Winter Weather Advisory 39, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
539 AM AKST Mon Dec 18 2017

.ANALYSIS AND UPPER LEVELS...
Generally quiet weather conditions prevail across the bulk of the
southern mainland this morning as a trough over the Gulf pulls
off to the east and upper level ridging builds in from the west.
Very localized areas of stratus or patchy fog are present across
portions of Southcentral and Bristol Bay, but for the most part
clear to partly skies prevail across the region. Father west, a
powerful storm force front is moving eastward through the Bering
accompanied by a wide swath of strong southerly winds. This front
is just beginning to impact coastal portions of the Kuskokwim
Delta with snow which should gradually change over to rain as the
front pushes inland today. Temperatures across the Bering are
generally warm enough to support all rain, with precipitation
currently clearing the Central Aleutians and spreading east into
the Pribilof Islands and the eastern part of the chain.

&&

.MODEL DISCUSSION...
Models remain in lockstep for the short term as the strong front
moves across Southwest AK today and then into Southcentral on
Tuesday. The biggest questions continue to revolve around small
scale details such as the Turnagain Arm wind and the role
downsloping will play in this event, which will have large
implications on the timing, amount, and type of precipitation for
the Anchorage area. There is greater forecast confidence heading
north into the Mat-Su region where precipitation should safely
fall as all snow. Changes were mostly minor for the morning
package, predominantly utilizing higher resolution NAM guidance
given the strong agreement.

&&

.AVIATION...
PANC...Fog has largely failed to materialize in the Upper Cook
Inlet region this morning. As a result, VFR conditions are
expected to prevail through Monday night though some patchy fog
still may linger in the vicinity of the terminal through this
morning. Gusty southeast winds will move out of Turnagain Arm into
the terminal by Tuesday morning as precipitation spreads into the
region with the arrival of a front. Anchorage should initially
stay downsloped at onset of precip, however ceilings should lower
below 5000 ft with possible MVFR cigs by the end of the TAF
period.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Other than a weak short wave bringing a few snow showers to the
Copper River Basin this morning, most of Southcentral Alaska
should have mostly clear skies today. Would call it mostly sunny
but sunshine is limited as we enter the shortest week of the year.
The front moving into the area Tuesday is the real item of
interest. There will be some significant snowfall in the Susitna
Valley with total snow amounts in the 6 to 10 inch range with a
some higher totals over a foot in the Alaska Range. The Winter
Storm Watch for this snowfall has been adjusted to a Winter
Weather Advisory for snow (Snow Advisory) tomorrow into tomorrow
evening. This was changed as confidence in snow amounts has
increased. Any areas that get over a foot of snow warranting a
Warning should take over 12 hours to reach that amount and
therefore a Snow Advisory looks to be a more appropriate product
as confidence is high in the snowfall and the amounts.

Anchorage and the Matanuska Valley are a bit more difficult to get
a good handle on for tomorrow. This issue is that models have been
consistent in bringing in snowfall with gusty Southeast winds.
This issue is that gusty Southeast winds almost always either
downslope the area into no precipitation or warm up the
temperatures enough that the precipitation is rain. The mid and
upper level flow is from the southwest which is favorable for
snow, but the issue is the lowest levels where a strong Turnagain
Arm wind and Knik River wind are expected to blow most of the day
tomorrow. Therefore, since I have high confidence in the wind
occurring, temperatures have bee adjusted warmer along with lower
chance of precipitation and the precipitation type being a rain
snow mix for most of the day. The best chance for snow is right as
the winds diminish and switch to the south-southwest in the late
afternoon to early evening. However, due to the short duration
when winds and dynamics are favorable for snow, accumulations will
likely be just an inch or two at most.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
A front moves slowly eastward across the southwest coast today
then picks up momentum this evening bringing widespread
precipitation across the region through Tuesday. Precipitation
across the Kuskokwim Delta and most of Bristol Bay is expected to
change over to rain as strong deep southerly flow ushers warm air
northward. Southerly flow remains gusty along coastal areas
through Wednesday as a second surface low races from the North
Pacific, although speeds slacken late tonight. Gusty winds are
slower to increase across inland as the frontal boundary remains
along the coast until this evening and then peak late tonight
before tapering off Tuesday afternoon.

&&

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

An active pattern continues through mid week as the current low
across the western Bering is followed by another strong North
Pacific system on Tuesday. Strong southerly flow across the
frontal boundary today shifts to the west behind the front.
Numerous snow showers behind the front develop along short waves
rotating around the parent low which slowly tracks to the Bering
Strait late Tuesday. A mix of rain and snow is expected along the
next front moving to the western and central Aleutians late
Tuesday night into Wednesday. This system will have a cooler core
as it taps a cooler air mass from the Kamchatka Peninsula on its
northward track.

&&

.LONG TERM FORECAST (Days Tuesday through Saturday)...

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...Winter Weather Advisory 145.
MARINE...Storm Warning 170 173 174 176 413 414
 		 Gale Warning 150 155 160 165 171 172 175 177 178 179 180
					  181 185 411 412

&&

SYNOPSIS AND MODEL DISCUSSION...CB
AVIATION...CB
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KH
LONG TERM...LF

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