Eagle Eye

I'm going to say it. Nowcasting event. Models are making little sense and still can't agree with each other. Best I can do is base it off model output unfortunately.

viking_smb Often doesn't matter it's not a full on downgrade signal yet in my opinion. ECM was already overdoing it and has corrected to normal. UKV has had 1 run off. Both still hitting as of now. This is why we're probably waiting till tomorrow to do tomorrow nights risk.

ChannelThunder Its very close to that potentially. But may just be a model wobble, there are still many UKV runs to come.

CoventryWeather Often these events are a couple hours out from what models suggest and this feels like it'll be earlier with initiation. Just a feeling though.

AROME appears to support the UKV fairly well.

Given the really bad modelling of this, I probably won't be issuing tomorrow nights risk till tomorrow afternoon. I'll just be doing tomorrow's small daytime risk tonight.

Andy Bown The first part is just frontal rain so I don't expect it'll be thundery but the second one should be if it does play out like that.

UKV 03Z upgrades it and has a signal it doesn't want to change. However, we'll see if the other models give in to this signal or not of the direction of travel.

00Z UKV update, but there's still a lot of chopping and changing on the models going on.

A sounding for Wednesday night. Fairly tall storms, that's what roughly 30,000 feet possible? Along with a hooked deep-layer shear with more than enough inflow for severe hail when considering it with the MUCAPE. Very interesting to keep an eye on Wednesday night's event with our first real big plume of some sort of variation this year i'm pretty sure.

Convective Outlook️ Wednesday (day 2) Wednesday night a shortwave trough advancing east of a frontal system should allow for storms to form over northern France potentially moving NW into southern parts of England. There are however uncertainties over positioning still so much so that they could instead be centred over Belgium and move into eastern parts of England, slowly weakening moving NW/W into the midlands. This is limiting the confidence over where to place the risk currently given lots of difference between the models. However, away from the modelling issues, this a high-end potential event but with a few bust risks as most do to be discussed later. A theta-W ejection from mainland Europe will either be centred over Germany and move NW into Belgium or further west on the border between Germany and France and hit northern France, and storms will likely initiate in one of those two areas. As they do, large upscale growth by deep-layer shear is likely into an MCS is likely with forcing being very high compared to a typical risk becoming very favourable for clustering into a system of storms, unless there's a downtrend in the forcing, this seems a very likely solution. Given the high theta-W airmass and surface inversion that appears to hold in place, a transition to elevated storms towards the evening as surface energy over northern France and/or the border and Belgium area weakens appears likely. With the remaining mid-level energy appears to be high as these storms move into a new area of what appears to be separate energy held in the mid and upper levels of the atmosphere slightly further north in the late evening. And this is where the transition upscaling into an MCS appears likely as well. The initial surface based convection over land becomes its own system as the trough forms with dry air undercutting the trough and driving the band of moisture convergence, influxing energy towards the trough itself and taking in energy from the surroundings. It's a very important part of the MCS development and also the potential to survive the channel, without the moisture convergence becoming centred around the developing trough, or becoming centred too early, it won't reach the UK. Currently most models are adamant that despite the multitude of scenarios with timing possible, the trough develops just late enough into the evening for the MCS to develop as an elevated MCS, rather than surface based and what shows that is that on some models without the whole storm being forced along the trough, the parts not on it fall apart as the surface energy weakens. This trough also brings in significant ascent beyond the boundary layer with buoyancy more typically associated with surface storm days in the height of summer with -5 LI's at the 850 hPa height. Therefore any convection could well force a vigorous updraft capable of significant hail just past an inch in diameter but mostly limited to that given the strong low-level shear in place that will take away some of the hail's strength. So despite favourable hodographs with a strong inflow region, along with widening updrafts because of combining cells and energy strong enough for quick updraft speeds, the hail is still limited to the lower end of the severe potential. This forced ascent should also help with quick upscale growth once the shortwave trough helps rapidly increase it and therefore most surrounding cells will be brought into the MCS likely as constructive mergers for momentum inducing events, so watch out for larger hail and more significant storms where these mergers take place, should the storms reach the UK (assuming they form which looks very likely despite the capping in place) and a small tornadic chance but the elevated nature of the MCS will likely keep that very minimal. Surface energy may still linger crossing the channel on the eastern side of the trough, along with strong deep-layer shear becoming more guided towards a hook there as well. This has lead to 50+ J/KG of 3CAPE with 1000+ J/KG of MUCAPE possibly forcing a Supercell for a time on the eastern side of the MCS or detached from the MCS itself, risking stronger hail and again a small tornadic chance. There could also be an elevated Supercell linked in with the actual MCS itself. The hodographs are fairly favourable for that, the actual MCS upscaling is more of an ascent nature than a speed shearing nature. 400+ (m^2/s^2) of SRH certainly shows the significant potential of this event but that depends on the shape of the hodograph which varies a lot from model to model as they tend to do at this point in the forecast. The higher end do have a very Supercell and hail favourable highly streamwise flow with a large inflow region though. However, there's still a weak tornadic potential with the surface lift just not being conductive on almost all models as the surface based Supercell looks very unlikely, just a small potential, most likely elevated if one does form. Lightning risk does look to exist fairly well with this, saturation doesn't seem much of an issue aside from a few areas on soundings. It's not great but strong energy and often I find lots of momentum builders as there are in MCS's can counteract that as long as there's enough energy, there does seem to be some sort of threshold past about 1,000 J/KG where the saturation can become less of an issue. This is mostly based off experience and modelling though. Saturation does appear to be quite good on the central southern coast on the models that go for the northern France initiation and also the ones that go for a Belgium initiation, hence that is the highest risk for lightning at the moment on the models. The more widespread lightning event currently modelled appears to be the Belgium initiation that the ECM is currently adamant on, but an uptrend in the northern France risk wouldn't take much to do the same. There's a fairly high-ceiling for this event, but it's still complicated in it's expected evolution and the surface based storm transition to elevated timings. Saturation could also become more of an issue if the models start trending that up.

