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.
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).
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.
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.
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