Reigate experienced an impressive deluge today when over 60mm* (tbc) of rain fell in a few hours causing flooding in parts of the town. This was a “surface water flood” caused by drains being overwhelmed by intense heavy rainfall, rather than a river flood, our local River Mole will react more slowly to this event and is unlikely to cause any problems. (*by 4.30pm manual rain gauge 64mm; radar netweather est 50mm; total 24 hour rainfall 74mm manual rain gauge; RGS AWS 36.6mm; Reigate AWS 50.8mm)
The entrance to Morrisons car park and the lower section of Bell Street were overwhelmed with water bursting from drain covers adding to the water running down the roads. It wasn’t just the roads that were flooded… such was the intensity of rain that Priory Park was also left with surface water pooling up several inches deep on the pitches, and even more in the sunken garden and around the cafe.
This rain event was accurately forecast by the MetOffice and MeteoGroup and several specialist severe and convective weather met agencies who all issued warnings of heavy thundery rain, some totals suggested over 60-90mm.
Two models, the MetOffice Euro4 and WRF NMM model output, turned out to be especially accurate with both putting exceptionally heavy rain precisely over our area at least 24 hours in advance and this was reported by RGSweather on twitter. The GFS model was less convinced about such heavy rain reaching beyond the south coast but, as always, these convective summer events are especially hit and miss. Today was a “hit” for Reigate but unfortunately would have probably caused a nuisance for businesses and commuters.
— Carolynne (@tea_in_reigate) August 24, 2015
The maximum intensity of the rainfall in Reigate was measured at 145mm/hr during the storm between 2-3.00pm. This intensity of rain is unusual in Surrey (highest rainfall intensity in recent years was 183mm/hr 20 Nov 2013) but we are told to expect more of our rain to fall in intense events like this with global warming. The last time flooding of this scale took place in town was 24 Dec 2014 but, having seen both events, I think this time was slightly “worse”, although given summer conditions the surface water may have subsided quicker. There are many areas for further investigation worthwhile here: how much rain, for how long and with what antecedent conditions (soil moisture / season / evaporation etc) and what duration of rainfall intensity causes flooding in Reigate?
The synoptic weather situation that caused this event was a trough over the Atlantic and a blocking HIGH over Scandinavia causing a warm unstable humid airmass to meet cooler Atlantic air along active fronts. The resulting LOW transported a lot of moisture over the SE and caused the rain in heavy thundery downpours triggered near the fronts. This situation is not likely to change much until later this week, so expect more rain but hopefully not as intense.
An unstable airmass with backing winds over the SE indicated by a skew t chart from the day.
Technically speaking this was part of a “trough disruption” event (see charts above) that started over the Atlantic during the weekend. A “trough disruption” is when an upper trough “breaks” and a southerly section of the trough proceeds purposefully east or NE leaving the centre of the trough behind as a semi-static feature. The isolated part of the trough can then behave erratically. In this case an unusually active and southerly dipping jetstream for the time of year also played a part in deepening the LOW at the base of the disrupted trough. Weather events can be “severe” and unexpected with models sometimes struggling to cope when trough disruptions occur.
The LOW pressure that brought heavy showers over the SE and Reigate formed in Biscay yesterday and travelled NE up the English Channel during the day. SE England therefore experienced SE / easterly winds in the morning, backing northerly and finally swinging westerly during the course of the day, fairly unusual for our part of the world. The anticlockwise change of direction in wind is called “backing”. Veering is the clockwise movement of wind over time or with height.
Here are some more pictures from this event on my google pics page: please use them but do credit rgsweather. https://goo.gl/photos/Rz9rpsAWozVaEtvB7
by evening it wasn’t any better:
— Noelle Vaughn (@noellevaughn) August 24, 2015
RGSweather photos featured on the BBC London news that evening (sorry, no sound)
Finally, we can expect more rain this week as the synoptic situation stays static with the Atlantic trough blocked by the high out East. This situation will keep us in a flow of humid moist air that, when it interacts with cooler Atlantic airmasses along fronts, is likely to cause rain. Add an unusually active jetstream and we have a decent recipe for a wet week.
links to other reports
the system went on to cause significant tornados in Netherlands http://www.dumpert.nl/mediabase/6677432/ac9ba4f6/poldernado_des_doods.html