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Eagle Eye

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    Storms and Convective discussion - May 2024

    in Storms & Severe Weather

    Posted

    Convective Outlook (tomorrow afternoons risk to come tomorrow) ⚡️ 

    An upgliding theta-E ejection on the east side and parallel to a northwestward moving warm front should allow for an MCS to approach the south coast overnight with Supercellular activity possible mainly with feeding cells on the eastern side of the storm. Frequent lightning, 40,000 feet tall storms, some severe hail and some flash flooding are all possible.

     

    Linear development should take place after initial surface development over France evolves from surface to elevated during the evening. With dry warm air, or on slightly less capped models a warm nose, intruding at the 925hPa level, a surface inversion layer forms and without surface heating as night falls, convection is very likely to transition to elevated well past the boundary layer where significant ascent begins to be forced with the development of stronger lapse-rates and a potential vorticity lobe stretches out to help with development into a full on linear MCS.

     

    As it hits the south coast, the far eastern extent of the MCS may develop more significant potential with slightly lower LCL’s and energy pushing in more from the southeast into the Kent and Sussex coast generally on most but not all models. Some models have the eastern extent a lot further west between Brighton and the Isle of Wight, but those models are beginning to become outliers compared to most. This is where 3000+ J/KG of MUCAPE is possible for the development of these cells and the eastern extent is more likely to see frequent lightning given more depth of MUCAPE and more MUCAPE generally. 

     

    The movement from northern France is northwest towards the central south to be hit first by the first part of the MCS but that appears to split or attempt to detach from what becomes the main MCS and weaken over the channel as it gets embedded into the warm front but may remain as thundery rain. That is not the main risk and occurs late in the evening. It is also not guaranteed to become much and may just be a feature of the warm front itself.

     

    Whereas the overnight risk comes next with MCS development further SSE, orientated less parallel and more head on towards the south coast moving northwestwards from northeastern France andor Belgium towards the central southern and southeastern coasts. Then it should move NW and further inland towards southern parts of the Midlands. With very lightning favourable saturation and significant MUCAPE, likely 1500-2000+ J/KG and cloud heights possibly up to 12km as it appears likely to hit central southern and southeastern coasts. This is where most risk comes from. The AROME and UKV are very strong on this occurring with pre-MCS cells forced with the change in wind direction forcing wind convergence in a line between Belgium and the SE on a few models and if this does occur, then very strong energy is available for pre-MCS cells to form and frequent lightning is very much a possibility along with severe hail especially as it eventually combines with the MCS.

     

    The most unstable buoyant layer has lifted indexes of around –9, which is extremely rare for UK risks and show the vigorousness of convection, hence the only reason why this isn’t a high risk is because of the worry over where the MCS hits. It would be in the SE but the Swiss models refuse to back down and they have been some of the best models, instead they focus on the central south with some convection extending east past Kent once it gets towards London way.

     

    A large inflow layer with significant energy and large buoyancy means that despite strong low-level shearing, hail is possible between 1 and 1.5 inches. This especially evident where cells feed momentum where they combine or if a Supercell does form. Rainfall totals could also be fairly upwards of 30mm in places and a lot of that falling in a short amount of time with a lot of moisture available. Therefore there exists risk for flash flooding.

     

    Overnight, initiation may occur north of the MCS as a forcing band develops perhaps due to gravity waves as the very tall potential of these storms gives that risk. Similarly, storms could trail behind to the south and east if the MCS is particularly strong enough. Some of which may last well into the morning because of the significant energy available.

     

    Eventually it should weaken wherever it is around 4am to 6am into thundery rain. This is where the potential weakens and any trailing cells are the main risk but only really on the UKV.

     

    There is still a lot of difference between the models but frequent lightning they almost all agree on. Along with some severe risk and generally quite significant storms. Along with the upscaling into an MCS at least of sorts on pretty much all of the models.

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    Storms and Convective discussion - May 2024

    in Storms & Severe Weather

    Posted · Edited by Eagle Eye

    My rough sketches on the development. 

    IMG_20240501_124126.thumb.jpg.f706da6e8139bb42d553a83173717ec7.jpg

    IMG_20240501_124207.thumb.jpg.ed7bbff2cc36804b60b7e5e2f4fc3b30.jpg

    IMG_20240501_123608.thumb.jpg.a9d96160bb43a9000af9cbc9ea8ecc91.jpg

    My expected frequent lightning areas. In the dotted surroundings. So the central south from the initial MCS, SE from combining developments and London ish area for the new developments but these don't seem to be as impressive. Thundery rain with semi frequent lightning mixed in between based off current modelling but only a rough sketch remember. 

    IMG_20240501_123805.thumb.jpg.fb3f13b92071efb01edb3cf6b8d484f5.jpg

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    Storms and Convective discussion - May 2024

    in Storms & Severe Weather

    Posted · Edited by Eagle Eye

     CoventryWeather

    Tonight's risk has been downgraded but if you're talking about tomorrow. Lapse-rates ahead of the MCS are very positive (so very poor for storm formation) and lift isn't there because of that. Limited depth of CAPE beyond the surface inversion means it's really unlikely to be anything before the MCS much at the moment.

    @Josh Rubio In a way yeah, they form parallel next to the warm front. But a long time after it first hits the south as the theta-E trails slightly behind. So the front should hit in the afternoon, then the storms hopefully late at night or early morning.

