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July 9, 2015

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Reigate Grammar School, UK. Local weather station and forecasts for education. Reporting on local and global weather and climate. RMetS education committee. Town VP2 updates website every 10mins, wind every 5secs. CoCoRaHS manual rain gauge. Data to Met Office + Weather Underground.  Status: all Reigate data to WoW and WU is good 😄

badge_2015Delighted to announce that Reigate Grammar School has won a new MetOffice and Royal Meteorology Society MetMark Award for excellence in weather teaching and promotion of weather understanding and climate awareness in schools.

Reigate experienced an impressive deluge today when over 60mm* (tbc) of rain fell in a few hours causing flooding in parts of the town.  This was a “surface water flood” caused by drains being overwhelmed by intense heavy rainfall, rather than a river flood, our local River Mole will react more slowly to this event and is unlikely to cause any problems. (*by 4.30pm manual rain gauge 64mm; radar netweather est 50mm; total 24 hour rainfall 74mm manual rain gauge; RGS AWS 36.6mm; Reigate AWS 50.8mm)

The entrance to Morrisons car park and the lower section of Bell Street were overwhelmed with water bursting from drain covers adding to the water running down the roads.  It wasn’t just the roads that were flooded… such was the intensity of rain that Priory Park was also left with surface water pooling up several inches deep on the pitches, and even more in the sunken garden and around the cafe.

This rain event was accurately forecast by the MetOffice and MeteoGroup and several specialist severe and convective weather met agencies who all issued warnings of heavy thundery rain, some totals suggested over 60-90mm.

Two models, the MetOffice Euro4 and WRF NMM model output, turned out to be especially accurate with both putting exceptionally heavy rain precisely over our area at least 24 hours in advance and this was reported by RGSweather on twitter.  The GFS model was less convinced about such heavy rain reaching beyond the south coast but, as always, these convective summer events are especially hit and miss.  Today was a “hit” for Reigate but unfortunately would have probably caused a nuisance for businesses and commuters.

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The maximum intensity of the rainfall in Reigate was measured at 145mm/hr during the storm between 2-3.00pm.  This intensity of rain is unusual in Surrey (highest rainfall intensity in recent years was 183mm/hr 20 Nov 2013) but we are told to expect more of our rain to fall in intense events like this with global warming.  The last time flooding of this scale took place in town was 24 Dec 2014 but, having seen both events, I think this time was slightly “worse”, although given summer conditions the surface water may have subsided quicker.  There are many areas for further investigation worthwhile here: how much rain, for how long and with what antecedent conditions (soil moisture / season / evaporation etc) and what duration of rainfall intensity causes flooding in Reigate?

2015-08-24_15-59-29

rainfall intesity Reigate

The synoptic weather situation that caused this event was a trough over the Atlantic and a blocking HIGH over Scandinavia causing a warm unstable humid airmass to meet cooler Atlantic air along active fronts.  The resulting LOW transported a lot of moisture over the SE and caused the rain in heavy thundery downpours triggered near the fronts.  This situation is not likely to change much until later this week, so expect more rain but hopefully not as intense.

August SE Reigate flood

August SE Reigate flood

An unstable airmass with backing winds over the SE indicated by a skew t chart from the day.

Technically speaking this was part of a “trough disruption” event (see charts above) that started over the Atlantic during the weekend. A “trough disruption” is when an upper trough “breaks” and a southerly section of the trough proceeds purposefully east or NE leaving the centre of the trough behind as a semi-static feature.  The isolated part of the trough can then behave erratically. In this case an unusually active and southerly dipping jetstream for the time of year also played a part in deepening the LOW at the base of the disrupted trough.  Weather events can be “severe” and unexpected with models sometimes struggling to cope when trough disruptions occur.

The LOW pressure that brought heavy showers over the SE and Reigate formed in Biscay yesterday and travelled NE up the English Channel during the day.  SE England therefore experienced SE / easterly winds in the morning, backing northerly and finally swinging westerly during the course of the day, fairly unusual for our part of the world. The anticlockwise change of direction in wind is called “backing”.  Veering is the clockwise movement of wind over time or with height.

