The worries are that the bright overhead lights will reflect off the wet surface of the track, and perhaps even the rain droplets themselves.
"It (Wednesday's rain) was so heavy that rivers of water flooded the track to a point where the tyres can't deal with it," said Sir Jackie Stewart. Full story
Related:
Drivers admits concerns over wet Singapore - Planet F1
'Heavy rain showers' for Singapore - Planet F1
Rain forecast for Singapore F1 - eTaiwan News
F1 drivers worried about rain under Singapore lights - motorsport.com