Mesoscale Discussion
Valid: Sun 23 Sep 2018 14:00 to Sun 23 Sep 2018 18:00 UTC
Issued: Sun 23 Sep 2018 14:04
Forecaster: GROENEMEIJER

DISCUSSION

Convection is developing along the cold front, which was located along a line from Paris to Koblenz to Chemnitz at 1300 UTC. An isolated thunderstorm is quickly moving eastward from the Nancy area.

Surface observations indicate that low-level humidity in the prefrontal air-mass is slightly higher than most model guidance, with dew point temperatures being generally 1-3C higher. Furthermore, surface temperature across Switzerland and southern Germany are 1-3C higher. This suggests that a bit more CAPE may be developing than forecast.

Overall, the developments seem to follow that described the forecast issued last night. There is some concern that the higher temperatures across Switzerland and South Germany will lead to higher wind gusts than simulated. That said, this effect may be mitigated by the higher coverage of prefrontal convection, which will act to cool the boundary layer before frontal passage.

Regardless, the scenario remains that convection along and ahead of the front will gradually intensify. The frontal passage will be associated by gusts around 25-32 m/s in many places, although slightly higher values cannot be discounted, particularly in areas that have been heated more than modelled.

Should any prefrontal storms become well-organized, which depends on whether the instability is able to compensate the high shear, they will bear a risk of tornadoes. Given the very strong low-level shear, any tornado that occurs may be strong. Tornadoes are not ruled out along the surface front either. This risk is the highest near the surface low centre in a zone across central Germany.