Uk Convective General Discussion & Forecasts, 16th August 2012>

[AWD]

Lets all head to Gloucester on Saturday! Lol!

[Nick F]

With rather warm/moist air moving north tomorrow, if strong enough forcing along frontal boundary stretching from SE Ireland to N England by noon and/or insolation can occur, then chance of some lively storm possible for Eire, N Wales and N England. Especially Ireland, where fax for 12z Fri shows frontal wave approaching - which will back winds and therefore increase low-level shear. More about this in tomorrow's forecast:

Valid: 17/08/2012 00:00 - 18/08/2012 00:00

Headline: ... THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FORECAST...

Synopsis

Slow-moving low pressure system W of Ireland and ridge over central/southern Europe will drive a SW'erly flow aloft and increasingly warm/moist southerly flow at the surface. A secondary low SW of the UK Thurs night will drift north across Ireland to be west of Scotland 12z Fri, with trailing cold front lying NE England to SE Ireland at 12z.

... REP OF IRELAND, N IRELAND, N WALES, N ENGLAND and S SCOTLAND ...

Latest surface analysis shows shallow secondary low will track NNE across Ireland overnight, with warm sector (characterised by dew points of 17-18C) lifting north across England and Wales and also across Eire as a wave develops further south along trailing cold front turning into a warm front lifting north over Ireland. Some 300-700 j/kg CAPE is indicated by GFS across Eire and N England Fri afternoon close to this frontal boundary, however, the development of storms maybe curtailed by extensive cloud cover and general rainfall from large scale ascent of warm sector and weaker lapse rates across N England.

However, should cloud breaks allow warmth to increase and/or forced ascent along the frontal boundary be sufficient - then there appears to be a risk of some storms developing. 55-65 knt 500mb SWerly jet and DL shear of 30-40 knts across N England/N Wales and 40-50 knts across Eire will overlap the potential instability indicated which will encourage any storms to organise into multicell structures, or perhaps bowing line segments all capable of producing isolated damaging winds and torrential rainfall leading to flash flooding.

In addition, frontal wave expected to develop near S Ireland on sfc analysis, will strengthen low-level shear across S/central Ireland early afternoon, so should any storms form here, they could develop mesocyclones, which helped by low LCLs and increasing dry air at 500mb, bring an increased chance of a tornado given sufficient low-level instability. Therefore a conditional SLIGHT risk issued for Ireland. Severe potential seems lower for N England due to weaker lapse rates/saturated profile indicated here and the likelihood of extensive cloud cover

[Allseasons-Si]

and this from Matt Hugo:

Doesn't look like much forcing so 2m T/Td likely to be main trigger, but these kind of fcst soundings are interesting - http://t.co/XdQmVI1S

Nice loaded gun sounding for Saturday for example at Fairford as an example http://t.co/1YFEAWIB

i cannot get my head around these skew t thingy bobs can someone explain or is there a tuition forum/part of forum to read about these graphs lol.

Edited August 16, 2012 by allseasons-si

[Nick F]

i cannot get my head around these skew t thingy bobs can someone explain or is there a tuition forum/part of forum to read about these graphs lol.

John's excellent tutorial should hopefully point you in the right direction:

http://forum.netweat...kew-t-diagrams/

Hot and humid airmass across England and Wales over the weekend (dew points 18-19C) and it looks like forcing along cold front could be sufficient for storms to develop along it on Sat and Sunday, though uncertainties over how quickly it will move east - GFS still has it Wales to NE England Sunday evening ... though trough drifting N/NE Sat night could bring some storms to the far SE corner and local wind convergence combined with weak shortwaves running through could trigger isolated storms across SE England and E Anglia Sunday afternoon. Storms could be severe given strong deep-layer shear indicated.

[Lynxus]

Lets all head to Gloucester on Saturday! Lol!

LOL That chart must be broken.

On another note. Breezy and sorta sunny at the moment.

