DISCUSSION: Flooded streets, hail covered ground, damaged buildings, and power outages were abundant throughout parts of western Europe yesterday. These were a result of an MCS (Mesoscale Convective System) that unleashed some heavy winds, severe thunderstorms, and large hail across Spain, France, and Germany.
The conditions were perfectly set up for this storm to brew: it began with some small thunderstorms that popped up near a trough in northwestern France that moved north/northeast into a more conducive environment for severe weather (one with stronger shear). This allowed those thunderstorms to become more organized. As they continued to move into areas with higher CAPE and instability, the weather quickly escalated to more severe levels and strong supercells formed. At this point, hail began to fall in the region; some hail as large as softballs had been reported throughout parts of Spain and France. As the trough and associated cold front continued to move east, more storms initiated with it. These smaller storms would basically be absorbed into the larger storms, and with the weak upper atmospheric winds and the slow push of the cold front, it created an MCS over Germany.
An MCS is a collection of multiple storms that create a massive, slow-moving, and very strong system whose duration can last for hours, producing immense flooding from intense rains, widespread damage from hail and winds, and power outages from concentrated and frequent lightning. An MCS, such as this one, has high cloud tops and a cold ring encapsulating the warm system, indicative by satellite and infrared radar imagery (see above, Fig 1, courtesy of kachelmann via Meteo Europe). To demonstrate the powerful quality of an MCS, a graphic below (Fig 2, courtesy of Meteo Europe) maps out the 4,400 strikes that occurred within only 2 hours over NW Europe from the MCS last night.
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©2017 Meteorologist Katie McCracken