2015-08-24_16-32-11

glowering clouds menace Reigate in the morning

Here are some more pictures from this event on my google pics page: please use them but do credit rgsweather. https://goo.gl/photos/Rz9rpsAWozVaEtvB7

by evening it wasn’t any better:

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RGSweather photos featured on the BBC London news that evening (sorry, no sound)

Finally, we can expect more rain this week as the synoptic situation stays static with the Atlantic trough blocked by the high out East. This situation will keep us in a flow of humid moist air that, when it interacts with cooler Atlantic airmasses along fronts, is likely to cause rain.  Add an unusually active jetstream and we have a decent recipe for a wet week.

2015-08-24_18-02-30

wet week ahead

links to other reports

http://www.kentonline.co.uk/herne-bay/news/tornado-spotted-in-kent-42169/

http://reigate.uk/heavy-downpour-causes-town-centre-flooding/

http://www.surreymirror.co.uk/East-Surrey-Floods-Reigate-church-evacuated-water/story-27667834-detail/story.html

http://www.surreymirror.co.uk/Reigate-floods-Petrol-garage-closed-buildings/story-27667710-detail/story.html

http://www.surreymirror.co.uk/East-Surrey-Floods-Deep-water-A25-Reigate/story-27667942-detail/story.html

http://www.surreymirror.co.uk/East-Surrey-Floods-Reigate-homes-evacuated/story-27667812-detail/story.html

http://www.surreymirror.co.uk/Morrisons-car-park-flooded-wettest-day-years/story-27667152-detail/story.html

the system went on to cause significant tornados in Netherlands http://www.dumpert.nl/mediabase/6677432/ac9ba4f6/poldernado_des_doods.html

2015-08-21_08-15-09

Reigate July 2015 statistics

  • Average temperature 17.4C
  • Tmax 35.1C
  • Tmin 7.4C
  • Total rainfall 51mm
  • Max gust 27mph
  • Sunshine  161 hours

July 2015 in Reigate started hot, reaching 35.1C on July 1st due to a Spanish Plume.  The first day of July broke the record for the hottest July day when Heathrow spiked at 36.7C as cloud cleared around the airport allowing sunshine to heat the surface through an already very warm airmass.  This caused a heat spike at Heathrow and elsewhere that allowed the MetOffice to announce 1 July 2015 as the “hottest July day since records began and the hottest day since 2003”.  This set climate / weather data skeptics in a whirl as they spotted that the nearby Kew Gardens weather station did not correlate with the same heat spike at the same time allowing them to claim it was simply due to hotter local conditions from the local airport tarmac, a change of wind direction or even heat from a passing plane.

The photos below show the locations of the Heathrow weather station adjacent to the Northern Perimeter Road west and the Kew Gardens weather station.

In response to the doubt, Mark McCarthy, a climate scientist at the metoffice, was quoted in this article explaining that the heat spike was neither due to passing aircraft, nor a change of wind direction or any micro-climatic influence from tarmac and that the temperatures from Heathrow are representative and so the record stands.  In discussion with RGSweather on twitter, Mark also explained that the last 10 years of daily Tmax observations from Kew Gardens and Heathrow differ only by <0.03C on average.  This debate was akin to the wider global warming debate where met agencies present data that is then questioned by skeptics who present data that appears to contradict the claim. In any case, July 2015 started hot!  Unfortunately, July temperatures then promptly collapsed across the UK and the CET came out around average as westerlies returned from an anomalously cold North Atlantic.  Despite a lack-lustre July in the UK, globally, July turned out as the hottest month ever recorded.

Reigate and the SE had an average month being just nearer the European heatwave that dominated July.  Overall the monthly temperature came out a tad below the CET long term average, continuing a run of 3 cool months.  Further north and west July became a decidedly cool wash-out, especially in Scotland where snowfall occurred over some mountains.  Night time minimums were cooler than usual and some records were broken for lowest ever July night time minima.  In Reigate we recorded a Tmin of just 7.4C at the end of the month.

Here are metoffice anomaly charts showing how Reigate and the SE stayed warmer and drier than elsewhere.

SE rainfall overall came out above average mainly due to thunderstorms delivering locally heavy precipitation.  Reigate at 51mm came out nearer average because most heavy rain missed us. Although not local to our area, a thunderstorm over the Brecon Beacons in early July killed two people on top of Pen Y Fan and Cribyn.  The photo below shows this thunderstorm.

