Wednesday is set to be the hottest day ever recorded in Wales, according to weather forecasters.
BBC Wales' weather presenter Derek Brockway said temperatures in some places could reach 36C (97F).
Wales' chief medical officer, Tony Jewell, issued guidance for parents to keep their children safe in the sun to avoid heat exhaustion or heatstroke.
A few hospitals reported slightly higher admissions than usual but most said numbers were at usual July levels.
Speaking on Tuesday, BBC Wales' Derek Brockway said: "Tomorrow could be the hottest day on record, with some places reaching 36C (97F).
"Thunderstorms are possible tomorrow night in some places.
"Thursday will be cooler but it will be very humid. Things could be hotting up again on Sunday."
Dr Jewell's 10-point guide includes keeping children inside during the hottest part of the day, dressing them in loose, cotton clothing, putting high-factor sunscreen on them and giving them plenty of non-fizzy, non-sugary drinks.
If children develop signs of heat exhaustion - including headaches, dizziness, nausea, cramps and high temperatures - parents should get medical advice immediately.
Radyr Comprehensive School in Cardiff has scrapped its uniform requirement for the day and told pupils - and staff - they can wear shorts and T-shirts or cool clothes instead in light of the heatwave.
St John ambulance has advised people to carry a "first aid" pack for the sun with them, including water, a hat, sunscreen, and snacks like bananas or sweets with them to prevent nausea.
Youngsters cool off in the River Usk
Dog owners were reminded of the dangers of leaving their pets out in the sun. The RSPCA said it had received 700 calls about overheated dogs across the UK, including two cases where the animals had died.
Mid and Wales Fire Service urged people out and about in the countryside to be careful after spending days tackling fires in the Port Talbot area.
Morriston station officer Craig Thomas said: "The exceptionally hot and dry weather makes our countryside vulnerable to grassland and woodland fires. I implore the public to be careful with cigarette ends, matches and barbecues.
"We have experienced a rise in grass and forestry fires in the past week. Fighting these fires is an arduous exhausting process. Our fire kit is heavy and you get warm very quickly when wearing it.
"We have told our crews that they should drink at least two litres of water each day, and also given guidance on work routines to reduce the risk from the heat."