Nicely done!
However, I would recommend the following structures:
Nowadays, the weather fluctuates more than in the past. As one can see, the rainy season in Thailand starts later and later every year, from June to August.
The second sentence is quite good, but I imagine there was more than one rainstorm that provoked the floods. If so, add an "S" to rainstorm. If not, keep it as is.