Archives For thunderstorms

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UK thunderstorms July 3-4 2015 analysis chart modified spanish plume

Very quick update on the (grand?) finale of the Spanish Plume that has ebbed and flowed this week. (directly above is the analysis UKMet synoptic chart posted this morning after the event.  “In the event” round up see foot of this post (please send photos, I didn’t get any ;-( )

Today, Friday sees a final push of warm/hot continental air into Southern England with Tmax 28C or more today.  The synoptic and upper air charts above show a Modified Spanish Plume set-up which is potentially conducive to MCS (big storms) pushing further north across the UK. The chart below shows this matches one of the three types of Spanish Plume identified by Lewis and Gray (2010) and further by Nat Melia (2012).

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modified spanish plume (after NatMelia 2012)

Winds charts below show a sheared wind environment with SE surface winds and a jetstream from the south.  This is conducive to thunderstorms and some rotation which organises storms and produces possible tornadic features.

Tonight thunderstorms are forecast to develop locally across S UK as elevated features and move north into N / NE England.  Models disagree on the track of these storms so some places are likely, as usual, to miss out.

The development as whole is debated too so could change during today.  Models show vastly differing rainfall patterns.

Nevertheless, the potential is there for severe thunderstorms, even a meso-scale convective system (MCS) which is a group of sustained thunderstorms.  Intense rainfall and large hail is also possible.

The charts below are a collection that illustrate the set up in more detail. These are posted for future reference and reanalysis of any event or non-event that occurs. Interestingly the storms arrive overnight (as usual) and surface based cape is therefore extremely limited with no solar heating. These are therefore likely to be elevated thunderstorms. During this potential episode overnight do stay weather aware in case you come across one of these potential beasts! Equally the set up is, as usual, not certain with convective forecasts so check weather professional media for updates.

The outlook is for cooler Atlantic (still tropical) air to sweep away the continental plume during the weekend.  Thereafter, mostly dry pleasant temperatures for the SE. Wetter further west with chance of showers as troughs drag fronts (mostly weak) across the north.

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GEFS outlook

In the event:

Big storms erupted quickly from 10-11pm and intensified as they drifted north / NE.  Reigate experienced some heavy rain and plenty of lightning. The storms produced over 93000 lightning strikes over the course of the night. The SE, always hit and miss with models for this event, scored only 17418 strikes. Below is an animation of radar and strikes.

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Locally Reigate had some early action from developing storms before and through midnight.  Storms then intensified further N/NE. Note the rainfall rate before midnight 135.6mm/hr giving approx 10mm (tbc) and associated pressure spikes with descending air from storms. Nice event, some spectacular lightning but not as much or as intense as further north. Pictures and video below from local photographer Simon Spiers who caught some amazing lightning shots around the area. Thanks for sharing.

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After a cool unsettled end to May with a strong zonal jetstream, early June weather prospects are getting interesting for Reigate as models build a tantalizing “heat wave” with potential for warm plumes and thunderstorms, quite a contrast to the start of the week!  The warming is just beginning to cook up on the mean temperature anomaly chart and recent GFS model runs shown below with Tmax temperatures in the high 20’s and some approaching 30C by next weekend/ Saturday especially. Here’s a review, rather than a forecast, of some of the synoptic features unfolding early June weather.  The scenario can change a lot by next weekend of course, so stay tuned, especially to twitter and the fantastic UK weather community (both amateur and professional) for updates.

In the short term, the end of May and start of June will continue to be dominated by LOW pressure to the north sweeping active fronts across the UK with attendant rain and wind, especially Sunday and more on Tuesday, though as usual Reigate and SE will be sheltered from the worst of this which will impact the NW mainly.  Tuesday’s Atlantic depression has an unseasonably low forecast central pressure of 972mb (UKMET) and 968mb (GFS) due to a strong jetstream across Scotland of 160mph.  Expect windy, gales in west and coasts, showery and unseasonably cool weather everywhere but especially in the NW during these episodes. Reigate on Tuesday could have gusts exceeding 40mph.  Update Sunday: MetOffice have issued a weather warning for gales countrywide for this event.

