Extreme Dutch Weather: Tuesday 28-06-2011; weak rotation, mammatus and someone spotted tornado near Kaatsheuvel

After a very hot day eventually from the south / south west a line of thunderstorms arrived. Weatherphenomena reported: downburst, short and weak tornado, 2-3 cm hail. Weatherphenomena spotted myself: weak rotating updraft, mammatus, funnel-like appearences, a good rainrate and some nice gusts. Later in the evening and during the night I thought some cells in the region were suspect to be supercells as well. In the morning there also was some convective activity; my wife made a photo of a weak shelfcloud.

A TV team from 'Uitgesproken EO' was with me in the afternoon and during the broadcasting they even went 'live'. I waited for the things to come at the 'Oude Maas' near Barendrecht.

Estofex gave a Level 2 for our region this day.

" ... Northern France, Benelux countries, extreme western Germany ...

Considerable CAPE has developed through an elevated mixed layer/moist boundary layer combination that is sustaining several mesoscale convective systems. The influence of their remnants will be relevant to the convective scenario to unfold on Tuesday.

Model guidance has not been consistent during Monday with regard tot eh location of the surface low that should develop over France, and move northward over the Benelux or southern North Sea during the day.

Greatest severe potential appears likely in case the low moves slow and a flow with an easterly component is maintained ahead of it, as this should result in the best overlap of instability and deep-layer shear possibly including sufficient helicity to support storm rotation. It is of concern that none of the available models simulates the MCS that is developing over the Bay of Biscay.

Will sketch a scenario here that some convection will be ongoing over the North Sea, English Channel, (north)western parts of France at daybreak and that mid and upper-level clouds to the east of this activity will have enough breaks for considerable surface-based instability (around or above 2000 J/kg) to redevelop. This should not be a problem courtesy of climatologically very high low-level moisture with dew points of 20-23 forecast in the presence of steep mid-level lapse rates.

The combination of such instability with moderate wind shear (around 15 m/s in the 0-6 km layer) should allow a couple of severe multicell storms to form during the afternoon with large hail and damaging wind gusts.

During the mid- to late evening, as forcing related to the approaching mid-/upper-level trough sets in, storm coverage should increase and develop eastward. The high moisture (40 mm of precipitable water) favours locally high precipitation accumulations and an attendant flash flood threat. "