More torrential rain is forecast for the weekend and on Monday to follow Friday's deluge across the county.
After a good few weeks enjoying the sunshine in parks and beer gardens, public health experts fear many will overlook the Covid-19 restrictions relating to social distancing indoors.
Northamptonshire's Public Health director, Lucy Wightman, admitted: "Covid-19 spreads much more easily indoors and just because you can, doesn’t mean you should. It is much safer to meet people outdoors if you can.
“With a change in the weather and the European Championship on TV, the temptation to meet indoors will be even greater than before.
"Coronavirus is circulating across the county and numbers are rising once again. We can’t ignore this and we must act to prevent further spread.
“Please remember that If you must mix indoors open doors and windows to let the fresh air in and stick to the Step Three rules by only meeting up indoors with friends and family you do not live with in a group of up to six or up to two households."
Numbers of new positive Covid-19 tests reported in the county have more than doubled to over 300 a week in the space of a month.
Five Covid-19 patients were being treated in Northamptonshire's two acute hospitals on June 15 compared to zero the week before.
And Kettering General Hospital recorded the county's first Covid-related death in 52 days on Tuesday.
Nearly three-quarters of all adults have now received at least a first dose of Covid-19 vaccine and more than half are fully vaccinated with both jabs.
Mrs Wightman added: "Both doses of the Covid-19 vaccine are important for the best possible protection against the virus and variants of concern — in particular the Delta variant, first identified in India.
"Getting vaccinated is the single most important thing we can do to protect ourselves, our loved ones and our communities against coronavirus, so it’s vital that everyone who is eligible should come forward for both vaccines as soon as they are invited.
"Vaccinating is something people have a choice about but this is our way out of the current restrictions.
"We recognise that people have concerns but local teams and GPs can talk you through those concerns and the risk is minimal compared to that of actually catching covid.
"We're seeing more and more younger people hospitalised who haven't had the vaccination and that's a real concern."
■ Anyone with Covid-19 symptoms — a new continuous cough, loss of or change in sense of taste or a high temperature should self-isolate immediately and book a test online HERE, via the Test and Trace app or by calling 119
■ All those aged 18 or over are eligible for a Covid-19 vaccination. Click HERE to book or contact your GP