Subtle/slight/complex

Hi,

Please take a look at the sentence below:

"Conventional computer models of the atmosphere have limited value in predicting short-live local storms like the Edmonton tornado, because the available weather data are generally not detailed enough to allow computers to discern the subtle atmospheric changes that precede these storms"

The word “subtle” is closest in meaning to

A. complex

B. regular

C. imagined

D. slight

Thank you for your help.