Ive been turkey 3 times in may and its iffy. some years it can be clear sky and sunshine with just chilly nights and sometimes you can have lots of rain and wind. you cant tell these days. the weather is changing I think
There is an excellent weather site on-line that allows you to print a forecast in the specific region for the period of your stay up to 14 days, (print day before you leave) We take it with us to help to plan excursions and shopping days or tan days by the pool. It also shows Historic Monthly Forecast Averages.
I too am off to Turkey next week. We often go at this time of the year and yes the weather can be variable but more often than not it is sunny most of the time. I really love the spring there as the flowers are fabulous. You will need the same clothes you would wear on an English summers day and something warmish for the evenings. Have a great time.