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.
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.
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.
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.
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.
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.
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.
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).
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.
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.
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).
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.
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.
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