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Convective/Thunderstorm Discussion Spring/Summer 2018
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http://www.convectiveweather.co.uk/forecast.php?date=2018-05-26
Day 2 Convective Outlook
VALID 06:00 UTC Sat 26 May 2018 - 05:59 UTC Sun 27 May 2018
ISSUED 22:40 UTC Fri 25 May 2018
ISSUED BY: Dan
Plume events are often fraught with uncertainty, primarily due to poor NWP modelling of deep convection generated by mid-level instability. In these types of situations, including today, it is best to take a broad-brush approach and try to highlight trends, rather than specifics - basically, expect the unexpected! The latest model guidance is much further north and a lot quicker with developments than data had been suggesting just 12 hours ago. It is likely some areas within the SLGT could remain lightning free, and the forecast may need to be altered if confidence improves - it rarely goes to plan...
There are reasonable signs for an area of elevated convection to be drifting across S / SE England at the beginning of this forecast period, continuing to move NW-wards through Saturday morning. Questionable as to how much lightning there will be with this activity given weakening instability, though forecast profiles do suggest some reasonable shear to help with some organisation. Either way, whatever activity there is will likely weaken with time as it continues to migrate across the Midlands and into Wales.
A secondary pulse of elevated convection may develop and move NW-wards across SW England late morning into the afternoon, which would have a better chance of producing lightning - but this is a weaker signal amongst model guidance.
This then leaves a window of opportunity during the afternoon for surface-based thunderstorms to occur. However, the atmosphere will be largely capped by a warm nose at 850-900mb, and so despite increasing instability it is possible very little will actually develop. Greatest potential perhaps across Dorset, Somerset and Gloucestershire, but this very much dependent on enough surface heating to break the cap, which is uncertain due to the potential for extensive mid/upper level cloud associated with earlier elevated convection. Should an isolated thunderstorm develop, it may be severe, capable of producing hail up to 2.0cm in diameter and localised surface water flooding.
The potential for thunderstorms then increases during Saturday evening and night, with numerous elevated thunderstorms likely to develop over the English Channel containing frequent lightning and perhaps some hail. The vast majority of model guidance would suggest thunderstorms will move to the NW, putting SW England and the West Country at greatest risk, broadly Isle of Wight westwards. However, ECMWF has been very consistent over several runs (now joined by 12z EURO4) to produce a much more bifurcated flow, which would then take a very active cluster of thunderstorms NE-wards over Hampshire, Sussex and towards the London area by the early hours of Sunday.
Given the large spread of possible areas affected, it is difficult to pin down a MDT area with any significant confidence - though worth stressing the ECMWF (and EURO4) tends to have a good handle of elevated convection, and its consistent signal over multiple runs is somewhat reassuring. As a result, have followed these trends for the MDT - though if other models are correct, this may need shifting westwards to cover more of Devon and Cornwall.0 -
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Update
UPDATE 06:49 UTC All areas extended north to cater for quicker arrival in latest model guidance. MDT expanded westwards to include Devon and Cornwall. Note: bifurcating flow tends to leave a gap between two main clusters of thunderstorms, so it is possible not everywhere in the MDT will be affected by lightning - but hard to specify exactly which areas might remain lightning-free0 -
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Big storm pushing closer to Ireland, a few hours off it would seem.
https://twitter.com/EUStormMap/status/10004440123843543040 -
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