Welcome!

June 22, 2013

LIVE

Gauges - FlashNow

Weather

Today  -  Yesterday  -  Trends

Extremes

This month  -  This year  -  All time

Online

Map  -  MetOffice  -  Twitter

Data

RGS data  -  WU data - WU forecast

Reigate Grammar School weather station, Surrey, SE England. School run website for local weather info and amateur weather observations and educational posts on weather. Forecasts are for educational and local information purposes! Please see UKMO for professional forecasts. Town location automatic VP2 wx updated to our own website pages every 15 minutes and “live” wind readings are updated every 5 seconds.  We also feed data to UK Met Office weather observations website (WOW) and to US based Weather Underground (WU). 

March 2014 was a contrast to last year.  The March average Reigate temp in 2014 was a warm balmy 8.7c, while 2013 it was only 3.5c.  March 2013 was cold, frosty and snowy, while March 2014 was warm and mostly dry and with good spells of sunshine.  March was also a welcome break from the winter storms as the jetstream broke up and meandered weakly north and south of the UK, rarely bringing in purposeful storms.

Like Reigate. the March Central England Temperature is 2c above the long term average at 7.6c.  This follows Jan and Feb 2014 being around 2c above average too.  This makes 2014 a significantly warm year so far for the UK.  Should these anomalies continue 2014 would be several degrees above average, making it a contender to be the warmest year on record.  However, this is extremely uncertain because we have a long way to go and the remaining months might work out being cooler!  It is quite likely that 2014 warmth will decline at some point, though when is not clear.

2014-04-01_22-57-19

 

Here are the raw figures for Reigate March 2014 (2013 in brackets).

Tmax 19.8c (16.1c)

Tmin -1.2c (-3.4c)

Average 8.7c (3.5c)

Total rain 29.9mm (67mm)

After the wild winter weather, March 2014 had no real weather drama in Reigate… it was calm with only moderate gusts of wind mid-month and mostly dry and quite sunny, with a total of 146.6 hours on our sunshine recorder.

Notable features of the month included several episodes of splendid altocumulus wave clouds.  These are caused when an upper air flows over hills or is caused to ruffle over an airstream below and the air starts to undulate.  In the upward undulations the air cools and condenses forming (more)  thicker cloud… the bands of cloud you see are usually perpendicular to the air stream.

A few interesting cumulonimbus showers with some thunderclaps occurred at times later in the month yielding some moderate spells of hail and also some fabulous double rainbows in the Spring sunshine.  The other notable feature came at the very end of the month when a Saharan dust storm covered the Southern UK and created a pollution incident exceeding 10/10 on the pollution scale in several parts of the UK including East Anglia.

 

The month was dry enough to allow some weather spring-cleaning.  We continued the process of calibrating the rain gauge using our own bespoke “trickle-ometer” courtesy of Ed in the physics department who developed our “calibratometer” to drip a tiny regulated flow into the VP2 buckets to get an accurate representation of a rainfall event without overwhelming the tipping buckets.  Results are not conclusive enough yet to warrant changing the VP2 settings… we need to run some more calibrations to calculate a precise % difference.  Meanwhile, we are using the CoCoRAHS rain gauge to check our readings and also a sister station in Reigate to cross-check measurements for the town to get a reliable average for rainfall.  Raw figures are available on the data page here.

As the wise calibration joke goes… “a man who has one watch knows the time, a man who has two watches is never sure”… the same is true for all weather instrumentation and calibration.

 

There is some interesting weather potential this week, but indications that the SE will see least of any drama which will be mostly further west. The well-established warm plume of S/SE wind from Africa/ Mediterranean and Spain has brought temps up to 19.4c in Reigate this weekend and 20c in London.  A warm sunny Saturday was especially pleasant.  The breezy S/SE wind is bringing Saharan dust falling over the UK, watch for this in any showers that might come our way on Monday.  Check your car for any dust.

