An under‐forecast snowstorm associated with a small but deep tropopause depression

Abstract

Three‐dimensional view of snow streamers producing heavy snowfall over southern England obtained from a T + 6h mesoscale model forecast valid at 1200 utc, 1 February 2019, showing a sub‐set of the computational domain 4km high and approximately 240 × 240km as viewed from the west. The 3D isosurface depicts a model reflectivity of 23dBZ, corresponding to a moderate snowfall intensity and has been shaded according to vertical air velocity according to the colour scale. Note the cellular and sloping nature of the streamers within the snow bands and the strong positive vertical velocities consistent with the presence of shallow ‘generating’ cells of upright convection at their top.

Snowfalls in the Cotswolds and Home Counties on 11 December 2022

“…Forecasting the extent and intensity of mesoscale snowfall in the United Kingdom remains a challenge for forecasters and NWP models. Non‐frontal snowfall is often associated with mesoscale low pressure systems and upper‐air features (anomalies of the dynamic tropopause; Smart and Browning, 2020). In addition, accumulated snowfall can be highly dependent on orographic factors and the intensity of precipitation (Pike, 2022).…”

“…Smart and Browning (2020) presented satellite imagery and NWP output depicting a disruptive snowfall event across southern England on 1 February 2019. The top row of Figure 3 may be compared with their fig.…”

Surface‐based convection inland in winter?

“…Of particular note is the circulation labelled V which had been running quickly eastwards. The presence of a so‐called ‘dark eye’ (in this case the blue area over the western English Channel) within an upper trough is indicative of a vorticity maximum aloft, the dryness being associated with a localised lowering of the tropopause (Smart and Browning, 2020). The circulation itself contained several substructures such as the separate filament swinging northeastwards around the circulation, marked by the distinct northern edge of the dry (blue/white) boundary reaching the south coast of England.…”