A Peculiar Precursor
At night, temperatures typically decrease due to radiational cooling at the surface and does so faster than the air above the ground. This results in warmer temperatures aloft.. This is called an inversion. This occurrence was observed during the overnight hours on the 28th and after midnight on the 29th. It would end up being enhanced due to a very warm and dry air-mass west of the Carolinas. The air seems to have originated over the deserts, west of the Central Plains. Subsidence, or sinking air (warmth results from the sinking) also played a part.
This was valid for 925mb (less than a kilometer abover the surface)
Prior to the arrival of the very warm air above the surface, it was predicted by computer models, like the RAP. Below are some modeled soundings for Danville, VA (KDAN) and Winston-Salem, NC (KINT) valid at 4AM (EDT) on the 29th. Soundings gives a representation of how temperatures and dew-points are changing with height/decreasing pressure. Both cities are east of the foothills. They reveal the remarkable vertical extent of this subsiding warm and dry air being advected into the region.
BUFKIT Screenshots: Temperatures(red) and Dew-Points(green) increase as the profile goes to the right
Note at the bottom of each sounding the sharp increase in temperature just above the surface. This is the inversion discussed earlier. Most night-time inversions near the surface aren't typically this pronounced. Basically, these soundings indicate that the air just above the surface was about 10 °C (or about 18 °F) warmer than found at the ground. VERY balmy air!
Mt. Airy and N. Wilkesboro are cities in the foothills and are higher in elevation than the cities referenced above. The following are temperature and dew-point profiles for both foothill towns during the early morning hours on the 29th...
Temperatures / Dew Points for N. Wilkesboro
Temperatures / Dew-Points for Mt. Airy
So despite it being overnight, temperatures in both cities skyrocketed during a very short period of time, accompanied with a marked drop in dew-points. In North Wilkesboro, this phenomenon occurred around 2AM EDT (6Z). But in Mt. Airy, it didn't occur until around 4AM EDT (8Z). My reasoning for this general occurrence is that these towns had a high-enough elevation for the abnormally hot and dry air to sink to the surface there.
My Personal Experience:
I was awoken around 4AM and told to go outside. It was one of the most eerie feelings I've had. A breeze accompanied this air too. The air was so warm! Little did I know that I'd experience another freakish weather event about 18 hours later ...
EPIC WINDS!
A large thunderstorm-complex broke out early in the afternoon in Illinois. During the next 11+ hours, it would wreak havoc on the Midwest and Mid-Atlantic States. Substantial wind damage reports resulted during this time across these areas.
Let's take a look at the ingredients that were in place for this storm.
The above graphic indicates that some very hot air was in place at the surface in these regions. Temperatures were over 20 °F above late-June norms in some places.
Instability was a big key as well. As the figures below show, significant values of this measure, CAPE, were present. In addition, DCAPE values over 800 J/kg is used to estimate the potential for downbursts and straight line winds. Again, substanital values were seen across these regions.
The "Lifted Index" gives an idea of how strong the instability is. Generally speaking, the lower the index, the quicker the increase in acceleration of the updraft. Very low values, like those displayed above, are generally regarded as being associated with severe weather.
Not all contributing factors, like wind shear, have been discussed. There are elements of the environmental wind-shear that result in the enhancement of systems like this(1). Bowing structures, like this particular event, have unique dynamics that help fuel them.
Across this area, favorability for intense updrafts and subsequent strong downdrafts were present. In essence, the right mix was in play in the atmosphere for long-term duration of the line. This article, which describes the event in better detail, cites a frontal boundary draped west to east as the trigger for the event. As a result, over 20 people perished from this system. Long-lasting power-outages were a big problem too.
My experience
Here in NW NC, severe thunderstorm warnings were issued around 45 minutes ahead of the storm's arrival! I actually had no idea that a storm was raging toward us until I heard the wind pick up outside. I'd had a long day of work, so I was oblivious to the current weather situation. During the event, I went outside (as we had no trees nearby our house). The wind was scary and incredible. There were maybe just a few rain-drops. The intense winds were picking up dust. A massive tree-branch on a neighbor's tree was nearly ripped off. Some power-outages happened and power-flashes could be seen in the distance. I was in awe. The kicker is, we barely felt the brunt of the storm. I haven't experienced many rare weather phenomena in my life, but it was amazing that two of the events on my list happened within 24 hours of each other. In fact, they probably are still at the top.
Sources:
1. "Mesoscale Meteorology in the Mid-Latitudes," Markowski and Richardson, page 250
2. See NWS link two paragraphs above
EDIT: 08/22/2017 - I believe I made erroneous statements about the reasoning why the storm wasn't as potent in NW NC as it was across Central Virginia.
Instability was a big key as well. As the figures below show, significant values of this measure, CAPE, were present. In addition, DCAPE values over 800 J/kg is used to estimate the potential for downbursts and straight line winds. Again, substanital values were seen across these regions.
The "Lifted Index" gives an idea of how strong the instability is. Generally speaking, the lower the index, the quicker the increase in acceleration of the updraft. Very low values, like those displayed above, are generally regarded as being associated with severe weather.
Not all contributing factors, like wind shear, have been discussed. There are elements of the environmental wind-shear that result in the enhancement of systems like this(1). Bowing structures, like this particular event, have unique dynamics that help fuel them.
Across this area, favorability for intense updrafts and subsequent strong downdrafts were present. In essence, the right mix was in play in the atmosphere for long-term duration of the line. This article, which describes the event in better detail, cites a frontal boundary draped west to east as the trigger for the event. As a result, over 20 people perished from this system. Long-lasting power-outages were a big problem too.
My experience
Here in NW NC, severe thunderstorm warnings were issued around 45 minutes ahead of the storm's arrival! I actually had no idea that a storm was raging toward us until I heard the wind pick up outside. I'd had a long day of work, so I was oblivious to the current weather situation. During the event, I went outside (as we had no trees nearby our house). The wind was scary and incredible. There were maybe just a few rain-drops. The intense winds were picking up dust. A massive tree-branch on a neighbor's tree was nearly ripped off. Some power-outages happened and power-flashes could be seen in the distance. I was in awe. The kicker is, we barely felt the brunt of the storm. I haven't experienced many rare weather phenomena in my life, but it was amazing that two of the events on my list happened within 24 hours of each other. In fact, they probably are still at the top.
Sources:
1. "Mesoscale Meteorology in the Mid-Latitudes," Markowski and Richardson, page 250
2. See NWS link two paragraphs above
EDIT: 08/22/2017 - I believe I made erroneous statements about the reasoning why the storm wasn't as potent in NW NC as it was across Central Virginia.
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