Weather extremes of 2006 - early heat and thunderstorms of May 4th

The high Temperatures and thunderstorms of the 4th May 2006 - a synoptic review

During Thursday 4th May 2006 a plume of air having originated over Southern Europe travelled north and brought much of the UK their first taste of summer. Temperatures reached over 27C in the SE and during the afternoon intense thunderstorms developed across SW Scotland.

The upper air situation 00Z on the 4th is shown in Fig 1 and shows the 300mb flow over the North Atlantic and Europe. A deep longwave trough is located to the west of the British Isles whilst a high pressure cell is located over Germany. The flow aloft is from the SSW across the UK and contour heights are falling.

At the surface a cold front was located west of Ireland and was moving east.

At medium levels warm advection was coming north across France and was well marked by increasing values in Theta-W at 850mbs. The boundary of this warmer airmass moved north during the night delivering some showery rain across Wales and southern England (mainly from AcCast).

During the day clearer skies fed into southern England. Partial thickness values suggested maximum temperatures would be in the order of 24-25C around the London area. However these values did not take into account the dry ground across much of the SE and so the heat flux was greater than anticipated. By mid morning temperatures had already exceeded 23C at Charlwood near London's Gatwick airport. During the afternoon 27.7C was achieved at both the London Met Centre and also at Northolt. Elsewhere temperatures widely exceeded 22C inland with values of 23-25C being recorded in places. The south coast, with a feed off the chilly English Channel, was cooler with temps pegged back to near 20C.

Attention now turned to the approaching upper trough and whether the advancing and increasingly unstable plume would become unstable* (see box below). Upper level charts and WV imagery indicated an embedded shortwave was moving NE towards Ireland and this was expected to destabilise the plume below with thunderstorms becoming organised late in the day, due to increasingly favourable upper level dynamics.

* The plume becomes unstable with the approach of a trough as increasingly cooler air aloft overrides increasingly warm air below and increases lapse rates and makes a profile unstable at the elevated level. Storms forming here do not form from the surface but instead form aloft at the top of the 'elevated moist layer'. (EML)

Study of ascents is often the best way to see what is going on throughout the entire depth of the atmosphere. On the 4th an 06Z Larkhill ascent was available, as shown in Fig 2. It shows a stable boundary layer (b/l) and is unstable above 725mbs. Elevated overnight showers would have been generated by this instabilty aloft. However given the fact that clearing skies were already sending temperatures soaring it was interesting to modify the sounding to see what would happen to a parcel of air being lifted that had been heated to 26C, giving forecasters an idea of what might happen later.

Fig 1 Upper air situation 00Z, 4th May 2006

Fig 2. 06Z 4th May 2006, Larkhill (Wilts) ascent - tephigram

Fig 3. Larkhill (Wilts) 06Z ascent 4th May 2006 -modified tephigram

Fig 4. 13Z 4th May 2006. Airmass derived Water Vapour Product

A modified Larkhill ascent is shown in Fig 3. It is modified to a temp of 26C and this allows massive CAPE to be released (1257j/Kg). However this is all very well but convection has to be iniated first. The air coming across from France was dry at the top of the boundary layer (890mbs-910mbs) and this meant air rising was forming patchy Cu, but then largely evaporating due to dryness of the atmosphere at this level. During the afternoon this dry air was mixed downwards with a lowering of dew points noted throughout the Thames Valley in particular. So despite the favourable "Loaded Gun" scenario portrayed by the Larkhill sounding there was insufficient moisture to allow convection to tap into this huge reservoir of instability aloft.

Further north however it was a different story. The 13Z Airmass Derived WV Product is shown in Fig 4. This is a very useful MSG product as it shows the position of PVA, Upper Troughs and Jet Streams. The image shows the expected shortwave moving NE, at this time located over SW Ireland with PVA driving NE. The warm stable air is located over England with the high WBPT plume contained inside the cloud that stretches from Western Scotland to the Bay of Biscay. The orginal cold front can be seen to the west of Ireland - this would weaken later in the day as the plume becomes the main thermal boundary. It was unfortunate that there was no 12Z Castor Bay ( N Ireland) ascent on the 4th. If there had been it would have likely have shown a lot of mid level instabilty over Northern Ireland and this environment would have also been in place over SW Scotland.

06Z guidance from the GFS and the UKMet Meso forecasts hinted at storm formation over NW Wales, the Irish Sea and into SW Scotland, just ahead of the approaching trough, which was to become the cold front. During the afternoon MSG imagery and radar indicated the first cells forming over Cardigan Bay and the Lleyn Peninsula, moving NNW into the Irish Sea. These storms were forming from the EML and growing upwards. They were not rooted in the boundary layer.

Storms subsequently developed over N Ireland with some locally heavy falls reported east of Belfast where a house was struck and damaged by lightning.

Fig 5 MSG UK HRV (high resolution visible) imagery for 1515Z 4th May 2006

Fig 6. Enhanced radar with overlay for Scotland at 20Z 4th May 2006

Fig 5 shows MSG HRV imagery for mid afternoon showing growing cloud tops across the Isle of Man and SW Scotland again developing upwards from the EML. Clearer skies can also be seen over England. Late in the day and into the evening the storms began to propogate east across SW Scotland and intensfied. Some intense thunderstorms began to backbuild across the Southern Uplands and into Argyll & Bute, with strong SW'ly shear aloft, allowing ventilation of the storms and allowing new growth to build to the SSE and move NNW whilst the overall motion was NE.

Fig 6 shows the evening radar across Scotland. Note the very heavy rainfall over Glasgow at this time associated with some intense thunderstorms moving through the Clyde Valley. MSG Enhanced Output indicated tops to over -70C across SW Scotland with tops to nr 30,000 ft.

Overnight the shortwave moved NE and relaxed and the storms and front weakened with nothing more than light rain / drizzle moving east across England displacing the warm weather with slightly fresher air.

Very few if any of the storms originated from the boundary layer and were mostly the result of differential thermal advection de-stabilsing a warm moist medium layer plume that had moved north from France and Spain in the preceeding 24 hrs.

The saying goes 'Three Fine days then a Thunderstorm'. Well in this case there was barely one! It was a useful reminder that upper layer mechanics are very important to our weather and that conditions at the surface sometimes have little impact on the weather we actually experience.

Copyright; Paul Blight, Assoc Fellow RMA

2006

e-mail pblightuk@yahoo.co.uk