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Reigate Grammar School weather station started recording weather data from August 2012.  Data is updated to our own website pages every 15 minutes and “live” wind readings are updated every 5 seconds.  We also feed data to the UK Met Office weather observations website and to US based Weather Underground (WU). Follow @RGSweather

Complex thundery weather in SE England this week made simple (ish)!

thundery weather made simples

Update: pressure building to North over UK Weds and highest risk thundery activity to very SE with highest euro temps.  We expected MUSE but only Justin Bieber turned up!!

OK, so thundery warm weather is on the way for SE and Southern England first half of this week (a “heat spike from Europe with a cool down by next weekend, sadly). What’s the cause? Without mentioning CAPE (convective available potential energy) or LI (lifted index) or instability… here goes a simple and quick version of what is causing the potentially explosive weather early this week… and if you get caught in one of the multi-cell thunderstorms it could deliver a potentially whopping amount of hail and rain in a short space of time.  The atmosphere is, at times, going to be unusually energetic.  Watch out for spectacular cumulonimbus clouds and anvil heads and possible mamatus clouds.
To get big thunderstorms you need moist warm air which wants to RISE into the atmosphere (called unstable: once it starts rising, it won’t stop… until it hits the tropopause which is about 10 km up and the top of our weather / ultimate lid on clouds). These moist rising thermals naturally occur most freely in LOW pressure where air is liable to rise when the surface heats up in the strong June sunshine.  A slack moist air flow and strong sunshine over the UK ticks this box.

As the thermals rise it cools down and any water vapour will condense and form droplets of rain and ice which are visible as clouds.  When water vapour condenses it releases more heat (latent heat) that, in turn, gives thermals more lift and drives them higher.  This causes the explosive effect of rapidly rising bubbling cumulonimbus clouds.  Watch out for these rapidly growing cumulus clouds as precursors to big thunderclouds this week (cumulonimbus).  The updrafts of rising air can rise vertically pretty quickly (updrafts at 45km/h or more), like a hot air balloon. 

All these elements are in place this week: moist slack air over the UK will be met by an overriding warm plume of relatively dry air from Spain and North Africa (Spanish Plume) at a warm front over the English Channel.  The final ingredient for really big multi-cell thunderstorms is a difference between wind speed at the surface and aloft, called wind shear.  If thunderclouds stay still the cold rain causes downdrafts of cold air which cut off the ririsng thermals killing the thunderstorm after a few hours. Fast moving air at altitude allows warm updrafts to continue being pumped into the moving cloud and continue the growth of and activity of the thunderclouds.  The cold downdraft gust fronts pushing ahead of thunderclouds can cause more warm air to lift and spawns more baby-thunderstorms, hence “multi-cell”.

A more technical view can be seen in the tephigram above.  This shows a cross section through the atmosphere and the steep reduction in temperature with height (lapse rate).  Notice that, at first, there is a “cap” or lid at low levels (below 500feet) which stops thermals from rising.  This inversion is necessary for the most explosive thunderstorm development.  The rapid warming of thermals at the surface eventually allows them to break through this lower lid (inversion) and to rise uninhibited to the top of the atmosphere giving a potentially very great vertical height to cloud formation and severe weather below. Hail forms in such thunderstorms because water droplets rising and falling with up and down drafts freeze at great height and fall to earth before fully melting.

Finally, tornadoes could even be formed if there is sufficient wind shear (change of wind speed / direction with height) to rotate rising air columns which eventually twist round into the vertical. Check the diagram and video below for a summary of these processes at different levels of complexity!

Mid-Summer model mayhem: Euro heat or Atlantic cool?

deep LOW to sink south

will this HIGH keep South dry?

Update Sunday: EURO-HEAT is winning out briefly this week! Possible for 30C mid-week with thunderstorms. More later mini heat wave!?

Forecasts for this week are swinging all over the place… from heat licking in from Europe over the SE and bringing attendant thunderstorms to a ridge of the Azores High building into the South and calming things down for a time, but keeping things in a westerly airflow and generally Atlantic and unsettled.

So, after possibly very heavy rain / thunderstorms for us in SE on Monday / Tues what will it be for the rest of this week… Euro-heat and thunderstorms or Atlantic cool and breezy (i.e. a continuation of what we have now)?

The forecast essentially depends on three things: the movement of the deep LOW shown hovering off the SW coast on the Sunday chart above, the strenght of the Azores High and the location of our good friend the jetstream (overhead at the moment, further south than it ought to be!).

