Archives For June 2013

Confidence now high for the HEAT WAVE starting weekend then continuing thru early next week and possibly beyond.  Updates later.

Early days still, but there is some indication of July warming up considerably from next weekend after a breezy mid-week. Models are coming together showing the Azores High building right across the UK from next weekend (7 July) bringing anticyclonic sunny and warm weather and a warm upper air flow. Some charts show temperatures in the south east exceeding 25°C and some even show 30°C! Whilst the UK is set to be much warmer than normal (positive anomaly), much of Europe is cooler than usual, still stuck in the trough of cool air.

warm UK 7 July

 It is worth remembering that the sun is very strong this time of year and, although a weather advisory seems tedious, it is worth slapping on the sun cream and adding a hat: slip,slap, slop!  Of course, our own heat wave will be nothing compared to the temperatures the SW USA is experiencing right now.  Record breaking heat is pushing temps up to the hottest ever recorded 54ºC.  That beats previously discredited records in Libya. ttp://

UV levels

Warm fronts do not usually grab the weather headlines. Warm air usually glides gently up and over cooler air in front and causes light rainfall. So it is tomorrow with a pencil thin line of warm front precipitation crossing the UK, slowly from west to east during the day.  

warm front friday cool poolA complicating factor is that the significantly warmer air advecting behind the front will meet the COOL POOL of air lingering in a trough over Northern Europe and the North Sea. This could intensify the warm front as temperature gradients and lift increase. Another factor of interest is the strong upper winds behind the warm front with only light winds at the surface,  Strong wind shear can encourage lift which causes more condensation, thicker cloud and more persistent rain.

cape and li fri 28 juneThe warmest air is kept in the SE where thundery showers are a low risk in the afternoon and evening but possible for a time after the warm front passes and introduces warmer air throughout Friday evening. CAPE values and lifted index values match a low risk of thundery outbreaks at any time afternoon through to early evening.  Temps will rise, unusually continuously during the course of the day from 12ºC at 9am to 20ºC still at 9pm: MUGGY! These factors indicate a warm front that may yield more interest than usual.  Eyes up, Brollies at the ready!

Good example of orographic rainfall this morning as a deep summer depression moves off into the North Sea and strong NW polar maritime winds bring heavy rainfall to the mountains of North Wales 23 June 2013. Note that the relief rain picks out the coast and the highest mountains where land rises steeply forcing more condensation and higher rainfall intensity. Note also the lower rainfall intensity in the valleys, Ogwen / Nant Ffrancon valleys can be clearly seen drawn out by lower rainfall!

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Reigate weekend weather will be windy and wet first thing Saturday with showers and bright intervals following after a well defined front moves away early. Typically, no deluge is forecast for Reigate and the East but early rain on Saturday could be heavy for a time around breakfast. Most rain will fall further west. The most significant feature of weekend weather for Reigate will be wind: gusting from the west over 35mph at times on exposed hills on Saturday, less on Sunday.  

A summer depression with a LOW central pressure for the summer (994mb) is passing straight across Scotland over the weekend. This has winds spiralling anti-clockwise around the centre of the LOW, for Reigate this means gusts over 30mph on the hills and generally widespread 20mph winds. Quite a breezy day for mid-summer. As the cloud breaks in the afternoon it will be good kite flying weather. More rain and cloud could be widespread on Sunday, some of it heavy and showery in the afternoon.
For next week things look much better as a large HIGH pressure builds across the entire Atlantic and pushes any more depressions well to the North over Iceland. This HIGH will settle over the west of the UK and build across the East as the weekend LOW moves out into the North Sea. We can look forward to some pleasant dry summer weather well into next week, reaching over 20C. But always with the threat of a shower in the east where winds will be from the NW or west.
So…total rainfall for the next week in Reigate is forecast to be a mere 10mm or so, hardly very much. At least 8mm of this is forecast to fall this weekend, making next week very dry indeed!

During the recent last spell of warm humid sticky weather with the warm southerly plume of potentially thundery downpours, Reigate got no showers at all! The capped inversion layer prevented any significant convection.  Despite immensely high forecast CAPES and lifted index, each forecast would predict heavy showers and potential thunderstorms but none would arrive as the last model runs would lower capes and LI.  Below the inversion, with no significant convection to break the cap, cloud simply spread out into dull strato-cumulus formations covering the sky and reducing the warming by the sun and the instability of the lower layer of the atmosphere was therefore reduced.  No cap was busted, no spectacular cumulonimbus were seen round Reigate.  Some occured nearby in Kent and the Channel and running up the N Sea.  Several models (NMM, GFS, NAE) consistently predicted showery rain almost everyday, some of it heavy, yet none arrived.  Other models were nearer the mark and picked up better on the lack of powerful convection beneath the inversion (UKMO, HIRLAM).  So, in Reigate, being so sheltered and pleasant, it never rains but it pause!


Interesting subtle switch in the weather coming up… surface winds today and tomorrow will slowly shift from a warm light easterly, round to southerly and thence to westerly usherring away the mini-Euro heat wave and bringing cooler normal Atlantic conditions by the weekend.
You might be surprised to know that, despite slack humid lazy conditions down here at the surface, the jetstream is alive and well and blowing warm air over SE England from Spain at a healthy 60mph+ at 5500m. This warm plume aloft is SO warm that it has created a temperature inversion at about 2-3000 feet, where temps actuallty increase with height. This effectively means rising thermals which could create thunderstorms do not break through the inversion cap. This is why we’ve had no spectacular thunderstorms…yet. The daytime temps have not risen high enough for the cap to be broken. Today this might change but it’s a low risk again and nothing may happen at all. Enjoy the last moments of continental heat… it’s drifting off today, albeit slowly, and being replaced gradually by cooler Atlantic air.

Before our westerly air returns let’s celebrate:

Wednesday 19 June could be a minor record breaker for Reigate, as we recorded an (unofficial) maximum temperature of 26.8ºC, one of the warmest temps in the UK! At 22:24 hrs it was still over 20ºC in Reigate… sticky.
This week we have stayed remarkably thunderstorm free despite high temps and humidity: the pressure has been rather high and storms have fed across the Channel further east up the North Sea and brushing Kent, missing Surrey entirely. Thursday may see our first thundery showers of this warm weather episode. Pressure is due to fall as a trough passes to the North of the UK and fronts pass over the UK from N France.  Whilst the warmest Euro air will move away to the NE the transition will cause instability and possible thundery heavy showers.
temp dipMind you, the precipitation forecasts from every provider have been wildly inaccurate this week, but this looks more certain than previous episodes. Finally, our mini-heat spike will fade away, quite slowly, over Friday and the weekend looks like a return to a wetter cooler westerly airflow, more like “normal”. 

One big benefit of the recent heat wave is spotting unusual Kelvin-Helmholtz wave cloud formations over Reigate (see photo above).  As the warm plume arrived from the continent it allowed layers of cirrostratus to condense in a more stable and cooler airstream below.  The fast moving warm plume churned the cloud into waves … like ocean breakers.  These are not the best examples but they looked pretty amazing and showed off the inversion cap preventing thunderstorms and convection on that occasion.

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!!

19-06-2013 06-55-47

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.

thunderstorm development

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!

thunderstorm form

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. 

jetstream in control

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!

15-06-2013 10-26-50 westerly stream

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

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

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!

lifted index thurs #

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!

monday 17 june