Warm plumes from Spain can also introduce unstable moist air and these produce thunderstorms and showers when moist warm air converges or fronts  undercut the plume with Atlantic air creating lift. There is good potential for heavy convective downpours of rain this week due to these scenarios.  Whilst not necessarily a classic Spanish Plume the synoptic situation is very similar to May 1998 when large super-cell thunderstorms drifted to the North and caused torrential rain and flooding in parts of the North.

2014-03-30_18-37-25

Various indicators are used to establish the potential for heavy convectional rain and thunderstorms.  At times the charts show several of these indicators at unsually high levels for end of March / early April this week.  CAPE (convective available potential energy), LI (lifted index) and ThetaE (potential equivalent temperature) … these are all technical charts that are commonly used to assess how likely thunderstorms will be.  ThetaE rarely goes above 19c in the UK so temps of +12c in April are unusual so early. Unfortunately, the development of convective rainfall and thunderstorms is difficult for forecasters to predict for any one location for a particular time.  Thunderstorms and showers are, by nature, hit and miss affairs: one place might get large hail and a deluge while, a mile down the road might remain sunny and dry.  Nevertheless, watch out for some potentially heavy rain, especially if travelling this week.  Watch out for interesting cumulonimbus clouds too!

Total rainfall remains highest in the west and away from the SE where pressure remains higher and frontal action is more limited.  So Reigate may escape the worst of all this convective rain action but still worth keeping an eye out for rogue storms that may well come our way drifting most likely from storms in the Channel around mid-week.

Finally, models are hinting at high pressure building back in for the school holidays after this wet unsettled week. Whilst not yet represented on charts very convincingly, the models suggest a possibility of some reasonable holiday weather in the UK with pressure rising and temps above the 30 year average on the GFS ensemble mean.

 

 

 

Soil moisture: possibly the most under-rated meteorological measurement!  Rarely do weathermen get animated about the extent of wet sod across the country.  Nevertheless, soil moisture, usually measured in centimetres of water in the top two metres of soil or as % saturation (see maps below), has been found to control continental scale weather patterns, summer maximum temperatures and even heat waves and the extent of droughts.  So we ignore soil moisture at our peril, especially as soil moisture also controls vegetation growth and death and the ability of farmers to grow food.

How wet the soil obviously relates to how much it has rained recently.  During winter, in mid-latitudes, soils usually become increasingly saturated with a surplus of water building up as inputs of precipitation exceed evaporation which is reduced in the cooler months and shorter days.  During the summer, soils tend to become increasingly depleted of their moisture content as evaporation (output) exceeds precipitation (input).  This input and output of moisture forms an annual balance known as a soil moisture budget and is shown in the graph below.

2014-03-28_21-36-57

High rainfall during the winter builds up a lot of water in the soil.  In the spring time a high soil water content “uses up” more energy from the sun in the process of evaporation.  The more energy “used up” in evaporation, the more energy is lost from the system to produce sensible warming at the surface. Hidden energy, or latent heat, is required to change liquid water into water vapour.  So “latent cooling” reduces the amount of energy available to warm the atmosphere as long wave radiation.  So local temperatures can be depressed over areas of wet soil especially during a spring when wetter-than-usual soils might take a long time to dry out.  It might also be expected that, after a wet winter, there could be a cooler period until such time that the soil dries out locally and more energy becomes available to produce a sensible heat flux at the surface.

In fact, the effects of soil moisture go far beyond these micro-climatic changes and can have impacts that are continental in scale.  In a 2007 study (see below) it was found that 25% extra soil moisture could reduce continental Europe-wide temperatures by up to 2c from average summer maximums. Likewise, a 25% reduction in soil moisture could raise temperatures across continental Europe by 2c in the study period.  It was also discovered that higher winter and spring soil moisture could raise summer precipitation levels and change continent-wide pressure patterns.

