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Wednesday was a tale of two seasons around my area. Readings in the far northeast never got out of the upper 40s while in SE Iowa highs hit at least 75 in Keokuk and 73 in Ft. Madison (a spread of 27 degrees over 190 miles). Those of you who remained in the 40s and 50s can blame Lake Michigan. Water temperatures are not much more than 40 degrees and when winds with an easterly component blow our way, temperature suffer thanks to the chill the water produces. Notice how the southern tip of Lake Michigan parallels I-80 and the Quad Cities (in a horizontal east west line). On days like Wednesday, the E/SE wind trajectory enhances a boundary laid out close to that I-80 position. On occasions when winds are more northeast, the boundary would likely be even further south closer to the Missouri Iowa border.

Without the impact from Lake Michigan, my southern counties are able to warm. Where the lake's involved further north, temperatures stay nice and fresh. This is a typical occurrence in spring until the water can warm up later in May and June. This streamline plot Wednesday evening (showing wind speed and direction), clearly depicts the boundary producing the 27 degree contrast in my region. You can trace the origins of the streamlines over my area directly back to Lake Michigan.

Below another view of temperatures around 4:00pm Wednesday. Visualize the streamlines above over these temperatures and it's easy to see why our day turned out the way it did. This is a fascinating mesoscale event that becomes a factor every spring when we get low pressure to the south or a big high pressure over the northern Great Lakes. It will remain an issue until we can warm Lake Michigan's water up.

In the big picture this could very well be a factor for us much of the next two weeks as the pattern remains intact in such a way that the overall wind fields will consistently have an easterly component to them. You can see that in the 16 day animation of pressure departures ending May 13th. There's either a low pressure passing south or a high pressure planted over the Great Lakes.

That's not a recipe for warm temperatures or dry weather. Here's the week 1 and week 2 temperature departures.

Week 1 (April 27th-May 4th)

Week 2 (April 5-May 13th)