[NL]

ESTOFEX have a level 1 risk for Ireland and also Northern England. Looks like their forecast is in line with Nicks, limiting factor again will be extensive cloud cover but if any breaks should occur then expect some pretty severe storms.

http://www.estofex.org/

[Coast]

ESTOFEX have two level 1 forecasts over the UK and Ireland to day:

Storm Forecast

Valid: Fri 17 Aug 2012 06:00 to Sat 18 Aug 2012 06:00 UTC

Issued: Thu 16 Aug 2012 18:22

Forecaster: TUSCHY

A level 1 was issued for parts of the UK mainly for a strong to severe wind gust and tornado event.

A level 1 was issued for Ireland mainly for an isolated large hail event.

SYNOPSIS and DISCUSSION

Downstream of an intense cyclonic vortex west of Ireland, WAA spreads to the northeast with a substantial increase of the 500 geopotential heights, reinforcing another omega-like pattern over Europe. A quasi-stationary longwave trough over E-Europe builds more to the south, reaching the E-Mediterranean with subtle height falls during the end of the forecast. Cool and unsettled conditions prevail over N-Europe.

Many places over N/E Europe see some thunderstorm activity, mainly daytime driven and sub-severe. Marginal hail and strong wind gusts accompany those storms.

A belt of strong SW-erlies (e.g. 30 m/s at 500 hPa) still affects Ireland/UK and Scotland. A first surface wave/trough and attendant pronounced warm front cross those areas from SW to NE during the day with pretty robust moisture advection forecast. Mid-level lapse rates remain marginal next to expected thick cloud cover, limiting any diabatic heating until noon. Thereafter, some clearing in the posfrontal warm sector is expected and forecast soundings indicate not much modification is needed for some MLCAPE build-up. HIRLAM and GFS both show another surface wave, placed just S of Ireland during the afternoon hours, again inserting some more backing in the BL flow. This assists in the infiltration of a better mixed LL air mass from N-France towards S-UK, where a decrease of dewpoints and an increase of the LCL can be seen during the afternoon. LCLs however remain well below 1000 m just along and south of the warm front over N-C UK. Both, the warm sector and the frontal boundary itself reveal great shear (both directional and speed shear), so if adequate clearing along/south of the warm front occurs, an isolated thunderstorm event is possible with a risk of producing severe wind gusts and an isolated tornado. Coverage probably remains limited with ongoing weak NVA over the area of interest. We also monitor the progress of a dry slot feature, which moves towards Ireland during the afternoon, so some better CAPE and enhanced thunderstorm activity is possible over Irland. 20 m/s DLS may allow a few storms to become better organized with isolated large hail and strong wind gusts. Therefore, a broad lightning area and a second level 1 were added. The thunderstorm and severe risk diminishes after sunset.

Seems like they and the NW forecast are the only ones so far this morning. 21st OWS have:

Mid level aviation chart for today:

Small areas of CAPE:

Even more rain......

Low level lapse rates:

But here's that tornado threat:

Which ties in nicely with the ESTOFEX forecast with thses blue ringed areas:

and some gusty winds in the SE:

But don't expect too much in the way of thunder today:

[Harry]

John's excellent tutorial should hopefully point you in the right direction:

http://forum.netweat...kew-t-diagrams/

Hot and humid airmass across England and Wales over the weekend (dew points 18-19C) and it looks like forcing along cold front could be sufficient for storms to develop along it on Sat and Sunday, though uncertainties over how quickly it will move east - GFS still has it Wales to NE England Sunday evening ... though trough drifting N/NE Sat night could bring some storms to the far SE corner and local wind convergence combined with weak shortwaves running through could trigger isolated storms across SE England and E Anglia Sunday afternoon. Storms could be severe given strong deep-layer shear indicated.

