|A strong westerly jet at mid and upper levels was moving into west-central continental Europe. An unseasonably strong surface cyclone over central England was moving onto the North Sea during the day. This low caused strong gale force winds near the Dutch shore. Postfrontal convection posed the main severe threat this day as the low level wind field strengthened significantly. A 30-33 m/s speed maximum at 850 hPa moved from the Channel region over the Benelux countries during the afternoon. Main convective modes were expected to be convective lines and probably mini-supercells. The latter posed a threat of tornadoes given that the low level wind field became favourable of updraft rotation as 0-1 SRH climbed to 200 m2/s2 some distance behind the cold front.|
In the early afternoon I went to an open field to watch a passing squall. It was not that active and storm organization was pretty poor. Some sort of arcus or shelf cloud was present, preceding intense rain. Soon there after I had to rush to work.
At work I saw the back of the squall moving east and it showed some vague mammatus clouds. At 15.45 LT I saw another active storm passing by with a nice flanking line,
showing crisp convection. Bases were low and somewhat turbulent. Once in a
while a small lowering was forming underneath the main updraft, sucking up
small fractus clouds. Suddenly I noticed the movement inside the first
updraft tower of the flanking line, just behind the main updraft. The
updraft as a whole showed corkscrew movement and pretty fast as well! I have
seen rotating lowerings in Holland before, but I've never seen such clear
rotation in an updraft tower.
At about 16.00 LT the storm moved off ENE too far and I couldn't follow the
storm anymore. So no chase, but I was glad I have seen this great storm.