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August 24, 2014

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Reigate Grammar School weather station, Surrey, SE England. School run website for local weather info and amateur weather observations and educational posts on weather. SkyWarnUK storm spotting. RiverSearch monitor for River Mole, Surrey drainage basin. Forecasts for educational and local information purposes. Town location automatic VP2 wx updated to our own website pages every 15 minutes and “live” wind readings are updated every 5 seconds.  Data to UK Met Office weather observations website (WOW) and to US based Weather Underground (WU). 

Quick update

More agreement amongst models as to firmness of the High pressure lasting into second week, at least part of it to 6/7 Sept. ECM latest run shows nice firm blob of HIGH NW of UK.  Only snag with this is IF the subglacial Bardarbunga caldera in Iceland decides to erupt it would be when upper air flow is northerly, direct to UK and Europe.  However, this is all speculative and of course the most unlucky scenario. Check this website for volcano updates http://www.ruv.is/ and the Icelandic Met Office http://en.vedur.is/earthquakes-and-volcanism/earthquakes/

Anticyclonic conditions are building over the UK in the coming week or so courtesy of ex-hurricane Cristobal which is tracking NE across Iceland this weekend. The ex-hurricane has morphed into a deep low pressure that is dragging warm tropical air behind it into the Mid-Atlantic thanks to the jetstream and this is helping to build high pressure across the Atlantic and over Scandinavia.

Ex-hurricane Cristobal leaves trail of construction

Ex-hurricane Cristobal leaves trail of construction

 

Whilst the black and white UKMet synoptic chart (below left) shows surface pressure, the colourful chart (below right) shows important UPPER air flow as well as surface pressure on a so-called “500hPa” chart.  Upper air charts often represent conditions at at 500hPa (hectopascal) and are commonly used by meteorologists to get a better idea of how the atmospheric conditions will develop over longer periods of time.  This is because upper air is less disturbed by surface features such as oceans and mountains and by the influence of day and night that can upset surface charts and make them awkward to interpret predictions over longer time periods.  Upper air charts most commonly show temperature at 500hPa, or 5km altitude, which is about “half way” up through the troposphere, well away from surface upsets.  Upper air temperatures usually correlate with surface pressure, but not always.  See below to compare 2 charts, one surface and one upper air, for the same time period.

Warm upper air, usually of tropical origin, is shown on the 500hPa charts in orange and red colours.  Cool polar air at height is shown in green and blue. Where the colours are tightly packed together it shows a steep temperature gradient and this is often associated with the jetstream.  Warmer air is less dense and, rather like fluffing-up a thick duvet,  takes up more “space” in the atmospheric column compared to the the thin “blanket” of cold Polar air.  As warm air increases the 500mb “height”, fluffing up the atmospheric duvet, more pressure is exerted on the surface which increases the air pressure.  Cloud formation is inhibited as air sinks, warms and dries out.  The HIGH being built early next week, initially over the Atlantic, is formed by a warm upper flow of Tropical air pumped into the Azores high by an upper SW jet following on the heels of ex-hurricane Cristobal.

2014-08-30_14-10-11

 

So HIGH pressure will bring settled weather across much of the UK, including here in Reigate and the SE, for about a week with some warm temperatures and light winds and generally pleasant weather with some sunshine.   However, not all models show a uniformly “clean” high i.e. an anticyclone that is free from cloud and blocking all incursions of cool or moist air.  Note the UKMET chart (up) shows some weak fronts wriggling across the HIGH and this could mean cloudy skies for some and even odd showers at times.  Rainfall charts also do not show completely dry weather throughout next week (see below).  The ECMWF model (above right) shows a weak trough / cut off low feature to the west of the UK mid-end of next week and eventually forming a low feature in Biscay.  If this scenario comes off it could temporarily spoil the HIGH and possibly send showers into the South as it drags in warmer humid air from a southerly direction later next week.  So the exact duration and nature of the high is still a little uncertain so check back and also check UKMO of course for updates.

