The long advertised heat and humidity finally made its presence known around the Midwest Thursday. Temperatures made into the 90s in many locations but it wasn't the heat as much as the humidity that made it so dog gone nasty. In the graphic below you can see the (PWAT's) precipitable water vapor that were more than 2" in spots.
In extreme northeastern Illinois they reached 2.9" which is about 3 standard deviations above normal...in simple terms "a massive amount of water vapor".
The way we measure that is with dew point observations. Thursday evening they looked like this over eastern Iowa. Some places were up around 80 which is highly elevated and in the oppressive range. They don't get much higher than that.
Take the high moisture and add it to the hot temperatures and you get stifling heat indices which in some parts of Iowa reached 110 degrees.
Tomorrow is likely to be as bad or worse as the core of the heat passes over the central Midwest. Here are some of the projected heat indices Friday afternoon. Des Moines sporting a reading of 114. Anything over 105 is considered quite dangerous. This is the type of summer weather you need to respect, especially if you are doing outside work that is strenuous. Water, water, water!
The excessive heat is widespread and warnings extend from the Plains all the way to New England.
As many of you know cold air is more dense than hot air. You can see that below in the sea level pressure anomalies. The blue indicates where pressures are lower than average depicting regions where warmth prevails.
The millions dollar question is when does the heat break and I'm happy to say the wait won't be long. A seasonally strong cold front is expected to punch through the Midwest Saturday night breaking the siege. With abundant moisture and instability in place strong storms with heavy rain will be a possibility late Saturday or more likely Saturday night. Cape values (convective available potential energy) of 4 to 5,000 j/kg Saturday evening shows the high degree of available energy for storms to tap.
At that time PWATs are still greater than 2" which is driving the high CAPE values
The high-res convective models show a stout line of storms Saturday night. Here's the 3k NAM
Once the storms depart, the heat breaks. In the meantime the focus remains on the heat and humidity which bakes the area through Saturday. Play it cool and root for that front. Have an excellent weekend and roll weather...TS