UK convective general discussion and forecasts

[weatherobsuk]

Anyone else see the anomalie on the UKMO and Sat24 vis satellites between 12-1pm? The Sat24 satellite looks to have misaligned itself, but the UKMO aswell? Do both systems operate from the same feed or something to do (realigning perhaps) for the G1-class (Kp=5) geomagnetic storm.

I noticed it looking at the band moving through the UK and thought it was odd the northerly movement briefly transitions southwards, before reverting back to its origional flow again.

[Coast]

We are going to try some specific threads for single days where convective events look more likely. The first one can be covered in the following thread dedicated only to potential and actual convective action between 00.00 - 23.59 hrs on 6th July 2011 i.e tomorrow.

Let us know how you think this goes after tomorrow and if you want to see more dedicated daily threads as storm action becomes more likely nearer an event (or not!).

This thread will continue for more general convective discussion and for events that are coming up in the days after any specifically identified daily potential.

[Adi F]

This is a good video - thunder is awesome! Pretty much captures what I experienced a week ago today

Sorry - wrong thread I know but it seems appropriate

VOLUME UP!!!

Much like what I was hearing on Tuesday, many of these sound like +ve CGs

LOL I have just asked him if he fancies going storm chasing!

[Biggin]

Thundery showers possible for tonight according to P Cockroft on SE Weather.

[Muffelchen]

like dropping a stone into water it creates a ripple effect, did you notice if the lightning radiated out from where the first lightning happened? or was it completly random?

Yes, slightly but the clump of showers was pretty small but, once one went, they all did. The effect was much too fast to be the collapsing downdraft hitting the ground and setting off new updrafts. We are talking minutes here not an hour or so. I do also seem to remember the programme mentioned above, which I believe was about sprites in the mesosphere/magnetoshere. The same effect was clearly visile from the satellite.

I had noticed the showers earlier in the evening out over the N Sea and had been keeping an eye on them. Nothing happened for an hour and half then they all went off within a minute or so. The whole thing only lasted 30 minutes but it just occurred to me that when one shower on a given day has thunder, they all seem to. Yet, when the structure appears perfect and there is no thunder in a shower, the next one doesn;'t seem to either. They all seem to go-off together.

I wonder if it has something to do with charges in one CB creating charges in a nearby CB by repelling electrons and, when CB1 discharges, the electrical imbalance caused, sets off CB2 etc.. I hasten to add this is pure speculation but it might help explain why Southern England exploded with lightning in about half an hour last week - too short a time for pulse storms to form from the outflow of dying storms.

Anyway looks like the next few days might be a test for this hypothesis.....

Edited July 5, 2011 by Muffelchen

[Harry]

Thundery showers possible for tonight according to P Cockroft on SE Weather.

Indeed, more so the further east you are it would appear (and as expected given the charts and indeed the national forecast).

I think thunder and lightning will be to a minimum although I am absolutely RUBBISH with these complex 'wave' patterns. To me they appear quite arbitrary with a random 'pulse of energy' running along a front - whatever that is! I can only assume it's a batch of nice muggy air, which if its the case certainly feels as if it's arriving at the moment...nowhere close to the humidity of early last week, but it's nudged up slightly today. If humid air does manage to squeeze in ahead of the front, then some livelier downpours are likely IMO, but thunder and lightning?? Hope rather than expect (just seen GFS is very keen indeed to rapidly intensify the front during evening areas right up the eastern side coupled with a 30-50% storm risk in some areas)

Edited July 5, 2011 by Harry

[Lt. Dan]

I think thunder and lightning will be to a minimum although I am absolutely RUBBISH with these complex 'wave' patterns.

I was going to ask about this as well, I cant find much information on what the 'wave' is or does, I see it mentioned somewhere else as a frontal wave, but all I can find for that description is mentioned in the development of a low pressure system??? confused.

[Harry]

I was going to ask about this as well, I cant find much information on what the 'wave' is or does, I see it mentioned somewhere else as a frontal wave, but all I can find for that description is mentioned in the development of a low pressure system??? confused.

