Convective Outlook: Wed 21 Jun 2017
What do these risk levels mean?
Convective Outlook

VALID 06:00 UTC Wed 21 Jun 2017 - 05:59 UTC Thu 22 Jun 2017

ISSUED 08:07 UTC Wed 21 Jun 2017


UPDATE 08:07 UTC Minor adjustments made based on analysis of latest guidance - potential for severe surface-based thunderstorm initiation very late afternoon into the evening hours, initially over NW Midlands / NW England, the focus then shifting E/NE across N England through the evening, perhaps lasting well into late evening before moving offshore to the North Sea. Very large, damaging hail, strong winds, very frequent lightning and perhaps a tornado will be possible with these. Large amounts of uncertainty still remain over whether surface-based convection will trigger, not least relating to cloud cover perhaps preventing significant insolation. Potential also exists farther south through Cambridgeshire/Norfolk etc, but questions over whether cap here can be eroded. Latest guidance continues to suggest increase in coverage of elevated deep convection over SW England and W Country towards the end of Wednesday night / Thursday morning.

UPDATE 06:08 UTC MDT over Cumbria shifted east (and hence merged with other existing MDT) given slightly faster eastward motion of thunderstorm cluster, now over NE England

Broad upper ridging (and anticyclonic flow) will still be in place across much of the British Isles on Wednesday, but as an Atlantic upper trough approaches it will advect an EML (elevated mixed layer) above a very warm and reasonably moist surface layer (afternoon dewpoints near 19-21C, ignoring the GFS which continues to over-estimate surface dewpoints, and hence grossly exaggerates modelled CAPE). 


Elevated thunderstorms will possibly be ongoing / developing at the beginning of this forecast period, especially over northern England and southern Scotland, with general areas of showery precipitation with embedded lightning moving erratically NE-wards across the remainder of Scotland.


Farther south, assuming plenty of insolation then strong heating will yield 2,000 - 3,000 Jkg-1 SBCAPE in a notably capped environment - the greatest instability developing during the second half of the afternoon and into the evening hours. Many areas will remain dry under such conditions, but it is just possible that enough forcing from a combination of sea breeze, topography, surface troughing and falling heights aloft during the evening hours may be able to overcome / erode the cap to allow explosive but probably fairly isolated thunderstorm development to occur. High-res models continue to offer little support for surface-based thunderstorms given the strength of the capping and dry layers in the mid-levels, but that said elements of medium-level instability release over northern England could become rooted in the boundary layer, and / or surface-based convection could develop.

Given the lack of any useful guidance (aside from 12z ECMWF, GFS has been consistently too moist in surface layers on recent days and continues to overplay the thunderstorm risk) it becomes hard to highlight any specific areas at risk with any confidence - but overlapping of favourable ingredients would suggest one or two isolated but severe thunderstorms would be possible during the late afternoon, more especially evening, hours across N or E England. Such storms could rapidly organise into multicells or perhaps supercells, capable of producing damaging hail up to 4-5cm in diameter, strong gusts of wind and, if surface winds can remain ideally backed, perhaps also a tornado. A severe thunderstorm cannot be ruled out elsewhere in England, but in general capping is likely to be too strong to be overcome.
It is quite possible that very little, if any, notable surface-based convection occurs - but worth stressing should it occur, then it could be quite noteworthy and potentially disruptive. A MDT has been issued for now where this is most likely to occur - but this may need updating with any further guidance.


Overnight, rapid height falls as an upper trough approaches combined with a cold front will allow medium-level instability release to help generate some elevated deep convection with the potential for a few thunderstorms - the risk beginning over SW Britain initially, then running NE-wards with time through the early hours. The exact track, extent and timing is very uncertain at the moment depending on the phasing of the upper trough with residual high WBPT plumes across S Britain, and further updates / tweaks may be necessary to the forecast.