Wow! Look at this graph of the water temperature at the Ludington buoy. The temp. fell from the mid 70s to near 40! The water temp. at 3:10 pm on Tue. was 72.7°. Just 7 hours later at 10:10 pm, the water temp. was down to 45.1°. By 2:50 am, the water temp. was reading 40.6°. You might think there was something wrong with the thermometer, but this is a classic case of upwelling.
Upwelling occurs when an offshore wind blows the surface water away from the nearshore area, allowing cold water to rise from the bottom of the lake to the surface. This process can happen relatively quickly. Even after a warmer than average summer in late August, we can see 40-degree water come to shoreline areas. It can be quite dangerous if you jump in this ice-cold water off a boat, not realizing how cold the water has become. The water will warm up again when the wind goes west and brings back the warmer surface water…though it’ll be tough to recover back to the mid 70s.
Update: Wed. midday – all beaches flying red flags for no swimming – waves are 3-6 feet. Water temps: Grand Haven 42°, Ludingon Beach 54°, Saugatuck 55°, S. Haven 57°, Hoffmaster 62°, Muskegon St. Park 64°.
Buoy water temps: Ludington 48°, Muskegon 52°, Port Sheldon/Holland 49°, S. Haven 56°, Stevensville 58°, Michigan City IN 73°, Wilmette/Chicago 73°, mid-Lake Michigan buoy 74°. Waves were running at 6.9 ft. at Michigan City (see pic. above from the GLERL station – from NOAA Coastwatch).