As much as one month's rainfall could soak the county in just two days, forecasters have warned.
The heavy douwnpours, due to hit tomorrow, could cause flash flooding, The Met Office said.
They have issued a severe weather warning, warning of the possibility of localised flooding and disruption to transport.
Frequent lightning and hail could also prove to be additional hazards.
The warning is in place from 1am on Thursday until 11.59pm on Friday.
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