Here's what my RE told me:
Your body tends to naturally select or favor one follicle a month and hold the others back while it matures. The estradiol supresses this mechanism, allowing multiple follicles to mature, and making your chlomid or other drugs a better shot at producing multiple eggs.
It is hard to predict. The higher levels of estrogen may promote the midcycle LH surge, which is what urine OPKs look for. If this happens, the surge might come a couple of days earlier. This is called positive feedback. On the other hand, the higher estrogen levels my inhibit LH (this is called negative feedback) delaying the surge. You will need to test for ovulation to find out.