Tuesday 6th October Convective Outlook

Tuesday looks to be the most unstable day that England and Wales has seen for some time . This post takes a look at the convective potential for the day, with the usual added level of educational detail for those interested in the way our weather works.

Tuesday’s Synopsis

On the southeastern side of a broad low in the mid-Atlantic, a smaller secondary low advances SW to NE across England through the course of the day. Then, as the broad low weakens and the secondary low clears away, pressure rises from the southwest through the evening and overnight hours.

Assessment of Convective Potential


For details on what vorticity is and how it relates to precipitation see:


Figure 1 shows model projections for vorticity and vorticity advection at 1200 GMT on Monday (1pm BST).

Figure 1: Tuesday 6th October ’15 1200 GMT charts from WRF-ARW showing vorticity at the 500 hPA level (left-hand image) and from GFS showing vorticity advection at the same level (right-hand image). Former is adapted from a chart obtained via modellzentrale.de, latter obtained via weatheronline.co.uk.

The secondary low has strongly positive vorticity associated with it, as one would expect. The advection chart illustrates its movement to the northeast, and this gives a good idea as to where convection will be able to initiate most readily tomorrow, regardless of any surface heating.

So this suggests widespread, frequent showers for Central Southern (CS) and Southeast (SE) England tomorrow between around 8 am and 4 pm. For Wales and The Midlands, convection looks less widespread but still with plenty of scope for numerous cells to develop.

Now it’s time to think about the likely intensity of these showers and the potential for them to become thunderstorms or at least bring a few rumbles of thunder.

CAPE (Convective Available Potential Energy)

Model projections for CAPE (not shown in this post) indicate that the highest values are likely to be seen across the West Country (WC) in general, perhaps peaking in Wales and The Midlands. This is largely down to the models predicting more in the way of cloud breaks on the west and northwest sides of the low that crosses England.

So it is that these regions have the potential to see some of the most vigorous showers, with the greatest chance of thunderstorms, despite the convective cells being less frequent than over CS and SE England.

Potential is nothing without sufficient moisture, however, so that’s what’s covered next, in Figure 2.

Precipitable Water

Figure 2: WRF-ARW charts for 1200 GMT Tuesday 6th October ’15, showing precipitable water on the left and 950-500 hPa wind shear on the right. Images adapted from charts obtained via modellzentrale.de

Across CS England, the precipitable water amounts look high enough to deliver some very heavy spells of rain, should the convection prove vigorous enough. This is possible if the sun manages to break through at all during the late morning.

The available moisture looks a little on the low side across The Midlands and Northern Wales as of 12 noon, but the higher values to the southwest reach these regions during the afternoon. Based on that and the other variables considered so far, some vigorous showers, with a few thunderstorms, can be expected to arrive or develop across Cornwall (perhaps Devon too) and Southern Wales during the morning, then advance to the rest of Wales, then The Midlands, during the rest of the daytime.

With decent sunny spells across Northern Wales and The Midlands, the ingredients are there for convection to be at its most intense across these regions.

Vertical Wind Shear

The right hand image in Figure 2 displays a real lack of much vertical wind shear across the areas of interest tomorrow. This means that for the most part, storms are unlikely to become organised, with the potential for severe weather very limited. Coupled with low values of helicity (to do with storm rotation, not shown in this post), I can’t see much reason to expect tornadic activity for the majority of the UK.

Possible exceptions could be across the far southwest and southeast for a short time during the morning, as some model guidance shows pockets of high vertical wind shear in these areas. We’re talking about a roughly 25% chance that such conditions will occur somewhere within one or both of the two regions, on a very localised scale. It could be in the center of a city or in the middle of an empty field… there’s simply no way of predicting that at all accurately.

CIN and Cloud Top Temperatures

CIN stands for convective inhibition, which represents layers of the atmosphere that can act as a barrier to rising air, preventing convection from initiating until if and when the barrier is broken through.

CIN looks very low on Tuesday, which will allow convection to initiate very quickly. Yet it also limits the intensity of cells – you see, convective potential is something that can build underneath a barrier much like how gas from a shaken bottle of fizzy drink can beneath a cap. Imagine removing the cap after you’re just started shaking the bottle – the release is far less explosive than if you keep the cap in place until many seconds of shaking have taken place.


Cloud top temperatures can be used to assess the liklihood of ice existing in the upper layers of cloud. The presence of ice in this way greatly increases the ability of a cell to produce both lightning and hail.

The cloud tops look to be well below freezing (as low as -40*C or so) on the west and northwest sides of the low pressure system. This aligns with where the highest CAPE is expected, which is no surprise as the most intense convection will nearly always have the lowest cloud top temperatures. Meanwhile, the temps across CS and SE England look to be near or even above freezing.

So, hail and lightning are most likely to be experienced across Southwest England, the West Country, Wales and The Midlands. Meanwhile, the risk looks very low across CS and SE England.


