Archives For spanish plume

2015-07-09_11-50-48

June 2015 Reigate summary

June in Reigate, Surrey continued the Summer 2015 theme of mostly cool, dry and sunny but with an unsettled start and THREE attempts at continental heat from Spanish Plumes in one month. (please note that the chart above hugely exaggerates the rainfall.. check the rain scale in mm.  I have yet to fix down the scales month on month!). 

  • Average Temperature  15.7C
  • Tmax 30.6C (25.6)
  • Tmin 5.5C (6.2)
  • Total rainfall 15mm (30)
  • sunshine 192 hours (175)
  • max gust 28mph

(Figures in brackets are from June 2014)

The month started unsettled with a deep low pressure 976mb crossing Scotland from a very much cooler-than-usual North Atlantic bringing brisk winds for the time of year and comparatively cool temperatures.  Another Atlantic LOW crossed Scotland through the first week 991mb (see satpic).

A weak first attempt at a Spanish Plume 5-6 June developed ahead of an Atlantic cold front and gave some limited thundery activity early morning on 5 June, these cells went on to become more significant further north east over E Anglia.

Pressure rose thereafter as an anticyclone built firmly over the UK.   This HIGH eventually slipped north and a heat LOW from Iberia brought the threat of a second Spanish Plume around 12-13 June.  This misfired and caused little convective activity over Reigate at least.  This turned out to be a significant mis-fire for convective forecasters, despite some limited activity here and there the overall level of activity was low and certainly nothing occured over Reigate except very late in the day when some congestus puffed up.

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Spanish Plume mis-fired 12 June 2015

High pressure built again with weak fronts skirting across the SE bringing some pleasant mid-level and upper level cloud, nice sunrise and sunsets and some good atmospheric optical phenomenon at times.

Towards the end of June a third attempt at a Spanish Plume yielded more heat and more purposeful thundery activity that eventually spilled over into decent thunderstorms into the start of July.  This was a modified Spanish Plume and more details can be found on the post written up here and here.

The end of June 2015 heat spike produced some 30C+ temperatures and in Reigate 30.6C was recorded on 30 June.  Overall the month was sunny but a shade cooler than average according to the CET central england temperature record.

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The UK anomaly charts reflect the overall dry pattern with notably little rainfall for the month.  In Reigate the total rainfall measured was 15mm.

Although this June was not consistently hot tp push up the CET (central england temp), we did nevertheless have episodes of unusual heat, especially at the end of the month with the end of June / early July heat spike.  This heat wave was more severe and prolonged in Europe.  A “cause” of the Euro heat wave, with significant heat in Spain and Portugal, was an OMEGA BLOCK or “shruggie”  that built through June and lasted into early July.  This Omega Block pattern may also be linked to rapid melting of the Greenland icecap that has been recently reported as heat builds there under high surface pressure.

A weather pattern that resembles an atmospheric version of the shruggie — ¯\_(ツ)_/¯ — is directing furnace-like heat toward Spain, France and England on Wednesday, with high temperatures near 40 degrees Celsius, or 104 degrees Fahrenheit, as far north as Paris.

The heat wave is also affecting southern parts of England, with temperatures in the upper 30s Celsius, or mid-to-high 90s Fahrenheit. Those temperatures on Wednesday were the warmest recorded in the UK in at least nine years, according to the UK Met Office.

Andrew Freeman, Mashable 1 July 2015

heat wave / mini!

heat wave / mini! south east England Surrey

http://www.metoffice.gov.uk/climate/uk/summaries

http://www.climatecentral.org/news/maps-greenlands-melt-season-19196

http://mashable.com/2015/07/01/england-france-spain-heat-wave/

synoptic chart for heat spike

synoptic chart for heat spike

In the event: July 1st 2015 Heathrow Airport recorded highest ever July Tmax at 36.7C. More here

http://blog.metoffice.gov.uk/2015/07/07/on-the-record-observing-a-heatwave/

Reigate and the southern half of Britain could see some very high temperatures from mid-week next week.  Technically this will probably struggle to become a “heatwave” because it looks like too brief a hot spell, ending by or through the weekend, to exceed the 5 day threshold for an official designation (see below). Nevertheless, a significant HEAT SPIKE is certainly on the cards.