 

 

2014-08-30_14-18-26

By later in the second week of September and beyond an active Atlantic with the possibility of more extra-tropical cyclones will probably start to breakdown any remaining high pressure. The chart above shows the chances of tropical cyclone formation (TCFP) in the Atlantic and, whilst % figures are low, this still represents a fair chance of cyclones forming and ultimately entering the mid-latitudes later in September.  So this might not be a durable blocking high or one that lasts.   Nevertheless, it is good enough to keep active Atlantic weather systems at bay for a week.

The positioning of the HIGH is also critical to how “warm” we will get in the UK.  This time of year the continent is still warm so high pressure over Scandinavia to the NE or East of the UK brings in some warm continental air on an easterly flow.  This is how the GFS and UKMO sees things developing initially  with a  high positioned over Scandinavia and Europe through mid-week.  This flow could yield temps by Thursday in the mid-20’s, up to 25c is possible for SE.  With sea surfaces at their warmest this time of year it could be a good time to head to the beach.  By next weekend most models (above) see some movement of the HIGH to the west and NW of the UK where it could introduce a cooler Atlantic flow from Iceland.  So peak temps are probably reached before the end of this week.

Further ahead several models suggest a breakdown mid-September to a more unsettled regime.  The CFS temperature chart shows a rather sudden decline Mid-Sept and then again in October.  Whilst  this is not surprising for the time of year, such steps in the CFS can indicate frontal systems and depressions as different air masses arrive across the UK.  Warmer than average Atlantic sea surface temperatures, in some places nearly 5c above normal, is likely to encourage the formation of active depressions as is usual for Autumn.

 

Monday 25 Aug

Bank Holiday heavy rainfall wash out!

2014-08-24_15-42-24

Nasty LOW

Like much of the rest of England and Wales, Reigate and Surrey and the SE will see an August Bank Holiday wash-out tomorrow with lots of rain through the day, likely to be heaviest in the afternoon.  This has the potential to be a moderately severe wet weather event with 48 hour rainfall accumulation possibly topping 20mm in our region with some models taking this even higher to 30-40mm in places but this is probably over-egging things.  The WRF and NMM models push totals to 40-50mm in places while Hirlam and Euro4 (UKMET) keep a lid on rain at 20-30mm.  Anything approaching 40mm in 48 hours would be highly unusual for us in SE in Atlantic frontal depressions but it’s by no means impossible.  Remember that a “wet day” for us in the SE usually adds up to only around 10mm, so any totals over 20mm tomorrow will seem a significant deluge. Check the rogues gallery of models below for the worst rainfall offenders.  By the way, always Check the UKMET forecast for a professional look at the situation, this site is for a local perspective on weather and to generate enthusiasm and interest and share some more understanding of the air around us, especially in these times of such uncertainty regarding weather and climate.  As we approach another autumn and winter please share any of your weather stories and photos here or follow us @RGSweather on twitter.  Twitter is an amazing portal for sharing, learning and discussing the amazing fast moving world of weather… join it if you can.

Either way, surface water local flooding is likely to be a feature on roads tomorrow and the UKMET has issued a yellow warning for heavy rain.  You can expect a cloudy, dull, blustery, cool and wet day: so maybe snuggle up with the TV guide and check out some bank holiday films :-)  and watch @RGSweather on twitter for exciting weather updates of course!

2014-08-24_16-20-53

The bank holiday soaking is courtesy of a low pressure in the Atlantic set to deepen to 981mb overnight as an unseasonably fast jetstream blasting through the Channel draws very moist air from the surface and lowers the surface pressure throwing all types of fronts our way and tightening isobars cross the SE during the course of the day. 

Today, the system lurked in the Atlantic as a significant band of cloud, as shown here by the VIIRS satellite at 13:00hrs.

2014-08-24_18-28-49

The system will bring rain first thing tomorrow to Reigate.  Initially rain looks to remain fairly light but will become progressively heavier through the day with even a low risk of heavy thundery showers as the cold front passes later pm and into Tuesday night (updated) before things gradually calm down Tues am. Whilst breezy through the day, max wind gusts are likely to arrive later in the afternoon but only 20mph.  This is certainly more a wet event than a wind event!

