Storms

[Andy H]

Now that winter is on its last legs a lot of people on here will be looking towards the summer thunder storms

after learning a bit this winter about forecasting cold spells what to we need to be looking for in regards to storms

[Cloudburst]

Now that winter is on its last legs a lot of people on here will be looking towards the summer thunder storms

after learning a bit this winter about forecasting cold spells what to we need to be looking for in regards to storms

CAPE is the best indicator of unstablity of the atmosphere, this is the basic ingrediant. Quite warm (about 25c+) and high humidity about 80+%. Also light winds can help to produce the big thunderstorms, and on a not that warm day convergance lines can help to produce them acting like a thermal (the wind from different directions meet and go up). TBH i dont look too much at the more complex charts, but i do look at Lapse rates as this can be a huge help in determining the cloud type/height and particle type within the cloud (i.e Rain, hail, ice crystals etc).

[Paranoid]

How high does CAPE usually have to be for tornadoes to form. I read somewhere that the CAPE value for the Oklahoma city tornado in 1999 had a CAPE of over 5000. Some of the CAPE we had a few days ago was around 100-200.

[Cloudburst]

How high does CAPE usually have to be for tornadoes to form. I read somewhere that the CAPE value for the Oklahoma city tornado in 1999 had a CAPE of over 5000. Some of the CAPE we had a few days ago was around 100-200.

Well its usually in the thousands as Supercells need alot of energy to form, but other factors also such as wind shear airmas type meeeting point etc also help to determin where they will form.

[Supercell]

I'd say around 2500 jKg^-1 with a lifted index of -8 plus in the main tornado producing thunderstorm - The Supercell. But, Tornados can form in far weaker systems.

[Cloudburst]

I'd say around 2500 jKg^-1 with a lifted index of -8 plus in the main tornado producing thunderstorm - The Supercell. But, Tornados can form in far weaker systems.

Yes tornadoes can form under minor Cb's or TCu as well, if the shear is great enough on a day

[BrickFielder]

Tornadoes are formed from quickly rising air which is forced to change direction as it rises. For them to be more than short lived then they need to be formed from a deep mass of rotating cloud such as a mesocyclones (as in supercells) .

So we look for steep low level lapse rates (temperature difference between the surface and a couple of thousand feet up) and high low level helicity and wind directional shear (change in direction of wind). Speed shear (strongly increasing wind speed with height) throughout the atmosphere helps. Typically high Cape environments have rapidly rising air and jetstreams or strong winds at 500hPa in the vicinity are a good estimate of speed shear. Proxmity to low pressure and fronts or troughs are good indicators for helicity and wind directional changes. There is a lot more to it and some people disagree on the exact criteria for Torndoes to form especially in the UK as compared to continental tornadoes. Go see the Torro site for more information on UK Tornadoes (see why the UK has more tornadoes per square mile than the US here )

Generally when we look for thunderstorm potential we look at the forecast SkewTs and John Holmes as done a good tutorial about these here and Supercell has written some thunderstorm details Here.

In more simple terms look at the Cape and LI charts and if Cape is above 500J and LI is less than -2 then there is a possibility of a thunderstorm. Lesser values can give thunderstorms but they tend to be weaker.

More complex charts to look at are dewpoint charts, relative humidity charts(dry upper levels) at all levels, wind charts at 300hPa,500hPa and 850hPa(strong upper winds), 850hPa to 500hPa lapse rate charts (26C+ at least) ,Convective inhibition charts (CIN), 500hPa height charts, vorticity charts and vorticity advection charts, thetae charts ,shear charts , helicity charts, low level winds(convergence zones) etc.

[the_white_stuff]

Ive read on NW somewhere last year that Cape as high as 11000kj and Li of -12 has been recorded in bangladesh. Is this correct because that must have been immense if a storm formed?

[kold weather]

In terms of supercells, I personally don't think that much cape is needed, prehaps 2-300 for tornadic supercell. All a supercell needs is a rotating Mesocyclone and that generally lasts for a couple of hours. If it meets that, then it'd be a supercell, regardless of whether it had large hail or tornadoes and if you have impressive shear and enough cape to get a shower gonig (In Spring overland, you'll generally get about 1-400 tops until late May) then you've still at least got a shot at a supercell.

The reason we assume (I believe) that supercells need so much cape is simply because of how they tend to form in the USA. When they do form here (maybe 1-2 a year i reckon), they usually do so in the Spring under very high shear but reasonably low amount of cape.