The study from 2007 used reanalysis of computer weather models to investigate the impact of soil moisture on the European 2003 heat wave, the warmest for 500 years.  This heat wave killed over 20,000 people and caused crop damage.  The study found that by re-running computer models just with different soil moisture values, the maximum temperatures and heat wave intensity varied greatly.  Dry soils during the spring increased summer heat wave intensities while wet soils reduced the maximum temperatures.  The difference was significant, and in some localized regions the intensity of heat anomalies varied by 40% simply due to different soil moisture content at the outset of the model runs.  The largest differences were mainly located over central Europe.  It seems that differences in soil moisture have most impact across central continental Europe and progressively less impact on summer temps with increased distance further north.

Not only did soil moisture control temperatures, it also had a control over continental pressure patterns.  Dry soils built pressure through the middle troposphere, while wet soils could lower pressure.  This has numerous positive feedbacks: wetter soils reduce pressure which increases cloud formation and summer rainfall that enhance the wet soils.  Dry soils build pressure, reducing cloud formation, reducing summer rainfall, further drying out soils.

2014-03-28_22-22-50

So, in summary:

  • If the soil-moisture deficit is high, the dry soils raise the sensible heat flux, producing a deeper, warmer, drier low-level atmosphere: raising temperatures and enhancing surface heating and drying.  Increased drought risk.
  • If soil moisture is high, the latent heat flux by evaporation and transpiration dominates, enhancing cloud formation and a tendency for cooling.  lower temperatures and enhancing rainfall and further wetting of the soil.  Increased wet summer.

2014-03-28_21-46-22

 

The full article is here: Fischer_heat_waves_2007 (1)

A ridge of HIGH pressure brought excellent fair weather cumulus cloud formation over the SE today.  The sat pic shows cloud streets of cumulus over the SE where local convection, capped by an inversion at 5000 feet created beautiful fluffy bubbly cumulus for much of the day. This came after the “coldest” night of the year at -1.2c in Reigate and -6c in the far north of the UK under clear skies and polar air.

Another point to note was the opposite wind directions at different altitudes today: surface SE flows giving way to NW flows above 7000 feet.  The skew-t chart below illustrates these features of a super-lovely day here.  The fairweather Cu convection was surpressed later in the day as an advancing occluded front spread high level cirrus and cirrostratus across the sky thereby creating an enormous regional inversion of warmer upper air that effectively stopped further convection and the cumulus gradually faded with incoming cirrostratus and altostratus later in the day.

 

skew-t chart fair weather CU formation

skew-t chart fair weather CU formation

 

The rest of the week for Reigate deteriorates and becomes cooler and damper and more gloomy.  A large cut-off low is set to form over the continent, while a HIGH over Scandinavia will combine with this COOL-POOL to drag NE and easterly winds over the UK.

Usually such winds bring cool, damp and showery weather across the East and SE, including Reigate, with moisture picked up from the N Sea and temps from a cool early spring-time continent often creating layers of cloudy gloomy stratocumulus widely.  Thurs/Fri may see brighter spells with showers.

A LOW from Denmark mid-week is modelled to drift across the N Sea and this is likely to exacerbate the cold by bringing an extra-cool shot of cold continental air (-6 at 850hPa) over the UK mid-week: snow is possible over high ground of the Pennines and NE at this time.

Later in the week and at the weekend a subtle shift in the location of the Scandi HIGH and Euro-low could allow warmer SE winds sourced from the Mediterranean to filter into SE UK edging temps to the upper teens once again.  This is a fair way off still but will make the weekend a more pleasant prospect if this comes off.

Model predictions for the start of April look uncertain still with ifs-and-buts over how settled it will be.  Several sugges high pressure building at times early on but other models put a LOW over the UK for this period… so charts definitely worth watching for Easter school hols.

2014-03-24_22-21-06

 

The weather has been mercifully quiet, with mostly warm and sunny conditions for Reigate over the last 2 weeks. This is gradually about to change but no particular drama is afoot other than some perky and windy frontal rain arriving in Reigate on Thurs night /Fri am, followed by showers and cooler breezier weather with hopefully some interesting clouds over the weekend!

