Nick F

Wondering why the hot weather, that teased us earlier this summer, is reluctant to return? Blame an unusually southerly tracking jet stream. View the full blog here

Notable shortwave visible as dark stripe in WV imagery approaching NW Britain and Ireland likely to support development of heavy showers and thunderstorms in this area this afternoon Storm & Convective Forecast Issued 2017-07-26 09:21:36 Valid: 26/07/2017 06z to 27/07/2017 06z CONVECTIVE / STORM FORECAST - WEDNESDAY 26TH JULY 2017 Synopsis Upper low and collocated surface low pressure system close to the west of Scotland today will drive a band of rain along a frontal system eastwards across the UK this morning. Post frontal airmass will become unstable across Scotland, Ireland and N. Ireland through the afternoon, supporting heavy showers / thunderstorms. … SCOTLAND, IRELAND + N. IRELAND … Water vapour imagery shows a marked shortwave trough in a belt of strong upper westerlies moving east towards NW Britain this morning in wake of frontal rain currently across much of mainland UK. This shortwave trough will steepen lapse rates and create forced ascent which will support the development of heavy showers and thunderstorms. Strong westerly winds aloft and backing of winds towards the surface with approach of shortwave will create fairly strong deep layer shear in the order of 30-50 knts … which will help organise heavy shower/storms into clusters and line segments. These may produce isolated strong wind gusts and also some hail. An isolated brief/weak tornado or waterspout can’t be ruled out with stronger convection which co-incides with an enhancement of low-level shear with approach of shortwave. Issued by: Nick Finnis

UK Severe Convective & Storm Forecast - Issued 2017-07-26 09:21:36 Valid: 26/07/2017 06z to 27/07/2017 06z CONVECTIVE / STORM FORECAST - WEDNESDAY 26TH JULY 2017 Click here for the full forecastCheck the full storm forecast here

A very intense shower just clearing here at my parents just SW of Ashford in Kent Before (looking north) with radar Following a kind of 'whales mouth' below looking south had torrential rain but no thunder

More funnel sightings today, this may even a weak tornado over the New Forest

Storm & Convective Forecast Issued 2017-07-23 05:59:56 Valid: 23/07/2017 06z to 24/07/2017 06z CONVECTIVE / STORM FORECAST - SUNDAY 23RD JULY 2017 Synopsis Upper and collocated surface lows across eastern England will slowly drift east across the N Sea during the evening. An unstable Wly veerying NWly to Nly flow will cover England and Wales. … ENGLAND and WALES … Cold mid-level temperatures of upper low atop moist diurnally heated surface airmass will create steep lapse rates which will support the re-development of scattered heavy showers and thunderstorms today. A zone of wind convergence shifting southeast across England and Wales during the day could be the focus for storms to organise and train along, bringing a threat of localised flooding where they do organise. Otherwise weak vertical shear will mean showers and storms that develop will be dis-organised and pulse-type and will limit the potential for severe weather. Any showers/storms may produce hail and, like yesterday, there could be a few funnels or even brief weak tornadoes / waterspouts – especially where surface convergence combines with buoyant and strong updrafts. Issued by Nick Finnis Also on Netweather here: http://www.netweather.tv/index.cgi?action=convective;sess=

Intense rain here near Croydon and one close CG seen with instant load thunder while looking out the window at our overflowing gutter. Need to clean that out ...

*UPDATE 11AM ... have included a MARGINAL risk of severe weather (flooding) for parts of N England and S Scotland ... recent radar imagery and obs indicate convergence zone south of frontal boundary focusing persistent feed of heavy convective rain - which is also producing thunder ... there is a risk of localised flooding from this slow-moving zone of intense rainfall. Storm forecast: http://www.netweather.tv/index.cgi?action=convective;sess=

