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Informal Case Studies
A Rare Moderate to Heavy Snow-Producing Standing Atmospheric Wave Event in Duluth MN, January 25, 2010
* Geographic and topographic maps created from the USGS/ESRI ArcGIS mapping software and database
Introduction
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. Personal measurements near the ridge crest in central Duluth showed that 2.0 inches of snow accumulated during the wave event on the evening of January 25, 2010 from approximately 9 PM to 12 AM CST (03 to 06 UTC January 26) . The wave occurred at the tail end of a major snowstorm that affected Duluth from January 23 through January 25. Some lingering light snow from the storm itself was still falling so the wave did not produce quite all of the two inches that fell. A Standing wave triggered by the topography of the Lake Superior north shore is a common event. A Standing wave producing several inches of snow in Duluth is not. Snow from these waves usually reaches the ground south and east of Duluth. When snow does fall in Duluth it is usually very light and near the immediate lakeshore, at least since the year 2001 based on personal observation.
Radar evidence of the wave
The following radar loop from the National Weather Service Forecast Office at Duluth, MN 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 builds back to the northwest while other snow elements on the radar move from north-northwest to south-southeast.
8:13 PM CST to 9:12 PM CST, January 25, 2010
The next 9 radar images show the snow band remaining stationary through 0544 UTC (11:44 PM CST). The band then shifts rapidly southeast and dissipates.
9:31 PM CST, January 25, 2010
10:07 PM CST, January 25, 2010
10:46 PM CST, January 25, 2010
11:15 PM CST, January 25, 2010
11:44 PM PM CST, January 25, 2010
12:25 AM CST, January 26, 2010
1:00 AM CST, January 26, 2010
1:58 AM CST, January 26, 2010
2:47 AM CST, January 26, 2010
Additional analysis
The wave developed in a northwest to north wind flow cold air advection environment behind the departing low pressure system which produced the snowstorm. The surface map and 500 mb analysis for 0000 UTC January 26, 2010 (6 PM CST January 25, 2010) show the locations of the surface and upper air components of the storm system shortly before the standing wave snow event developed in Duluth.
Surface analysis - 6:00 PM CST, January 25, 2010
National Weather Service Weather Prediction Center
500 mb analysis - 6:00 PM CST, January 25, 2010
National Weather Service Storm Prediction Center
Conditions favorable for a standing wave to form
1. An elongated hill with a rounded top rather than a sharp narrow top and rises gradually on the upwind side then drops quickly on the downwind side
The topographic map shows that the ridge running through Duluth has the ideal shape to trigger a standing wave. The darker shade along the lake-shore and the rim of the Saint Louis River Valley denotes a more abrupt change in elevation.
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 charts, show winds 10 to 15 knots from the northwest at the 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.
925 mb analysis - 6 PM CST, January 25, 2010
National Weather Service Storm Prediction Center
850 mb analysis - 6 PM CST, January 25, 2010
National Weather Service Storm Prediction Center
700 mb analysis - 6 PM CST, January 25, 2010
National Weather Service Storm Prediction Center
3. A Generally stable atmosphere
Some stability in the atmosphere extending at least above the height of the hill is important. In a stable atmosphere, buoyancy forces will try to restore any displacement of the air by the hill especially when the air sinks abruptly on the downwind side. The restoration process sets off a vertical oscillation of the air. The oscillation is represented as the wave. No detailed stability analysis here but some degree of stability near the surface is likely given the strong cold advection occurring at the surface. The radar loop does show some north to south oriented streaks of snow showers falling from stratocumulus clouds with bases above the hill so some instability is indicated in a layer above the hill.
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 north to northwest wind flow, indicate enough moisture to support cloud and snow formation with the wave.
Concluding remarks
The magnitude of the standing wave snow event in Duluth on the evening of January 25, 2010 was extremely rare. When a standing wave does produce snow in Duluth, the snow is usually very light and does not extend as far inland as in this event. An interesting characteristic of standing waves is that they tend to tilt upstream with height. The wave may have been tilted more than what is typical which could increase the odds of snow falling in Duluth but that is just a consideration. The main purpose of this summary was to document the occurrence of the event. No other standing wave event from Winter 2001-2002 to Winter 2017-2018 has produced as much snow as far inland as this event.
References
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
Additional reading
Brady, R. H. and J. S. Waldstreicher, 2001: Observations of Mountain Wave-Induced Precipitation Shadows over Northeast Pennsylvania, Wea. Forecasting, 16, 281-300.
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.