A moderate risk now of some significant rainfall totals in some areas as a disturbance associated with the low to the SW moves across the UK during Thursday.
This system has some residual ex-tropical air entrained in it's circulation, characterised by high PWs and theta-e values up into the 48 or 50 range which is indicative of a very moist and potentially unstable airmass.
There remains some doubt over the location of the heaviest rain but WRF and lately NAE are delivering in excess of 30mm every 3hrs in places which if realised would cause local flooding issues.

Best guess is the SW and up into S Wales during Thurs and then to the NW later. Convective indices would also deliver deeper cells with attendant thundery activity.

Originally Posted by: nsrobins 

The PW is the precipitable water and is a measure of the amount of moisture in the lower atmosphere. The values are normally including in Skew-t sounding forecasts. The higher the value, the more likely it is for large amounts of rain given other conditions. Roughly I look for values of around 2.00cms as a base level. The sounding for Devon at 18Z Thurs shows a PW of 3.55 which is very impressive and indicates a moist, rain-laden lower atmosphere. With theta-e and some thermal heating lifting this air, and a comparison with modelled pecipitation, heavy rain would be in my forecast.

Theta-e values are a guide to instability. It is defined as the temperature a parcel of air would have to reach for all the water vapour in that parcel to condense out. In stable conditions this value increases with altitude, in unstable conditions it decreases with altitude. If you look at the chart for 18Z Thurs the theta-e at 850HPa is around 50C - higher than normal. At 700Hpa it is around 47C, indicating a reasonably high risk of convection and instability from an already raised state.

Originally Posted by: nsrobins 

Thank you for explaining this nsrobins Neil, it is very much appreciated I'd better get the pumps out

Originally Posted by: Osprey 

Originally Posted by: Osprey 

Heavy rain should start just after midday in this area...Glad to be on top of a hill..... 

Originally Posted by: ARTzeman 

Originally Posted by: Osprey 

The PW is the precipitable water and is a measure of the amount of moisture in the lower atmosphere. The values are normally including in Skew-t sounding forecasts. The higher the value, the more likely it is for large amounts of rain given other conditions. Roughly I look for values of around 2.00cms as a base level. The sounding for Devon at 18Z Thurs shows a PW of 3.55 which is very impressive and indicates a moist, rain-laden lower atmosphere. With theta-e and some thermal heating lifting this air, and a comparison with modelled pecipitation, heavy rain would be in my forecast.

Theta-e values are a guide to instability. It is defined as the temperature a parcel of air would have to reach for all the water vapour in that parcel to condense out. In stable conditions this value increases with altitude, in unstable conditions it decreases with altitude. If you look at the chart for 18Z Thurs the theta-e at 850HPa is around 50C - higher than normal. At 700Hpa it is around 47C, indicating a reasonably high risk of convection and instability from an already raised state.

Originally Posted by: nsrobins 

All kicking off nicely over Biscay and W.France. Looks like 2 MCS's have developed with a 3rd forming across Spain.

Originally Posted by: Sevendust 

Yep, could be interesting morning for our friends on the S. coast if these systems can maintain their integrity.

Originally Posted by: Sevendust 

I think it will hold together alright. The most interesting part is the intenseness of the precipiation though it does appear to be moving through any one place quite quickly so after a couple of hours it should quickly improve with a second phase later today and tonight.

Originally Posted by: Sevendust 

Fair assessment Martin. Still some divergence where the heaviest rain will fall but the first warm-frontal induced ppn is now in real-time, with some embedded intense cells involved and recent sferics being detected.
Another area moves in after dark, this likely heaviest in the CS region and into the London area later.

The WF system is characterised by the theta-e bulge I described yesterday, although it does look like the activity will be mostly elevated in nature.
The second system however is rather more interesting as more surface based cells may be involved, with attendant risk of increased lightning and with shear parameters strengthening I wouldn't rule out some rotational elements in the mix,

Originally Posted by: Sevendust 

Looks like they've got the timing all wrong again. Some very heavy stuff nudging the south now though.

Originally Posted by: Matty H 

Are we likely to have this here as well Neil?

Originally Posted by: Osprey 

As it stands the greatest threat now for you is from the first system now entering the Channel.
The second potentially more lively area looks set to affect IOW eastwards and then towards London and EA.

Originally Posted by: Osprey