Why Such A Warm Weekend

To answer this question, we have to look at what's happening above our heads.  A blocking high pressure system has been stationed over the Great Lakes, while an upper level low has been spinning out west.

Air throughout the atmosphere is sinking under high pressure.  When air sinks, it compresses causing the air to warm.  Sinking air makes it difficult for clouds to develop and this is typically why when under high pressure there are very few clouds around.

The opposite is true for low pressure.  Low pressure causes rising air throughout the atmosphere.  As the air rises from the surface, surrounding air tries to 'fill in' the lower pressure.  This causes clouds to form which eventually lead to rain and thunderstorms (or if it's cold enough, snow). 

Since we've been under high pressure for several days, our temperatures have warmed each and every day.  We continue to add warmth into the atmosphere which continues to heat our surrounding air.  Monday the Great Lakes will still be under the influence of high pressure, but the cut-off low to the west will be inching closer.  This will cause a little more cloud cover to develop by suppertime Monday evening and ultimately bring an end to our 80 degree stretch (but not before we hit another 80 degrees Monday afternoon).  Thundershowers will be a little more likely by Tuesday morning.