Storm & Convective Discussion - 1 September 2015 onwards

[vladthemert]

Last night was quite a show heavy rain and lightening.  Just read Supercell storm chase following it towards north sea.  Also read saw footage of the Tornado, Funnel cloud or land-spout at Sleaford.  Reading a debate on how they developed what even were them.  Seems few locations had them Sleaford, Northampton, Tewksbury and Leamington Spa.  

[Flash bang flash bang etc]

The models seem to be struggling to capture the deep convection associated with the system - many of the higher resolution ones seem to have little precipitation associated with it until it reaches The Channel, which is a bit odd but may be related to issues with data assimilation at the beginning.

Anyway, France's very high resolution AROME model develops quite a lot of downpours within the leading area of general heavy rain:

...but the positioning is further west than most models have it. If the low tracked that far west, the southeast would be at risk of seeing some of the stronger wind gusts on the eastern side (up to nearly 50 mph perhaps).

There's then a fairly small region in which heavy showers and possible thunderstorms develop across S'rn England (see below), which advances quickly NE.

This is just one of a remarkably wide variety of options being put forward by the models tonight, with most taking the low and areas of rain a bit further east, some keeping the leading area of rain further south, and projected rainfall totals between 6am and 6pm that on the local scale range from less than 10 mm to around 70 mm, depending on the exact distribution of those downpours.

So there will probably be a fair few on here who see a 'damp squib' event, while others experience a day to remember.

I'm not sure about 'a day to remember' - more likely a lot of pulsy storms randomly popping up and infrequent and scattered lightning.

Here's hoping something more spectacular takes place - but as I recall these tropical storm events usually spawn short-lived convective activity which is usually impossible to track on radar, let alone chase. A squall would be nice, but they move by so quick it's hardly something to sit back and enjoy.

Time will tell...

Still excited about the potential anyway :-)

Edited September 15, 2015 by Flash bang flash bang etc

[Nick F]

CONVECTIVE OUTLOOK WEDNESDAY 16TH SEPTEMBER

 

 

Valid: 16/09/2015 0000z to 17/09/2015 0000z

 

Synopsis

Longwave upper trough overspreads NW Europe, with a number of shortwave troughs rotating around it within a belt of very strong upper winds, containing jet streaks of 180mph+. One shortwave trough and left exit of a jet streak combine to deepen a low over Biscay Tuesday/early Weds which contains the tropical airmass remnants of ex-TD Henri. This fairly deep low for early Sept tracks NE across SE England/E Anglia Weds afternoon, the unstable warm sector on east side of the low crossing N France and clipping SE England and E Anglia Weds PM.

 

… SE ENGLAND and E ANGLIA …

 

Unstable warm sector over France, characterised by theta-w values of 16C+, spreads across far SE UK Wednesday afternoon, with GFS indicating 400-800 j/kg CAPE available. Left exit region of strong jet stream will overspread this warm sector over SE UK, producing strong divergence aloft and thus convergence at the surface to fill the void. This strong forcing for ascent of the warm moist airmass combined with some 40-50knts of 0-6km deep layer shear will support well-organised thunderstorms, perhaps even one or two supercells, capable of producing severe convective weather.

 

Main severe risk appears to be excessive rainfall (15-25mm in a short space of time) from thunderstorm downpours given high moisture content of atmosphere in warm sector. Also there could be isolated incidences of large hail (1-2cm in diameter) from stronger storms. Low-level shear is likely to be fairly strong for a time as winds will be backed ahead of the low moving NE in relation to SWly flow aloft ... so combined with moist surface airmass and likely low LCLs (cloud base), this will create conditions ideal for supercells with mesocyclones/rotating updrafts which maybe capable of producing a tornado. There is also a risk of isolated strong convective gusts (40-50mph). Given these severe risks, but uncertainty over how far NW unstable warm sector will advance over SE UK and thus how widespread storms will develop, have issued a SLIGHT risk for now.

 

Issued by Nick Finnis for Netweather:  http://www.netweather.tv/index.cgi?action=convective;sess=

 

[Flash bang flash bang etc]

Lots of Sferics way out to to the south-west of the UK near BoB.

A good start indeed...

[Singularity]

Yes, promising at the moment. I wonder how well the tropical air has retained its identity across all those miles.

 

When I said 'day to remember' earlier I was thinking in terms of large rainfall totals, as for me more than 40 mm in a day (let alone 12 hours as may be experienced) is a very rare occurrence. Yet in light of Nick's convective outlook (thanks Nick), there could actually be some memorable storm rotation/strong winds or even a tornado for a few lucky/unlucky (cross out depending on your p.o.v.) people.

 

The main issue I can see is a lack of surface heating, with large scale uplift forced by the low having to compensate. The Channel may be warm enough to give things an extra kick in the far south though.

 

Shame this thing's making the run-in during the middle of the night, as it would be fun to compare it's track with the model predictions and attempt to anticipate any last minute changes needed for nowcasting. As it is, sleep can't be ignored for the likes of me without landing a heavy blow (v. high metabolism), so I'll have to turn in and wait to see what tomorrow morning brings.

