A Rare Standing Atmospheric Wave Snow Event in Duluth MN, January 25, 2010
A standing wave is an atmospheric wave that remains stationary relative to the hill that triggers its formation. Duluth is located at the western tip of Lake Superior on the north shore of the lake. The city is built along a chain of hills capable of inducing a variety of atmospheric waves. A standing wave appears to have produced an inch or two of snow in Duluth on the evening of January 25, 2010. The snow fell toward the tail end of a major snowstorm that occurred from January 23 through January 25. Standing wave events are not unusual over the ridge that runs along the north shore of Lake Superior. The ridge also extends southwest beyond the tip of the Lake. Rarely however does snow reach the ground in Duluth, except sometimes light snow in parts of the city near the immediate lakeshore. This event produced moderate to briefly heavy snow farther inland.
Radar Evidence of the Wave
The following radar animation shows developing snow produced by the wave. A band of snow is intensifying along the lake shore and southwest beyond the tip of the lake. The band is building slightly back to the northwest against the southeast movement of other bands of light snow and snow showers.
The next 9 images show the band associated with the wave remaining stationary through 0544 UTC (11:44 PM CST). The band then shifts rapidly southeast and dissipates.
The wave developed in a cold air advection environment with northwest to north winds behind the departing low pressure system which produced the snowstorm. The 0000 UTC January 26 (6 PM CST January 25), 2010 surface map and 500 mb analysis show the locations of the surface and upper air components of the storm system.
Conditions Favorable for a Standing Wave to Form
1. An elongated hill with a rounded rather than sharp narrow top
The ridge that runs through Duluth has a near ideal shape to support standing wave formation. The topographic map shows the long ridge parallel to the north shore of Lake Superior. The elevation rises gradually on the northwest side of the ridge, levels off, then falls off quickly to the lake. A hill that rises gradually on the upwind side and drops abruptly on the downwind side is also favorable for standing wave development.
2. Winds 25 Knots or Greater in the Lower to Middle Troposphere and Perpendicular to the hill
The following sequence of images show additional upper air analyses at 925 mb, 850 mb and 700 mb. These analyses, along with the surface and 500 mb, show winds 10 to 15 knots at the immediate surface, then 20 to 30 knots from a northwest to north direction from 925 mb up to 500 mb. A near perpendicular wind direction into the ridge existed through a great depth of the atmosphere.
3. A Generally Stable Atmosphere
No detailed stability analysis is available. Increasingly stable atmospheric conditions may be inferred from the cold air advection, especially at the surface and 925 mb.
4. Enough Moisture to Support Snow Formation
Scattered areas of light snow lingering behind the storm, especially the north-south streaks of snow showers in the low level wind flow, indicate enough moisture to support cloud and snow formation with the wave.
The amount of snow attributable to the standing wave is not certain. This author measured 2 inches of accumulation from the start of the enhanced snow to the time it ended at the measurement location. Light snow occurring independent of the wave added to the measured amount.
The reasons why accumulating snow fell so far inland was not addressed. The degree of upstream tilt, a typical characteristic of standing waves, could have been a factor. The purpose of this summary was mainly to document its occurrence. Remember that snow on radar that appears attached to terrain features is not guaranteed evidence of a wave event. Upslope winds can also enhance snow relative to higher terrain.
Surface and upper air maps compliments of the National Centers for Environmental Prediction (NCEP).
Radar images compliments of the National Weather Service.
Lake Superior map compliments of nationalatlas.gov now called nationalmap.gov.
Dan Miller, Science and Operations Officer, National Weather Service, Duluth, MN
Forecasting Staff, National Weather Service, Duluth, MN
The "Mountain Waves and Downslope Winds" Module on the MetEd website
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Smith, R. B., 1979: The influence of mountains on the atmosphere. B. Saltzman, Ed., Adv. Geophys., 21, 87-230.
[ Specifically note pages 88 and 89. ]
Kirkwood, P. D., D. M. Gaffin, and S. S. Parker, 2002: An Unexpectedly Heavy and Complex Snowfall Event across the Southern Appalachian Region. Wea. Forecasting, 18, 224-235.