Equatorial bubbles occur sporadically, so that the resulting scintillation that disrupts transionospheric RF propagation is unpredictable. Furthermore, this randomness prevents the systematic observation of the formation of bubbles necessary for making progress in their prediction. However, recent results indicate that a necessary condition for bubble formation is bottomside spread F (BSSF); that BSSF is a function of the F region dynamo electric field; and that this field is dependent on magnetic activity and on season. This work measures these conditions using an array of ionospheric sounders in conjunction with other spread F observations, all of which are ground-based, in the western American sector, during solar maximum. Three spread F levels are observed: total BSSF and strong BSSF, both measured near the dip equator, and macroscopic bubbles measured at the equatorial anomaly. The three independently measured thresholds form a hierarchy in which macroscopic bubbles are a subset of strong BSSF, which is a subset of total BSSF. Each level decreases as a linear function of Kp averaged in the 6 hours preceding observation during March, April, September, and October, and during November--February; however, during May--August the levels are small and are independent of Kp. As a result, the probability of occurrence of each level, most importantly of macroscopic bubbles, is a quantitative function of Kp and season. The seasonal variation of Kp dependence indicates that the suppression of irregularities results from the decrease in the maximum prereversal vertical drift velocity of the equatorial F layer and therefore from the decrease in the eastward electric field.