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#Reigate A beginners guide to the Spanish Plume!

The “Spanish Plume” forecast to arrive from Friday and into Saturday is a special weather set-up for producing thunderstorms in the UK, as seen below.



It is a rather complex weather pattern which needs some special ingredients to prime the atmosphere but also some fine tuning of local conditions to cook-up any big thunderstorms.  Spanish Plume thunderstorms are based on convection (thermals of air rising due to surface heating) and storm formation is notoriously difficult to forecast accurately, often being altered by local factors below the resolution of most forecast models.  Some places may see a tumultuous thunderstorm of epic proportions while other places not far away will see nothing at all. So an understanding of the plume will help figure out how much at risk you are from any convective action!

If you are in a rush then read the BBC weather summary here (bit shorter!)


<sub>Plume weather


Jetstreak pushing warm southerly air</sub>

The large scale “synoptic” ingredient that is priming the atmosphere for the possibility of storms is a moderately deep (for June) low pressure that has arrived from a cool NW direction, and will sit in the Atlantic to the SW of the UK and west of Spain.  This LOW pressure system has cool moist unstable Atlantic polar maritime air wrapped round numerous fronts. The low pressure system provides the main synoptic ingredient to kick off a Spanish Plume.  It spins winds anti-clockwise around the central low and this means that, if the LOW is deep enough (which this one is for June at an unseasonable 988mb) and located in just the right place, powerful upper winds driven by a lively jetstream will reach the UK from a warm southerly direction ahead of any fronts as the LOW inches towards the UK.  HIGH pressure over the Mediterranean will also help to push winds out of North Africa, over Spain, into France and finally reach Britain across the English Channel.  This southerly wind is what is known as a “plume”.  It is characteristically a dry airmass, often loaded with Saharan dust, that arrives in the UK at relatively high levels, above around 1000m, as it rides over local boundary layer weather nearer the surface.


<sub>Winds at 5000ft: dry


Winds at the surface: moist


Big low showing the plume


Warm airmass for the UK


dust also lofted our way…</sub>

By itself, a warm dry wind wafting up from Spain will not necessarily create any big bangs.  Other, smaller scale ingredients (herbs and spices if you like) are required to finally cook-up a good thunderstorm, with perhaps large hail, cg (cloud to ground) lightning and thunder and even funnel clouds or a tornado. Thunderstorm triggers are needed to push the air UP.  Thunderstorms require some critical ingredients: heat, moisture and lift. Additionally, convergence of winds, hills and wind shear and nearby FRONTS can also contribute to the thunderstorm cook-up.

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!


<sub>High CAPE j/Kg and low LI


= High storm risk


A warm day expected</sub>

Moisture: local SE winds are set to be quite humid with a relatively high water content: 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 feed 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.


<sub>Morning capped


Afternoon instability</sub>

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}


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 cloud formation.  This cap is known as a temperature inversion and the cap on Saturday is pretty solid, so little convection is expected early on.  However, during the day the sun will heat the surface and start to break down the cap.  A strong cap will build energy and heat below and, if the surface heats up to a threshold then the cap can be broken, usually in the afternoon and 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.  Another mechanism that is capable of lifting air rapidly is the proximity of fronts which can mechanically lift the air as airmasses 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.


<sub>Strong cap am


Breaking down pm</sub>

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!


<sub>Converging moist winds over the UK – saturday


Storm relative helicity? OK, a map showing possible supercell formation</sub>

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 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 likely position of the front.  The first plume of instability arrives in N France on Friday and some might arrive S England Friday pm with low risk thunderstorm activity imported across the Channel from France.  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.



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

_{Bron: Rgsweather} | Gewijzigd: 7 juni 2014, 10:14 uur, door Ries

Uitbraak van Severe Weather in Europa

De zinderende hitte, die zich opbouwt boven het midden van Europa zorgt ervoor, dat er een meerdaags severe weather event plaatsvindt. Deze eindigt op woensdag. Een koufront glijdt in de-warmste-lucht-tot-nu-toe-dit-jaar. Hierdoor krijgen de gebieden vanaf Frankrijk tot het westen van Polen en het noorden van Italië te maken met zeer krachtige onweersbuien.

Deze uitbraak van noodweer begint op maandagmiddag in het westen en noorden van Frankrijk, in België en over Nederland. In de avond trekt dit gebied weg richting het noordwesten van Duitsland. De krachtigste onweersbuien, die maandag laat in de middag en in de avond gaan vallen, kunnen destructieve windstoten, grote hagelstenen en verblindende stortbuien veroorzaken.

Volgens meteoroloog Tyler Roys kunnen er ook tornado's voortkomen uit de buien. Deze zullen zich vermoedelijk concentreren in de strook van het noorden van Frankrijk naar het noordwesten van Duitsland.



Vervolgens trekken de zware onweersbuien op dinsdag richting Denemarken en het midden van Duitsland maar naar Zwitserland en het naburige deel van Frankrijk. Deze buien met eveneens destructieve windstoten en stortregens zullen vooral in Denemarken en het noorden van Duitsland wijdverspreid voorkomen. In de overige gebieden hebben ze een meer lokaal karakter. Ongeacht de grootte van de complexen: er is maar één zware onweersbui nodig om veel schade te veroorzaken.

Op woensdag ligt het risicogebied voor zware buien vanaf het westen van Polen tot net noorden van Itali:e en het zuidoosten van Frankrijk. Al met al zullen in sommige delen van het midden van Europa weer tot recordhoogtes stijgen. Dinsdag zal bijvoorbeeld in Berlijn het kwik oplopen tot een zinderende 34C.

© onweer-online
^{Bron: Accuweather}

De reports die zijn binnen gekomen bij het ESWD met het Pinksterweekend. Op 7, 8 en 9 juni kwamen er maarliefst 278 reports binnen, waarvan meer dan 200 in de Benelux en een deel van zuid-west Duitsland richting noord-oost Duitsland. 


_{Kaart: ESWD} | Gewijzigd: 13 juni 2014, 12:30 uur, door Ries