November Witches bring snow to North America

The Gordon Lightfoot song The Wreck of the Edmund Fitzgerald commemorates the sinking of the eponymous bulk carrier, which was lost in high winds and seas on Lake Superior on November 10th, 40 years ago, as a rapidly deepening low crossed the Midwest and the Great Lakes.

Great Lakes, USA. Image by NASA / CC BY 2.0

As the lyrics of the song mention, such storms are popularly called November Witches and they are a reasonably regular feature of mid to late autumn in North America.

They have their source in Colorado Lows – depressions re-developing in the lee of the Rockies.

Pacific lows move east or southeast across the western USA then weaken as they encounter the disruption of the Rockies.

Lee troughing provides an environment in which re-cyclogenesis is possible over Colorado or NE New Mexico, especially with sub-tropical high pressure (Bermuda High) pushing warm, moist Gulf air northwards to the plains and Midwest.

If the jet stream is a little more southerly, with a sharp trough southwards across the West/Rockies, then deepening occurs in the vicinity of the Texas/Oklahoma Panhandle, and the storm can be called a Panhandle Hooker – because its track is from the Pacific NW SE’wards to the Rockies before it “hooks” leftwards.

Both types move NE’wards towards the Great Lakes and can be a headache for forecasters, bringing risks of supercells/severe storms/tornadoes, widespread strong winds, heavy rain and heavy snow.

The hooker type can in winter bring snow or ice storms as far south as N Texas.

There is such an event occurring at the moment and it has already brought snow to the Rockies in Wyoming and Colorado, and blizzards are going to develop across the High Plains on its north-western / western flank.

Given that there is not a strong source of frigid air to tap into (Canada is currently unusually mild), the snow threat is low for the Midwest and Northeast but there are certainly severe storm and flood risks.

Even without severe storms there should be gusts of 50-60mph very widely.

The relative warmth of the Great Lakes can augment the strength of these depressions, as can a convergence of a Colorado Low/Panhandle Hooker with an Alberta Clipper running ESE’wards – such as in the Great Lakes Storm of November 7-10, 1913 (aka the White Hurricane), which was the most destructive natural disaster ever on the Lakes: 19 ships destroyed and more than 250 people killed, due to 90mph gusts and 11 metre waves.

Another example: the Armistice Day storm of 1940, a much colder, snowier, and also widely fatal event, with temperatures in Chicago dropping from 17C to -3C in seven hours. Minnesota experienced snow drifts 6-7 metres deep and 154 people were killed, 66 of them in ship sinkings on Lake Michigan.

Most recently is what was dubbed the Octobomb of October 25-27, 2010, which became the deepest low ever recorded over Minnesota, with 955.2mb measured at Big Fork.

(Press Association)