Small Craft Advisory 894, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
520 PM AKST Sun Nov 24 2019

.ANALYSIS AND UPPER LEVELS...
Moist air overrunning the the Copper River Basin, which brought
significant snowfall overnight, is weakening as cloud tops are
warming. Otherwise, the broad upper low extending from the
southwest interior continues to progress very slowly east.
Instability associated with the cold upper center brought
widespread showers from the Bering Sea to over the Gulf of Alaska
with lesser aerial coverage over land. Showers were mainly snow
across the Bering Sea and the Mainlands and more mixed rain and
snow over the Gulf waters. Gusty outflow conditions continued
across the western Gulf and Barren Islands as cold air squeezed
through gaps in the Aleutian Range. Satellite imagery shows a
developing low and associated front just entering the far western
Bering Sea.

&&

.MODEL DISCUSSION...
Models overall have similar and reasonable solutions with the
storm force front moving east across the Bering Sea tonight
through Tuesday. The bigger differences, which affect
Southcentral, center on the potential of a wave developing along
the front over the north Pacific and it pushing moisture into
Southcentral Tuesday and Wednesday. The ECMWF and 12z NAM develop
this low and spread overrunning snow into the Southcentral(west of
the Copper River Basin) Tuesday night and Monday. The GFS is
significantly slower and on the dry side for the MAT_SU Valleys.
We are currently leaning toward the ECMWF and NAM solutions.

&&

.AVIATION...
PANC...Other than a few passing showers this afternoon, VFR
conditions and light winds should persist.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)...
An upper-level low moving across Southcentral will continue to
produce bands of scattered snow showers along the Kenai Mountains
north over the Anchorage Bowl and Matanuska Valley through the
evening hours. As this low continues to track east over the Copper
River Basin then southeast toward the Alaska Peninsula, a ridge
will will begin to amplify in its wake. The resulting northerly
flow will begin to usher in much colder air across the region
beginning tonight. This colder air will remain in place into mid-
week before a storm system moves in from the west for Wednesday.

In addition to the colder air, the resulting pressure and thermal
gradients will produce gusty northerly outflow winds across the
Southcentral Coast, as well as through mountain passes. The winds
will ramp up later tonight and peak Sunday night before slowly
winding down Monday as the upper-level ridge ridge crests
overhead.

Then all eyes turn to a north Pacific low phasing with a second
system further upstream over the Bering. This system is expected
to then push into the western Gulf on Tuesday and over much of
Southcentral by Wednesday. With plenty of arctic air in place,
this system has the potential to produce widespread accumulating
snow from the eastern Kenai Peninsula north to the Susitna Valley.

With that said, though, there is still quite a bit of uncertainty
regarding the timing and evolution of this complex system, not to
mention exact snow amounts. Of course, we will be monitoring this
closely as this could impact travel during the Thanksgiving
holiday.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
An area of ridging lies over the eastern Bering, with some
shortwave ripples in the mean flow sparking isolated snow showers
in southwest Alaska this afternoon. As the ridge moves eastward, a
960 mb Kamchatka low will bring a storm force front across the
Bering over the next 48 hours. Arrival in southwest Alaska will
take place Monday evening with coastal flooding possible in the
Kusko Delta coastal area Monday evening through Tuesday evening
and north of Cape Newenham Tuesday morning. Coastal flooding will
be at its peak during the high tide cycles. Expecting some areas
of snow and blowing snow Monday night and Tuesday in the Kusko
Valley with a rain snow mix in the Kusko Delta. Models have backed
off of precipitation quantities in the last 24 hours and the
extent of snow to be blown around will depend on the mixture of
rain and snow. Southwest Alaska will see some isolated snow, but
the majority of precipitation stays to the north.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The 960 mb Kamchatka low will move from the western Bering this
evening with storm force winds in the central Bering by Monday
evening. Storm force winds will be short-lived and diminish by
Tuesday morning as the system passes the Pribilof Islands, but
gales will impact the entire Aleutian chain tonight through
Tuesday morning, as well as the eastern Bering into Tuesday
evening. Expect areas of snow showers behind the front with a
strong push of cold air.

&&

.MARINE (Days 3 through 5)...
Southerly gales are expected to spread from west to east across
the Bering and Aleutians Thursday and Friday as a front works its
way across the area. Northerly Gales are expected south of the
eastern Aleutians and Alaska Peninsula on Thursday and Friday with
strong inflow into a low over the southern Gulf.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Shortwave energy initially over the eastern Aleutians/Bering Sea
into the North Pacific on Wednesday should progress eastward and
weaken while a mid-latitude system to the south remains separated
from most of this energy and eventually drops southeastward.
Toward the end of the week and weekend the pattern will become
more dominated by a deep upper low moving into or just inland from
the extreme western/northern Bering. The overall trough containing
the upper low should extend south into the mid-latitude Pacific,
with the trough axis most likely reaching the eastern Bering
around next Mon. Expect a downstream ridge aloft to cross the
mainland around Fri-Sat and continue into western Canada. For the
southern Alaska mainland this will lead to a warming trend
Wednesday and Thursday, with warmer than normal temperatures
continuing through the weekend. Precipitation type will therefore
be an issue for most of the extended forecast.

Operational model runs and individual ensemble members display a
decent amount of spread and run-to-run variability to temper
confidence in the forecast, and thus it is a little below
average.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Coastal Flood Advisory 155 161.
MARINE...Storm Warning 177>179.
 Gale Warning 127 130>132 138 165 170 173>176 180 181 185.
FIRE WEATHER...NONE.

&&

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