Summary Including
Chance of Lightning Within 30 Miles of a Given Location

Frequent showers affect CS and SE England from early morning through to mid or late afternoon, but with hail and lightning unlikely (less than 5% lightning chance).

Showers are less frequent but more vigorous across Southwest England and Southern Wales during the morning. I give these regions a 10% lightning chance for this period. The risk then reduces for the afternoon as the showers diminish. There is the outside chance of a very localised severe wind or tornado event.

The rest of Wales and The Midlands may start off largely dry with decent sunny spells, but this is likely to create an environment that enhances the intensity of convection arriving from the southwest from early afternoon onward. For the afternoon period I give a 25% lightning chance, with a 5% chance of experiencing hail at a given location. Please note that hail is unlikely to be more than the usual pea-sized variety.


Convection clears or fades out from the southwest during the evening and overnight period, with Wednesday looking largely dry.

Check out this post from mid-September to learn more about the variables considered in this post:



Thank you for reading, and watch out for those downpours tomorrow!

As ever, let the team at Weather Sci know if you have any feedback or comments :)

Early October: Blip or Breakdown?

Much of the UK has spent the past week or so basking in abundant sunshine, so much so that places such as my own are now into record territory for the total hours of sunshine. If you’ve recorded some exceptional sunshine totals at your location, we’d love to hear the stats 😉

This stunning weather – albeit a bit windy in the south during the last two days – has been brought about by a strong area of high pressure stretched out west to east through the UK. It’s extent, and the dry air within, can be seen in my composite image below (Figure 1).

Figure 1: Overlay of GFS det. run charts for 1st October 2015. Shown are surface level pressure (SLP; red contours) and 850 hPa relative humidity values (blue shaded contours; this level is some way above the surface and serves as a good guide to airmass characteristics). Adapted from images obtained from weatheronline.co.uk

Those pale colours represent air with humidity values of less than 20% which is particularly dry for the UK, though values will nearly always be higher at the surface.

Through the weekend, this high moves east and a little south, with the winds across England turning from their current easterly direction to a southerly, while across Scotland it goes from westerly to southerly. Daytime temperatures may rise slightly on the already decent values seen this week, but there may be more in the way of cloud as the high will no longer be right over the UK and the air over Europe does look to be a bit less dry. Humidity will be on the rise and you may notice that a bit when out and about.

By Monday, low pressure in the Atlantic is attempting to push in from the west, but the most recent model guidance suggests that it will only manage to make a brief impact before being sent packing as high pressure holds its ground just east of the UK. You can see the standoff in Figure 2.

Figure 2: Overlay of GFS charts as described for Figure 1. Adapted from images obtained from weatheronline.co.uk.

Notice that sharp step up in relative humidity running north to south through the UK. That marks a frontal boundary on which there looks to be a fair bit of rain due to the presence of relatively warm air that has plenty of moisture associated with it.

The Atlantic lows look to have further goes at shoving that high pressure away as the week progresses, but it’s not clear how successful they’ll be. A look at the latest GFS ensemble forecast for London gives a good overview of the situation, as can be seen below (Figure 3).

Figure 3: GEFS line graphs for London, compiled into one catch-all image from charts obtained via wetterzentrale.de. The top group of lines are for surface pressure, the middle are for 850 hPa temperatures (a good guide to how warm or cold an airmass is), and the bottom are for rainfall. I’ve annotated the graphs to highlight key characteristics.

Notice the strong agreement on a fall in pressure on Monday, coinciding with the arrival of substantial amounts of rain along that frontal boundary (and from showers behind), this fueled by some relatively warm air getting involved.

After this it becomes a bit of a mess in terms of surface pressure, with some ensemble members (‘runs’) bringing high pressure back across the UK as early as Tuesday while others take until the weekend, or even keep low pressure in charge right into the following week. this disagreement is reflected in a thinning out of the rainfall spikes, but enough remain to suggest that we’ll be lucky to stay dry for the whole time between 6th and 9th October.

Notice how high the 850 hPa temperature rise during that time – it looks like it will tend to be on the warm side away from the rain, regardless of how unsettled it is. With a good bit of sunshine coming through, temps could reach the low 20’s Celsius in some parts of the UK (this most likely in eastern England). Under persistent rain, though, the mid-teens is about as high as you can expect.

From the 9th or 10th we see a distinct change of emphasis, toward one of higher pressure but cooler air affecting the UK. The averages of all the runs (white lines) suggest only occasional rain, near average temperatures (day and night combined) and fairly settled but not strongly so. Yet given that we do see a fair few runs with strong high pressure back on the scene, and very few keep low pressure in play, it seems that the average outcome might be underplaying the settled signal. The same can’t be said for the temperatures though – there’s so much scatter, with options ranging from summer-like warmth to a wintry chill in the air (but probably not wintry precipitation!), that if anything the average is meaningless – the model just can’t make heads or tails what’s going to happen that far ahead in time. This is fairly typical once you get beyond about a week ahead of the initialisation time.


