Convective Weather UK

[BrickFielder]

Forecasts suggest some moist warm air being advected northwards ahead of the cold front which shows up on the theta charts and pwat charts.

My feeling is that although there may be some embedded thunderstorms in the cold front there is a suggestion of thunderstorms slightly ahead of the cold front.

Some problems which shows up are the forecast dewpoints which may not be that deep, mid level cloud cover and convective inhibition. The question is will temperatures reach the required levels with the cloud cover. My feeling is that over the cities it just might although the forecast 26 C might be a stretch.

Forecast SkewT's show a marked warm level at 500hPa above which is marginal instability and hence cloud cover. There is not much evidence of a dry air anywhere, but winds are nicely backed in the lowest levels.

I am not convinced by the low topped super cells argument by Estofex although there may be some similarities with the birmingham tornado scenario. There is a risk of tornado given the low level shear, but unidirectional winds above and slightly higher cloud bases reduce the risk. Given the UK can produce some unusual conditions with storm splitting and the like a severe storm cannot be ruled out, but I just get the feeling temperatures might be just on the low side. Still its worth watching even if no storms develop at all which has to remain a possibility.

[Guest]

Yes, agree with that BrickFielder, instability looks to be lacking with a saturated profile right through to the upper levels, but rather moist boundary-layer and increased daytime heating would, I think, generate modest instability ahead of the front. Latest GFS does model marginal MLCAPE across parts of the country, wouldn't be surprised if this turned out to be somewhat higher in reality. It's also keeping the heaviest of the rainfall (which is also progged to be convective) towards the west over Wales and north and east over northern Midlands and northern England. Am waiting for the latest NAE output to come out.

If I remember rightly wasn't it the NAE which has consistently pushed the rain much further east than other precip models?

Be interesting to see what the latest output shows.

[Nick F]

I am not convinced by the low topped super cells argument by Estofex although there may be some similarities with the birmingham tornado scenario. There is a risk of tornado given the low level shear, but unidirectional winds above and slightly higher cloud bases reduce the risk. Given the UK can produce some unusual conditions with storm splitting and the like a severe storm cannot be ruled out, but I just get the feeling temperatures might be just on the low side. Still its worth watching even if no storms develop at all which has to remain a possibility.

Yes, not convinced either of supercells. There is 30-40 knots of DL shear modeled, but rather overcast skies with lack of insolation along with lack of drier air aloft looks like limiting instability, certainly GFS on the 06z is still very reluctant to build any.

I don’t see too much similarity to the Birmingham tornado day in that the July 2005 event saw a deepening low move up from the SW which created strong sheer on its approach across the Midlands. Today we have a waving frontal boundary which may see increasingly backed winds across central and NE England. But, for tornado potential you really need some dry mid-level air above a warm moist boundary layer (profiles look saturated quite a way up) – and this ingredient along with good insolation looks to be absent today. But, still a lot of uncertainty to what may happen, and if any storms do form just ahead of the cold front across W Midlands up to N Yorks, then with low LCL heights near the cold front wouldn’t rule out a tornado given increasing shear as the wave moves N/NE along the front.

All-in-all, this is rather tricky to forecast with any certainty, with a lot of complex interactions going on that GFS and NMM may not be picking up, so will be case of watching it evolve on the radar to get any idea what may happen!

[johnholmes]

All-in-all, this is rather tricky to forecast with any certainty, with a lot of complex interactions going on that GFS and NMM may not be picking up, so will be case of watching it evolve on the radar to get any idea what may happen!

hit the nail on the head there Nick

Watch the radar, plot the 850, 700 and 500mb winds on it, check back this afternoon for what the actual skew-t's show just ahead of where the waving cold front is.

where is it, best guide to that will be the current 06z Met O Fax chart then their 12z, or if you are okay with actual charts, follow these links

http://www.weatherweb.net/aviations/3huka3.gif

or

http://meteocentre.com/analyse/map.php?hour=0〈=en&map=UK

that is the best advice it seems to me for anyone on here.

[Phil UK]

I think, Nick F, a highly unlikely chance of a repeat of the July 2005 Birmingham tornado IMO. Heavy showers, embedded thunderstorm as the front pushes back west, pulling in warm and humid air.

Although there is some convection kicking off from where I live, I think anything goes from now till dusk, and maybe beyond.

