Temperatures and heat index readings in the 90s are not something uncommon in northern Illinois and southern Wisconsin during the summer months, but the first time it happens is usually a little bit of shock to the body because we're not used to that kind of warmth and humidity. Temperatures will warm into the upper 80s through the weekend but with dew point temperatures in the low 70s, the heat index will warm into the low to mid 90s.
A strong ridge of high pressure will build over the center of the U.S. Wednesday and Thursday, shifting northeast into the Midwest and Great Lakes by Friday and Saturday. As the jet stream lifts north, scattered thunderstorms will be possible as disturbances move through the flow of the jet stream. As moisture is pulled north under the ridge dew point temperatures will also warm. The dew point temperature is a direct measurement of how much moisture is in the atmosphere. Whenever the dew point temperature rises over 65 degrees, it's considered to be fairly
muggy. Dew points by the weekend are expected to reach the low 70s.
With the returning moisture, however, will also come the chance for isolated thunderstorms for the weekend. The placement of the jet stream will be key in exactly where thunderstorm development will occur. A pattern very common during the summer months is often times called the 'ring of fire' pattern. Sinking air under high pressure heats the atmosphere fueling thunderstorms that ripple through the jet stream around the heat dome. If the outer edge of the jet stream lines up across the Great Lakes it could put parts of Wisconsin and Illinois in the more favorable area for thunderstorm development. If it doesn't, then we'll have more of an influence from the sinking air possibly rising temperatures into the low 90s through the weekend.
Either way it's going to get a little more uncomfortable as the week progresses. If you plan to spend a lot of time outside later this week just remember to take frequent breaks as the humidity increases.
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