How do scientists forecast space weather why?

Well that's a not easy task. The specialized forecasters analyze near-real-time ground- and space-based observations to assess the current state of the solar  environment which includes the areas from the Sun to the Earth and points in between. Space weather forecasters also analyze the pattern of solar activity. 

The problem here is that according to aufbau principle before 3d orbital 4s subshell comes.after filling 3p orbital with electrons you have 4 electrons left, you place two in 4s with opposite spin and the other two in 3d in any of its two orbitals of total five orbitals with the same spin.