Temperatures are expected to approach 30C (86F) on Monday as a heatwave reaches Northern Ireland.
People have been advised to stay out of the sun, despite there being no heat warning in place in the region.
In the Republic of Ireland, a yellow weather warning is in place and red and amber weather alerts for extreme heat have been issued across Great Britain.
The Met Office said Northern Ireland had its hottest day of the year so far on Sunday with 27.7C (81.9F) recorded in Armagh, but that is expected to be exceeded on Monday.
The Met Office said: “Parts of the west around Fermanagh, west Tyrone and south Armagh could be closer to 30C on Monday.”
The highest temperature ever recorded in Northern Ireland was 31.3C (88.3F) in Castlederg in County Tyrone last July.
Dr Brid Farrell, deputy director of public health at the PHA, urged people to stay out of the sun during the warmest hours.
She told the BBC Good Morning Ulster programme: “What we are recommending is that people try to stay in cool areas or else stay cool by drinking plenty of fluids.
“In particular, we would be keen that people avoid being outside between the hours of 11am and 3pm.
“Stay out of the sun, use sunscreen.
“Actually we are in a fortunate position that we are not going to reach the temperatures currently being seen in England, Wales and the south of Scotland.
“Severe temperatures can effect old people and young children more severely so everybody should look out for them.”
Dr Farrell added: “We are going to encounter more extremes of weather, both very hot and very cold in the next couple of decades.
“That is all due to climate change.
“It is something we have to get ready for and be prepared for and know what to do when the temperatures reach very high levels.
“I think for the next 48 hours we should be able to manage and health services will cope but for the future this is going to become a problem.”
Lifeboat and lifeguard services are expected to be busy with families heading to the beach to try to stay cool in the sunshine.
Carl Kennedy from the RNLI told the BBC: “We would expect today to be a pretty busy day for our lifeguards.
“We have got wall to wall sunshine and there will be huge numbers of families coming to be beach today.
“The lifeguard priority is on people in the water, so we will be keeping an eye on people and kids playing around the water.”
On Sunday RNLI lifeboat crew rescued five adults and four children off the Co Antrim coast after their boat hit a submerged object and started taking on water.
The group was taken to Ballycastle Harbour where they were helped by coastguards.
Health services are also expected to be stretched during the heatwave with the public urged to call for an ambulance only in an emergency.
The Northern Ireland Ambulance Service said it expected an increase in the number of less urgent calls it receives through the 999 system.
A spokesperson said: “To help us protect the most vulnerable, we would ask that the public only call if an emergency, but not to hesitate to do so if necessary.”
Animal welfare charity the USPCA has issued a warning to pet owners about the weather conditions.
Colleen Tinnelly, USPCA development manager, said: “It’s crucial that your pet has access to a shaded rest area and has a constant supply of clean and cool water – your pet’s water dish should be refilled regularly with cooler water or ice cubes during the day.
“You can also freeze some treats for your pet to enjoy – such as carrot or apple bites, chopped watermelon and banana, or peanut butter which can be placed inside toys and provides great enrichment for them.
“It can be very tempting to take our dogs out with us on walks or family days out however this can be very dangerous for them.
“One of the most important pieces of advice we can give members of the public is to never leave them in a car.”
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