Some very high, possibly record breaking upper air temperatures are due to arrive aloft (forecast 23C 850hPa temperatures for South are quite unheard of in recent years) and, if it’s not too cloudy, 2m surface temperatures could soar to over 30C and possibly even nudge up to 35C.

This is not a forecast, and models will ebb and flow with the event intensity up to the wire, but it’s a review of some factors that will play a part in this heat spike episode.  The synoptic set-up, on charts below and top, shows a blocking HIGH developing over Scandinavia (an omega block) and a trough in the Atlantic with a heat LOW over Iberia. This set-up brings the well known “Spanish Plume”.  Typically this involves a warm, dry upper air flow from the South, drawn up by a perky northward limb of the jetstream and accompanied by an easterly or SE surface flow.  The combination is associated with heat and thunderstorms, though not necessarily extremes of either.

Whilst the Scandinavian Omega Block persists, the easterly continental wind and drier conditions should prevail over the UK but we are close to the edge of the anticyclone and in the line of fire from Atlantic fronts nibbling at the edges and thermal LOWS from Spain running north under the jetstream. The interaction of the advancing cool Atlantic air with the warm upper flow and the increasing surface heat can crank up convective instability as the plume migrates north across France to the UK.  The CAPE and skew-t charts below show the possibility of (elevated) thunderstorms by mid week.  Interestingly at times there is a strong cap near the surface, but great instability aloft, so storms likely to be elevated AcCast (altocumulus castellanus) with potentially strong lightning shows but maybe little rain getting to the surface, at least at first / Wednesday. CAPE is through the roof on some runs but too dry at lower levels and too strongly capped to yield widespread storms as shown below on the skew-t chart.

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upper level instability for potential elevated thunderstorms

Later in the week cool Atlantic air from the west is likely to interact with the plume, descending behind cold fronts and this process can cause CAPE values to increase bringing the chance of more organised thunderstorms that usually herald an imminent invasion of a cooler westerly regime.  Cool tropical maritime air behind fronts typically descends and causes increased lift as it runs into the unstable plume. Recent runs show the GFS wants to hang on to the heat longer while the ECMWF brings back westerlies more promptly by the weekend.  (update: now reversed!) This is not, therefore, likely to be a completely dry hot episode, because thunderstorms threaten especially after any really hot days.

The cross section below shows the flow of upper air clearly swinging round from a southerly direction.  Note the surface flow from the SE.  This combination, brief though it is, raises the risk of unstable conditions and thunderstorms indicated by the raised lifted index (LI) and Total Totals at the foot of the chart.  The average weekly 2m temperature anomaly charts below show how brief the heat might be… the second 5 day average returns back to normal.

Despite the likely short duration, it could be a notable period due to other factors playing a part.  Important ingredients that can be thrown into the mix of heat and thunderstorm potential are sea surface temperatures and soil moisture content. Despite a cool Atlantic, the seas immediately surrounding southern Britain are currently anomalously warm and ripe for transport of heat and thunderstorms across the Channel.

Similarly, dry soil enhances potential heat build up by reducing evaporative cooling and possibly kicking up temperatures higher as less heat is “used up” evaporating soil moisture.  Also, dry soil can enhance instability due to the rapid surface heat build increasing lapse rates.  I’m not quite sure how models handle these factors when producing their 2m temperature forecasts.

Locally, it’s worth noting the frequency of summer days exceeding 30C in Reigate, Surrey. Since 2009 only 29 days have reached or exceeded 30C and only 2 of these have been in June, most in July.  Also note the increase in hots days recently, though of course this is not a big enough sample to be significant.

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It’s still too early to be precise about when and how hot and how thundery it might get next week but the ingredients are at least in place for some very interesting weather!.  Further ahead this looks like a brief heat spike as westerlies resume promptly.  However, the overall pattern seems to favour a blocking HIGH over Europe which could continue to feed the UK with warm Southerly or at least SSW winds for a while.  A cool Atlantic also favours higher pressure so this ought to reduce the chances of very wet conditions, especially here in the South East. Phase 6 of the MJO is also correlated with blocking HIGH patterns over Europe.  So July ought to continue warm, dry and occasionally sultry but with possible thundery episodes.

