The Met Office has issued a "yellow" weather warning as an Atlantic storm sweeps in, bring ing "persistent" downpours and gusty winds.
A severe weather warning was issued for the region, including Glasgow, from 9am today until midnight tomorrow.
Western areas and higher ground will be worst hit.
Chris Burton, a forecaster for Meteogroup, the weather arm of the Press Association, said Scotland would be waterlogged today and overnight.
But conditions are set to brighten by Thursday.
Mr Burton said: "There is a storm front moving over the West of Scotland, where it will remain quite static – it's not going anywhere.
"Rain will be persistent through the day and overnight but it will begin to fizzle out on Wednesday.
"The problem is that the storm front is not moving so the rain will keep coming throughout today. It's not going to be heavy but it is going to be raining for a long time."
Water is expected to lie, causing a flood risk to low ground.
But Thursday will be dry and bright with some frost and plenty of sunshine but will become cloudy later in the day.
Rain and wind is set to return on Friday and last throughout the weekend.
Mr Burton added: "Temperature-wise it's not going to be too cold but we are expecting Thursday to be the highlight of the week before the rain comes back on Friday and lasts throughout the weekend."
The Met Office warning for today said: "Outbreaks of rain will become persistent and heavy at times.
"Falling on already saturated ground, the public should be aware that this rain could lead to some localised disruption and the possibility of flooding."