The LOW is forecast to form a trough over Spain and sink south over the Med producing showers there.  This will leave a complex of fronts in N France with heavy rain moving north over the South of UK on Monday and Tuesday (heavy showers, poss thundery Monday pm).  If the Mediterranean LOW yo-yos back to the UK later in the week it will bring a plume of heat with it but latest model runs show a ridge extending from the Azores HIGH shutting out any Euro-heat and keeping the UK in a westerly airflow pretty much under the jetstream… meaning continued unsettled weather especially in the North, for the coming week.  Essentially this means the jetstream (below) stays in control of UK weather and brings a succession of LOWS with rain and wind and brief calm dry spells, especially in the south of the UK which fairs better.  If the Azores HIGH can build into the South this will calm things down here for a longer period.  This looks most likely at the moment from midweek onward until another LOW pressure comes in on Friday – the longest day. 

Yo-Yo LOW key to our weather on the run up to the longest day

warmer but wet

classic cold front clears on Saturday

heat leak from Europe

Heat from the Med

SE licked by continental heat

Mid summer model mayhem! Update Saturday am: Rapidly changing situation… recent runs (2 charts below) show much reduced risk of any thundery activity later this week; perhaps some Monday pm and eve but ridge building over South should calm things down but shut door on any big heat from Europe. Keeping air flow westerly and unsettled. Things are changing on every run so check back for updates!

This week could see a warm plume of air from southern Europe causing temperatures to rise significantly in the South especially.  This may also bring thunderstorms. A real mix this week of wet, windy, dry and sunny, warm, humid and thundery weather could be on the cards. Check back for updates as things get clearer! 

The North and Midlands saw some very thundery downpours today, whilst the south experienced unseasonably high winds over 50 mph in places.  Reigate had peak gusts of 33 mph being sheltered by the Downs and Priory hills surrounding the town here!  Anyhow, it could potentially be the turn of the SE to see some (more?) severe weather later next week, but it’s a long way off so stay tuned for updates and on @RGSweather on twitter.

The weather really is not going to play ball next week despite being on the run-up to the longest day of the year.  Some decent warm weather is possible mid-week but much of the rest looks unsettled, showery at times and occasionally windy too: it’s hardly looking summery except the potentially warm maximum temps mid-week prior to any thundery interlude later on.  Whilst Reigate temperatures could be set to rise to 23ºC by middle-late in the week as air from a heating Europe is fed our way, any potential temperature “spike” is likely to be regrettably brief and spark off showers and even thunderstorms as pressure stays low.  This remains uncertain and will change but models are suggesting a potential “heat leak” from the continent over the SE for a short time later next week but, as suggested, if this happens it will be accompanied by very heavy thundery downpours for the SE.

Before all that potential excitement, the start of the weekend will be dominated by the passage of a classic depression with a perky cold front carrying some moderate to heavy rain crossing the entire country overnight Friday and clearing the east coast on Saturday morning.  A strong jetstream will push this through quickly.

rain clearing Saturday

This will leave a breezy day of sunshine and isolated showers hanging around for most of Saturday, though the UKMO plays these showers down. Sunday looks cloudy and mostly dry until later in the day when another low, this time approaching from further south west, brings potentially heavy thundery showers to Reigate on Monday afternoon as it lingers near the SW.  Rain could linger, lighter, through Tuesday.  This low is not due to cross the UK, instead it will sink south and then yo-yo back up later in the week.

yo-yo low

Now, there is a chance of some big thunderstorms getting into SE England around mid-week.  Most models agree that a trough will develop as the LOW sinks south.  The jet stream will cut off this low as it floats around Spain for a day or two basking in heat and warming through.  The story thereafter becomes uncertain but the cut-off low could yo-yo back north and feed some continental heat and energy back up north to the UK by Weds/ or Thursday. Heat advecting into a cool UK will increase lapse rates (decrease in temps with height) and could set off whopping convective showers and thunderstorms.  Another measure of thunderstorm potential is called lifted index (shown below) The Lifted Index (LI) is defined as the temperature of a rising air bubble when it reaches about 5,500m subtracted from the actual temperature of the environmental air at 5,500m. If the Lifted Index is a large negative number, then the parcel will be much warmer than its surroundings, and will continue to rise = convection! Thunderstorms are fueled by strong rising air which remains much warmer than the air surrounding it (like a rising hot air balloon) thus the Lifted Index is a good measurement of the atmosphere’s potential to produce severe thunderstorms. The map below is the latest GFS run showing LI across Europe on Thursday.  Note the salmon red colours in UK!

