The Middlesex-London Health Unit says its first heat warning of the year will come into effect on Saturday and is expected to last until Monday, though the general heat and humidity are expected to continue well into the workweek.
The health unit issues heat warnings when either the daily high reaches 31 C or more and low of 20 C or higher for two consecutive days, or when the humidex value reaches 40 C or higher for two consecutive days.
The weekend forecast just reaches that threshold, with Environment Canada calling for highs of 31 C both Saturday and Sunday.
“The high UV index that we’re expecting to accompany these hot and sunny conditions will bring challenges of its own that people need to be aware of,” public health inspector Randy Walker said in a statement.
“It will not only be key to stay hydrated and take breaks from the sun, it will also be very important to cover exposed skin with light, loose-fitting clothes, and to use sunblock with an SPF of at least 30 to protect exposed skin.”
Additional measures to avoid heat-related illness can be found on the health unit’s website.
This is the first heat warning from the MLHU this season, but the health unit has already issued two heat alerts: on the Friday before the May long weekend and the Tuesday following.
While it’s not expected to be hot enough to sustain a heat warning beyond the weekend, it’ll still be plenty hot and humid throughout the week.
The forecast is calling for highs of 29 C Monday through Wednesday, and 28 C on Thursday.
Anthony Farnell, chief meteorologist for Global News, says it wouldn’t surprise him if the London region set four new high temperature records by the middle of next week.
He says the heat is coming from western Canada and the western United States, but it “does moderate some as it encounters the still chilly waters of all the Great Lakes.”
“But of course, when you have an area surrounded by water like us here in southern Ontario, you also add some humidity because of that,” he added.
Farnell says he anticipates thunderstorms over the weekend in the Muskoka and Kawartha Lakes area, but he believes “the ridge is just too strong” for storms to develop in London.
“What that means is there’s just so much sinking air that you don’t get anything more than a few clouds. It basically creates a cap on the atmosphere because it’s so hot,” he explained.
“So until that cap breaks — which may happen as early as Tuesday or Wednesday next week — I think we’re in the clear as far as big storms go. But oftentimes these heat waves end with a bang, and that’s something that we’ll definitely be watching for later next week.”
Farnell is expecting a “hot month overall,” to cap off a drier than usual spring.
“Even with the recent rainfall, it’s definitely been a dry spring… the fact that there isn’t that much moisture in the ground, it means that when the sun shines and you get these big ridges of high pressure overhead, it’s a lot easier for the temperature to climb.”
Brandon Williamson with the Upper Thames River Conservation Authority says water levels are down throughout the watershed and the growth of vegetation will be slowed.
“Usually you don’t see this type of drying conditions until sort of July, August. A lot of times the vegetation growth will slow down in that period of time just because there’s not as much water for them to uptake and nutrients as well.”
He says “it could stunt their growth a little bit for a lot of vegetation” but “most native species should fare OK through this.”
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