Storms and Convective Discussion - 20th April 2019 onwards

[biddie]

Wont be till later tonight here

[Summer Sun]

Day 1 Convective Outlook

VALID 06:00 UTC Mon 24 Jun 2019 - 05:59 UTC Tue 25 Jun 2019

ISSUED 12:34 UTC Mon 24 Jun 2019

ISSUED BY: Dan

UPDATE 12:34 UTC Minor adjustments to the various threat level, no major changes to forecast expectations. Elevated thunderstorms increasing in coverage over southern Scotland and northern England, drifting to the N/NE. Scattered surface-based thunderstorms expected in parts of Ireland this afternoon. Elevated thunderstorms / MCS expected later this evening over the English Channel and into CS / SE England, moving into Home Counties / East Anglia later in the night - though perhaps not affecting northern parts of the MDT until the very end or even after this forecast period ends (06z Tue)

The forecast evolution for Monday daytime is very uncertain and rather complex, and hence carries low confidence. However, confidence is much higher by Monday night, especially compared with the convective potential on Sunday for example. 

... ENGLAND MONDAY - DAYTIME ...

On Monday morning, a few elevated thunderstorms may be ongoing across parts of the Irish Sea and northern England, along the rear side of the frontal rain. This will continue to lift northwards into southern and eastern Scotland and out across the North Sea as the day progresses.

In its wake, a second shortwave impulse will arrive by late morning or the afternoon, and so a second round of thunderstorms is possible, particularly over northern England - but possibly some areas farther south too, depending on the timing/phasing of this shortwave. At a glance, forecast profiles look generally capped to surface-based convection, and so it is quite likely much of this activity will be elevated. That said, isolated surface-based convection cannot be ruled out, especially where elevated convection can root within the boundary layer, or orographic lift / upslope flow plays a role in thunderstorm development (such as the Pennines).

The environment is likely to be well-sheared, with both reasonable speed and directional shear present, enabling cell organisation. Cloud clover is likely to be a major issue, but if sufficient surface heating can occur, some fairly substantial CAPE may develop. As such, hail up to 2cm in diameter and flash flooding may occur. This potential may be somewhat contaminated by hints of some frontal (dynamic) precipitation running up from the south at times too, along the frontal boundary.

... NORTHERN IRELAND / REPUBLIC OF IRELAND ...

Scattered thunderstorms are likely to develop over parts of Northern Ireland and the Republic of Ireland on Monday afternoon / early evening. Cloud may also be an issue here, so storms will tend to develop where the best surface heating occurs combined with areas of low-level convergence (such as sea breeze). With closer proximity to the upper low, the environment will be less-sheared and so "pulse type" convection is most favoured - but with weak steering winds, slow storm motion could result in prolonged heavy downpours leading to some localised flooding. Hail up to 1.5cm in diameter may also accompany the most intense cells.

... SCOTLAND ...

Periods of frontal rain will affect central, southern and eastern parts of Scotland for much of this forecast period, reinforced by embedded convective elements for North Sea coastal counties in particular - especially later in the afternoon and into the evening. Some embedded lightning may occur locally, with the main threat being from localised flooding.

... CS + SE ENGLAND / EAST ANGLIA / HOME COUNTIES - MONDAY EVENING / NIGHT ...

During Monday daytime a heat low will form over Spain and western France, with low-level winds on the northern flank of the low becoming more backed with time. As a result, advection of a very warm, moist airmass will occur towards the Bay of Biscay which will serve to increase the horizontal thermal gradient in the area.

By late afternoon / early evening, a shortwave will approach from the southwest, forcing a wave to develop along the frontal boundary and increasing the potential for dynamic rainfall. On the eastern flank of the frontal wave, isentropic upglide will result in scattered elevated thunderstorms developing over NW France during the evening hours. Strong deep directional and speed shear will allow thunderstorms to become organised, with unimpeded warm, moist inflow from easterly low-level winds. Upscale growth into an MCS (mesoscale convective system) is possible as the thunderstorm complex drifts NNE / NE across the English Channel towards CS / SE England, then later past London and into East Anglia. Hail, gusty winds, very frequent lightning and localised surface water flooding are all hazards associated with this complex.

The exact track of these thunderstorms is a little uncertain, and an adjustment of ± 40 miles may be required to the MDT area. However, it must be noted that most lightning activity will tend to be focussed on the eastern flank of the precipitation envelope, with a period of more generic frontal rain likely on the western flank as the wave runs north along the frontal zone.

http://www.convectiveweather.co.uk/forecast.php?date=2019-06-24

[sparky1972]

still far to cloudy here, really hampering any chance of storm activity here i think.

[P-M]

2 minutes ago, sparky1972 said:

still far to cloudy here, really hampering any chance of storm activity here i think.

It's been cloudy here all day but there's storms just to the south of me. They'll be elevated in nature rather than surface based but storms definitely around these parts.

[Mesoscale]

Very humid here with a few light drops of rain, bring on the storm....... if it happens.....? 

[snowfall09]

Storm Forecast
Valid: Mon 24 Jun 2019 06:00 to Tue 25 Jun 2019 06:00 UTC
Issued: Mon 24 Jun 2019 03:02
Forecaster: VAN DER VELDE

A level 1 and 2 were issued across western France and southern UK mainly for severe wind gust, large hail and tornado chances, and in Ireland for spout-type tornadoes.

A level 1 was issued across southeastern Europe mainly for large hail and extreme rainfall.


SYNOPSIS

The strong southerly flow east of an Atlantic low has advected a layer with steep low to mid level lapse rates out of the Sahara and Spain northward into central France, where high low level moisture converges in a thermal trough. Meanwhile, southeastern Europe continues to be under influence of an upper cold pool, with relatively weak flow but significant CAPE.

