On the Occurrence of Low-topped Supercells across the British Isles

IntroductionIt has long been recognised that tornadoes are fairly common across the British Isles, with an average of around 40/year, and extreme years, over 150. The islands are also affected by other types of severe convective weather each year, such as damaging straight-line winds and hailstorms.

The modes of convection which spawn these events have not been studied in great detail until recent years. The author is proposing that some of these events are attributable to low topped supercell-type storms.

A supercell is defined as a storm which possesses a deep, persistent rotating updraught called a mesocyclone.

A “low-topped” (LT) supercell is one where the equilibrium level is much lower compared with the “large” supercells which affect portions of the central US each spring. Such storms have been documented in the Pacific Northwest, as well as in the Great Plains

Discussion• Both storms studied show evidence of supercell behaviour.

• Whilst thermodynamics may seem unsupportive of supercells (especially in storm 1), the strong shear through the convective layer may support low-topped supercells.

• During autumn/winter, the UK is frequently subjected to high vertical wind shear acting on unstable airmasses

• Tornadoes and large hail occur each autumn/winter, often associated with strong deep-layer and low-level shear - the most spectacular example being 105 tornadoes in one day across England and Wales, on Nov 21st 1981

• Parcel buoyancy is low in these seasons, yet strong wind shear through the buoyancy may promote updraught rotation and possibly supercells

• Both storms occurred on days where TORRO forecasters had recognised that a severe weather threat existed

• Severe thunderstorms and tornadoes have traditionally not been part of standard severe weather warnings, leaving residents oblivious to the threat of severe convective weather

• TORRO have pioneered severe thunderstorm and tornado forecasting in the UK via the issuance of experimental “advisories” and “watches”, since 1992.

• In 2004, 26 tornadoes occurred within a TORRO forecast.

Paul Knightley –Director of TORRO Severe Storms Forecast DivisionAssistant Forecast Manager, PA WeatherCentre Ltd, 292 Vauxhall Bridge Road, London, England. SW1V 1AE

paul.knightley@torro.org.uk

Aim• To show that conditions favouring LT supercells can occur across the British Isles.

• To highlight two recent examples of storms which exhibited supercell-type behaviour

• To highlight the fact that such storms pose a threat to life and property in the British Isles each year

• To show that such storms can and should be forecasted.

Method Two severe thunderstorms which both produced

tornadoes were studied using

Upper-air and surface data

Radar imagery

Photographs of the storms

The first storm (Storm 1) occurred on the 21st March 2004. It produced:

Hailstones up to 20mm in size, and,

A tornado which caused damage in the Filkins area of Oxfordshire (South Midlands), rated as T3.

The second storm (Storm 2) occurred on the 28th July 2005. It produced:

A tornado rated as T3 around Peterborough (eastern England).

Another tornado at Moulton, Lincolnshire (eastern England).

Conclusions

• LT supercells can pose a significant threat to life and property in the UK

• Lack of national Doppler radar prevents a climatology of such storms

• Despite limited resources and a non 24/7 operation, TORRO has achieved respectable results in the forecasting of severe convective storms

• A fully resourced weather company working alongside TORRO should be able to achieve even better results

• Thought should be given as to how to integrate tornado watches into standard severe weather warnings

Tree damage near Filkins, Oxon, March 2004 (from Storm 1)

© Brendan Jones

Tornado and rear-flank downdraught (RFD) notch near Peterborough, 28th July 2005 (Storm 2)

© Doug Fidler

Surface observations across the Midlands around the time of the Peterborough tornado.Image © PA WeatherCentre

Estimate progression of the triple point between a

“synoptic” cold front, and a “insolation-enhanced” warm

front across the UK on 28th July 2005

Image © PA WeatherCentre

Surface observations across the Midlands around the time of the Peterborough tornado.Image © PA WeatherCentre

Estimate progression of the triple point between a

“synoptic” cold front, and a “insolation-enhanced” warm

front across the UK on 28th July 2005

Image © PA WeatherCentre

Storm 2

Tracks of the left- and right-moving supercell storms, from storm initiation, on July 28th 2005. Purple track is the left split, and the black track is the right split. Image © PA WeatherCentre.

Radar image at 1630 on Thursday 28th May 2005,

showing left and right supercell members over

Lincolnshire.Image © PA WeatherCentre

Nottingham 12Z tephigram on 28th July 2005. The ascent has

been modified with observed air and dew-point temperatures at

2m which were measured in the vicinity of the storm. Note the

strong veering in the lowest 1km. SBCAPE around 1500 J/Kg.Image © PA WeatherCentre

T3 damage caused by tornado at Peterborough.

© Stuart Robinson

Estimate progression of the triple point between a

“synoptic” cold front, and a “insolation-enhanced” warm

front across the UK on 28th July 2005

Image © PA WeatherCentre

Strongly-enhanced image of the Filkins storm, looking east.

Original photo © Brendan Jones

Tracks of Storm 1 (light blue line) and another cell (black line) on 21st March 2004. Notice the deviant motion to the right of the mean flow. The other cell (black line) moved with the mean flow.Image © PA WeatherCentre

AcknowledgementsPA WeatherCentre, Michael Dukes, Terence Meaden, Brendan Jones, Laura Gilchrist, Doug Fidler, Stuart Robinson, the staff and members of TORRO

Herstmonceux tephigram for 12Z on March 21st 2004.

Note the strong speed shear near the surface. SBCAPE is

around 300 J/Kg.Image © PA WeatherCentre

www.torro.org.uk www.paweathercentre.com