Discussion: As the US has dealt with copious amounts of severe weather over the past several months, across the pond Europe severe weather season continues as well. There will be a localized region of possible severe convection across far northwestern France this afternoon/evening (Tuesday July 22, 2019) and across portions of south to southwestern UK into the overnight. This region will be the focus of this article.
Rapid destabilization will develop across extreme northwest France during the afternoon hours Tuesday. A moist air mass with dewpoint temperatures in the mid and upper 60s Fahrenheit lead to moderate CAPE across this region. In the mid and upper levels of the atmosphere, a short-wave trough approaches by the late afternoon and evening hours just to the northwest of France. This system along with potential surface boundaries may generate enough lift to allow for isolated surface thunderstorm development. Some model guidance point to convection staying just to the north mainly in the English Channel perhaps in due part to the short wave being just slightly off to the northwest. A cap in the lower portions of the atmosphere (a lid of warmer air that limits rising air and hence lowers storm chances if this feature cannot be overcome) will be another possible limiting factor to convective development. Real-time observations and trends in satellite data will likely be needed to determine convective likelihood in this region. Regardless, if storms do initiate during this time, even if isolated, there will be the potential for a few strong to severe cells with frequent lightning, strong gusty outflow winds, and large hail as the parameters used to forecast deep moist convection are rather robust in this scenario.
The image above is a forecast sounding from the GFS for far northwest France during the late afternoon hours. A couple of key features are present in this vertical profile of the atmosphere. Abundant CAPE and moisture (high low-level dewpoint temperature values) is present and hence a very unstable environment has developed in this forecast scenario. The bulk wind shear (Sfc-6km) is modest in the lower to mid-30 knot range, good enough in theory for multi-cells or possible/transient supercells. Another important factor is present, the cap (numerically represented by the negative CINH values in the sounding, graphically as the bulge or relatively warmer temperatures near 850 mb). The question is whether air can push through this feature to generate robust surface-based storms. The amount of daytime heating as well as if any surface boundaries/convergence can develop and how strong will determine if this can happen. Otherwise either elevated storms will develop with slightly less severe tendencies or very little develops at all.
While uncertainty may exist regarding convective initiation and coverage across far northwest France, this is not the case across portions of the UK. Storms will either initiate during the late afternoon hours (which looks less likely and would be less widespread) or hold off and push into the region during the evening to overnight hours and as a result will be elevated. In addition to anonymously moist air and elevated CAPE, the robust shortwave will impact portions of western and southwestern UK creating the necessary lift for strong to severe convection. Frequent lightning, strong gusty winds, and even large hail will remain a possibility across southern perhaps into the central portions of the UK. Regardless of the widespread nature of the strong winds and hail, areas of heavy rainfall may be an issue given the high moisture content and the possibility for storms to congeal over the UK.
The images above represent the four major ingredients for deep moist convection: moisture, instability, lift, and wind shear. The diagrams (courtesy of pivotal weather) via the GFS model are timed out to be either during the late afternoon hours or the overnight. One thing is for certain and that is the moisture present across W/NW France and the UK as this system approaches is anonymously high. The instability or convective available potential energy (CAPE) is modest along with the wind shear which is mainly limited to coastal northwest France and into S/SW United Kingdom. Notice as the shortwave approaches it appears to remain to the northwest and in closer proximity to the southern and western portions of the UK with stronger lift and therefore more widespread storm coverage. The high wind shear and CAPE values will allow for the potential for large hail and gusty outflow winds where storms do develop, especially across far southwest UK and far northwest France.
The broad picture of this discussion was guided via the European Storm Forecast Experiment or “ESTOFEX”. The maps included in this discussion are mainly from model websites and not affiliated with the organization.
This organization is similar to the Storm Prediction Center in that it forecasts for convective storms across Europe however they do not issue the watches, warnings, etc. The forecasters are volunteer with the purpose being to education and provide awareness. The ESTOFEX does issue convective outlooks, forecast discussions, mesoscale discussions, etc. If you want to check out the website the direct link is below where you can explore European convective forecasts daily among a few other neat sections of the site.
To learn more about European convective storms, weather patterns, and more, click here!
©2019 Meteorologist Joe DeLizio