Convective Outlook: Wed 20 Jul 2022
LOW
SLGT
MDT
HIGH
SVR
What do these risk levels mean?
Convective Outlook

VALID 06:00 UTC Wed 20 Jul 2022 - 05:59 UTC Thu 21 Jul 2022

ISSUED 07:54 UTC Wed 20 Jul 2022

ISSUED BY: Dan

An upper low will slowly meander eastwards across southern England and the English Channel during Wednesday daytime. At the surface, a warm and moist airmass persists with dewpoints 15-17C. Diurnal heating will result in some mixing across East Anglia and SE England with dewpoints reducing a little here, but they should remain in the 15-18C range across the Midlands and central southern England. Assuming cloud breaks sufficiently to maximise surface heating, instability will increase with 500-1,000 J/kg CAPE possible in southern Britain by mid-late afternoon. The relatively slack surface flow here will encourage low-level convergence and the potential for scattered showers and thunderstorms to develop through the afternoon and early evening hours. Similarly, a separate convergence zone in eastern England may also provide the focus for heavy showers and the odd thunderstorm. One concern is exactly how deep convection can become, given evidence of warming in the upper troposphere.

Beneath the main upper trough axis shear will be fairly weak, although northwesterly mid-level flow will increase across central southern England later in the afternoon as the upper low slowly clears eastwards. Showers/storms may be rather messy and pulse-type at times, but some slight organisation of cells later in the day in southern England is possible. Have tentatively introduced a MDT where thunderstorms may evolve into a cluster late afternoon into the evening. Biggest threat will be localised flash flooding given slow-moving torrential downpours falling on very dry ground. The most intense cells may produce some sub-severe hail (1.0-1.5cm diameter), while a couple of funnel clouds/weak tornadoes will be possible given vorticity stretching in the vicinity of low-level convergence. Showers/storms will only slowly fade during the evening hours, and given the proximity of the upper low to East Anglia/SE England there may continue to be further development overnight.