It is typical in areas with good nectar sources for Monarchs to stick around and fatten up for their journey. If the weather is not good for flying that will also keep them grounded.
What about the mechanics of migration?
"As long-distance aviators, monarchs must know how to ride a good wind, and how to duck and cover in a bad wind, says Taylor. As we talked, he described the monarchs outside his office: "Today, the butterflies are low, out of the wind, they can't move into this wind. Tomorrow the wind will change to the northwest and they will use it to move to southeast. The wind will then change further and come from the north on the next day, aiding the butterflies as they progress southward. They ride these winds, and have all sorts of sensing capabilities to take advantage of thermals and winds speeds. This is like a military aircraft -- it's got all sorts of onboard computers." .... (access the website to read more)