The movement and location of this YO-YO low is critical in determining whether this happens at all.  If it matches the GFS it could cause significant thundery weather by dragging in a “lick” of very warm air over the south east and east of the UK, if it runs with the ECMWF it will probably stay south, moving further east and fill over the continent and be potentially less interesting.  Worth watching the yo-yo low!

Jetstreak threatens to bring heavy rain to South tomorrow

LOWS develop to North of jet

saturated atmosphere right to the top

windy at 5500m!

jetstreak

pressure pattern at 500hPa

A jetstreak is a thread of extremely strong winds embedded into the jetstream. The jetstream is now in charge of our weather and is directing a family of LOWS across the NW of the UK this week.  Tomorrow, Wednesday, a strong jetstreak is forecast to develop overhead across the south and SE of England and reach a maximum strength Wednesday afternoon through to evening and overnight to Thursday early hours. At 5500 metres and above wind speeds will exceed 120 mph and this will lead to rapidly rising air motion which, in turn, causes excessively deep cloud formation throughout a saturated atmosphere and … heavy rain.  In Reigate, totals of over 10mm are forecast tomorrow, not terribly much really, but most of this will fall in a few hours in the afternoon. It will also be quite windy at ground level, possibly exceeding +35mph gusts.  Temperatures will be strangely warm during all this (+19ºC) as the air is largely tropical in origin, so it could feel quite balmy and humid if you happen to rush between shelter or as you battle with your umbrella.

Needless to say, the thread of heavy rain in the south, shown left, associated with the jetstreak could move north or south and we might miss the big action altogether. Nevertheless, put summer on hold for a while and grab the wet weather gear!

Rapidly developing lows to the left exit region of this jetstreak will also cause heavy rain to the North as well.This system is associated with the decaying remnants Tropical Storm Andrea which soaked the east coast of the USA last week before crossing the Atlantic to throw some of her dying fury at the UK.  She is rather weary but still capable of lashing out, let’s wait and see what she has left tomorrow! 

Andrea’s arrival peps-up the Atlantic weather party!

Ageing Andrea on the attack

dry over Europe at last

warmer, wetter

Atlantic weather streams in

dry hangs on in East

zonal upper flow

return of the jet

wet end of week for SE

Post-Tropical storm Andrea arrives in the Atlantic this week and sets the scene for a significant switch back to a more Atlantic dominated weather pattern (called “zonal”: where a stronger west to east flowing jet stream brings in Atlantic weather from the west). Andrea is the first tropical storm of the 2013 Hurricane season and soaked the east coast of the USA from Florida northwards last week.  Her arrival in the Mid-Atlantic coincides and perhaps helps to nudge our weather back to a more “zonal” flow and a positive North Atlantic Oscillation (HIGH over the Azores and LOW over Iceland = “normal”!). Andrea is ageing and much-weakened since thrashing the US east coast.  Whilst she is unlikely to storm ashore by herself, she is set to merge forces and deepen a slack LOW in the Atlantic and gather strength ready to sweep fronts across the UK, especially the NW which will get most of the rain this week.  The “Andrea” system will arrive in SE England on Wednesday with windy conditions as isobars tighten.  Reigate and SE is likely to escape the worst of the rain, at least early in the week, as HIGH pressure builds, at last, over Europe (giving them a drier week after record breaking floods and rain in Germany, Austria and Czech Republic). update Mon: some rain arr Tues with fronts. Most rain Wednesday.

The plus side of this switch back to a westerly weather pattern is that the east will see the back of the “spoiler” cool NE wind which brought some gloomy cloud cover in the mornings and colder than expected “warm dry days” this weekend and last week where promises of 25°C never happened in the SE.  With a warmer flow swinging around from South and SW temperatures should climb away this week to a real 20°C and actually feel properly warm as humid air is fed over the SE from the balmy South West rather than the cold North Sea.  Update Monday: temps will fall away to mid-teens Tmax as rain spreads more quickly east than first models predicted. For Reigate and the SE the heaviest rain is likely to be at the end of the week as heat builds over Europe and possibly sees convective storms associated with a front over France bringing in heavy rain for Friday / Saturday.   This warm feed of air has the potential to be a Spanish Plume event causing significant thundery activity in the SE.  Check back later for updates on this exciting potential! (looks less likely on latest runs; but wetter weather mid-week more likely now!)