968mb would be close to the lowest June central pressure to impact the UK since 2000**, though the MetOffice chart below showing 972mb is probably nearer what will happen in reality.  Highest winds (40mph inland, 50mph coastal) in Reigate and SE are expected Tuesday am as a daughter frontal wave-low whizzes across the Midlands trailing her bigger parent.

**

 

By the end of this week models build a meridional (wiggly) flow in the jetstream as the North Atlantic Oscillation (NAO) turns negative which means pressure over Iceland rises relative to that over the Azores which falls below average. This weakens the pressure gradient and reduces jetstream strength.  The NAO is not a driver of weather but is an indicator of Atlantic patterns that controls incoming weather for the UK and Europe.  A negative NAO often means fewer Atlantic based LOWS with a less aggressive and frequently more meandering polar front jetstream, with more chance of continental weather impacting the UK as pressure builds to the north (in winter this can bring cold weather from the continent).

A negative North Atlantic Oscillation usually indicates a weaker jetstream and one that meanders with high amplitude waves across latitudes, a so called meridional pattern (rather than zonal which blows purposefully west to east across Atlantic along strong pressure gradient between Azores and Iceland: dragging in frequent LOWS).  A meridional jetstream can slow-down and fix weather patterns into place, especially if a HIGH builds to the north as a so called blocking pattern.  Such a pattern looks possible with current model runs, though with steep temperature gradients building over a heating up USA/Canada, a return to an active unsettled Atlantic pattern seems possible later into June which could breakdown any blocking pattern and finish off our balmy continental flow, but that is way off so remains to be seen!

The meandering jet developing from mid week will encourage a warm / hot continental easterly / SE flow for the UK as pressure builds initially to the north east and pressure lowers to the south from Wednesday.  A cut-off LOW to the west of Iberia sandwiched between the Azores HIGH and the HIGH further NE is also a prime ingredient to waft warm unstable Spanish plumes our way as the HIGH pressure drifts east over Scandinavia (see above charts).

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Spanish Plume June 2015

The threat of heat and thunderstorms peaks next weekend, notably on Saturday, with temperatures peaking as high as 30C. The 850hPa chart above shows the warm plume arriving from Spain.  Upper air temperatures exceeding 15C would yield hot daytime temperatures approaching 30C in sunny conditions.  Such warm plumes of continental air, meeting Atlantic air injected from the cool HIGH offshore, could lead to unstable thundery episodes (more on Spanish Plumes here) On the other hand the pressure is quite high in the East so this could suppress convective action here, the detail will be critical.  On the skew-t chart below spot the steep lapse rates, negative lifted index and high CAPE, high dew points (moist air) and precipitable water content below, all lively indicators of an unstable atmosphere.  It’s a long way off though so things can change a lot and frequently do!

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This set-up is the source of excitement over “heat” by the end of next week: a warm continental plume.  Pressure in this scenario would be highest in the north of the UK so the SE could see more unsettled conditions.  Interestingly, the current Madden-Julian Oscillation Phase 1 and 2 (a tropical disturbance pattern used to forecast patterns in medium range) correlates with this emerging pattern, with P1 and P2 often linked to HIGH pressure to north, LOW over Europe and an unsettled S/SE UK.

Despite this, a prolonged heat wave does not seem to be a strong possibility.  Technically a UK “heat wave” is when daily maximum temperatures for more than five consecutive days exceed the average maximum temperature by 5 °C.  For the SE this usually means exceeding 30C daytime Tmax and 16C night time Tmin, whilst forecasts for the end of next week are warm, sustained heat of this nature does not seem likely.  Pressure and 850hPa temperatures rise this week with some models in the high 20’sC Tmax. Both medium and longer range models suggest either a flat-line or fall in both as June progresses.