Phil.

[Nick F]

Have updated the N-W storm forecast to include a SLIGHT risk area:

With the following addition:

16Z UPDATE TO FORECAST ISSUED YESTERDAY EVENING*

12z Nottingham and Camborne ascents indicated that airmass across central and northern England would likely become unstable with surface heating and with lift. Recent radar returns show some thunderstorms have developed towards NE England and also further SW across Cheshire in the last hour, these storms have formed near a wind convergence zone ahead of waving cold front to the west, winds backing SE near this zone will increase Storm-relative helicity with any storms moving NE, and hence have issued a SLIGHT risk for tornadoes, large hail and strong wind gusts possible with any storms in the area highlighted on updated map. Otherwise, previous discussion applies.

Otherwise as previous forecast:

http://www.netweather.tv/index.cgi?action=convective;sess=

[The watcher]

I know its a little soon, but tommorow for me is looking quite good. Good Cape and LI, Cold air aloft, and a convergence line directly overhead (subject to any changes though). I think tommorow is definitely my best chance of the year so far for anything with real "oomph" to it. Anyone know what shear is looking to do in my area tommorow? I know DP's are forecasted a bit lowish 12-14c, humidity around 85-90%......

[PersianPaladin]

Heavier rain moving in now, and I'm concerned there could be some local flooding if it keeps up.

[dogs32]

Yes, risk of heavy showers and thunderstorms tomorrow afternoon and into evening for most of Ireland, SW England, Wales, Midlands and Nern England. Approach of upper trough from the WNW and associated upper cold pool coupled with sufficient low level moisture and daytime heating, will generate several 100j/kg of SBCAPE across these regions. Low level convergence across central-eastern Ireland will likely provide focus for thunderstorm development, while over southern UK upper trough disturbance from a SW will aid convective development, mainly towards the west. Vertical wind shear looks weak at best over areas of best instability, so doesn't look like thunderstorms would become organised. However convergence zone over central-eastern Ireland could contribute to spinning updrafts in any stronger cells that form.

Upper trough and associated upper cold pool looks to then be situated over England during Thursday, so a risk of slow-moving heavy showers and thunderstorms exists for this period as well across a good part of England and Wales.

W09...are you saying there is more chance tomorrow...or the same as today's chance..

[dogs32]

cheers for you reply W09

[Nick F]

Valid: 21/07/2010 06:00 - 22/07/2010 06:00

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

Synopsis

Broad upper trough to the west of the UK disrupts to form a cut-off upper low across Ireland early Wednesday, this upper low will slowly move E across western UK during the forecast period. At 00z Wednesday, waving cold front expected to lie Newcastle-Leeds-Bournemouth, this frontal wave will clear east coast by 09z, meanwhile shallow low/trough will move south across western areas with occlusion following south in Nerly flow across Scotland, N Ireland and N England in the evening.

... SW ENGLAND, WALES, N and W MIDLANDS, N ENGLAND, S SCOTLAND and IRELAND ...

Overnight storms will clear N England in the early hours of Wednesday morning, as cold front moves east out into the N Sea.

Post frontal airmass will become unstable in the north and west of the UK through Wednesday, as upper low/cold pool across Ireland and western UK will create steep lapse rates as surface warms with insolation during the day. With a moist boundary still present in the west tomorrow, GFS projects several 100 j/kg SBCAPE across Ireland, Wales, Midlands and N England in the afternoon. Therefore expect scattered heavy showers and thunderstorms to develop across these areas during daylight hours, aided by lift of approaching upper trough. Dry mid and upper level air above warm moist low-level airmass will help enhance potential instability and thunderstorm development across E Wales, the Midlands and N England ... and with the same areas seeing 30-40 knots of deep layer shear for a time in the afternoon, organisation of storms into multicell/line segments capable of producing hail and marginally severe strong wind gusts seems possible. Local wind convergence, light sfc winds and fairly low LCL heights (below 1000m) may aid the development of one or two weak tornadoes and funnels with any stronger updrafts. Storms should fade after dark.

http://www.netweather.tv/index.cgi?action=convective;sess=

[Cloud 10]

Some good potential showing up on the gfs 18z for central areas on thursday which will no doubt be watched closely!

