Parts of Ireland will enjoy highs of 22C on Saturday on what will be the hottest day of 2020 so far.
But the weather chiefs are predicting that a "northerly airflow" will arrive on Saturday night, resulting in much cooler temperatures for the following days.
Despite the warm conditions being forecast, some areas are set to get thundery or heavy showers at times on Saturday.
The northwest and east will be slightly colder, while winds throughout the day will be mostly light.
A Met Eireann forecaster said: "Saturday will be mostly warm and sunny, although showers, some heavy and with a risk of thunder, will develop during the afternoon.
"Warm for most, with highest temperatures of 18C to 22C, but cooler in the northwest and along eastern coasts, with maximum values of 14 to 17 degrees.
"Winds mostly light, north or variable, increasing moderate in northern parts later in the day."
It will be as cold as 4C in northern areas at night, and temperatures will plunge even further on Sunday and Monday nights to as low as 0C.
The forecaster added: "Becoming much cooler and blustery in a northerly airflow Saturday night and Sunday.
"But dry and bright with a touch of frost possible Sunday and Monday night, remaining mostly dry and settled through the rest of the week with temperatures gradually improving."
Meanwhile, Friday will also be warm with daytime highs of 21C, with the northwest and coasts expected to be cooler with top temperatures of 17C.
Most areas are to get long spells of sunshine, while Ulster is set for the majority of any rainfall.
The forecaster said: "Today will be cloudy and misty at first, sunny spells will develop fairly widely by late morning.
"Whilst most areas will be dry, some isolated showers will develop, mainly across Ulster.
"It will be a very mild or warm day for most with highest temperatures of 17 to 21 degrees, cooler in the northwest and on coasts, with maximum values here of 14 to 17 degrees, in light variable breezes."
They added: "Tonight will be mostly dry, with clear spells. Isolated showers may persist for a time.
"In calm conditions, patches of mist and fog will develop. Minimum temperatures 6 to 10 degrees."