Models also suggest a weakening of pressure and some play with a thundery breakdown bringing in wildly high CAPE values (convective potential) into June. This suggests a breakdown as pressure falls. Some CAPE values forecast are extreme for the UK and would not be out of place in a Mid-West tornadic supercell! However, often these scenarios fail to materialise as forecast and often the instability and thunderstorms simply brush past the SW of the UK or stay in France, perhaps clipping Kent alone.

Models in the second week of June seem to suggest the HIGH builds out to the NW, regressing from the location over Scandinavia.  This would put the UK on the cooler side of the HIGH with a northerly flow, thus ending any heat spike. This is just one GFS run and at the unreliable end of the model but a possible solution.

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Finally, over the long term average June is rarely “hot” for a prolonged period and this brief hot spell declining into “warm” would seem to match the emerging pattern.  On average, the hottest day of the year falls in June only 25% of the time and June has shown climate trends of becoming rather duller and wetter bucking the trend of overall warming for other months due to climate change.  The frequency of Atlantic westerlies, on average, also picks up in June making sustained “heat” a rarity.  Nevertheless, a brief warm spell is likely as shown by the end of this week as shown by increasingly convincing model trends. Thereafter, the location of the HIGH and jetstream activity will probably control unfolding June weather events.

Update from @wansteadweather supports idea that June may not live up to any hot start, HIGH dominates but the position may feed average to cool flow from N.  Will be interesting to see how this pans out!

http://www.metoffice.gov.uk/learning/learn-about-the-weather/weather-phenomena/heatwave

http://www.metoffice.gov.uk/public/weather/forecast/gcpg7rs0t#?tab=fiveDay&fcTime=1432944000

Quick update here focusing on SE especially: please note this applies mainly to Reigate in Surrey and is an amateur analysis for educational purposes.  For updates through the storm please see @RGSweather on twitter for the Bertha story as it unfolds for us in Reigate and SE.  This is called NOWCASTING (as opposed to “forecasting”).

Ex-Bertha is turning out to be rather interesting meteorologically!  A convective potential has emerged today, which means there is more possibility of thunderstorms of some significance as the LOW passes across the UK, especially to the south of the system.  Convective gusts of 50-60mph could be possible and the odd tornado cannot be ruled out, though no need to panic because these are quite common and not usually powerful or damaging in the UK.  So it is still the case that the overall impact of this storm is still not likely to be extra-ordinary or wreak widespread havoc Daily Express style.  It is more likely to be underwhelming for most.  Nevertheless, rainfall totals in a short space of time for some places might be high and there could be some interesting weather phenomena associated with active fronts.

UPDATE Sunday 7:30am

Estofex and TORRO have issued severe convective weather warnings for the S UK. Estofex Level 2 storm warning is most unusual for the UK and TORRO do not issue tornado watches lightly.

(back to yesterdays update:) The UKMO fax chart below for Sunday midday shows a “triple point” of three fronts meeting near the SE (warm front, cold front and occlusion) Between the warm front and cold front the warmest humid air is wrapping into the centre of the LOW in the warm sector: this contains much of the moisture to fuel the storm as condensation releases latent heat driving up parcels of air.  On top of this a conveyor of cooler drier Polar air that flows over the cold front and warm sector and this increases lapse rates further encouraging lift throughout the system. The warm air eventually flows to the core of the storm as it occludes.

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All the time the jetstream to the south is lifting air off the ground (by a process called divergence in the upper atmosphere) and lowering the central pressure causing air to converge into the centre of the LOW… this results in the surface wind rushing into the centre.   Converging air at the surface has nowhere to go except up.  Rising air, especially where tropical air meets polar air at the fronts, creates condensation, thick cloud and potentially plenty of rain.  The potential water available in this storm is large.  In addition, cloud top temps, with the influx of cold air aloft, are likely to be as low as -50C causing ice to form in turbulent air that can create charge up thunderstorms.  Such storms are only a risk and may not happen at all.2014-08-09_20-59-21

For Reigate and the SE it seems we can expect more rain during the morning than was previously the case in earlier models and forecasts.  Latest models suggest widepsread rain in the SE of up to 20mm and discrete patches of high totals possibly exceeding 50mm in the SE.  This is about a month of rain in one day, so local flooding could be a problem.

