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Heads up, Reigate!  Or rather… wrap up, stay in and keep warm: SE weather for the end of this week looks awful and, whilst nothing approaching the US Mid-West, it could be described as potentially severe for the SE.

Pick words from this list and repeat and then remind yourself it’s nearly June! “COLD, heavy persistent rain, windy, wet snow, sleet, hail, heavy thundery showers, freezing wind chill, 35 mph northerly gusts… at the end of May!

Thursday: bright start but with cold northerly winds +20mph and heavy showers building in the afternoon.  Feeling chilly at only 10ºC max.  Overnight temperatures fall to 3ºC and any showers could even fall as icy sleet or even wet snow on the Downs. 

Friday: things actually look worse than earlier this week: COLDER northerly winds will drag temperatures right down to 3 or 4ºC in heavy showers and a maximum of  8 – 10°C with temperatures falling lowest in showers.  The centre of a nasty deepening LOW pressure will track directly across the SE on Friday and we can expect persistent rain, some heavy at times and accompanied by really cold winds (for the time of year).  Later in the day heavy thundery showers have the potential to bring hail and sleet.  Significant instability is forecast (*CAPE >350 j/kg; before the Oklahoma tornado CAPE measured 5000 j/kg), currently the highest this year in the SE, which means there is a fair chance of severe weather events in the form of thunderstorms.  Watch out for mountainous cumulonimbus cloud formations extending to the height of Mount Everest and possible funnel or mamatus clouds under dark bases.
Freezing levels will be down to 600m and during any heavy showers evaporative cooling could potentially cause heavy rain to turn to sleet as snow forms above 1000m and heavy rain draws down colder air.  Evaporative cooling happens when enough of the heavy rain evaporates on the descent to take heat out of the air and freeze supercooled droplets or sleet into proper wet snow.  Strong winds gusting 35mph will make it feel truly nasty.  This all sounds ridiculous for May… but the potential is there..let’s hope it’s not quite this bad!

Friday night will be a chilly and wet experience outside.  Temperatures will fall to 4ºC but feel near freezing in northerly winds. Not good for camping.

The weekend recovers reasonably well with a weak high pressure ridge bringing bright and dry spell Saturday behind the Friday storm. The fine weather  could hang on through Sunday but showers could threaten as pressure falls away as a deep LOW approaches from the NW to threaten Monday with rain.  The rest of half term looks unsettled but a bit warmer and drier at the end and best in the NW of the UK where HIGH pressure is likely to ridge across.

*CAPE = convective available potential energy: how buoyant a parcel of air is and this determines speed of vertical extent of convective cloud formation.

In the end: Max 8.4C; Min 4.3C; Rainfall 8mm; gust 24mph NW; So, in the end, pretty foul and unpleasant but fortunately no observed wet snow or sleet anywhere.  Timings of light rain onset were good; heaviest rain pm and cold temps made it feel more like January. No thunderstorms heard but lightning was recorded in SE on ATD lightning detector; CAPES fell to just over 100j/kg in the last run of GFS in the morning, which is a little disappointing for extreme weather enthusiasts!

Last week our total rainfall in Reigate was 14mm, some models are predicting 40mm+ this week! Much of this will fall in heavy frontal rain mid-week and in thundery showers Weds/Thurs thereafter.  The jetstream is developing a meridional flow at the moment, which means it is meandering wildly and developing slow moving weather systems which sit around for long periods; the more usual zonal flow is when the jetstream flows west to east and drags weather systems briskly across the UK and keeps weather moving. Meridional flow gives rise to interesting weather: LOW pressure with heavy showers is the basic scenario this week: great for growing awesome cumulus clouds!

LOW pressure sits right over the UK all this week bringing sunshine and showers but with an added twist mid-week when a deepening short-wave trough is predicted to cross southern regions bringing potentially heavy frontal rain across the whole of the south, including Reigate. Thereafter, on Wednesday heavy thundery showers with hail could develop as winds slacken off.  Watch out for tall cumulonimbus clouds and rainbows! Any showers could build into single cell thunderstorms which will be slow moving in the slack winds and persistent and cause localised flooding. Around Reigate these showers will probably be quite isolated and some places will miss them completely but if you get caught in one you’ll know it! Temperatures remain uninspiring at 15C, although climbing slightly through the week. The end of the week remains very uncertain but could develop more heavy rain for us in the SE as the cut-off LOW over Europe feeds a miserably cool NE wind our way.
As always, this is early days and the end of the week looks especially variable and interesting!
Please check @RGSweather for updates!

25-12-2012 17-24-45 xmas day 12pm 2012

A heavy band of thundery showers caused hail, thunder and lightning over Reigate at 8.30am Christmas morning. A trough passed over the region and brought intense rainfall reaching 93mm/hr at 8.08am. Temperatures then dropped suddenly during the showers from 8C to 5.7C and winds gusted to over 25mph. Winds veered from Southerly to Westerly as the LOW pressure tracked north and out into the North Sea. The blue dashed line on the charts shows what happened as the trough passed over. The map shows the synoptic situation and the chart at the bottom shows lightning strikes occuring over Reigate at the time the storms passed over.

Over 16mm of rain fell on Christmas Day bringing the total for December to nearly 90mm.  With further rain forecast it is likely that Reigate rainfall will top 100mm by the end of the month: an “extreme” monthly rainfall for our area.  Our wettest day this half of the year was September 23 with 27mm of rain in one day.

25-12-2012 08-17-20 xmas day lightning and hail storm