Is seeing rain after performing Istikhara, before the dawn prayer, an indication of goodness and a positive sign?

There is no evidence linking rainfall to a preferred outcome after performing Istikhara. The one performing Istikhara must put their trust in Allah and proceed with the matter for which they sought guidance. If it is facilitated, then it is good; if it is not facilitated, then it is not good. Similarly, the feeling of relief and contentment in one's heart regarding the matter for which Istikhara was performed is, by the permission of Allah, a sign of goodness in it. Ibn Taymiyyah said: "If one seeks guidance from Allah, then whatever expands his chest and becomes easy for him among matters is what He has chosen." Ali al-Qari said: "And after Istikhara, one proceeds with what his heart feels expanded towards, an expansion free from personal desires."