Mmmm hmmmm!!! That would be sweet justice, but sadly we only seem to get storms from cool Westerlies or North Westerlies these days...the day of the hot and humid thundery breakdown has quickly become legend for our part of the world.

Am excited though about a heatwave weekend...in the absence of thunder, I will happily take Med style bliss

[Coast]

the day of the hot and humid thundery breakdown has quickly become legend for our part of the world.

Amen to that brother! (as has French imports!) I'm still not sure about Sunday afternoon, or the morning.......

It's so damn close that I hardly want to get my hopes up!

Can you imagine the 'humdinger' this could bring?

It doesn't need to move very far North to become 'our' storm!!

HiRLAM is almost in range and shows something similar:

Here's the only downer that could kill it, the CIN is in the wrong place again!

Not sure if I should get excited or keep my powder dry until it happens?

[legritter]

Just waiting for radar to liven up a bit across Southwest and West country. How far east any action will come is hard to tell at present [9.15 am ] any thoughts gang?

It does look like an awfull amount of rain for parts of the Southwest, Wales also Ireland and Northwest Britain, could be serious local issues with flooding in areas that keep the rain going. As for the potential Sunday, BBC weather did mention front pepping up but would not commit to any specific locations, so it's a waiting game yet again but potential there. I think we all need our fix of a good old fashioned train of storms in a broad band very very slowly heading North from France, not breaking up over the Channel and arriving during night time.

Any thoughts please? legritter

[Coast]

Looking at the SkewT sounding for my local area at the peak time, we still need that CIN to decline with some nice surface heating and a bit of moisture, the CAPE is otherwise quite good.

Typical only.

Fig. 1.1. Skew-T log p thermodynamic diagram, on which temperature and dew point temperature data from a radiosonde ascent are plotted. The diagram shows isobars, isotherms, lines of equal potential temperature theta (isentropes or dry adiabats), lines of equal pseudo-equivalent potential temperature theta-ep (pseudo-isentropes or moist adiabats) and lines of equal mixing ratio r (isohumes). An ascent curve T(p) of a parcel from the surface having an initial temperature of 26°C and a dew point temperature of 21°C has been constructed.

http://www.estofex.o...uide/1_2_2.html

Maybe we will get enough of that......

Above (15.00hrs Sunday), The 'Solar Radiation' actually reaching the surface divided by the amount of solar radiation which would reach the surface in a dry atmosphere (i.e. in the absence of clouds and water vapor), expressed as a percentage. This parameter indicates the degree of cloudiness, i.e. where clouds limit the sunlight reaching the surface.

Above (15.00hrs Sunday), heat transferred into the atmosphere due to solar heating of the ground, creating thermals. This parameter is an important determinant of thermal strength (as is the BL depth).

But it starts to disappear after that:

GFS Soaring index may give some hope, but it's one to watch develop or retreat back to France!!!

[Paul Sherman]

Dare I say it.......................but it looks like possible Night time Lightning Opportunities in the SE On Sunday evening from Elevated Thunderstorms.

One to watch, temps possibly still in the High 20's by Sunset as well seeing as 31-33c could be acheived on Sunday.

Cameras ready guys!

[Coast]

Dare I say it.......................but it looks like possible Night time Lightning Opportunities in the SE On Sunday evening from Elevated Thunderstorms.

That's buggered it right up then!!!! It's really quite close isn't it?

[Snow free zone]

To add to the last few very informative posts - thank you.

Rain stopped here and there are breaks appearing in the cloud. This does seem to be a change to the meto forecast, which had us under heavy rain until this evening.

Edited August 17, 2012 by Snow free zone

[Nick F]

Some slight adjustments to today's forecast - highlighted below:

Valid: 17/08/2012 00:00 - 18/08/2012 00:00

Headline: ... THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FORECAST...

Synopsis

Slow-moving low pressure system W of Ireland and ridge over central/southern Europe will drive a SW'erly flow aloft and increasingly warm/moist southerly flow at the surface. A secondary low SW of the UK Thurs night will drift north across Ireland to be west of Scotland 12z Fri, with trailing cold front lying NE England to SE Ireland at 12z.

