MNT: Slightly re-order calculation to eliminate warning
MNT: Slightly re-order calculation to eliminate warning

Original code issued a warning about an overflow in exp when passing in
masked data. By re-ordering the multiplication, we can keep a different
masked value and eliminate the overflow and the warning.

dopplershift/MetPy,Unidata/MetPy,Unidata/MetPy,dopplershift/MetPy,ShawnMurd/MetPy