The 21-year-old, who became the youngest English triple-centurion when he hit an unbeaten 303 against Essex three weeks ago, moved smoothly to 39 not out from 61 balls as Northamptonshire reached 194 for 4 in the 68th over.
But only just over an hour's play was possible before the downpour and, with pools of water surrounding the square, umpires Peter Willey and Alan Whitehead were forced to abandon play for the day.
Resuming on 137 for 4, Sales and Russell Warren added 57 in 65 minutes, the former hitting a six off Jeremy Snape and seven fours in another impressive display.
Warren struggled to find his touch but still played a valuable supporting role before the predicted rain started to fall at mid-day, halting proceeding moments later. Sales, who passed 1,000 first class runs for the season during his knock against Essex, now needs another 68 to reach four figures in the Championship.
The first day of Sussex's game against Surrey at Hove and Leicestershire's visit to Lord's to play Middlesex were both washed out yesterday, with no play possible.Reuse content