Winter Storm Watch 5, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
508 PM AKST Fri Dec 29 2017

.ANALYSIS AND UPPER LEVELS...
There is a vertically stacked low centered just south of the
central Aleutians. There is a leading front that stretches from
the south central Bering Sea southeastward across the eastern
Aleutians and western Alaska Peninsula into the northeast
Pacific. There is a "back door" cold front over the western
Aleutians and the western portion of the central Aleutians. There
is an upper level ridge over the western Bering, with
northeasterly flow at the surface. Weak upper level high pressure
is over the southern mainland. Some offshore flow exists at the
surface, but plenty of low and mid level clouds are over the area.
The upper low over northwest Alaska is retreating northward.

&&

.MODEL DISCUSSION...
The numerical models are in decent agreement through Sunday
afternoon. There are still some differences in how they handle
various surface lows from the Alaska Peninsula west through the
Aleutians and southern Bering, but less than has been the case the
last few days. This also impacts the speed of a front moving
across the Gulf on Sunday. Forecast confidence is near normal.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist. Wind shear
will develop Saturday afternoon as southeast winds aloft increase
with an approaching weather system.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Generally cloudy and benign conditions will remain into Saturday
as most of the area remains under the influence of a weak surface
high. One exception to this will be over Northern Cook Inlet where
a weak shortwave and convergence boundary will bring some light
snowfall to the Western Kenai coast, generally from Anchor Point
northward. This may even bring a few flurries on its very eastern
edge into the Anchorage Bowl this evening.

The much more impactful weather will then spread into the Gulf
beginning Saturday with widespread gale force winds and rain mixed
with snow. The models continue to struggle with the placement or
even existence of a weak triple point low near Kodiak Island from
this system that is hampering the overall forecast confidence,
however there remains good agreement that this system will bring
widespread snow to the Gulf Coast and in particular the Eastern
Kenai that will see persistent onshore flow through Sunday with
temperatures generally cool enough to support snow. This front
however will not have enough of a moisture source to produce
warning level accumulations to that area, but it will be slow
moving enough to bring a foot or more of snowfall to the Eastern
Kenai Peninsula through Sunday, especially through the Turnagain
Pass area that could receive even more. On the lee side of the
Chugach (Western Kenai northward to Anchorage/Matanuska Valley)
will remain generally dry from this event due to strong cross
barrier flow and downsloping, with gusty winds through Turnagain
Arm and the Matanuska Valley. For the Anchorage bowl, with strong
flow down Cook Inlet there is very good confidence that these the
Turnagain winds will not reach into town or the hillside, and
instead will likely see breezy northerly winds.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
A trough across the Aleutians pushes a front across the Southwest
on Saturday spreading precipitation south to north through Sunday.
The main challenges with this system are precipitation types and
gusty winds. The light northeasterly flow pattern shifts east to
Southeast across Bristol Bay and the Kuskokwim Valley as the
surface low moves the front north tonight. Winds across the
Kuskokwim Delta are expected to keep a northeasterly direction
with the higher gusts along the coast which may cause visibility
reductions in snow as the front passes through. Offshore flow
keeps a shallow surface cold pool over much of the region while a
strong low level southeasterly jet surges warm subtropical air
over the region through Saturday. Snow will develop along the
front and change over to rain for much of Bristol Bay. The
highest precipitation amounts will be confined to east facing
slopes in this flow pattern. The next front is expected to move up
from the North Pacific Sunday into Monday bringing another round
of strong winds and precipitation along another warm core system.

&&

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

An unsettled weather pattern continues through early next week as
a broad trough sets up over the Bering and pinwheels several low
pressure systems about its core. Frequent wind direction changes
are expected along with numerous rain and snow showers. Gale force
marine winds will be common in this pattern, although winds taper
down along the central waters as the low centers near the
Pribilof Islands on Sunday. Cold air wraps along the Western
Bering on the backside of low bringing numerous snow showers along
upper level disturbances rotating through the flow.

&&

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

The long term forecast period begins with New Year's Eve, which
will see a low over the Aleutians and a front draped from
Southwest AK across the Kenai Peninsula and into the Gulf.
Although this isn't an unusually strong system, it does bring warm
air to the state, which will start our transition to a more
active pattern. The warming temperatures aloft should bring mixed
precip to Southwest AK, changing over to rain in the lee of the
mountains. For Southcentral, this system looks like it will be
primarily snow and possibly some quite heavy snow over the
mountains on the Kenai Peninsula. Precipitation will remain mostly
in the mountains, so not looking for a snow event for most
communities along the road system.

The next system approaches on a very similar track beginning
Tuesday, bringing stronger winds and warmer air. Precipitation
will most likely fall as rain for most of the forecast area,
including Southwest AK. In Southcentral, the current track and
strength of this system has the possibility to bring a Chinook
wind event, which would enhance our already warm temperatures for
an early "January thaw". The first chance for a return to cooler
temperatures begins on Wednesday, when the low in the Gulf slides
northward, bringing cooler temperatures on the west side
(backside) of the low. However, with recent model performance,
confidence in the 7-day forecast is low.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 119 120 132 136-140 150 155 160 165 170 172 173 175
         177-181 185.
         Storm 130 131.
         Heavy Freezing Spray 180 181.

&&

SYNOPSIS AND MODEL DISCUSSION...BL
SOUTHCENTRAL ALASKA...DEK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KH
LONG TERM...LF