BKRPDM wrote: ↑Thu Aug 12, 2021 10:49 am
To jaded or MW, or anyone who might know:
*Customarily, storms of a severe nature seem to lesson in intensity towards the late evening or overnight. I wonder why this hasn’t been the case lately….
*Has this been a relatively rough storm season? It seems like we’ve had some calm summers the last few years. I do remember some stormy summers in the early 1980s.
My apologies for the late reply. I, too, was one of the many without electricity. I had generator power, but lost cable and internet with the Thursday morning storms. Everything's been restored today.
As MW said, what we saw was mostly elevated convection. We can explain convective instability going back to thermodynamics. Density is inversely proportional to temperature at constant pressure, so the warmer the temperature, the less mass per unit volume it occupies. Thus, the relatively weaker the gravitational force is for that particular parcel of air. Cooler parcels are then more dense and heavy than warmer parcels. So, the natural tendency towards an equilibrium is for relatively cool air to sink towards the surface, and warm air to rise up aloft.
Since pressure decreases exponentially with height, we can't make the assumption that pressure is constant with vertical motions, and thus the relationship between density and temperature is a bit more complex. That's why we use lapse rates, or the rate at which temperature decreases with height, to diagnose convective instability. The steeper the lapse rates, the sharper the vertical temperature gradient is, and the more unstable the atmosphere becomes. Since, as a parcel rises, it cools, the relative humidity of the parcel increases and reaches saturation, creating clouds and eventually precipitation.
With surface based instability, the sun heats up the ground, which heats up the adjacent lower layer of the atmosphere much more quickly than the air aloft. Evidence of surface based instability generally manifests in the formation of cumulus clouds (usually at the top of the boundary layer). If the atmosphere above the boundary layer is unstable, the cumulus clouds continue to grow eventually into a precipitation producing cloud, and maybe even cumulonimbus clouds. This is the typical mode of thunderstorm development in the daytime.
At night, the ground cools off pretty quickly, and it cools the surface layer more quickly than the air aloft. Thus, the lowest levels of the atmosphere are pretty stable. This stability can be reduced if we're in a deeply tropical airmass with high dew points because of the higher specific heat capacity of the water vapor-rich air. So, the surface is less stable than it otherwise would be if we were in a drier air mass. The added moisture also makes it easier for a parcel to reach saturation, enhancing cloud and storm processes. However, this alone isn't enough for nocturnal convection. We need the presence of unstable air above the nocturnal stable layer. This is typically done by strong winds bringing in warmer, moister air just above the boundary layer, known as the low-level jet, driven by larger scale meteorological processes. Thus, above the surface, we end up triggering deep convection like we saw overnight Wednesday into Thursday morning.
Typically, elevated convection is not severe. This is because the denser air in the nocturnal stable layer dampens the transfer of winds aloft down to the surface. However, if the surface stability isn't quite strong, the dampening effects are weaker and stronger wind gusts can still mix down to the surface. Another possibility from elevated convection is that the air aloft is so deeply unstable due to steep lapse rates that it aids in the formation of large hail, which often occurs in the great plains.
As a quick aside, another mode of elevated convection is what occurs in the cool sector north of a warm front. Warm winds like to move above a warm front, creating a layer of elevated instability triggering precip, regardless of time of day. That's often why you see widespread precipitation on the north side of the warm fronts around here. The relative dryness south of the front in the warm sector can often be explained by a thermal inversion aloft (warm air above cool air creating stability), or a lack of a trigger to initiate convection.
As to the local climatology, I have limited expertise in that. I defer to MW, Rate This, WOHO who have lived here much longer and know more about how this summer compares to decades past. I will say that this is the most active summer that I've experienced since moving here in 2012 (although I was not here during the summers of 2014 and 2015). I lived here in the 90s as well, but I was a child then and really only 1997 and 1998 stick out to me as active years from my lived experience.
