Mesoscale Discussion
Valid: Tue 18 Jul 2017 13:00 to Tue 18 Jul 2017 21:00 UTC
Issued: Tue 18 Jul 2017 13:48
Forecaster: TUSCHY
MESOSCALE DISCUSSION for
... N-France and S-UK ...
Latest remote sensing data indicates two ongoing thunderstorm clusters, one affecting SW-UK and a second one lifting from N-Brittany to the ENE. This activity developed ahead of a NE-ward drifting and deamplifying short-wave in the 300-500 hPa layer, which is also confirmed by latest WV data, showing a rather tight moisture gradient approaching the W-English Channel. This wave is forecast to move NE during the following hours towards CNTRL-UK.
Latest surface data places a warm front over far SW-UK, the E-English Channel towards Belgium. With a gradually consolidating and northward drifting surface low (in response to stronger hight falls to its west due to ongoing amplification of an upper-level trough), this front is forecast to lift north at a leisurely pace. Latest VIS loop indicates full sunshine ahead of both clusters with a deepening Cu field over NE-France in respone to moisture pooling along the warm front. Diabatic heating lowered surface dewpoints a bit, which are now around 15 °C and temperatures in the mid twenties over SW-UK and lower thirties over NW-France (ahead of both clusters).
12Z soundings show strong and even strengthening capping over SW-/S-UK, whereas full mixing and a deepening and dry subcloud layer are present over N-France.
Numeric continues to diverge substantially regarding the track of the thunderstorm clusters with one model suite taking the convection more to the north/northeast towards CNTRL-UK (probably in response to the passing short-wave), whereas another suite (with numerous limited area models) brings the activity more to the NE towards the E-English Channel.
As the MCS over NE-Brittany moves E/NE, it will take profit of uncontaminated inflow of warm/moist air and CAPE of 1-2 kJ/kg along the coast. A dry subcloud layer should support strengthening cold pool activity and a forward propagating MCS to the NE is expected. In addition, channeled NE-erly flow over the English Channel creates most intense LL outflow/inflow convergence along its NE/E-side, so a constant motion in this direction is expected. Onshore convection has a chance to root into the BL with a severe wind gust and large hail threat mainly over N/NE-France. A confined swath of severe to isolated damaging wind gusts is possible affecting the CNTRL-/E-English Channel and surrounding areas during the following hours! Towards SE-UK, a cooling BL offshore and a strengthening cap should force convection to become more elevated betimes with an excessive rainfall and large hail risk. However ongoing intense diabatic heating and expected strong NE-erly inflow over far SE-UK may keep this activity near surface based with an ongoing severe wind gust risk from onshore moving convection. During the night, this activity spreads towards Belgium and offshore towards the S-North Sea with a lowering severe risk.
The ongoing activity over SW-UK is forecast to lift north and northeast betimes in response to the passing short wave. Despite latest VIS data indicating surface based activity just offshore of SW-UK, the general mode should be an elevated one with an isolated excessive rainfall and large hail threat.
