Small Craft Advisory 224, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion...CORRECTED
National Weather Service Anchorage AK
518 AM AKST Sat Mar 7 2020

.ANALYSIS AND UPPER LEVELS...

An upper level ridge is currently over southern Yukon Territory
with a negatively-tilted trough to the west over the Alaskan
mainland. The trough-axis extends southeastward from Southwest
Alaska to south of 40N in the North Pacific. The subtropical
jetstream is oriented from southwest to northeast from the
subtropical western Pacific through the central Bering/Aleutians
and then curves southeastward into the subtropical eastern
Pacific. An upper low is currently developing west of the
Aleutians and east of the Kamchatka Peninsula as this is where the
left-exit region is located in the aforementioned subtropical
jetstream, thus strong Positive Vorticity Advection (PVA) is
present in this location to develop cyclonic flow aloft. The
subtropical jetstream meets the polar jetstream in the central
Bering and diverges from the polar jet over the western Gulf. As a
result, GOES-17 water vapor imagery shows an area of stretching
deformation aloft as the subtropical jet sharply dips
southeastward and the polar jet bulges northeastward over the
AlCan border. Teleconnections support the pattern to become more
zonal with a strengthening jetstream as the Arctic Oscillation
(AO) becomes strongly positive and thus increases the pressure
gradient between the north pole and mid- latitudes in the northern
hemisphere. At the same time, the Pacific/North American Pattern
(PNA) and the Eastern Pacific Oscillation (EPO) remain weak, thus
the strongly positive AO is the dominant driver our synoptic
pattern and thus resulting in a transition to strong latitudinal
(zonal) flow in the upper levels.

At the surface, a 988 mb low is currently located west of the
Aleutians and east of the Kamchatka Peninsula and is moving
northeastward. An occluded frontal boundary extends from the
central Bering to the Kamchatka Peninsula and is moving northward.
A 1036 mb ridge of high pressure extends northward from the
subtropical central Pacific to the Pacific side of the central
Aleutians. A 1003 mb low near 47N 145W continues to slowly move
southeastward. A weak 1009 mb surface low currently west of the
Haida Gwaii Archipelago is slowly moving northwestward. A 1032 mb
surface ridge of high pressure is currently over Northwest
Territory. Radar imagery shows snow showers moving northwestward
across the northern Gulf and into Prince William Sound. Another
area of snow showers is moving northeastward across northern
Bristol Bay and the Kuskokwim Delta.

&&

.MODEL DISCUSSION...

Models are in good agreement with the upper level synoptic pattern
into the start of next week with a strong zonal jet developing
over the southern Bering and the overall synoptic pattern
becoming more zonal. This means there is consistency with the
central to northern Bering being a primary area for the potential
for lows to develop and strengthen as the strong zonal jet to the
south will make this area have strong PVA. Guidance is also
consistent with the jetstream gradually moving southeastward
Monday into Tuesday allowing for a more active weather pattern
for southcentral for the start of next week. Ensembles show the
AO rapidly weakening towards the end of next week and thus
guidance is showing the pattern slowly becoming more meridional as
a weaker jetstream will allow for troughs and ridges to develop.
Significant differences arise in amplitude and orientation of
these troughs and ridges, however there is consistency in that the
overall synoptic pattern will change later next week.

At the surface, models are in good agreement with the overall
synoptic pattern through the weekend and are in fair agreement on
Monday as minor discrepancies arise on low positions and
intensities. Discrepancies increase on Tuesday and Wednesday with
surface features and thus, forecast confidence will remain low
thereafter.

&&

.AVIATION...
PANC...Conditions overall will remain MVFR for this TAF package
as visibility will remain between 3-5SM as light snow moves across
the area. Ceilings will fluctuate between VFR and MVFR this TAF
period.

&&

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

The first of two of upper-level shortwaves will bring periods of
snow to Southcentral through the weekend. The first is tracking
east across Southcentral this morning, and is currently producing
areas of light snow from Cook Inlet north across the Susitna
Valley. Light snow will also develop later today across the Copper
River Basin. Snow totals will remain light with a dusting to as
much as two inches of snow across the region.

