Convective Outlook Spring 2008 (Technical)

[BrickFielder]

When I looked at some of the forecast SkewT's last night it looked like all we would get is some elevated storms and it is still very much touch and go today. A number of factors need to come together for storm initiation and I am not sure all the right ingredients will come together in one place.

Certainly we have moist air coming in across east anglia with dewpoints already up around 15C.

Places like farnborough are giving dewpoints around 15C so the moisture may spread a little further south than the models predict. This moisture is forecast to advect north during the day.

Another ingredient needed will be moderately steep mid level lapse rates.

The cold front which produced last nights storms was expected to move through during today so we will need another forcing mechanism as a trigger. Fom satellite pictures and looking at the 06Z Met Office fax chart this looks like coming from a trough moving in from the south east.

As to where the trough is likely to move then it is likely to extend northwards.

Looking at some forecast SkewT's then it certainly looks like on top of the moisture some insolation(sunshine) is required to lift temperatures so that surface based convection can take place.

The difference between surface and mixed layer cape tells its tale.

Don't assume though that sbcape pinpoints where storms will develop because you still need the forcing trigger which if models are to be believed will be to the west of the highest surface based cape. 06Z Larkhill sounding had a marked dry layer at 600hPa which may well spoil the party and wind profiles look a lot more benign than models might suggest.

Still touch and go whether we get any storms today but we should expect some heavy rain. Wales, the south west, Scotland and Northern Ireland should get a fair amount today. I will be watching in particular the area of heavy rain in the channel to the south of the UK as I think it will stop moving east and will swing up through southern counties and either up into the midlands or wales.

I would guess in this instance that the MetOffice modelling is closest to what will happen but it is over to the satellite and radar now. Chances of tornadoes forming are very much reduced with low level helicity moving into Scotland with the cold front, but there may just be enough low level speed shear across some inland areas especially since surface winds seem so ligth today. Far to many ifs though for more than a very very slight risk.

One final thought is that the KNMI satellite analysis for this morning is different to the Met Office one.

KNMI full size Satellite reports available here

[BrickFielder]

Lobe of vorticity over northern france and bay of biscay will move north.

This is forecast to cross the south west. Rememebr it is the approaching increase in vorticity (Positive vorticity Advection - PVA) which is likely to provide a trigger for convective activity.

There is a problem though apart from cloud cover and this is that mid level lapse rates are not that impressive and particularly so in the east. Typically 850-500hPa lapse rates below 25C are not condusive to storms, but it does depend on the specifics of the profile.

Surface based Cape pretty much says it all with storm development limited to the south west.

If we got a storm a little further east perhaps towards Bristol or a little east of there then we have an interesting wind profile forecast with quite a lot of wind veer. This would perhaps be condusive to mid level mesoscale cyclone development (low topped super cells maybe) although without the low level jet (strong wind at 850hPa) no tornado are likely to form.

Some high temperatures will probably be needed and its going to be late in the day if at all. Its probably one of those days where conditions almost come together but not quite and we end up with just a few heavy showers for the south west and southern wales.

[BrickFielder]

Not that impressed with todays prospects of storms to be honest as 850-500hPa lapse rates are just not really high enough anywhere but Ireland.

Some heavy showers may occur in the far south west and south west Wales along with some low level convegence showers along the south coast but I doubt whether we will get any thunder and lightning.

Similar picture for Saturday and Sunday with north or north east wind not particularly conducive to high surface temepratures and mid level lapse rates not impressive.

Monday looks interesting with warm moist air coming up from the south. Haven't we seen this before recently with GFS over cooking things a little in terms of instabillity and we end up with heavy rain moving north into the midlands. Dewpoints are forecast to be around 17C which we know is more likely to be around 15C at most. I think the risk here might be heavy slow moving rain and hail similar to what we have seen in recent weeks.

[BrickFielder]

Risk of a heavy shower tomorrow with a tilted upper trough over western parts.

Interesting wind pattern at 700hPa with slack winds and a small scale low and high pressure. It does not seem to affect vertical velocities on current forecasts, so I am unsure what to make of it at the moment. At lower levels we have the suggestions of a convergence zone.

Upper level vorticity charts show PVA (positive vorticity advection) over the western half of the UK.

While the moisture is all in eastern parts.

Somewhere in the middle we may just see a heavy shower , although tops may be limited by fairly relaxed lapse rates at upper levels. The usual caveats about GFS temperatures apply, high dewpoints will be in a wafer thin layer so we need to etsimate by knocking it down a peg or so and surface temperatures inland will be under estimated espcially with slack winds.

Lowish cloud bases , slack winds, convergence zones and steep low level lapse rates indicative of strong updrafts suggest there may be a risk of weak tornadoes formed from vertical vorticty sources. These spout type tornadoes often never reach the ground and are usually of limited duration. A lot of things must come together and I am not convinced they will at the moment and a considerable limiting factor must be the limited cloud top heights.

At this point about all we can say is that there is potential for some heavy showers which are unlikely to be severe, develop into thunderstorms or pose a significant tornado risk, but the models may change a little as we get closer to tomorrow.

[BrickFielder]

Convergence zone over England and Scotland is setting up further east than GFS predicted at the moment so things may be a little different to forecast. 500hPa height looks critical with convergence over the UK indicating some subsidence. This shows up nicely on 500hPa vertical velocity charts although we should note 06Z makes less of it.

Forecast SkewT's shows how this can limit cloud tops.There is also a worry of high level cloud with some moist air right at the top of the troposphere.

Forecasts SkewT's for northern areas of England and Scotland don't have this limitation but mid level lapse rates are unimpressive and equally likely to limit cloud tops.

Most likely place for a thunderstorm seems likely to be Northern Ireland with perhaps an outside chance for somewhere like Leeds or the welsh marches.

[BrickFielder]

Keep an eye on Wednesday with a cold front moving south and the Uk coming under a jet streak exit. Models suggest low level convergence zones will develop towards the north east of England and along the south coast near the Isle of Wight. Front at the minute looks like it may be an anafront and you must wonder if it is possible to get some line convection ahead of the front or even if the leading edge of the front contains some thunderstorms. More closer to the time.

[BrickFielder]

Jetstream pattern for Wednesday has changed somewhat from yesterday with a complex pattern which would suggest potential over northern areas and south eastern counties. This will almost certainly change for tomorrows charts.

The best way to look at this is to consider two seperate areas of thunderstorm risk. The most likely area to see thunderstorms at the moment looks to be in a line from around Carlisle to Hull mid afternoon with the area around carlisle and the borders favoured due to steeper 850-500hpa lapse rates. There is no suggestion at the moment that any storms developing will be severe with a fairly gentle wind profile but with low level winds perhaps strong enough and updrafts too weak for vertical vorticity sources to develop.

The second area of interest is just ahead of the cold front coming south or near to the surface low which is currently suggested to develop over the UK.

Instability looks a bit weak but GFS always tends to limit low level temperatures by spilling cloud too far ahead of fronts in this situation. Charts closer to the time are likely to make the situation clearer but at the minute it looks as if this is limited to heavy showers with cloud tops being rather limited. Lack of drier air aloft also would tend to put a damper on any prospects of storm development.

More tomorrow.

[BrickFielder]

Models change again with upper level vorticity held back further north. I still think the north east may get some heavy showers late on into the afternoon.

Development of the low pressure system over the UK looks less likely now, but I am watching how the rainfall over northern ireland is moving south faster than I thougth it would. A watch the radar and satellite rather than models day I think.

[BrickFielder]

Those in Scotland may like to keep an eye on Wednesday afternoon.