Small Craft Advisory 924, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
445 PM AKST Tue Dec 3 2019

.ANALYSIS AND UPPER LEVELS...
A vertically stacked low over the northern Gulf is slowly weakening
as it exits to the south and east. As a result, snowfall across
Southcentral is tapering off. Gusty offshore flow continues along
Prince William Sound and across the northern and western Gulf
thanks to tight pressure gradients and persistent weak cold air
advection. Cloud cover remains extensive which is holding
temperatures steady. With an arctic air mass in place,
temperatures will begin to cool as ceilings lift or cloud cover
diminishes.

Conditions are quiet over Southwest Alaska, with a couple weak
features transiting aloft over top of a short-wave ridge over
Bristol Bay. Aside from a few showers near the coast, conditions
are dry and seasonably cold with a few areas of fog.

An inverted trough extends northward from a low passing south of
the Eastern Aleutians and southern Alaska Peninsula, with an
upper level short-wave trough and cold air advection just upstream.
This is leading to a fairly narrow band of snow tracking from
the eastern Aleutian Islands eastward across the southern Alaska
Peninsula. Surface temperatures are mostly above freezing, so
expect only minor snow accumulation as this band moves through.

Meanwhile, a low tracking into the far western Bering Sea is
pushing a warm front northward across the western Aleutians and
western Bering Sea. A cold front follows not far behind. This is
leading to strong southerly winds and rain across the
western-most Aleutian Islands.

&&

.MODEL DISCUSSION...
Models remain in good agreement with large scale features through
Wednesday night. Solutions then diverge Thursday through Friday as
upper level pattern amplifies. There are are differences with
track and depth of lows moving into the Bering as well as whether
a surface low develops farther east and affects Southwest Alaska.
There are enough commonalities in the overall pattern to depict
clear trends in the forecast. Chief among these will be another
warm-up across Southwest Alaska, with precipitation transitioning
from snow to rain beginning on Friday.

&&

.AVIATION...
PANC...Mid to upper level moisture will diminish as the Gulf low
pulls away. While there is some low level northerly flow, it
doesn't appear strong enough to advect out all of the lower level
clouds. Also, forecast soundings indicate increasing low level
stability as near surface temperatures continue to cool and
temperatures at around 5000 feet hold steady. Therefore, expect
to hold in an MVFR cloud deck through tonight. If there is any
clearing of clouds in the area, then could see an IFR or lower
cloud deck form and lurk in the vicinity of the terminal. In any
case, a strengthening of low level flow on Friday should allow
for ceilings to lift, and possibly even to scatter out.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3;
Tonight through Friday)...

An upper-level trough and associated surface low will continue to
track east-southeast toward the Alaska Panhandle tonight. As it
does, a drier, northerly flow will develop as high pressure builds
over the interior. Although moisture associated with the trough
will diminish over the Kenai Peninsula north to the Susitna Valley
by the morning hours Wednesday, periods of light snow/flurries
are expected to linger across Cook Inlet and the Kenai Peninsula
through the overnight hours. The challenge to this forecast is
whether enough there is enough clearing overnight to allow for
additional radiational cooling tonight. This could lead to the
development of fog across portions of Southcentral overnight.
Right now, the thought is that there will likely be some fog along
Cook Inlet, in the valleys of the Kenai Mountains, and the
southern half of the Copper River Valley.

Temperatures will continue to trend downward through the forecast
period with some of the coldest readings thus far this season,
given the fresh snowpack and strong cold air advection. The
caveat to this will be some lingering low stratus in the valleys
both tonight and tomorrow night which could help moderate the
cooling trend a bit. Nonetheless, expect overnight lows well
below zero across interior Southcentral and near zero as far south
as Anchorage and Kenai Wednesday night into early morning Thursday.

Continued gales over the Western Gulf and brisk outflow winds
across the eastern Kenai Peninsula and North Gulf Coast (Copper
River Delta) will slowly diminish on Wednesday.

A pattern change may then be in store for Southcentral by the
weekend as guidance is keying in on the possible development of a
low over the AKPEN swinging a warm front from the western Gulf
Thursday north to the Southcentral coast on Friday. If this
holds, precipitation type will be a challenge as southerly winds
increase and advect warmer air back across the region.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Dry and cold conditions persist through tonight as a ridge over
the interior forces a dense air mass offshore. A shift in the
pattern is expected Wednesday afternoon as the next front moves
toward the coast. A gale force front comes ashore Wednesday
evening with blowing snow and reduced visibilities thanks to gusty
winds Wednesday night and Thursday morning. A Winter Weather
Advisory was issued with the afternoon package for blowing snow
which will reduce visibilities across much of the Kuskokwim Delta
and Bristol Bay regions Wednesday evening through Thursday
morning. Precipitation should taper off Thursday afternoon. A
reinforcing shot of warm air with rain will push ashore in Bristol
Bay Thursday night as another frontal boundary comes in from the
north Pacific.

&&

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

For tonight...a better organized shortwave diving over the Alaska
Peninsula will initiate numerous snow showers as it exits to the
North Pacific. A gale force front currently moving to the central
Bering will bring a mix of rain and snow as it moves east then
moves ashore Wednesday night into Thursday morning.

The next front quickly forms in the Western Aleutians early
Thursday morning. Rain will develop along the front as it crosses
to the central Aleutians spreading northeastward to the Pribilof
Islands by Thursday night. Snow and rain showers continue in the
forecast for the western half of the Bering through Friday morning
as low pressure centers over the northern Bering.

.MARINE (Days 3 through 5: Friday through Sunday)...

The focus continues to be on low pressure in the Bering Sea, since
the Gulf of Alaska remains quiet until Sunday when some small
craft advisory winds will be present. Low pressure in the Bering
is still expected to produce Gale Force winds. We can be confident
in this but that's about it. Models are very inconsistent
starting Saturday, so placement and movement of low pressure is
still quite fuzzy. Particulars with low pressure in the Bering
will need to be handled the next couple of days. For now expect
gales, but be ready for timing and placement of them to adjust in
the coming days.

&&

.LONG TERM FORECAST (Days 3 through 7: Friday through Tuesday)...

The long term starts with cold air in place over much of our
forecast area. A longwave low pressure trough will also exist over
the Bering Sea. That begins to change Saturday with the
reamplification of a high pressure ridge to our east, forcing flow
out of the south from the bottom of the longwave trough out west.
This has the potential to change our weather back to about what it
was a couple weeks ago, with a warm air intrusion over southcentral
AK (warming temperatures) and melting snow. However, model guidance
uncertainty is quite high, with any consistency falling apart
Saturday. So, while ridge amplification changing our pattern looks
probable, the question remains where it will set up and how much
warming we will get. So, there is low confidence past Saturday and
it's just too early to tell.

During the pattern change over southcentral, broad low pressure
will continue to hold in the Bering, which will bring multiple
rounds of inclement weather through the Bering and southwest AK.
This will be from smaller lows circulating within the longwave
trough. uncertainty is quite high in the Bering as well, so
particulars of individual low pressure systems will need to be
figured out when they enter the short term timeframe.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 155 161.
MARINE...Gale 130 131 174-178 180 181 185.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...SEB
SOUTHCENTRAL ALASKA...TM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...LB/KH
MARINE/LONG TERM...BB