Deep low develops near SW England over May Bank Holiday

DIAGRAMS TO BE ADDED AND ADDITIONAL CONTENT  SHORTLY

Introduction

The late May Bank Holiday Weekend 2007 became noteworthy as a deep depression lay close to southern England. In sharp contrast to the warm Easter weekend the weather was cold, wet and windy in many places. Rainfall totals of over 75mm (3inches) were reported in parts of the south and maximum temperatures struggled to reach double figures.  This article examines the synoptic background to the development.

Development  

The origin of the depression was on Friday 25th May over the Davis Straight where a developing shortwave upper trough was moving E-SE into the Atlantic. A weak surface low developed in response to this over and to the SE of Greenland. At the same time a strengthening jet was moving SE out of Northern Canada.  Initially the low was forecast to move SE on the warm side of the developing jet, however as the weekend progressed the shortwave trough was expected to sharpen and force the jet to buckle, thus helping to create marked development as the low crossed onto the cold side of the jet.

The low looked set to combine with increased baroclinicity and vorticity advection and deepen. All these factors seemed likely to combine to steer it, as a deepening low across the southern UK over the Bank Holiday weekend.

The problem faced by forecasters was the degree of development and the locations of the heaviest rain. Model oscillations of the event up to Friday were large, particularly from the GFS & UKMet office solutions, however there was a much more consistent story from the ECMWF. The reasoning behind this is quite likely to lie in its later cut-off time and 4D VAR Assimilation techniques which place it at an advantage; the upper trough and jet both originated from data sparse areas of the North Atlantic and the latters later cut-off time would certainly be useful in order to capture late obs from aireps (aircraft reports) made over N Canada and the Davis Straight. The ECMWF consistently brought a deep low (around 990mbs) over southern England/ the English Channel and then east into the near continent.

Models were also in disagreement about how quickly the low was set to exit. On Friday some guidance kept the wrap around close to eastern England on Monday likely leading to a cold miserable and wet day for many, whilst others cleared it away to leave (a still cool) but somewhat brighter and showery day. Low daytime maxima were another consistent signal.

By the morning of the 26th the strong NW jet was diving SE, SW of Iceland. At the surface a flabby elongated low stretched from 53N 20W to 57N30W. There were few upper reports over the jet's source region at midnight (over Greenland) and hence there remained rather large uncertainty over the subsequent evolution. The low's development was certainly not looking like a classic one and would involve a number of factors coming together over the subsequent 24hrs and possible development in-situ over the UK. In this regard WV & IR Imagery held few clues at this time; however WV imagery did show a marked extension of the jet and trough south of Iceland with a mass of growing cloud tops in the same region. It appeared that this trough extension and sharpening over the next 24hrs combined with a mixture of Vorticity Advection, increased diffluence aloft and an increase in baroclinicity would all very likely help in the development of a deep surface low in the SW approaches overnight and into Sunday.

There remained larger than normal positional differences between the respected NWP Guidance. The ECMWF remained “rock solid" in forecasting a track through the Channel at between 985-992mbs, whilst the GFS and UKMET oscillated between more northern and southern forecast tracks (both their 06Z runs on Saturday took the low further south, closer to the northern French coast).

Overnight the main development took place, and indeed was somewhat more marked than models anticipated, 00Z WV/IR Imagery is shown in Fig 1, it can be seen the low has crossed over onto the cold side of the (unusually) strong NW Jet and was undergoing cyclogenesis. This cyclogenesis co-located at the base of the upper trough which was still sharpening.

By 05Z on Sunday the low had reached maximum depth west of Seven Stones at around 981mbs and was moving slowly ESE and starting to fill.  The T+06 Frame from 00Z model cycle from the major centres did not do a very good job in resolving the depth of the low although positionally they were in the right place.  For example the 06Z analysis of the GFS did not respond to the plentiful obs and produced a centre just SE of the Isles of Scilly around 990mbs, some 9 mbs too shallow.  The UKMet was only marginally better.

The extra deepening that occurred seems to have stemmed from the earlier uncertainties with regards to the strength of the Jet. 00Z Aireps depicted an unusually strong 145 Knt jet core moving SE, (the jet running some 10 knts or so above 12Z guidance from Saturday due to the data sparse background over Greenland)  and this, when combined with the sharp upper trough and the increased baroclinicity as the residual high WBPT European airmass was engaged, all acted to deepen the low considerably.

Morning WV Imagery indicated the "dry eye" becoming co-located over the surface low indicating that maximum depth had been achieved and that slow filling of the low should commence. Surface winds were still lagging behind the development, however these soon increased and gusts to over 60Knts were observed over Brittany.

By Sunday morning rain had become extensive across England with overnight totals near or above 13mm across parts of the SW. After a cool night temperatures were struggling to reach double figures with most places seeing 9-12C - some 7C or so below average.

As the morning progressed large confluent pressure rises were now occurring on the western side of the low (up over 7mbs/3 hrs). At this stage a sting jet like feature now seemed to affect the western and southern flank of the wrap around with gusts above gradient strength, Gusts to over 60knts were reported from the Celtic Sea SE towards Brittany. At 10Z a gust to 67mph was reported from the Isles of Scilly and an 81mph gust from the Seven Stones Light Vessel nearby.  Later in the day the strong winds would move east, with 71mph reported at Brixham (South Devon).

The Sat/Radar Composite showing the expanse of rain over England and the depression swirl is seen in Fig 2.

The Met Office Synoptic Analyses from 14Z and 16Z are shown in Figs 3 & 4. These showing the slow moving occlusions over England and Wales and the depression moving slowly ESE towards the N French Coast.

By 16Z the deep low was located inland South of St Malo in Brittany, circa 989mbs and slowly filling. Large pressure rises around its southern and western flank were driving gusts above 60knt into the Bay of Biscay and around Southern Brittany (Meteo France issued an Orange Alert). A continued feed of both high WBPT and Vorticity was feeding north around the low and feeding the frontal zone which became slow moving over the British Isles.  The 1630Z Satellite/Radar Composite is shown in Fig 5.

Heavy rain now covered much of England and Wales, with marked PVA enhancement of the southern coastal rain band making for some particularly heavy rain moving into central southern England (see Fig 6 which is a composite of 1&2 KM Radars from Jersey, and those in Southern England).  In the clearer air heavy showers and thunderstorms affected the Channel Islands and the north French Coasts.

The forecasts from both the GFS and UKMet had both slowed (again) the movement east of the rain band. The 12Z GFS finally got a good analysis of the low, with a deeper centre forecast to clear ESE into France, filling at the expense of a new centre developing over Germany, this low centre effectively blocking the eastward movement of the occlusion and barcoclinic zone and reinforcing the positive feedback Vorticity which in turn maintained the status quo over England.  The rain continued into the evening and night across England. Fig 7 shows Chenies/Thurnham 1&2 KM radar at 23.45Z.

By Monday morning totals were over 3 inches (75mm) at St Catherine’s Point, whilst the rain continued over the northern Home Counties and London

24 hr Rainfall to 06Z (GMT) Monday (mm)