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 National Radar Mosaic from SPC Mesoanalysis

7:30 AM CDT Oct 26 10:30 AM CDT Oct 26 1:30 PM CDT Oct 26 3:30 PM CDT Oct 26
7:30 PM CDT Oct 26 10:30 PM CDT Oct 26 1:30 AM CDT Oct 27 4:30 AM CDT Oct 27

 

Composite Surface Map: Radar, Temperature (32, 34, and 36F), and Sea Level Pressure

7 PM CDT 10 PM CDT

These two maps illustrate the rapid cooling of the surface temperatures in the early evening across a narrow area of northern Minnesota. This cooling allowed the precipitation to change over from rain to snow, and the snow rates were heavy enough to allow accumulation to begin with surface temperatures around 33 degrees.

 

250 millibar Constant Pressure Upper Air Maps (from Storm Prediction Center)

7 PM CDT Oct 25
7 AM CDT Oct 26
7 PM CDT Oct 26
 7 AM CDT Oct 27

Individual observations from sounding locations are plotted. Wind speeds are shaded. Streamlines are drawn in gray. The 250mb level is generally near the top of the troposphere, so this type of chart helps meteorologists analyze jet stream winds.

On the 7 PM map from October 25th, you may notice a very strong observed wind at the Salt Lake City, Utah sounding location. Here is that sounding (image from SPC). That evening's sounding recorded a 208 mph wind (181 knots) about 10,700 meters above ground level. The strong dynamics associated with this jet streak likely contributed to the rapid deepening of the surface low.

 

Data From the University of Minnesota-Duluth Buoy

Color shaded and interpolated graph of water temperature trends at UMD Buoy 10/25 to 10/29. Plot of individual depths' water temperatures at the UMD Buoy 10/25 to 10/29. Plot of wind speeds and gusts at the UMD Buoy 10/25 to 10/29.

Thanks to Jay Austin for permitting us to use these graphics on our story. Data is from this website.

The water temperatures are measured at twelve different depths from 1m below the surface of the water to 40m below the surface of the water. The strong offshore winds associated with the strong low that passed through the area on October 26th contributed to cooling through a great depth of the lake as upwelling occurred, and the warmer near-surface water became more mixed with the deeper, cooler water of the lake. The peak gust on the graph is in fact approximately 49 knots, taken at a height of 3m above the water surface. The peak sustained wind was 35 knots.

Data From the Duluth Water Level Observation Station

To the right is a graph taken from the water level observation site near the Duluth Harbor that shows some interesting trends during and after the height of the storm on October 26th. Here is what is plotted:

  • Top Panel:  the water level in comparison to the low water datum (LWD).
  • 2nd Panel:  a graph of wind speeds and directions. Arrows indicate direction and red dots indicate speed on y-axis.
  • 3rd Panel:  a graph of air pressure in millibars.
  • 4th Panel:  water temperature graphed in blue, and air temperature in red. Both are in Fahrenheit.
  • Bottom Panel:  relative humidity (%)

The first thing to notice is that the graph begins as the low is passing Duluth. After the low passed, southwest winds increased into the overnight from October 26th to October 27th. At the same time, the water level fell at Duluth by approximately 1.5 feet over the course of about 12 hours.

This is not an uncommon effect on large bodies of water when strong winds develop and persist from a certain direction for awhile. Essentially, the water gets "piled up" on the downwind end of the lake. After the winds decreased slightly, and shifted to the northwest on October 27th, the water level gradually started to rise. It was still generally below the low water datum and this was likely due to the strong northwest winds blowing the water more towards the Wisconsin shoreline of Lake Superior.

An oscillation is also evident starting in the evening on October 27th. The fluctuations in water levels would be due to the excess water that had "piled up" on the other end of the lake "sloshing" back towards Duluth. Of course, the water doesn't immediately return to the west end of the lake and instantly settle. The fluctuations will continue until the low amplitude wave gradually damps out.


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