Convective Outlook
VALID 06:00 UTC Fri 20 Jul 2018 - 05:59 UTC Sat 21 Jul 2018
ISSUED 20:04 UTC Thu 19 Jul 2018
ISSUED BY: Dan
Upper trough over the British Isles and Western Europe will gradually dig southwards, while slowly nudging southeastwards. On the forward side, a plume of high ThetaW air (and associated mid-level instability) will attempt to drift northwards from France towards SE England - but probably struggling to make much progress across the English Channel. As such, a few residual showers from mid-level origin are possible over SE England and perhaps East Anglia on Friday morning (but little, if any lightning) - though some model guidance (i.e. 12z EURO4) suggesting this may form into a more organised area of dynamic rain, which may have implications for potential convective developments later in the day.
Meanwhile, a slack pressure pattern over southern and eastern England will promote convergence zone development. In the absence of any significant upper forcing, this CZ may provide enough of a trigger for a few isolated to well-scattered heavy showers to form, particularly so during the late afternoon and into the evening hours in a zone from Dorset/Hampshire NE-wards towards Cambs/Lincs (including W London). PWAT 25-30mm and slow storm-motion will bring the risk of locally high rainfall totals and the threat of surface water flooding, especially (a) if one occurs over an urban area, and (b) considering very dry ground and high surface runoff following prolonged dry spell.
It is worth noting that there is still some considerable uncertainty over whether any showers will develop - forecast soundings exhibit a slight warm nose at 700mb which may limit the depth of convective cloud, though this may cool slightly during the evening hours as the upper trough approaches from the NW. Even if air parcels are able to pass this layer, CAPE profiles look rather skinny and given a distinct lack of shear, such convection will tend to be rather pulse-type in nature - so while there could be a few heavy showers, they may not produce much in the way of lightning. Risk of lightning considered 20% for now (and therefore below SLGT threshold) - there could be some scope for an upgrade to SLGT for parts of Berks / Bucks / Northants and perhaps W London if confidence improves a little.
Farther north, any isolated lightning from medium-level instability release over Ireland will likely ease fairly quickly on Friday morning as instability weakens - as such, no threat levels have been issued (see forecast for Thursday for more information). A LOW threat level also issued for SW England for Friday night / early hours of Saturday with some subtle hints of marginal mid-level instability release with an area of showery rain sinking S/SE-wards.
