Potentially historic derecho in Ontario and Quebec this past Saturday

While some severe weather was expected this past Saturday, few expected the potentially historic derecho that unfolded. It started in Indiana and Michigan in the morning and then quickly tracked through the Windsor-Quebec City corridor, Canada's most populated area with almost half of the country's population, for the rest of the day. It caused at least 10 deaths and 925,000 customers were without power yesterday evening. Interestingly though, it largely spared Montreal (which seems to be a relative convective hole in general) as it passed just to the north. Farther south, a few more scattered severe storms brought strong winds and borderline large hail in a few areas in far northern New England, but those paled in comparison to the derecho. Those storms eventually merged with the derecho over the Eastern Townships of Quebec in the evening. The Northern Tornadoes Project is still conducting storm damage surveys.

A few selected METARs:

Detroit, MI: KDTW 211353Z 20027G41KT 3SM +TSRA SQ OVC035CB 23/18 A2994 RMK AO2 LTG DSNT SW-N RAB49 TSB52 SLP133 OCNL LTGIC ALQDS AND OHD TS ALQDS AND OHD MOV NE T02330183 PNO $

Kitchener, ON: CYKF 211623Z AUTO 28032G71KT 220V290 3/4SM +TSRA BR SQ SCT022 BKN036 BKN046 OVC086 18/17 A3000 RMK LTNG DIST ALQDS PRESRR SLP161 DENSITY ALT 1500FT

Ottawa, ON: CYOW 211952Z 28038G65KT 1/4SM R07/P6000FT/U R32/4000VP6000FT/D +TSRA OVC050CB 20/17 A2992 RMK RA5CB3 NOTE WIND SPEED PRESRR SLP135 DENSITY ALT 1100FT

Quebec, QC: CYQB 212305Z 25036G54KT 210V290 3/4SM R06/1800V2200FT/N +TSRA OVC013CB 21/19 A2985 RMK CB8 PRESRR SLP110 DENSITY ALT 1200FT

It appears to be a serial derecho whose winds were greatly aided by strong winds aloft. Derechos are not so uncommon in southern Ontario, but are quite rare in Quebec. Quebec probably has not had a derecho since July 4-5, 1999 (which was one of the farthest north progressive derechos recorded).

Source: SPC

With strong instability meeting up with a quasi-stationary front and WSW flow at mid-levels parallel to the front, the environment was at least somewhat conducive for derechos, based on Johns and Hirt (1987). A lack of height falls or a true mid-level dry intrusion as well as the mid-level flow being slightly more backed than with most derechos were possible negative factors though. 

 

Source: SPC

 

Derechos are almost never easy to predict, especially given their rarity in this area, and this one was no exception. Even the 12z HRRR and NAM 3-km (convectively-allowing models) from that morning failed to show anything severe:

Source: weather.us

Reality is shown below:

Source: weather.us