The Status Yellow high temperature warning has also been extended to Saturday.
Severe thunderstorms could bring heavy downpours, localised flash flooding and large hailstones over the coming hours.
Met Éireann is issuing a status yellow warning for thunderstorms for six counties across the Midlands and the North West.
Roscommon, Galway, Sligo, Mayo, Leitrim and Donegal fall under the warning from 3pm.
However the national emergency co-ordination group says storms could develop anywhere in Ireland.
Its also advising people to stay vigilant for heat stress, with record high temperatures possible in parts today.
Alan O’Reilly from Carlow Weather says the storms are expected to hit tonight and tomorrow:
Authorities are urging the public to be aware of two weather related risks over the next 48 hours - high temperatures and heavy thunderstorms.
The National Directorate for Fire and Emergency Management met today to prepare for the weather conditions.
The yellow alert for high temperatures has been extended until Saturday morning.
A status Orange warning for Cork, Kerry and Waterford for thunderstorms is due to end about now, while Connacht and Donegal are under a yellow thunderstorm warning until tomorrow morning.
Cathal Nolan from Ireland's Weather Channel says those thunderstorms could be potent :
It's set to be the hottest day of the year - and potentially the hottest day ever in Ireland.
Met Eireann is predicting temperatures of around 27 degrees today, but there is a chance record high of 33.3 degrees registered at Kilkenny Castle all the way back in 1887.
People are urged to stay hydrated, remain out of the sun and be careful if swimming in lakes, rivers or at the beach.
Met Eireann is also warning of an increased risk of forest fires.
Alan O'Reilly from Carlow Weather says in addition to the heat, we're also in for some very severe thunderstorms:
The status yellow high temperature warning for the Midlands has been extended until Saturday.
The elderly, children and people with health issues such as diabetes are at particular risk of dehydration and heat stroke.
Keith Leonard Chairperson of National Emergency Coordination Group says people should check on the elderly today:
Today temperatures could rise to between 25 and 31 degrees in parts of the country, with the highest temperatures in the midlands.
Holly O’Neill, Meteorologist with Met Eireann says people need a break from this unusually hot week:
Given the continuing spell of warm weather and the beginning of the summer holidays for schoolchildren, ESB would like to remind the public of the dangers and potentially serious consequences of swimming and other recreational activities in any ESB reservoirs and watercourses.
Henry Bouchier, Manager, Hydro Operations said: “It is important that people take note of the safety warnings which are visible on signs posted around ESB reservoirs. These areas are unsafe for swimming due to deep, fast-flowing waters, fluctuating water levels, and uneven terrain, which present significant hazards to the public."
To ensure your safety and the safety of others, always choose safer swimming locations such as swimming pools or beaches with designated lifeguards.
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