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Convective Outlook - Sunday 26th May 2024

Updated: May 29

Verification: Rating: 8.0/10



Discussion: An occluded front associated with an upper low will bring extensive cloud cover and showery outbreaks of rain to many parts of the British Isles early on Sunday. These showers are expected to gradually clear to the north, potentially breaking up due to insolation (surface heating) as the day progresses. This could result in the heating of the moist low-level airmass, especially from late morning onwards. Combined with a mid-level cold pool drifting NNE throughout the day, this will allow for 500-1000 J/kg of MLCAPE across parts of England and eastern Wales.


Numerous heavy showers and thunderstorms are likely to develop, particularly in convergence zones, most likely around Northern and Eastern England. Limited deep-level shear will prevent the development of severe and long-lasting thunderstorms, favouring pulse-type storms instead. However, a slight improvement in shear (DLS >20 kts) across northern England and parts of the East Midlands could support somewhat longer-lasting thunderstorms in these areas. As these cells mature and drift NNE, the greatest lightning risk will be in the Midlands, northern parts of East Anglia, and Northern England. A moderate risk has been issued for some of these areas. Diurnal heating could also trigger isolated thunderstorms in central southern England (most likely around Salisbury Plain) as the mid-level cold pool begins to move northward from 11z.


The strongest cells may produce hail (0.5-1 cm) and gusts of 40-50 mph. However, the lack of substantial shear will reduce the risk of severe weather. Additionally, low PWAT (precipitable water) will limit the risk of localized flooding, although some areas might still experience localized flooding due to the training effect of showers.


In Ireland, scattered heavy showers are also likely across central and southern parts due to diurnal heating and low-level convergence zones. However, the lack of CAPE will limit the amount of lightning in these regions. Wrote: @Handry_Outlook


Technical Discussion: Surface heating a moisture convergence behind a clearing frontal system should allow surface storms to form across the UK and Ireland, with lightning from them potentially becoming frequent at times. 200-800 J/KG of SBCAPE should allow for widespread pulse-type storms to occur, likely strongest in the midlands and northern parts of the country. Perhaps also in eastern Ireland. There's also a chance of some small hail, funnel clouds, and small-scale flooding. Strong lapse-rates help lift buoyancy and allow for some vigorous convection to begin taking place early afternoon, with storms cycling throughout the day. Also helping favour some small hail development and the low LCL's give risk to a few funnel clouds. Most lift stays at the surface and doesn't translate to strong post-boundary layer lift as most surface heating days tend to be, especially when there's a relatively moist mid-level area, also lowering the risk of lightning. Similarly lowering the risk of lightning is the short convective cycle where most energy is held below freezing which also lowers hail risk as well. There is still a chance for modification of the airmass for air parcels to have more post boundary layer lift if the mid-levels cool, that would also increase hail risk. Similarly, the energy could increase as sometimes people can do saturation I find is overestimated and if there's drier air in the mid-levels with a stronger northwards push of the frontal area - there's strongly dry air in the SW during the afternoon but with weaker energy due to other circumstances - then the energy and lightning risk increases. The main convergence on the best factors is in the NE and Lincolnshire with drier air at the mid-levels wrapped up in a stronger decreasing pressure compared to the heating flux into the post boundary layer area because of cooler uppers. With converging vorticity reaching up to around 9-10km. This naturally increase the lapse-rates and buoyancy. Along with allowing for nearly 2000 J/KG of MUCAPE in some scenarios. Despite weak shearing this should allow for 10km storm heights and the strongest lightning and hail risk. Along with some wind change with height increeasing the risk in the isolated tornadoes there.


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