I hope you can make the most of the weekend’s fine weather when it comes around – just in case it doesn’t come back later next week!

Settled Spell: How Long and How Warm?

High pressure is building strongly across the UK and looks like dominating our weather for some time to come.

Gauging just how long is in truth a matter of taking into account a vast array of computer model runs as well as their reliability based on long-term verification statistics and so on. It’s a lengthy, complex process that forms a significant aspect of professional weather forecasting and involves whole teams of highly qualified people.

However, those who aren’t specialised in the subject, or can only spend a little time each day looking at the prospects for our weather, can get a decent idea of what’s most likely to happen by making use of just one or two model’s ‘ensembles’. These are produced by varying the conditions at T+0 (the start time of the run) in ways that are very small, yet can have significant consequences for how the weather is projected to behave. So it is that a range of possible outcomes is generated which gives us an indication of how likely certain types of weather are to affect the UK at a given point in time.

It’s possible to find handy line graphs plotting how variables such as temperature, rainfall and pressure vary among the different runs. Figures 1 and 2 show examples for London and Aberdeen – taking into account both ends of the country – that use ensemble data from the Global Forecasting System (GFS).

Figure 1: Mean Surface Level Pressure and Precipitation plots (overlaid) of the GFS ensemble forecast (GEFS) for London initiated at 00:00 on 25/09/15. Image adapted from a chart obtained via theweatheroutlook.com
Figure 2: Mean Surface Level Pressure and Precipitation plots of the GFS ensemble forecast (GEFS) for Aberdeen initiated at 00:00 on 25/09/15. Image adapted from a chart obtained via theweatheroutlook.com

We can clearly see  the strong rise in surface pressure during the coming five days, reaching values of between 1035 to 1042 mb, which is about as high as is usually observed in the UK. There’s then a trend to lower the pressure, both for both locations there aren’t many runs bringing precipitation until we get past 5th October.

When thinking about how likely is is that our settled weather will be broken on a given date, the number of runs bringing rainfall out of a possible 22 can be used to estimate the chances (e.g. 1 in 22 is just under a 5% chance). Unfortunately, counting the ‘spikes’ of rainfall often proves difficult to do very accurately. The same goes for the surface pressure lines when looking for the dips in pressure that indicate areas of low pressure moving through.
So I tend to just go with an estimate, which is what I’ve annotated the two graphs with.

I’ve drawn the following conclusions from Figures 1 and 2:

  • Settled conditions are pretty much guaranteed for most of the UK until the end of September.
  • There is a chance (at most 25%) of a weak area of low pressure affecting the UK from the south at some point during the first five days of October.
  • For the same period, the main Atlantic storm systems are likely to stay clear of the UK, with the eastern side of Scotland tending to see nothing more than a few brief showers on one or two days.
  • Unsettled conditions become far more likely by the end of the first week of the month, but while the storms could roll in, it’s equally likely that they stay away with high pressure again in charge of our weather. This remains the case looking as far ahead as 11th October!

The same method can be used to analyse how warm or cold it’s likely to be. Using plots for surface temperatures such as the example in Figure 3 (see below the bullets), I reach the following conclusions:

  • Up to the end of September, it seems to be about 50/50 as to whether it turns out pleasant by day with near average temperatures (perhaps a little above in the south), or a bit on the chilly side. I believe fog is the culprit here – where it lingers, temperatures will struggle to rise.
    By night it looks chilly, but with frost unlikely – just a low chance from the 30th for Scotland.
  • 1st-5th October, there’s a good chance it remains pleasantly warm by day at both ends of the country, but there is some suggestion of a few chilly days occurring, and again fog is the likely cause.
    There’s a slight chance of overnight frost in the south (around 10%), and a higher chance in the north (around 30%), but the dominant signal is for nighttime lows to remain similar to those seen during the final days of September.
Figure 3: Example plot of (near) surface temperatures for London, generated using GFS ensemble data via theweatheroutlook.com. This chart will update with time so may look different if you’re reading this many days after publication.

Often, it’s safe to assume that the indications from the ensembles for a given location serve as a decent guide for conditions in the surrounding area. In fact, the whole of the UK can often be assessed using one location in the south, one in the north, and one in the middle.

On this occasion, the dominance of high pressure for the next 10 days, perhaps more, has allowed me to leave out the middle station, as conditions there aren’t likely to be much different to those in the south.

There are other model ensembles out there which can be used, but the GFS ensembles are the most widely available. I find this site particularly useful:


– just click on the ‘GEFS’ tab and use the buttons below the image to generate some line graphs using one of three variables, or use the buttons on the left to look at charts for individual runs.