Rain will arrive tonight, after midnight, and persist throughout the morning.  Wind speeds, probably 30-40mph max gusts inland, possibly more gusty in any thunderstorms, will increase towards the middle of the day and potentially be highest as the cold front moves away which, on current models looks like early afternoon.  Strongest gusts will be associated with any thunderstorms.  The good news is that by pm the cloud should break rather rapidly, however, scattered showers could follow in the brisk westerly. This regime will continue for much of the early part of the week.

Needless to say, apart from the rain potential, Reigate is less at risk from tstorms than further N during this episode (Reigate storm shield!)

Even now much still remains uncertain about this storm and it is causing lots of interest and headaches for both professional and amateur meteorologists.  The nature of the fronts may produce some organised squall like features and some organised thunderstorms for places but predicting these is extremely difficult.  Any such storms can have the potential to deposit a lot of rain in a short space of time.

Late this afternoon Saturday Bertha split in two: one rain system moving north, the other pushing more ENE.  This was unexpected.  Currently the rain moving north across Ireland is the more significant but things can change.

 

Let’s finish with the UKMO forecast for Reigate.  It shows lots of rain, potential for thunderstorms and some unsually strong winds for the time of year.  As leaves remain on trees this might cause loose branches to fall and peak rainfall totals, if met, could cause some local flooding.  Certainly nothing to panic about but do look out for any interesting weather features and send them in to @RGSweather!  Sadly for @ridelondon the prospects are not terriibly nice in the morning.

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Photo mosaic of the squall line that passed over Reigate on 10 Aug afternoon: quite a feature!

and tornado reports of damage from various locations including Hull, plus other news here:

http://metofficenews.wordpress.com/2014/08/10/sundays-rain-and-wind-data/

http://www.bbc.co.uk/news/world-europe-jersey-28746723

 

 

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.

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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

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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:

Heavy rain alert and UKMO weather warning issued for Reigate and SE Thursday 3 Oct from afternoon and overnight through to small hours Friday.

An unstable plume of warm moist air will edge across the SE during Thursday afternoon and bring the potential for heavy rain as fronts provide lift to the air to produce possible thunderstorms through Thursday pm and evening.  Rainfall totals from showery outbursts will vary a lot locally, probably most further towards the south coast and further east, but could be as high as 20mm over the 24 hour period almost anywhere in S and SE corner during this time; most places will see a lot less, 6-10mm being widespread.  Winds will be 15mph initially from the SE but in heavy showers gusts could reach 25mph. The wind will veer to the S overnight. It will be increasingly humid during the day with Tmax of up to 20c, staying rather warm overnight but cooler and much brighter on Friday at 17c as cooler polar air follows the cold fronts with a few showers lingering as fronts move away leaving a brighter showery day.

Many ingredients for producing decent thundery activity are met during this period: including…

  • high dewpoints rising to 17c in the warm sector and as the cold fronts approach on Thursday evening: high dewpoints indicate a saturated atmosphere with plenty of moisture available for cloud and rain formation
  • warm air advection: which means a warm plume of air will be provided for several hours during Thursday pm but will be replaced by cooler air on Fri am.
  • wind shear: winds at the surface will intially be SE but at height will be a brisk warm southerly.  This change in direction and speed with height storms will not “stagnate” over ground they have just cooled with a soaking, so that thermal activity will be maintained with further warm air available to lift and grow bigger cumulonimbus clouds which roll through rapidly.  On the other hand, high rainfall totals will be subdued as showers move over relatively quickly.
  • Jetstream overhead (arriving later on Thursday evening) providing strong upper air divergence: this means air will be dragged off the surface as air rushes UPWARDS to fill the gap left by the rapidly disappearing air aloft.  This convergence on the surface means rapid uplift… perfect for building thunderstorms!
  • Some ingredients are missing or rather lackluster.  The most obvious one is a lack of heating during the day.  Thursday will be mostly cloudy and, being October, the heat from the sun will not provide as much buoyancy as in mid-summer. This is a critical ingredient for big thunderstorms.  This episode is also short duration and a cold front will rapidly replace the warm humid plume with cooler polar air, increasingly less showery, during Friday.
some ingredients are good to go

some ingredients are good to go

jetstream 3 oct

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!

