Risks of swimming in such conditions include cold water shock and unseen dangers such as strong currents or objects under the water that a swimmer could become tangled in, he added.
Anyone who sees a person in distress in the water is asked to call 999 immediately, and, if possible, use the What3words app to describe the exact location.
They are also advised to tell the person in the water to float on their back and if possible throw a buoyancy aid instead of entering the water themselves.
Temperatures of more than 33C (91F) are forecast across the county for much of the week, and have prompted closures and cancellations.
The Chester House Estate said it would close to visitors from Wednesday to Friday in light of the “exceptionally high temperatures”.
Wednesday’s Corby 5, part of the East Midlands Grand Prix, external of running events, has also been cancelled.
Organisers said they hoped to find a new date for the annual run, which started and finished at East Carlton Park.
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