[Stuart]

And for today from UKASF

Storm Forecast Issued: 2010-07-20 20:49:00

Valid: 2010-07-21 00:00:00 - 2010-07-21 23:59:00

Regions Affected

Wales, SE Northern Ireland, N + W Midlands and parts of NW England (Wales, Northern Ireland, S Scotland and majority of England, excluding SE/E, are included in the WATCH)

Synopsis

Complex area of Low pressure "Quendeline" (FU Berlin) situates itself across the United Kingdom, dominating the weather on Wednesday.

Overnight heavy rain and thunderstorms associated with the cold front continue to drift across the North Sea during daylight hours towards Scandinavia.

In the Returning Polar Maritime airmass, numerous showers and thunderstorms are expected to develop as instability increases, aided by daytime heating, these mainly across central and western Britain and most of Ireland. Showers/storms may perhaps become locally severe in nature, with gusty winds, moderate-sized hail and a chance of a weak tornado, particularly over Wales and western England.

The showers/storms are expected to decrease in coverage during the evening hours, becoming mainly confined to the coasts overnight where the thunder/lightning risk continues.

http://www.ukasf.co.uk/module-Storm/mode-forecast/id-315/

[Coast]

ESTOFEX also showing their forecast for today, which includes that area over western and central Britain.

Storm Forecast

Valid: Wed 21 Jul 2010 06:00 to Thu 22 Jul 2010 06:00 UTC

Issued: Tue 20 Jul 2010 23:12

Forecaster: PUCIK

A level 1 was issued for Central France to Northwestern Germany mainly for severe wind gusts.

SYNOPSIS

At midlevels, a large trough will reside over the Eastern Atlantic. On the forward flank of this feature, strong flow, reaching 20-25 m/s at 500 hPa will be observed. To the east, mostly insignificant pool of higher geopotentials will be present.

At the surface, a shallow occluded surface low will be centered over the British Isles by Wednesday morning, with its center shifting towards Scandinavia. A cold front will progress across France, Western Germany and Northern Spain during the forecast period. Ahead of it, a modified warm airmass will lie over most of Europe. With abundant daytime heating, airmass will become conditionally unstable over much of Eastern, Central and parts of Southern Europe.

DISCUSSION

... Southern France to Western Germany ....

A narrow belt of mostly weak instability is predicted by models ahead of the surface cold front, with ECMWF showing only marginal degree of destabilisation and GFS putting values of MLCAPE mostly below 1000 J/kg. Overlap with strong southwesterly flow will be likely and wind shear in the 0-3 km layer is expected to reach 15-20 m/s, with the highest values generally on the western fringe of the "instability belt". Flow will be mostly parallel to the surface front and only weak storm relative winds will exist at the anvil-level of the storms.

Another detrimental factor will be lack of synoptic scale forcing, except for the front itself. Moreover, a scenario with the precipitation being carried ahead of the progressing linear storm is possible. There is still a chance, however, that in 15-20 m/s flow at 700 hPa, quickly traveling bowing segment will form with a chance of producing severe wind gusts. Over Southern France and Southwestern Germany, marginally severe hail can not be completely ruled out, as higher degree of instability will occur over these regions. A low end Level 1 is introduced for these regions.

Other info to sift through for your own thoughts on today:

Latest:

[Jane Louise]

Thanks Coast.

I think it's going to be more North Wales and Cheshire today!

Definitely looking good for widespread thunderstorms tomorrow.

[ZONE 51]

GFS 06Z CHARTS for thursday(tomorow) - il leave it to the experts to explain whats developing on thursday

Edited July 21, 2010 by nimbilus

[Nick F]

Valid: 22/07/2010 06:00 - 23/07/2010 06:00

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

Synopsis

Upper low over western UK/Irish Sea early Thursday will slowly edge SE over southern England towards Nern France. At the surface, occlusion lying from N Ireland to S Scotland will move slowly south across N England and weaken by 12z, to the S of frontal zone - an unstable slack cyclonic flow with low pressure centred over S England at 12z before moving ESE to be centred over Nern Belgium at 00z Friday.

... ENGLAND and WALES (except far N England) ...

