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Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
434 PM akst sun Dec 17 2017

Analysis and upper levels...

A weakening positive tilt trough over southern Alaska is shifting
east this afternoon. Embedded within the larger trough are
numerous smaller scale and weak disturbances, one along the north
Gulf Coast and another moving across the Kuskokwim Delta. An
unstable environment remains over the Gulf of Alaska and along the
immediate coast where snow showers are ongoing. There is a weak
surface trough axis along the north Gulf Coast, otherwise, the
gradient over southcentral is rather weak. Out west, ridging
building into the Bering is keeping a tight northwest flow
gradient over the southwest coastline. This is ahead of a
developing bomb cyclone over the far western Bering Sea as a
potential vorticity anomaly is in the process of intersecting a
modest warm front. Himawari satellite imagery is already showing
the classic signs of a powerful low, including a developing
emerging cloud head out of the left exit of the upper jet.

&&

Model discussion...

The numerical models are in unusually good agreement with the
details of a powerful bomb cyclone over the western Bering
Sea...and the associated front which will cross the Bering Sea and
move inland Monday and Tuesday. The biggest discrepancy in the
models is how much inland precipitation will fall across parts of
southcentral owing to a low Level Cross-barrier flow component
which will act to diminish precip values for some locations.
Places like Anchorage, the Copper Basin, and parts of the northern
Kenai Peninsula will likely see some impact of cross-barrier flow
and possibly even some low level warming which may result in mixed
precip on the onset of the event (a mix of rain and snow).

&&

Aviation...
panc...fog is likely to redevelop during the overnight as mid
level clouds clear out and the low level flow remains weak. This
will likely bring in IFR to LIFR ceilings and visibilities which will last
through the morning. However, confidence is only moderate since
the coverage of fog is not expected to be widespread.

&&

Short term forecast southcentral Alaska (days 1 and 2)...
a weak surface low currently located in the Gulf of Alaska will
slowly weaken tonight while moving off to the east, bringing an
end to the scattered snow showers along the coast by the early
morning hours. A transient ridge will quickly build in behind the
departing low tomorrow morning. Weak flow with clearing skies and
abundant low level moisture will allow areas of dense fog to
reform tonight for locations along the Cook Inlet and Knik Arm.
Besides the fog, skies will be mostly sunny and temperatures will
be noticeably colder compared to the first half of the month.

The ridge will depart quickly to the east as the next storm system
comes barreling through the area from the west by Monday night. A
front will move into southcentral by Tuesday morning and a triple
point low is expected to form south of Homer and move north. Warm
air is expected to advect up from the south and easterly winds
will increase through Turnagain Arm and the Knik River Valley.
This will likely keep the typical downsloped areas, including the
western Kenai Peninsula, Anchorage, and Palmer, mostly dry and
warmer through the day on Tuesday. It is not until the low passes
to the northeast late Tuesday afternoon that precipitation will
develop over these areas. Colder air filtering in from the northwest will
likely change precipitation over to all snow, though total
accumulations should be no more than 1-2". In the Susitna Valley,
favorable midlevel flow from the SW will provide a good push of
moisture up the Cook Inlet brining a decent snow event to
locations along the Parks Highway, especially north of Willow,
where total snowfall is expected to be between 6-8" with localized
amounts near a foot by Tuesday night. A Winter Storm Watch has
therefore been issued for this area. Hatcher Pass will also see
significant snowfall, with 1-1.5 ft of snow falling through the
day on Wednesday.

The low will track across the northern Gulf and Prince William
Sound spreading precipitation across coastal areas by late Tuesday
morning. Precipitation should mostly be rain or mixed, though
colder air trapped in Valdez will lead to all snow, with nearly a
foot of snow expected to fall in town through Wednesday morning.

&&

Short term forecast southwest Alaska (days 1 and 2)...
weak ridging has moved in over southwest Alaska and will remain
overnight bringing a possibility of fog to the area. Then on
Monday morning a front will push in from the west at first
bringing snow to southwest Alaska. This front has a long fetch of
warm air with it that will transition the snow over to rain for
areas at and near the coast. By Tuesday afternoon the front will
have pushed off to the east with cold air coming in behind it.

&&

Short term forecast Bering Sea/Aleutians (days 1 and 2)...
over the eastern Bering, high pressure is quickly tracking to the
east. Out west there is a deep low over the far western Bering
with its front draped through the western Aleutians. As the front
moves through the central Aleutians this evening it will bring
strong gusty winds reaching warning level. As it passes early
Monday morning the winds will diminish.

The front is pulling up warm air from the south causing it to
bring rain to the islands. It will track east through the area and
make it to the eastern Bering and Aleutians Monday morning. Cold
westerly flow will follow behind the front bringing snow showers
to the area. Then on Tuesday another warm system will approach the
western Aleutians from the North Pacific.

&&

Long term forecast (days Tuesday through saturday)...

The long term forecast remains on track for Tuesday, with snow
expected in the Susitna Valley and along the chugach range toward
Valdez. Current snow amounts look to be close to a foot north and
east of Talkeetna, which will bring some much welcome ground cover
in those areas. Details are slightly less certain through Valdez,
but right now snow amounts are expected to be very similar in this
area as well. Unfortunately for snow lovers, this will be a non-
event for Anchorage and the Kenai Peninsula, with an inch or less
expected.

After Tuesday, most of the state will experience a quick shot of
cold air (a.K.A. Return to seasonable temperatures) before the
North Pacific ridge builds back in, starting Thursday. The
orientation of the ridge makes a big difference in the forecast,
but models keep trending towards a more negatively tilted ridge,
pushing low pressure systems further westward. Friday and Saturday
a new, strong system moves up from the Gulf of Alaska and has the
potential to bring an atmospheric river for the southcentral
region. Currently, Kodiak appears to be in the crosshairs, but if
the solutions wobble again, the southern Kenai may see the brunt
of the system. Unfortunately a combination of warm southerly flow
and downsloping across the southcentral area are currently
indicating more of a wind and rain storm, instead of an early snow
for Christmas.

&&



Afc watches/warnings/advisories...
public...west-southwest 145 & npw 187.
Marine...storms 170 173-177 179 411-414.
Gales 150 155 160 165 170 171 178 180 181 185.
&&

$$

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