A second shortwave quickly follows on the heels of the first,
helping to redevelop snow across Cook Inlet and the Susitna Valley
later this afternoon and across the rest of Southcentral by late
Saturday evening. By late Saturday night into Sunday,
southeasterly gap winds will increase as a surface front
associated with the low over interior Bristol Bay moves from the
western Gulf into the northern Gulf. As the front lifts north, a
surface low will develop along it south of the Kenai Peninsula.
The resultant flow shift to the southeast for a time, helping
initiate downsloping along the lee of the Kenai and Chugach
Mountains and limiting precipitation amounts for places such as
the Anchorage Bowl for a time early Sunday. But as the surface low
and upper-level trough lift northeast Sunday afternoon, the flow
will turn back to the southwest and usher in another round of snow
from the western Kenai north across the Matanuska Valley as
slightly colder air advects in. Given the persistent southerly
flow and a weak trough stretched across the Susitna Valley through
the weekend, a longer duration snowfall is expected from west of
Wasilla to Broad Pass. In total, 8 to 16 inches of snow is
possible for the Susitna Valley with 6 to 12 inches possible,
mainly west of Wasilla, for the Matanuska Valley through Monday
morning.

&&

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

Snow showers have continued across southwest Alaska as observed on
radar. These showers are ahead of the low pressure system that
remains over the eastern Bering. This low has formed after the
parent low became sheared and split, with one low propagating
into the North Pacific and the other low remaining over the
eastern Bering. Showers will remain as a front associated with
the next low pressure system over the central Bering, catches up
to the low that was left behind in the eastern Bering. Expect
these conditions to continue through Sunday afternoon as this
system moves east across the region. Behind this, northerly flow
aloft will take over as it taps cold and dry arctic air from the
north and brings it into the southwest through Tuesday. Wind
chills will likely return to the Kuskokwim Delta coast, although,
they don't look to currently meet advisory criteria.

&&

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

A break in the weather continues across the central Bering between
the low in the eastern Bering and the low that is moving across
the western Bering. Similar to the low in the eastern Bering as it
moved across the area, weather conditions will be identical
through Monday morning. The backside of this low will bring
northerly flow aloft which will propagate cold temperatures and
dry conditions from the arctic across the Bering and to the
Aleutians. Snowfall looks to continue for the western Bering
during this time. Behind this feature, high pressure will build in
before the next system starts to impact the western Bering
Tuesday morning.

&&

.MARINE (Days 3 through 5: Monday through Wednesday)...
Gulf of Alaska: A weak low over the Gulf on Monday morning will
intensify and shift to the northern Gulf by Tuesday as cold air
moves into the western Gulf. Northwesterly gales will develop
through the mountain gaps, specifically through the Kamishak Gap
and the Barren Islands, and the southern end of Kodiak Island
starting Monday night, and persist in those same areas through
Wednesday. By Wednesday the winds will be more northerly.

Bering Sea/Aleutians: Northerly gales will be possible between
the Aleutian Islands from Dutch Harbor to Atka Monday morning. The
front causing the gales will weaken, though gusty north to
northwest winds will persist from Dutch Harbor east through
Tuesday. A front south of a low near Kamchatka may produce
southwesterly gales Monday evening across the western Aleutians.
The front weakens overnight, but gales will be possible on
northwesterly winds over the open waters of the western Bering
through the day Tuesday. A much stronger front will produce gales
with storm force gusts over all of the western Bering and
Aleutians Wednesday afternoon and evening. The widespread gales
will spread into the central Bering Wednesday night.

&&

.LONG TERM FORECAST (Days 4 through 7: Tuesday through Friday)...
A highly amplified pattern will be set up across Southern Alaska
on Tuesday morning, with a low over the northeastern Gulf near
Yakutat. A high will be over the eastern Bering centered near the
Pribilofs, and a low will be moving into the western Aleutians.
This pattern will favor continued cold air, strong gusty
northwesterly winds, and a drier pattern across Southern Alaska on
Tuesday morning. The northeastern Gulf low will persist through
Wednesday, keeping conditions unsettled along the north Gulf
coast. The high over the Bering moves into mainland Alaska on
Wednesday. Thus, the winds will gradually dissipate from west to
east through Thursday.

A strong front will bring another round of warm air, southerly
winds and rain to the western Aleutians on Wednesday afternoon.
While the southern end of the front will move slowly eastward
across the central Aleutians on Thursday, the northern side will
move east much more quickly. Snow, potentially mixed with rain
near the coast, will move into the Kuskokwim Delta Thursday
afternoon, and overspread the rest of Southwest Alaska Thursday
night. A large point of uncertainty for Friday will be how much of
that moisture makes it over the Alaska Range and into
Southcentral. Both the GFS and EC do show some snow moving into
the Cook Inlet region, including Anchorage, during the day, and
lightly persisting into Friday night as the upper level front
continues eastward.

Temperature-wise, cold, well below normal temperatures will
persist through Thursday across Southcentral while things warm up
to around freezing across Southwest Alaska.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 111 145.
MARINE...Heavy Freezing Spray Warning 127 185.
         Gale Warning 178.
FIRE WEATHER...NONE.

&&

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