9th Sept 2011 - Storm/convective Forecast Discussion And Reports

[Coast]

Looks like there may be some action today with a level one from ESTOFEX for Ireland and a watch area over a broader area of the UK:

Storm Forecast

Valid: Fri 09 Sep 2011 06:00 to Sat 10 Sep 2011 06:00 UTC

Issued: Thu 08 Sep 2011 17:43

Forecaster: TUSCHY

A level 1 was issued for Ireland mainly for an isolated tornado/severe wind gust report and isolated large hail.

SYNOPSIS

Wave train marches eastwards and places W-Europe beneath strengthening ridge/WAA. Intense trough over E/NE Europe remains in place with ongoing CAA. No real focus is detected for any organized thunderstorm activity.

DISCUSSION

... N-France, W-Germany, Benelux, Ireland, Scotland and United Kingdom...

Robust upper trough/LL cyclone west of Ireland allows gradual moisture recovery as a well mixed maritime air mass from the Atlantic spreads eastwards. However, no signals can be detected for a more robust return flow from the W-Mediterranean with offshore pointing LL streamlines along the coast of S-France well into the night hours. An active warm front spreads northeastwards during the forecast but with latest MIMIC-TPW measurements indicating a TPW minimum over the Iberian Peninsula and further to the SW, concerns arise regarding the moisture content of the warm sector (fostered by yesterday's soundings from that area). This may allow the warm sector to become prone to strong diabatic heating, BL mixing and hence limited BL moisture compared to current model outputs. Finally, EML axis remains well to the west, so forecast CAPE magnitude of more than 800 J/kg MLCAPE (GFS) in some places remains questionable.

Isolated thunderstorm initiation will probably be bound to the warm front, with some elevated thunderstorm activity. Those elevated storms probably also affect parts of the UK and Benelux during the night hours and hence we expanded the 15-% area well to the north. With increasing speed/directional shear, a few updrafts may organize somewhat with an augmented hail risk. However, marginal MUCAPE precludes any level area for now.

There are hints that some surface based instability may evolve over UK around sunrise (the 10th) with strong directional/speed shear in the BL. If this trend indeed already starts in my forecast period, a level 1 may be needed due to an isolated tornado/hail risk. However, current indications are that the main SBCAPE plume sets-up right after 06 Z.

A level 1 was added for Ireland for the 12Z-18Z time frame, as influx of moist air, cool mid-levels, a coupled jet configuration and strong LL shear/modest DLS create a favorable environment for organized thunderstorms. Surface based storms are forecast with an augmented tornado, severe wind gust and isolated large hail risk. The risk diminishes after sunset, as SBCAPE vanishes (becomes more elevated).

Surface based initiation over central France and further to the south seems unlikely with current model data (no real mesoscale focus, pretty dry BL, increasing cap, weak EML). If initiation becomes more likely, a level 1 may be needed due to increasing DLS from the SW.

... NE-Europe ...

Placed beneath a cold-core low, daytime driven showers/thunderstorms are forecast. Weak shear and moderate instability allow pulsating storms with marginal hail and strong wind gusts. Activity eases after sunset.

UKASF have a different area covered:

Areas Affected: Wales, Midlands, E Ireland

Synopsis:

A warm front will surge northward into Scotland during the morning putting much of the Slight risk in a broad warm sector with above average temperatures and high surface dew-points for the time of year.

Discussion:

Warm, humid air combined with weak vorticity pulses (short-wave troughs) will move through the warm sector and may just be enough to produce a few isolated sharp showers during the afternoon. Temperatures will peak into the low to mid-20s C and allow for SBCAPE values of around 500-600j/kg by the afternoon. Shear values remain low, except over Ireland where 0-1km shear is in the 15kt range, however instablity looks weakest in this area.

Showers will likely be isolated in nature (given the lack of cold air aloft), however if a few can develop, they could develop enough to produce a few rumbles of thunder. This area will move into the E Mids later in the afternoon/early evening.

21st OWS have an area approaching Ireland later:

Their MetO version has it more like the UKASF:

Aviation weather more complicated but Katia is now showing:

This has me interested for later, especially with the very high dewpoint and humidity today:

Nice!

Lapse rates around London and the Home Counties:

and a possible tornado indication for that area:

It's not all about you just yet Katia!!! wait your turn while other weather features get a chance!!!

