Convective Outlook
VALID 06:00 UTC Mon 15 Aug 2022 - 05:59 UTC Tue 16 Aug 2022
ISSUED 06:07 UTC Mon 15 Aug 2022
ISSUED BY: Dan
An upper trough will extend southwards from the Atlantic to Biscay, the exact timing/shape subject to some error which appears to have the greatest impact on the forecast evolution across Ireland (faster evolution will restrict convection to just near the south coast, for example). Cooling of the mid/upper troposphere will continue to extend across all parts of the British Isles, atop a very warm and increasingly humid low-level airmass - resulting in more widespread unstable profiles and hence risk of showers/thunderstorms compared with previous days. The general pattern is rather complex, and highly sensitive to differences in key aspects such as the thermodynamic profile and cloud cover, which vary amongst model guidance.
In a broad sense, much of England, Wales, southern Scotland and central/southern Ireland has a risk of heavy showers and thunderstorms developing, but the exact placement is uncertain - while persistent and heavy rain affects western and northern Scotland. Many showers/storms will be forced by low-level wind convergence (such as sea breeze moving inland) or orographic forcing, meaning some coastal areas, where steering flow does not carry inland showers offshore, may remain void of showers (e.g. southern England). Some capping issues exist, especially across England where a well-mixed hot airmass still persists but with lower surface temperatures compared with recent days. The mid/upper troposphere also appears rather moist which may also hinder buoyancy of rising air parcels. Shear is weak (<20kts) near/under the upper trough axis, which generally means any thunderstorms that can develop will become outflow dominant, generating cold pools which may then trigger daughter cells nearby. Slightly stronger shear is likely over southern and central Scotland and northern England, which may aid cell longevity somewhat and enhance hail size. The greatest concern is arguably the weak steering flow and potential for multiple cells moving over the same area, both posing the risk of flash flooding on dry, hard ground. Generally sub-severe hail and gusty winds will be possible with the most intense cells, with particular focus on northern/eastern England where inverted-V profiles could result in some notably strong gusts of wind locally (perhaps >50mph).
Using a blend of model guidance some key themes become apparent:
- a NW-SE line of heavy showers/thunderstorms developing in the vicinity of the Pennines and southern Scotland during Monday afternoon and evening, drifting northeastwards
- Heavy showers/thunderstorms developing between Dartmoor and Exmoor during the afternoon hours, where a MDT has been introduced (primarily because of better cross-model support)
- Clusters of heavy showers/thunderstorms in southern and eastern Ireland in response to a marked CZ, although the exact placement (how far inland) is a little uncertain. This area in particular has the greatest potential for significant rainfall accumulations, with some CAM guidance suggesting scope for 50-90mm.
Shower coverage elsewhere is less clear and varies significantly both between models and between runs of the same model. A broad SLGT has been included, but it is inevitable some areas (perhaps even large) may remain void of lightning activity. Showers/storms will persist well into the evening hours in some areas, but a gradual decay is likely towards late evening and overnight. However, renewed activity is possible near Cornwall/Scilly overnight, perhaps also central southern England into the south Midlands/south Wales, with the potential for other showers/thunderstorms to drift out of northern France towards S/SE England - as PVA rounds the base of the Biscay trough and lifts north towards the English Channel. Therefore some areas, even inland, that remained relatively quiet during the daytime hours could actually experience more activity overnight.