Just a had a rumble of thunder from heavy shower passing over in SE25

Storm & Convective Forecast Issued 2017-07-22 06:50:16 Valid: 22/07/2017 06z to 23/07/2017 06z CONVECTIVE / STORM FORECAST - SATURDAY 22ND JULY 2017 Synopsis Upper vortex and collocated surface low will drift east across England and Wales on Saturday. Airmass will be unstable across England and Wales and perhaps southern Scotland, with surface heating, leading to heavy showers and scattered thunderstorms. DISCUSSION … Cool mid-level temperatures of cold pool / upper low crossing England and Wales (500mb temps AOB -20C) will create steep lapse rates with surface heating in sunny spells. Heavy showers and thunderstorms will likely be ongoing across southwestern/southern coastal areas in the morning driven by warm SSTs, then diurnal heating will build cumulonimbus clouds elsewhere across England & Wales, high enough for charge separation to bring scattered thunderstorms. Given steep lapse rates, hail will be generated in heavy showers and thunderstorms, isolated up to 1cm in diameter, with any locally strong buoyant updrafts. Vertical shear will weak, so showers/storms will be disorganised / pulse-type convection. However, frequency of showers/storms towards the south coast and convergence near slow-moving frontal boundary across Wales, N England and Scottish borders could focus convection and lead to a localised flooding risk. However, organised severe weather is not expected. Most storms should fade after dark, as diurnal heating dissipates and airmass becomes stable. However, there may be sufficiently steep lapse rates from departing upper low atop moist surface airmass across SE England to keep heavy showers and isolated thunderstorms going after dark here, with an isolated risk of surface flooding. Issued by: Nick Finnis

Just hope there's no further flash-flooding from this squall moving across Cornwall, not exactly moving through too fast.

Storm & Convective Forecast Issued 2017-07-20 21:37:59 Valid: 21/07/2017 06z to 22/07/2017 06z CONVECTIVE / STORM FORECAST - FRIDAY 21ST JULY 2017 Synopsis A collocated upper and surface low will move in off the Atlantic across Ireland, becoming slow-moving. Associated occluding cold front will push east across England and Wales during Friday, lying Cumbria to East Sussex by 00z Saturday. SUMMARY … A band of heavy rain will accompany the occluding frontal boundary pushing east across, with some convective elements that may produce isolated thunderstorms along its length, with a risk of strong wind gusts and isolated flooding, as it moves in from the west across SW England, Wales and NW England during the day. A lower risk further east as the heavy rain reaches here in the evening and overnight. Also there is a risk of thunderstorms developing across Ireland in the afternoon, with a risk of localised flooding and isolated tornadoes. DISCUSSION … ... SW England, Wales, Midlands, NW England ... Models indicate fairly strong upward motion in association with lobes of strong positive vorticity wrapping around upper low moving in the west – particularly along/ahead frontal boundary pushing slowly east. A tongue of fairly warm and moist air drawn north ahead of the occluding cold front pushing east from the Irish Sea / SW approaches will produce some marginal low-level instability ahead of the front – due to weak lapse rates. But surface and upper support for strong lift of warm moist conveyor indicated by models along front, overlapped by strong deep layer shear of 30-50knts provided by divergent flow aloft of southerly jet streak parallel to front may be enough to produce some linear / squall convection. Main threats from this line convection, if it develops, will be brief but intense rainfall leading to flash-flooding. But strong low-level jet winds aloft may be brought towards the surface by downward momentum in any convective lines leading to a risk of strong convective gusts of 40-50mph … so have issued a MARGINAL risk for severe (flooding and strong winds). Lightning is expected to be isolated / sporadic with these frontal convective lines … given weak lapse rates forecast. ... Ireland / N. Ireland ... Post frontal airmass across Ireland / N. Ireland in the afternoon will have steeper lapse rates due to surface heating in any sunshine beneath cold pool of upper low aloft. So there is greater risk of thunderstorms/lightning developing here, though small hail and localised flooding will be the main threat here. Worth highlighting that the cold core upper low, low-level instability, steep lapse rates and low Lifted Condensation Level (LCL) here will be favourable conditions for brief funnel/tornadoes where surface convergence forms in the low pressure system crossing the area. Have issued a MARGINAL risk for localised flooding and isolated tornado risk. Issued by: Nick Finnis

UK Severe Convective & Storm Forecast - Issued 2017-07-20 21:37:59 Valid: 21/07/2017 06z to 22/07/2017 06z CONVECTIVE / STORM FORECAST - FRIDAY 21ST JULY 2017 Click here for the full forecastCheck the full storm forecast here

Is a MCV (Mesoscale Convective Vortex) - look at the swirl or circulation!

