Convective Outlook: Wed 20 Oct 2021
LOW
SLGT
MDT
HIGH
SVR
What do these risk levels mean?
Convective Outlook

VALID 06:00 UTC Wed 20 Oct 2021 - 05:59 UTC Thu 21 Oct 2021

ISSUED 07:00 UTC Wed 20 Oct 2021

ISSUED BY: Dan

... KEY POINTS ...
  • Daytime: Locally damaging wind possible from N Wales - N England - Humber (E Yorks/N Lincs)
  • Daytime: Marginally-severe hail and isolated tornadoes possible from any discrete cells in the Midlands, East Anglia and perhaps SE England
  • Evening: Locally damaging wind in NW Ireland
  • Evening: Locally damaging wind and isolated tornadoes near English Channel coastal counties
A rather complex pattern will evolve through Wednesday and Wednesday night, as a broad upper trough swings southeastwards across the UK/Ireland. A strong southwesterly mid-level jet will cover southern Britain, with one or two shortwaves running east-northeastwards on its northern flank through the day. The associated cold pockets aloft atop unseasonably warm, moist low-level air will yield substantial CAPE (for the time of year) in a strongly-sheared environment. These will provide the focus for enhanced activity, as scattered heavy showers and a few thunderstorms, sometimes clustering into more organised comma features, migrate along in the flow. Particular attention is given to the Midlands / Wales / SE Ireland in the morning, shifting eastwards into East Anglia, the East Midlands and perhaps also parts of northern England. Model guidance varies over the exact evolution, but there is scope for an intensifying comma feature to track across northern England that could produce locally damaging winds and perhaps an isolated tornado. In some ways, this is reminiscent of 17 November 2016 that produced a narrow zone of wind damage from Wales into the West Midlands. Much will depend on the exact evolution and phasing of various key ingredients, but particular concern is given to a corridor from N Wales / NW England and across to the Humber.
Further south, cells may be a little more discrete (but given the strong uni-directional shear could end up merging into line segments) in an environment that would be supportive of one or two supercells capable of producing large hail to 2.0cm in diameter and strong gusts of wind. There could in theory be an isolated tornado, although the main mitigating factor will be a rather strong near-parallel low-level flow (lack of any pronounced backing). Convection may be somewhat shallower in depth towards far SE England during the day under the main core of the jet aloft.

Also during the day, several boundaries are expected to merge with the southward-advancing cold front in northern Scotland. There is likely to be some rather strong low-level shear within the vicinity of the frontal boundary, and broken line segments are expected to develop capable of producing brief spells of very heavy rain and squally winds - especially from Orkney, down the western side of Scotland and into Northern Ireland and NW Ireland. There may be very little, if any, lightning but several small-scale lows may develop with these features that could result in some locally enhanced wind gusts capable of producing some damage - but the exact detail/evolution is somewhat uncertain, although NW Ireland appears most favourable.

Finally, near the base of the sharpening upper trough a deepening surface low will approach SW England late afternoon and slide eastwards across southern England through the evening/night hours. Frontal rain will wrap-around this feature, but given the strong forcing aloft coupled with substantial CAPE due to warm SSTs beneath cold air aloft, some embedded deep convection will be possible capable of producing enhanced torrential rain, some sporadic lightning (albeit confidence low on coverage) and locally damaging gusts of wind/isolated tornadoes - especially close to English Channel coasts.

Overnight there could be a few isolated lightning strikes over the North Sea and near adjacent coasts from the Northern Isles and across NE / E Scotland to NE England - but the risk is considered too low to warrant a SLGT.