Media references:

http://www.metoffice.gov.uk/news/releases/archive/2015/hot-july?utm_source=twitterfeed&utm_medium=twitter

http://www.theguardian.com/uk-news/2015/jun/25/mini-heatwave-forecast-for-uk-next-week-temperature-30c

http://www.dailymail.co.uk/news/article-3138816/Glastonbury-Wimbledon-UK-weather-Britain-set-hottest-temperatures-year-week.html

http://blog.metoffice.gov.uk/2015/06/25/hotter-weather-for-the-start-of-july/

http://www.theguardian.com/uk-news/shortcuts/2014/sep/17/continental-blow-torch-warm-weather-britain

http://www.telegraph.co.uk/comment/11733731/Met-Office-caught-out-over-its-hottest-July-day-ever-claim.html (twaddle)

http://www.express.co.uk/news/uk/587218/Heatwave-UK-weather-forecast-summer

https://notalotofpeopleknowthat.wordpress.com/2015/07/05/new-data-from-ruislip-casts-more-doubt-on-heathrow-record-temperature-claims/

http://www.carbonbrief.org/blog/2015/07/met-office-wind-data-dispels-doubt-about-cause-of-heathrow-high-temperatures/

05 june plume fax

Analysis chart on the day: tstorms arrived early Fri SE and grew across East Anglia

A brief, rather marginal, teasing Spanish Plume is developing in the next 48 hours and might cause the first significant thundery activity of the year in the SE Thursday into Friday this week.  A Spanish Plume (written up here previously from 2014) is a plume of warm air that rides north, out of Iberia, through France and over the UK.  It is often accompanied by a preceding south easterly from a warm or hot continent.  This mix typically causes humid weather and thunderstorms, but plumes vary in quality!

This particular episode is marginal for the UK, and taunting forecasters, because the heating episode is very brief and most of any heat and resulting instability is forecast to remain over the continent impacting N France and Benelux with decent thunderstorms before they migrate into the North Sea. The UK might see little of this in comparison.

2015-06-03_19-49-53

plume edges into UK

Whilst it is not certain that thunderstorms will develop over the UK, some action is likely in the S/SE and East Anglia during Friday. A MetOffice warning was issued earlier today but note the very low likelihood of moderate impact.

It’s worth recording here because it is a similar synoptic set up to the first plume event of June 7 2014 which created relatively perky thunderstorms to the SW and elevated rumbles and thundery showers over SE.  The charts below show streamlines at different levels associated with the plume on Friday.

The plume on Friday is a heavily cut-down version of the “heat wave” that the charts flirted with last week.  The heat wave is not going to happen but a day of warm / hot conditions will brush fleetingly past the SE.  Upper air over 15C is set to push in from the continent through Thursday night into Friday morning and will quickly waft over the S/SE and raise temperatures to Tmax 27C before quickly being pushed east by cooler Atlantic air arriving later Friday.

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Brief heat

This warm “heat” spike for the SE is caused by HIGH pressure (currently over the UK) slipping NE towards Scandinavia, followed closely by an Atlantic LOW edging in from the west.  The result is a humid surface S/SE flow from the continent with an upper airflow directed from Iberia meeting cool polar maritime from the Atlantic.

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The convergence of these airmasses can cause steep lapse rates (rapid drop in temperature with height) and instability, in which saturated air rises freely to a great height creating tall thunder clouds.  The charts below show some of the parameters involved: note the relatively high (but brief) CAPE values and PWAT (water content).  The limiting factor might be low SST temperatures over the Channel which could subdue any imported storms from France.

Instability can be caused locally if the sun comes out and heats the surface sufficiently to erode the “cap” and release parcels of warm air that rise, condense and form cumulonimbus clouds. Upper air is cold enough to cause hail as updrafts keep driving precipitation aloft into freezing cloud top temperatures below -30C.  If it stays cloudy we can still get elevated thunderstorms imported in unstable upper air from the continent.  Skew-T diagrams show the relatively unstable situation forecast for Friday.  A dry slot / hydrolapse (where dew point departs markedly from air temperature in a mid layer) can increase potential instability because evaporative cooling in any dry mid-layers can increase instability by increasing the difference in temperature between rising thermals and environmental air (though 700hPa dry layers I think are more significant than the lower layer shown below).

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skew t plume Friday 5 June 2015

It is most likely that the plume will be cut off rapidly before heat can establish and cause whopping storms.  Nevertheless, heavy thundery showers are likely here and there.  The latest GFS charts from lightning wizard suggest a low risk of meso-scale convective systems earlier on Friday being imported from France into the south coast in the LOW pressure moving North from Spain.

