Its best to try and keep the majority of snow out of your exterior central air unit. This can help prolong the interior components and protect them from excess water when this snow melts. Some people will cover the AC unit with a tarp. I am not a fan of this, it can trap excess humidity causing condensation due to a lack of airflow through the cooling fins. The preferred method is cutting a piece of plywood slightly larger than the unit and laying it on top. You will need to put a brick or rock on it to hold it down as the wind will likely blow it off if not held down. Some units also have an oil heater installed in them so shutting off the power at the breaker or the disconnect can save on electricity in periods of non-use. This also protects your unit from someone accidentally turning it on in the winter which can damage the compressor.