Chase July 10: III
2010.07.18
A massive destructive shelf cloud in the West of Venlo (Netherlands), exact location can be found here…
The “n” th chase with Peter and Bart this week and we finally got to see something… not that we never see anything but sometimes models and reality are so different that using your eyes and some internet (if there is any) are the only options left open… Sometimes the lack of internet is good, you are bound to use your eyes and taking decisions in those moments are completely different than www related choices…(radar, models and other electrical influences…)
We started early with a drive towards Eindhoven. Just before leaving Belgium we stopped at exit 26, an excellent spot with a wide view from where we easily could gain Eastern kilometers in no time. When I say early, well… it was…left 10 AM for a nice wait near Retie… ideal for some macro photography, there was plenty of time… After a last check on the radar images we decided to move on, due to the lack of activity in the part that was moving towards us. We left the neighborhood of Retie around 17 PM for a ride towards Venlo and further if necessary… Of course, as usual, as we should know since long… once a convective system started its progression, especially with the amount of jet, it is difficult to stay ahead of its progression… We unanimously stopped the chase van here…
Arrived at our photography location around 17.34 pm with a short wait as future. Shanti (our chase dog) disappeared into the car after 15 minutes wondering whatever we were doing watching from a protected little corner… Witnessed a very disturbed, messy shelf cloud due to the collision with another expanding thunderstorm next to it which produced it’s own “shelf”. A fight of air, moist, temperature and progression… we were lucky not to be in the path of the new born…
Left at 18.02 PM in hail, rain and severe wind gusts…
Tried to get in front, at moments I was pretty confident… 
…but and as usual, especially in shelf cases, once the system passes overhead there is no way of getting back in front… Learned the lesson multiple times but we always seem to make the same mistakes… Stuck in rain and lightning, massive amounts of water on the roads, here and there a tree laying down it’s last moments on the road, we searched for some internet…
And found some… only to realize that it was over… Hopeful signs of a electrical shower somewhere in the North of France… on our way to Gent…
A shelf cloud is a low, horizontal wedge-shaped arcus cloud. Unlike a roll cloud, a shelf cloud is attached to the base of the parent cloud (usually a thunderstorm). Rising cloud motion often can be seen in the leading (outer) part of the shelf cloud, while the underside often appears turbulent and wind-torn. Cool, sinking air from a storm cloud’s downdraft spreads out across the surface with the leading edge called a gust front. This outflow undercuts warm air being drawn into the storm’s updraft. As the cool air lifts the warm moist air, water condenses creating a cloud which often rolls with the different winds above and below (wind shear).
As Peter reported already a few days ahead, the 21 of July would be a good day for a chase. I left around 2.30 PM for Gent. Late as usual, when I arrived Peter was quite excited. Bart was already there and together they were checking out the latest radar and sat images.
Models are insofar constistent with each other, that they all produce precipitation in the late afternoon and/or evening across the Benelux and NW Germany, and some further east along the warm front as well. But some models produce only little convection. Apparently, the overall forcing for upward motion is not very large. Still there is confidence that significantly more than just one or two storms will develop. Because of the excellent kinematic and thermodynamic parameters, the storms have a significant potential to become severe, and a level 2 is issued as a consequence. The storms will likely include supercells. Storms across the Benelux and northern Germany will be capable of producing large hail and damaging gusts, and also tornadoes. The tornado risk will be largest with those cells that tap most of the helical low-level flow: those that ingest the most backed low-level winds. The storms are expected to continue throughout the evening, and new storms could even develop overnight. However, those will probably be elevated and because of low-level cooling, they will be less likely to produce severe winds or tornadoes.)