Small Craft Advisory 889, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion...CORRECTED
National Weather Service Anchorage AK
920 PM AKST Sat Nov 23 2019

.ANALYSIS AND UPPER LEVELS...
Broad upper cyclonic flow with embedded waves is bringing
continued unsettled weather across much of southern Mainland,
Gulf of Alaska, and the Bering Sea/Aleutians today. In the east,
there is a fairly organized area of showers(mainly rain) over the
Gulf of Alaska and North Gulf Coast moving north associated with a
surface low and weak frontal boundary. Out west, cooler air
moving over a warm water surface is producing a broad area of
mostly open cell cumulus clouds associated with snow showers.
Satellite imagery shows that these clouds/showers extend across
much of the Bering Sea, Aleutians, and the Alaska Peninsula.

&&

.MODEL DISCUSSION...
Models overall are in generally good synoptic agreement through
Monday across Southcentral with shower placement the only real
struggle. Out west, timing issues continue somewhat with winds
and a precipitation shield with next front moving across the
Bering Sea late Sunday into Monday.

&&

.AVIATION...
PANC...VFR conditions will persist. Some gusty southerly winds are
possible this evening, otherwise light winds are expected.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)...
Cyclonic flow around a surface low south of the Copper River
Delta will continue to push moisture in the form of coastal rain
showers and higher elevation snow showers over Prince William
Sound through the overnight hours. As a supporting upper-level
wave lifts north this evening, additional moisture will move into
the Copper River Basin, producing areas of light snow through
Sunday afternoon. Upstream, additional upper-level shortwaves will
slide from the AKPEN across the northern Gulf.

The first shortwave will move across the Barren Islands tonight,
with strong westerly winds developing in its wake. The
combination of persistent wind and cold air advection will produce
bands of snow showers across the mouth of Cook Inlet, with
periods of light snow for the Homer / Seldovia area through
Sunday night. The second wave will be the main upper-level low as
it begins to shear apart and transition to an open wave as it
moves across Southcentral. This feature will also enhance snow
shower activity for portions of the Susitna Valley, the western
Kenai Peninsula and Anchorage Bowl on Sunday. Right now, it looks
as though most of the snow showers will fall along the upslope
side of the mountains, but there remains the potential for a
light dusting of snow over lower elevations.

As the trough exits and a ridge builds to the north and west,
colder air will begin to spill south from the interior with a
noticeable cooling trend Sunday and Monday. In addition, the
increasing pressure and thermal gradients between the cold ridge
inland and the warmer trough over the eastern Gulf will produce
gusty northerly outflow winds. These winds will be strongest
through the bays and passes of Prince William Sound, as well as
Thompson Pass and the Copper River Delta. Winds will also remain
quite through Kamishak Bay and across the Barren Islands.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Cold temperatures and snow showers continue across Southwest
Alaska this afternoon associated with an upper level trough. On
Sunday, snow showers begin to diminish across the region,
resulting in little, if any, new accumulations for the Southwest
through the overnight hours as the upper level trough exits the
region. However, high temperatures will stay well below freezing
on Sunday for most of the area, with only the Alaska Peninsula
seeing temperatures near or slightly above freezing.

The main focus of today's forecast is the next big pattern change
beginning on Monday when the next big system enters for forecast
area. A front, associated with a storm force low in the western
Bering, will begin to impact the Southwest coast by Monday morning
when stronger winds begin to move inland over the Kuskokwim Delta
and Bristol Bay coastline. The front will begin to bring
precipitation to the Kuskokwim Delta by late Monday morning and
work its way inland to the remainder of the region through the
afternoon.

With cold temperatures already in place, precipitation will
initially start off as snow on Monday, but will quickly transition
over to rain as this warm front brings in warming temperatures.
There is high confidence that this new system will be a warm one,
but there is still uncertainty is the exact timing of when snow
changes over to rain and whether there will be a period of
rain/snow mix as the front moves inland. This leads to less
confidence in the amount of snowfall that will result before
switching over to rain, but areas across the Kuskokwim Valley
could likely see a few inches. The timing of the snow to rain and
snowfall amounts will continue to be looked at over the next few
forecasts when models come into better agreement.

Coastal flood watches have been issued for the Kuskokwim Bay
coast from Hooper Bay south to Cape Newenham due to higher
confidence in gale force winds, and likely storm force gusts, that
will impact the area. Water levels could rise 4 to 7 feet above
normal high tide beginning with the late evening high tide on
Monday. The highest surge potential is expected with the high
tides Tuesday morning through Tuesday afternoon.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
A front over the eastern Bering this afternoon will begin to
weaken and diminish through Sunday morning. The focus quickly
shifts to a strong warm front entering the western Bering and
Aleutians for Sunday morning. The initial gale force front will
reach the Pribilofs by Monday afternoon, while storm force winds
begin to move over the western Aleutians. There could be a few
snow showers on the leading edge of this system, but widespread
rain is expected to accompany this warm system as it moves across
the region. Colder air and potential snow will follow behind this
front over the western Aleutians and Bering as colder air begins
to move into the region.

&&

.MARINE (Days 3 through 5)...
Offshore flow tapers off Monday night, with small craft winds
expected across much of the Gulf. Locally elevated outflow winds
through channeled terrain may continue longer. A gale force front
moves into the Gulf by Wednesday morning.

For the Bering, storm force winds in the central Bering Monday evening
will spread eastward through Tuesday morning as the front moves
inland. Expect gales through Tuesday in the eastern Bering before
winds taper to small craft by Wednesday.

&&

.LONG TERM FORECAST (Days 3 through 7)...
An active pattern is expected to continue next week as an upper
level trough over the southern Mainland slowly moves eastward
Monday night. Meanwhile a large frontal system in the Bering lifts
over Southwest Monday evening into Tuesday bringing gusty winds
and heavy precipitation. As this system weakens, the remnants move
eastward and reach Southcentral Alaska by midweek. By this time,
a wave over the northern Pacific lifts northward and merges with
the remnant front. This will produce the next round of active
weather for the southern Mainland late Tuesday into early
Wednesday. A warm front reaches the Alaska Peninsula and Kodiak
Island Wednesday morning and continues moving northeastward into
Southcentral by Thursday morning. With an ample moisture supply
from the Gulf, high precipitation rates are likely. The biggest
forecast challenge will be precipitation types and where the
rain/snow line sets up. By the end of the week, another strong
Bering low looks to usher in colder air over Southwest.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Coastal Flood Watch 155 161.
MARINE...Gale Warning 130>132 150 160 173 175 177>179 185.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...RC
SOUTHCENTRAL ALASKA...TM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AH
MARINE/LONG TERM...KO