Small Craft Advisory 933, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
508 PM AKST Thu Dec 5 2019

.ANALYSIS AND UPPER LEVELS...
The pattern is in the very early stages of amplifying. Out over
Western AK, the first in a series of fronts from the Eastern
Bering is moving inland today. This front brought snow and blowing
snow conditions to several places across the YK Delta and Bristol
Bay. These conditions continue to improve across the area as the
front falls apart while moving inland. Over the eastern half of
AK, high pressure continues to hold on. This airmass brought some
of the coldest temperatures of the year this morning with Gulkana
reaching about negative 10 and Skwentna touching as cold as
negative 19. Some patchy stratus and fog, especially along Knik
Arm, kept some of the populated locations from materializing all
the cold air. This stratus continues to linger, but is slowly
mixing out with some mid level clouds moving in and decreasing
stability.

&&

.MODEL DISCUSSION...
Models are struggling greatly with some of they key surface
features by Sat morning. The most important feature is a strong
surface low just west of Bristol Bay. The exact location, timing,
and strength of this low continues to vary from model to model and
run to run. This morning's 12z ECMWF run came into much better
agreement with the American models in terms of placement. However,
many components of this forecast (Anchorage winds, temperatures,
and coastal erosion) are contingent on the exact details of this
low. Thus, the forecast will remain consistent from previous
forecasts until we have better fidelity on some of these finer
details.

&&

.AVIATION...
PANC...Low stratus and fog continue to linger around the area, but
should remain away from the terminal for the most part. Snow will
start late this evening and bring deteriorating cigs and vis down
to at least MVFR conditions. The exact start time of the snow
could vary by an hour or 2. Light northerly winds will persist.

&&

.MARINE (Days 3 through 5)...
The biggest features of concern in this period will be a strong
decaying low over the Northern Bering and a developing system over
the Gulf of AK. First, in the Northern Bering, there is high
confidence in a strong system that will be weakening starting on
Sun. Lingering gale force winds will be moving out of the area
with this system. A broad trough of cold air across the Bering
will keep a small area of gales across the Western Aleutians on
Sun, but these will move out of the area to the south on Mon. Then
in the Gulf of AK, a developing system is riddled with uncertainty
at this time. However, we are trending things into the gale force
wind range to start the week with this forecast package. It is
certainly possible this system could bring storm force winds
(particularly in the Barrier Jet region), however, we will wait to
see a few more model runs before we make that call.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 3)...

The first of a series of fronts will move across Southcentral
tonight as the weather pattern becomes more amplified and more
active than the past couple of days. As this front lifts to the
coast tonight, areas of light snow will develop from the Kenai
Peninsula north across the Anchorage Bowl and Mat-Su valleys. The
snowfall will be generally light, but minor accumulations of 1 to
3 inches are possible into Friday morning. This front then stalls
along the coast as a second, more potent frontal systems lifts
from the AKPEN to the northern Gulf by Friday night. A deeper,
more moist southerly flow will accompany this second system,
making precipitation type a bit more challenging as warmer air is
advected across the region. Snow should change quickly to rain
late Friday night along coastal eastern Kenai Peninsula and
western Prince William Sound as a strong southerly surface wind
develops. The question is how much of the cold air already
entrenched over the interior locations erodes with this system, as
guidance is providing wildly differing solutions. Furthermore,
this is dependent on the placement of the surface out west and the
orientation and progression of the front. Again, there is quite a
bit of uncertainty with this event.

Given this, the thought right now is that enough warm air will
work up the Inlet to change snow to rain Saturday morning. Farther
north, the colder air should remain in place until the afternoon,
when it when a rain/snow mix becomes more likely for Anchorage and
the Matanuska Valley. Strong southeasterly gap winds will also
increase ahead of this front with gusty Turnagain Arm and
Anchorage Hillside winds developing Friday night and peaking
Saturday morning. Winds are currently forecasted to gust 40 to 50
mph, but this could be conservative given model trends.

