Weather extremes of 2004- 17th December: Stormy south

The Synoptic development of the storm on 17th December

During the 17th December a rapidly developing low tracked ESE across the Southern half of the UK and into NE France. The low brought wind gusts to near 100mph and caused damage across S England and N France. The low, whilst not as deep or as intense as the Boxing Day storm in 1999 bore some quite striking similarities.

At 00Z on the 17th a fairly flat open wave was situated several hundred miles west of Ireland and moving very quickly east. At upper levels a very powerful jet stream was in place with a jet streak heading towards the UK. Aireps from overnight on the 17th measured the jet as being significantly stronger than models had expected. A report from a plane to the NW of Ireland in the early hours of Friday morning reported a peak flight level wind of over 200 Knts (over 230mph), some 15 kts above model expectations.

During the morning the low moved across Ireland and began to deepen. Large pressure falls began to occur over to the SW of the UK. Milford Haven saw a fall of 12mbs in the 3 hrs to 7am whilst Chivenor saw a fall of 11mbs in the 3 hrs to 8am. Water vapour imagery was beginning to look rather ominous with a dark (dry) slot appearing to the SW & W of the developing low. (a dry / dark slot is essentially dry upper air (200mbs or above) that is forced down to the surface and essentially brings the stronger winds aloft down to the surface as gusts)

By 09Z gusts were already above 60knts in SW Cornwall and the large pressure falls were spreading ESE into C S England. During the morning the strong jet streak continued to propagate ESE and at the same time the depression was crossing underneath it and into its left exit. During the late morning the low was over central southern England, already near 985mbs, with the steepest pressure gradient found to the SW of the main low. Largest pressure falls were found to the ESE of the main low. The low now under the left exit experienced further deepening and by 12Z was situated close to SE England around 982mbs. Gusts on Dartmoor reached 72 Knts or 83mph

Fig 1 (opposite, top) shows the synoptic chart at 12Z - in all probability the low was slightly further NE than the analysis suggests, a better position was probably over N Kent at around 981or 982 mbs.

Fig 2 (opposite,below) shows the corresponding 12Z WV image from the University of Dundee (reproduced with grateful thanks) and shows the marked dry slot over central southern England and the Channel towards Cherbourg. The low by this time was reaching maximum depth.

The strong gusts that were reported from underneath the "dry slot" were very typical of a "sting jet". Research has shown that a developing low being overriden by a strong jet and that is still undergoing deepening may have the ability to have a "sting" in its tail. In effect the low is sucking in more air in its developmental process than can be physically expanded through it's windfield. The dry upper air brings down strong upper winds to near the surface and because the air is unstable these get into the boundary layer as gusts and can generate a large average wind /gust ratio.

During the early afternoon the low, having just about reached its peak intensity, moved across NE France. Some exceptional gusts occured around noon in N France and over Central Southern England. Lee on Solent reported a gust to 78mph, the peak low level wind gust from the shores of the UK. Across the Channel, Alderney Airport (Channel Isles) saw gusts to over 70mph. It was much worse on the N French coast with Cap la Hague on the Cherbourg Peninsula reporting a midday gust of 80 Knts (92mph) whilst at 13Z Cap de la Heve nr Le Havre reported a 96mph gust.

In the Channel Storm -force 10- warnings were hastily issued during the morning and gusts exceeded 82mph at the Greenwich Light Vessel.

During the afternoon and evening the low tracked into NE France and Luxembourg slowly filling as the jet overran and left the low in a less developmental situation. The rain over Belgium turned to snow during the late afternoon. Paris did not see the level of winds in the exposed north, however with gusts to over 70mph the mean/gust ratio was very marked here, the rapidity of the gusts causing damage to some buildings.

A good example of a quickly developing depression which came under the influence of the sometimes very powerful North Atlantic jet stream. Had the depression come under the jet's left exit for a slightly longer period of time then we could easily have seen another (even more) damaging 1999 type of storm.