family affair is right..
The Great Lakes lie in kind of a climatological battlefield. During the time you can comfortably cruise, (June-Sept) they are affected by cold fronts that bring warm air up on Southerly winds ahead of them and the winds generally veer NW to NE behind them with a sometimes significant drop in temps. Some fronts are not that strong and bring little change with their passage. When a strong Bermuda HI or a localized hi pressure system parks over the lakes, the winds can be calm for days, although this is rare.
On the whole, the lakes are more cantankerous than their saltwater counterparts. Because of the freshwater, the waves are closer together and somehow, quicker..
Run the Southerly shore of Erie from Buffalo West. Except for the passage of the aforementioned cold fronts, the prevailing winds are S to SW. with onshore breezes some afternoons. This will give you reasonable running conditions along a lee shore past Cleveland to Sandusky and to the Islands. There, you will find the infamous, Erie chop, more often than not. It seems to be a combination of the islands, the ferrys running to-and-from Put-In-Bay and Kellys and the winds + boat traffic. The wave action is kind of like a swimming pool on Memorial Day Weekend filled with kids.
When you get thru the Detroit River and Lake St. Clair, run the East shore of Huron unless you really want to see the North Channel. This route will give you another lee shore for the most part. Underneath the Mac Bridge and into Lake Michigan..Here begins the gamble. Either shore will be unpredictable, depending on the fronts and the weather systems. Stay overnite at Mackinaw City Municipal Marina. Study the weather, maybe ride the ferry out to Mackinaw Island for the day; eat some fudge and make your cruising plans. Again, the Wisconsin side is generally your lee side. Fortunately both shores have ports of entry every 18-20 miles or so with good marinas where you can wait out rough days. The dunes on the Northern lake Michigan shoreline are a sight to behold, by the way.
Your 258 Journey? Up to a 2 ft sea and 10-15K winds you should be fine. Past that, as Family Affair says, drop the tabs & slow down.
Best months? July-Aug. After that, the stronger Westerlies and NW winds pick up as the fronts get stronger.
Remember, the lakes get rough fast, but calm down as quickly. Be prepared.. That water is cold, even in the summer.
Enjoy..Marvelous cruising.