Our two week dry spell (well, 13 days) ended briefly today with a light shower.  As the blocking high sinks away to the S, a cooler NW flow will swing by over the weekend ushered in by an active cold front on Thurs/Fri which will mark the arrival of some cooler days and frosty nights through to early next week.

The charts show a typical early spring flip-flop in temperatures as warm air from the SW on the back edge of HIGHs is replaced by cool NW and even N winds as LOWS sweep across the North of the UK.  The back-edge of the LOW moving over Scotland in the next few days will drag down chilly Polar air especially on Saturday and Sunday.

Daytime temps will struggle to double figures which, when last weekend hit 20c might feel a bit of a shock!  This cool airtstream, fresh from Polar regions, will also be warming through from below and therefore get rather unstable: this means thermals will have a tendency to rise, and keep rising through the chilly upper air, creating towering cumulus clouds and showers.  The chart below shows a typically unstable temperature height diagram with some ingredients for a showery day capable of building some nice cumulus clouds: steep lapse rate, saturated airmass through a large column of the atmosphere, some wind sheer (change of wind speed or direction with height).

2014-03-18_21-35-31

 

By the afternoon especially on Saturday cumulus congestus clouds might even be tall enough for some sharp showers, possible hail in the cold upper air and the odd clap of thunder is not impossible.  Nevertheless, total rain predicted is low and there will be sunny intervals. April showers, but a month early!  Next week sees a continuation of flip-flop spring weather.  April has the possibility of some high pressure returning with sunny and dry weather but the exact timing of this remains uncertain.  There is also some indication of unsettled pattern in April too, so take your pick!

LOW over Scandinavia

LOW over Scandinavia

High pressure persists this week over the UK as shown by the chart and the amazingly clear satellite pic showing great swathes of clear skies over a remarkably snow-free Europe today 11 March. It’s worth remembering that on this day last year we saw the lowest wind chill temps of the year in Reigate at -11.9c early on 12 March with -10c at 9:30am on 11 March during an episode of extremely cold NE winds which brought a blizzard to the Channel Islands.

Back to this week… the high pressure has brought delightfully sunny days and Tmax temps to Reigate over 19c last weekend , especially warm and spring like on Sunday. Since then a cooler NE/E breeze around the edge of the high as it slipped north and east has kept us cooler and occasionally more gloomy with anticyclone gloom and stratocumulus blanket on some days.

The outlook this week remains settled but with a distinct drop-off of temps over the weekend and especially into next week.  This is due to the HIGH slipping west and the jetstream dragging down cooler NW winds fresh from Greenland.  A LOW moving across N Scotland and over to Scandinavia this weekend will start the process of dragging down NW winds that will make next week cooler, especially further east where it looks like the N Sea will get a full blast of chilly air at times.

Nevertheless, it is unlikely to become terribly unsettled down here in the SE as the longer term picture looks like high pressure hangs on in the south keeping us mostly dry.  The north of the UK might see more frontal action as LOWS bring fronts across more northerly parts.  Models seem to be keeping the south relatively dry for another week to 10 days.

After the wettest winter on record, groundwater levels in the chalk aquifers are the highest EVER in the SE and the Mole Valley particularly.  There are still 50 flood warnings in the SE and locally flood water continues to pour across the A25 between Reigate and Dorking near Brockham.  This groundwater flooding, though of course nothing like as damaging as the fluvial river flooding during the winter along the R Mole and the Thames, could last for some months yet as the winter rain continues to percolate into the chalk water table and flows out along geological boundaries at inconvenient locations such as normally dry river beds and bournes and across impermeable surfaces such as roads. The map showing soil moisture below shows how wet the soil remains after this extremely wet winter.

Quick stats summary for February and a medley of photos below…

Tmax 13.3c

Tmin -0.2c

Average temp 9.8c (5.9c 2013)

sunshine hours 101.7hours

Total rainfall  139.1mm (42mm 2013)

Strongest wind gust 43 mph

The sun rose for the first time over Svalbard, Arctic on Feb 15 after months of darkness.  Whilst the news from Reigate was not quite as dramatic, the weather in Surrey continued wet and stormy for much of the time.. check the sat pics below for the amazing run of spectacular low pressures that swept across the UK.