(ps, be the way, the cape is far from the be all and end all, we've been under 2000kj and -4LI and yet seen nothing simply because the cap is just too much.

[Paul Sherman]

Yes Bangladesh has recorded Cape as hign as 11000 before and has the World Record for the Biggest Hailstones in the world, I think somewhere in the US A few years back had a Lifted Indice of -16

I would just settle for 1000 and -5 for the Uk, would certainly give us a good show, France has some great Cape and LI's over the Summer Months. Some parts of South Central France last year had Capes of 2500kg and LI 's of -8 couple that with temps at 92f and Dp'S OF 72F And you can see why France gets Supercells.

Paul Sherman

[kold weather]

In terms of the UK, and this applies mainly to the south and the Midlands, a high Cape total for here would be between 1000-1500kj, and a very high and pretty rare total would be above 2500. in a typical Spainish plume you'll be looking at cape totals between 1000-2000kj with lifted index down to -5/6.

[Cloudburst]

In terms of supercells, I personally don't think that much cape is needed,

(ps, be the way, the cape is far from the be all and end all, we've been under 2000kj and -4LI and yet seen nothing simply because the cap is just too much.

Yes the CAP can be strong, and you could see it as alwyas present (the boundary of Troposphere and Stratosphere). But regradless, where ever in the world you get the right moisture, instability, lift, and vertical shear, you have the ingredients for a Supercell. I disagree with having a supercell with less than say (at the bare minimum) 700 kj, as in order for the cloud height to be right (i.e gettng into the top part of the jet) you require more bouyent enery in the form of CAPE. If you got a supercell under about 100kj i would say it was very rare, as otherwise surely you would get them all the time every time the cape increased.? OK so other factors such as Helicity are important but without CAPE you cannot get the required upward motion. This may help:

:

Oh and ofocurse theres the bulk richardson number (equation).

Edited March 29, 2006 by ¤CloudBurst¤

[kold weather]

Indeed, your diagram is no doubt correct for the big powerful supercells in the USA. However in a technical sense, if nothing more you can have a supercell with very little cape indeed providing you've got the forcing in place as well as the shear, then cape almost becomes a small sideline factor when you've got all the other facotrs in place, thats the reason why supercells are rare in the UK.

Its also probably the case that you get different sizes of supercells. Over in the UK we tend to get the shallow supercell type when we do rarely get them as described by Bren J (from UKWW and TWO)

http://www.eots.co.uk/

(under reports, then the tornado+thunderstorm)

It's a intresting topic mind you Andy and one that has been debated many times.

[Paul Sherman]

What was Interesting last year on the Plains was the fact we got plagued by too many High Based Storms which failed to deliver Tornados. The Childress Storm maxed at 65,000 feet and Granted it produced about 5 spin ups (Nothing of Note) occured and many chasers (Stu Included) were saying at the time the Storms/Supercells were just too High. The same happened in Nebraska on 10 & 11TH May, these storms were struggling to rotate and quickly became HP, All we ended up doing was trying to miss the 3-5" Hailstones. The biggest producer on May 12th near Plainview last year was relatively low compared to the others at about 39,000 feet which was a Cyclic Producer.

Paul Sherman

[Nick F]

Yes the CAP can be strong, and you could see it as alwyas present (the boundary of Troposphere and Stratosphere). But regradless, where ever in the world you get the right moisture, instability, lift, and vertical shear, you have the ingredients for a Supercell. I disagree with having a supercell with less than say (at the bare minimum) 700 kj, as in order for the cloud height to be right (i.e gettng into the top part of the jet) you require more bouyent enery in the form of CAPE. If you got a supercell under about 100kj i would say it was very rare, as otherwise surely you would get them all the time every time the cape increased.? OK so other factors such as Helicity are important but without CAPE you cannot get the required upward motion. This may help:

:

Oh and ofocurse theres the bulk richardson number (equation).

I think the amount of vertical wind shear is often more important than CAPE availability for supercells to form. If vertical wind shear is high, then deep and persistant rotating updrafts can allow supercells to form, regardless of CAPE. About 15-20 m/s shear from the surface to 6km aloft is normally required for supercells to form, and the resulting high vertical velocities within updrafts often exceed what would be created by the release of CAPE on it's own, so CAPE is only part of the mechanism in supercells that cause vertical velocity.