[Harry]

This is arguably without the question the most exciting (and serious) period of weather in 2011 so far IMO.

The worst of the snow was end of the 2010 in the main.

There have of course been floods...but the coming few days could be far more widespread and potentially deadly than what we've seen so far (not sure why I've been led to insert a smile but I have lol)

Edited September 9, 2011 by Harry

[Coast]

This is arguably without the question the most exciting (and serious) period of weather in 2011 so far IMO.

I think it may be and over such a short space of time!

[Sprites]

This is arguably without the question the most exciting (and serious) period of weather in 2011 so far IMO.

The worst of the snow was end of the 2010 in the main.

There have of course been floods...but the coming few days could be far more widespread and potentially deadly than what we've seen so far (not sure why I've been led to insert a smile but I have lol)

I would prefer a level 2 estofex for scattered severe thunderstorms lol

[Harry]

I would prefer a level 2 estofex for scattered severe thunderstorms lol

Me too!!!

[Purplepixii]

awww a lolcat - not seen one of those in ages!

My area has a 31% chance of storms at 13:00 and a 45% chance at 16:00. It's warm & muggy at the moment albeit overcast. Ground is very wet and sun did pop out but it's now gone dark again! Current temp 19.5oC with 91% humidity.

A storm would be nice pleeeeeeeeeeeeeeeeeeeease!

[Erbyskerby]

Just had three rumbles here (and a fourth as I type). Fairly distant. Am in bed poorly so can't investigate further - always the way isn't it?!

[Harry]

Estofex updated forecast, expanding Lvl 1 zone NE and SE

http://estofex.org/

[Coast]

Just had three rumbles here (and a fourth as I type). Fairly distant. Am in bed poorly so can't investigate further - always the way isn't it?!

Send it up the coast please!

Estofex updated forecast, expanding Lvl 1 zone NE and SE

http://estofex.org/

and a little more South as well please!

DISCUSSION

... Parts of UK and Scotland, 21 Z onwards...

Not much has changed regarding confidence in thunderstorm initiation over the area of interest. Models still diverge significantly regarding magnitude of CAPE (surface based or elevated) and potential initiation (even significant run-to-run inconsistency ). Nevertheless, GFS is the model, which persistently showed a build up of some SBCAPE after 21 Z over UK and Scotland. Forecast soundings from this area hint at a very moist lower troposphere with weak lapse rates below 800 hpa and gradually steepening ones atop that layer. Not much T/Td bias needed for surface based initiation and we now think that any residual boundary or mesoscale convergence zone may allow for adequate moisture pooling to assist in surface based initiation. Initiation may be fostered by a prefrontal convergence zone, which moves in from the west after midnight, but strength of this boundary remains vague at best.

Also initiation in the southern part of the level 1 may be more sporadic due to increasing WAA/capping.

The kinematic environment improves through the period with LL shear exceeding 15 m/s and SRH-1 approaching 200 m^2/s^2. Forecast hodographs also reveal strong loops, so any more persistent updraft may organize/rotate. The LCL remains at or below 600 m, so an isolated tornado event will be possible and hence we decided to expand the level area to the northeast/southeast.

A more certain risk will be the severe wind gust risk, as 850 hPa winds increase to 20 m/s and more. Although LL lapse rates remain weak, a few severe wind gusts are well possible (which also justifies a larger level 1 area). Marginal hail accompanies stronger storms and if expected MUCAPE values verify, a few large hail reports are in the realm of possibility.

Only cosmetic changes were performed for the rest of the outlook. Not much thunderstorm activity expected in those areas. Also, the level 1 for Ireland remains unchanged.

[Erbyskerby]

Hailing now fairly heavily - thunder is increasing. I thought it was out to sea but it appears to be behind me ie coming up from the other coast. Power has just gone off for a few seconds - house alarms are going off up the road. Well I wasn't expecting that!

[Purplepixii]

Hailing now fairly heavily - thunder is increasing. I thought it was out to sea but it appears to be behind me ie coming up from the other coast. Power has just gone off for a few seconds - house alarms are going off up the road. Well I wasn't expecting that!

Meteox showing storms your way indeed http://www.meteox.co.uk/h.aspx?r=&jaar=-3&soort=satradar

Temp here now up to 22^oC with wind making it feel like 24.2^oC and climbing. Sun is out but sky looking very dark grey.