Into Monday morning: update:

Satellite pics show a rapidly developing occluded low pressure with tightly wrapped fronts.  The dark are on the sat pics represents high altitude air that is subsiding (sinking) into the depression.  This dry intrusion can cause increased instability around the cold front where it is forced to rise once more nearer the surface, increasing cloud formation.  Sometimes is can cause thundery doinwpours and some of the models are picking up on this by increasing rainfall totals even further for the SE as the frontal system passes over.  Interestingly this does not count as a “bomb” cyclogenesis which requires a 24mb fall in pressure over 24 hours (1mb per hour).  This has managed about half that pressure fall so far. 

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.

2014-08-09_20-43-56

 

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.

2014-08-09_21-06-51

 

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

 

 

Even in her hey-day Hurricane Bertha was not a powerful or well organised storm and she disappeared as a discrete tropical storm a few days ago. What is left crossing the Atlantic is a significant “blob” of heat and moisture that she dragged into the dog-days of the mid-latitudes.  Unfortunately for meteorologists the energy injected “intravenously” into the mid-latitudes upsets super-computer weather model forecasts. Models cannot handle the excitement!  The result is that various weather forecasts have struggled to agree and have been producing significantly differing outcomes as to the strength of Bertha and to when, where and IF she “makes landfall” in the UK.  Usually, the differences in model forecasts gradually reduce and forecasts increasingly agree, but in volatile atmospheric conditions it often goes down to the wire, like on this occasion!  This is why meteorologists have been unable to confidently pin down the exact track and strength and impact of Bertha on the UK.

Today there remains uncertainty, despite being only 48 hours out from her arrival.  The main cause of uncertainty is how she eventually interacts with an unseasonably strong and southerly jetstream.  Where a low pressure like Bertha arrives and moves under a jetstream makes a big difference to where she goes and whether she strengthens or weakens. The so-called “left-exit” region of a jet exerts most influence on deepening low pressures into significant storms.  Where the jet is slowing down or speeding up or curving can create extra lift, dragging air off the surface causing pressure to drop.

Here are three of the possible forecast outcomes from the GFS (including WRF/NMM which are similar), UKMET and ECMWF models. (Thanks to @BigJoeBastardi for the ECM model output from @WBAnalytics)

#1 Bertha could travel more or less down the eye through the English Channel from the SW.  This would take most of her highest winds and gales through France but would clip the south and SE of England with some heavy rain as the low crosses directly over this area.  The highest winds in these tight LOW pressure systems usually occur to the south of the LOW centre, associated with the warm and cold fronts.  The heaviest rainfall is modelled to fall to the north of the low associated with the occluded fronts and low pressure centre, hence the heavy rain clipping South and SE England. This outcome is currently favoured by the BBC and UKMET office models.

#2 Various other models, the GFS and WRF-NMM amongst them, favour a more northerly route and take the LOW from the SW approaches, through St George’s Channel, across Wales and into the heart of England before exiting into the North Sea.  Broadly speaking, this would be a worst-case scenario because the UK would take the brunt of both heaviest rain (to the north) and the highest winds (to the south).  Nevertheless, for @ridelondon and #Reigate and the SE this might be the preferred route because any fronts and poor weather would pass over our area relatively quickly on Sunday, the worst of it probably from around 9am through to 2pm.  It would also leave only trailing fronts depositing up to 10mm or so of rain across our region and some gusty conditions, wet for a while but nothing too drastic to speak of really.

#3 A third model, the ECMWF has, until recent runs been quite the outlier amongst all competing tracks, sending Bertha further south through France in earlier runs.  More recent ECM runs have played catch-up with GFS and now sends the LOW NE through the UK.  Significantly, ECM drops pressure to 987mb in the Irish Sea.

The outcome could, of course, be somewhere in between these two or something totally unexpected!  It is worth also pointing out that extra-tropical storms arriving in the UK are not uncommon and this one is likely to underwhelm in many places inland where gusts are unlikely to exceed 40mph and, in the SE maybe 30mph gusts will be the widespread maximum.  Coasts, hills and exposed areas are likely to see the worst of it with gusts up to 60mph or more.  Even here it will not be anything like as potent as our storms last winter where winds exceeded 90mph on occasions.  Nevertheless, with leaves on the trees there may be some blow-downs and loose branches and rain in some places might disrupt travel but it is doubtful that wide-spread chaos will ensue as compared with the morning after St Jude last October which was a lot more potent. See below.

