VALID 06:00 UTC Sun 09 Feb 2020 - 05:59 UTC Mon 10 Feb 2020
ISSUED 21:47 UTC Sat 08 Feb 2020
ISSUED BY: Dan
A very active day of weather is expected across the British Isles, as a deep area of low pressure tracks eastwards close to northern Scotland. The strong north-south pressure gradient will lead to widespread strong winds, regardless of any additional convective component - and as such we are primarily interested in the additional damaging convective gusts / tornado potential on top of the pre-existing strong wind field (which is already covered in warnings by the Met Office and Met Éireann).
The environment will be strongly-sheared, with strong winds throughout the vertical, largely uni-directional. A strong LLJ (90-100mph winds at 850mb) will traverse and strengthen eastwards across England and Wales during Sunday, aiding advection of warm, moist low-level air immediately ahead of the surface cold front. A notable temperature/dewpoint gradient will exist across the cold front, with a slight wind veer. Model guidance suggests 100-300 J/kg CAPE will be available, while the forward motion of the front will aid in forced ascent. A remarkable 40-50kts of shear will exist in the lowest 1km!
All-in-all, the atmosphere will be primed for the development of one or more squall lines, fracturing at times to evolve into LEWPs. Some sporadic lightning is possible in places, hence the introduction of a SLGT. Transfer of high momentum air aloft down to the surface in downdrafts could lead to brief damaging straight-line wind gusts at ground level of 75-85mph (especially East Midlands into East Anglia). Such outflow combined with local topographical features could distort the low-level wind field sufficiently to generate bookend vortices and hence pose a risk of a few tornadoes, perhaps locally strong. A SVR has been introduced to highlight this risk. Of course, it is incredibly difficult to pinpoint exactly where this may occur, and the vast majority of the area will not see any tornadic activity - but should any fractures develop within squall lines, which seems likely, then this will increase the risk of a tornado on a local scale.
By mid-evening most squall line activity will have cleared to the English Channel and nearby Continent, leaving a rather more classic setup of cold air aloft and frequent showers piling into western areas, but moving well-inland on strong steering winds and more organised troughs in the flow. A few sporadic lightning strikes will therefore be possible on Sunday night, primarily over open waters and near exposed western coasts.