All my sheep are wool sheep (as is the norm over here....Wales). Our temperatures rarely reach 90, and, if so, it's usually in July. We try to get our sheep sheared by the beginning of July. Our winters are not cold by some folks standards, but we do get a lot of frosts and periods of being 'snowed in'.
Most lambs have not been traditionally shorn in July, but recent evidence shows that they grow quicker if they are shorn in July/August time (after birth between mid March and end of April). We don't do this as yet, but it may become the norm in the future.
Sheep have to be dry to be shorn, and best to avoid a period where weather is forecast to be cold.
I don't know how cold your winters are, but, over here, shearing wouldn't take place beyond the start of September, and wouldn't ever happen as early as March.
So, short answer to your question is, what temperatures and how much precipitation is forecast? Also what breed of sheep are they as some breeds are much less cold-susceptible than others?
Shear them as often as you like.....and they won't be needing those blankets.It reaches at least 90 everyday and winter (maybe a week) only gloves down to like 60 maybe 50 at night but during the day it goes up to like 80