Convective Outlook Spring 2008 (Technical)

[Tony Gilbert]

A small risk of thundery showers for England and Ireland through Sunday.

Unseasonal mild airstream from north of Azores is funneled across the UK and Ireland. Depression central Atlantic and west of Ireland allows for this transportation. Some clear sky's expected to induce solar heating through the day. