The A83 was closed by a landslip in the early hours of the morning near the top of the Rest and Be Thankful.
It is the ninth time the road has been closed in this way in the last five years.
The local diversion route recently constructed on "the Old Military Road" is now operational following the closure. The convoy system and traffic signals are leading to delays.
Traffic Scotland is advising motorists to plan their journey accordingly
The alternative is the 50 mile plus detour by Crianlarich and Dalmally.
The majority of material brought down by the rain has been caught in recently installed debris fences, but "slurry" has covered the road, maintenance company Bear Scotland said.
Engineers are inspecting the area and are concerned that the hillside remains unstable so they are working to open the nearby road with temporary traffic lights to allow a local diversion.
The relief road runs a few hundred feet below and roughly parallel to the A83. It was upgraded last year at a cost of almost £4 million after landslides shut the A83 for more than 30 days in 2012, hitting business and local residents.
The latest incident follows a landslide last week when almost 60 tonnes of debris was brought down on the road at Butterbridge.
A spokeswoman for Bear said: "The initial geotechnical inspection has been undertaken and established that the failure is high on the slope, above the present cloud level.
"Given existing knowledge of this area there is significant concern relating to the stability of the hillside so the A83 will remain closed at present.
"On that basis, the adjacent Old Military Road is currently being prepared to allow a local diversion route to be implemented under traffic signal convoy control. Delays are still expected on the route due to the nature of the convoy system."
There are three flood alerts in Scotland with heavy rain overnight affecting mainly Ayrshire and Arran, Dumfries and Galloway and the Borders.
Plans are already being drawn up to plant 250,000 trees on the slopes above the A83 to prevent landslips.
According to experts, the roots of the trees would bind the soil and also help heavy rainfall soak down into the ground rather than running off towards the road, taking part of the hillside with it.
Since 2009 the Scottish Government has invested £4.9 million to combat the problems on the road, which runs between the head of Loch Long and that of Loch Fyne.
This includes the construction of the local diversion route at the Rest and Be Thankful.
But twice since it opened, the A83 has been closed by landslips further north, and the diversion was unable to be used. Motorists were again left with the options of the 50-mile detour, or travelling by way of the Gourock to Dunoon ferry.
Transport Scotland is still seeking a long-term solution and is working with Forestry Commission Scotland (FCS) and local landowners to develop a strategy.
A spokeswoman said: "Tree planting to help stabilise the slope above the A83 is one of many options being looked at. We have established this would have to be done in conjunction with other measures and are discussing this with our stakeholders."