It has been several years since we have had to write about this issue at this time of year. Dry weather can affect both soil applied and postemergence herbicide performance. All soil applied herbicides require rainfall to mobilize them for effective weed control. In general, rainfall should occur within 7 to 10 days after application or before weed emergence. As a general rule of thumb, a ½ inch of rain is considered the minimum depending on current soil moisture levels and the herbicide used; ¾ to 1 inch is ideal. The less mobile materials (Prowl, Atrazine) and deeper germinating weeds (e.g. yellow nutsedge, cocklebur, velvetleaf, ragweed, etc.) will require more rainfall for effective mobilization and activation into the seed germination zone. Keep in mind that many small seeded annual weeds can germinate with minimal moisture. Thus, sometimes you may observe weed emergence before enough rainfall has activated the herbicide for effective kill.
If 10-14 days have past without rainfall following a Pre-treatment and weeds are starting to break, start planning for a post herbicide application (or where possible, consider using a rotary hoe or that old cultivator that you have been thinking about trying again). Some "reach back" or “recharge” can be expected on small annual weeds (esp. broadleaves) with some herbicides when rainfall occurs, although depending on this may be a little like gambling. In particular, the HPPD (Group 27) herbicides (Balance, Corvus, Lumax, Lexar, Instigate, Prequel, etc.) tend to have better “reach back” potential then some other herbicides and escaped grass control is probably of greater concern. The Group 5 herbicides (Photosystem II inhibitors) like atrazine, simazine, and metribuzin will also control small emerged susceptible broadleaves via root uptake. As for post herbicide applications, remember that small annual weeds are easier to kill than large ones and examine adjuvant options to maximize activity under dry weather conditions. Delaying the herbicide application until after a rainfall rarely increases your chances of success unless you're making that decision while watching a large storm front move in from the west.