The Chicago Tribune has a very interesting graphic (left) showing the affects of the ongoing mild winter. As I have written earlier, the current mild spell is really a continuation of a trend dating all the way back to October. Beginning then, every month so far has been above normal. This warmth has been observed virtually across the lower 48 states.
An interesting off shoot of this mildness is that the Great Lakes have higher water levels than last year. Part of this comes from the very wet spring and summer in the lakes area. But part is a result of the warmer winter which reduces evaporation of the lake water.
This reduced evaporation is somewhat counter intuitive. One would think warmer air temperatures would induce more evaporation. In the summer, this is true. But in the winter the opposite. Why you ask? In a normal cold winter, you have very steep temperature lapse rates between the warmer lake water, and the very cold air aloft at say 5000 feet. These very steep low level lapse rates produce very strong surface evaporation as the warm moist air near the water surface readily rises. You also usually have very strong winds with the cold air, again to increase evaporation. The winds and cold result in heavy lake effect snow well inland, which of course is evaporated lake water. This year, we have had neither the cold air aloft, or the very strong winds. As a result, much less evaporation. The graphic shows the typical evaporation rates of 30-40%. The mild winter has reduced this.
For the coming week, above normal conditions continue for the Chicago metro area. Highs Monday through Thursday will be in the 45-50 range, or 15-20 degrees above the normal high of 31. Warmest day of the week expected Tuesday with a high near 50. Friday will still be a little cooler, but a high near 40.
A little bit of light snow very late tonight/early Sunday morning, and again possible very late Sunday night/early Monday morning as the warm air moves in. Any snow will quickly melt Monday with highs in the 40s. A little rain possible Tuesday night.
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