[Cloudburst]

I think the amount of vertical wind shear is often more important than CAPE availability for supercells to form. If vertical wind shear is high, then deep and persistant rotating updrafts can allow supercells to form, regardless of CAPE. About 15-20 m/s shear from the surface to 6km aloft is normally required for supercells to form, and the resulting high vertical velocities within updrafts often exceed what would be created by the release of CAPE on it's own, so CAPE is only part of the mechanism in supercells that cause vertical velocity.

I get your point but, surely without CAPE it would be extremely difficult for air parcels to get to the height from which they could mature into a supercell?

Edited March 29, 2006 by ¤CloudBurst¤

[kold weather]

These low cape supercells in the Uk nearly always occured in a fairly chilly NW flow and in Spring as well. Generally you've got a reasonably chilly atmopshere above 900mbs and it cools quite rapidly from then on. With alot of sunshine that you get in that sort of airflow then you get reaosnably high temps on the ground. This result in some decent laspe rates and there is generally a fair amount of instablity (though not that much cape) and this along with the little cape and also the low convective inhabitation temp meant that it wouldmn't take much cape to get the cell deep enough.

What was key in the report on my last post was the very high shear at the lowest 150mbs. It's also worth noting that term "shallow" supercell which suggests that the supercell was smaller then those on the plains and therefore did not need so much cape to get going.

[Nick F]

I get your point but, surely without CAPE it would be extremely difficult for air parcels to get to the height from which they could mature into a supercell?

Supercell storms can form in environments where CAPE is as low 500 j/kg, but the over riding factor for their formation is a mechanism, such as high wind shear, for the development of the mesocyclone (or vertical rotating updraft) -which is the defining feature of a Supercell. The starting mechanism is often a cold front which moves into an environment of high dew points (Theta-W), and often where cold dry air overides high amounts of low level moisture - but high vertical wind shear is often the most critical part for the mesocyclone to form- as the vertical velocity created is often enough to lift warm and moist air aloft and it it prolongs the rotation of the mesocyclone.

[BrickFielder]

Hanstrum et al in there paper The Cool-Season Tornadoes of California and Southern Australia show that in maritime environments such as the UK with low Cape values and tendency towards low topped supercells that the critical factor for tornado development is low level shear. They identify these conditions as having tornadic potential.

negative 700hPa surface lifted index

near-surface convergence

surface–850 hPa (0-3km) shear greater than 11 m/s (22 knots 25mph).

I am not totally happy with these as predictors as it has been shown that high speed shear below 1000 meters tends to prevent supercell development while directional shear below 1000 meters enhances supercell development. I would use the following.

0 to negative surface lifted index

100J + Cape

0-500m directional shear present. (can be caused by hills cliffs etc)

0-500m speed shear < 3m/s (light low level winds)

500-1500 speed shear > 10m/s

I think the abundance of low level directional shear (i.e. we don't live on a flat plain) means in the UK we more likely to see tornadoes than in the US even though they are significantly weaker.

[Supercell]

Bangladesh would be a fantastic place to chase tornadoes. It's just a shame that the necessary means of touring around Bangladesh are inadequate to say the least.

Some unconfirmed reports of the inconceiveable F6 devastating the countryside with coconut-sized hail. The thought of the structure of the mesocyclone and the reach of the anvil of a spectacular beast of a storm enveloping the sky. Exciting times.

Edited March 30, 2006 by Supercell

[Paranoid]

How powerful are tornadoes in Bangladesh and Eastern India usually. am i interpreting the above post correctly that some people have reported an F6 in Bangladesh?

[Paul Sherman]

Possibly. But the Largest Hailstone recorded in the world fell in Bangladesh on 14th April 1986. It Weighed 2.25 pounds This Hailstorm killed 92 people at Gopalganj

Paul Sherman

[Paranoid]

Ouch, i wouldn't like to be hit by that. Until today, i thought the largest hailstone fell on Coffeyville, Kansas, which weighed around 750g.

Edited March 30, 2006 by Paranoid

[Thundersquall]

Remember that fantastic night-time display that occured last year?

It was set-off as a cell moved northwards from France and was very impressive.

It gave Bristol a brilliant electrical show and reports of damaging hailstones.

Further north, very heavy rain formed as the cells exploded in size very rapidly.

The CAPE, I remember, was much lower than would usually be expected for such a storm (around 800 or so).

Edited March 30, 2006 by Thundersquall

[Paranoid]

I think i remember that, saw about 20 flashes before i heard a bang. Very strong gusts of winds and the rain ripped loads of the cherry blossoms off the tree outside my house.