PlanetsCentral

Voltage_change =
   2 * Random(6)
   ...i.e. 0 to 10 in steps of two
Radius_change =
   Random(4)
   ...i.e. 0 to 3
New_voltage =
   Old_voltage + Voltage_change + Additional_gain

New_radius =
   Old_radius - Radius_change
   ...if Old_radius > Radius_change
   1 - (Old_radius - Radius_change)
   ...otherwise

The Additional_gain consists of four bonuses that are given to each storm. Note that, because growing storms always have odd voltages, adding one will turn the storm into a weakening one. All four conditions are checked, so the bonus may be anything between zero and four.

Additional_gain =
   +1   ...if second rule was used for New_radius
   +1   ...with a 1% chance
   +1   ...with a 2.5% chance if New_voltage > 320
   +1   ...with a 10% chance if New_voltage > 500