It rained buckets in the Copper Country on June 11, 1954. A daily-record 2.31 inches of rain was measured at the Houghton County Airport (CMX) between Hancock and Calumet. Lesser, but still substantial rains fell farther south. Escanaba reported 0.84 inches while just under an inch fell in Marquette. The rain was caused by a persistent southwesterly flow aloft that brought a wave of low pressure up from the southwest. The low passed through Wisconsin so the Upper Peninsula remained in the cool sector of the system.
The 2.31 inches at CMX is not only a daily record but a monthly record, too (between 1953 and 1999). In comparing locations, daily rainfall records and monthly averages during the summer are substantially lower in the Copper Country than in U.P. locations to the south. For instance, the June average at CMX is 2.85 inches. At Ironwood, the monthly average is 4.16 inches, while in Iron Mountain it??s 3.48 inches. The wettest June day at Ironwood is 3.81 inches set on June 24, 1946. At Iron Mountain, the daily extreme is similar to Ironwood??s??3.78 inches on June 17, 1951.
What is the reason for the difference? In summer, rainfall is most often of a convective nature. That means that warm, moist air is lifted high into the atmosphere to produce showers and thunderstorms. Inland locations like Ironwood and Iron Mountain only need warm, humid air and a trigger to produce this sort of precipitation. To the north in the Copper Country, this process is often interrupted or at least inhibited by the cold water of Lake Superior which creates a stable layer of cool air close to the surface. This is especially true early in the season when the water is the coldest. Of course, this isn??t always the case. Severe thunderstorms and heavy rain can occur. It??s just that there is the tendency for storms to weaken when encountering the cool air surrounding the Keweenaw Peninsula. The same holds true for locations near the Lake like Marquette and Munising.
This early summer, the inhibition process is well established with chilly water remaining after the record-breaking winter (Image Above). Again, it??s not that it will not rain; it??s just that the cold water will tend to temper thunderstorm activity to some degree.