Well, a wave itself (in it's most basic terms as that is what I am most familiar with) is where warm air cuts back into a cold/occluded front. The main front is moving eastwards, but a small portion of the front has warm air trying to nudge back westwards.

I've just stepped outside by the way and the humidity has definitely nudged up a notch...much more sweaty betty feeling than yesterday!

Edited July 5, 2011 by Harry

[Lt. Dan]

Well, a wave itself (in it's most basic terms as that is what I am most familiar with) is where warm air cuts back into a cold/occluded front. The main front is moving eastwards, but a small portion of the front has warm air trying to nudge back westwards.

I've just stepped outside by the way and the humidity has definitely nudged up a notch...much more sweaty betty feeling than yesterday!

Cheers Harry, that makes sense actually looking at the attached pic. Must be the wave they have marked over wales earlier?

[Staffordshire]

Hey any potential here? I'm a very busy body at the mo and jut popping out, could someone post so i can see when I get back please?

Regards

Staffs

[Harry]

Cheers Harry, that makes sense actually looking at the attached pic. Must be the wave they have marked over wales earlier?

The black line over Wales is a trough (like a mini-front but not introducing an airmass of particularly different temperature/humidity).

The wave is the small red warm front line sitting behind the secondary Low pressure symbol ('L') over the South coast. I've reproduced it in child like fashion lol (attached) with wind arrows showing what's happening in the broader picture - I couldn't paste and edit your image.

I wasn't expecting this secondary low to develop. Furthermore, the chart you have posted indicates the front as a Cold Front, whereas the MetO have the front as an occluded front.....interesting! Given the fact that there is a temperature gradient of between 8-10C either side of the front, a Cold Front IMO does seem more appropriate, even if earlier in the cyclogenesis cycle it was an occluded front.

Edited July 5, 2011 by Harry

[Lt. Dan]

The black line over Wales is a trough (like a mini-front but not introducing an airmass of particularly different temperature/humidity).

The wave is the small red warm front line sitting behind the secondary Low pressure symbol ('L') over the South coast. I've reproduced it in child like fashion lol (attached) with wind arrows showing what's happening in the broader picture - I couldn't paste and edit your image.

I wasn't expecting this secondary low to develop. Furthermore, the chart you have posted indicates the front as a Cold Front, whereas the MetO have the front as an occluded front.....interesting! Given the fact that there is a temperature gradient of between 8-10C either side of the front, a Cold Front IMO does seem more appropriate, even if earlier in the cyclogenesis cycle it was an occluded front.

Yes, sorry, my geography was way off there! The small warm front they have marked was meant to be what I was looking at. I noticed before that there sat analysis does differ from the fax charts at times - not always easy when you are trying to learn more about this stuff!

If the warmer air starts cutting back into the cold/cooler air then this could cause a secondary low to form? If my thinking is correct, that's almost how a low starts to form anyway?

[Neilsouth]

Seems to be decaying rather alot..

[Harry]

Yes, sorry, my geography was way off there! The small warm front they have marked was meant to be what I was looking at. I noticed before that there sat analysis does differ from the fax charts at times - not always easy when you are trying to learn more about this stuff!

If the warmer air starts cutting back into the cold/cooler air then this could cause a secondary low to form? If my thinking is correct, that's almost how a low starts to form anyway?

Exactly that...where you get warm and cold air trying to force past one another, the warm air rises causing a drop in pressure near the surface.

However, where my knowledge goes some what rusty, is whether or not the secondary is forming the wave, or the wave is forming the secondary low! Probably a bit of both...curious to find out actually.

Next 6 hours could be interesting though - could be some quite intense local downpours around across the SE/EA initially, then spreading up the east coast.

Seems to be decaying rather alot..

Nah if you run the loop it's been pulsating slightly.

It's from around 5-6pm its meant to get its act together...if the MetO and GFS are correct anyway

Furthermore, would keep a close eye on developments immediately behind the frontal system...appears to be a line of intensifying precip running out of NW France atm...nothing to get excited about just yet but this could be what MAY bring the odd rumble tonight...this is what MetO invent seems to point at anyway

Edited July 5, 2011 by Harry

[Coast]

Just a reminder that we are trying some dedicated threads for daily convective potential. The first one for tomorrow is here:

[VillagePlank]

How do i go about understanding that chart?