Later in the day any action is likely to migrate further east into the SE / East Anglia.  Most action will be on the continent where some significant thunderstorms are likely. The charts below show how rapidly the cold air makes progress east.  Once this arrives the plume is cut off and convective activity associated with it ends.

Looking ahead, pressure is expected to rise into the weekend in the south so it will be dry and pleasant, but not hot. Into next week things look similarly benign with high pressure mostly in charge in the Atlantic feeding a flow of cooler air keeping us a tad below average in terms of temperature. Into mid-June things become more unsettled.

2015-06-03_23-15-34

http://rammb.cira.colostate.edu/wmovl/VRL/Tutorials/SatManu-eumetsat/SatManu/CMs/SPl/backgr.htm

The “Spanish Plume” is a special weather set-up which can produce thunderstorms in the UK, as well as France and Benelux counties and beyond.

2014-06-07_08-06-18

A Spanish Plume is when hot dry air from the Spanish Plateau moves north towards the UK.  This occurs when LOW pressure exists in the Bay of Biscay and a HIGH pressure builds further east over Europe or N Europe / Scandinavia. An “omega block” is one such pattern but several different types of Spanish Plume are recognised.

As the warm dry Spanish air moves north from Iberia it picks up moisture from the Bay of Biscay, increasing the humidity of the lower layers but retaining dry air aloft.  The combination of these two airmasses, moist surface flow and dry upper flow, creates instability and potential thunderstorms. The unstable airmass can further lift over any relatively cooler air residing over N France and the UK (isentropic lifting). The lifting of the air column allows thunderstorms to grow even further.

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Jetstream and upper flow from the South

Thunderstorms are directed north on the upper winds and jetstream from the south where, given an approaching cold front, lapse rates can increase further and the intensity of storms can grow over France, Benelux and the UK and further into Europe.  The Spanish Plume is a complex weather pattern which needs some special ingredients to prime the atmosphere. Like all thunderstorms, Spanish Plume storms are based on convection in an unstable atmosphere which occurs when thermals of air rise uninhibited to a great height creating tall cumulonimbus clouds.

Unfortunately, thunderstorm formation is difficult to forecast, often being altered by local factors below the resolution of many forecast models.  The localised nature of storms also means that some places may see a tumultuous thunderstorm of epic proportions while other places not far away will see very little impressive action or nothing at all. Quite frequently plumes will skirt to the east of the UK and miss us entirely, storms staying on the continent, as initial forecasts of a “direct hit” are corrected east.  Cool sea surface temperatures in the English Channel can also deflate imported storms from France. So some understanding of the plume might help figure out how much we can expect from any convective action forecast with a Spanish Plume event!

For a wonderful time lapse of the plume passing over the UK watch this…

saturdays weather

Read below to find out the ingredients for a Spanish Plume but, if you are in a rush, then read the BBC weather summary here (it’s a bit shorter!) 🙂

Setting the stage:

The large scale “synoptic” pattern required to prime the atmosphere for a Spanish Plume is for a cut-off LOW pressure to locate around the Atlantic coast of Spain, ideally moving into the Bay of Biscay. The LOW in the Atlantic therefore sits to the SW of the UK.  An upper level trough nudges this LOW east across a hot Iberian plateau and France picking up humidity across the Bay of Biscay.  Strong solar heating over Spain creates a thermal heat LOW circulation and local thunderstorms here.  The stage is set for a Spanish Plume!

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Spanish Plume set-up

Warm plume followed by cold front chaser!

How does this “plume” head towards the UK and create thunderstorms here? Prior to the arrival of the upper warm Spanish plume in the UK, a humid surface SE flow from the continent feeds heat and moisture into the UK at low levels, steepening lapse rates. SE winds in the UK in summer frequently herald thunderstorms due to the low level advection (movement) of heat and moisture from the continent, both critical to feed thunderstorm formation.

Meanwhile, the upper SW flow (jetstream) ahead of the upper level trough pumps warm air from the Spanish Plateau northwards. This plume rides a conveyor belt of air rising towards and over a warm front as it travels north in a warm humid sector. Now an approaching cold front from the west is needed to really kick off storms by destabilizing the atmosphere!

