Rare Severe Thunderstorm Warnings issued in November

Rare Severe Thunderstorm Warnings were issued by the NWS Denver/Boulder forecast office late on Friday November 2. The first warning went out at 10:35 pm for Southeast Boulder, Northeast Jefferson, and Broomfield Counties, most of Denver, and western Adams and Arapahoe Counties. The second warning was issued for Southeast Larimer and western Weld Counties. Wind speeds of 60 to 70 mph were reported, and a highway sign was blown down along I-25 near Timnath. The storms formed in weak instability and lift associated with low pressure moving through the region, and the convection helped mix strong jet stream winds down to the surface.

Severe Thunderstorm Warnings issued November 2 2018

These were the first Severe Thunderstorm Warnings issued by the NWS Denver/Boulder office going back to 1986, when comprehensive reports began. While rare, Colorado severe thunderstorms in November are not without precedent:

In 1922, a tornado on November 4 killed 4 and injured 25, when it hit a farmhouse in Lincoln County 20 miles north of Sugar City at 5 am,
On the same day another tornado struck Pleasant Valley School eleven miles east-southeast of Holyoke at 9:30 am, killing a teacher in her "teacherage" home,
On November 17 1999 a trained spotter reported a thunderstorm wind gust of 60 mph two miles west of Fruita in Mesa County,
On November 11 2005 a trained spotter reported a thunderstorm wind gust of 65 mph one mile west of Arboles in Archuleta County.