Storms and Convective discussion - 16th June 2021 onwards

[dr weather]

N Somerset may be in the firing line, will video anything that gets interesting.

[Mapantz]

Not expecting much here, apart from a bit of rain..

[Gordon Webb]

latest from Convective Weather

Cut-off upper low continues to linger near the Brest peninsula on Sunday, with lobes of PVA rotating anticlockwise up from France and into southern Britain. The steering flow along its eastern and northern flank will allow advection of a relatively high Theta-W airmass from France into southern Britain in close proximity to a frontal boundary and within a zone of broad divergence aloft. As such, outbreaks of heavy rain are expected to spread gradually northwards across S/SW England on Sunday morning, extending into SE England around/after midday and then into the S Midlands / S East Anglia / S Wales later in the day. Embedded elevated convection will enhance rainfall rates, as was the case on 18th June for example, leading to some locally high rainfall totals and the risk of surface water flooding. Very moist profiles and generally skinny CAPE suggests lightning will probably be rather limited, with perhaps the exception of the eastern English Channel and adjacent areas. 

Ahead of this band of rain, some early elevated convection / showers will be possible over East Anglia and the East Midlands. Assuming sufficient cloud breaks can occur by late morning and into the afternoon, surface heating could lift 2m temperatures to 21-22C with dewpoints of 16-17C possible. A strengthening northeasterly wind will allow advection of this warm, moist low-level air west/southwestwards towards the northern Home Counties and south Midlands/M4 corridor, where an area of enhanced convergence may evolve by the afternoon hours. If all ingredients come together, surface-based thunderstorms could develop just ahead of the main rain band, perhaps also aided by some modest orographic forcing in the area. Forecast profiles exhibit fairly unidirectional winds, although some backing of the low-level winds will be possible. Speed shear is fairly low due to similar strengths at most levels, but stronger flow in the upper troposphere is available if deep convection can grow tall enough. Overall, this setup bears a resemblance to 16th August 2020 which produced some extreme rainfall totals due to very moist profiles (with a large proportion of the cloud layer below the freezing level) and persistent back-building - and a similar situation could evolve over the south Midlands / M4 corridor, albeit with a shorter window of potential (so probably not as much rain!). Current NWP guidance suggests anywhere from 50-70mm to as much as 100mm may be possible very locally, with hourly accumulations of 20-30mm. A SVR has been issued primarily for the risk of exceptionally high rainfall totals and associated flash flooding, but this will be fairly localised and not affect the whole area highlighted. If surface-based convection can develop, then low cloud bases and vorticity stretching near convergence zone could produce an isolated tornado. However, the situation is very finely-balanced, and insufficient surface heating or a faster northward motion of the rain will probably eradicate this potential.

 

Also, depending on quickly rain can clear from Cen S / SE England (and indeed how extensive such rain/cloud is in the first place), there may be scope for surface heating here later in the afternoon that could foster surface-based convection too. In some ways, faster motion of rain northwards decreases the potential in S Midlands but increases it in SE England, and vice versa. Any thunderstorms will probably weaken eventually towards mid/late evening, but a new wave of rain with embedded elevated convection will likely push out of France towards southern England during the overnight period as the next PVA lobe rotates around the main upper low. Again, lightning may be rather infrequent/isolated given moist profiles and skinny CAPE.

[Mapantz]

A few sunny spells developing here. Temps  / Dew points responding.

[SteveB]

I'm right in the middle of the warning zone,  and within the black circle. 

It really doesn't feel like anything other than any other typical day here! I have a feeling it will stay that way too....

[camturbo]

11 hours ago, dr weather said:

N Somerset may be in the firing line, will video anything that gets interesting.

That would be good if you could. I'm not that far out of the warning zone really so may hear a few rumbles if anything does occur. 

[Jamie M]

Storms are off for tomorrow then 

[alexisj9]

51 minutes ago, Mapantz said:

A few sunny spells developing here. Temps  / Dew points responding.

Just got fog from the north sea, or light rain and fog when a bit makes it over 

[Harry]

Cloudy all morning thus far with odd spot of rain. Much much more humid than yesterday evening.

[offerman]

12 hours ago, Kirkcaldy Weather said:

similar to what @staplehurst mentioned, my focus is on SW England for the most intense rainfall, CAPE looks better further east but I'm unsure of storms firing tho wouldn't rule it out. Looks like it will be from that front and also wind convergence 

 Blimey , is that for real. It’s right bang over where I live for a change . 
 

[ChannelThunder]

42 minutes ago, Jamie M said:

Storms are off for tomorrow then 

If that's for home-grown storms, they may as well have chopped the Isle of Wight out of the warning area altogether!

[staplehurst]

13 hours ago, staplehurst said:

Tomorrow exhibits some similarities to 16 August 2020, which produced some extreme rainfall totals in south Norfolk (240mm in 6-8 hours). There is a paper currently being published on the meteorology of this event.

It seems likely a convergence zone may develop in the afternoon somewhere near or just north of the M4 across to Hertfordshire. North of this boundary, a brisk northeasterly wind will, in theory, allow a continuous supply of 'heated' air from East Anglia (where the best potential for cloud breaks is likely by midday and into the afternoon), and this will result in pooling of warm, moist air along this boundary during the afternoon. In @Paul Sherman speak, "spreads" could be 72F/61F over East Anglia.

