September 2, 2010, Wake Low

September 2, 2010, Wake Low

Image to the right: sea level pressure observations reported at the Duluth International Airport ASOS observation (KDLH) the morning of September 2, 2010.

What Is A Wake Low?

Here is the definition provided by the American Meteorological Society:

In meteorology, a surface low pressure area or mesolow (or the envelope of several low pressure areas) to the rear of a squall line; most commonly found in squall lines with trailing stratiform precipitation regions, in which case the axis of the low is positioned near the back edge of the stratiform rain area.

Wake lows are a phenomenon that is not completely understood in meteorology at this point. It is basically a small-scale, localized area of low pressure that tends to form behind a decaying area of thunderstorms. Strong wake lows will not be observed with every round of storms, but occasionally the right conditions come together. As the pressure fluctuations become more extreme, the stronger the pressure gradient. Gradients in pressure are what drives the winds in the atmosphere.

Below is a regional radar image from the Storm Prediction Center, with 2-hour surface pressure changes overlaid in colored lines. Sea-level pressure is overlaid in thin solid black lines. You can see the area of strongest pressure falls (blue) behind the decaying thunderstorms in the Duluth area:

What Happened In The Duluth Area?

At the Duluth International Airport, a peak wind of 45 knots (51.8 mph) was recorded at 6:10 AM. This was accompanied by a surface pressure drop of 6.1 millibars in only 28 minutes between 5:38 AM and 6:06 AM. You can see the rapid fluctuation in pressure in the graph at the top of the page (right). Such rapid fluctuations of surface pressure are common in strong wake low cases. Across the rest of the area, there were also strong wind gusts. They are listed below:

Peak Wind	Time (CDT)	Location
57.5 mph	6:15 AM	Blatnik Bridge / Pier 20
51.8 mph	6:10 AM	Duluth International Airport (KDLH)
44.0 mph	6:14 AM	Sky Harbor Airport (KDYT)
44.0 mph	6:20 AM	Park Point
43.8 mph	6:18 AM	Duluth Water Level Observation
42.6 mph	8:02 AM	Devils Island CMAN
39.1 mph	6:55 AM	Two Harbors Airport (KTWM)
36.2 mph	6:02 AM	4.2 NW Downtown Duluth
35.7 mph	6:04 AM	Hermantown Middle School
35.5 mph	6:34 AM	2.2 NE Poplar
34.4 mph	7:40 AM	Silver Bay Marine Observation
34.0 mph	6:35 AM	I-35 at Garfield Avenue in Duluth
34.0 mph	6:37 AM	Weather Station in Two Harbors
32.2 mph	7:20 AM	Port Wing Marine Observation
31.1 mph	6:14 AM	Lake Superior Maritime Visitors Center
31.1 mph	6:36 AM	North Shore Community School
30.0 mph	7:13 AM	Silver Bay Airport