When meteorologists look at heat domes, we usually look at the 500 mb level. More explanation for that on the following pages. 

Earlier this week, the upper levels of the atmosphere were at some extremely lofty levels associated with this heat dome.  A large 600 dm closed contour as shown below.

It looks to me like they peaked Monday evening, which is when I copied this map from the European ensemble. There are several factors that go into what causes record heat and just because you have a record 500 mb height level which is a measurement several miles above the ground, that doesn't always equate exactly to the hottest temperature at the surface (the amount of moisture at the surface and in the soil, the terrain and elevation play a role that can contribute 15+ degrees for instance. However, generally speaking, the greater the 500 mb heights are, the hotter the temperatures will be at the surface, all other things being equal.

Looking at the extremely impressive, huge 600 dm contour below, every meteorologist thinks "near record or record heat,  0% chance of rain anywhere close to it. 

Here in Evansville, IN, we were inside the 600 dm contour on Monday with a high of "only" 92 degrees, which was not close to the record.

A big reason for that was because our dewpoint was extremely high too, at 77 deg. It's much harder to heat humid air than dry air. Had the dewpoint been 65 or less, we would have likely been able to hit 100 with this same heat dome!

                                         500 mb with vorticity below

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The 500 mb heights are falling here now. Below is what they will look like on Friday, when we are just outside the 594 dm contour.  That's still VERY impressive. However, the ground is drying out and dewpoints will be a bit lower and the core of the hottest air at the surface will be blown in from the southwest, so we have a shot at 100 on Friday.

You can see the big kink in the height contours in the Upper Great Lakes then. That's the 500 mb,  upper level part of the perturbation which is sending a pretty decent surface cold front down from Canada that will cool the temperature down 15+ degrees next week! 

 The 2nd map shows vorticity, which is a measure of spin. 

The value of vorticity is not as important as is the rate of change by which vorticity increases or decreases.

Downstream from (ahead of) areas of maximum vorticity are the locations of the greatest rate of increase in vorticity. This implies an area where air converges at the low levels and, therefore, rises into the atmosphere, possibly leading to precipitation. We call those areas PVA or positive vorticity advection.

You'll note on the map above with the vorticity and the one below, that inside the strongest part of the dome, there is no vorticity or spin. With very warm air aloft associated with  heat domes, it "caps" the heat below from being able to continue to rise with enough gusto to cool and condense moisture into clouds. 

This capping of the atmosphere from extremely warm air aloft,  effectively defeats the rain making process inside the dome and anywhere close by with the very warm air aloft.

So here's the deal with this cold front that cools us off at the surface this weekend:

There will be enough lingering very warm air aloft from the dome, when the cold front undercuts it at the surface so that the normally strong lift from the cold front will be suppressed. We need cooler upper levels (lower 500 mb heights) to help the air at the surface to be more buoyant/rise with enough gusto and cool to condense out moisture for clouds/rain making.

The higher the 500 mb value, the more stable the air because of warmth aloft.

The lower the 500 mb levels, the more unstable the air.

Bottom line is 500 mb levels will still be high and unfavorable for much rain in Indiana with this cold front.

                                        500 mb with vorticity below