|Prince William Sound Low|
Effects on Anchorage Bowl
Anchorage, Alaska is located in South Central Alaska, northeast of the Alaska Peninsula, due north of
the Kenai Peninsula, northwest of Prince William Sound and the Alaska Panhandle, and nearly due south of
Mount McKinley. The Anchorage "Bowl" is a strip of coastal lowland extending up the slopes of the Chugach
Mountains to the east. To the south, lies the Turnagain Arm, a fjord with some of the world's highest
tides. The Knik Arm, another tidal inlet, lies to the west and north. The following locations encompass
the Anchorage bowl; Anchorage, Elmendorf AFB, Fort Richardson, Eagle River, Birchwood, Peters Creek,
Eklutna (most northern town), Girdwood, and Portage (most southern town).
When most people think of Alaska they think of lots of snow for most of the year, however; Anchorage only
averages about 70.5 inches of snow a year. Alaska winters begin in late October and usually end in early
April with the most snow occurring from December to February.
Forecasting snow in the Anchorage bowl can be difficult at times due to the geographical features that
surround the region. The mountains to the east can easily hinder moisture moving into the bowl and a
majority of the time precipitation will only fall on the windward side of the range. Wind flow along
the Chugach mountain range usually produces a downsloping feature that brings strong warm air advection
to the region. When the wind flow is from the north-northeast, the flow passes through valleys and
passes which allow moisture into the bowl. The atmospheric feature that sets this type of flow up
is known as the Broad Longwave Trough with Transient Shortwave Troughs regime.
The Broad Longwave Trough with Transient Shortwave Troughs regime usually sets up 15-20 times a year with
most of the occurrences happening during the winter. The winds aloft tend to be light and the pattern
usually sets up with the trough line along or just west of 150W. This regime is the primary snow
producer for the Anchorage bowl and 1-2 times a year a Prince William Sound Low will develop as
one of these transient shortwave troughs move through the region and into the sound. When a
Prince William Sound low sets up the forecaster for the Anchorage Bowl will have to track the
system very closely since the position of the low can have a great impact on snowfall amounts.
The broad longwave trough will need to be oriented to the west of Anchorage for a Prince William Sound
low to develop and have any influence on Anchorage. This allows the low to approach the Anchorage
bowl from the southwest. As the system approaches the winds will shift to a more north to northeasterly
direction bringing in needed abundant low level moisture.
The broad longwave trough lying over western Alaska provides instability, moisture advection, and cold air
advection to the Anchorage bowl. The longwave trough has very little movement and tends to remain for a
week or more. Maintaining continuity of the trough axis is critical with this pattern as it often
retrogrades. The weather to the west of the trough axis is usually fair, stable, and cold while
the weather to the east is unsettled and unstable.
The typical shortwave trough propagating through the nearly stationary longwave trough will normally
track from the Bering Sea toward mainland Alaska then into the north Gulf of Alaska. These shortwave
troughs can bring rapid weather changes in short time as they progress across the state. The shortwave
troughs that the forecasters in the Anchorage bowl are concerned about are the ones that track into
the Prince William Sound. Depending on the actually track of the low the bowl can see up to 20
inches of snow in a very short time, usually less than 24 hours.
Typically before the shortwave trough approaches, there will be significant cold air advection over the
Anchorage bowl. The cold air usually pushes south into the Prince William Sound causing increased
instability as this colder air lies over the relatively warmer waters. The approaching shortwave
trough will then tend to move into this region of greater instability. As the shortwave trough
moves into the northern Gulf of Alaska a surface low will develop. The shortwave trough will slow
and typically begin to fill as it continues to push east into anti-cyclonic flow, where the longwave
trough begins to transition into the longwave ridge, and the surface feature will move into the
Prince William Sound. The upper level feature will continue to push east to northeast and fill
or dissipate while the surface low will stall in the sound as it gets caught up in the mountainous
terrain on shore.
The tracking of the low will be a major factor on determining how much snow will fall in the Anchorage
bowl. If the low tracks in the eastern half of the sound very little or no snow will accumulate in
the bowl. If the low tracks in the western half of the sound snow will occur with the greatest snow
fall amounts happening when the low tracks along the western shore of the sound. The models,
unfortunately, do not normally handle the tracking of the surface low very well so the regional
forecaster really needs to use metsat and radar to help with the tracking of the low and
After the upper level feature has outrun the surface feature the surface low will begin to fill
as it no longer has upper level support. Snow usually begins as the surface low develops or moves
into the north Gulf of Alaska with the heaviest snow and greatest snowfall amounts occurring as the
system begins to fill. The faster the system fills the greater the snow fall amounts will be. Snow
fall continues until a significant synoptic change occurs, the low fills entirely, or until low level
ridging or upper level divergence becomes prominent.
After reviewing snow fall amounts associated with Prince William Sound lows at Elmendorf AFB it appears
that 10-25 inches of snow are not uncommon. Due to the system begin very uncommon and hard to forecast
for there is very little written about the feature. After talking to some civilian Air Force forecasters
that have been forecasting the weather for the Anchorage bowl for nearly 10 years I have come to
the conclusion that there is still much controversy as to what an actual Prince William Sound low
is and how they develop. After my review and 3 year forecasting experience at Elmendorf, I believe
that the Prince William Sound low is one of the hardest weather features to forecast for. The low
position, the strength of the upper and low level features, and the amount of moisture available
all play a role in the potential development of a possible Prince William Sound low. I believe
the biggest help with trying to forecast this feature will be using the metsat and radar to
track the system and incoming precipitation. The metsat loop will assist with the tracking
of the low especially when the low position can have such an influence on the snow fall
amounts. The radar will assist in tracking the heaviest snow.