The elevated mixed layer(EML) was first discussed in pioneering work by Toby Carlson in the mid to late 1960s.
        He noted that the EML resulted in a "lid" which prevented thunderstorm activity.  His synoptic meteorology book entitled
        "Mid-latitude Weather Systems" has a chapter devoted to the "lid". This book is a must read for meteorology students.
        Most meteorology books are highly theoretical and are frankly no fun to read. Carlson's book actually reads like a book
        and is interesting throughout. I was fortunate enough to enjoy Carlson's advanced synoptic meteorology course while
        attending Penn State University in 1991. While many of the courses I took in graduate school were just about deriving
        equations, this course was very applied and I learned a lot. Yes, we learned the components of the important equations
        and applied the equations to practical problems,  but the whole course was not based on proving ones mathematical
        prowess.

        The EML is important for the following reasons:

        1. The EML prevents deep, moist convection until high instability is achieved. In the absence of deep, moist
            convection,  warm and moisture laden air can flow poleward in an unimpeded manner. Daily evapo-transpiration
            also adds moisture to the boundary layer further enhancing theta-e.

        2. The EML tends to keep storms isolated. When deep moist convection occurs in a capped environment, it tends to
            be in localized areas of enhanced convergence such as along out flow boundaries, dryline and terrain features, or of
            course along frontal boundaries. Isolated storms tend to be more severe than widespread storms since there is less
            competition for available warmth and moisture.

        3. The EML along the southern edge of the westerlies prevents deep vertical mixing. Deep vertical mixing is
            a CAPE destroyer. It is very difficult to maintain high mixing ratios when very deep mixing is occurring. The cap
            provided by the EML confines the moisture to a shallow layer, preventing the mix-out effect. This effect is most
            apparent in late-spring and summer when the southern edge of the westerlies retreats to 40-45N. The high dewpoints
            will usually be along the southern edge of the westerlies where the lid is the strongest and where cold fronts
            stall out. Moisture convergence is also greatest along the southern edge of westerlies, typically just poleward of
            stalled out fronts or outflow boundaries where evapo-transpiration is at a maximum from vegetation and previous
            rains. The mixout effect can also occur beneath the strong capping inversion in early-spring when moisture return
            is extremely shallow, particularly when strong synoptic-scale disturbances are involved.

        EML's develop when arid regions heat up and deep, dry adiabatic lapse rates extend from the surface to between 450mb
        and 600mb. Elevated mixed layers can occur any time of the year. Of course an EML can occur along with a very stable
        boundary layer too. But this page is devoted to EML's that result in capping inversions above a warm and moist boundary
        layer. I  have found EML soundings in all seasons and all areas east of the Rockies. The
 

Some EML sources include:

        1. Dry, elevated terrain of the interior, western United States
        2. High plains of the United States
        3. Sierra Madre Occidental of Old Mexico
        3. Western desert areas of southern Africa
        4. Desert areas of northern India
        5. Parts of Spain, France and Saharan north Africa

        In the cool season, the elevated mixed layer (with positive instability) can occur over the southern United States.
        Nothern old Mexico and the southern Rockies are the source region this time of year. A tornado outbreak occurred
        across Arkansas and western Tennessee on November of 1994. An elevated mixed layer off of old Mexico capped
        the moist, low-level air over Little Rock at 12Z.

        The Rockies' EML can be found from time to time along the east Coast. Here are a few notable examples from
        prominent tornado cases:

        Albany( August 28, 1973)  F4 killer tornado in Columbia county, NY and Berkshire county, MA
        Washington Dulles(July 10, 1989) Tornadoes family moved SSE from eastern NY to New Haven, CT to Long Island
                                                          to Putnam county, NY
        Hatteras (March 28, 1984)  At least 2 tornadic supercells, one of which produces many tornadoes over a 5 hour period,
                                                         killing 57 people

 
        Of course elevated mixed layer soundings are commonplace in spring and summer:

        North Platte (July 10, 1977)  F3 tornado in Cherry county, NE and Bennett county, SD
        Flint (August 28, 1990)   Plainfield F5 tornado
        Green Bay (September 6, 1995)  F2 tornado Rice county, MN
        Dodge City (June 6, 1990)  Limon, CO tornadoes
        Friona, TX (June 2, 1995)  Large tornadoes near Dimmit and Friona, TX
        Amarillo, TX (March 19, 1982)  Long-track F4 tornado TX, OK panhandles
        North Platte (May 17, 2000)  Large tornado at Brady, NE
        Dodge City (May 16, 1995) Tornadoes in western Kansas

The EML is present over East India and Bangladesh from late-March until early-May. I will post some of these soon.