Small Craft Advisory 138, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
458 PM AKST Sun Feb 11 2018

.ANALYSIS AND UPPER LEVELS...
A water vapor imagery loop tells the story of the upcoming active
weather. A southwesterly jet stream with deep subtropical
connection streams toward the mainland. The precipitable water for
the column within this stream is around an inch, which would be
well over 200% of normal for this time of year. The core of the
jet is around 150kt and is driving a warm front northward toward
the southern Mainland. The surface low associated with the jet and
upper level trough is well west, approaching the eastern Aleutians
and Alaska Peninsula. Given the deep southwesterly flow and
moisture tap, the big questions accompanying the forecast will be
temperatures and precipitation type. Currently a light down-Inlet
flow is in place while middle and high clouds from the approaching
system overspread the area. The warm/occluded front is beginning
to impinge on the southwest mainland, along with gusty
southeasterly winds, bringing temperatures into the mid-30s. A
harbinger of atmospheric conditions to come, perhaps? This
forecaster thinks so.

&&

.MODEL DISCUSSION...
Synoptics of the event are well agreed upon, and as always, the
devil is in the details. Model disagreement, and hence,
uncertainty comes from timing issues of the southeasterly winds,
and depth of the warm air accompanying the front. Guidance is in
agreement with shunting the warm sector to our east, giving us an
occluded front passage.

&&

.AVIATION...
PANC...All the uncertainty with the TAF will be due to winds and
precipitation type, which will be pretty tied together for the
forecast. While the precipitation will likely start as snow, the
Turnagain Arm wind will start to bring warmer surface air to the
area. How warm, and how deep the warm air will be is the question.
As long as the southeast wind stays on the lighter side (<10kts)
snow could linger, but as the winds increase precipitation may
mix or briefly switch over completely around 09z before ending
around 12z. Before this occurs, a short period of LLWS was
introduced this evening until the SE winds mix to the surface.
Confidence is then much higher later in the period with the
redevelopment of IFR snow beginning Mon morning as the low level
flow weakens and cold air filters in.

&&

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

A very complex weather setup is approaching for the next 36 hours
across Southcentral. A plume of moisture working its way northward
up the warm side of a series of lows is moving into the area. This
will be the catalyst for the first round of precipitation for the
area. The 00Z Anchorage sounding shows an above freezing layer
from 980-900 mb, which would be plenty thick enough to change any
snow over to rain, but that layer also happens to be dry,
indicative that there will be lots of room for that column to cool
as snow sublimates into it. Thus, right now it looks like chances
for rain are small, but not impossible initially as precip
develops early this evening. The snow will increase in intensity
area- wide through the evening. Then after midnight, as the storm
itself lifts to the north and makes its closest approach,
increasing winds out of the southeast will begin a brief period of
downsloping, bringing temperatures above freezing near Turnagain
Arm and the Knik River winds. This will result in the
precipitation largely ending for the rest of the overnight except
over the Susitna Valley and coastal locations, though there is a
chance that snow may mix or switch over the rain before ending
early Monday morning.

Later on Monday morning, the cold front will move into
Southcentral from the west. This will turn the winds to the
southwest just ahead of the front in the morning and then
northwest behind it in the afternoon. Meanwhile, an upper level
trough will also move overhead, causing upper level temperatures
to drop and increase instability. Thus, the increasingly unstable
conditions will result in a return to precipitation in the form of
off-and-on snow showers. The cold air associated with the front
will be only slightly colder. With such weak advection of cold
air, it is expected that temperatures Monday will remain at or
above freezing area-wide. Thus, significant accumulations from the
snow showers will be limited to only the heaviest ones, where
snowfall rates exceed melting rates. Behind the trough's passage
Monday afternoon, the winds will turn to the northwest throughout
the atmospheric column. That will put the Hillside and Matanuska
Valley in a relatively rare upslope regime, which with only slowly
diminishing moisture will lead to the snow showers continuing
there into the evening, resulting in higher accumulations
especially as temperatures drop after sunset.

The snow showers are expected to taper off entirely after
midnight. Towards sunrise Tuesday morning, clearing skies may
result in some localized fog development.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
(Tonight through Tuesday evening)

Warmer and wetter is the theme for the next 24 hours across much
of the region, as an area of low pressure tracks from the AKpen
towards Russian Mission. Area radars show precipitation
blossoming over all of southwest Alaska. As seen in model
forecasts and area observations, southeast to south winds are
drawing warmer are across the region with snow quickly changing to
rain. Still, some of the higher elevations may pick up a quick
few inches before precipitation becomes all rain. With the warmer
air in place, look for relatively pleasant temperatures are
expected overnight as the warm advection continues, with most
locations only dropping into the upper 20s to lower 30s.

As the system departs our area to the northeast on Monday, colder
air will filter back into the region with afternoon highs only
increasing a few degrees. The active pattern looks to continue
though, as a short-wave transient ridge moves across the area
Monday night. The drier air associated with the ridge will allow
for clearing skies with lows back into mainly mid teens to mid
20s. Another frontal system will approach the coast by Tuesday
evening, bringing more wet weather back to southwest portions of
the mainland.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
(Tonight through Tuesday evening)

An active pattern looks to continue over the next few days for the
entire region. As one storm moves across the eastern portions of
the Bering, a short-wave ridge will quickly move from western portions
of the sea/Aleutians tonight, eastward to the coast of southwest
Alaska by Tuesday evening. This will briefly put the ki-bosh to
any precipitation as the ridge crosses overhead. However, this
will be short-lived, as another strong front sweeps west to east
on its heels, accompanied by widespread precipitation and gale
force winds (locally storm south of the chain between Atka and
Kiska).

&&

.LONG TERM FORECAST (Days 3 through 7)...
The extended portion of the forecast begins Tuesday night with
quieter conditions across Southcentral Alaska beneath a transient
upper level ridge. This will promote modest offshore flow and
diminishing precipitation into the middle of the week. Meanwhile,
a split upper level jet structure looks to maintain one storm
track through the Bering Sea and another through the Gulf of
Alaska during the coming week. This will send a pair of systems
into the Bering on the periphery of the longwave trough on
Wednesday and Friday respectively, keeping active weather across
much of the region through the bulk of the week. Another system
will move across the Gulf on Thursday, with most of the impacts
looking to be confined to coastal areas and the panhandle at this
time. This synoptic pattern would suggest rather benign conditions
remaining over Southcentral for much of the work week, with a
shortwave trough moving through and providing a brief chance of
precipitation around Thursday. Otherwise, much of interior
Southcentral looks to remain on the dry side into next weekend as
upper level ridging redevelops toward the end of the week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 145.
MARINE...Storm 177
         Gale 120 130-132 150 155 170 172-176 178 179 185.

&&

SYNOPSIS AND MODEL DISCUSSION...MTL
SOUTHCENTRAL ALASKA...JW
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD
LONG TERM...CB