Uk General Convective Discussion & Forecasts, 13th March 2013 onwards

[Supacell]

Friday still looking interesting from a convective pov on the GFS and ECM with an area of LP sat over the southern half of the UK bringing slack winds and some heavy showers and thunderstorms to the Midlands southwards, which would be slow moving in central parts under the centre of the low and light winds. Certainly no severe threat though with pulse type thundery showers the likely scenario from this set up. Could still be the first widespread convective day of the season though with some good CAPE and LI values and hail likely in the showers too.

As Weather09 says above, the UKMO has the low pressure to our NE by Friday which would ensure NW'ly winds and is nowhere near as good for thunderstorm potential as GFS/ECM, infact thunderstorms very unlikely from the UKMO.

Edited April 9, 2013 by Supacell

[dave48]

Thundery showers on Friday for a few of us by the look of it, midlands/east anglia best placed at the moment for lightning, according to gfs.

Edit: Could certainly be some fun for Wales into the South West too, although isolated.

Edited April 9, 2013 by dave48

[Coast]

Latest GFS says storms isolated and more to the SW:

[Supacell]

Unfortunately the 12z GFS is falling more into line with the UKMO idea of NW'ly winds for Friday. GFS still showing a storm risk in the SE quadrant (SE, EA, E Mids) but I am expecting that to move E over the next few runs to be honest. If the low moves to be to our east for Friday then the storm risk is all but gone (for Friday). We need the low to move through just a little slower to still have us under the S'ly winds at peak daytime heating to provide the storms.

If anything, the best possibilities now look to be later tomorrow across the SW, W Country and maybe up into S Wales and the SW Midlands but with fairly meagre CAPE it would more likely be heavy showers with the odd rumble here and there - although worth keeping an eye on for upgrades tomorrow as I have a feeling this could provide the odd surprise.

[East_England_Stormchaser91]

Hmmmm.. Surely if this came to fruition, we would be seeing summer arriving early but with a big bang to add bonus with it !

Edited April 10, 2013 by East_England_Stormchaser91

[Panayiotis]

Hi I love this forum during the seasons where thunderstorms become more active and I learn so much so thanks for sharing your info!

I have a question however; back in March(15th) I checked the V.Vert 700hPa for the day after (16th) on the 0.005GFS on meteociel and it showed my area under deeply negative values(red/black), so when the 16th came I found that when my area was forecast to have deeply negative values is when I had a couple of thunderstorms:

http://www.meteociel...3&mode=15&map=0

So does anyone know what it is used for and how you use it?

Example:

Edited April 10, 2013 by Panayiotis

[MP-R]

Best chances in the southwest and South Wales tomorrow it would appear. Lightning symbols also shown on the BBC forecast.

[AWD]

Best chances in the southwest and South Wales tomorrow it would appear. Lightning symbols also shown on the BBC forecast.

Yup. Small amount of Cape;

and a trough over southern central areas mid afternoon;

could provide some weak, mid-level storms to develop.

[Bobby]

Some potent showers developing in the SW, no lightning detected yet though, the trough is crossing us now bringing some heavy rain. Some strikes over Ireland though.

Edited April 11, 2013 by Bobby

[A.J]

first sferic of the day near the mainland.....in the Bristol Channel south of Cardiff to be more precise

[AWD]

Cluster of cells spreading NE up the Bristol Channel coast.

Reports in the South West thread of thunder from some of these.

Edited April 11, 2013 by AWD

[Convective]

Nice cell getting going now over Bristol. Plenty more on the way by the looks of things!

[Chris K]

Big flash of lightning and a quick hail shower here. Comes 10 mins after a few rumbles from some dark sky a few miles NE.

[MP-R]

There was a brief very intense cell that moved across about an hour ago here with heavy hail and overheard thunder and lightning. Two flashes and rumbles followed as it tracked northwards. Just general rain now.

[Ben Sainsbury]

Nice day of storms. 2 storms passed to my NW with one with what I thought was a lowering/rotation and some thunder and the other just general rain. Later another shower passed with an obvious gust front and the sky went very dark but only rain afterwards. Any storms tonight for Bristol?

[MP-R]

^^ I also noticed some rotation this afternoon, proceeded by a period of decent but not big hail. Then the actual thunderstorm followed after that.

Is nocturnal cooling going to do its job tonight or is that it until tomorrow?

[Convective]

Nice day of storms. 2 storms passed to my NW with one with what I thought was a lowering/rotation and some thunder and the other just general rain. Later another shower passed with an obvious gust front and the sky went very dark but only rain afterwards. Any storms tonight for Bristol?

 

 

^^ I also noticed some rotation this afternoon, proceeded by a period of decent but not big hail. Then the actual thunderstorm followed after that.

Is nocturnal cooling going to do its job tonight or is that it until tomorrow?

