Report on the 'Bracknell Storm' of the evening 7th May, 2000

All times GMT
Location: Bracknell (Wooden Hill/Great Hollands), Berkshire. NGR: SU 854 667 Lat: 51DEG23MIN30SEC(N); Long: 00DEG46MIN28SEC(W)
Height(amsl): 80 metres (262 feet)
(Details of observing site, etc., on the main web site)
Thunderstorm evening 7th May, 2000 intensity of thunderstorm judged by flash rate.

Data / information prior to storm onset:-
night minimum temperature (6th/7th) read at 09z=11.7 degC
day maximum temperature (7th) read at 08/09z=23.5degC*
(* although read 'next-day' the maximum did occur afternoon of the 7th)

Following comments made at the 'morning' observation (i.e. 07/0900GMT):
" bc (1 okta Ac cast - early pr(slight) from Ac cas, 3 oktas 'other' Ac) "
rainfall amount period 06/0900 to 07/0900=trace (this from the aforementioned Ac cas event).
No other information available for the period leading up to the storm - no other showers, ts etc. observed.

07/1715 Cb (multiple cells) developing to east - distant
07/1800 first thunder heard
07/1840 tl(slight) with slight rain soon becoming moderate rain.
07/1900 tl(slight) with heavy rain/sharply gusty wind (F5)
07/1930 tl(moderate) with heavy rain(R)/slight hail (<5mm dia)
07/1935 tl(moderate) with moderate rain/moderate hail (>5mm dia.. beginning to carpet the ground. Within one minute, hail diameter average 1 cm, with some individual pieces 1.5cm dia. Squally wind - F6/7, but no sign of 'tornadic' activity i.e. no rotational motion evident in lower cloud layers)
07/1940 tl(moderate)R (hail ceased)
07/1950 tl(slight)R h(hail started again <5mm dia).
07/1955 tl(slight)R (hail ceased)
07/2015 tl(slight)slight rain.
07/2020 tl(slight) rain ceased.
07/2110 estimate last thunder heard. Lightning still visible to WSW - notably vivid.
07/2200 last lightning 'obvious' (though view to west is blocked by houses, trees etc.)

Total rainfall in period 07/1840 to 07/2020=47.1 mm (some (considerable?) loss probable due hail and intense rainfall components)**(see note below re: rainfall measurement) Majority of this fell in period: 07/1900 to 07/1955 (55 minutes)

At no time did the hail completely cover ground***(see notes re hail below).

Although lightning rate was never great ( circa 2 or 3 flashes / minute at height of storm), individual CG strokes quite intense/bright. Overall, judge that CC heavily outnumbered CG.

>>> Comments re: immediate area around observing site/house Minor local flooding only. Your observer engaged at one point dashing out in the intense rain unblocking drains and 'shoo-ing' water away from garage.

>>> Comments re: wider 'local' area -- Local small lake and several underpasses were flooded in the Great Hollands, Birch Hill, South Hill Park areas, with some properties flooded requiring fire brigade assistance. The whole area is a naturally low-lying region, which before the development of this part of Bracknell (from 1970 onwards) would have been part of the Windsor Forest - all 'older' established habitation being at higher elevations of Easthampstead and Bracknell itself. The building of underpasses also helps to 'funnel' large quantities of water through and to inhabited areas. The poor drainage was not helped by the large quantities of hail that blocked drains.

** Note re: rainfall measurement -- At one point the fall of hail must have blocked, if even only temporarily, the collection of rainwater, both during and after the hail-fall. This caused increased 'splash-out', particularly as my gauge, although a standard 5-inch is not of the Met.Office pattern which has a fluted projection above the funnel. There may be considerable underestimate for this reason AND CARE SHOULD BE USED IN USING THIS FIGURE IN ANY STUDIES ETC. Given the rate at which the rain was falling at this time, at least 5 mm of rain may have been 'lost', and perhaps 10 mm.

*** Note re: hail (hail diameters at my site were measured).. although at this site maximum diameter was circa 1.5 cm (less than 10% of fall, probably nearer 5%), from television news footage, the usual 'golf-ball' size hail was reported (can't confirm or otherwise), but from the pictures then certainly the hail size was larger than here. I think we can claim at least 2 cm diameter in the Birch Hill / South Hill Park area (from same footage), with a possible 2.5 cm. FOR INDIVIDUAL ELEMENTS. ( There must be some doubt though as I have not yet come across anyone who has reported damage to cars - which these large sizes would presumably impart.)

The storm was possibly twin-celled. Certainly when I first observed it (1715Z), it had two distinct large Cu or Cb tops associated - non cirriform (i.e. Cb calvus OR Cu congestus -- difficult to tell whether striated or not at the distance).

From the damage reports, and subsequent radar imagery, this was a complex cellular storm, rather than a single-cell, or 'supercell' .. but more detailed analysis would be required to establish this aspect. Also note that my wife is subject to severe asthma attacks under stress. This was one such time, and at the height of the storm, I was engaged in making sure she was coping with the situation and noting the vagries of the storm assumed a much lower priority. You should allow for this fact if any of my comments appear at variance from others, or conflict with common / scientific sense.

*** Report from Lorna (my sister/finishing duty in the NMC): She cycled home that evening, leaving HQ MetOffice (Deepfield Road/SU 873 694)) circa 1900 GMT, arriving home in Forest Park (Crofton Close)/SU 885 676) circa 1925GMT. This journey would have been effectively a traverse across the storm track. At departure (roughly 1900z), rain (heavy) only at HQ, but hail soon encountered down the end of Deepfield Road (Bullbrook)/SU 881 695). Hail/heavy rain mixed from then on to the Running Horse roundabout (jc. of LilyHill Drive, London Road, Broad Lane)/SU 883 690 necessitated her sheltering in the underpass there. The hail was "painful". Hail/rain eased, and she set off again to travel through Harmanswater (Ralphs Ride)/SU 882 680 when the hail was encountered again (not clear if this is a separate event from above, or a renewal of the original). At her home, the hail element was easing considerably, but still raining. Hail covering the ground 'in patches' and covering other outdoor objects e.g. cat's hutch, garage roof etc. She estimates the size as 'around 1cm', with maximum depth on the ground at the worst point (London Road, Running Horse areas) of some 3 cm - but 'floating on a film of icy water'!

*** Report from my parent's home (Crouch Lane, Winkfield/SU 923 728) ... no rain or hail (i.e. not affected by the storm).
*** At Forest Road, North Ascot (B3034)/SU 918 708, rain only, (heavy with standing water etc.), but no hail.