Chicago Weather: Temps in 70s ... finally!

The Chicago area is enjoying a taste of spring on the last day of April.

Yes, it was the fourth coldest April on record, but all is forgiven as Monday is the warmest day of the year so far.

Temperatures reached the mid 70s, even on the lakefront, and this is only the second time this year we have seen the 70s.

And 80 degrees is coming on Tuesday!

Monday also brought plenty of sunshine and winds, which could reach 30 mph in some areas in the afternoon.

As a result, there is a Red Flag Warning until 8 p.m., which means there is an elevated fire risk due to the high winds and dry vegetation. Outdoor burning is discouraged.

DRY AIR

We are experiencing a rare very dry air day in Chicago. Relative humidity has dropped to 15 percent in Chicago. The lowest relative humidity every recorded since 1871 is 13 percent. That has happened only five times since 1871.

How rare and why?

If you look at the statistics, about once every 15 years do we ever see relative humidity drop below 20 percent in Chicago. These dry air events can really only happen in the spring or fall when we have warm air and little in the way of moisture sources.

Monday's air is mixing down from above, bringing down drier and warmer air aloft to the ground. As air sinks it warms and also lowers the humidity level. This also is creating the gusty winds we are seeing now.

There was also no source of moisture on Monday. No moisture from the Gulf of Mexico has made it this far north. And the other BIG factor is the dormant vegetation. With very few trees and plants growing yet, they are not adding moisture to the atmosphere like they typically would later in Spring or in Summer.

THE REST OF THE WEEK

Temperatures will stay in the 70s Wednesday through Friday.

Rain chances return to the area Wednesday, Thursday and into Friday morning. Some areas could see 1 inch to 2 inches of rain by Friday morning. There will be the chance of thunderstorms Wednesday and Thursday.