Convective Outlook: Sat 03 Sep 2022
LOW
SLGT
MDT
HIGH
SVR
What do these risk levels mean?
Convective Outlook

VALID 06:00 UTC Sat 03 Sep 2022 - 05:59 UTC Sun 04 Sep 2022

ISSUED 06:49 UTC Sat 03 Sep 2022

ISSUED BY: Dan

A sharp prefrontal trough will migrate gradually northwards across England into southern Scotland on Saturday. The low-level wind convergence along the trough axis, combined with modest heating of a moist low-level airmass (dewpoints 14-17C) yielding around 500 J/kg CAPE, will provide an environment increasingly favourable for scattered heavy showers and perhaps a few thunderstorms, especially with time (meaning northern England/southern Scotland at greatest risk, towards late afternoon and into the evening hours). There will likely be issues with mid-level capping restricting the depth of convection in places - and so lightning activity is questionable - but this should be less of an issue towards far northern England and southern Scotland. Here, reasonable speed shear and subtle veering with height may create an environment with around 30kts DLS, which may aid cell organisation for a time. Greatest threat for local surface water flooding is SW Scotland where multiple cells may train over similar areas. Eventual upscale growth into a band of rain is anticipated through the evening and night hours, and so lightning activity will likely wane. However, an uptick in elevated convection is possible in E / SE Scotland during the early hours of Sunday, spreading inland from the North Sea. Confidence is not overly high on much in the way of lightning occurring, but tentatively introduced a SLGT here.

In the post-frontal environment, scattered showers are likely in C / W / S Ireland in a strongly-sheared environment. Limited CAPE and the potential for clusters of showers to morph into areas of showery rain casts some uncertainty over how much lightning activity may actually occur. By the overnight period model guidance diverges substantially as a pronounced dry intrusion engages with the trailing cold front over the Celtic Sea, inducing a wave or even a developing surface low. Strong forcing could result in a few active thunderstorms somewhere over the Celtic Sea, which could impact SE Ireland and/or SW Wales/SW England, but there is a lot of uncertainty over whether this will even occur let alone areas potentially affected. ECMWF and derived products have been consistent in a more active feature/greater pressure falls, and therefore favour a track further to the west into Ireland, whereas other model guidance is less-developed and further east (into more of Wales/SW England). Given strong low-level shear and backed winds in the vicinity of the cold front, there is a threat for both SLW and isolated tornadoes - and a SVR has been introduced for this risk. Some local surface water flooding may also be possible with pockets of more intense convection, especially where this trains over similar areas.