VALID 06:00 UTC Sun 06 Jun 2021 - 05:59 UTC Mon 07 Jun 2021
ISSUED 06:14 UTC Sun 06 Jun 2021
ISSUED BY: Dan
A weakening frontal boundary will drift slowly northeastwards across Britain on Sunday, associated with moist low-levels and patchy rain. Through the day diurnal heating and resultant convective overturning will likely cause initially extensive cloud to gradually thin and break and turn increasingly convective in nature with time. A mid/upper-level shortwave trough will also drift slowly northeastwards through the day, the associated cold pocket aloft aiding to slightly steepen mid-level lapse rates and increase instability - with scope for 300-600 J/kg CAPE in places. The net result is initial patchy rain will translate to scattered heavy showers by the afternoon and evening hours, particularly focussed along convergence boundaries (sea breeze and peninsula) and orographic forcing (especially Wales).
A marked dry intrusion aloft will spread gradually northeastwards with time, although the exact shape/timing varies between model guidance and this may have some impact on highlighting specific areas with the best lightning potential. In theory, the leading edge of this dry intrusion holds the best potential, as ultimately the following substantial subsidence aloft will tend to restrict the depth of convection. Therefore a favourable overlap of low-level forced ascent and leading edge of dry intrusion will be key for identifying region(s) with potential for deepest convection / lightning potential. Either way, numerous showers are likely to develop, but only a few may produce some sporadic lightning. Given light winds, shear in the low-levels is relatively weak and so many showers will tend to be of pulse-type variety - however, if deep convection can grow tall enough to utilise the stronger mid-level flow, then these may be able to last longer and produce some hail. The very moist low-level environment (dewpoints of 14-15C) and associated low cloud bases suggests at least the potential for a few funnel clouds or weak tornado, especially near convergence boundaries. Showers may continue to develop well into the evening hours before a gradual decay overnight.