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    Storms and Convective discussion - May 2024

    in Storms & Severe Weather

    Posted · Edited by Eagle Eye

    Convective Outlook⚡️ 

    Overnight on Wednesday, severe thunderstorms look fairly likely to hit parts of the south coast bringing the risk of frequent lightning, low-end severe hail and some flash flooding risk. Before that, some daytime showers are possible across England and Ireland with 200+ J/KG of SBCAPE possible. The daytime risk looks fairly limited on the lightning front without much energy but a few lines of showers may grow strong in Ireland whilst lacking in much lightning threat along a PV lobe. Forced ascent into clusters along convergence zones in Ireland and less organised showers across the midlands ahead of and connecting to the approaching frontal system. Generally weakening into the evening as surface energy weakens. Then the main risk is the likely linear system evolving overnight. Initially forming in a large theta-E pool east (ahead in this scenario, or rather being upglided next to the warm front) of the warm front, perhaps connecting into the warm front itself. That may weaken the western portions of the system, but only over time and unlikely to make it's effect too much before the coast. Most of the energy is associated with the eastern extent of the system where the severe risk is as well, with development into the system acting as momentum inducers. Also, bringing the risk of elevated Supercellular development given hooked deep-layer shear and strong energy. If so , then the severe hail risk is very much there because of the increasing severity of updrafts associated if a Supercell does form, as they take up more energy into the storm than non-Supercelular storms do, and given the buoyancy available, the threat for 1-1.25 inch hail is there despite not particularly amazing background signals such as the mediocre lapse-rates and the strong low-level shear. With the MCS itself, winds are unlikely to be severe but 200+ J/KG of DCAPE may risk some strong wind gusts but it's not particularly looking severe on the models. However the below cloud winds do suggest that they will take away somewhat from the hail risk. Initial development of the MCS should take place in the evening over upper central portions of France, the exact positioning still contested about. With surface storms evolving into an elevated MCS as night falls and they enter 1000+ J/KG of MLCAPE associated with the intensifying trough and moisture convergence towards the MCS as an inversion undercuts the surface-850's portion of the system and the moist but cooling air beyond the inversion takes advantage of the ejecting Theta-W plane upgliding along the warm front. Dry air entrainment into the mid-levels means that the saturation weakens past the surface towards 50-70% which is more than enough for frequent lightning to take place as the saturation signal doesn't interfere with it.

    Dry air forces itself behind the MCS leading to moisture diverging away from the surroundings and converging into a tight band wrapping up into the MCS itself and increasing the 850's lift and ascent occurs rapidly as this occurs in the evening or overnight depending on speed, near the northern France coast. This is where severe weather is likely to be at its strongest but some severe weather may linger into southern England as I said earlier.

    Exact alignment is still very poorly modelled, however, the steering currents tend to align with any Belgian or northern France developments hitting the southern coasts, with a more Belgian development perhaps having more energy to feed from but will move more WNW rather than the NW of northern French development, but that's generally the difference between the SE and southern parts of east Anglia being hit full on or relying on new storms forming further east in the channel which has been a scenario on a lot of models, likely because of the sheer amount of energy eventually out-strengthening  the convective inhibition on some models, but we have decided not to extend the moderate into the SE, because of the large model split over Belgium MCS development or relying on further channel development.

    Convective height looks unlikely to be an issue with 10-12km of cloud height development and given the previously mentioned kinematics such as forcing and deep-layer shear, it may become even taller. With most unstable ascent being -8 LI's on some soundings. Very rare for the UK and hence the severe potential. 

    As it hits the south coast, there is a pool of 100+ J/KG of 3CAPE extending across most of the south coast and that could well force surface storms ahead or within the system itself and an increase in strength again. Very interesting surface conditions with sea level pressure falling ahead of the trough of itself forcing that surface energy, which whilst it may take up some of the MCS's energy, this is a different pool of energy and rather the MCS is moving with it's separate energy pool so I find that an unlikely bust scenario. However, very limiting lapse-rates means that the lift is suppressing convection in this area and I don't see it overcoming the surface inversion.

    There are some models that are still limited in it's width development andor whether it can properly cross the channel itself. Hence why it's currently a moderate risk and a rather small one at that based off not particularly expected strength as most places hit by this, especially on the eastern extent in the south and southeast should get frequent lightning, but rather the most likely place to be hit.

    Given the moisture that is available, there is risk of torrential rain. and hence the flash flooding risk. Some of that may evaporative with the distance between surface and cloud base and limit that to another low-end severe risk though. The main risk is frequent lightning currently.

    The general movement of this appears to be along the south coast across all the way to Wales by 6 am on most models. However, weakening as they do. There may be a few re strengthening attempts in local areas based off kinematics in certain areas but that's very difficult to forecast so a slight generally covers that.   

    That'll do for now. A very weird scenario as I keep saying. This is only my best attempt at a discussion. 

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    Storms and Convective discussion - April 2024

    in Storms & Severe Weather

    Posted · Edited by Eagle Eye

     Southern Storm

    Thank you for your kind words. My team and I have decided to do tomorrow's forecast tonight and update it again tomorrow evening so they'll be an update coming soon. I'll be doing another discussion as there has been slight changes from yesterday. Also, about the timing, I've noticed that a lot of my classic storms have occurred 1-3am, that's pretty much exactly what I'm forecasted. Im not saying this is a definite classic but at least it's beginning to have the timing of one, just now hoping it actually does happen. 

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