Also BBC is forecasting storms here for Thursday.

Hi Lauren,

Sorry for not getting back to you yesterday - too much beer and fun, (un)fortunately. Easiest way of doing this is for today, in London, at 12z. The 'z', by the way stands for 'zulu' time. It's not quite accurate, anymore, since zulu time should mean GMT, not BST, but that's by the by.

Here's the skew-t for today:

What this shows is the (forecast) atmopsheric profile of the atmosphere. You'll note three lines on this chart: a blue one, and red one, and a grey one. these lines are superimposed against temperatures at the bottom and air pressure at the right.

The blue one is dewpoint. We all know what 'dew' is; it's that wonderfully beautiful stuff you see on the fields one summers morning at dawn. On skew-t's, however, is a bit more technical; the dewpoint is the temperature that a wet piece of air must be cooled so that the wetness in it condenses out into water - or dew.

Dewpoint, sometimes referenced as T_d, is strongly tied, then, to how moist the air actually is. If T_d is equal to the temperature then the air saturated - ie it can't 'hold' any more water: you'd see on your weather station 100% relative humidity. More specifically, the T_d, is entirely related to the specific humidity of air (ie how much water, by weight, is in a kg of air)

Moistness (is this starting to sound like '70s porno movie?) is very important. Why? Well, when water changes state heat is released. This is easier to understand than most people realise. Take a deep breath ... if you want to change the state of water, let's say into steam - boiling the kettle - everyone understands the need to put energy in, otherwise it just won't happen. You can sit and watch your kettle, but unless you run electricity (energy) through the element, nothing is going to happen. Steam is effectively (not quite) water vapour. Now, because of all sorts of rules, we can't destroy or make energy we can only move it around.

This means that the energy required to convert water into steam is still retained in the steam (it hasn't gone anywhere) If that steam then condenses out into water, what happens to that energy? It is released in the form of heat; I think the scientists around here call it 'latent' energy. If I may be so bold, think of it this way: energy + water = water-vapour, therefore, water = water-vapour - energy: the energy has to be released (or subtracted in this example), by simple algebraic manipulation. God is a mathematician. Ever wondered why it feels slightly warmer just before it snows? .... now you know why.

We need energy for thunderstorms. Just to avoid confusion - heat, or temperature, is a measure of energy. It measures how much the molecules in a substance, in this case air, are vibrating (this is a porn movie!)

We can get that energy from water (as above) or from the sun. Technically, of course, the sun is the source for all energy (it's what turned the water into water vapour), but for this I will pretend it's different. The sun does not heat the air, it heats a surface - ie oceans and lakes and rivers to evaporate water into water vapour or land. That's why[i/] it's generally cooler out at sea - the energy from the sun is stored in water-vapour and not transferred as heat into the air.

On the land, by conduction (if you heat a burger and then put a bun around it then bun will warm by conduction), the sun heats the land, and the land heats the air. We measure that heat by temperature, which is the red line. We need temperature for thunderstorms. Most of the time, we need high temperatures for a good light-show, but this isn't always the case. As long as the temperature is higher lower, and lower higher (sorry, couldn't help myself) then, as we all know, hot stuff (OK, no more porn references) always moves to cold stuff. That's why thundersnow exists. In this case, air therefore rises since it's warmer low down, and cool higher up. The more the difference the more rapid the air will rise. Since we only have about 1/3rd of the day of the sun heating, we want that to be quite rapid. Real weather geeks (not me - I am a rank beginner) call this difference with height the lapse rate. High lapse rates mean more vigorous movement (lol). You can see the lapse rates on a skew-t by how steep, to the left, the dewpoint line and temperature line is.

