Convective Outlook: Mon 21 May 2018
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Convective Outlook

VALID 06:00 UTC Mon 21 May 2018 - 05:59 UTC Tue 22 May 2018

ISSUED 05:28 UTC Mon 21 May 2018


A shortwave will slide across SE England on Monday morning, rotating around the northern periphery of the main upper low over France. This will be accompanied by slight warming of the low-levels, while also cooling aloft. Some elevated convection seems likely during the morning hours over SE England, though the depth convection may be somewhat limited - hence uncertain if there will be much lightning activity.

Nonetheless, this shortwave becomes the focus for deeper convection during the afternoon and evening hours as it continues to drift westwards. Model guidance varies in coverage and location of showers / thunderstorms, but 500-1,000 J/kg CAPE and low-level wind convergence/orographic forcing will likely aid the development of at least some isolated to widely-scattered showers and a few thunderstorms. A marked dewpoint boundary over the northern / western Home Counties might also aid initiation. A SLGT has been issued where better multi-model consensus exists, though isolated thunderstorms may be possible as far north as NW England, for example.

Deep layer shear is rather weak, and so convection will be poorly-organised and pulse-type (an individual thunderstorm will only have a life cycle of less than 1 hour for example). Main threat will be localised surface water flooding (due to slow motion of showers) and gusty winds (particularly when showers/storms collapse at the end of their life cycle). Hail up to 1.0cm in diameter will be possible with the strongest cells. Main uncertainty involves just how many showers may actually develop, and then whether they become deep enough to produce much in the way of lightning given the weakly-sheared environment. Some uncertainty also exists over the exact W-E position of the SLGT area in E Wales / W Midlands, which may need to be adjusted.

Any showers / thunderstorms that do develop will begin to fade towards sunset as nocturnal cooling of the boundary layer commences.

Upper low over NE France will drift close to SE England on Monday night. On its northern flank, higher 850mb WBPT airmass will attempt to advect across the North Sea from the nearby Continent, though some uncertainty as to how far west this will reach, depending on the shape/location of the upper low.

Elements of medium-level instability are likely to spread west from NE France / BeNeLux towards East Anglia and SE England, and so it is possible some elevated showers or thunderstorms may drift in this general direction during Monday night, but probably with a weakening trend as instability reduces as one comes further west. For now, have refrained from issuing a SLGT, but one may be required if conditions improve.