Farmers across many parts of the UK are bracing themselves for thunderstorms and outbursts of heavy rain after weeks of drought and high temperatures.

The sudden change in the weather, expected to affect eastern areas hardest but spreading to the north and Midlands over Friday, is likely to cause problems of flooding and potential crop damage.

While farmers with parched fields and wilting crops have been hoping for rain, land that has been baked dry and hard in the two-month heatwave will be unable to absorb the moisture. Rain running off the land as a result could cause localised flooding and, especially if accompanied by hail, could further damage crops that have suffered in the heat.

Robert Martin, who keeps cattle near Carlisle, said: “Any rain I get over the coming days will be well overdue – my fields are now brown, having been green eight weeks ago. It takes continuous rain over a prolonged period for it to have any benefit, as the ground is short of about 4-5 inches of rain. It takes about two weeks for grass to go from brown to green, then a further period for the grass to grow enough leaf to be grazed by animals.”

He said the weather was causing headaches and expense for livestock farmers: “All my cattle are on full winter ration to try to hold milk volumes, as grass growth has stopped due to the heat. This adds extra expense to my cost of production for the milk my business produces.”

Rather than bringing instant relief, the abrupt change may add to farmers’ woes, with many looking anxiously to the long-term forecast for the rest of the year. George Dunn, chief executive of the Tenant Farmers Association, said: “With much of the ground hard-baked there is doubt about the extent to which any rain will be able to penetrate the ground sufficiently, particularly if it comes in large quantities over a short period of time.”

He said the problems created by the drought would continue into the autumn.

“Most livestock farmers I am talking to have already eaten into what they should have been leaving to feed their livestock in the winter,” he said. “We need a benign autumn with some steady precipitation and warm temperatures if we are to avert a winter crisis. Inevitably, crop yields and quality are going to be well below average, with potatoes, vegetables and horticultural crops in particular difficulty.”

The Met Office forecast that temperatures on Friday were likely to break the UK July record and possibly the all-time UK record. Highs of 36C were expected on Thursday and 37C on Friday.

Steve Ramsdale, chief meteorologist at the Met Office, said: “The heatwave conditions are expected to peak across many places from the English Midlands eastwards on Thursday and Friday, with temperatures into the mid- to high 30s celsius. It’s likely that we will see a new temperature record set for July and possible that we could break the UK record of 38.5C if conditions all come together.”

Eastern parts of England, where much of the country’s grain is grown, are facing intense thunderstorms from Friday afternoon, before the fresher conditions make their way east across the UK for the weekend. Rain is forecast to be heaviest over eastern and northern England, where there is a Met Office weather warning in force.

Farming leaders warned people in rural areas to prepare. The Country Land and Business Association (CLA), which represents landowners, farmers and rural businesses, said its members were concerned by the extremes in current weather conditions. “The ground is so hard right now because of the prolonged dry weather that the predicted sudden heavy rain just won’t sink into the earth enough to make a difference to parched crops,” a spokesperson said.

“There is a high risk of localised flooding because the rainwater will not be absorbed by the dry soil. Our advice to farmers is to have a contingency plan in case of flooding, be prepared to move livestock if necessary, and check insurance documents.”

Farmers have also been worried by the risk of wildfire, caused by carelessness or deliberately, which can spread rapidly in the parched conditions.

Farmers and rural dwellers have been advised to consult the Environment Agency’s flood warning website. The agency has warned that intense rainfall on dry or compacted ground runs off quickly, making river levels rise rapidly. This means it is possible to have floods even during prolonged dry periods.

Moving livestock away from areas that might flash-flood is a priority, as is checking that farmyard drainage channels are clear and able to function, while ensuring slurry and silage reception pits have the capacity to take sudden rainfall.

Pollution could also be a problem, as rapid runoff from intense rainfall on to compact soils can also cause pollutants to be washed into water courses. Swimmers enjoying the hot weather were also cautioned that bathing water quality can be reduced temporarily after heavy rain, so they should check for signs at the swimming site or online.

The Environment Agency is meeting groups such as farmers to advise on conserving water and planning for the effects of prolonged dry weather, and its teams are monitoring local conditions and making necessary preparations.

Any abrupt change in weather could disrupt the harvest in affected areas, as some crops such as grains that would normally be harvested in August are being brought in early, as they have ripened rapidly in the prevailing sunshine.

Lesley Chandler, who farms 200 acres in Oxfordshire, expressed concern over the possibility of hail. “That could knock the heads off corn and damage maize crop leaves and any root crops and brassicas with large leaves,” she said. “The soil will love it, provided it is not too torrential, when it will lead to flooding with soil in the water.”

Urban areas will not escape the damage, as surface water flooding is possible in areas affected by sudden heavy rains. Prolonged dry and hot spells can compact sewage in underground pipes, and sudden downpours can flood the pipe network, leading to outflow to the surface.