Weather extremes of 2004- Storms and deep Channel low of 18th-19th August

The synoptic background of the Channel low   The synoptic background to the low which moved up through the English Channel overnight on the 18/19th August 2004 was not straightforward. The air associated with the low had tropical origins. A week earlier Tropical Storm 'Bonnie' had been lashing the Florida Panhandle with wind and rain, and during the following days the energy from Bonnie (in terms of latent heat) moved northeast and merged with a North Atlantic depression.This was steered east and lay in the east Atlantic by 00Z on 18th with abundant moisture and heat energy associated with it.   The 00Z UK Met Office analysis centred the low at approximately 48N 15W with central pressure of 982mbs. It appears, from what happened subsequently, that the inner core of this low may well have been deeper than this initial analysis suggested. During the day the low drifted ENE towards Brest. There were no fronts left associated with it - in essence it was a convective low with shower bands around its circlulation. The low was initially forecast to track up the English Channel during the subsequent evening and night, whilst slowly filling. However morning forecasts hinted at a more organised band of showers moving east as the low retained its circulation for longer.   It is worth noting at this point that the 'GFS' model refused to pick up on the low at all, weakening it quickly as it moved ENE. The grid resolution of the models is such that they would not have been able to pick up on the tight inner core- if one existed- in any case. The UKMO's 'Mesoscale' model does have the required smaller grid space, however it feeds off the UK Global Model; hence errors from the initial global model analysis would have fed into the Mesoscale model in any case.   During the evening water vapour imagery showed discrete forcing aloft, moving east in tandem with the low pressure centre. Several bands of heavy showers swung NE into Brest whilst others developed ahead of the forcing over Cherbourg and Normandy, these later moving NE into the SE of England. Pressure falls from Ushant Lighthouse (On the Isle of Quessant - Brittany) started during the afternoon as expected, however during the evening they increased rapidly indicating the core of the low was tight and deeper than the 18Z analysis had indicated (subsquent information suggests it was at least 5mb too shallow) By 19Z on the 18th Pressure falls over Brittany were large, down 7.2mbs in 3 hrs at Ushant at 20Z and down 3.1mbs from the previous hr. The winds picked up rapidly too, with a gust to 49 knts at 20Z.   Slightly further south at the Pointe Du Raz (an exposed coastguard station in Western Brittany) the winds increased still further. In the 2 hrs from 17Z to 19Z the pressure fell 4.1mb and the wind increased to be gusting as high as 60 knots. An hour later at 20Z the wind became even more extreme, a mean wind of 54 knots was recorded along with a gust to 68 knots (78mph). Even inland the wind become strong with gusts to the mid 40 knot level. At 21Z, the station of La Tulet ( an island off the south coast of Brittany) saw a gust to 62 knts, however there were no significant pressure falls to report in the region prior to the wind increasing - it seems that the small tight circulation did not move near enough to cause a fall in the pressure, however it was falling rapidly 60miles to the NW and so a large pressure gradient became established across this region.   The low tracked east during the night. Winds at Jersey increased through the evening. At 23Z the pressure tendency at Guernsey Airport was down 5.3mbs in the previous 3 hrs, whilst at Ushant lighthouse it had risen as much as 11.4mbs in the same period. A huge isallobaric gradient therefore existed between the Channel Islands to the east and Quessant to the west. The low continued its track east, with large pressure rises to its rear. The strongest winds seem have been located just to the SE of the main low, Boulogne gusting to 54 knts at 06Z and 07Z. However, the very highest gust recorded appears to have been in the Dover Straits with Sandettie Light vessel reporting 74 knots (85mph) at 05Z. An hour later the mean wind was 49 knots -or storm force 10. Storm force 10 warnings were hastily issued for the corresponding sea areas (Dover and Thames) as the low tracked east.   The low continued to fill through the morning, whilst maintaining a tight centre. The lowest pressure at the Sandettie Light Vessel was 995.4mbs. Had the low tracked slightly further to the north, over the SE of England with such winds and gradients, then the story might have been a very different one- and deemed rather more newsworthy!