Globally, NOAA and JMA (Japanese met agency) reported that July 2015 was the hottest month ever recorded.

More from the metoffice on JUly 2015 here: http://www.metoffice.gov.uk/climate/uk/summaries/2015/july

Originally posted on xmetman:

This is the mean 1200 UTC surface temperature for the first 5 days of July 2015 and shows a cold North Atlantic and a warm central Europe (+10°C). It also shows a very cold northern Russia (-8°C), and cooler than average temperatures again are evident over the northern Sahara region (-3°C). I know the 5 day mean is a bit short to create anomaly charts but it does give the bigger immediate picture that is lost if you look at an entire month.

1200 Mean Air Temperature &amp; Anomalies 01 Jul to 05 Jul 2015 1200 Mean Air Temperature & Anomalies 01 Jul to 05 Jul 2015 (courtesy of NOAA & NCEP Reanalysis)

View original

2015-07-09_11-50-48

June 2015 Reigate summary

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

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

(Figures in brackets are from June 2014)

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

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

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

2015-07-09_11-19-46

Spanish Plume mis-fired 12 June 2015

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

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

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

2015-07-09_11-27-25

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

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

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

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

Andrew Freeman, Mashable 1 July 2015

heat wave / mini!

heat wave / mini! south east England Surrey

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

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

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

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2015-07-04_07-01-46

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).

2015-07-02_18-09-48

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.

2015-07-03_06-57-26

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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2015-07-04_06-04-15 2015-07-03_22-48-22 2015-07-03_22-43-40 2015-07-03_22-39-19

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.

2015-07-04_07-10-28 2015-07-04_07-11-02

synoptic chart for heat spike

synoptic chart for heat spike

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

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

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

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

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

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

2015-06-27_08-35-10

upper level instability for potential elevated thunderstorms

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

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

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

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

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

2015-06-25_20-44-03

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

Media references:

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

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

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

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

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

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

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

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

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

2015-06-20_07-45-37

June has been a cool month so far and is on target to be the coolest June since 1991, or certainly close.  It has also been dry with just 28% of monthly rain so far for SE England.

In contrast to the June trend, mostly due to the Azores HIGH lodged firmly to the SW and dragging in a cool dry NW flow (bit unusual for a summer regime), today a humid moist warm sector crosses Reigate and this will bring a minor hiccup to the dry weather of the last week when there has been no rain at all in Reigate.  Humid air is due to arrive in the warm sector and this is indicated by modestly raised dew points (high teens) and PWAT (precipitable water) exceeding 30mm, showing potential for some heavy rain around but this is hit n miss. (update: metoffice weather warning issued in morning for heavy thundery rain)

Of interest are surface winds which appear to converge in a zone, associated with a cold front, across the south later today (spot the twisty wind barbs below): convergence is where winds arrive quicker than they leave a region and, as winds “pile up”, this often promotes LIFT (upward air motion) that encourages convection and storms. But it’s a slack scene and by no means a classic stormy picture. Scattered slight risk of heavy showers is likely to be about it.

converging winds

converging winds

Skew-t charts (cross-sections through the atmosphere showing temperature, wind speed, direction and humidity etc) also show reasonably high CAPE >600j/kg (convective available potential energy) and some negative lifted index in central southern England. These values are indicators of instability: which means air is free to rise to a great height, condense into tall clouds possibly forming cumulonimbus.  Note the change of wind speed with height, such wind shear also acts to duct air from the surface.  Well, the ingredients are there for heavy showers later today but they were also present in the much heralded plume last week and that came to nothing, catching out professional forecasts as well as amateur enthusiasts. There were a few notable heavy thundery outbreaks last week but many convective forecasters and storm enthusiasts were stung by the lack of activity and model predictions appeared to founder.  The scenario today is decidedly less “stormy” so storm fans should not get excited either!

2015-06-19_21-44-47

Heathrow Saturday unstable showery chart.. dry-ish mid level slot too?

Of note is a dry slot at 700hPa mid levels (shown well on the chart below) that can induce evaporative cooling.  Evaporative cooling is caused when moisture evaporates and reduces temperature.  This reduced temperature at mid levels of the atmosphere can create higher CAPE as warm parcels excitedly find themselves rising through ever cooler environmental air. If sunshine heats the surface this can promote heavy showers and possible thunderstorms, albeit scattered and only a slight risk today. In addition a weak jetstream moves to a position later pm where any showers will find themselves on the left-exit region of the jetstream.. this is area known to further lift air from the surface, like a hoover dragging air upwards into divergent air aloft.

