What is an Elevated Mixed-Layer?

The origin of the EML

An elevated mixed-layer (EML) is a region of extremely warm and dry air typically located between about 800mb and 600mb (~6,500ft to ~16,000ft ASL) in the atmosphere.  In the United States, EML's originate and develop across the Rocky Mountains region near the desert southwest and across the high plains.  Air moving up and over the Rocky mountains warms and dries significantly.  This allows for very steep lapse rates (change of temperature with height) to exist between the surface and 600mb in the atmosphere.  

Importance of the EML and severe thunderstorms 

The EML is also known as a capping inversion because of the warming and drying of the atmosphere where the EML begins.  Remember, air will continue to rise as long as the surrounding air is cooler than the temperature of the parcel.  Once the air begins to warm, this process is no longer occurring, thus the air parcel no longer rises.  This process inhibits clouds, showers, and thunderstorms from developing which allows for the sun to continue shining strong.  If the low level airmass is warm to hot and there is a rich presence of moisture (high dewpoints) the combination of the very steep lapse rates, high surface temps, and high dews leads to the development of extreme instability, a key ingredient in the development and strength of thunderstorms.  

If some sort of lifting mechanism, such as a cold front, potent shortwave, etc, pushes in and provides enough lift to break through the cap, thunderstorms would begin to initiate and become very strong very quickly.  If the capping inversion is too strong then one of two things can happen; 1) No activity develops and all the instability goes to waste or 2) Only a few storms develop.  This can actually enhance the severity of the storms that develop because with only a few storms around the updrafts are not fighting each other.  

How can EML's be observed?  

An EML is easily observed on a skew-t forecast sounding which gives you a look at the entire atmospheric profile.  Below is an example of what a true EML looks like on a forecast sounding:

In the above sounding you can see the region of extremely warm (where the black temperature line jets off to the right) and extremely dry (where the green dewpoint line shoots off to the left) air.  

EML advection into the Northeast

While EML's don't often survive the journey into the Northeast, every now and then a plume of EML air can break off from the source region and ridge up over ridging at 700mb and remain in tack all the way into the Northeastern United States.  However, ridging at 700mb does not always guarantee EML advection into the Northeast.  In a research paper published by meteorologists Michael Ekster and Peter Bannacos, both identified 36 EML events and the corresponding 700mb pattern in place 36 hours prior to the event, 24 hours prior to the event and the observed day of the event.  As you can see ridging at 700mb appears dominant across the southeastern United States extending northward into the mid-At;antic and into the Northeast: