Convective Outlook
VALID 06:00 UTC Sun 20 Feb 2022 - 05:59 UTC Mon 21 Feb 2022
ISSUED 08:07 UTC Sun 20 Feb 2022
ISSUED BY: Dan
A diffluent upper trough will drive a cold front southeastwards across Britain and Ireland through Sunday. Surface dewpoints of 8-9C are expected ahead of the cold front, with air temperatures likely to peak around 11-14C. A reasonable wind veer is simulated along the cold front (although not as sharp as some other events so far this autumn/winter season). There are also hints a wave may develop near northern England/north Midlands and exit to the North Sea. In any case, this setup bears quite a few similarities with 25th January 2014 squall line. High-resolution model guidance suggests the potential for broken line segments (perhaps even more than one line), initially embedded within a broader precipitation shield, but through the afternoon it is anticipated much of this will be eroded as a pronounced dry intrusion races eastwards across Wales and the Midlands, accompanied by a strengthening pre-frontal low level jet. This may provide extra impetus for convection to intensify as it moves across the East Midlands into East Anglia and SE England, and this in conjunction with strong forcing and modest CAPE suggests arguably a better chance of some lightning activity than compared with many other line convection events so far this season.
It should be noted that confidence on much in the way of lightning is still rather low, since convection will be limited in vertical extent generally below 5km (16,000ft), but a couple hundred J/kg CAPE condensed into this comparatively shallow layer should, in conjunction with strong low-level forcing, provide some fast upward motion - a low-end SLGT has been introduced, although realistically the risk is considered below SLGT threshold. The strong low-level shear/vorticity in the vicinity of the frontal boundary then subjected to vertical stretching suggests the risk of one or two isolated tornadoes, especially given increasing speed with height and resultant looping hodographs. In addition, high momentum air from strong flow aloft may be brought down to the surface in squalls, bringing the risk of brief damaging 70mph gusts (perhaps locally higher).
Showers will quickly follow the cold front and will persist through much of the remainder of the forecast period. These are likely to produce hail in places (and snow) and perhaps a few odd lightning strikes, more especially near exposed western coasts and where showers are forced over hills as they move inland. The main focus will be western Ireland where a favourable overlap of CAPE and shear will exist, but also potentially western Scotland. Other areas of interest during the evening/night hours include NW England and the Bristol Channel into Somerset/north Devon, although the strong low-level flow will probably prevent any substantial convergence developing. In all cases, high momentum air may be brought down to the surface in the vicinity of showers bringing an ongoing threat of damaging winds. A SVR has been introduced for the risk of localised damaging gusts of wind, but also the chance of one or two isolated tornadoes.