Next you have all sorts of faint dotted lines. These are the laws of physics - the adiabats. Essentially these describe (in a perfect vacuum) how much heat should be released or gained so that the amount of energy is equal- ie how much temperature (or energy) should be put into or taken out of the the system at a given pressure so that amount of energy in the air is constant. We have wet stuff, and dry stuff on this diagram (top right) It's probably easier to describe a box with some marbles bouncing around inside. If you make the box smaller then the marbles bouncing around become more rapid (more energy) if you make the box bigger then the bounces will become less frequent (less energy) In order to keep the energy of a bit of air constant you will have to add/remove pressure (the box size).

The grey line is interesting for lovers of clouds. On this chart you really want to start at the lifted condesation level. This is the point above your head that the air will reach 100% relative humdity - ie it will condense out into water. You work this out by assuming that at a given ground temperature the air has no moisture (of course, not true) so you follow the dry adiabatic line and you connect it to a line from the dewpoint at the surface by following the mixing ratio. Too much physics for why this is the case. Handily this chart marks that point.

From the LCL we must assume that if the air is at 100% RH it must form clouds. And since it's readily and properly moist (here we go again!!) we then follow the wet adiabat until that line touches the temperature line. Here it's at some 350hPa (hPa=mb) that's some 8000m up! We are expecting big clouds.

If you coloured in the area between the grey line and the red line and assigned a number to it - that number is CAPE, or convective active potential energy. People, including me, get really excited by it because the difference between the grey line and the red line indicates how much energy by water changing state with height (I think - not sure). It is an indicator of the potential for thunderstorms.

If you took the temperature at the LCL level and subtracted that from the temperature (following the wet adiabat) at 500hPa then that will give you the LI, or lifted index. This is essentially the lapse rate from cloud bottom to about 5500m up. The lower the number the more rapid air will rise.

I hope most of this is right - got a hangover from hell this morning. Hope it helps.

Anyway, from skew-t you can see where the cloud base is (cloud-ground lightning strikes more common with a low cloud base) how big clouds are likely to get, how much energy is about, and how unstable the atmosphere is. There's a whole load more than you get from this but this post is too long already.

EDIT: any mistakes, please let me know!

Cheers,

Mark

Edited July 6, 2011 by Sparticle

[Stuart]

Last Updated: 19:44 Monday, 11th July 2011

Valid: 00:00 Tuesday, 12th July 2011 - 23:59 Tuesday, 12th July 2011

Areas Affected:

None

Synopsis:

A surface area of high pressure (1027mb) situated just to the north and north-west of the UK is forecast to dominate the weather across many areas of the UK during the day. Mid and upper level conditions are also anticyclonic, with 500mb temperatures for example up to -15C to -20C in association with warming from subsidence across the UK.

Discussion:

The predominantly fine and settled conditions are forecast to continue across many areas of the UK during the course of Tuesday. Latest data and forecast soundings signal that parts of Wales, W England and perhaps Ireland may well experience some slight instability, but only up to 10,000ft at very most. A few scattered light or moderate showers may develop, particularly in association with any localised convergence zones for example, but there is little or no significant evidence to suggest that thunderstorms will develop across the UK during Tuesday.

As an addition a highly convective area of low pressure is forecast to affect many areas of the near Continent during Tuesday. This area of low pressure has been signalled for numerous days, but the majority, if not all of the forecast models keep the main convective activity and thunderstorms threat over in the near continent. Some precipitation may well affect parts of the far south and south-east during Tuesday in association with this system, but again there is little or no evident to suggest convection and/or thunderstorms will be present. This feature will be monitored however in case the convective threat does end up being further north and west, which as a result may then affect Sussex and Kent for example.

http://ukasf.co.uk/storm-forecasts/37

[Jane Louise]

A little potential for tomorow, so therefore I have started a new thread for a slight storm chance!

http://forum.netweat...on-and-reports/

[Stuart]

Weekend dont look to bad for storms at the mo but still to far out to forecast

[ZONE 51]

Taking a look at wednesday(today)-here we are looking at and focusing on convective risk not any general rain areas.

-data GFS/NAE-18z

Western areas at risk of convective showers, but my attention is pulled towards SW England, lets take a look at the charts..

