Small Craft Advisory 44, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
348 PM AKST Sun Jan 14 2018

.ANALYSIS AND UPPER LEVELS...
The fairly active pattern continues across the Gulf and Mainland
Alaska. The upper long wave trough extends from eastern Russia to
a low center moving north over coastal Southwest Alaska, then
digs southeast across the northeast Pacific. A fairly zonal (west-
east) jet is evident along 40N on the southern periphery of the
upper trough. While to the eastern a southerly jet is present
owing to the pinch between the amplified ridge across the West
Coast/western Canada. Dynamics with this jet are reinforcing the
board baroclinic zone (frontal boundary) extending from the
northeast pacific, across the eastern Gulf, to across the North
Gulf coast, and Copper River Basin. Steady rain continues across
the Gulf coast with snow at higher elevations above 1200 feet.
Overrunning has kept the snow fairly steady across the eastern
Copper River Basin. With very warm temperatures aloft over below
freezing air near the surface it is likely that some freezing rain
is falling across the southeast Copper River Basin. Some
lingering showers continue across Southcentral and Southwest
Alaska as the stacked low center over Southwest Alaska heads
north. Snow to mixed showers were evident across much of the
Bering Sea.

&&

.MODEL DISCUSSION...
Models are in fairly good agreement with the old stacked low
moving north near the Southwest Alaska Coast. They are also
in good agreement and reasonable with a developing low along the
baroclinic zone in the northeast Pacific. Significant model
differences arise with the next low to developing along this
boundary Monday which heads toward Kodiak Tuesday. The ECMWF and
Canadian Global have a much faster movement than the GFS/NAM. The
ECMWF has had overall the best run to run consistency and a faster
movement does fit better with the strong southerly upper jet.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist through
tonight. MVFR to IFR conditions are expected with snow developing
during the morning hours on Monday.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
An upper level short-wave and surface low tracking northward
across the eastern Pacific will curve westward toward Prince
William Sound tonight. This will push all of the precipitation
which has been falling over Eastern Prince William Sound and the
Copper River Basin westward toward the Kenai Peninsula, Anchorage,
and the Mat-Su. There will be a sharp delineation between
accumulating snow and no snow at all, and right now it looks like
that line will set up in the vicinity of a line from West
Anchorage up to Talkeetna (along the Parks Highway corridor).
Areas north and east of this line have the best chance of
accumulating snow late tonight through Monday morning. Have
generally gone with an inch or two of accumulation for this area,
though if the snow were to push farther west amounts could be a
bit higher.

Meanwhile, the atmospheric river (AR) which has been pointed
toward Yakutat will shift westward toward eastern Prince William
Sound. This will bring a period of heavy precipitation to the
southern Copper River Basin and eastern Prince William Sound. The
AR will be in a weakening state as cyclogenesis over the
northeast Pacific begins to cut off the northward flow of warm
moist air, so it looks like about a 6 hour window of heavy
precipitation. One of the biggest forecast challenges is
precipitation type over the southern half of the Copper River
Basin. Area observations indicate all snow at the moment, but as
the low moves up to Prince William Sound warmer air will move in
aloft, over the coastal mountains and into the Copper River Basin.
A period of freezing rain looks likely for the southeastern
basin, perhaps reaching as far north as the Glennallen area. Have
backed off a bit on snow totals for this area, but stuck with all
snow along the Richardson Highway approaching Thompson Pass,
thinking the warmest air will remain east of here.

The new low forming over the Northeast Pacific will quickly head
into the Gulf Monday night into Tuesday. There is quite a bit of
uncertainty in time of arrival, but based on a low track toward
Kodiak Island, lots of precipitation and warm air will spread
north and west to Southcentral. Have trended the forecast toward
warmer temperatures and more rain than snow across the region. The
population centers may end up being downsloped for much of this
and avoiding rain falling on snow and ice covered roadways.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
The much-talked about Bering Sea low continues its slow northward
push across eastern portions of the sea, with its associated
occluded front well north of the region. In its wake, a warmer
downsloping southeast wind has brought warmer air to much of the
interior, along with some scattered showers. Winds have been
rather gusty along the Alaska Range, and along the immediate
coast, where they've topped 45 mph in a few locations.

As the system continues to move away from the region tomorrow,
precipitation coverage and wind speeds will be on the decrease, as
some subtle ridging builds into the region from the east. However,
this will be short-lived, as another low approaches Kodiak Island
for early Wednesday morning. An inverted trough retrograding
westward from the low will act as a focus for precipitation to
develop under a developing deformation band, with early
indications showing the potential for some light accumulations of
snow, depending where the low tracks.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
As the eastern Bering low moves north of the region over the next
few days, strong northwesterly flow will continue to funnel
colder air from eastern Siberia into the region. This colder air
over the relatively warmer ocean will create sufficient low-level
instability for showery type precipitation to develop, with
locations generally north and west of a Nikolski to Nunivak Island
line seeing snow showers, and rain showers south and east of
there for Monday. This rain snow line will edge further southeast
for Tuesday, as cold air advection continues to strengthen.

Low end Gales across the eastern Bering will diminish to Small
Craft criteria or less by Monday. An area of low pressure moving
into the western Aleutians Tuesday evening will bring increasing
chances for a more steady precipitation pattern, with winds
increasing to low end Gale force.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The long term forecast beginning Wednesday features a broad upper
level trough extending across the western two thirds of the state
with a highly amplified long wave ridge centered over the western
US extending up through central Canada and into the Yukon. As the
upper level trough digs east to encompass the whole state, the
upper level ridge will be pinched off over Canada and the jet
stream will return to more of an west/east zonal pattern, with the
main jet core setting up well to the south of the state, nearing
35N. What this means for the sensible weather this week is a
transition from warm and moist southerly flow to more of a colder
and drier pattern over the mainland as the storm track is
directed well to our south.

At the surface, a low over the northern Gulf will move north into
Bristol Bay and weaken during the day on Wednesday, though not
before bringing accumulating snow to the SW mainland. As this
system fizzles out the pattern becomes much quieter over the
mainland as cold air is allowed to filter south from the Arctic.
This will bring temperatures back to normal for this time of
year, likely dropping below normal by the end of the week,
especially out west. By Thursday, the models fall out of agreement
and solutions become unclear regarding specific features that may
track into the area, though at this point models are all
generally showing the storm track still staying to our south.
Models come back into agreement by the weekend as the next low
tracks across the northern Pacific and into the western Bering,
which could potentially be the next weather system to impact the
mainland.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 141.
MARINE...Gale Warning 119.

&&

SYNOPSIS AND MODEL DISCUSSION...RC
SOUTHCENTRAL ALASKA...SB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD
LONG TERM...KVP