Expecting Scattered Elevated Showers and Thunderstorms Over the Channel Islands
VALID 06:00 UTC Tue 18 Jun 2019 - 05:59 UTC Wed 19 Jun 2019
ISSUED 20:51 UTC Mon 17 Jun 2019
ISSUED BY: Chris/Dan
Firstly, the usual caveats apply to a destabilising plume event. There is an inherently increased level of uncertainty in these type of situations, more than most other synoptic events, and even at relatively short time frames. Therefore it is likely parts of this forecast will undoubtedly change from initial thoughts, and further updates/changes may be required. It is often suggested to take a more broad-brush, ingredients-based approach rather than concentrate on specifics until observational data (radar, satellite etc) provide more convincing clues on how the forecast may evolve. We have attempted to outline the key aspects of this forecast period - but this is subject to change...
Broad upper troughing will reside over the Atlantic on Tuesday, placing the British Isles under southwesterly flow aloft. Advection of a high Theta-W airmass will likely be occurring on Tuesday morning from France into southern England as a lead impulse drifts northeastwards. Elevated convection will likely occur as a result, initially over the Channel Islands before drifting towards southern England - however, this appears likely to turn increasingly frontal with time as baroclinicity increases on the western flank, with any scope for renewed elevated convection likely to be focussed towards CS / SE England, and later East Anglia. Lightning activity with this first round of activity on Tuesday daytime is uncertain, given somewhat limited instability and subtle forcing. There is a low risk of isolated surface-based development occurring over SE England in the afternoon / early evening should enough insolation materialise.
Later in the day, backing flow as a degenerating shortwave over Biscay approaches will tend to advect an even warmer, moist low-level airmass into southern England, while heights continue to fall aloft. This will be accompanied by stronger forcing for ascent, on the forward side of the approaching shortwave. It seems likely scattered elevated showers and thunderstorms will develop on Tuesday evening over the English Channel, perhaps organising into an MCS while drifting north / northeastwards across CS / SE England, and then across parts of East Anglia and/or Home Counties overnight. Depending on the forecast evolution, there may be a couple rounds of thunderstorms. Lightning activity with these storms will likely be fairly frequent, with the threat of localised surface water issues should multiple cells move over the same area, and perhaps hail close to 1.5cm in diameter.
There is inevitably some level of uncertainty in these complex setups, with the exact timing and location of thunderstorm potential subject to change. The latest 12z/Mon suite of model guidance has generally trended farther west than earlier guidance from the past few days (with more backed flow allowing cells to track south-north than southwest-northeast), and so the SLGT has been adjusted accordingly. However, it is unclear how reliable this signal is - should this trend persist, then the MDT may also need to be adjusted. Worth noting the 12z EC, which has been remarkably consistent in recent runs, is now much slower with the main shortwave not arriving until Wednesday morning, and therefore produces minimal thunderstorm activity during this particular forecast period.
Elsewhere, scattered heavy showers are likely to develop where low-level convergence or topography will aid forcing - most notably over Wales / NW Midlands / Yorkshire, southern Republic of Ireland and NW Republic of Ireland / Northern Ireland. Later in the night, scattered heavy showers will arrive from the Atlantic onto coastal parts of western Scotland and northwest Ireland as cold advection occurs associated with the sharpening Atlantic upper trough. In all cases, convection will be limited somewhat in depth and so the lightning risk is considered rather low.