Blitzortung going berserk atm!

Plenty of warmth aloft tonight spreading north out of France to fuel this elevated storms threatening the south coast, 850mb temps of 17-18C as per radar with T850 overlay

Looks like it has its own circulation the storm system NW of London ... interesting! A MCV indeed. hoping the storms over the Channel get a move on, as got be up early in the morning ...

Look like we may see a new area of storms developing over the English Channel as another 'surge' of the high theta-w plume spreading out of France interacts with shortwave, shown by dark stripe on WV image below: storm development may focus along theta-w plume 'axis' , shown by streamlines on chart below, as the plume spreads north overnight

Can see on the winds at different levels, steering winds generally mid level say 700-500mb, more backed Sly in the west to SWly over to the east, The 12z radiosonde ascent for Herstmonceux in E Sussex (SE England) shows steering flow SWly ... whereas Camborne in far SW England the steering flow is more Sly

Looking at recent satellite, looks like two storm systems with an arc SE across northern France along northern edge of plume of further Tcu and potential storms to spread north Plenty of mid-level instability above warm nose at 800-850mb upstream looking at 12z Brest and Trappes (Paris) radiosonde ascents - some shear evident with winds backed at the surface - so upscale growth into one or two MCS likely.

00z ECMWF to throw into the hat too for midnight. Differences between all the models, why we should nowcast storm development and movement

Thinking that storms could arrive across SW England as early as late morning / early afternoon. Already storms off Brittany. A look at the 00z Brest radiosonde ascent shows plenty of mid-upper level instability, warm / dry nose 800-950mb keeping convection well elevated.