The LOW pressure system has cool moist Atlantic air wrapping behind the cold front (seen on satellite pics below as the white line of cloud extending from the mother low to the NW).  A warm surface conveyor is driven NE in the developing warm sector ahead of the cold front, guided by upper level winds and the jetstream.

The rising warm plume has cool Atlantic air moving in aloft ahead of the cold front.  This cool air aloft can override the warm plume, increasing lapse rates and destabilizing the atmosphere further.  Sometimes this scenario can cause huge thunderstorms called meso-scale convective systems (MCS) or sometimes supercells.

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Any solar heating will further rapidly destabilize the atmosphere creating surface based storms where temperatures rise. Storms are often squeezed into a narrowing band or chain ahead of the cold front.  The cold front itself often weakens as air tends to sink after the passage of the thunderstorms.  The cold front can pass over unnoticed behind the thunderstorm event, its main job is to advect cold air over the rising warm plume to increase lapse rates.

HIGH pressure over the Mediterranean can also assist the process by advecting warm / hot air out of North Africa, over Spain, into France and finally reach Britain across the English Channel.  The more heat the better.

Sounding unstable 

By itself, a warm moist wind wafting up from Spain will not necessarily create the biggest bangs.  Other, ingredients are required to cook-up a good thunderstorm, with large hail, cg (cloud to ground) lightning and thunder and even funnel clouds or a tornado. Thunderstorms need an unstable atmosphere to trigger the air to rise rapidly UP. Such triggers can be spotted on a skew-t or sounding chart.  Cooler dry air sinking from aloft ahead of the upper trough can destabilize the atmosphere by increasing lapse rates and CAPE (convective available potential energy). This occurs because dry air at mid levels (4000m-7000m; 400-600hpa) can cause evaporative cooling of the atmosphere.  Such cooling aloft will mean that rising parcels remain unstable to a greater height and rise freely.  The sounding below shows all the ingredients that caused a supercell storm (note dry slot at 500mb). (this is for Texas, but same applies!)

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thunderstorm sounding

2015-07-01_11-22-09

unstable sounding for Glasgow airport July 1 2015

Another important factor in thunderstorm formation especially for the UK is TIMING.  The arrival time and combination of unstable air masses, fronts and moisture is what makes or breaks storms. Any element that is premature or delayed can be the death of expected storm formation. The best example of this is whether unstable air arrives during a sunny warm day or at night, when surface based heating is absent.  In the case of the Spanish Plume event 7 June, the unstable air arrived at night over a comparatively stable boundary layer surface air mass.  The only storms that resulted were elevated thunderstorms embedded in the unstable upper air ahead of the cold front (which oddly brought warmer conditions later in the day as cloud cleared and sun came out but too late to coincide with the impressive instability of the plume).

A cap in the sounding can also enhance convection.  A temperature inversion can hold heated air near the surface building convective energy which is held to the ground until the cap is broken by daytime heating… if it is sunny enough, cloud cover will spoil the event.  Busting the cap can create explosive and dangerous thunderstorms, rarely seen in the UK.

Here is a summary of some other ingredients as it applies to the Spanish Plume like the one on 7 June 2014 over the UK:

Heat! the SE of England is due to get warm or even hot on Saturday with Tmax temps well over 25c in sunny spots.  This will warm the air at the surface and, like a hot air balloon, these air “parcels” will want to rise (called lift). Warm air is also being moved into the country from the south by a process called advection.  Charts showing the heat energy providing the potential for air to rise are called CAPE: convective available potential energy.  In the UK we are pleased with CAPE values of 200-500j/Kg for some thunderstorms.  On Saturday we might expect values of up to 3000j/Kg. The other measure used to assess thunderstorm potential is the LIFTED INDEX (LI) this measures the difference in temperature between a parcel of air lifted to 5000m and the temperature of the air around it.  Negative figures show the air is buoyant and ready to rise.  Values of -8 or -9 are unusually low and show a very unstable airmass with the potential for plenty of lift!

In a Spanish Plume event the upper air contains enough energy and moisture to produce elevated thunderstorms even in the absence of surface heating: this means moderate thunderstorms can occur at night and with extensive cloud cover. The morning moderate storms experienced in the SE on Saturday 7 June were all elevated thunderstorms because extensive cloud cover throughout the morning meant an absence of surface based heating to kick off more purposeful convective activity.  Here’s an excellent blog explaining elevated thunderstorms compared to surface thunderstorms that would occur due to surface heating.

