What are "November Witches"?

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  • This image shows the sea level pressure across the USA at midnight GMT on Friday 13th November 2015. A low pressure system is expected to track north-east across NE USA and into Canada through the end of this week.

    This image shows the forecast snowfall amounts (in cm) between midnight on Thursday 12th November and midnight Tuesday 17th November 2015 (times in GMT). Snow is expected to fall across the high ground in the west of the USA.

    "November Witches" can bring heavy snow to parts of the UK, and a low pressure system at present is currently bringing snow to the Rockies in Wyoming and Colorado, and blizzards are expected to develop across the High Plains on its north-western/western flank.

    "Colorado Lows" and "Panhandle Hookers" bring the risk of supercells/severe storms/tornadoes, widespread strong winds, heavy rain and heavy snow. The "Hooker" type storms can in winter bring snow or ice storms as far south as N Texas.

  • What are "November Witches"?
    12.11.2015 16:06


    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.  


    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.   

    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. This is from the NWS La Crosse (Wisconsin) report at the time:

    "The greatest losses were on Lake Michigan which felt the full fury of the southwest gales.  Three steamers were sunk, a number of others were grounded, and several smaller boats were lost.  Possibly because early in the day the wind was from the southeast and increasing, some of the captains navigated their vessels near the east shore of Lake Michigan; the sudden shift to southwest gales later proved disastrous, as the steamers were practically helpless because they could not run before the storm nor withstand the battering which would result from heading into gale and high waves.  

    The three freighters that foundered all sank off Pentwater, near Ludington, Michigan, with loss of life as follows:  William B. Davock, 33; Anna C. Minch, 24; and Novadoc, 2.

    Other drownings occurred when the fishing tugs Indian and Richard H. and the motor cruiser Nancy Jane, with a total of 10 persons aboard, were lost on the southern end of Lake Michigan.

    Ships reported as driven ashore or on reefs, in addition to many smaller boats, were:  Sinaloa at Escanaba; City of Flint at Ludington; Conneaut on the north shore of Straits of Mackinac; Frank J. Peterson on St. Helena Island (reported as abandoned on November 21).

    […]

    The effect of the sustained southwest gale on the water level of Lake Michigan is indicated by reports of a drop of 4.8 feet at Chicago, a rise of 4 to 4.5 feet at Beaver Island.  A lowering of the Fox River by about 5 feet, the result of south and southwest winds, forced paper mills and a power plant to suspend operations at Green Bay, Wisconsin."

    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.

    By: Stephen Davenport & Gemma Plumb