On the ground in Reigate this meant higher than average rainfall of nearly 140mm for the month, compared with 42mm last year.  February rainfall for the whole of SE England was 273% above average, so the Reigate total was not too far off that figure and highlights our generally sheltered location relative to other places in the region (i.e. the coast). Whilst not quite as bad as the Dec/Jan storms for Reigate, February wind gusts locally in the town peaked at over 40mph mid-month, and were considerably higher over the Downs (50mph+).

A notable feature of the month were big showers and occasional claps of thunder in cumulonimbus clouds (Cb) produced in unstable streams of polar maritime air often sweeping in behind occluded fronts.  These Cb shower clouds sometimes produced splendid rainbows and clearing skies at night, ideal for star gazing.  An unusual aurora (from solar coronal outburst) late in the month caused spectacular displays of the northern lights to be seen all over the UK.  In Reigate the aurora could be seen as a faint red glow away from the lights of London.  Although plainly less spectacular than the amazing aurora displays further north, this was nevertheless a special sight considering the rarity of such events.

http://www.surreymirror.co.uk/Excitement-Northern-Lights-seen-Surrey/story-20727615-detail/story.html

Weather is chaotic and numerical weather models are not perfect. The forecast for Reigate today went rather awry, though not completely.  It was forecast to rain heavily, perhaps on and off, but the forecast was for heavy rain more or less throughout the day. Check UKMO forecast from yesterday below.  Some models brought 24 hour totals of 20-30mm to SE at points on the lead up to the event.  The cause of the forecast deluge: a small scale low tracking NW to SE with a tightly wrapped occluded front crossing the area once, then lingering nearby to deposit more rain during the day before drifting off southeast. Once the front had passed through early am, it turned out to be a splendid day with sunshine and bright spells throughout, until rain later.  So what went wrong/right?

The front passed over as forecast during early am dropping 6mm on Reigate before 8am.  It then sat N of London most of the day while further south convection over Sussex caused significant Cb clouds and showers (some thundery) to spark off from midday.  For us in Reigate, we had a splendidly bright day with glorious sunshine by 8am and bubbly cumulus clouds thereafter, the odd spot of rain but nothing significant until early afternoon when the front migrated south east.  So for most daylight hours Reigate was dry, quite the opposite of the forecast.

The photos above and graphics below suggest a possible reason for this.  Reigate sat in a sort of “Goldilocks Gap” between the persistent frontal rain further north and convective rain nearer the LOW further south. It is notable that the convective showers built mostly over the land, showing almost April-shower tendencies to build on warmer land surfaces than the now-cooler sea. The occluded front sat close to Reigate, frontal wave clouds and cirrus were visible above and to the north for most of the day.  This may have helped suppress convection.  As warm tropical air is lofted over an occluded front it spreads out and forms a cirrus veil, this often suggests a broad inversion of warmer air aloft that effectively suppresses uplift of thermals: the cirrus acts like a lid.  So cumulus clouds over Reigate and the N Downs stayed small and harmless.  Not far south, in Sussex, thundery downpours developed as the buoyant air lofted uninhibited by any inversion.  You can see this on the radar image below.

Reigate was therefore dry for most of the day perhaps because of our location in a sort of Goldilocks Gap (our word) that was just far enough from the occluded front to avoid persistent rain and just near enough to benefit from the inversion to prevent convective showers. Met-Magic!  The graphics and photos try to explain this further.

This is just one possible reason why slight changes in the tack of a LOW will render a forecast completely wrong, even in the middle of a LOW pressure when all hope of a nice day might be thought lost.  Further ideas are most welcome to extend this.

Update for Reigate: tricky weather picture but here goes… 

Friday: wet cloudy day, mostly light winds for us, recent model runs now bringing in slightly less total rainfall accumulation tomorrow for Reigate.. around 10mm likely, more if showers perk up pm.  Rain arrives early hours Fri am with persistent frontal rain as occlusion wraps almost full circle around the SE, later in the day more convective showers are likely. All this drifts slowly off to the S later pm.