 

Correct me if I'm wrong someone but I believe it's generally too early in the season for nocturnal cooling to do a good job. You generally find that after a good hot summers day when the storms are full of energy then the sun setting cools the air causing the convection/ condensing within the clouds to increase leading to higher density clouds and thus more collisions and more lightning. So from my point of view, no.. that's it for the night apart from the odd rumble from a few sharp showers.

 

The concentration of CAPE and lifted index looks to be further east tomorrow so Bristol doesn't look like it will see as much as today.

[Darren Bown]

Is that a hook echo? Or just a random shower?

[East_England_Stormchaser91]

Looks like a big squall line coming into the SW as I speak, wonder if that will deliver any thunder tonight, forecasts suggest to me that it may well do for the SW and central S England

[Stuart]

Synopsis:

Upper trough over Ireland will continue to migrate slowly eastwards across Britain during Friday, albeit weakening/filling progressively. Ridging will build from the southwest during the day, limiting initially widespread convection to the far East by the end of the day.

 

Discussion:

... SW ENGLAND, W COUNTRY, S WALES ...

For these areas, main lightning chances during this forecast period will be associated with a well-marked shortwave trough which will cross the area during the early hours of Friday morning in a northeastward direction. Convection should be sufficiently deep (for example ELTs occasionally reaching -40C) for some sporadic lightning activity - unlikely to be widespread, however, especially due to the elevated nature of such convection (nocturnal surface inversion etc).
During Friday, while there may initially be some post-trough convection, strengthening ridging during the day should gradually cause convective showers to weaken and decay, leaving a largely dry second half of the afternoon and evening here.


... N ENGLAND, MIDLANDS, EAST ANGLIA, CS & SE ENGLAND ...

An initially well-marked shortwave trough moving northeastwards across the area during the morning will begin to weaken, and turn more showery. Better prospects of insolation, coupled with cooling aloft and steepened lapse rates, will generate 500-800 J/kg CAPE, enabling widespread showers and a few thunderstorms to form during the day.
Showers/storms will be slow moving due to the slack winds present, hence increasing the risk of prolonged heavy downpours with hail likely in many showers, potentially up to 1.5cm in diameter, which may cause some local disruption on roads etc. Showers/storms should decay rapidly after sunset as both diurnal heating and upper forcing subside.

 

http://www.ukasf.co.uk/storm-forecasts/233

[Jane Louise]

Posted Thurs 9pm

Moderate Risk of Thundery Hail Showers Primarily Central and SEastern Regions 12Z-19Z Fri 12th April 13

Weak low pressure system continues to move eastward. Upper long wave trough introduces steep upper lapse rates increasing lift. Surface troughing once again expected to bring convective showers to many parts with hail and thunder.

A small risk of some thundery activity during the early hrs of Fri morning for Cornwall. Though prime attention is focused on central to Thames Valley regions later in the period where a moderate mid level divergent jet stream moves further east and could well increase 0-6km shear values enough for the development of an isolated organised storm cell. Current indications based on WRF and GFS overlay of CAPE to upper shear suggest maybe Oxford through to North London from around 3pm. This is not to say the parameter is there for a supercell but moreover one or two organised stronger storms with moderate sized hail will be possible. The trigger for this later activity looks to be a combination of 500 mb PVA advection and surface convergence. 0-3km helicity values look rather weak and the full vertical forecast wind profiles look fairly straight lined.Bucks and Fairford forecast sounding show CAPE right up to 350mb with freezing level around the LCL. So in short;... tall cloud tops with deep cooling down to cloud base might suggest some larger hail reports possible.

http://www.ukweatherworld.co.uk/forum/index.php?/topic/97625-convective-outlook-fri-12th-april-13/

[Ben Sainsbury]

Why was their rotation with the Bristol storm?

[Convective]

Second forecast of the 2013 season. Also.. 36mph winds recorded from that system just crossing the southwest coast now.

 

[Supacell]

Depending on how things look tomorrow morning I may go out storm chasing for the first time this year. Looks like the the Midlands, Lincolnshire, East Yorkshire, East Anglia and the SE will be well placed tomorrow late morning onwards. If I was to pick somewhere from this afternoon/evenings runs I would probably say somewhere like Northamptonshire.

Worth noting that there would likely be hail in a lot of these showers/storms too.

EDIT: GFS 18z has it slightly further south, so more counties just to the north and east of London looking best. I shall wait for tomorrow mornings runs and post my thoughts then.

Edited April 11, 2013 by Supacell

[MP-R]

Correct me if I'm wrong someone but I believe it's generally too early in the season for nocturnal cooling to do a good job.

You may well be right there. Perhaps had their been sunshine from the word go then showers could have kept going longer into the night. We do have the whole spring and summer to go though.

Looks like some beefy showers again for the West Country tomorrow morning and early afternoon before clearing by mid afternoon with a largely bright and breezy end to the day ahead of the weekend's washout.