too bad it was my last game of league and two open bowling games afterward shot a 708 of you put those three games together. I have not been able to string three good games in a row all year. My high series has been a 622 because of it with throwing a few 240+ games and a 264 earlier this year. Tonight made me feel a lot better about my bowling

Not trying to burst your bubble, but are you counting the last game of leagues and the two open bowling games? Actually, that's not how you would count a series. league and open bowling are two different things...

But, then again you were talking about just string'n three good games in a row....

Yeah I know it isn't legitimately a 700 series due to all 3 not being league, but I have a hell of a time stringing 3 good games together. This is the first time in 2-3 years I have strung 3 good games together, so I am actually feeling good about my bowling again

I finally feel like changes I made to my timing and trying to stop squeezing the ball have made a difference. I feel like I am throwing the ball much better than I have been

Congratulations.  Hopefully you will get a boost of confidence and exceed that sometime in a near future league session.

For some reason, my average always starts out decent at the start of winter league, drops once the weather gets colder, and then I start shooting good sets again when spring time rolls around. And my summer leagues are consistently 10-15 pins higher than winter leagues

I am not an expert so take all of this with a grain of salt, but it could be:

1.  Just a coincidence unless it is over several seasons, then it may more likely be one of...

2.  Due to weather changes and its effect on the lane surface/condition.  You may have to learn how to adjust to the changes in the lane conditions that you see during the colder weather.  This may be especially true if you live in a colder or humid climate.

3.  Physical changes in your body, especially the thumb.  If your thumb changes because of the temp/humidity changes, you must use tape to compensate. 

http://bowl4fun.com/ron/btm04_files/btm4.htm

http://www.itbca.bowlingknowledge.info/index.php/2013-fall/95-tiered-taping-a-process-to--grip-pressure-and-improve-swing-release

http://www.bowlingthismonth.com/article/tiered-taping/

4.  Least likely, but it could be changes to the ball thumb hole due to weather.  This may be due to keeping the ball in a car trunk before bowling or temp changes wherever the ball is stored between sets.  The ball including the holes are expanding/shrinking or humidity is changing how the thumb exits.

5.  The summer averages could also be higher because the center is using a different pattern than the winter leagues and it better matches your game.  Or, you are more relaxed because it is a summer league and you don't take it as seriously as your winter leagues.  If so, you need to work on your mental game.