Torrential rain, lightning, hail and strong winds may bring a risk of localised flooding to North East Lincolnshire on Saturday, the Met Office has warned.
A further alert has also been issued for heatwave conditions for Friday and the weekend.
At present, both of the alerts extend to a large proportion of the country with the areas most likely to be affected remaining uncertain.
The chief forecaster for the Met Office said: "Following a build of very warm and humid air from mid-week onwards there is the potential for the development of large thunderstorms over the UK from Friday night.
"In this situation large amounts of energy are available in the atmosphere and this coupled with high moisture content can lead to torrential downpours along with frequent lightning activity, large hail and locally strong gusts.
"In this situation many elements need to come together at the same time to form these thunderstorms and so there remains large uncertainty in their development. This alert is therefore likely to be updated in the coming days."
In response to the heatwave warning, the North East Lincolnshire Clinical Commissioning Group (NELCCG) has issued advice to residents on how to cope with the hot weather.
Dr Peter Melton, local GP and Chief Clinical Officer of NELCCG said: "We want people to enjoy the sunshine but for those who are vulnerable we advise that they avoid going out between 11am and 3pm, the hottest times of the day, and to drink plenty of fluids. Children and babies in particular need to stay hydrated.
"If you are with someone who does feel unwell, get them somewhere cool to rest and give them plenty of fluids to drink. If they start to display symptoms such as breathlessness, chest pain, confusion, weakness, dizziness or cramps don't leave them alone, but seek medical help.
"Having a cold alcoholic beverage can be a favourite pastime in summer in Britain however we advise people to avoid drinking excessive amounts of alcohol in the hot weather as this can lead to higher, more dangerous levels of dehydration. Drinking plenty of water is the best way to rehydrate." Five day weather forecast for Grimsby
