[Tony Gilbert]

9.30am Sun

A bit early to make any accurate forecast but worth a heads up for Tues Ireland and UK.
A double whammy for convective potential with the chance of an early morning squall with potential for severe gusts and a post frontal thundery regime for the remainder of the day. A very moist mild air lokks to advect north ahead of Tues. Also my attention is particularly drawn to the predicted sharply divergent upper jet stream. If the models keep the front left exit region in line with the cold front then we could have the potential for very strong and deep lifting dynamics for southern UK. ATM UKMO Fax chart shows a sliding triple point. If the TP slides south too early we might be left with a rather messy bag of wind and rain along the occlusion.
The post frontal troughing looks to bring scattered thundery showers across much of the UK. A marginally sheared environment could well see forward leaning anvils with the potential for one or two storms to become quite strong for a time.
There is currently some variation in the models with some pointing to the jet dropping south over the continent and away from the UK.
Anyway certainly one to keep an eye on!

This post has been edited by Tony Gilbert: 15 April 2012 - 08:46

[Tony Gilbert]

4pm Mon

Risk of Squall line developing 00Z-12Z Southern UK (Moderate gusts possible non severe)

Moderate Risk of Scattered Thundery Showers UK & Ireland 12Z-21Z

Moderate Risk of a Severe Thunderstorm Devon, Somerset, Dorset, Hants 12Z-18Z

Based on the GFS and NMM model we have a good chance of an embedded well defined squall line developing across the southern regions of the UK. Though with relatively weak low level shear we might only expect some moderate level gusts of around 20kts. Though having said that there is a chance that the squall may become more convective as the mid level dry incursion overruns the surface lifting dynamic from central regions of UK westward. A mean average calculation for true convective gust would then be around 40kts!

Later in the day we can expect some post frontal troughing to initiate a number of scattered thundery showers across the UK & Ireland. Particular attention is given to a zone central southern UK (red box) where a strong upper divergent jet stream looks to enhance any building storm within this sector. Vertical shear will be sufficient for the development of a severe thunderstorm capable of producing large hail and possibly a tornado. Strongest vertical shear will exist within the mid to upper levels with the potential for mid level mesocyclone development. Any sustained rotation will have the potential to hold hail aloft for longer and create a full separation of updraft to downdraft. Whilst there would appear to be weak low level shear in place the potential for RFD could augment inflow into the updraft and hence a defined risk of tornado development will be possible.

If the forecast remains as is then I will be likely to take the afternoon off to monitor this development out on the road. I will start a separate thread regarding others able to chase within the red box!

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This post has been edited by Tony Gilbert: 16 April 2012 - 15:13

[Astromadhatter]

Thank you for the outlook for Tommorow. Sadly i have no car till the middle of may so cant chase:(, but i will be keeping a very good eye on whats happening here in bournemouth. I look forward to your updates:).

[Tony Gilbert]

Astromadhatter, on 16 April 2012 - 18:31, said:

Thank you for the outlook for Tommorow. Sadly i have no car till the middle of may so cant chase:(, but i will be keeping a very good eye on whats happening here in bournemouth. I look forward to your updates:).

Too early to pin point any particular location though steering winds look to run WNW to ESE so any activity will probably initiate inland and move past to your north toward Southampton or that region, maybe! I won't start a thread on storm chasing because I cannot guarantee what time I can get away from work.

[Tony Gilbert]

UPDATE 8.24 am Tues

As per earlier forecast though with Severe Thunderstorm (red box) cancelled.

Updated models increase 500mb humidity from 40% to 90% this will likely have the net effect of decreasing potential instability within the mid levels. In short, the air temp at this level will be slower to cool as the upper trough moves eastward. Hense any upward moving parcel will likely achieve less buoyancy!
In addition to this 500mb winds have decreased from 45kts down to 30kts. This combined with 850mb winds increasing from 22 kts to 30kts will have the effect of dramatically reducing vertical shear within the mid levels of any storm cloud. Hense mid level mesocyclones now look unlikely.
Ironically, the increase in 850mb shear will substitute the low level shear which has now increased and would normally favour tornado develop except that I'm not now sure that convective cloud updrafts will be sustained long enough to support such development.

All in all the location of the originally red box should still support an isolated stronger storm cell or two later today but with a lesser risk level.
Based on current model output there looks to be a line of convergence possible from Yeovil through to Winchester later this afternoon.

This post has been edited by Tony Gilbert: 17 April 2012 - 07:35

[PaulKn]

TORRO CONVECTIVE DISCUSSION 2012/001

A TORRO CONVECTIVE DISCUSSION has been issued at 1200GMT on Tuesday 17th April 2012

Valid from/until: 1200 – 0800GMT on Tuesday 17th/Wednesday 18th April 2012 for the following regions of the United Kingdom & Eire:

Cent and S Eire

Wales

Midlands

E Anglia

S England

Channel Islands

THREATS

Isolated tornadoes; hail to 10mm; CG lightning; heavy rain; gusty winds

SYNOPSIS

Deep upper low and associated occluded surface low will move from west of Ireland to S Wales in this forecast period. An unstable rPm airmass will be in place across the area. Reasonable shear this afternoon (0-6km around 30 knots; 0-1km 15-20 knots) along with a slightly unstable atmosphere should allow some organisation of cells into short lines, and perhaps some brief rotation within the more robust updraughts. Hail up to 10mm diameter and wind gusts to 45-50mph are possible, along with a low risk of one or two brief tornadoes. CG lightning is also likely.

Overnight, the low will move towards Wales. A tightening gradient on its southern flank will increase the chance of some 50mph gusts across SW England, and the increasing low-level flow to the east along with some backing may cause the tornado risk to continue along and just inland from the south coast (dotted area). The remainder of the area is expected to see reduced instability, and with much reduced shear, any showers should be disorganised.

Forecaster: RPK.

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This post has been edited by PaulKn: 17 April 2012 - 12:08