The forecast is for scattered showers, some thundery later.  So, as usual, some places could miss them though it’s a moist air flow so some rain is likely most places.  Rainfall totals will vary between almost nothing to possible >10mm.

2015-06-20_08-04-35

more unsettled to end June

Sunday looks cloudy but mostly dry as showers clear off tonight. Next week and towards the end of June there is a threat of more rain, possibly pretty heavy on Monday as Atlantic LOWS nibble away at the Azores high that has dominated our weather recently.  Monday sees a frontal wave low sticking wet conditions across the south which could yield high rain totals. Thereafter, mid week sees the Azores HIGH ridging back in with pleasant warm and dry conditions but this looks temporary as Atlantic LOWS nibble away with wetter westerlies always trying to edge back in.

2015-06-20_11-36-22

Azores ridges back in, but looks temporary

The outlook is therefore occasionally unsettled, especially in the north further from any ridges, though with the risk of heavy showers at times in the south.  Gradual warming trend into July with possibility of a brief warm or hot SE flow to start the month, as shown below on the 850hPa temperature GEFS chart: note the “plume” (oh no) with regressed Azores HIGH and thermal heat LOW over Spain. Way off, but worth watching :-)

2015-06-20_12-26-56

brief hot spell to start July?

2015-06-11_18-35-59

synoptic chart spanish plume event on 12/06/2015

Review: In the event: this was largely a fizzle! Few thunderstorms emerged until later and the SW missed out completely, which was the favoured area for action by some forecasts. One or two heavy tstorms impacted Sussex / Kent and East Anglia later in the day and into evening.  Surrey largely missed out, probably due to too much cloud cover, which left insufficient surface based heating to trigger home grown storms.  Also, a lack of soil moisture is significant too and humidity was left crashing by the afternoon.  Tmax Reigate barely scraped 25C here and, at 18C, dew points were on the low side all day, with RH similarly unimpressive hovering as low as 67% for a good while.

2015-06-12_18-20-06

complete lack of humidity in SE by evening

Imported storms attempting to cross the Channel generally couldn’t make it and, those that did, clipped Kent and Sussex late in the evening.  An interesting local convergence line in the evening popped up convection late on but this was mostly not thundery and missed Reigate anyway.  This was always forecast as an isolated tstorm event and heavy showers did occur but nothing of note over Reigate.  Many forecasts were found wanting and many convective storm specialists and enthusiasts will probably be smarting over this partial non-event.  Possibly the biggest “miss” in forecasting for a number of years.  In reviewing the charts below it’s worth noting that the individual ingredients were modelled but did not come together on this occasion and others were not solid: GFS CAPE and dew points significantly lowered in runs prior to the event.  There was little wind shear to drive storms along in a very slack flow. It wasn’t a classic Spanish flow from the SSW – drier easterlies dominated. Finally, the initial flurry of Channel storms left a good deal of cloud clag across the region and did not permit surface heating.

original post:

Southern England, including Reigate, is likely to have a first proper convective event of the year tomorrow including moderate to possibly even severe isolated storms with some heavy rain in a short space of time and a chance of hail, thunder and lightning.   Not everywhere will get a storm but, if you do, it could be a pretty big one.  For us in Reigate thunderstorms are most likely towards the afternoon and into Friday evening, though they could “pop” almost anytime during the day, especially if it is sunny and heat builds early.  If it stays cloudy through the morning the set-up could fizzle out spectacularly!  Expert convective forecasters say that this is a highly complex situation and forecasts, even at this late stage, are prone to inaccuracy when pinning down potential storms like these.  This post is not a forecast but outlines some key ingredients for thunderstorms and sees how tomorrow is set up to deliver the goods, at least some of them.

A MetOffice weather warning has been issued because the rain or hail could be heavy in a short space of time and cause local flooding and some disruption. The worst conditions are not expected here but further to the SW and S Wales. Severe thunderstorms have been occurring in France and Spain all day and, indeed across S Europe for much of the week, ours will be pretty moderate by comparison.

