When Mary Poppins uses her umbrella to fly it is unlikely that anyone would be able to replicate this using normal wind speeds and a normal sized umbrella. Firstly you would need a force upwards that equals your weight downwards. If her umbrella is about 2 meters across and if marry weighed 50kg, using a website i found that she would need about 16.8kg per meter. You would need an updraft speed of around 15 m/s or about 29 knots. That is a big updraft - not one you would normally see
outside of a thunderstorm-showing that the physics in the movie is not as real as it seemed to my 6 year old self.