The Met Office has issued an amber warning, the second most serious kind, which covers half of Wales as well as most of England for Sunday.
In Breconshire and Radnorshire, the weather warning has very narrowly missed Ystradgynlais and Llanwrtyd Wells, however, high temperatures are still expected.
Rhayader falls directly on the border of the area covered by the warning on July 17.
At the moment, and with almost a week to go until the warning kicks in at midnight on Sunday, temperatures are predicted to reach a boiling 29 Degrees Celsius in Hay-on-Wye and Talgarth.
Areas like Brecon, Builth, Crickhowell and Llandrindod are all looking at reaching 27 Degrees Celsius.
All of the areas in the weather warning’s area, which lasts until 11.59pm on Sunday, are predicted to experience temperatures of at least 26 or 27 Degrees Celsius.
According to the Met Office, the head of extreme events and health protection at UKHSA Dr Agostinho Sousa said: “Heat-health alerts have now been issued to the majority of the country, with temperatures set to remain consistently high throughout the duration of this week.
“Most of us can enjoy the hot weather when it arrives, but it is important to keep yourself hydrated and to find shade where possible when UV rays are strongest, between 11am and 3pm.
“If you have vulnerable family, friends and neighbours, make sure they are aware of how they can keep themselves protected from the warm weather.’’
The extreme weather warning comes as schools are preparing to break-up for the summer holidays.
Hot weather is expected all week and follows days of warm weather of over 20 Degrees Celsius, high pollen and glorious sunshine.
With tourists and families expected to flok to attractions and beauty spots, the Met Office has issued advice on how to stay safe in the sun.
Accompanying the warning, it said: “Population-wide adverse health effects are likely to be experienced, not limited to those most vulnerable to extreme heat, leading to potential serious illness or danger to life. Government advice is that 999 services should be used in emergencies only; seek advice from 111 if you need non-emergency health advice.”
The Met Office has also advised that “substantial changes” may be needed to working practices and daily routines.
“Significantly more people are likely to visit coastal areas, lakes and rivers leading to increased risk of water safety incidents”, it said on its website.
The Met Office added: “Delays on roads and road closures are possible, along with delays and cancellations to rail and air travel, with potential for significant welfare issues for those who experience even moderate delays.
Here is the Met Office advice on how you can stay safe when the hot weather arrives:
- Look out for those who may struggle to keep themselves cool and hydrated. Older people, those with underlying conditions and those who live alone are particularly at risk.
- If you live alone, ask a relative or friend to phone to check that you are not having difficulties during periods of extreme heat.
- Stay cool indoors: Close curtains on rooms that face the sun to keep indoor spaces cooler and remember it may be cooler outdoors than indoors.
- If going outdoors, use cool spaces considerately.
- Drink plenty of fluids and avoid excess alcohol.
- Never leave anyone in a closed, parked vehicle, especially infants, young children or animals.
- Try to keep out of the sun between 11am to 3pm, when the UV rays are strongest.
- Walk in the shade, apply sunscreen and wear a wide-brimmed hat, if you have to go out in the heat.
- Avoid physical exertion in the hottest parts of the day.
- Make sure you take water with you, if you are travelling.
- Check the latest weather forecast and temperature warnings – you can find these on TV, radio, mobile app or website.
- During warm weather going for a swim can provide much-welcomed relief, take care and follow local safety advice, if you are going into open water to cool down.