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I know that a majority of thunderstorms form in the daytime due to convection from sunlight heating in a moist, unstable airmass. Once the sun begins to set, these storms dissipate.

But on some mornings (in the summer, of course) I'll check my local NWS radar (in northern Ohio) before I set out for work and see that it's alive with storm cells that have popped up from seemingly out of nowhere. They are not existing storms that moved into the range of my local radar, but brand new cells that just appeared about an hour before. These storms usually contain a lot of lightning and bring sustained heavy rain, sometimes to the point of minor flooding.

What causes early morning storms to pop up like this when it's typically the coolest time of the day and convection is at a minimum? It's something I've been thinking about for quite a while now.

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    $\begingroup$ could be passage of a synoptic scale event that is unrelated to diurnal heating $\endgroup$
    – gansub
    Aug 31 at 1:14
  • $\begingroup$ wavy.com/blogs/… stationay front in Ohio couple of weeks back. $\endgroup$
    – gansub
    Aug 31 at 5:12
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Do you have a specific date and time that comes to mind? There are a couple of different causes of nocturnal convection that I immediately think of. I'll list a few, and expand on the one that I think is most likely to be the culprit.

  • As one commenter named, a synoptic scale feature, such as a cold front or occluded front could produce a lifting mechanism that taps into the residual layer.
  • Remnants of a tropical cyclone. Given the scenario you described, this is unlikely.
  • Mesoscale Convective Systems that begin to form during dusk, and propagate well afterwards. Given what you described, this may not be the case.
  • Bores and Atmospheric Gravity Waves that initiate convection. This is definitely a possibility, but it might require convection upstream.
  • The likely culprit is the influence of a low level jet (LLJ). The LLJ is formed by the temperature difference between the Rockies and the Plains. The temperature gradient driving the LLJ might be an oversimplified reasoning for the driving of the LLJ, but all of the mathematics and theory behind it, such as the alignment of the geostrophic and ageostrophic wind, deserves a full blown lecture. Why ramble on about these LLJs? They can initiate convection through low level convergence.

What all of these have in common is that they are all called forced convection. This paper that I found also lists a couple of mechanisms, some of which I mentioned, but others I am not that familiar with (elevated convergent layers, MPS circulation).

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