Fri-Sat some cloud and rain may persist late overnight, any showers turning briefly wintry over hills as temps cool off, gradually drying up through to early Sat am.

Saturday: Some light rain may persist from the remnants of slow moving front edging SE, though this may clear more quickly for a dry mostly cloudy day. Cool at Tmax 6c

2014-02-27_21-36-54

Overnight Sat-Sun: a new LOW crossing the north of the UK will sweep a front across our area and bring more rain overnight, due to clear Sunday am, though, again, latest UKMO run shows light rain a possibility during Sunday as this front weakens and becomes slow moving over the SE.  Any remaining rain

Yet another LOW pressure system, this one larger, is set to move in to UK quickly overnight Sun-Mon and bring more persistent and widespread rain overnight Sun-Mon and showers Monday.

2014-02-27_21-51-51

The longer term picture is rising pressure towards the end of next week so that next weekend could be much more spring-like and pleasant. Check back for details.

A strong jetstream to the SW of the UK will manufacture some tricky weather over the next 48 hours and continues to make details for the weekend uncertain.  Here’s an outline of the likely scenario for Reigate and SE but the advice must be to watch the forecast if you are doing anything weather-related!

The jetstream is located to the SW of the UK and is blowing strongly from NW to SE.  This means the UK is sitting in that vulnerable zone to the cool north-side of the jetstream where active small scale depressions can run-up rapidly and deepen off the left-exit region of the jetstream where temperature and pressure gradients are greatest.  These depressions are small scale but can produce a lot of rain and their track is sometimes rather erratic: and tiny changes in track can make a big difference to the weather experienced!

Two LOWS are predicted to spring out of the jet from Fri-Sun: the first during Friday – will track directly over SE England and drop possibly 10mm of rain onto Reigate.  As they track broadly S and E of UK, these LOWS will, in turn, swing the winds into a much cooler northerly or NE direction.  Reigate is unlikely to get any strong wind because the LOW pressures are due to pass almost overhead… meaning we are likely get mostly light wind but potential for heavy showers as the cool polar air “warms through” during any sunny periods which will cause convective uplift and showers.  There are also occluding fronts wrapped round these systems that could bring more persistent rain at times.  The second LOW emerges on Saturday and is currently due to track off in a more southerly direction through France but could bring rain across S coasts.  This one needs watching carefully as any movement north will cause a much wetter and unsettled Saturday than is currently on forecasts: which currently show Sat largely dry but with some poss showers later. Showers are possible as the cool air warms through and sparks convective showers on either Sat or Sun. Higher pressure ridges between these lows will bring cold nights, possibly frosty in places if cloud clears.

The complicating factor is that pretty cool air is wrapped up in these depressions, almost cool enough for sleet and wintry precipitation at times later on Friday for Reigate and during almost any showers that rotate around the remnants of the LOWS during Saturday and even into Sunday.  Whilst wintry ppt is unlikely for Reigate, further north and over hills in Wales and N England there will be snow, probably patchy. short lived temporary wet snow at lower levels but accumulations possible over higher ground.

The only way Reigate will see any fleeting proper snow is if we get any HEAVY rain during the passage of fronts later on Friday and any showers left behind in low pressure Sat/Sun.  All rainfall during this period will start as snow aloft, in fact almost all our rain in the UK starts as snow up high and simply melts as it descends.  However, if rainfall is heavy enough it can actually COOL the air sufficiently so that further precipitation falls as snow.  This is called evaporative cooling: some rainfall always evaporates on descending to the ground, even in cold weather.  If enough rain evaporates then this will cool the air sufficient for freezing levels to fall to the surface and then, bingo, snow will descend to the surface as well!  So, whilst rain can NEVER “turn to snow” (completely impossible!), it is possible for heavy rainfall falling through a sufficiently cool air mass (around -5c at 850hPa), to cause snow to reach the surface.