Upper low/cold pool drifting slowly SE across across Wales, southern and central England will create steep lapse rates across these areas during Thursday. A well-defined surface wind convergence zone lying SW-NE from SW Wales to Yorks in the morning will move SE to lie from SW England to E Anglia by evening, this convergence zone will pool moisture and with surface heating will likely develop instability - GFS gives CAPE of several 100 j/kg towards this zone. Therefore, heavy showers or thunderstorms are likely to develop in this zone and also scattered showers/storms may develop to the S ahead of the zone during the morning. Showers or storms are likely to be slow moving with light upper winds/weak shear, and therefore high rainfall totals in a short space of time may lead to localised flooding in places; also fairly steep lapse rates and decent CAPE may see moderate-size hail, along with gusty winds in downdrafts and frequent cg lightning. A weak tornado or two and funnels are possible with stronger updrafts in cells forming along the convergence zone.

http://www.netweather.tv/index.cgi?action=convective;sess=b52390a696c842d0be60aee34de3ae35

[Coast]

ESTOFEX going for a general watch in western and central areas and a specific target over Pat's house!

Storm Forecast

Valid: Thu 22 Jul 2010 06:00 to Fri 23 Jul 2010 06:00 UTC

Issued: Wed 21 Jul 2010 21:03

Forecaster: GATZEN

SYNOPSIS

Low geopotential expands from the arctic sea to the British Isles. Within the south-westerly flow that stretches from the west Mediterranean to north-eastern Scandinavia, several jet streaks are present. Warm air remains over most of southern and eastern Europe ahead of a cold front that will reach from northern Spain to the central Alps, western Poland and north-western Russia at the end of the period. Except for the regions north of the Black Sea and western Poland, quite rich low-level moisture is forecast. Steep lapse rates are expected from Russia to Poland and southern Germany, over the west Mediterranean region, and over northern Scandinavia. Forcing can be expected especially along the frontal boundary during the period, where QG forcing will be present ahead of the main trough.

DISCUSSION

Sweden to northern Finland

A vort-max actually over the Bay of Biscay will travel north-eastward crossing northern Germany in the morning hours and is expected to affect central and northern Sweden and northern Finland during the day, where the mid-level jet streak will likely exceed 30 m/s. A frontal wave that builds in response to this trough is associated with strong warm air advection into eastern Scandinavia, and low-level moisture is forecast to increase to 11-13 g/kg mixing ratio. This is expected to yield CAPE given rather steep lapse rates especially near the trough centre over the northern portions, while rather warm mid-level air will keep the equilibrium level quite low over the southern portions.

Strong QG forcing is forecast that will be strongest near the warm front over the northern portions until the afternoon hours. The advected air mass will be unstable with CAPE values about 600 J/kg, and thunderstorms are likely. These are forecast to organize rapidly in a strongly sheared environment with about 15 m/s bulk shear and 100 m²/s² storm-relative helicity in the lowest 3 km. Right moving storms that will move eastwards are forecast to produce tornadoes as low-level vertical wind shear is about 15 m/s and adequate low-level buoyancy and favourable hodographs are also in place. Strong tornadoes may be possible due to the strong low-level winds reaching 25 m/s at the 700 hPa level. Large hail, excessive precipitation, and severe wind gusts are also possible.

In the afternoon and evening hours, strong linear forcing is forecast along the approaching cold front. A linear and mostly parallel stratiform MCS is expected that will spread eastward ahead of the trough axis. While the winds will further increase to 35 m/s at the 500 hPa level, bowing segments are expected along this line that will pose a threat of severe wind gusts and tornadoes. Excessive rain will be also possible. The MCS is forecast to produce thunder and severe wind gusts up to the Arctic Sea during the morning hours, while warm mid levels will limit the potential of deep moist convection further south.

Central British Isles

Underneath the through centre, weak tornadoes are not ruled out given weak vertical wind shear and quite good low-level buoyancy and convergence.

UKASF are also targeting a central area for consideration

Storm Forecast Issued: 2010-07-22 00:56:00

Valid: 2010-07-22 00:00:00 - 2010-07-22 23:59:00

Regions Affected

Wales, Midlands, East Anglia, CS + SW England, Yorkshire and parts of NW England (all of England and Wales are included in the WATCH)

Synopsis

Complex area of Low pressure "Quendeline" (FU Berlin) situated over the United Kingdom, continues to dominate the weather across the country on Thursday.

In the Returning Polar Maritime airmass, numerous showers and thunderstorms are expected to develop widely as instability increases, primarily aided by daytime heating, but also continuing during the night-time period in places.

