[Tony Gilbert]

8.45am Tues

Slight Risk of Thundery Showers Central & South Central UK 15Z-19Z Weds 23rd May 12

A bit early to be sure ATM but worth a heads up at this point.

UKMO fax chart forecast weak surface troughing along a north south central line of convergence. Synopsis shows this feature sandwiched between an Atlantic low and a weak surface depression over the continent with strong ridging east of Norway. With clear skies and warm advection from the south we might expect to see a number of isolated thunderstorms develop by late afternoon. Moisture looks somewhat lean ATM though the prime trigger looks to be surface convergence!

A note is made regarding Sea Breeze Convergence Zones;- GFS suggests up to 23 deg C possible inland. Conversely the SST are a mere 10 degC ATM . The net effect will likely develop a number of convergence zones along the coast where the models predict an offshore vector (primarily south coast).


Will update later tonight!

This post has been edited by Tony Gilbert: 22 May 2012 - 08:07

[Chris Alder]

Tony, what do you make of the DP's currently predicted by GFS? They look to be significantly over-egged at 18-20C across most southern England tomorrow and Thursday?

[Tony Gilbert]

Chris Alder, on 22 May 2012 - 08:44, said:

Tony, what do you make of the DP's currently predicted by GFS? They look to be significantly over-egged at 18-20C across most southern England tomorrow and Thursday?

Hi Chris. Today has seen 16 degC dew points in some central regions so 18 deg C DP's are not out of the question for tomorrow. Though TBH anything less will likely cause the CAPE to mix out into the atmosphere! Strong capping will exist for eastern regions according to GFS. NMM ATM sees very little or no convection, though both WRF and GFS maintain convective development late in the day. With 1000 J/KG CAPE possible the outlook could go off with quite a big boom if convection can get going! Moisture remains a concern and I would currently rate the risk potential at around 50/50!!

This post has been edited by Tony Gilbert: 22 May 2012 - 15:43

[PJB]

The KNMI Hirlam breaks out showers from the Chilterns through to the Pennines and Southern Uplands of Scotland. This implies that temp and DP along with convergence and uplift from high ground will be required to trigger convection and only those areas where the additional lift is provided for initiated by the high ground will see the forced convection.

NAE is even more limited with really only isolated showers / storms over the PEnnines and into S and then Central Scotland.

GFS has more in southern england, but its bias towards slightly higher dewpoints than reality probably explains that.

paul

[Tony Gilbert]

Yes, forced accent by topography may well trigger some storms on Weds. Though, I have noted that some models seem to be more sided toward this type of convection than others. Based on the three model examples as per above it would appear that outside thePennines the best chance of a trigger might well occur within surface moisture convergence! The best convergence probably based on an west to east Sea breeze convergence line just inland from the south coast!

Over the few years that I have monitored similar various model output I have seen a discrete pattern under this type of synopsis, whereby GFS will often initiate convection too early and others under estimate the power of surface convergence! ...I suppose it all pretty much relates to a mixed bag of information and as to which model we may lay better weight with!

This post has been edited by Tony Gilbert: 22 May 2012 - 18:55

[Tony Gilbert]

8pm Tues

As per earlier forecast with slight risk of Thundery Showers within the white box as per map.

Moisture level whilst lean would seem to be workable. This is also reflected within the level of predicted dew points. Prime risk looks to be small hail with a chance of a funnel cloud/ weak tornado report. Such reports may occur anywhere within the span of the white box but an enhanced risk zone marked in red is added as per map.

Overal outlook for good convection for me remains just 50/50 ATM. It is noted that a dry intrusion from 700 mb upwards is likely to move eastward through the period increasing potential instability. GFS predicts vertical accent up to 300 mb which would increase the potential for sferic activity. Particular attention is given to the potential for high surface temps near relatively cold seas. The resulting sea breeze convergence could well be strong and hence an additional risk for FC / weak tornadoes may become a real factor here for a time primarily Hants through to East Sussex.

