Small Craft Advisory 227, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
512 AM AKDT Sun Mar 8 2020

.ANALYSIS AND UPPER LEVELS...

The highly amplified pattern is in the process of gradually
transitioning into a zonal pattern. Teleconnections are the main
driver for this pattern change as the Arctic Oscillation is
strengthening in the positive phase. This is significant as it
will increase the pressure gradient between the north pole and
mid-latitudes, thus increasing the wind speed inside of the
jetstream over the North Pacific. At the same time, the
Pacific/North American Pattern (PNA) and Eastern Pacific
Oscillation (EPO) remain relatively weak, thus more evidence that
the AO will be the dominant driver of the overall synoptic pattern
becoming more zonal as the jetstream winds increase. The
jetstream is oriented northeastward from the western subtropical
Pacific to the central Aleutians and then curves southeastward
across the western Gulf and extends to the eastern subtropical
Pacific. With the current jetstream orientation there is a
positively-tilted upper level trough over the western Pacific and
an upper ridge over the central Pacific. A second positively-
tilted upper trough is currently over the eastern Pacific. An
upper low is currently spinning up over the central Bering due to
this region being located in the left-exit region of the jetstream
where upper level diffluence is present combined with the maximum
Positive Vorticity Advection (PVA). Due to the PVA maxima being
located in this area, cyclonic flow is developing aloft and
helping support the development of low pressure.

At the surface, a 996 mb low is located over the central Bering
and is moving northeastward. A 1036 mb ridge of high pressure
extends northward from the central subtropical Pacific to the
central Aleutians. Radar imagery shows snow moving northeastward
across Cook Inlet and the Kenai Peninsula into the Mat-Su Valley.
Snowfall intensity varies across the Anchorage Bowl as the
eastern portion of Anchorage is experiencing downslope and thus
cutting down on snowfall rates. Light snow is also moving
northeastward across the northern Gulf and into Prince William
Sound. Radar also shows light snow moving northeastward across
Bristol Bay, the Kuskokwim Delta and northern AKPEN.

&&

.MODEL DISCUSSION...

Models are in good agreement with the upper level synoptic pattern
for the start of this week and then fair agreement later this
week. Ensembles show the strong positive AO rapidly weakening
later this week and this is reflected in guidance when analyzing
the 300 mb wind pattern. The start of the week begins with a
strong zonal jetstream across the North Pacific due to a strong
positive AO, this strong zonal jet will gradually move
southeastward and result in the pattern across Southcentral to
become more active Monday night into Tuesday. This is primarily
due to Southcentral being in the left-exit region of the 300 mb
jet, thus the combination of diffluence aloft and PVA will be
present to help induce cyclonic flow. Guidance is also in good
agreement with a 500 mb trough moving through Tuesday. This is
significant as the cold air in place as the 300 mb jet is located
to the south and will allow for the precipitation type to most
likely be snow. The 500 mb trough and vorticity provide a
mechanism to trigger the lifting of air parcels and increase the
chances of precipitation ahead of the trough-axis Monday night
into Tuesday. Later this week the jetstream will significantly
weaken and allow for a more amplified (meridional) pattern to
develop as weaker flow will allow for troughs and ridges to build.
There are amplitude and orientation differences in guidance,
however there is consistency that the pattern will change and
allow for a strong upper level ridge to build across Southcentral
Alaska. This upper level ridging over Southcentral Alaska will
allow for less cloud cover and a gradual warming trend in temperatures.

At the surface, models are in good agreement with the synoptic
situation for the first half of the week. Models are highly
consistent in the position and orientation of high and low
pressure systems and thus forecast confidence is above average.
Models begin to diverge in the second half of the week with the
position and orientation of these features but all show a general
consistency in the overall trend with strong surface high pressure
ridging moving over Southcentral which is congruent to the upper
level pattern. This combination of surface and upper level ridging
will persist through next weekend and allow for a general trend of
clear skies and warming of surface temperatures. Forecast
confidence for the extended period remains above normal due to
guidance having an above average consistency in the patterns at
the surface and in the upper levels.

&&

.AVIATION...
PANC...Conditions will gradually transition from VFR to MVFR
through 08th/16z as light snow will gradually lower ceilings and
visibility. Expect IFR conditions between 08th/16z and 09th/04z
as light snow will reduce visibility between 1-3SM, though
ceilings will remain in the MVFR category. Conditions will then
become VFR after 09th/14z as visibility increases to between
3-5SM.

