Convective Outlook: Tue 13 Sep 2022
LOW
SLGT
MDT
HIGH
SVR
What do these risk levels mean?
Convective Outlook

VALID 06:00 UTC Tue 13 Sep 2022 - 05:59 UTC Wed 14 Sep 2022

ISSUED 06:07 UTC Tue 13 Sep 2022

ISSUED BY: Dan

A cut-off upper low over Biscay will slowly extend eastwards across northern France on Tuesday, the flow aloft attempting to advect a very warm, moist airmass northwards (>18C 850hPa Theta-W). At the same time, a surface cold front will tend to straddle southern England, undercutting this warmer air aloft. Multiple waves of precipitation may develop close to this thermally-active frontal boundary during this forecast period as subtle disturbances aloft encourage weak ascent - model guidance varies in how much precipitation will be present along the frontal boundary. To the south of the front, over the English Channel and northern France, a notable EML will begin to slowly erode as the upper trough extends from the west. Nonetheless, there is substantial instability above the EML to generate elevated thunderstorms at almost any time through this forecast period, although the risk is arguably higher during the evening/overnight period due to increasing synoptic-scale ascent over the area. Despite fairly weak cloud-layer shear, the magnitude of CAPE above the EML/800-900hPa warm nose would promote some fairly active thunderstorms.

The bifurcating upper flow suggests any storms will make slow progress northwards, and will likely weaken/split west and east on approach to southern England - therefore most activity will likely remain offshore, and also affect the Channel Islands. There is, however, some subtle differences amongst model guidance as to how far north the frontal boundary may stall, which may still put southern England at a greater risk of thunderstorms if the boundary is further north than anticipated. If convection develops widely over northern France then this may also impact the thermodynamic profile over the English Channel (and could result in a decrease in potential).