Thereafter, things look to be set in a cooler unsettled cyclonic pattern with LOW pressure systems storming off the Atlantic bringing wetter conditions for the rest of June but still much uncertainty on this one. 

In summary: this week the wind will clock round each day from a cool easterly (from North Sea) through Southerly to humid SW and weather is likely to become more mobile but with dry weather hanging on longest in SE and feeling warmer too!  What’s not to like?  It’s Summer, enjoy!

links Andrea on NOAA 
http://www.nhc.noaa.gov/refresh/graphics_at1+shtml/143437.shtml?gm_track#contents

Reigate May 2013 summary: a cold-rinse cycle!

May Reigate 2013

After a good start with temperatures climbing to 22ºC in the first week, May 2013 was dominated by the UK being stuck in a TROUGH (low pressure) of relatively cool air producing occasionally convective heavy showers and unsettled weather. We called it a “cold rinse cycle” and, nationally, it was the coldest May since 1996. Whilst Reigate escaped the worst of this, some parts of North and West and NE of the UK suffered downpours and thundery showers.

A meridional jetstream flowing north-south fed cool upper air down to Europe from Polar regions making the UK and France rather cool while Eastern Europe (on the northward limb of the jet) was bathed in warm air from the Med: heat wave! The other effect of a meridional jetstream is that weather systems get stuck and don’t move. So it turned out that the trough over Europe did indeed not budge far for the whole of May. With HIGH pressure shifting north this week, the Euro-trough continues to linger to the south of the UK for the start of June. The persistent LOW pressure over Eastern Europe has caused storms and extensive flooding in Czech republic and Germany, while the warm northward limb of the jetstream has caused a heatwave further north in the Arctic (Finland reached >30C!).

 

The trough of low pressure over Europe was persistent and air pressure in the SE of the UK stayed pretty low throughout mid-May.  The northerly jet stream pegged back temperatures too, allowing strong May sunshine to kick off showers.As you’d expect, there was a good correlation between lowest pressure and episodes of heaviest rainfall. Nevertheless, periods of showery activity were very tricky to forecast and the GFS model seemed to over-do the rainfall over SE England on several occasions, seemingly not at ease with an easterly flow. Nationally, Spring 2013 has been the coldest for 50 years.

Reigate summary stats:

• Highest temp 22.7ºC on 7 May
• Lowest temp 1.7ºC on 3 May
• Av temp 10.8ºC
• Rainy days 16
• Total rainfall 65mm of which 14.5mm fell on 28 May

Interestingly, June seems to be starting in a similar way with HIGH pressure drifting to the North and a trough sticking over Europe… time will tell as to how unsettled it gets after this first week of super June weather!

Find full May stats on data page

Also, visit our page on MetOffice for some interactive graphs

Heat builds in the UK this week…but risk of a twist at the end!

1. Saturday warm across Europe

2. warmer…warmer!

3. climbing temperatures this week

4. HIGH to North, LOW to S = easterly winds

5. rain is across the Channel

Temperatures through this week look increasingly summery with dry weather courtesy of our summer friend the “Azores High”.  Coincidentally, the meteorological summer starts today, June 1st.  Temperatures are due to mostly increase this week in Reigate possibly topping out at 25°C by next Saturday but the building heat comes with a twist!  The HIGH pressure we are enjoying over the UK right now is due to slip North and then West. UPDATE: this will take its time… breakdown looks like Mon/Tues next week, weekend fine!

This will expose us increasingly to LOW pressure over Europe and a feed of warm air from a LOW in Biscay.  Summer heat combined with LOW pressure can yield thunderstorms as thermals lift into the atmosphere creating big cumulonimbus clouds.  A feed of warm air (warm air advection) will also encourage this thundery activity.  

The continental LOW over Europe has been a breeding ground for thunderstorms and heavy rain which has plagued the continent recently.  With high pressure possibly leaking away from the SE UK by next weekend, models are suggesting that, with a stronger sun and lower pressure, heavy thundery rain could invade from the LOW pressure to the South.  So, enjoy our summer friend this week, the Azores HIGH, because it could break down and allow convective storms and fronts to move across the UK from the continent by next weekend.  

Increasing storm risk

The above graphs show rainfall and CAPE.  CAPE is a measure of convective available potential energy and is a predictor of showery and thundery activity. CAPES so far this year around Reigate have been max of around 300 j/kg: giving a few thundery showery days during May.  Some model runs predict CAPES of 800j/kg later in June which could yield some bigger thunderstorms.  Nevertheless, recent tornadic supercells in Oklahoma have readily built CAPES in excess of 6000 j/kg so ours are small-fry by comparison.  Also, this is along way off and models do get things spectacularly wrong, so storms are certainly not a certainty but worth considering as an idea. 

