The Met Office has confirmed the precise timing for thunderstorms to arrive in Wales on Thursday, with a yellow weather warning in place for large parts of the country from 6am until 10pm.
Areas under the weather warning
The warning covers Blaenau Gwent, Bridgend, Caerphilly, Cardiff, Carmarthenshire, Ceredigion, Conwy, Denbighshire, Flintshire, Gwynedd, Isle of Anglesey and Merthyr Tydfil. Also affected are Monmouthshire, Neath Port Talbot, Newport, Pembrokeshire, Powys, Rhondda Cynon Taf, Swansea, Torfaen, Vale of Glamorgan, and Wrexham.
The Met Office said: "A broad band of thunderstorms and heavy rain will move north across England and Wales, perhaps becoming slow moving in the afternoon and evening."
Rainfall totals and hazards
Accumulations of 20-30 mm in three hours can be expected fairly widely, with up to 50 mm in a 12-hour period possible in places. There is a small chance that some locations may see more than 50 mm in three hours and more than 75 mm through 12 hours, especially over east-facing high ground.
Lightning and hail of 1-2 cm will also be hazards, and in a very few places strong gusts may present a hazard, most likely in the south of England early on Thursday morning.
Timing of the worst weather
Weather maps show heavy rainfall of more than 32mm per hour will first fall on parts of Newport at 5.30am. By 6am, parts of Cardiff, Newport, Pontypridd and Chepstow will be affected.
The meteorological service said where flooding or lightning strikes occur, there is a chance of delays and some cancellations to train and bus services. Spray and sudden flooding could lead to difficult driving conditions and some road closures, and some communities might become cut off if roads flood. Flooding of homes and businesses could happen quickly, with damage to some buildings from floodwater, lightning strikes, hail or strong winds.
Advice from the Met Office
Offering advice, the Met Office said: "Consider if your location is at risk of flash flooding. If so, consider preparing a flood plan and an emergency flood kit.
"Prepare to protect your property and people from injury. Before gusty winds arrive, check to ensure moveable objects or temporary structures are well secured. Items include: bins, garden furniture, trampolines, tents, gazebos, sheds, and fences.
"Give yourself the best chance of avoiding delays by checking road conditions if driving, or bus and train timetables, amending your travel plans if necessary.
"People cope better with power cuts when they have prepared for them in advance. It's easy to do; consider gathering torches and batteries, a mobile phone power pack and other essential items.
"If you find yourself outside and hear thunder, protect yourself by finding a safe enclosed shelter (such as a car). Do not shelter under or near trees, or other structures which may be struck by lightning. If you are on an elevated area, move to lower ground.
"Be prepared for weather warnings to change quickly: when a weather warning is issued, the Met Office recommends staying up to date with the weather forecast in your area."