DISCUSSION

...Western Europe...

A level 1 over central France illustrates mainly the uncertainty of storm initiation, where MUCAPE over 1000 J/kg meets 500 m2/s2 0-3 km SREH and 20-25 m/s DLS. Td2m forecast to be 19-21C. Across western France and southeastern UK various models produce precipitation likely from elevated convection along the cold front, with weaker CAPE. The lifting action of shortwave troughs also reaches central France, and relatively weak CIN is calculated on the convergence line. However, models hesitate producing any storm in the otherwise severe weather supportive environment, except for an evening/night MCS developing most likely over Normandy, moving NNE-ward.

Initially convection will start as clusters of cells including rotating ones (supercells) even if elevated base, which can produce very large hail and severe gusts. If rooted in the boundary layer of the surface convergence line, then with 0-1 km shear exceeding 10 m/s and the strong SREH also tornadoes are possible. However, such collocation is not what COSMO model suggests, which keeps nocturnal development west of the line. But if indeed clustering into an MCS over the late evening, a squall line may form with the potential of organized severe wind gusts.

[Thunderstruck]

Light showers here currently, and it’s also been overcast for much of the day. Don’t know how that will affect things later on but it does still feel very humid and stormy...

[ballybabe21]

Lots of Rumbles here in Newton Aycliffe. No visible lightening. 

[StormChaseUK]

Clouds starting to bubble up in the sunshine of east wales. Very humid 

[JoeShmoe]

Been a few storms already today in Edinburgh and another one now (near the airport)

[Josh Rubio]

Are these storms which are expected tonight elevated or surface based? Because there certainly hasn’t been any surface heating here as the sun hasn’t made one appearance so far today. Full cloud cover with the mist not budging one bit. 

[Ben Sainsbury]

1 minute ago, Josh Rubio said:

Are these storms which are expected tonight elevated or surface based? Because there certainly hasn’t been any surface heating here as the sun hasn’t made one appearance so far today. Full cloud cover with the mist not budging one bit. 

They are going to be elevated!

[P-M]

Thunder in Washington first rumble but no lightning seen gone very dark though. 

[Aiden2012]

Ohh showers building near the wash, hopefully this is it haha

[Harry]

Some gorgeous looking undulatus beginning to appear over London 

[P-M]

Ah at last! I can stop moaning now. Nice cracks of thunder heard. Band seems to be intensifying slightly to my South

[Nice Ice Babe]

Thunder in Gateshead now. Woke the dog up. His first thunder storm. 

[Met4Cast]

22 minutes ago, sparky1972 said:

still far to cloudy here, really hampering any chance of storm activity here i think.

It shouldn't - Unlike last night where the storms were the home grown variety, tonights storms should initiate and build across northern France and then move across the channel to the UK so heating etc on UK mainland isn't quite so important. 

Re: GFS, ignore it. This is a highly complex situation & the GFS just doesn't have the resolution to resolve it, hence the massive blob of PPN. Interestingly the AROME has shifted Eastwards on the 06z update

ARPEGE a little further West but with London still largely in the main risk zone

The NetWx-SR model is much further West than just about every model, it does seem to have quite a westerly bias this one and generally I don't find it all that reliable, but it is one of the few Hi-Res models we have

[P-M]

Tipping it down now with intermittent thunder 

[Mercury Rising]

Currently hearing a few rumbles in sunderland the air smells fresh with rain 

[Harry]

8 minutes ago, Met4Cast said:

It shouldn't - Unlike last night where the storms were the home grown variety, tonights storms should initiate and build across northern France and then move across the channel to the UK so heating etc on UK mainland isn't quite so important. 

Re: GFS, ignore it. This is a highly complex situation & the GFS just doesn't have the resolution to resolve it, hence the massive blob of PPN. Interestingly the AROME has shifted Eastwards on the 06z update

ARPEGE a little further West but with London still largely in the main risk zone

The NetWx-SR model is much further West than just about every model, it does seem to have quite a westerly bias this one and generally I don't find it all that reliable, but it is one of the few Hi-Res models we have

I'm very pleased the trend is to bring the storms in sooner (may give me a few hours to get some shuteye).

Prefer the Arpege over the Arome - up until then I've largely been on the eastern flank.

Interestingly Euro4 going with a westerly bias (as of 06z) - although this is for 06:00 so much later arrival compared to AROME, ARPEGE etc

Edited June 24, 2019 by Harry

[Derbyshire_snow]

Same old here storms to the north east south and west we had tropical rain for 10 minutes that burst rain covers and created a water feature in the middle of the road and caused a bit of minor flooding in the space of 10 mins had that carried on 30mins + you would have been looking at severe flash flooding it was that intense.

[Sparkiee storm]

I have a question that I haven't seen anyone ask or answer, and I don't even know whether it can really happen, but somewhere I heard that if storms are strong enough e.g. a supercell/MCS, they can move a completely different direction to the steering wind making it quite irrelevant, if this is true some people possibly may have a few surprises tonight, but like I said I don't know if it is true.

Any facts, opinions or thoughts about this? 

Edited June 24, 2019 by Sparkiee storm

[Chris.R]

So it’s sunny and dry here and has been dry since the warm front last night. It’s 23°C now but I  know it will be sunny/dry all day. 

Friday now my next chance.

 It seems East is best again this week. Hate being pessimistic like that but it’s proved to be true again. Oh well. 

Edited June 24, 2019 by Chris.R

[Biggin]

I prefer to trust in Nick

https://www.netweather.tv/weather-forecasts/uk/convective