Convective Outlook: Mon 10 Jun 2019
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Convective Outlook

VALID 06:00 UTC Mon 10 Jun 2019 - 05:59 UTC Tue 11 Jun 2019

ISSUED 17:09 UTC Mon 10 Jun 2019


UPDATE 17:09 UTC SLGT extended across S Wales (as mentioned below), and an additional SLGT introduced to SE England / East Anglia for this evening as pulses of elevated instability are released (also see below)

A rather complex pattern evolves across the British Isles on Monday, and while confidence is high in the macroscale detail it is considerably lower for mesoscale detail. The broad upper pattern sees an upper trough over the British Isles, with an upper low initially over Ireland drifting southeast to the Brest peninsula by Monday evening. At the surface, an area of low pressure centred over the Low Countries at midday will push frontal rain across parts of England and Wales from the southeast, with notable warm advection occurring over the southern North Sea on Monday night.

Highest confidence during this forecast period exists over these areas, where a combination of cold air aloft combined with surface heating will yield 300-600 J/kg MLCAPE. This, combined with low-level wind convergence and topographic forcing will enable scattered showers to develop, especially from late morning and through the afternoon, before gradually decaying during the evening hours.

Some sporadic lightning seems likely, especially over Ireland where instability will be greatest. Hail up to 1.0cm in diameter will be possible from the most intense cells, though in general shear will be quite weak and so "pulse-type" convection will tend to be the most favoured mode.

Showers (and possible isolated thunderstorms) will be ongoing across parts of Devon and Cornwall at the start of this forecast period. Otherwise, depending on how widespread the frontal rain is, there is some scope for sufficient heating of the reasonably moist low-level airmass (dewpoints 9-11C) to yield potentially 300-500 J/kg MLCAPE by Monday afternoon. Topographic forcing and low-level wind convergence could aid in the development of scattered showers and a few thunderstorms, but the extent is rather uncertain.

A blend of various model guidance would suggest the areas most at risk would be Cen S / SW England, and perhaps also S Wales - though should any lengthy sunny breaks develop elsewhere (such as SE England) then an isolated shower/thunderstorm would also be possible. A conditional SLGT has been introduced, though it is possible either (a) no thunderstorms develop at all, or (b) others develop elsewhere outside of the SLGT, but the lack of confidence limits the ability to be more precise at this stage.

Guidance varies in the exact shape and location of the surface low by Monday evening, and this will ultimately affect the potential for lightning activity over the southern North Sea and adjacent land areas through Monday night. Advection of a high ThetaW airmass will occur along the eastern and northern flank of the aforementioned low, which may push into East Anglia late afternoon / evening, and then areas farther north overnight. Forcing aloft combined with isentropic upglide may encourage the release of mid-level instability, with the potential for a few elevated thunderstorms to occur - probably staying largely offshore over the North Sea. Nonetheless, some locally very heavy rain will be possible. Given the uncertainties, have refrained from issuing a SLGT for now.