Moisture! local SE winds are frequent precursors for thunderstorms in England. This is because, in summer, SE winds are often humid with a relatively high water content, advecting (moving) lots of moisture into the SE: high dew points illustrate this with some reaching 20c on Saturday… a muggy humid day. This moisture will be required, of course, to form clouds.  To form really big clouds you need a lot of water in the atmosphere.  Once water vapour starts to condense it releases latent heat and this heat gives additional lift to convection and feeds thunderstorm formation.  Interestingly and perhaps counter to what might be expected, a dry plume of air mid-way up through the atmosphere is also an important ingredient to the production of big thunderstorms.  Drier air at mid-levels aloft gives the atmosphere added instability for the production of thunderstorms.  The reason for this is that dry air cools more rapidly with height than moist air (because rising moist air releases latent heat when clouds inevitably form and this additional heat reduces the rate of cooling of saturated air with height).  The rate of cooling with height is called the “lapse rate” and the lapse rates or temperature gradient on Saturday is steep. So moisture is a critical ingredient to storm formation because it controls instability and cloud formation.

The skew-t charts show temperature change with altitude.  They are called Skew-t because the temperature lines are skewed off the vertical slightly.  Whilst they are initially odd to look at, focus on the red and blue and dashed lines: if the red and blue lines are close together it means the air is saturated (cloudy).  If they are far apart then the air is dry. If the red line skews to the right then this is known as an inversion where temperatures can increase or stay the same with height.  Such an inversion will prevent thermals rising and the formation of clouds from surface convection, a critical ingredient for big thunderstorms.

more analysis of Skew-t

more analysis of Skew-t

The other ingredient the skew-t chart shows above is plenty of wind shear with height, in this case speed shear.  Wind shear is the change of wind speed and/or direction with height.  Increasing wind speed with height has the effect of dragging air off the ground like a hoover. On Saturday there is a jetstream moving overhead during the day and this will cause divergence aloft and encourage more air to lift off the surface. Strong directional wind shear, when winds turn at angles through the atmosphere and when different winds directions meet around fronts, is also a trigger for tornadoes.  The UK quite regularly has tornadic conditions in lively convective thunderstorm weather but the ingredients for tornadoes are a even more fickle!  Our tornadoes are considerably less powerful than those in the USA but can be of great interest and still cause damage by uprooting trees, damaging rooves and chimneys and even tipping cars.

Lift! like a hot air balloon, a warm bubble of air will only rise if it is warmer than the air around it.  On Saturday morning a CAP will exist in the atmosphere that will prevent air lifting far off the surface, thus preventing extensive vertical lift. The cap is known as a temperature inversion and the cap on Saturday is pretty solid, so little convection is expected early on, except possibly in unstable upper atmospheric layers which might cause elevated thunderstorms.   During the day, if there are suitable breaks in cloud cover, the sun will heat the surface and this will start to break down the cap.  A strong cap has the effect of building energy and heat below and, if the surface heats up sufficiently then the cap can be broken suddenly.  This usually happens in the afternoon when a sudden explosive thunderstorm could be produced.  In the mid-west of the USA tornado chasers call this process “busting the cap” and it produces the possibility of extreme weather.  On charts the cap is shown as convective inhibition that acts against convection but, at the same time, can be an ingredient for extreme weather.  Another mechanism that is capable of lifting air rapidly is the proximity of fronts which can mechanically lift the air as different air masses converge.  On Saturday, a slow moving cold front is forecast to be located along a north/south axis through the Midlands and S England, moving gradually east… this could be the focus of most activity if the cap remains solid further east.

Convergence of winds! winds converging at the surface are another local or regional factor that enhances convection. Converging winds at the surface tend to slow down and pile up, like lorries slowing down uphill and causing congestion.  When surface winds pile into each other (converge) they can only go one way… UP!  Convergence often occurs at the coast where winds coming from the sea slow down due to friction over the land.  This is one reason why the south coast often produces thunderstorms or enhances them as they cross the Channel (so long as the Channel sea surface temperature is warm enough – another story!). The other map below shows areas where there is a sufficient combination of wind shear, heating and energy to possibly start to rotate a thunderstorm: rotating thunderstorms are called supercells and are capable of producing tornadoes.  These are the “big daddy” of thunderstorm cells and would be an awesome site for anyone able to catch a photo before or after the inevitable heavy rain, hail, lightning, darkening skies and thunder!

