[PaulKn]

There are some suggestions that weak buoyancy may develop just ahead of a cold front, which is expected to surge eastwards overnight tonight and through tomorrow morning. Strong wind shear will accompany this feature. From portions of Eire, across to Wales, central and northern England, there appears to be the chance of line convection developing, with an attendant tornado risk.

Further south, as the line moves south-eastwards tomorrow morning, shallow line convection may continue.

[John Mason]

Fax charts suggest the cold front will be approx from Berwick-Lancaster-Bangor-Fishguard at 0600 (=0700 BST) tomorrow (Tuesday).

Cheers - John

[Sam Jowett]

Is convection on the cold front across Wales and the Midlands worth taking note of during tomorrow morning? The charts at http://www.lightningwizard.com/maps/ intimate any convection will be able to take advantage of some powerful shear conditions. Strikes me as the kind of conditions that can spring a rather strong and fast moving tornado or 2, particularly as the polar airmass crosses the relatively warm Irish Sea...

[Tony Gilbert]

9.20pm Mon

Moderate Risk of Severe Convective Gusts Wales, Central and Eastern Regions 06Z-18Z

Slight Risk of Isolated Short Tracked Tornadoes UK as per red box

Risk of Thundery Showers later in period as per yellow box

Sharpe and probably unseasonal 'Ana' cold front expected to develop within the wake of a weaker upper feature by early morning Tues. UKMO fax charts show a typical rapid forward progress with streamlines at an acute angle to the orientation of the frontal boundary. Models show very moist air lifted abruptly upward and into the upper layer of the atmosphere with a dry incursion lower down at 700mb. Strong subsidence looks to follow on behind the cold front where further strong gusting is likely to develop. These conditions are typically associated with 'ana' type cold fronts. Convective gusts are calculated with a maximum strength of around 60kt where some minor structural damage may occur within the strongest embedded convective cells.

There will also be a slight chance for an isolated brief tornado event or two between segments as per red box. This risk zone is calculated based on the likely occurrence of strong dynamic lift close to the baroclinic zone close to surface where forward rolling vorticity can be lifted into the vertical where updrafts are strongest. The nature of this type of development puts the strongest shear values low down within the storm cell and hence any low level mesocyclone development is likely to be short lived but quite intense. The red box ATM suggests the potential for these conditions to come together more efficiently than the broader risk zone assessed (white box)

Worth remembering that the prime risk for Tues is born directly from the energy within the lowest layers with cloud tops not necessarily that high or even needed to be high to develop severe weather. Hence the risk of thunder, apart from region just east of Irish Sea look to be rather sparse. Though can't rule out the odd sferic!

[Sam Jowett]

I'm surprised how localised you've shown the tornado risk Tony? I'd have thought a risk was present across Wales between 06-09Z and Central England N of the M4, S of a line from the Wirral to the Humber between 09Z-12Z... at least this is the timings indicated on Lightning Wizard. Dynamic forcing looks to weaken a bit as the front moves E across England and convective forcing ought to be strongest near the sea in these conditions. How come I'm reading things a little differently to you?

[PaulKn]

TORRO TORNADO WATCH 2011/007

A TORNDAO WATCH has been issued at 22:40GMT on Monday September 5 2011

Valid from/until: 22:40 - 17:00GMT on Monday September 5/Tuesday September 6 2011, for the following regions of the United Kingdom & Eire:

Parts of (see map)

Eire

N Ireland

England

Wales

S Scotland

THREATS

Tornadoes; wind gusts to 65mph; occasional CG lightning

SYNOPSIS

Upper low will approach NW Scotland through the period, slowing and deepening as it does so. At the surface, a well-marked cold front will move aross the British Isles, with strong warm advection just ahead of it. Strong 500 hpa height falls /up to 10-11DAM in 12 hours/ will overspread the cold front, promoting lift. Despite very moist profiles, some steepening of lapse rates is likely aloft, especially as the front splits and drier air overspreads the moist zone ahead of the surface front, increasing potential instability.

As the upper low slows, the front will also tend to slow somewhat as the mean tropospheric flow backs. This will serve to increase low-level convergence along the front, especially as it moves across England and Wales later tonight and through Tuesday. Indications are that a LEWP type structure may develop. Strong low-level wind shear /0-1km shear of >30 knots/ and low LCLs, combined with -ve LIs at 700 hPa and up to 150J/Kg of CAPE suggests that tornadoes are possible. Whilst the overall risk seems rather widespread, an enhanced risk, perhaps with several tornadoes, appears likely to extend from Wales, into the Midlands and E Anglia, along with parts of Cent S and SE England late tonight and through Tuesday.

Much of the LEWP activity may be low-topped, with little lightning present. However, there may be a higher chance of lightning across parts of Wales and N England for a time, and it cannot be ruled out elsewhere.

Forecaster: RPK

[Tony Gilbert]

Hi All,
been trying to update this thread since early this morning but was completely shut out like many other members during the transition to the new forum layout. Let's hope things settle down soon. ATM I still have very limited posting capability.
Today's risk was very much less interesting by this morning and the models had shifted to the negative in many respects. Though sadly I was unable to update this situation! I'm also currently unable to view any images within any forum?
PS. I do understand that these transitions can be awkward at best!

This post has been edited by Tony Gilbert: 06 September 2011 - 20:03