approach 75 mph, but these should only last an
hour or two. This weather front will stall over the central
Aleutians as another low forms along the front and moves up from
the south on Thursday.

&&

.LONG TERM FORECAST (Thursday through Tuesday)...
Starting Thursday, a ridge of high pressure will be firmly in
place over the mainland, which will change the track of the low
pressure systems traversing the forecast area. Instead of moving
in from the west, the next two low pressure systems will approach
from the south and bring a lot more moisture, which will cause
heavy precipitation along the North Pacific coast, including the
oceanward side of the Kenai Peninsula. The first weather concern
is for southwest Alaska on Thursday and Friday, as the approaching
low will bring a strong low level jet and very tight gradient
over the Alaska Peninsula and Bristol Bay area. This has the
potential for warning level winds over the northern AK Pen and
inland Bristol Bay, so we have issued a high wind watch for those
areas. Temperatures all along the west coast will be very warm, so
snow will eventually turn over to rain as far north as the
Kuskokwim Delta.

The precipitation pushing onshore will be mostly rain for the
Kodiak area and the southern portion of the Kenai peninsula, but
there are some interesting developments further northward. Models
have been hanging on to a pocket of cold air over Southcentral for
several days now, making precip type particularly challenging in
this area. 12z run of the GFS and EC indicate that the surface low
will be further westward, which would keep that cold air in place
and possibly bring snow to the Kenai Peninsula and northward over
the weekend. However, the 18z run of the GFS has moved the low
eastward again, indicating more of a rain/snow mix. Behind this
low is some significant cold air, which brings a chance for snow
for all portions of the forecast area before Christmas Day.
Either way, the forecast looks to be messy for the weekend before
a return to more seasonable weather early next week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Storm Warning 145.
 High Wind Watch 161.
MARINE...Storm Warning 173 175 176 177 179 181.
 Gale Warning 160 165 170 171 172 174 178 180.

&&

SYNOPSIS AND MODEL DISCUSSION...JA
SOUTHCENTRAL ALASKA...SB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AD
LONG TERM...LF

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE ANCHORAGE AK
1041 PM AKST TUE DEC 19 2017

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

1230 PM     SNOW             TALKEETNA               62.34N 150.09W
12/19/2017  M9.0 INCH                           AK   PUBLIC

            NEAR THE CHULITNA RIVER

0127 PM     SNOW             KENAI                   60.55N 151.21W
12/19/2017  M1.7 INCH                           AK   NWS EMPLOYEE

0430 PM     SNOW             14 ESE SKWENTNA         61.94N 150.99W
12/19/2017  M5.5 INCH                           AK   PUBLIC

0612 PM     SNOW             BIG LAKE                61.52N 149.95W
12/19/2017  M6.2 INCH                           AK   NWS EMPLOYEE

            STORM TOTAL SNOWFALL

0612 PM     SNOW             6 ESE MULDOON           61.18N 149.59W
12/19/2017  M2.5 INCH                           AK   PUBLIC

            1.5 HR SNOWFALL TOTAL

0700 PM     SNOW             NE ANCHORAGE            61.22N 149.87W
12/19/2017  M3.0 INCH                           AK   NWS EMPLOYEE

            4-7PM SNOWFALL TOTAL

0725 PM     SNOW             SUTTON                  61.72N 148.88W
12/19/2017  M7.0 INCH                           AK   TRAINED SPOTTER

            AT THE TIME OF THE OBSERVATION IT WAS STILL SNOWING.

0747 PM     SNOW             2 N RABBIT CREEK        61.11N 149.83W
12/19/2017  M2.0 INCH                           AK   NWS EMPLOYEE

            STORM TOTAL

0747 PM     SNOW             1 ESE EAGLE RIVER       61.32N 149.55W
12/19/2017  M3.5 INCH                           AK   NWS EMPLOYEE

            STORM TOTAL OVER 4 HOURS

0800 PM     SNOW             1 NW GLEN ALPS          61.11N 149.71W
12/19/2017  M3.0 INCH                           AK   TRAINED SPOTTER

            SOUTHERLY BREEZE WITH SOME PATCHY FOG OBSERVED

0854 PM     SNOW             8 SW THOMPSON PASS      61.06N 145.91W
12/19/2017  M7.0 INCH                           AK   TRAINED SPOTTER

            SNOWFALL BETWEEN 4-7PM

0854 PM     SNOW             VALDEZ                  61.08N 146.30W
12/19/2017  M13.8 INCH                          AK   TRAINED SPOTTER

            FALLING AT A RATE OF 1.6IN PER HR

0920 PM     SNOW             2 SE EAGLE RIVER        61.30N 149.52W
12/19/2017  M3.0 INCH                           AK   NWS EMPLOYEE

            STORM TOTAL

1012 PM     SNOW             1 E RABBIT CREEK        61.08N 149.78W
12/19/2017  M1.0 INCH                           AK   PUBLIC

            STORM TOTAL

&&

RA