Convective Outlook
VALID 06:00 UTC Wed 10 Jun 2020 - 05:59 UTC Thu 11 Jun 2020
ISSUED 06:52 UTC Wed 10 Jun 2020
ISSUED BY: Dan
A disrupting upper trough will result in a deepening upper low sliding southwards across the British Isles on Wednesday, primarily centred over Ireland / Irish Sea and reaching the northern Bay of Biscay by 06z Thursday. Its timing potentially coinciding with diurnal heating results in a very messy forecast evolution, with a mixture of dynamic and convective rainfall in various places. Two main frontal boundaries are identified during Wednesday daytime - one lingering over eastern England into Scotland, and another straddling Irish Sea coasts. As the upper low deepens, forcing aloft will increase - especially by the evening hours. The biggest concern over convection potential in most areas is extensive cloud and resultant limited surface heating. Large disparities between model guidance results in lower than normal confidence for the local detail during this forecast period.
... SE ENGLAND / EAST ANGLIA / E MIDLANDS ...
The leading front will potentially linger over similar areas for much of the day, and will be characterised by periodic outbreaks of rainfall falling from primarily mid/high-level cloud with embedded elevated convection. Fairly extensive rain will ultimately result in a cool boundary layer and little instability. However, if the rain becomes more patchy, and if any breaks in the cloud can allow some degree of surface heating, then there may be scope for surface-based convection to develop. Forecast profiles suggest cloud tops to around 15,000ft (-15C ELTs) may be possible if sufficient surface heating can occur in the vicinity of low-level convergence. While a few isolated lightning strikes would be possible from heavy showers that may develop, the saturated profiles and weak shear suggests individual cells will struggle to maintain intensity and be slow-moving, hence the greatest threat is localised surface water issues.
Late on Wednesday night, elevated convection is likely to result in scattered showers arriving from the North Sea into East Anglia and perhaps SE England. Some uncertainty over how much lightning activity (if any) this may produce, but the risk does increase here during the early hours of Thursday,
... SW ENGLAND / WALES / W MIDLANDS / NW ENGLAND / SW SCOTLAND ...
Rather extensive cloud and patchy outbreaks of rain seem likely during Wednesday daytime, but some cloud breaks may develop and allow surface heating to occur - potentially yielding up to 500 J/kg CAPE in places. Scattered showers may develop with time, particularly over Wales, Cumbria and SW Scotland where orographic forcing will aid in forced ascent given an otherwise fairly slack surface pattern. Forecast profiles suggest that a warm nose at 550-600mb, which may initially limit the depth of convection to around 14,000ft, will be gradually eroded as the mid-levels continue to cool and moisten as the upper low approaches and deepens. Consequently, deeper convection could evolve towards the second half of the afternoon. Weak shear and saturated profiles suggests lightning may be fairly localised. Nonetheless, a couple of low-end SLGTs (30%) have been issued.
Towards the evening hours, the increased forcing aloft is likely to engage with the frontal boundary over the Irish Sea, resulting in more widespread outbreaks of heavy rain developing over Wales and SW England. This is likely to be a mixture of dynamic and convective rainfall, and may ultimately lead to an increase in lightning activity in the vicinity of south Wales and SW England during the evening hours - especially where convergence is maximised close to the axis of a developing surface low. However, weak shear, saturated profiles and limited CAPE casts much uncertainty over how much lightning activity may actually occur.
... NORTHERN IRELAND / REPUBLIC OF IRELAND ...
With frontal rain having largely cleared to the east by late morning, here a much more clear-cut forecast evolution is expected. Increasing insolation as cloud continues to break beneath cold mid-levels as the upper low slides slowly southwards atop, will result in numerous showers developing in an environment with 300-500 J/kg CAPE but weak shear. A few sporadic lightning strikes will be possible during the afternoon and early evening, before showers gradually weaken and clear southwards during the second half of the evening. A low-end SLGT (30%) has been issued.