 

The best advice is to watch weather warnings and updates carefully on Sunday for any changes.  RGSweather will be posting NOWCAST updates on twitter for Reigate and environs.  Here is the latest from NWS/NCEP showing storm force winds in the sea around the UK, note the unseasonably LOW central pressure on 985mb. In comparison, St Jude storm had a lowest overland central pressure of 976mb back in October 2013.

2014-08-08_19-19-29

Interestingly, the longer term weather impact of Bertha into next week is probably better modeled than her immediate track on Sunday.  She is forecast to move slowly up the North Sea and merge with and deepen her “parent” LOW to the N of Scotland.  This will introduce cooler unstable showery NW winds to the UK for the early part of next week. Showers and some more organised bands of rain are likely to be frequent visitors, especially to the NW and west coast.  It will also feel cooler and more breezy for the whole country.  So, put the balmy warm days of summer on hold next week.

http://www.bbc.co.uk/weather/features/28707422

http://www.bbc.co.uk/weather/features/28707422

 http://metofficenews.wordpress.com/2014/08/06/ex-bertha-more-likely-to-miss-uk/

Update: Fri pm

More model agreement now on a northerly track.  Potential for UK worst-case scenario. Wind and rain crossing the country.  Inland 30-40mph, coastal and hills poss 50-60mph.

2014-08-08_20-37-10

Reigate and UK weather looks increasingly unsettled through the next week or so as the jetstream goes up a gear and becomes much more active and autumnal, possibly culminating early next week with the delivery of our first extra-tropical storm of the season, Bertha, on a fast jet from across the Atlantic. More on her later.

 

This week a couple of significant secondary LOW pressure systems are forecast to spin up rapidly out of the SW bringing wet breezy weather in the South.  Wednesday early morning looks especially wet but with a possible rapid clearance behind the active front.  Rain totals could be significant and warrant warnings nearer the time, watch the UKMO for these.

Friday looks a similar performance, even wetter for some, with another secondary depression developing rapidly from a parent LOW over the mid-Atlantic near to S Iceland.  This secondary low emerges quickly from the south bringing potentially wet weather but it is further off so things could change…

Hurricane season in the Tropical Atlantic basin starts in June and ends in November.  The National Hurricane center in Miami keeps an expert eye on every development in the tropical Atlantic but forecasters in the UK also take a great interest in them too.  This is because hurricanes can impact the UK as extra-tropical storms.

Several tropical disturbances look like they could potentially affect UK weather in the next few weeks, Bertha being the first.  Of course, the UK has never seen a “true tropical hurricane” (i.e. with an eye, wind sustained no fronts etc etc) but we certainly get the remains of tropical storms (called extra-tropical storms when they leave the tropics and arrive in the mid-latitudes).  Many / most extra-tropical storms never make it to our shores (e.g. Humberto), many also die a death or veer off long before they have any real impact on the UK.  Some arrive as late summer or autumn depressions often characterised by wet and breezy weather: this could be Bertha next week.

potential Bertha tracks

potential Bertha tracks

Whether or not extra-tropical storms arrive on UK shores they can still inject a lot of heat and moisture and energy into the late summer and autumn mid Atlantic at mid-latitudes.  This can perk up UK weather significantly and spoil the dog days of August. This can cause trouble for forecasters as models become unreliable.  Bertha is significant because models seem to be agreeing, at this stage anyhow,  that she will arrive near the UK sometime late next weekend or Monday.  Some models show her deepening into a significant summer storm (GEM), others show her moving south into Biscay and producing a Spanish plume of warm/not thundery weather.  In any case, expect more rain this week and next and do keep an eye on forecasts as the weather gets interesting, I would not be surprised to see a flurry / slurry of weather warnings for heavy rain at times.

Finally, the overall outlook remains rather unsettled for us as the charts above show: slightly below average temps with no heat wave on the cards yet (although heat cannot be ruled out at times as a plume event is possible) and plenty of rain spikes with over 150% more rain than normal shown on the NCEP charts.  As a rough guide, daily rain totals in Reigate exceeding around 10mm would be considered a really wet day… so there are several potential candidates for some wet days in the next week or so.

