We've made 3 trips that were either the first or second week in March. Only once did we have an afternoon rain shower but it didn't last long. Most days, t-shirts were fine (with maybe a light jacket for mornings/night);although, one day we started out wearing tees & a cold front came through in late afternoon. It got very windy & COLD in a hurry so we wound up buying sweatshirts from a vendor stand in Epcot. In March of this year, a couple of mornings were pretty nippy so we wore sweatshirts over tees & just pulled the sw/shirts off when it warmed up. When the sun set, it felt good to put them on again!
As for crowds, not bad at all during this time. We were able to walk on most rides a few years ago but this year the crowds seemed bigger. By using FP, we still didn't have to wait for rides though.