Archives For thunderstorm

2016-03-27_08-43-08

Before we get to Katie, it’s worth mentioning recent and current weather for Easter Sunday: the first convective day of 2016.  A cold pool of polar upper air followed the impressive cold front yesterday which developed a very long squall line.

Thundery conditions followed the cold front in an unstable Polar airmass with notable CAPE (convective available energy) and negative 2 Lifted Index for the time of year – both indicators of potential thundery heavy shower activity.  The limiting factor on multicell thunderstorms locally was lack of wind shear and an anticyclonic jetstream, both subduing anything truly impressive or sustained. Nevertheless, some hail and thunderstorm activity is rife across the UK today in this unstable polar maritime airmass.

Storm Katie is riding a powerful jetstream and deepening rapidly over the Atlantic through today.  She is some 600miles away to the SW but approaching and deepening rapidly as she undergoes rapid cyclogenesis (RaCy) falling from 999mb to 975mb:  24mb in 24 hours, qualifying as a “bomb depression”.

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Katie emerged out of Canada earlier this week as cold continental air met humid sub-tropical maritime air.  She engaged with an active jetstream to be launched across the Atlantic.  Storms tend not to move fast and deepen rapidly at the same time, so her rapid progress across the Atlantic will now slow as she now expends more energy lowering pressure on her approach to the UK.

The evolution and track of this storm is similar to St Jude October 2013, but Katie is forecast to be less powerful, due in part to the slacker pressure rise on her departure into the North Sea.** (update: in the event Katie pipped StJude, see below!)

Katie also has some similarities to the October 1987 but again is significantly less powerful than that rare 200 year event.

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The water vapour sat pics below show the evolution of Katie and cirrus cloud associated with the jetstream.

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Baroclinic leaf in the Atlantic, jetsream cirrus to the UK

 

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dark slot shows rapid intensification stage as strat air descends into LOW, showers over UK

Storm Katie is forecast to deepen through the SW approaches to below 980mb on an expected track overnight Sunday-Monday from Biscay into the Bristol Channel through to exit around the Wash.

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The pressure gradient between the Isle of Wight and the Wash could be as much as 12mb… the highest of our Wight-Wash Oscillations recorded, and greater than the 10mb WWO recorded in StJude, if this comes off.

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However, the pressure rise behind Katie is expected to be less rapid and sustained than St Jude, so this critically reduces the potential for highest gusts, sting jets aside.

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The highest wind speeds for Storm Katie will be south of the low core in the occluding warm sector in the small hours of Monday morning.

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Max gusts of over 80mph are possible in the Channel, 70mph along Channel coasts and 60mph further inland.  For Reigate this means a brief episode of gusts possibly exceeding 50mph and more over exposed hills.  This could wake people up momentarily as gusts roar through trees and round houses.

Rainfall will be significant too and some hi res models put down >40mm over parts of the North Downs, showing the significance of small hills in generating orographic rainfall.

Rainfall more widely is more likely to be around 20-30mm, enough for local flooding. The MetOffice have issued a yellow warning for Storm Katie.  Check the weather impact matrix showing moderate likelihood of moderate impact (low yellow).

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Update: Amber warning issued for Storm Katie.

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The shipping forecast is less equivocal and shows storm warnings right around the UK associated with Katie and other low pressure systems.  Storm Force 10 is possible in the Channel for a while. http://bbc.in/1RtwvyS

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Furthermore, evolution so far matching StJude and the Wight-Wash pressure gradient forecast seems to be matching or beating St Jude.

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Below is a Meteosat picture (courtesy Dundee sat receiving station) showing Katie moving into the North Sea, deepening again slightly to 972mb,

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Meteosat visible photo of Storm Katie moving into North Sea on Monday 28 March

Post-Katie we enter a broadly zonal westerly flow but turning cooler as slack winds swing northerly through the week as LOW pressure moves into North Sea/Scandinavia. Unsettled and showery activity continues through the week but with pressure slowly rising from the west by Thursday. No further significant storms are expected for the time being. Into the start of April, pressure over the Atlantic is expected to rise and progress over the south of the UK into Europe to give a settled period for the start of April.

RGSweather is off to East Iceland, so expect occasional tweets and updates from cooler climes.

Storm Katie: In the Event:

various meteorological impacts show Katie exceed StJude in power but both fall well short of October 1987 Great Storm.

  • Max wind gusts Needles 105mph (St Jude 99mph)
  • Reigate max wind gust 51mph, widely 50-60mph and 70mph on North Downs (Kenley) Gatwick 59mph
  • Redhill aerodrome recorded 70mph gust at 04:50
  • Lowest pressure: 971mb (St Jude 2013 976mb; Oct’87 958mb)
  • Reigate lowest pressure 976mb (St Jude 980mb; Oct ’87 est
  • Reigate rainfall 20mm
  • Wight-Wash Oscillation 14mb (St Jude 12mb, Oct ’87 storm 20mb!)
  • Rapid rise in River Mole catchment – no significant flooding but overtopped bankfull stage.

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Note, compared with the Oct ’87 storm Katie and St Jude are found wanting.  The Wight-Wash Oscillation for the 200 year storm of October 1987 was a whopping 20mb! See the chart below for the exit of the Oct 87 storm through the Wash on 16 October 1987.

Finally, which of these big SE storms are most powerful? Play SE storms TRUMPS to find out..

