Multiple font sizes in plot title

Try:

import matplotlib.pyplot as plt

plt.rc('text', usetex=True)
plt.title(r'\fontsize{30pt}{3em}\selectfont{}{Mean WRFv3.5 LHF\r}{\fontsize{18pt}{3em}\selectfont{}(September 16 - October 30, 2012)}')

plt.show()

That \r might want to be a \n on your system.

I used Joel’s answer to address your question.