Showers/storms may perhaps become locally severe in nature, with gusty winds, moderate-sized hail and a chance of a weak tornado, particularly over Wales and western England.

The showers/storms are expected to decrease in coverage during the evening hours, becoming mainly confined to eastern counties of England overnight where the thunder/lightning risk continues.

Other charts and info for today:

[Nick F]

A look at the 06z Larkhill (Wiltshire) radiosonde ascent suggested a readily unstable profile with just the smallest amount of surface heating, hence the heavy showers developing from day break across SW and S England:

Adjust the surface temp of a rising parcel to 20C (rising parcel shown by red line), and some decent instability is likely with several 100 j/kg of CAPE (shown by the yellow area). With cloud tops to 350mb/8000m or around 26,000ft – thunderstorms likely with convective temp of only 20C, some moderate hail possible too.

[Coast]

Update from ESTOFEX:

Forecast Update

Valid: Thu 22 Jul 2010 11:00 to Fri 23 Jul 2010 06:00 UTC

Issued: Thu 22 Jul 2010 11:08

Forecaster: GATZEN

A level 1 was issued for the southern British Isles mainly for tornadoes.

SYNOPSIS

Please refer to the main outlook, issued at Wed 21 Jul 2010 21:03 Z.

DISCUSSION

England

Underneath the through centre, weak tornadoes are not ruled out given weak vertical wind shear and quite good low-level buoyancy and convergence. As models do agree well on this situation, a level 1 was issued for the noon and afternoon hours.

Nothing from TORRO yet......

[ZONE 51]

The fax charts show another convergence line/zone setting up across se england friday ,also with negative lifted index and some CAPE with cold uppers and ground heating i would think theres a possibilty of heavy downpours with hail or thunderstorms

??

[Coast]

ESTOFEX have all the action just over the Channel from us today:

UKASF have it over the SE corner with a slightly larger watch zone:

Storm Forecast Issued: 2010-07-23 00:40:00

Valid: 2010-07-23 00:00:00 - 2010-07-23 23:59:00

Regions Affected

SE England, parts of E Anglia (Lincolnshire, E Anglia, Home Counties, CS & SE England are included in the WATCH)

Synopsis

A ridge of High pressure extends across the majority of the United Kingdom, with a surface Low situated on the nearby continent.

Slight instability is likely to lead to the development of scattered showers and isolated thunderstorms over Southeast England during the morning and afternoon, these quickly decaying during the early evening. Given the airmass, small hail is possible in some of the showers, but severe weather is unlikely.

TORRO have nothing for today yet (if they feel it warrants it)

Here are the usual charts for you to decide:

[Nick F]

Can't rule out the odd isolated sferic amongst the showers likely to develop across SE England later this morning - early afternoon, mainly Kent/Sussex. But IMO probability too low to issue a convective forecast, GFS shows negligible CAPE across the SE corner. Perhaps a funnel or two can't be ruled out early afternoon - as sea breezes develop inland from south coast against the general light background N'erly flow.

00z GFS shows a cut-off upper low/cold pool with a pool of CAPE drifting S on Wednesday which may ignite some heavy thundery showers irf it came off, but the potential is too far off to take seriously. Otherwise looking like a quiet period coming up atm convective-wise.

[Guest FireStorm]

So the possibility of no storms continues for an area that usually does pretty well for storms every year! I'm beginning to think they just might not happen. At least the old man will be happy - he hates storms

[NL]

ESTOFEX has most of the British Isles under a low thunderstorm risk today (15%).

SYNOPSIS

A large mid-level trough has taken position over central Europe. Western Europe is now in a cooler weather pattern with northwestern winds, while warm moist air is found over the Balkan, Poland and Baltic states. The main surface low moves slowly northward over Poland. From this low, a cold front bends southeastward to Romania and back to the west, while a warm front stretches across the Baltic states and southern Finland, slowly advancing northwestward. Balkan, Italy, Spain and British Isles are locations with an unstable aimass where a mid-level shortwave trough passes which will likely help to activate convection.

http://www.estofex.org/

Happy to see low expectancy of storms today as the annual Sunderland airshow starts today, looking forward to seeing the USAF B-52 Stratofortress which makes its very first visit here in the NE.