The Met Office has issued a 15-hour yellow warning for rain from midnight, covering almost the whole of Wales as well as part of Shropshire.
Forecasters are warning up to 100mm of rain could fall during the warning which runs from midnight until 3pm tomorrow.
"Rain is expected to set in during Wednesday evening, turning particularly heavy and persistent overnight especially over southwest facing hills," the warning says.
#Measuring the #River #Severn at Montford, just upstream of Shrewsbury today. 250 tonnes of water per second coming down the river system here at speeds of well over 2 metres per second! Keep up to date with the latest #levels and #information here - https://t.co/SG1o6b9YJi pic.twitter.com/sOBwWZiu2o— Rob Davies (@RobDaviesEA) March 15, 2023
"40-70 mm is likely quite widely, with the wettest spots possibly seeing in excess of 100 mm of rain. The rain will be coupled with generally windy conditions which is likely to further reduce visibility from spray. Conditions will slowly improve through Thursday afternoon."
While the weather covers all of Mid Wales, including towns such as Welshpool, Montgomery and Newtown, only a small fraction of Shropshire - the area around Clun - is included.
In its West Midlands forecast for this evening, the Met Office says: "Remaining wet and windy with rain, heavy at times, continuing to move across the region during the evening and overnight. Persistent rain could lead to the risk of flooding. Milder. Minimum temperature 5C (41F)."
For tomorrow, it continues: "Cloudy with further outbreaks of patchy rain and drizzle, becoming heavy at times during the afternoon. Mild with winds easing later. Maximum temperature 14C (57F).
This morning two flood warnings remained along the River Severn in Shrewsbury, with the level expected to peak at 3.4m to 3.6m at Welsh Bridge. Flood barriers have been installed at Frankwell with the car park closed. The warnings come after heavy rainfall and large amounts of melting snow on the Welsh hills.
Flood alerts remain in place for the River Dee catchment from Whitchurch to Chester, the rest of the River Severn in Shropshire, the Severn Vyrnwy confluence and the Tern and Perry catchments.
Meanwhile, police in Shrewsbury have reminded people not to move cones blocking off closed roads.
The Shrewsbury Police Twitter account posted photos from the weir, showing Sydney Avenue submerged beneath overflowing water from the river, along with a post that read: "For those that keep moving the cones to try and get through, yes the road is closed!"