Convective Outlook: Mon 05 Sep 2022
LOW
SLGT
MDT
HIGH
SVR
What do these risk levels mean?
Convective Outlook

VALID 06:00 UTC Mon 05 Sep 2022 - 05:59 UTC Tue 06 Sep 2022

ISSUED 07:07 UTC Mon 05 Sep 2022

ISSUED BY: Dan

Upper low will remain anchored to the west of Munster on Monday, with several shortwaves rotating around the main parent upper low. The first shortwave will drive an area of occasionally heavy rain across Scotland during the morning hours, but this should weaken and slowly clear northwards. The associated waving cold front will have largely cleared to the North Sea, but bends back westwards as a rather diffuse warm front lifting north across East Anglia and the Midlands at 12z, reaching Scotland by the evening. Surface heating of a moist low-level airmass (15-18C dewpoints) may yield several hundred J/kg CAPE by the afternoon hours across portions of England and Wales. In the absence of any large-scale forcing, most CAM guidance is rather unenthusiastic in developing many showers/thunderstorms; however coarser-resolution parameterised global model guidance continues to suggest the potential for a few scattered showers/thunderstorms developing in response to diurnal heating (and perhaps also aided by sea breeze convergence). Examination of forecast profiles suggests some hints of capping, with CAPE also fairly tall and skinny. Therefore it is hard to ascertain if many showers/thunderstorms will develop during the afternoon, but given unstable profiles and modest speed shear it is possible isolated/well-scattered activity could occur generally south of Merseyside-Humber initially, but expanding north of this line during the evening.

During the late afternoon/early evening, the next shortwave will arrive from the south and may be accompanied by a small surface low approaching from northern France. This seems tied to a quasi-warm front as advection of 16-18 850hPa Theta-W plume occurs into southern England. Surface heating coupled with cooling aloft as the shortwave approaches may yield CAPE in excess of 1,000 J/kg CAPE across parts of SW / Cen S England, Wales and the SW Midlands. CAM guidance varies, but there is a signal for multiple lines and/or clusters of very active thunderstorms to develop over these areas during the second half of the afternoon and through the evening hours as low-level convergence increases. Given increasing wind shear during the evening (30-40kts with low-level winds becoming more backed) some of these cells could become very lively and organised, with a supercell possible if cells can remain semi-discrete. This brings the threat of large hail, locally greater than 2.0cm in diameter, gusty winds and perhaps an isolated tornado - a SVR has been included to highlight these threats (main focus Dorset, Somerset, Wiltshire, Gloucestershire and SE Wales) alongside local surface water flooding, and also across portions of Ireland for gusty winds and an isolated tornado, primarily the afternoon and early evening hours here.

Thunderstorms will continue to migrate northeastwards across the Midlands and southern England through the evening and night hours, perhaps joined by additional cells approaching from the English Channel and NE France, and perhaps also expanding into more of N/NW England for a time. Later in the night there may be an uptick in elevated convection spreading into E/SE Scotland from the North Sea.