A significant maximum in cosmic rays occurs one day before solar sector boundary crossings. Therefore, previous work showing a maximum in thunderstorm frequency one day after solar sector boundary crossings may result from high cosmic ray counts. This is evidenced by a maximum in thunderstorm frequency three days after high cosmic rays and, also, by a maximum in thunderstorm frequency three days after minimum Kp. These maxima are of higher statistical significance than those found when solar sector boundary crossings were key days.