[Erbyskerby]

Very interesting - lightning map was bare and then boom quite a few strikes in a short period. Think it has passed me by now - hopefully someone else can enjoy it further up the line. Still quite muggy though - might explain my migraine.

[liambailey09]

I think things could get very interesting later on but is that cap still supposed to be in place over England today?

[gottolovethisweather]

Very interesting - lightning map was bare and then boom quite a few strikes in a short period. Think it has passed me by now - hopefully someone else can enjoy it further up the line. Still quite muggy though - might explain my migraine.

Hi Erbyskerby,

Confirmed many rumbles form the holiday camp where my mother and father are. Was an impressive storm judging by the Netweather radar and it was at it's most intense between Truro and south of Padstow as it tracked NNE'wards across the land. The cell originated due south of the Lizard so you are quite right with your assumption that it came from the orther coast.

Cheers

gottolovethisweather

[Andy Bown]

Yes that storm developed very rapidly over Cornwall, I wonder if more will form in response to the heat of the day in this very tropical air.

[todd1970]

Anyone know whats happend to the met-office invent predicted rainfall radar..they seem to have moved/binned it ? from their site.

[Coast]

Yes that storm developed very rapidly over Cornwall, I wonder if more will form in response to the heat of the day in this very tropical air.

Pretty oppressive in our part of the World this afternoon:

I think things could get very interesting later on but is that cap still supposed to be in place over England today?

Time for a NW Extra subscription!

[Erbyskerby]

Just a quick question re the netweather extra radar - I use my ipod to view this forum. Would i be able to see the radar on it or does it need flash?

[Weather Wizard]

Sorry but do the darker colours (i.e. red) mean that there is a hugher CAP and so less convection is likely to occur or is it the other way round

Thanks

Also Torro have a watch for all of England out and UKASF have a promissing looking map for tommorow for most of England.

Edited September 9, 2011 by Weather wizard

[Coast]

Just a quick question re the netweather extra radar - I use my ipod to view this forum. Would i be able to see the radar on it or does it need flash?

On my Windows PC it doesn't want to run flash - I think it's a stand alone and may well work on other OS's

[liambailey09]

Pretty oppressive in our part of the World this afternoon:

Time for a NW Extra subscription!

Thanks coast, might have to put NW extra subscription on my xmas list!

[Coast]

Sorry but do the darker colours (i.e. red) mean that there is a hugher CAP and so less convection is likely to occur or is it the other way round

Higher numbers of CIN, more inhibition (cap)

CIN is the amount of energy required to overcome the negatively buoyant energy the environment exerts on an air parcel. In most cases, when CIN exists, it covers a layer from the ground to the level of free convection (LFC). The negatively buoyant energy exerted on an air parcel is a result of the air parcel being cooler (denser) than the air which surrounds it, which causes the air parcel to accelerate downward. The layer of air dominated by CIN is warmer and more stable than the layers above or below it.

The situation in which convective inhibition is measured is when layers of warmer air are above a particular region of air. The effect of having warm air above a cooler air parcel is to prevent the cooler air parcel from rising into the atmosphere. This creates a stable region of air. Convective inhibition indicates the amount of energy that will be required to force the cooler packet of air to rise. This energy comes from fronts, heating, moistening, or mesoscale convergence boundaries such as outflow and sea breeze boundaries, ororographic lift.

Typically, an area with a high convection inihibition number is considered stable and has very little likelihood of developing a thunderstorm. Conceptually, it is the opposite of CAPE.

CIN hinders updrafts necessary to produce convective weather, such as thunderstorms. Although, when large amounts of CIN are reduced by heating and moistening during a convective storm, the storm will be more severe than in the case when no CIN was present.

CIN is strengthened by low altitude dry air advection and surface air cooling. Surface cooling causes a small capping inversion to form aloft allowing the air to become stable. Incoming weather fronts and short waves influence the strengthening or weaking of CIN.

http://en.wikipedia....tive_inhibition

[Weather Wizard]

Thanks

Edited September 9, 2011 by Weather wizard

[MP-R]

Fingers crossed for here. This is a little similar to August 21st last year which had a thunderstorm from an almost identical setup following a partly cloudy humid day.

[loopydreamer]

on raintoday a little dark dot of precip has just appeared in the channel south west of portsmouth! if that develops further that is on course to hit around brighton