What is the significance of seeing rain in winter after seeking Allah's guidance (istikharah) regarding matters of work, marriage, and friendship, with work matters being eased but marriage matters becoming difficult?

If you perform Istikhara regarding marriage to a specific woman and it does not materialize, this is a sign that the matter has been averted from you, and this is an aspect of the acceptance of Istikhara. There is no need for an overt sign for this, and seeing rain has no connection to Istikhara. Rely on Allah and proceed with the matter. If it is facilitated, then it is good; and if it is not facilitated, then there is no good in it. Among the signs of a sound Istikhara are: a righteous dream, a feeling of ease in one's heart, and the facilitation of the matter.