She said the party's policy of spending £5 billion on flood defences was necessary, particularly as the climate emergency would make such events more common.
In an update on its flooding response issued yesterday, the agency said more heavy rain is expected on Thursday and Friday.
Across the country, 34 Flood Warnings - meaning flooding is expected - and 75 Flood Alerts - meaning flooding is possible - remain live.
The Met Office forecasts a "fair amount" of rain across England and Wales this week.
Tonight, outbreaks of rain will spread across the north and west of England and Wales.
Conservative leader Johnson, campaigning to remain in Downing Street after next month's general election, made the decision after chairing a meeting of the government's emergency response committee.
Today, the Met Office forecast "rain at times, but not as heavy as recent days", before "further rain" on Saturday in England and southern Scotland.
Further showers over the next 48 hours could bring "severe" flooding to areas where water levels are already at an all-time high, according to the Environment Agency (EA).
Snow has fallen in some parts of the United Kingdom, causing trouble on the roads.
The EA says it is working with emergency responders and local authorities to guard folks and properties in Yorkshire, Derbyshire, Nottinghamshire and Lincolnshire.
Nearly 50mm of rain fell near Tal-y-Maes, Wales, in the 24 hours to Thursday evening, and the 49.6mm recorded in Brecknockshire was nearly matched by Ross-on-Wye in Herefordshire, which had 48.8mm over the same period.
Pumps have also been deployed across five of the worst-hit parts of the United Kingdom to help remove flood water.
Almost 100 soldiers have arrived in South Yorkshire to aid communities which have been cut off by flooding.
More than 800 properties have been affected by the floods, with people evacuated from homes in Bentley and Fishlake, near Doncaster, the agency said.
The UK experienced its coldest autumn night of the year on Wednesday night and east Gloucestershire, parts of Wiltshire and south Wales were pictured blanketed in snow on Thursday morning.
The weekend will see temperatures increase slightly with the wet weather subsiding.