[Flash bang flash bang etc]

I'm trying to finish some work and eagerly watching radar but that cheeky ol' devil called Sleep is telling me to quit for the night. Will be in Reading tomorrow but the plan is to work fast and get finished by 1pm and get out so I can do a little chase - if conditions are favourable.

Good luck to all u storm fans and take care everyone in the rain and wind!

Edited September 15, 2015 by Flash bang flash bang etc

[East_England_Stormchaser91]

Where's all the rain at?!

Only convective activity so far is happening way down over La Rochelle!

Still, we have all day yet.

[VillagePlank]

 

Interesting weather day, today!

[Speedway Slider]

Are we really in for all this heavy, ex Henri rain/thunder after all?? The rain radar seems to indicative of a Kent Clipper now, more than a main England/south central Wales event? Has it all slipped further south/east?

[Nick F]

Are we really in for all this heavy, ex Henri rain/thunder after all?? The rain radar seems to indicative of a Kent Clipper now, more than a main England/south central Wales event? Has it all slipped further south/east?

 

The heavy rain spreading north bit delayed as per 00z GFS run, but still spreading across across E Wales and England (south of a line from north Wales to North York Moors). Persistent rain arrives late morning southern England, then spreading further north during the afternoon and evening.

 

GFS still brings unstable high theta-w plume across SE England and E Anglia, bringing that risk of potentialy strong storms in highly sheared enviironment, but again, delayed until mid/late afternoon.

Edited September 16, 2015 by Nick F

[NO LONGER POSTING]

Well low cloud is on a Misson going south and upper winds East quite something

[Mark Parsons]

Are we really in for all this heavy, ex Henri rain/thunder after all?? The rain radar seems to indicative of a Kent Clipper now, more than a main England/south central Wales event? Has it all slipped further south/east?

I think the situation is a developing low over Biscay moving ne towards us so it will come to together quickly this morning, don't be fooled!

[Evening thunder]

Shame the SW is not in the risk zone like it was shown to be a few days ago.. especially as this originated over the Atlantic. Seems the SE and the continent even get the best storms/interesting weather from maritime sources lol 

[William Grimsley]

Good luck to everyone in the south east, though can't seem to see how this will work seen as nothing seems to have ignited yet, still I think the south west still has some risk this afternoon with the associated heavy/torrential rain.

[NO LONGER POSTING]

Good luck to everyone in the south east, though can't seem to see how this will work seen as nothing seems to have ignited yet, still I think the south west still has some risk this afternoon with the associated heavy/torrential rain.

Storms are not suppose to get going to the afternoon and perhaps linger into the evening

[William Grimsley]

Storms are not suppose to get going to the afternoon and perhaps linger into the evening

Ah well, no doubt all of you in the south east are going to get mullered as per usual and we'll end up getting a damp squib. Time will tell!

[Lauren]

If they can wait until I'm home so I can enjoy it, that would be fab.

[Dami]

Ah well, no doubt all of you in the south east are going to get mullered as per usual and we'll end up getting a damp squib. Time will tell!

Bedford will also miss out i'm sure, so I feel your pain. But it did rain here yesterday, I was quite surprised.

 

But after 3:30pm please IF i'm gonna be lucky.

[William Grimsley]

The rain seems to be starting to develop in the Channel now. Eyes on the beautiful 500 metre rainfall radar!

[William Grimsley]

Developments over Brittany now just moving off into the English Channel. These showers may turn thundery.

[stainesbloke]

Hmm, looks like a completely different scenario to what was being forecast even late last night. That was showing torrential rain across the S/SE all morning moving up into the Midlands, followed by intense thunderstorms into the SE. Looking at the radar....no rain! Some over N France and some showers over NW France but that's it, no idea what that means for the rest of the day but will be extremely surprised to see anything thundery.

[TwisterGirl81]

wELL IT LOOK

 

Developments over Brittany now just moving off into the English Channel. These showers may turn thundery.

 

Yes I thought we weren't going to get any rain in the southwest to speak of but then I went on bbc weather forecast animation and some heavy showers could be here by this afternoon

[William Grimsley]

wELL IT LOOK

 

 

Yes I thought we weren't going to get any rain in the southwest to speak of but then I went on bbc weather forecast animation and some heavy showers could be here by this afternoon

Yeah, definitely some heavy rain/showers this afternoon. Just to let everyone know, ignition has now started over Jersey. Off we go south westerners!

[Winter Cold]

well just shows how even in 2015 with all this technology that the weather can still make fools of us all. Nothing like the BBC forecasts of yesterday evening. The heavy rain band should now have been just about hitting Sheffield but its not even hit the south coast yet!!

 

Cant see any storm threat now, as those were shown to be in the SE this afternoon after the rain has passed. At this delayed rate itl be evening and dark by that time now so cant see any convection happening.

[A.J]

The remnants of TS Henri are causing weather havoc here in deepest Wiltshire, we're being absolutely inundated by torrential invisible dry rain lol