The wind should get stronger the further north you go from Fortaleza, also in August. Although it can be possible that in the next state of Piaui (very north) the wind is not that strong yet.
If you stay north of Fortaleza, you should have almost a wind-guarantee for any day of your stay. South of Fortaleza you should usually find a bit weaker but often very steady wind. The chance of wind might be a bit lower than north, but it is not always like this. It is highly dependent on where you go. If it is your first time Brasil I would head to the north. Though it is a matter of taste.
You get the most out of your stay when you rent a car and just drive up the beaches. Looking for Flat-Water you can stop at Taiba Lagoon and a must stop is Paracuru. To get a bit more of a brazilian atmosphere, stop at the beautiful beach of Lagoinha and spend at least a night there. They have a Lagoon which can be reached by a short downwind. If you would like to stay brazilian style (simple but great brazilian hospitality) try „Lagoinha Kitepoint“. It is also the place where kitesurfers meet each other on the beach.
Looking for Lagoons I would defintely stop at Ilha do Guajiru next. With your own car you will have the possibility to explore several spots if you want to. You can have a look here: http://www.guajiruinfo.com/
(information about Ilha itself but also about the sorrounding area).
There will be a few flatwaterspots around Jericoacoara but the next favourite for me, would be the village of Barra Grande which is situated a couple of hours north of Ilha do Guajiru. Just, it is a loong drive..