Thunderstorms need 3 things to get them going: moisture, heat and lift.  Here’s a quick review of some of these ingredients thrown into the Reigate and Surrey weather mix tomorrow with some charts to illustrate.

thunderstorm characteristics

thunderstorm characteristics

1. Heat

You may have noticed that today warmed up considerably reaching nearly 25C in Reigate . Tomorrow will be warmer still. This is due to a warm “plume” of air arriving from the continent, from as far away as S France, Spain and the Mediterranean.

This imported heat alone will raise the “airmass” temperature to over 15C at 850hPa (1500m).  Any sunshine, of course, will further heat the surface and this could raise temperatures on the ground to over 25C.  This is a critical ingredient for thunderstorms: air needs to be warmed so that it will rise into the atmosphere.

In conditions of potential severe weather it is useful to have sunhsine to heat the surface.  The cloud cover tomorrow looks broken and, if it remains like that through the morning, this will build bigger afternoon storms.

Whilst heat at the surface is a good thing to create warm rising bubbles of air, a comparatively cooler atmosphere through which the warm parcels of air can rise, is also a useful ingredient.  The air high up is unusually cool at the moment and this will create steep lapse rates… a rapid reduction of temperature with height.   Lapse rates can be shown on charts like below and on skew-t diagrams: these look tricky but show a cross-section through the atmosphere.

Steep lapse rates encourage warm parcels to stay warmer than the surrounding air.. so they will keep rising creating tall clouds such as cumulonimbus. It’s also useful to have droplets freeze at the top of clouds: it’s these ice particles that bounce around through the cloud, rising and falling to build a charge that causes lightning.

thundery skew t

thundery skew t

The red line is the “environment” air temperature: imagine taking the temperature of the air at regular heights as you rise up on a balloon flight: you’d expect the temperature to go down… usually by about 0.6C per 100m.  Now consider how a warm rising air parcel will rise, expand (as less pressure), cool and condense at a different rate: this is the dashed line which shows how rising parcels sometimes stay warmer (tomorrow) than the environmental air right the way to the top of the chart at some 30,000 feet.  So long as the rising parcel stays warmer relative to its surrounding then it will rise! The ultimate height of some well developed cumulonimbus clouds exceed 10km.  Freely rising bubbles of warm air (thermals) is known as an “unstable atmosphere”: like heating soup on the hob.. bubbles rise through it.

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Channel sea surface temperatures

The Channel is only 12-13C sea surface temperature at the moment and this can subdue thunderstorms attempting to cross from France. Nevertheless, storms currently approaching the south coast are pretty active still but are not expected to reach far inland to the SE as pressure is still comparatively high.

2. Moisture

Without moisture there will be no clouds and certainly no thunder.  The humidity and high dew points on the charts above shows how tomorrow there is plenty of moisture being advected into the country on the humid plume after the warm front passes north.

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theta e charts show airmasses well; spot the plume!

The synoptic chart shows the warm moist wedge that is due to pass over the UK tomorrow; it has large quantities of precipitable water (over 30mm) which could fall all at once in the right conditions.  Water vapour is a key ingredient: as water vapour condenses it releases latent heat which can drive upward lift in thunderclouds yet further (saturated air cools less slowly, so increases instability).

3. Lift

Lift can be any forcing mechanism that encourages air upward.  Surface based heating (diurnal heating) is important tomorrow, but so are fronts.  An advancing cold front sometimes cools and dries out the upper atmosphere and this can increase lapse rates dramatically and encourage further lift.  Charts showing CAPE (convective available potential energy) and lifted index can both show the tendency for air to lift.  Higher CAPE numbers are good, negative Lifted Index numbers are good for storms too. LOW pressure is also important to encourage the mass ascent of air.  An upper ridge early tomorrow could inhibit thunderstorm development in the SE until the trough arrives later.

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CAPE and lifted index

A further source of lift is surface convergence of airflow.  Convergence is where air arrives in a location quicker than it leaves.  When surface winds converge air “piles up” at a location and has nowhere to go except UP.  Surface convergence, with divergence aloft, is a good set up for lifting.  Sometimes hills or coastal breezes can cause convergence and enhance lift too.

What’s missing tomorrow?

