Mainland stocks may be headed for a "sizable correction" after a so-called momentum indicator for the Shanghai Composite Index advanced to the highest in at least five months.

The 14-day relative strength index, or RSI, for the Shanghai Composite climbed to 83 this week, above the 70 threshold that signals to technical analysts an asset or market is poised to fall. The indicator compares the magnitude of recent gains to losses.

The last time the Shanghai gauge's RSI breached the 80 level, in February, the stock measure sank as much as 13 percent in following two weeks.