Yet another strong frontal system is then expected to move into
either the western or northern Gulf late Saturday into Sunday.
Again, there is little certainty as to position and timing
between the forecast models; however, the persistent southerly
fetch would indicate a continued upward trend in temperatures and
any lingering snow giving way to rain.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...

Remnants of a front that brought rain and snow to southwest Alaska
are exiting the area this evening. Meanwhile, another frontal
boundary in the eastern Bering and north Pacific is expected to
move ashore Friday morning and will bring a mix of snow, freezing
rain, and then rain to the area. A winter weather advisory has
been issued for the Kuskokwim Delta for blowing snow during the
day Friday with some areas receiving 4 inches of new snow. As warm
air advection replaces the cold air in place, a transition period
of freezing rain to rain will occur Friday evening in the
Kuskokwim Delta. The frontal boundary pushes through southwest
Alaska on Saturday with strong winds and rain. Precipitation will
taper off Saturday afternoon as the parent low moves northwestward
out to sea.

&&

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

The Bering Sea and ALeutians will enter a very active pattern as a
large and strong upper level trough shifts eastward over the
region. A warm front will track across the central and eastern
Bering Sea through Friday, bringing rain and gale force winds. A
more powerful near storm force low will track north from the NPAC
through Bristol Bay and north toward the Bering Strait Friday
evening through Saturday. Rain will turn to mixed rain and snow
showers as cold air wraps behind this low across the entire Bering
Sea.

&&

.MARINE (Days 3 through 5)...
The biggest features of concern in this period will be a strong
decaying low over the Northern Bering and a developing system over
the Gulf of AK. First, in the Northern Bering, there is high
confidence in a strong system that will be weakening starting on
Sun. Lingering gale force winds will be moving out of the area
with this system. A broad trough of cold air across the Bering
will keep a small area of gales across the Western Aleutians on
Sun, but these will move out of the area to the south on Mon. Then
in the Gulf of AK, a developing system is riddled with uncertainty
at this time. However, we are trending things into the gale force
wind range to start the week with this forecast package. It is
certainly possible this system could bring storm force winds
(particularly in the Barrier Jet region), however, we will wait to
see a few more model runs before we make that call.

&&

.LONG TERM FORECAST (Days 3 through 7: Sunday through Thursday)...

The long term continues to be a mess, quite frankly. Models are
decidedly poor in performance, changing enough each day this week
to have low confidence in them. Therefore, our forecast for the
long term will be one of persistence and hedging towards the
trend this winter of cycles of warming and cooling.

What this means for Southcentral is that we will continue to
forecast a warm-up to start the long term, with sustained warmth
through the week. This is sticking with this season's trend
mentioned above by bringing us out of our current cold snap.
Models are showing a series of lows and general southerly flow
which is why we can at least expect that warmth to continue. Even
with their fickle nature this week, they would be hard pressed to
completely shift this pattern. Preliminarily for next week, there
looks to be a compact low moving north along the gulf coast Monday
into Tuesday. Then, a larger and powerful low looks to enter the
western Gulf Wednesday into Thursday. Bottom line...warm, wet and
possibly windy weather for next week especially along the coast.

For the Bering, Aleutians and Southwest AK...the longwave trough
holds over the area which will continue to circulate low pressure
systems within it through early next week. Models are hinting at a
shortwave ridge entering the Bering Wednesday, briefly calming
things midweek. Another large and powerful looks to enter the
western Bering and Aleutians Thursday. All the while, the previously
mentioned powerful low in the western Gulf that develops midweek
will have at least some play on weather in southwest AK with
easterly winds. However, the AK range will probably block most
weather. Details are yet to emerge for out west in all locations,
but do expect unsettled weather for the foreseeable future.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 155.
MARINE...Gale 119 130 136 150-170 172 176 178 180 181.
FIRE WEATHER...NONE.

&&

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