The synoptic background to the storms of June 19th 2005 & associated flooding

The synoptic background to the Thunderstorm outbreak on the 19th June which led to terrible flooding in the Helmsey area was typical of a spanish plume event that covered the whole of the UK. However the more severe weather was restricted to a relatively narrow corridor across N England and parts of the West Midlands.

General Situation

During the summer months elevated plumes of warm air that originate over the Spanish plateau can move north over the Bay of Biscay and Western France and eventually up into the British Isles, these plumes contain high dewpoints and also 850mb temperatures. Perhaps the best indicator of all combines the two (WBPT or Theta-W).

During the 18th and 19th high 'theta-W' values moved north over Biscay and France and up across England. Values peaked on the 19th and were at or over 19C stretching in a zone across the West Midlands and into NW England. These values are quite high and are not often exceeded across the British Isles. The moist low level air was also derived from some very warm moist TM air that had originated in the deep tropics several days earlier and which had moved NE on Thursday and Friday.

Early in the day an upper trough was noted in WV Imagery (a dark slot indicative of elevated PVA on its ascending side) approaching the western side of the UK.

This situation is nicely shown in Fig. 1 which is a WV/IR composite from Met 8. It has the advantage of showing both upper level forcing and also cloud development from the IR band.  The long wave upper trough is shown to the west whilst high pressure is located over the continent. The area of High 850mb WBPT is shown also. This image shows that thunderstorms initiated as the upper forcing moved over the plume (bright white over darker pink), but there was one other required ingredient- convergence, to begin initial lift. 

During the morning a convergence zone was initiated over East Wales and up into Cheshire. This provided the final piece of forcing such that convection was initiated and quickly broke through the weakening cap, located from Birmingham northwards. Huge CB's exploded upwards very rapidly after this and thunderstorms with hail occurred as a result, shortly afterwards.

As the afternoon progressed thunderstorms developed widely along this convergence zone from Shawbury (Shrops), NE to Newcastle. Some of the storms were severe with golf  ball sized hail reported and 30mm of rain in one hour at Hawarden in Cheshire.   

Imagery and Reports

Fig. 2, 3 and 4 are IR, HRV and radar images respectively for various points during the afternoon. The radar confirms rates in the region of 32-64mm/hr falling (no doubt spot rates far exceeded these values in few locations).

Fig 2. is the Enhanced Infra Red Met-8 image over NW Europe, it shows the very cold cloud tops which exploded into life during the early afternoon. By 3pm the temperature in the tops was below -60C, that equates to above 40,000ft. In fact overshooting tops may well have exceeded 45,000 ft, very high for the British Isles.

Fig 3. is the HRV image over England showing the huge CB's developing and running NE wards. The stark contrast between these CB's and only small clouds further south is very marked.

Fig 4. is the N England Regional Radar and shows intense precipitation falling in the source region of the river that runs through Helmsey; peak rainfall rates were probably over 100mm/h over parts of the catchment area.

Fig 5. is the 12Z Synoptic Chart with overlaid 10M streamlines.  It can be seen that storm initiation was along the well marked confluence zone from West Midlands then running NE. As the afternoon progressed this confluence zone moved SE as did the storm initiation zone.

Storm Initiation or not?

Ascents on the 18th July were mostly stable across the UK, due in the most part to high pressure and well subsidised air over the UK, very moist air was also still prevalent though a gradual drying process was accelerated during the day as drier air was advected north from France, however the lowest level 10M air was kept quite moist as noted by the relatively high dew points.

During the early morning of the 19th, as the shortwave approached, contour heights fell across the UK with a general fall in temperature aloft. At the same time warm air advection was continuing to increase the temperatures at 850mb across England. This was most marked in the SE (where it would go on to produce a large CAP), however it was more modest further NW, as indicated by the ascent made at Nottingham in Fig 6. This shows only a very small convective inhibition area and a large area of CAPE that can easily be released given a rise in temp to around 29-30C. Once through this small CAP, the ascent indicated tops would not stop rising until reaching the tropopause and then overshoot att 40,000 - 45,000 ft. (nb: hatched area indicates CAPE)

Fig, 7 is the 12Z Ascent for Albemarle and whilst not appearing so unstable, this is in fact not the case. The upper trough had not got this far NE by lunchtime, therefore only skinny CAPE was shown, however as the afternoon progressed cooler air at 400-500mbs was advected NE and as a result (move the line in your head) a much larger area of CAPE is available for release. Both ascents show the potential for hail (with a lot of dry mid level air and large CAPE).

Despite the high temperatures further south, the Herstmonceux ascent (not shown) portrayed a large CAP and would have required a temp exceeding the mid 30s to break it and even then there was little moisture below 850mbs to initiate convection as dry air had progressively invaded the south of England from France. 

Summary

The storms that formed on the afternoon of the 19th were severe and the result of destabilisation aloft of a conditionally unstable atmosphere. The vital triggers were:

• Upper forcing from a shortwave trough moving NE at peak daytime heating
• Surface convergence
• Warm CAP at 800-850mbs enough to produce a 'loaded gun' which allowed maximum insolation and heating before convection was initiated.

The resulting storms were severe in terms of hail and the violent rain rates in a few locations, also the flooding across Yorkshire. The storms however were confined to a relatively narrow corridor, the zone shifting east during the remaining part of the day.

For an article about the effects of the associated storms please see the page just here.

(C) Paul Blight June 2005