Small Craft Advisory 265, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
424 PM AKDT Sat Mar 21 2020

.ANALYSIS AND UPPER LEVELS...

The main driver of the weather is an upper level trough that is
transiting from west to east across the mainland. This is coupled
with a low that is moving from the Barren Island into the Gulf of
Alaska. Both features will continue to track east southeast
overnight. The southwest mainland has a ridge building in behind
the trough with west to northwest flow pushing inland over the
Bering Sea coastal region. West of this we have a a gale force low
out near the Kamchatka Peninsula. The gradient between this low
and the high pressure over the Bering is where we have near gale
force wind over the extreme western Aleutians and western Bering
Sea offshore.

Snow is over Southcentral Alaska to the Copper River Basin. The
snow over southwest Alaska is tapering off as the ridge builds
over the region. This may lead to some fog in the region. Over
Kodiak Island to Prince William Sound warmer air is resulting in a
mix of rain and snow or all rain.

&&

.MODEL DISCUSSION...

Good model agreement continues even though we are in this
progressive weather pattern. The main challenge will be timing
the end of the precipitation over the south central region to the
Canadian border. Following this, the challenge is how strong and
when the outflow winds begin and end. The current thinking is to
stick with the previous forecast with some minor adjustments.

&&

.AVIATION...
PANC...IFR/MVFR conditions will rapidly improve early this
evening. The main issue will be whether we see fog as skies clear
this evening. Most likely, if there is any fog, it will be brief,
then likely over the Inlet. North wind off the surface will likely
keep conditions mixed enough to reduce any fog impact.

&&

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

An area of low pressure east of the Barren Islands will continue
to track east over the northern Gulf tonight. Widespread, light
precipitation is accompanying this feature, with a southeasterly
fetch out ahead of the low pushing moisture in the form of a mix
of rain and snow from Cordova west over Prince William Sound. As
the low moves east, areas of light snow will develop across the
Copper River Basin, continuing through the overnight hours.

Behind the low, light snow (with some rain mixed in along the
southern Kenai Peninsula coast) is continuing across the Kenai
Peninsula north over the Mat- Su Valleys. This area of
precipitation is being enhanced by a trailing upper- level
shortwave trough that will push across the region later this
evening. As it does, the snow will quickly shut off, but not
before an additional coating to inch of snow accumulation.
Lingering low- level moisture along Cook Inlet may also produce
some patchy fog from Kenai north to Anchorage overnight.

Strong northerly outflow winds will also begin to develop
overnight as the low shifts east a ridge over interior Alaska
quickly strengthens. These winds will increase in intensity
through favored gap locations (Passage Canal, Valdez City and
waterways, Thompson Pass, and the Copper River Delta) Sunday
morning and remain quite gusty through Monday before slowly
tapering off as a ridge over the AKPEN moves into the western
Gulf, shifting the northerly flow aloft to a westerly flow.

The northerly flow will also help keep things relatively clear and
dry over much of the area Sunday and Monday, with any shower
activity limited to the higher terrain. Bands of mid and high
level clouds may periodically move across the northern reaches of
the forecast area Monday into Tuesday as two weak shortwaves move
through the interior of the state.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...

Things will be rather quiet and mild in the coming days in
southwestern Alaska. A stacked low in the Gulf will track
eastward Saturday night and a strong upper level ridge builds in
behind it over much of the Bering Sea. This will also keep
southwest Alaska in an onshore wind pattern and thus keep
temperatures on the mild side, leading to possible fog Sunday and
Monday morning in the Bristol Bay region. Guidance suggests likely
snow showers Monday afternoon over the Middle Kuskokwim Valley
under a series of shortwaves troughs. The breakdown of the
dominant ridge will usher in a fairly active pattern Tuesday
onward with several upper level disturbances moving through the
mean flow. Southwest Alaska should expect precipitation chances as
these disturbances move ashore early Tuesday. Temperatures
hovering in the 20s and 30s will likely deliver a mix of rain and
snow with the warm air moving ashore ahead of these fronts.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...

High pressure will dominate the first half of the period over
much of the Bering as a stacked low in the Gulf of Alaska tracks
eastward. Precipitation will develop Sunday afternoon as a front
lifts out of the north Pacific into the western Aleutians and then
spreads northeastward into the rest of the Bering by Monday
afternoon. The Pribilofs will see rain chances beginning Monday
morning with shortwaves passing through the collapsing ridge. The
breakdown of the dominant ridge will usher in a fairly active
pattern Tuesday onward with several minor upper level disturbances
moving through the mean flow and bringing more mixed precipitation
to the Pribilofs and Aleutians.

&&

.MARINE (Days 3 through 5, Mon through Wed)...

In the Gulf, winds should not exceed small craft through Wednesday.
Due to slight differences in the models, low end Gales are possible
for the western Aleutians on Thursday with the rest of the Bering staying
at or below small craft criteria.

&&

.LONG TERM FORECAST (Days 3 through 7, Mon through Sat)...

Strong confidence remains that an upper level ridge will remain over
the southern part of Alaska through the middle of the week. Tuesday
morning, a North Pacific low will get caught up in the backside of
the ridge, which will bring it northwest across the western Aleutians
and into the Bering Sea. This low will weaken the ridge, thereby flattening
it, and looks to force the low southeastward across the eastern Aleutians
and right back into the North Pacific by Thursday. Following this, a
zonal flow aloft will be the dominant feature into Friday night. After
this, an area of low pressure is on track to enter the western Bering,
although, strength and timing remain the forecast challenges for this
feature. It is a possibility that this low will weaken into a trough
as it makes its way towards the Alaska Peninsula Saturday morning and
into the Gulf on Sunday.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gales 127 130-132 136 150 176 351 352.
FIRE WEATHER...NONE.

&&

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