Convective Outlook️ Tuesday (not the Wednesday one yet) During Tuesday early hours, a low pressure will approach from the aouth of Ireland introducing heavy & intense rain across S parts of Ireland which would of cleared North by the early morning period across Northern Ireland. The odd isolated lightning from this front cannot be ruled out throughout the early hours of Tuesday. Heading into the early morning period, we are expecting to see an increase of Cape of anywhere from 100J/KG-400J/KG across S parts of Ireland with intense showers increasing in activity. This may bring the odd one or two thunderstorms in the morning across S & SW parts of Ireland, however lightning will mainly remain isolated & sporadic. Into the late morning/early afternoon, Cape will drastically become more widespread mainly across the AOI "Area Of Interest" Zone up to 300J/KG - 600J/KG which will increase the risk of lightning across Ireland. We are interested that Cork up to past Galway will see the most activity within Ireland. This risk will continue into the evening, with further sporadic lightning expected, however one or two thunderstorms may develop for a time. Looking at the late afternoon/early evening period, were expecting to see a convergence line form dangling down across the Central spine of North England down to just East of Birmingham. Heavy & intense showers are expected to from of from this alongside with Cape around 100-300J/KG which may allow a few lightning strikes to occur. Confident remains low on if any lightning will occur, however there is a risk. Into the late evening/ night time period, the risk of heavy showers will head north clipping parts of Cornwall which may bring a few lightning strikes due to Cape around 200-400J/KG, however confident is quite low on this. Our attention then shifts towards parts of South East England into East Anglia, where surface based thunderstorms are expected to develop across NE France where 500+J/KG sits which these storms will advect north towards extreme eastern parts of East Anglia. These thunderstorms are likely to become elevated throughout its course north. There is a low risk of it slightly being further W than expected clipping E England & SE England, which is why a low risk covers these areas just in case. There is also a chance these storms may not end up clipping EA. Great Yarmouth looks to have the greatest chance of seeing elevated thunderstorms throughout the night.

Shaunado 1000+ J/KG of MUCAPE and don't take notice of SBCAPE or MLCAPE. MLCAPE takes a sample of the atmosphere I think fairly low down and lifts it, the changes in saturation and variables with height can mess with it. It's not a surface based system so don't worry about that. - 5 LI's past the boundary layer is more than most of what we got last year , this has more than enough trigger for it to form.

viking_smb Update coming tonight. I suspect we're going for a small slight for now but that may move in area depending on 15Z UKV and model updates. This looks to be a moderate eventually as long as it remains on the models.