Winds aloft are fairly unidirectional (mostly from the ESE), and would in theory steer any storm cells to the WNW or NW, while inflow winds near ground level will be perpendicular (from the NE) and of similar speed to the storm motion. This will help restrain cold outflow / gust front from advancing too far away from the parent cell, and in theory aid the development of daughter cells in a similar location, such that *somewhere* could get multiple cells training over the same area. Profiles are very moist, with a substantial amount of the cloud below the freezing level, so these could deliver a LOT of rain over a period of a few hours. BUT this is highly dependent on many factors, namely the convergence zone developing in the first place and sufficient surface heating, both of which could be scuppered if there's too much cloud/rain or it advances too quickly northwards.

We often see in the U.S. how the shape/progress of convergence boundaries can get distorted by rain-cooled air from earlier convection, etc.

The main difference between this event and last August is the magnitude of instability is lower, and the window of potential shorter - so both will mean rain totals will be lower. Either way, currently keeping an eye on the south Midlands / M4 corridor for the risk of some fairly large rainfall totals (various NWP guidance suggests 50-70mm possible, KNMI Harmonie fancies near 100mm on some runs). Perhaps Bucks / Oxfordshire / Berkshire / N Wiltshire / E Glocs most at risk, but need to keep an eye on trends during Sunday to see if this potential still exists. This of course is not including the more widespread heavy rain that will be affecting other parts of Cen S / SW England.

Note: I'm not necessarily promising loads of lightning because profiles will be very moist, but scope for some torrential downpours and flash flooding etc.

With each model run the progress northwards of rain has become slower, and as a result the area at risk is further south. Currently the winds are broadly NE'erly along the south coast, however over the next few hours convergence is expected to develop as coastal sites develop more of SE'ly onshore wind. We've seen some sunshine lifting temperatures to 20-21C, dewpoints are around 15-16C. 

Therefore, eyes on Dorset and Hampshire into the afternoon where we could see some surface-based developments, the focus then gradually shifting northwestwards through the afternoon. If storms can develop, as mentioned in previous post, scope for 70+mm very locally, these totals probably focussed on Wiltshire (but also perhaps extending into some adjacent counties) where training of multiple cells over the same areas could occur. Very much conditional on enough surface heating and sufficient convergence developing though!

Edited June 27, 2021 by staplehurst

[alexisj9]

20 minutes ago, staplehurst said:

With each model run the progress northwards of rain has become slower, and as a result the area at risk is further south. Currently the winds are broadly NE'erly along the south coast, however over the next few hours convergence is expected to develop as coastal sites develop more of SE'ly onshore wind. We've seen some sunshine lifting temperatures to 20-21C, dewpoints are around 15-16C. 

Therefore, eyes on Dorset and Hampshire into the afternoon where we could see some surface-based developments, the focus then gradually shifting northwestwards through the afternoon. If storms can develop, as mentioned in previous post, scope for 70+mm very locally, these totals probably focussed on Wiltshire (but also perhaps extending into some adjacent counties) where training of multiple cells over the same areas could occur. Very much conditional on enough surface heating and sufficient convergence developing though!

Yep, like I said fog off the north sea, it's feels cold, but also muggy.

[Mapantz]

Gone pretty dark here. A cell has developed to my South, around Langton Matravers .

[offerman]

Development currently heading toward Salisbury. 

[WeatherWatchmanG]

South east is getting frekkles lots of random strikes on the map now.

[Mapantz]

I can't see rain being an issue here today..

[alexisj9]

2 minutes ago, WeatherWatchmanG said:

South east is getting frekkles lots of random strikes on the map now.

I'm watching the rain band, it's lightning up in Belgium and France now, wonder what it'll do here.

[Chris Lea-Alex]

I'm going to head c/w on the M25 and towards the m4. Good idea?

[Edit] I've not left yet, decided to have a dominos

Edited June 27, 2021 by Chris Lea-Alex

[alexisj9]

3 minutes ago, Chris Lea-Alex said:

I'm going to head c/w on the M25 and towards the m4. Good idea?

No idea. Whole thing looks like it'll be random, on where gets a storm.

Edited June 27, 2021 by alexisj9

[Chris Lea-Alex]

1 minute ago, alexisj9 said:

No idea. Whole foods Hong looks like it'll be random, on where gets a storm.

It is a bit of a messy setup I'll admit, but I'm gonna roll the dice!

[Allseasons-Si]

Found another cam in Southampton facing the Isle of Wight just in case.

 

 

Edited June 27, 2021 by Allseasons-si

[ChannelThunder]

13 minutes ago, Allseasons-si said:

Found another cam in Southampton facing the Isle of Wight just in case.

 

 

Just for info from a local, that's not the IOW you can see across the water, it's the other side of Southampton Water just north of Hythe, so it'd be a good cam for anything New Forest way

[matt111]

10 minutes ago, Allseasons-si said:

Found another cam in Southampton facing the Isle of Wight just in case.

 

 

I know that cam very well as I mod on the chat there. That’s not facing the IOW. It’s more to the SW, however there are other cams on the channel, one of which Cruise Cam is usually facing more towards the island although there’s a big cruise ship in the way at the moment. Based on its current track it looks like the cell to the SE will pass just to the north of there unless it expands slightly. 

[Allseasons-Si]

OK thanks guys,..at least it's another cam to add to the mix