&&

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

Areas of light snow continue to fall this morning across a large
area of Southcentral Alaska due, in part, to a front lifting north
from the western Gulf. As it does, east to west gap winds will
increase today, especially along Turnagain Arm. These winds will
be further enhanced by an increase in the pressure gradient due to
the development of a surface low near the Barren Islands this
morning. As the low and its associated front track south of
Seward this morning, a southeasterly flow in the lower levels of
the atmosphere will increase and further establish downsloping
along the lee of the Chugach and Kenai Mountains through mid-
morning. However, as the low moves toward western Prince William
Sound and the trailing upper-level shortwave moves over Cook
Inlet, the gaps winds will diminish along with the downsloping
winds. A steadier snow will again develop across these areas,
including Anchorage and Palmer, where 2 to 4 inches of snow could
fall.

This second round of snow will persist from Cook Inlet north
across the Mat-Su Valleys through Monday morning as the trough
slows its eastward progression. By late Monday morning the snow
will taper to flurries, but weak cold air advection and a light
westerly wind underneath the trough may still allow for light
snow to linger across the Anchorage Hillside north to Palmer and
Sutton (due to upslope flow) through Tuesday morning before the
main trough axis slides east and a drier and colder northwesterly
flow develops.

For the Susitna Valley, especially from Willow to Talkeetna, a
stationary surface trough combined with a steady southerly flow
aloft will keep sufficient moisture and lift over the region to
produce a steady snowfall through Monday. It is in these locations
that the highest snow totals are expected, with an additional 6
to 12 inches possible through Monday morning. The southerly flow
will also bring upwards for another 6 to 12 inches of snow for
Hatcher Pass as well.

As these features finally move east on Monday, light snow will
spread into the Copper River Basin aided by a moist, southeasterly
flow ahead of the main trough. Here, several inches of snow could
pile up, especially along the Richardson Highway through Tuesday.
As the trough departs toward the Alaska Panhandle and a ridge
builds back over the interior of the state, expect another shot of
colder air and gusty northerly winds across Southcentral by
midweek.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Weak upper level pieces of energy in general zonal flow aloft will
continue to propagate through Southwest Alaska today into tonight,
leading to continuation of light snow with light snow accumulations.
A second elongated stronger piece of upper level energy will move
through Southwest Alaska tonight into Sunday morning with
westerly flow. This will enhance the snowfall intensity across the
Alaska Range due to upslope flow as the air is lifted and
condensed. Areas of Southwest Alaska will begin to dry out during
the morning Sunday from west to east as cold air advection begins
to move into the area with some clearing later in the day Sunday.

A digging trough Sunday night into Monday will bring a return of
arctic air to the region. Wind chills will also return with this
blast of arctic air, however, winds and the arctic air won't
align for advisory level criteria.

&&

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

An elongated strong piece of upper level energy will exit rapidly
across the eastern Bering leaving a stationary front at the
surface connected with a weak surface low in the central Bering.
As a result, current weather conditions will persist across the
Pribilof Islands and central Bering.

The weak surface low over the central Bering will drift slowly
southward beginning this afternoon and continuing through Monday
afternoon. As this low moves southeastward, a stronger pressure
gradient will develop between Dutch Harbor and Adak allowing gusty
northerly winds to develop as colder air moves in.

An upper level trough digging southward across the eastern Bering
will bring northerly flow to the eastern Aleutians and southern
Alaska Peninsula. Along with this digging trough will be cold air
advection moving down from the north. A stronger pressure
gradient at the surface due to the departing low and building
surface high pressure will allow for the strong gap winds through
the normal areas of the eastern Aleutians and Alaska Peninsula
Monday through Monday night.

&&

.MARINE (Days 3 through 5)...

A low will track east through the Gulf into Tue. This, combined
with cold air advection in anticyclonic flow over the Bering will
bring gale force northwest winds over the Barren Islands and south
of the AKPEN. Expect elevated offshore winds out of the Copper
River Delta. Winds over the Gulf will diminish Wed into Thu. A
gale force low will approach the western Aleutians Wed into Thu,
bringing southerly gales to the Aleutians and Bering Sea.

&&

.LONG TERM FORECAST (Days 3 through 7)...

A building Arctic high will continue cold, mostly dry weather with
clearing skies over the Southern Mainland Tue and Wed. Meanwhile,
a shortwave trough will traverse the western to central
Aleutians, bringing a quick shot of precipitation. With cold high
pressure inland, expect increased winds out of the usual gaps and
passes over Southcentral: Valdez waters, Copper River Delta, and
over the Barren Islands. Moving forward, a broad low will approach
the western Bering Thu. This will transition Southwest AK to a
southwesterly flow regime, and Arctic air should get shunted
eastward toward the Alcan border. Expect a warming trend to
initiate from west to east Thu into Fri. The associated front will
traverse the Aleutians and reach the Southwest Mainland Sat,
ushering in moisture from the south and potential for
precipitation.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 111 145.
MARINE...Gale Warning 173 174 175 176.
	     Heavy Freezing Spray Warning 179 185 414.
FIRE WEATHER...NONE.

&&

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