So the action of winds converging at the surface and diverging aloft or doing the reverse, is important to thunderstorm activity. The charts below (website ) are a cross-section through the atmosphere from S-N and W-E across the UK before and during the plume.  The streamlines showing wind vectors illustrate the turbulent nature of the air during the plume.  There are elements of both lift and subsiding air at different elevations showing the complexity of the plume event.

Even with so many ingredients primed for thundery weather here in Reigate on Saturday it still remains a risk potential rather than a certainty.  Local factors might inhibit convection, too much cloud in the morning could reduce surface heating and the cap may not be broken, so no thunderstorms! It’s a matter of waiting to see the conditions nearer the time to make a final forecast and often it remains uncertain until the very day.  Nevertheless, there is a possibility of explosive convective thunderstorm action on Saturday, mostly in the Midlands and the East, if the CAP is BUST!  If this occurs then instability of the atmosphere will rapidly build extremely tall cumulonimbus clouds up to 10,000 metres tall.  The UKMO issued an uncharacteristically early warning of heavy rain for Saturday on the basis of this Spanish Plume event. Others will doubtless follow, but it is safe to ignore the daft scaremongering of the Daily Express but do keep a watchful eye on any dark clouds!

Although an accurate forecast is beyond the scope of this post and best left to the professionals, one broad indicator of possible storm pattern over the course of the day is set by convergence of winds and the likely position of the front as it progresses west to N/east.  The first plume of instability arrives in N France later on Friday and some might arrive S England Friday pm with low risk thunderstorm activity imported across the Channel from France (as it happened this occured only in the SW).  The charts for Saturday below show how the main location of biggest storm activity could move broadly from west to NE during Saturday and clear off into the N Sea overnight. (as it happened the front moved much quicker and clear S England and most parts of Central England by lunchtime leaving a bright afternoon with bubbly Cu).

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The animation here also shows the total rainfall expected to accumulate during the 48 hour plume event.

Whatever happens, do watch out for altocumulus castellanus in clear skies or asperatus undulatus (pic above) or shelf clouds and cumulonimbus developing ahead of storms as the plume arrives on Saturday and then watch for any cumulonimbus clouds exploding if the cap is bust later on!  Please send in your photos of any interesting weather phenomenon to RGSweather : on twitter and facebook and email.

Comments and any additional information always welcome!

sources: many thanks to these sites:

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.

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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.

 

 

 

Quick update Tuesday: Friday sees upper trough sinking over Ireland with dramatic loop in jetstream rapidly developing a surface low East England tracking NW taking rain with it. Worst of rain set for NW. Looks wetter for Reigate on Friday but potentially reasonable for rest of weekend, even temps recover somewhat during day. Changeable and interesting, check back.   

The late summer mini-heat wave currently building nicely mid-week with temperatures up to 28ºC possible for Reigate looks likely to end with rain on Friday and then some heavy thundery showers developing through the weekend and possibly into early next week.  A cold front is moving south bringing cooler surface air on Friday and this will interact with a warm plume of upper air coming the opposite direction from Spain at the weekend.  

The result could be heavy thunderstorms. Whilst convective showers are typically hit and miss, it is probably safe to assume it will be a wet weekend at times in some places in the south.  Total rainfall for Friday through to Monday for Reigate could potentially exceed 20mm.

unstable sunday 8 Sept

So, periods of heavy rain and possible thunderstorms are looking more likely for Reigate and parts of Southern England from Friday through the weekend (especially Sunday / Monday) as the cut-off LOW over the Bay of Biscay winds up and pushes a warm upper air flow across the south which will interact with a cold front moving across the region on Friday. Indicators like CAPE and Lifted Index which measure instability (the propensity for buoyant air to LIFT / convect upwards, condensing and forming cumulonimbus clouds) are all set for some potentially perky convective thundery activity at points anytime from Friday through to Monday. Heavy thundery showery rain is possible during this period.  Nevertheless, such activity is prone to miss places entirely and drift off at the last minute so… check back for updates!