For the very biggest storms more of all the above is good.  Moderate storms and isolated severe storms might arise but there is a lack of wind shear to organise storms into supercells.  Wind shear is increasing wind speed or change in direction with height.  Tomorrow is a slack flow until a moderate jetstream appears later in the evening.

lacks shear strong upper flow

lacks shear strong upper flow

Wind shear has the effect of hoovering air up from the surface and separating the warm storm inflow from the cold outflow. An organised storm will thus keep hoovering up warm air which feeds further development into a severe storm or supercell (which starts rotating and is a precursor of tornadoes).

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organised thunderstorm formation

With little wind shear the inflow feeding the storm can be disrupted and stopped by cold air descending from the tops of thunderclouds and cutting off their heat supply.  Such storms die naturally after a few hours and are known as single cell storms.  Multicell, supercell or meso-scale convective systems (MCS) require some shear to keep them strong and well fed. The lack of isobars on the synoptic surface pressure chart below indicatea the slack flow.  This can cause high rainfall totals because  storms sit and soak the same place rather than move on.

slack plume

slack plume

The overall synoptic development of this plume shown below is good-to-go for storm action tomorrow for some places in the south and SE including Reigate. A separate article discusses Spanish Plume development here.

So, some factors favouring storm formation tomorrow include:

  • Heat: strong advection of very warm humid air across southern England.
  • Low pressure: allows air to rise on mass.
  • Winds from the SE / ESE: can import storms from France in a NW direction. (they often miss us and go off to Kent otherwise)
  • Moisture: humid air across the south throughout the day.
  • Lift: sunshine will hopefully lift temperatures and permit thermals to rise to develop home-grown storms later in the day.

Do watch out for the King of Clouds… cumulonimbus or any of the development clouds like altostratus castellanus, and share pics and stories of any storms that come your way. Meanwhile,check professional forecasts for updates of course if you are making decisions.

Note: this is not an expert convection article, but an educational outline of storm formation. Comments always welcome.

Try Lightning Camera app https://play.google.com/store/apps/details?id=com.pluto.aftershot&hl=en

Amazing clear blue skies this weekend in Reigate! Subtle changes at first this week, then more significant deterioration by the end… probably ;-)

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HIGH pressure stays in charge through the first half of this week but with more cloud building and a few scattered showers possible.  We are on the cool side of the HIGH with an airflow from N/NE and occasionally brisk cool winds on the coast circulating round the HIGH.

HIGH pressure UK early June

HIGH pressure UK early June

Easterlies / NE winds are due to build on occasions this week especially on the south coast as super-geostrophic wind circulating round the HIGH draws cool air from the North East.  Cool nights are likely in a generally cool airmass when skies are clear of cloud and heat escapes.

cool nights.. frost up north?

cool nights.. frost up north?

Could even be a touch of frost up north on first few nights early this week.

The thickness chart above shows how the east side of the HIGH has a less “thick” airmass which is cooler as measured between 1000-500mb height). This means that, despite the overall high pressure, surface warming during the day can increase lapse rates lifting thermals into the cool air which can spark scattered showers as land warms through.  There have been big thunderstorms in Europe along the front separating this cooler airmass from the building heat of the Med.

Later in the week warmer thicker airmass is due to fold round the top of the HIGH but this might bring more cloud.  Things are on the change by the end of the week as a LOW forms in Biscay and threatens to edge north / NE to bring possibly wet weather to the SE by Friday.  The MetOffice fax chart shows a triple point crossing into the South by Friday and these can yield a lot of rain. Spanish Plume potential brewing … (updated Tues)

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metoffice fax chart plume

Whilst this is unlikely to be a full-on classic plume plume (update Tues… yes it could be best for some time!) it could sweep unstable air into the south and possibly cause thunderstorms. Heavy rain is possible too.

Thereafter the scene into the weekend looks more unsettled as a trough over the UK replaces the HIGH as it regresses (moves west) into the Atlantic. An Atlantic LOW from the NW looks on the cards for the weekend while pressure remains low. June has started cool (especially due to cool nights) and will continue to be overall cooler than average for the next 10 days. Note the change visible on the charts below.

There is general agreement for this deterioration amongst models.  So this week is likely to see a change to less agreeable conditions: lower pressure and a bit more rain at times and staying rather cool for summer!

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model agreement