An unseasonable depression will bring significant rainfall to much of the UK on Monday.  The SE will escape the worst of the heavy rain crossing the rest of the country further NW in a deepening LOW moving NE across the country through Sunday night and Monday.  Many parts of the UK will experience large rainfall totals exceeding 40mm and with flooding in places.  The SE is blessed by the late arrival of any fronts (weakening too) and the movement of the system to the NE driven by a brisk southerly jetstream 20,000 feet overhead.  It simply passes us by.  Nevertheless, Reigate could see 5mm or so of rain, mostly arriving in the form of showers in the afternoon, some heavy: so there is a twist in the “tail” of this depression.

The sounding shows the instability of the air as the front crosses.  Any warm parcels of air lifting through the atmosphere (dashed line) as the cold front approaches will be warmer than the surrounding incoming cool airmass (red line) so will continue to lift and this is a so-called “unstable” situation likely to form tall cumulonimbus clouds and heavy rain and possibly fast moving thunderstorms.  The dew point (blue line) is also very near to the air temperature so the atmosphere to a great altitude is humid / near saturated and likely to condense large amounts of water vapour into cloud droplets.  Moderate rainfall totals on a brisk air flow are possible.

frontal sounding shows instability

frontal sounding shows instability

The far corner of the SE may see no rain at all during most of Monday but an area of instability and rapid upward velocity (lift) associated with the passage of a cold front approaching from midday could spark off heavy showers and thunderstorms in the SE and Reigate.
Temperatures in the SE will be higher than anywhere else in the UK as we enjoy a warm southerly flow of upper air in the warm sector of this depression with a strong southerly jetstream starting the day over Reigate (>70mph). Temperatures at 10,000 feet will fall below freezing over Reigate during the course of the day as the cold front brings in much cooler NW air flow across the UK.  The rest of the week looks less unsettled, temperatures recovering to normal though with more rain at times. Typical August!

Spectacular thunderstorms, lightning and flash flooding drifted west to east across the UK today on a blustery unstable SW airstream but the SE and Reigate missed out on any cumulonimbus and we experienced no thunderstorms at all which would please most people but rather upsets weather enthusiasts looking for interesting weather!

Lumpy cumulus congestus and some perky showers with bright blustery conditions in beautiful bubbly convective skies which cleared at night were all the SE could manage. North and West of the region saw amazing convective cumulonimbus with towering cells producing hail thunder and lightning with flash floods here and there.  Spotting the clues as to why the SE missed out is tricky… a combination of factors reduced the Tstorm risk.
The general synoptic situation was higher pressure in SE, soundings with lower CAPE values (600 v >1000) and higher lifted index were also evident.  Lower 700hPa humidity in the SE was noticeable but possibly irrelevant.  The ThetaE map showed considerably higher values away from the SE too.

Soundings for the SE compared with a thundery location (Fairford) to the west show Reigate with less instability through the lower and higher elevations, less depth of saturated air, a stronger cap and less steep lapse rates.  Significantly lower dew points in the SE would also reduce humidity and water available for cloud development.  Winds were also SSW and thunderstorms developing in the SW early on (Bath and Bristol first) simply drifted to the NW of the area and clipped the North of London missing the SE altogether.  More broadly the overall synoptic situation had higher pressure for the SE as well which would inhibit convection.  I’ll add to this as more thoughts and ideas arise… why did the SE miss out?  Thanks to @MillinMan and @marknealweather with their expert ideas for this quick review! More ideas most welcome @RGSweather. Thunderstorms: edgy stuff but our weather today nothing like as extreme as that in Italy… watch this http://www.youreporter.it/video_Tromba_d_aria_a_Trezzo_Sull_Adda_-_29_Luglio_2013

sounding reigate

sounding tstorms fairford

 

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.