Storm & Convective Forecast Issued 2017-07-17 22:18:34 Valid: 18/07/2017 06z to 20/07/2017 06z DAY 1 & 2 CONVECTIVE OUTLOOK - 18TH / 19TH JULY 2017 Synopsis A negatively-tilted long-wave upper trough extending SE from S Greenland toward Bay of Biscay will approach to W of Ireland on Wednesday. Low to mid-level flow will back S to SEly ahead of trough and will draw in a very warm / moist and increasingly unstable high theta-w plume north out of France later on Tuesday and through Wednesday, as a surface low drifts north out of W France / Biscay. Elevated thunderstorms on the northern edge / warm front of the plume are likely to develop Tuesday evening/ night and move N across England and Wales, reaching Scotland by late Wednesday morning. Hot and humid airmass in wake of these storms across England and Wales may support further storms in the afternoon DAY 1 CONVECTIVE / STORM FORECAST – TUESDAY 18TH JULY 2017 VALID TUES 18TH 0600 TO WEDS 19TH 0600 Areas concerning … WALES, S ENGLAND, MIDLANDS, E ANGLIA … Threats …Flash-flooding, frequent cloud-to-ground lightning, Hail … Summary … Isolated to scattered thunderstorms, locally strong, will bring a risk of intense downpours, frequent lightning and isolated hail to parts of southern Britain from Tuesday evening and overnight into Wednesday morning. … Discussion … Theta-w plume spreading north out of France Tuesday evening / night will be characterised by steep mid-level lapse rates associated with elevated mixed layer (EML) aloft originating from SW Europe. This will bolster mid-level instability in the warm conveyor spreading north from France through the overnight period, with lift of warm conveyor aided by a low-amplitude shortwave trough moving NE ahead of large scale upper trough to the west. This instability and lift supporting development of isolated to scattered elevated storms spreading north with the northern edge of the advancing plume. Scattered storms initially developing and moving north across SW England during the evening, forming clusters and then moving north through parts of Wales and the W Midlands overnight. Storms are likely to develop and drift north further east during the night across parts of central-S and SE England too – though coverage here maybe more isolated as per recent model runs. Southeasterly low-level flow veering and strengthening to S or SWly aloft should result in 30-40 knts of effective shear … which combined with MLCAPE values of 500-1000 j/kg … will support organised clusters of thunderstorms – which may grow upscale and merge into one or two small convective complexes. Have issued a MARGINAL threat for intense/torrential downpours producing 20-30mm per/hour locally leading to flash-flooding. Frequent cloud-to-ground lightning may accompany stronger storms. Also there may be isolated incidences of large hail and strong wind gusts. -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- DAY 2 CONVECTIVE / STORM FORECAST – TUESDAY 18TH JULY 2017 VALID WEDS 19TH 0600 TO THURS 20TH 0600 Areas concerning … ENGLAND, WALES, S and E SCOTLAND: Threats …Flash-flooding, frequent cloud-to-ground lightning, large hail, gusty winds. … Summary … An area of scattered thunderstorms, locally strong and forming one or two storm complexes, will spread north across England and Wales Wednesday morning, before reaching S and E Scotland in the afternoon, bring a risk of intense downpours, frequent lightning and isolated hail. In the wake of these elevated storms, a very warm or hot airmass will support further development of isolated thunderstorms across England and Wales in the afternoon. … Discussion … … England and Wales … Elevated thunderstorms are likely to be ongoing at breakfast time on Wednesday, spreading north across England and Wales during the morning. These storms will continue to bring a threat of flash-flooding and frequent cloud-to-ground lightning, though hail threat likely diminished. In wake of these elevated storms drifting north, a warm and humid airmass will be in place which, with surface heating in increasing sunshine by the afternoon, will become unstable with CAPE values of 500-1000 j/kg inland widely, with E Midlands and E Anglia forecast values of 1000-1500 j/kg by GFS. Despite a lack of focus for lift to trigger storms and weak deep layer shear, surface heating and strong instability should support scattered thunderstorm development late morning onwards through to the evening, mainly N of M4/London – where SWly flow will not be modified by maritime influence so much. Strengthening SWly flow aloft may allow storms to merge and organise into linear segments – with a risk of isolated large hail, given large CAPE values, risk of intense downpours leading to flash-flooding and also strong wind gusts. … Scotland … Elevated storms are likely arrive and track NE across southern and eastern parts later in the morning and through the afternoon before clearing in the evening. These storms may produce intense downpours leading to flash-flooding, frequent lightning and gusty winds. Issued by: Nick Finnis

UK Severe Convective & Storm Forecast - Issued 2017-07-17 22:18:34 Valid: 18/07/2017 06z to 20/07/2017 06z DAY 1 & 2 CONVECTIVE OUTLOOK - 18TH / 19TH JULY 2017 Click here for the full forecastCheck the full storm forecast here

Thanks for that info, I don't doubt that you provide some credible reasoning for why elevated storms produce intense lightning. The theory I mentioned was not my own, but from what I've read from a Meteorologist somewhere on an American forum, which I admit may not be widely accepted. Lightning production is a fascinating subject and certainly very complex and perhaps still a phenomenon, like tornadoes, which may not be fully understood. One thing from being out in the U.S. chasing storms is observing that lightning is often most powerful / prolific in the updraft area of the storm that is often rain-free too. Strong updrafts, sometimes in the mesocyclone area which produce tornadoes, I've seen some frequent positive bolts - perhaps the region of the storm where the updraft is pushing up at greatest speed into levels below freezing creating charge separation. Also, some of the most dangerous positive bolts are well-known to arc at long heights from the anvil of a strong storm well away from any rain/hail core, indeed sometimes away from the storm - resulting in those 'bolts out of the blue' or in blue skies. Basically if you can hear thunder - you are always in very slight danger, if exposed, of being struck by lightning.