Attached thumbnail(s)

• 

This post has been edited by Tony Gilbert: 22 May 2012 - 19:12

[Sam Jowett]

Hmmm, looks pretty tight. Hirlam suggests parcel depths of 1500m and applying that to a GFS forecast sounding indicates trigger temperatures in excess of 25ºC will be needed in the absence of any forced ascent. With the lack of decent dynamic forcing, I'd expect the odd isolated shower/storm at best. Given forecast soundings show a rather dry atmosphere aloft from the S Midlands southward, I'd expect the greatest risk across the Pennines where the topographical lift and synoptic convergence can combine forces. Whilst wind shear isn't great, around 10-20 knots is available between the top and bottom of the cloud layer where a weak easterly is found at the base of the cloud layer and a weak westerly towards the top, could be enough to separate updraft and downdraft enough to allow a quasistationary mulitcell in a favoured spot.

[Tony Gilbert]

Sam Jowett, on 22 May 2012 - 19:26, said:

Hmmm, looks pretty tight. Hirlam suggests parcel depths of 1500m and applying that to a GFS forecast sounding indicates trigger temperatures in excess of 25ºC will be needed in the absence of any forced ascent. With the lack of decent dynamic forcing, I'd expect the odd isolated shower/storm at best. Given forecast soundings show a rather dry atmosphere aloft from the S Midlands southward, I'd expect the greatest risk across the Pennines where the topographical lift and synoptic convergence can combine forces. Whilst wind shear isn't great, around 10-20 knots is available between the top and bottom of the cloud layer where a weak easterly is found at the base of the cloud layer and a weak westerly towards the top, could be enough to separate updraft and downdraft enough to allow a quasistationary mulitcell in a favoured spot.

Hi Sam, yes pretty much 50/50 as mentioned above though not sure if vertical shear will have any significance for weds whatsoever. We need to remember that similar outlooks in the past when and if they develop will be based primarily on zone of convergence or topographic lift. At best we can only expect to see pulse type storm activity. Though ironically, whilst hard to predict where each storm will initiate; if you are in the right place at the right time then you are likely to see a vertically building congestus over maybe a 45 minute period before it collapses inward upon the updraft. If there is a confluent vector at surface at this time then maybe even a funnel or two.
PS. Regarding the dry atmosphere aloft it can only serve to assist any rising parcel above its LFC if sufficient moisture exists within the boundary layer

This post has been edited by Tony Gilbert: 22 May 2012 - 20:40

[Sam Jowett]

Yes, point taken... so long as entrainment of that dry air doesn't stifle the updraft...

[Tony Gilbert]

Sam Jowett, on 22 May 2012 - 20:50, said:

Yes, point taken... so long as entrainment of that dry air doesn't stifle the updraft...

This is true, Sam. If the boundary lay depth is limited and the dry air is ubrupt just above the boundary layer the updrafts will merely mix out.

[Tony Gilbert]

8.50am Weds

As per earlier forecast though risk for convection across central south UK would seem to be only Slight to Marginal.

Hirlam, WRF, and NMM continue to show limited or no convection breaking out across southern UK. Whilst in complete contrast GFS maintains a strong nucleus for convection.
The GFS is renown for over estimating dew point/ CAPE potential though IMO it would be counter productive at this stage to remove the risk potential for the said region.

Probably best to now wait and see what the result are. Though overall opinion is currently low!

[Sam Jowett]

A quick lunch break scan of the setup looks more promising than I was last night if I'm honest, partially because I'm currently recording a 16.4ºC dew point in the central Midlands. I haven't time to be particularly thorough, but at the very surface at least, the GFS forecast isn't a million miles away from reality (though this doesn't mean the SMZ is deep enough to support the convection GFS shows). DMC will still require a trigger temperature of around 25ºC and a lifting mechanism to get things started, but makes me more confident of something being triggered over (particularly the southern) Pennines. Tops to 250hpa/-50ºC is amongst the deepest potential this year...

[PJB]

Anything will be quite isolated due to the lack of moisture in the middle / lower layers , esp 700-800mb, it will need focussed ascent from both convergence and high ground ascent to trigger much.

[Tony Gilbert]

Notts, Camb and Herst 12z skew-t soundings show a stout cap at low level. This could well be the main restriction today.
Regions subject to orthographic lift may will able to pass through this layer!