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http://www.bbc.co.uk/news/in-pictures-35910151

http://www.bbc.co.uk/news/uk-24708614

http://www.chichester.co.uk/news/update-more-than-600-incident-in-sussex-as-storm-katie-batters-the-south-1-7299324?utm_medium=twitter&utm_source=dlvr.it

http://www.telegraph.co.uk/news/weather/12205613/Storm-Katie.html?frame=3601967

http://www.bbc.co.uk/news/uk-england-35909651

http://www.dailymail.co.uk/news/article-3512119/Storm-Katie-batters-Britain-Gales-100mph-dozens-flights-cancelled-UK-wakes-Bank-Holiday-washout.html

http://www.theguardian.com/uk-news/2016/mar/28/storm-katie-flight-diversions-wind-rain-batter-southern-england-and-wales

http://www.mirror.co.uk/news/uk-news/see-full-carnage-storm-katie-7644830

local coverage

http://www.dorkingandleatherheadadvertiser.co.uk/Storm-Katie-Trees-power-Surrey-storm-hits-PHOTOS/story-29006658-detail/story.html

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.

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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

05 june plume fax

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

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

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

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plume edges into UK

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

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

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

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Brief heat

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

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

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

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skew t plume Friday 5 June 2015

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

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

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

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

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).

2015-05-30_20-18-19

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!

2015-05-30_09-33-32

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.

2015-05-30_12-11-00

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

May 19 thundery showers was not a “severe” weather day but 10mm of rain from 7 showers in 7 hours, several with some moderately intense rainfall rate (30-40mm/hr), small hail and episodes of thunder, was of interest locally and deserves a review of some charts leading up to the event.

Often our area of the SE seems to miss convective action but this time showers perked up as they converged across East Surrey and some developed into reasonably impressive cumulonimbus clouds (cb).  As a bonus mammatus clouds were spotted at the end of the day (see photo above). Below is a summary of some key ingredients for the day.

2015-05-19_21-43-21

Synoptic chart thundery 19 May 2015

Low pressure: This synoptic set-up saw an unusually cold plunge of upper air flood across the country during May18-19. The synoptic chart above shows the invasion of cold polar maritime air behind a cold front, itself not especially dynamic across the SE. The LOW over the North Sea dragged this unseasonably cool air from the NW: the basic ingredient for instability on this occasion.

meandering jetstream

meandering jetstream

Note the u-bend wiggle in the jetstream around the LOW pressure in the chart above. The northward limb of the jet on the inside of the U-bend can become the location for instability, though this was not a fast jetstreak occurrence.  The base of the trough also crossed the SE of the UK during the day encouraging lift.  Showers were forecast on the charts days before.

Steep lapse rates: Very cold upper air temperatures fell to below -30C at 500hPa (5500m) across the UK.  In Spring a cold airmass like this can become unstable over land especially if the sun warms the surface to create thermals of warm air able to lift through the cold environmental air. The 850hPa temperatures show comparatively warm air at 1500m which increases the lapse rate.  The morning of May 19 was sunny and the surface heated with 5 hours of May sunshine to a modest 14C, just enough to release thermals and rising parcels of air.  Convection requires heat as a key ingredient to steepen lapse rates and create thermals, rising parcels of air.

Low lifted index: LI forecast for Heathrow (below) showed a LI of -3.  The lifted index is a measure of instability in the atmosphere and shows how readily bubbles of warmed air will rise from the surface to a great height (convection).  It is the difference in temperature between the environmental air at 500mb level (around 5500m) and the temperature of the theoretically lifted bubble of air (parcel).

2015-05-20_20-21-04

lifted index (UK rarely exceeds -5)

Negative LI numbers are good for thunderstorms because they show that the parcel is significantly warmer than the air around it at altitude, which means that at 5km the air is still bouyant. http://www.weatheronline.co.uk/reports/wxfacts/Thunderstorm-Probability.htm

2015-05-19_06-12-28

Moderate CAPE: convective available potential energy is the energy available to push air vertically up and create those surging bubbles of cumulonimbus clouds.  The higher the CAPE the greater uplift potential (above).  Note it is potential energy and a high CAPE often confounds convective activity if other ingredients are lacking, such as heat or moisture.  19 May had marginally unstable values for the UK of 800j/kg.

In the USA Tornado Alley forecast CAPE values exceed 2,000 j/kg in super cell outbreaks.  CAPE on a skew-t chart is the area between the parcel trajectory and the environment temperature.  In the SE of the UK we often just get “skinny cape”, where rising parcels are only just warmer than the surrounding environmental air, whilst in Tornado Alley “fat cape” is frequent which indicate parcels much warmer than the surrounding air, increasing bouyancy.  http://www.weatheronline.co.uk/reports/wxfacts/CAPE—Convective-Available-Potential-Energy.htm

The colourful charts above from lightningwizard.com show streamlines of wind and air mass equivalent potential temperature.  Streamlines show where the airmass is coming from and, if they converge at the surface, can indicate potential for lift.  Theta e charts show moisture and temperature characteristics of airmasses.  Where there is a contrast of colours between high theta e and convergence at the surface and lower values and divergence of air at altitude this can bring on convective weather. The moisture chart shows that the airmass supplied sufficient moisture to the UK to provide for cloud formation and release latent heat.  Dew points stayed relatively low and this showed moisture (and heat) locally was lacking for bigger storms.  Compare these forecast charts with the actual occurrence of lightning on the day.

curved forecast shows showers

curved forecast shows showers

Forecasts for the day (Euro4) showed typical lines of rainfall accumulation which indicate showers.  The actual radar showed “popcorn convection” (@metmanjames) over a wide area drifting east.

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Recommend following @convectivewx for UK convective forecasts.  Their forecast from 3 days ahead was spot on.

2015-05-17_19-54-49