October is the wettest month of the year– at least at the National Weather Service site near Negaunee. The October average precipitation is 4.04″, besting September (3.96″) by less than a tenth of an inch. May is the next wettest month with an average of about 3.37″.
October is wet because the air gets cold enough to begin generating lake effect showers– rain and sometimes even snow. Plus, the main storm track across North American usually drops downward, closer to the Upper Peninsula, as October progresses.
Some big, multi-day rainstorms can occur with these low pressure systems. Most recently, just over three inches of rain (3.02″) was received at the NWS on October. 9-10, 2018. Both the 0.84″ on the 9th and the 2.18″ on the 10th still stand as daily records. On the 11th and 12th in 1983, 2.63″ feel, with another 1.03″ over the next three days, including the first measurable snowfall on October 14, 1983.
One of the greatest October storms occurred in 1959. Low pressure developed over North Dakota on October 23 and moved eastward into the Great Lakes while deepening. An October daily-record of 4.06″ came down in the City of Marquette on October 24, with 5.21″ of precipitation from October 23-25, including over three inches of snow.
I bring up these past rainstorms because there will be another low pressure heading toward the Great Lakes late this week into the weekend. That means a good chance of more rainy and cool weather.