ChannelThunder Sorry yes meant northwest. So used to things moving northeast.

Day 3 risk for Wednesday that'll be updated by the time we get to Day 2 and Day 1. We're going to start doing this for events well signalled in advance. On Wednesday, an upper low is expected to form over Germany and Benelux, ushering in a relatively warm Theta-E airmass along the northern and eastern parts of the occluded front. Forecast models indicate relatively high CAPE values (700 - 1500 J/kg) late on Wednesday, a rarity for May according to climatology. By late afternoon/early evening, a shortwave from the southeast will trigger the development of a wave along the frontal boundary, increasing the potential for dynamic rainfall. Isentropic upglide on the north-eastern flank of the frontal wave will lead to scattered elevated thunderstorms over southeast England during the evening hours. Strong deep directional and speed shear will facilitate organized thunderstorms, fuelled by warm, moist inflow from easterly low-level winds. The potential for upscale growth into a Mesoscale Convective System (MCS) exists as the thunderstorm complex drifts northeast across the Midlands. Hazards associated with this complex include hail, gusty winds, lightning, and localised surface water flooding.

Convective Outlook️ During the early hours of Monday, a low pressure will track into Ireland which will clear into Scotland throughout the morning. Into the late morning / early afternoon period, Cape levels will start to once again pick up across Ireland at around 200J/KG to 400J/KG allowing intense showers to form across the Central spine of Ireland. These showers may develop into 1 or two thunderstorms however, lightning will mostly remain isolated / sporadic. Heading into the late afternoon/early evening period, these showers will track in a NE/E direction towards E parts of Ireland & Northern Ireland which will also bring the risk of isolated / sporadic lightning. Further shower development can be expected across.

Incredible

The outbreak went very significant overnight unfortunately

Convective Outlook️ A mass of rain from a low pressure system should clear by late morning or early afternoon in the south and mid afternoon further north leaving behind some energy forced with a wind convergence zone in England. Meanwhile in Ireland similar lines of convergence should form and help convection with rising low-level temperatures amidst slightly falling pressure. Bands of forcing behind the clearing low pressure over England should allow for some showers to be forced where energy can rise the most. This is most likely in areas where the sun can come out, just behind the strongest of the forcing bands which hug close to the low, but so does the cloud. Limiting shower depth and the strength of convective tower potential, unless clearing can become more widespread in the higher forced areas mainly in the SE and east Anglia where showers also have more moisture to feed off for that convection. Lapse-rates are strong enough for some small hail to form, but given the weak CAPE, I can't see much forming. Nor can I see much in the way of lightning because of the saturation associated with the area behind the low pressure system. In Ireland the forcing bands are more connected with the potential for convection because of less favourable conditions for thick cloud cover. Therefore stronger lift is likely to occur here and force a few showers. Stronger energy should take up more of the moisture even though it's quite weak and may lead to some heavy rainfall associated with these and they could also form into a proper line of convection by the afternoon if enough shearing were to be present. If they do, I suspect they could sustain further east than the models suggest hence why it's been extended into the Irish Sea and western Wales just in case, with a very small chance of a Supercell but shearing is lacking much strength to really push that risk. However, most likely is that these are to remain rather weak and lightning is still a fairly small risk for now. Low-level lapse-rates are strong here and so small hail is possible however unlikely to be much more than 1/4 of an inch as the MLCAPE barely reaches past 300 J/KG.

WeatherArc It's broken the Nadocast scale. It's the 80% counter which wasn't even on the chance box.

And so it begins.

WeatherArc Shear is more significant than most models showing and saturation even better for what you'd typically see. This has that feel that it'll be nothing or very significant.

Taking a look at yesterday versus today. Substantial low-level shear, but given the weak energy at the surface, it appears that the very low LCL's significantly helped with tornadic production. Something to keep an eye on for events that feel like they might not be so big just because of the other signals. Comparing that to today's risk, significantly more shearing and fairly similar CAPE, along with a more typical saturation profile. However, the LCL's vary a lot, theoretically this has higher end potential, but I suspect there will be less significant tornadoes, but there's more favouring long track potential.

More to add to the incredible photos.