CAPE pockets around with some extra in the SW enough for some convective interest!

-CAPE afternoon

GFS breaks out some areas of precipitation over Ireland and Southwest England

-precipitation afternoon

NAE indicates the same areas

-ppn

the lifted index chart here shows some slightly unstable air over western areas and also the upper temperatures at 500hpa are very cold

-lifted index

-upper temps at 500hpa

next the vertical velocity chart at 700hpa, i have circled the areas of interest-with SW England the focus again

-vertical velocity 700hpa

looking at GFS wind flow for the afternoon-i have highlighted an areas of possible convergence here

-wind flow

and last another two GFS charts these are CAPE/LI and ppn

Watch - HEAVY SHOWERS IN WESTERN AREAS - SW ENGLAND THUNDERY DOWNPOURS MAYBE INTO SOUTH WALES

-

note - these are my thoughts on the convective risk and things might change by the morning..

Edited July 13, 2011 by ElectricSnowStorm

[Coast]

My eye was drawn this morning to a thunderstorm symbol on the BBC South East today forecast for Sunday 17th!

Not sure what particularly warrants that currently, but that animated symbol has been a rare beasty in recent times

UKASF today:

Forecaster: Matt

Last Updated: 19:21 Tuesday, 12th July 2011

Valid: 00:00 Wednesday, 13th July 2011 - 23:59 Wednesday, 13th July 2011

Areas Affected:

None

Synopsis:

A surface ridge of high pressure (1020mb to 1024mb) is forecast to persist across most areas, in conjunction with a ridge also at mid and upper levels, albeit a relatively weak feature. An upper trough is evident just to the east and south-east of the UK and this is in association with a thundery area of low pressure at surface levels that is predicted to clear away to the east of the UK, with little or no affect on the UK weather.

Discussion:

Another predominantly fine and settled day is expected for most areas of the UK beneath the ridge of high pressure. The various forecast models and forecast soundings signal little or no risk of any thunderstorms or significant convective activity. However, there remains a threat of some limited instability across parts of Wales, SW England and Ireland, but this would only result in relatively shallow instability and as a result isolated or scattered light or moderate showers are expected at most, particularly where any localised convergence zones develop.

Of interest, an analysis of the various forecast soundings from across the UK signal a noteworthy EML generally above 850mb to 700mb. A reasonably pronounced subsidence inversion is also forecast again near the 850mb levels with much drier air above this level. This could well signal a day where Cu clouds develop and then potentially spread out beneath the inversion layer bringing quite cloudy conditions during the afternoon period.

[Stuart]

I am keeping my eyes on the weekend

[Nick F]

Looks like there could a few sharp showers across Devon and Cornwall this afternoon with the odd rumble of thunder not to be ruled out and perhaps across south Wales too.

The weekend looks potentially interesting storm-wise, as an area of low pressure moves off the Atlantic and becomes slow-moving across the north then the east. GFS atm shows quite a strong jet rounding the base of this low on Saturday across southern UK, and where the left exit of this jet engages some instability - could be some potential for severe weather,

... though still too far off to firm up on specifics. But both Saturday and Sunday should see plenty of convective activity with an upper low over the UK, especially northern, central and eastern areas.

[DeepSnow]

Im going to Binbrook, Lincolnshire on Friday and coming back Saturday night, is there any chance of me seeing a storm up there?

It would be just my luck that as i'm over there for two days that I get a storm back home here!!

[Thundery wintry showers]

I think that over most of Scotland and north-eastern England (north of Yorkshire), going by past experience of these setups, the emphasis will end up on overcast skies and frontal rain. The strong north-easterly winds off the North Sea particularly won't help regarding the instability, while there will also be slow moving fronts draped around the low pressure system.

I think anywhere in eastern England to the south of the depression centre looks quite favoured for thundery activity though, with East Anglia probably most favoured of all. It looks like those areas will spend significant lengths of time away from the fronts, and with some resident instability and the possibility of some sunshine coming through, I would expect some fairly significant convective towers to go up.