*UPDATE 10Z FRI 17TH AUGUST*

Some adjustments to the forecast and forecast map, frontal boundary not advancing so far N and NE, so inclusion of more central England and S Wales for risk of storms ... warmth from insolation further south could be ingested north into storms across these areas and further north this afternoon with a threat of excessive rainfall into parts of N Midlands, N Wales and N England - so included SLIGHT risk here. Slight risk still applies to Ireland for excessive rainfall, though risk box shrunk to take in account less northerly advance of wave and less risk of tornadoes.

... PREVIOUS DISCUSSION ...

... REP OF IRELAND, N IRELAND, N WALES, N ENGLAND and S SCOTLAND ...

Latest surface analysis shows shallow secondary low will track NNE across Ireland overnight, with warm sector (characterised by dew points of 17-18C) lifting north across England and Wales and also across Eire as a wave develops further south along trailing cold front turning into a warm front lifting north over Ireland. Some 300-700 j/kg CAPE is indicated by GFS across Eire and N England Fri afternoon close to this frontal boundary, however, the development of storms maybe curtailed by extensive cloud cover and general rainfall from large scale ascent of warm sector and weaker lapse rates across N England.

However, should cloud breaks allow warmth to increase and/or forced ascent along the frontal boundary be sufficient - then there appears to be a risk of some storms developing. 55-65 knt 500mb SWerly jet and DL shear of 30-40 knts across N England/N Wales and 40-50 knts across Eire will overlap the potential instability indicated which will encourage any storms to organise into multicell structures, or perhaps bowing line segments all capable of producing isolated damaging winds and torrential rainfall leading to flash flooding.

In addition, frontal wave expected to develop near S Ireland on sfc analysis, will strengthen low-level shear across S/central Ireland early afternoon, so should any storms form here, they could develop mesocyclones, which helped by low LCLs and increasing dry air at 500mb, bring an increased chance of a tornado given sufficient low-level instability. Therefore a conditional SLIGHT risk issued for Ireland. Severe potential seems lower for N England due to weaker lapse rates/saturated profile indicated here and the likelihood of extensive cloud cover.

Issued by: Nick F - Forecaster for Netweather

[Lynxus]

Damn thats some heavy looking rain down the SW.

Biblical ?

No sferics yet though

[Coast]

Latest Meto for Sunday.....

[AWD]

Latest Meto for Sunday.....

That shortwave over the far South East/East Anglia could provide some interest!

Edited August 17, 2012 by Active Weather Dude

[Darren Bown]

That rain in the SW although not thundery looks torrential!

[Coast]

That shortwave over tha far South East/East Anglia could provide some interest!

Smash that into a plume and?

[Carl46Wrexham]

BBC Weather 11.56 (Philip Avery) warnings upgraded to amber for Wales, North West England,Yorkshire and Humber..3 inches of rain possible in places.

Edited August 17, 2012 by Carl46Wrexham

[johnholmes]

If one looks at the latest movement in the radar, and its intensity, then one can see why Met are updating their warnings. The winds at cloud height, 10-18000ft are approx SW 30-40+ knots. They will probably be updating into central and northern England and more eastern parts of the north Midlands along with those already updated in my view.

[legritter]

Thanks Coast for posting Sunday fax, very interesting situation could develop for some lucky posters. Back to now that clump moving north looks very potent for North Devon parts Somerset and Wales. I wouldn’t be surprised if one or two locations get 20 mm an hour from that.

Cheers

[Carl46Wrexham]

Chief Forecaster at the Met Office states there is scope for 25mm rainfall in an hour locally with some places looking at 50-75mm within the period.

Not good at all.

[Coast]

Are we prepared for a really muggy afternoon Sunday if nothing else? (Dewpoints)