Nice early June: good effort in the UK, but warmer in Northern Iceland!

850hPa temps: check Iceland

UK dry, Europe wet!

Iceland warmer than usual, UK cool

fohn effect #2

northern iceland feels the heat

fohn wind over Vatnajokull ice cap

anticyclone over UK

Good news! HIGH pressure is building across the UK for early June and should last well into next week. The “Azores High” is moving our way and this will bring settled, calmer, warmer, drier, brighter and occasionally sunny weather to Reigate and much of the UK. Some rain is possible as fronts brush across the west and a trough over Europe brings a low risk of thundery showers later next week for the SE but this is not certain at this stage. The weather is looking mostly good for early June with temps reaching Tmax 20ºC.  

Despite this the SE will remain average or cooler than average for the time of year due to being on the cool side of the HIGH: winds will be from a cool east or NE crossing the North Sea (still cool 8-9C) as the HIGH migrates to the North and East of the country.  

High pressure is where air sinks over a wide area. Sinking air is caused by cooling either from Tropical air chilling over comparatively cool oceans, like the “Azores High”, or air cooling as heat is lost from frigid continents in winter, like the continental HIGH over Asia./ Siberia.  Subsiding air inhibits convective updrafts of air (thermals) which create cloud and rain, so HIGHS are usually dry. HIGH pressure systems, also called anticyclones, also BLOCK the passage of low pressure cells pushing frontal systems away to the edges.  So, HIGHS are more or less the “opposite” of LOW pressure which has dominated our weather for most of May. In LOW pressure systems (cyclones or depressions) air rises, cools and condenses to form clouds and rain.  

Air flows from HIGH to LOW: that is, winds blow from areas of HIGH pressure to LOW pressure.  Due to the rotation of the Earth wind is deflected to the right in the Northern Hemisphere.  This is called the coriolis force and creates the familiar pattern of spiraling winds circulating anti-clockwise into LOWS and clockwise out of HIGHS in the Northern Hemisphere (it’s the opposite in the Southern Hemisphere).

 

fohn effect #1

The HIGH is forecast to move to the NE of the UK and this will bring in cooler NE and E winds to the SE: but light winds always. Remarkably, on Tuesday the relative location of HIGH pressure to the East and LOW to the SW of Iceland will mean Northern Iceland could be warmer than Reigate! The reason is an unusual weather phenomenon called the Föhn Effect.  Iceland has big ice sheets and mountains in the south and central region,including Vatnajokull, which is the biggest ice cap in Europe and has the highest mountains in Iceland.  The southerly / SW wind building over Iceland, created by the movement of the HIGH to the east and movement of a LOW to the South, will roll up and over these mountains and ice sheets and create LOTS of rain and snow!  On the windward slopes, in the teeth of these SW winds, saturated air cools as it is forced to rise over mountains.  However, as it it cools there is a release of latent heat due to condensation.  On the leeward side of the mountains and ice sheets the comparatively dry air sinks and warms up at a faster rate than it cooled. This is because the descending drier air warms more rapidly than the ascending cool air cooled (due to that latent heat being released)! (Still with me?!)  The result is a warm, dry wind streaming down from the mountains of central Iceland and bathing Northern Iceland in temperatures over 18ºC. It is rather like an “extreme” rainshadow effect experienced to the east of mountains in the UK, for example.  Föhn winds are associated with most high mountain ranges which experience stable saturated airmasses being forced to rise over them, such as the Alps and the Rockies.  In the Rocky Mountains, the Fohn wind is called the Chinook or “snow-eater” which melts snow rapidly across the plains at this time of year.

UP and away…ride with a weather balloon!

follow the balloon

UP through cloud layer

cumulus cloud viewed from above

edge of space just before balloon burst

Severn Estuary from very high up

Weather information is captured from satellites, surface weather stations, ships, planes and also weather balloons which are launched regularly from sites all over the world.  Weather balloons have instruments which continuously capture data during their ascent through the atmosphere on several weather parameters including temperature, wind speed, humidity, pressure etc. The map shows the track of a balloon launched today, that ascended to 40,000 m over Swindon. The car icon shows the chase vehicle used to collect the instruments when the balloon eventually falls to earth.  You can track more weather balloons live at http://spacenear.us/tracker/