[Tony Gilbert]

Posted 7.20pm Tues

Slight Risk of Isolated Thunderstorms 09Z-21Z Ireland & Central UK Regions

Confidence Level; Medium

Low remains just NE of UK in quasi stationary state. Long wave upper trough moves east generating lift and increasing lapse rates. Strong jet stream with a west to east axis increases upper support to convection building closer to the southern sector of the convective box. UKMO currently forecast a surface wave follow by post frontal troughing later in the day.

WRF much less confident than GFS ATM. GFS builds strong instability as lapse rates increase through the day with up to 900 J/KG predicted. Front left exit region of the jet stream look divergent and should increase lift for a time primarily central UK. A number of mid level PVA max's look to run along this diffluent zone increasing upper dynamic lift further. GFS also predict a layer of dry air above the boundary layer where cooling of cloud tops may be enhanced for a time. Low level speed shear somewhat limited but certainly some veer along a line from N.Wales through to the Wash. There may be some scope for a tornado risk in association with the surface wave though this will need to be assessed on Weds.

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This post has been edited by Tony Gilbert: 17 July 2012 - 18:34

[Tony Gilbert]

Above title should read Weds 18th July. If a moderator could please edit this. Thankyou

[Sam Jowett]

The Hirlam charts at lightningwizard.com certainly show some quite strong potential across central England, including shear profiles capable of supporting mesocyclone and tornadogenisis, together with the passing diffluent left jet exit. Looks a pretty potent setup really across much of Wales, central England and the RoI with multicell/supercell development possible and therefore a potential for large hail (supported by the drier air aloft) and a tornado or 2. Instability should be released with modest surface values of 18/14, which look comfortably achievable given the moisture already in attendance over the region. I'd have expected more buzz with this setup really... am I missing something?

Should be some singlecell storms across N England and S Scotland too, where shear profiles are much less conducive to severe weather...

[Sam Jowett]

00Z model runs appear to maintain the potential for severe storms today, primarily N of the M4 corridor to a line from N Wales to central Lincs, so little really to add to last nights comments. Getting sufficient brightness for storm initiation will likely be key, though as noted, trigger temperatures and dew points are pretty modest really...

[Ian Williams]

Yes, I quite like the look of Leicestershire, Nottinghamshire, Rutland and especially Lincolnshire, and possibly north east Anglia for a few funnels looking at the helicity values. Surprised Estofex has no storms forecasted though at this present time.

[Tony Gilbert]

Posted 8.40am Weds

As per earlier forecast with additional risk;

Slight Risk of Tornado Development 13Z-21Z as per red box (experimental)

In reality there is a small risk of a brief funnel/ tornado anywhere within the white box today. Though particularly so for boxes marked in red. 'Experimental' because model resolution in general are not able to pin point such small zones. Though having said that I have basically overlayed specific conducive conditions from GFS to WRF in line with the overlapping periphery of the upper jet to create a more concentrated risk zone. This target is based on what the models are predicting now but will obviously be subject to change.

Spatial coverage today look less than earlier forecast. So whilst the south and north will see heavy rain, any convection looks tied into central regions only. Many places will stay dry today. Though where storms develop we should see some multi cell development for a time. Possibly an isolated supercell just south of the Wash by around 4pm.

The overriding restrictive factor today looks to be generally weak dynamic forcing at surface and only restricted DLS where instability is present.

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This post has been edited by Tony Gilbert: 18 July 2012 - 08:02

[PaulKn]

TORRO TORNADO WATCH 2012/005

A TORNADO WATCH has been issued at 0800GMT on Wednesday 18th July 2012

Valid from/until: 0800 - 2000GMT on Wednesday 18th July 2012, for the following regions of the United Kingdom & Eire:

Parts of (see map)

Wales

Northern parts of S England

Midlands

E Anglia

Southern parts of N England

THREATS

Tornadoes; hail to 45mm diameter; wind gusts to 60mph; CG lightning

SYNOPSIS

A cold front will move slowly eastwards across the area today with waves likely to run along it. It may well take on a somewhat split characteristic across northern parts of the WATCH area, with the surface front lagging behind mid-level cooling/drying. Reasonable instability of 300-800 J/Kg is expected in the moist airmass, especially where sunny spells develop. With 500 hPa winds of around 50 knots, and some low-level veering, shear is expected to be sufficient for organised convection, including the risk of supercells.

With the main forcing coming from both the cold front and shear vorticity along the northern side of a mid-level speed max, and the fact that both of these mechanisms will be reasonably parallel with the mean tropospheric flow vector, convection may well organise into several bands orientated WSW to ENE. Small waves running along these bands may allow rotation to develop, including embedded supercell structures. In addition, some more isolated storm activity may develop too, including supercells.

Given a moist low-level airmass and fairly low LCLs, and reasonable low-level shear, a few tornadoes are possible. In addition, any supercell which develops will bring the risk of reasonably large hail, perhaps up to around 40-45mm in diameter. Strong winds and CG lightning are also possible.

Please note that this has been issued early due to commitments which means that monitoring of the developing situation may not be possible today.

Forecaster: RPK