RaVeNStOrM

THUNDERSTORM T0027
Date: Thursday 5th August 1999   Time: 14:08 (Multicell) 16:30 BST (Supercell)
Location:
Macclesfield, Cheshire UK
Path: Multicell: Overhead / Supercell: 5-8 miles Northwest

Synoptics:
Deep upper trough overriding surface cold front with RP Maritime behind. Slack gradient wind.
Duration: 40 minutes (Multicell)
Type: Linear Multicell & Supercell
Average lightning type: C-C (Multicell)
Average discharge rate: 105 seconds (Multicell)
Footage Quality:
VHS



 

14:08 - T0027 Part 1 - Electrified Multicell on Cold Front
An elongated occlusion had passed northwards during the morning, leaving behind a returning polar airmass but with a pronounced upper trough (driving the fronts) hanging over Macclesfield. A line of convection, aided by surface heating, looked like it formed along the axis of the upper trough. This allowed for this fairly active electrified thunderstorm. A linear tail of torrential rain swept up from the West Midlands across Macclesfield giving a few C-Cs and distant C-Gs with decent thunders.

 

The cell that passed over Macclesfield gave some low-amp C-C overhead and C-G over the Pennines in the distance to the east. It cleared up allowing a view of a frontal anvil overhang spanning the entire northern sky. Following this, a few non-thundery showers (with weak anvils) developed to the southwest and passed overhead, giving further torrential downpours.

It was clear there was cold advection as a timelapse of these follow-up cells showed the boundary layer wind was westerly, but mid-level was southwesterly, and high level was still south to southeasterly. This is important to note for the wind shear environment during what was to follow a couple of hours later...

16:30 - Developing Supercell with mesocyclone and Gustnado/Tornado event
I decided to tie this in with the T0027 cluster, even though it occurred a few hours later separately, mainly because I didn't observe any lightning activity. What I did observe was what appeared to be a mesocyclone, with a very low base, uncharacteristic to the rest of the day's convection. It was positioned to the northwest of Macclesfield at 16:30 perhaps 5-8 miles away, and the clouds around it were moving in all directions at all heights. From what I can gather looking back at the footage, there was a low-level northerly, a mid-level southwesterly, and upper level southeasterly. There could have been lucky convergence with the either outflow boundaries from other showers, or from the surface low itself. A NOAA 850hPa wind analysis below shows actually, through the axis of the trough, there was likely a wind minimum with a maximum either side of it. It is likely that this formed right in the heart of the trough, allowing high low-level CAPE and mesoscale wind boundaries (such as outflow from prior storm) to converge together.

NOAA 850hPa Streamlines Reanalysis

Whatever the environmental conditions, it was enough to produce convection with what looked like a classic supercell base. A rapidly rising (to the naked eye) updraught was evident on the right hand side of the mesocyclone, so I focussed the camera initially on that. It was obvious there was either horizontal shear through convergence as the mesocyclone appeared to be rotating, especially in time-lapsed footage.





Low cloud close to Macclesfield was moving in a southerly direction while the mesocyclone drifted northeastwards. What looked like a roll-cloud with swirls was seen in the local skies, supporting large wind shear. As I watched the mesocyclone twist in the distance, with my camera trained on it, the updraught on the right hand side went completely vertical and I saw ragged condensing cloud start to form under it all the way to (from?) the surface. This was in a similar position as the tornado spawned from T0014 the previous year 1998, though being on the flat Cheshire plain I doubt locality had anything to do with this coincidence.

The ragged cloud seem to tighten into a funnel-shape and twisted for a minute or so. Then, a SECOND swirl of condensation developed to the right of the primary, this time starting from the ground up and appeared to be a bit closer than the first. It was clearly on the deck given I was looking down a plain, so I deem this either a well-defined Gustnado or Tornado. Characteristics of both types were evident. I zoomed in with the camera and could see pronounced and multiple sub-vortices to the surface as it spun around. The primary funnel at this time was still just about visible but started to dissipate.

About 6 minutes after the second 'nado, both funnels ascended into the base. By this time the mesocyclone was moving behind trees and further northeast. A few further local showers followed overhead as the structure of the suspected supercell started to become more evident, demonstrating a very dark base with arcus clouds (similar to T0014 and suggesting outflow was present) and also an irregular mushroomed anvil. Interestingly, no thunder was heard during this whole event; perhaps too far away or not mature enough.


 

Tornadoes or Funnels Argument
Like T0014, I have had a video of this event online for public witness for over a decade (before remastering it in 2020 here). There was, in comments, much debate about whether these were tornadoes or funnels (no mention of Gustnadoes), mostly stating a funnel becomes a tornado when you see it lifting debris, which is not necessarily true. In the case of these "funnels", viewing sufficient debris lifted from unpopulated pasteurised land at a few miles distance would be challenging! The footage clearly shows the primary and secondary condensation funnels connecting all the way to the surface, with me looking down a plain from higher altitude, and were rotating. If you were to stand directly underneath them I would be surprised if your hairstyle didn't change.

In summary I think a very rare event for the UK has been documented here. Twin tornadoes/gustnadoes, in the UK, without having to even chase them... (bedroom window!) what are the odds?! Has a multi-vortex ever been documented in the UK? Am I the first? I would welcome anyone's thoughts on this for discussion. As a time-served meteorologist, I am still a little perplexed as to what I witnessed on this day.

CHARTS (Credits)


NOAA 500hPa Reanalysis NOAA 850hPa Temp Reanalysis NOAA 850hPa Streamlines Reanalysis


SATELLITE IMAGES (Credits)
VISIBLE 05.08.1999 09:09

INFRARED 05.08.1999 09:09
COLOUR 05.08.1999 09:09

VISIBLE 05.08.1999 14:44
INFRARED 05.08.1999 14:44
COLOUR 05.08.1999 14:44

VISIBLE 05.08.1999 16:04
INFRARED 05.08.1999 16:04
COLOUR 05.08.1999 16:04


VIS and IR images at 1444Z (1544 BST).


VIS and IR images at 1604Z (1704BST)


SFERICS (Credits)
BBC Radar for Thunderstorm BBC Radar for Thunderstorm

BBC Weather Grabs featuring Phil Avery (Credits)
BBC Radar for Thunderstorm BBC Radar for Supercell BBC Top Temp and Satellite

 


 
Mark Seltzer  www.electricsky.co.uk

 

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