For the first time in the 2018 season, a risk of widespread thunderstorms across Ontario could occur on Friday, May 4th, 2018 – some potentially severe.

Synopsis
During Friday morning, many regions across Southwestern, Southern and Central Ontario will experience widespread moderate rain: an isolated non-severe thunderstorm cannot be ruled out. As the morning and day progresses, conditions will become drier, sunny at times, more humid and potentially lead to some severe thunderstorms. The severe thunderstorms that do occur will bring mainly a wind threat.

A somewhat secondary line of isolated severe thunderstorms could also sweep into eastern sections of the Niagara region as a linear set-up of storms will form near upstate Pennsylvania and western New York state; where this line of storms exactly forms will be incredibly relevant for this region. Should it form quickly and near the CAN/USA border (as current atmospheric parameters suggest), the region will be impacted; a formation more eastwards will reduce the risk and impact substantially, to virtually nil.

Potential Severe-Threat Timing
Afternoon to early evening from west to east.

Scientific Analysis
The greatest risk for isolated severe thunderstorms can be depicted in the ‘darker orange region’.
A slight risk for isolated severe thunderstorms can be depicted in the ‘lighter orange region’.
Mainly non-severe thunderstorms can be depicted in the ‘yellow region’. One or two severe thunderstorms cannot be ruled out in this region, particularly along HWY 402, HWY 403 near Brantford and HWY 401 near the London-Woodstock corridor. Later on Friday, it is possible along eastern sections (ex. Kingston to Brockville and 50-60km east of Ottawa), an isolated severe thunderstorm is also possible.

Sufficient atmospheric energy at the immediate surface, as well as, 100mb and 300mb above the surface (SB, ML and MU CAPE) could be adequate to lead to severe thunderstorms. While energy is not astronomically high, and lower in comparison to the more traditional summer months, relatively strong dynamic forces should act as a sufficient trigger for isolated severe thunderstorms. Convective inhibition (CIN) at the surface as well as 100mb above the surface (ML-CIN), should reduce the risk of severe thunderstorms, and thunderstorms all together along the immediate northern shoreline of Lake Ontario: this will lead to cells rapidly weakening as they form near or just east of Lake Huron and usher eastwards. A relatively moderate lifted index (LI) will aid in dynamic instability leading to the greatest threat of isolated severe thunderstorms particularly near or just inland of Lake Huron and SE Georgian Bay. Storm relative helicity (SR) remains weak at the 0-1km threshold, and slightly stronger at the 3km threshold; this will allow for an increased threat of isolated severe thunderstorms in more of a linear format (although not encompassing a large area). Cyclonic updraft at the surface is virtually non-existent at the 0-1km threshold thereby limiting any tornado touchdown to virtually nil.

Other parameters including: Bulk Richardson Number, Cross-Totals, Total-Totals, Thompson Index and shear at the high atmosphere (6km) support isolated severe thunderstorms; although not super-cells; with a risk for strong wind gusts mainly to 90-100km/h as being the primary threat.

Non-conducive lapse-rates and wet-bulb zero (WBZ) coefficients will mitigate the hail risk overall to pea-size to nil.

Precipitable water (PWAT – surface to 300mb) is somewhat low for ‘torrential downpours’, therefore, residents should be spared a ‘flooding risk’ as typical with some severe thunderstorms.

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Ultimately, the deepening of the approaching low-pressure system will be well-timed with strong uni-directional winds aloft, 500mb height-falls, and a jet-streak. Further, a sharp cold front is expected to eject through Southern Ontario between ~2-6pm thereby providing a trigger for these thunderstorms.

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Across much of Southern Ontario, residents can expect winds gusting to 80km/h for many regions and locally up to 100km/h, irrespective if regions see thunderstorms or not.

Tie down loose patio furniture, patio umbrellas and chairs.

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Remember, summer thunderstorm forecasts  – severe and/or non-severe – represent a plausible scenario based on extensive weather calculations, parameters and up-to-date atmospheric data. Nothing is for certain, but outlines a possible or likely scenario based on the aforementioned points.

Please stay tuned, as we at TCW are monitoring this situation attentively.

Drive safe and always be alert during hazardous conditions.

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