Some places in Japan have recently seen more than 250mm IN ONE DAY in tropical storm Nakri, so our worst weather is still nothing compared with some global extremes. In addition, think of Japan as Super-Typhoon Halong bares down on the southern islands this week.

http://www.accuweather.com/en/weather-news/halong-becomes-super-typhoon-j/31488164

Update:

Unusually Bertha appears on Atlantic UKMO chart as a tropical storm status and is downgraded about 12 hours later to ex-TS Bertha.  The charts for late Sunday / Monday look astonishingly autumnal with a perky jetstream invigorating the storm under a trough. Nothing certain yet, as exact track and intensity is variable still but this is a turn up for the books.  Last year no ET-hurricanes made it to the UK, Humberto drifted off harmlessly in mid September and actually built us a nice HIGH pressure as it made “landfall” over Iceland.  Perhaps Bertha might do the same thing?

 

2014-08-03_22-02-19

July summary for Reigate, Surrey

Tmax 30.2C on 23 July

Tmin 8.4C on & July

Total rainfall 31mm, with half of that falling during storms on 8 July.

Sunshine 201 hours

Max wind gust 28 mph on 26 July

July 2014 was notable for significant thunderstorms and a particularly spectacular meso-scale convective system (MCS) that drifted up from France and across the Channel on 18 July and then again on the 28 July with attendant shelf clouds and torrential rain and amazing lightning displays across much of the SE.  Whilst Reigate saw some thundery activity neither storm impacted significantly in Reigate, skirting further east, but did cause some spectacular lightning.  Flooding and hail was notable along the south coast, especially Brighton.  RGSweather was on holiday (typical!) so missed the action directly but will collect some links to add below. Any local photos available gratefully received.

http://www.surreymirror.co.uk/Thunderstorms-floods-perilously-close-Reigate/story-21947695-detail/story.html

http://www.surreymirror.co.uk/Video-pictures-Storms-Surrey-night/story-21657193-detail/story.html

http://www.severe-weather.eu/recent-events/spectacular-squall-line-over-the-english-channel-and-south-uk-july-18-2014/#more-7015

2014-07-14_15-55-50

 

Quick June summary for weather in Reigate: data only this month.

Tmax 25.6c

Tmin 6.2c

Precipitation 30mm

Max Gust 21mph

Sunshine 175 hours

Analysis and more weather posts after the summer holidays.  Thanks for reading!

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.

2014-06-07_08-06-18

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.  Like all thunderstorms, Spanish Plume storms are based on convection which means that thermals of air can rise uninhibited through the atmosphere creating tall deep cumulonimbus clouds.  Unfortunately, thunderstorm formation is notoriously difficult to forecast accurately, often being altered by local factors below the resolution of most forecast models or subtle changes in wind patterns which can subdue expected instability even up to the last moment.  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.  So an understanding of the plume might help figure out how much you 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…

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!) :-)

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.

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, cool or warm water bodies (the Channel), orographic influences (hills) and wind shear and any nearby FRONTS can also contribute to the thunderstorm cook-up.

Perhaps the most important factor in thunderstorm formation is TIMING.  The precise 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).

Here is an overview of the other main 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 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 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).

2014-06-05_20-51-20

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:

  • weather online
  • netweather.tv
  • lightning wizard
  • manunicast (new)
  • RASP BlipMap
  • RGSweather
  • nullschool wind flow charts
  • meteosat dundee uni sat pics
  • metbrief UKMO synoptic charts

 

Reigate weather in May 2014: summary

  • Tmax 26.3c
  • Tmin 1.5c
  • Total Precipitation 69.2mm
  • Total sunshine hours 150.5 hours

May was a month with some convective interest but no explosive thunderstorms for Reigate despite one or two possible moments with CAPE values exceeding 1000 kj.  The rapid in-situ development of an impressive Cumulonimbus over the town on a southerly flow on 22 May proved to be interesting for cloud watchers and delivered a well structured gust front but only one crack of thunder and some fleeting cg lightning.

Elsewhere in Europe record breaking rainfall over the Balkans caused tremendous floods and there were numerous funnel clouds and spouts from France through to Italy in unusually convective conditions.

Here are some thundery pics of the synoptic situation in late May.

More pics showing the synoptic situation during thundery epsiodes across the UK and Europe.