[Tony Gilbert]

Posted 10.10am Sat

Slight Risk of Scattered Thundery Showers UK & Ireland Sun 29th July 2012

Confidence Level; Medium

A fairly low risk outlook with regards to severity, though certainly worth an early heads up for any would be chaser!

Weak surface low over Scotland remains quasi stationary throughout period. Occlusion and some surface troughing expected to initiate a broad rash of scattered convection from quite early on in the day. Some showers could well become thundery as surface heating steepens low level lapse rates. Relatively deep short wave upper trough overspreads much of UK and Ireland generating upper lift. Limited surface moisture would tend to suggest quite an isolated scattering of showers with many parts staying completely dry. A relatively weakly sheared environment with straight winds likely at all levels. Though ATM the WRF model is picking up on a line of convergence along the South Downs.

So all in all too early to be specific about favourable regions. Though worth noting that the expected reduced surface humidity values will likely develop clear glaciated congestus with classic charcoal underbellies which could be particularly photogenic. Cloud tops expected to reach 350mb with very little CIN. So a few classic anvils should be able to develop later in the day on the larger convective cloud.

PS. Being Sunday there may be some potential for chasers to arrange a meet up at certain points. If the outlook remains promising I'll start a separate chase thread where members at favourable locations can PM their mobile numbers to each other!

This post has been edited by Tony Gilbert: 28 July 2012 - 09:33

[Tony Gilbert]

Update 7.40pm Sat

As per earlier forecast.
Noted unexplained intermittent loss of data from NMM, GFS and WRF over the last 24 hrs. That aside, and based on the brief detail between these blackouts there would appear to be some moderate level for some thundery activity through Sunday.

A broad scope for isolated pockets of convection for most of UK & Ireland. Not easy to isolate zones for better convective potential though have been able to make some indication as per yellow box for stronger risk of isolated thundery showers. The overall prospect of straight lined winds on a vertical profile will likely limit the outcome to no more than GCs and small hail reports. Prime conditions at play would look to be sharp low level lapse rates, increased potential instability and steering winds that can reduce the risk of cold pooling at surface!

On a local scale for me there would seem to be some defined potential around Winchester by 1pm moving ENE toward London later in the convective period.

I have started a chaser thread for those that may be interested http://www.ukweather...chasing-sunday/

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This post has been edited by Tony Gilbert: 28 July 2012 - 19:07

[Tony Gilbert]

Update Posted 8.30am Sunday

As per earlier forecast though with risk now increased to MODERATE for yellow box zones.

Models remain fairly consistent with this one. Already some isolated sferics detected Bristol Channel. Surface trough southern UK counties expected to develop further thunderstorms though indications are now that these will more likely develop into clusters as opposed to the scattered radar returns we are currently seeing to the west. Some good clear skies between showers looks to maintain some good solar heating today. With a relatively low freezing level we can expect to see numerous reports of small hail and convective gusts calculated at around 20kts. A marginally sheared environment should allow for storms to climb to the tropapause creating anvils. The relatively sustained ascent looks to allow for strong outflow locally as storms collapse initiating further activity to the southern side where new storm cells will develop. This shows up nicely using the WRF convergence parameter. Winds look to remain straight lined today so not sure about funnel activity. Though having said that it is quite possible for us to see the odd FC where storm outflow interacts. The best chance for this is on new storm development toward the south of any larger decaying storm cell.

Further north; N.Ireland and SW Scotland can expect thundery showers today though with less shear available. Prime risk small hail.

A chase thread has been started. Experimental red crossed markers might indicate good starting positions for wud be chasers today.

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This post has been edited by Tony Gilbert: 29 July 2012 - 07:52

[Matt D]

Pronounced line of heavy showers now extending from Gloucester to Plymouth (close to surface trough) and this will extend eastwards across the rest of S England this afternoon. Small clusters of scattered heavy showers already breaking out ahead of this over Cent S England and pushing ENE towards London.

Good chance of a few lively thunderstorms along A303 corridor into London and East Anglia this afternoon, ahead of this main trough. Fewer showers along the south coast this afternoon as main action pushes inland. Will be keeping an eye out from the London/N Downs today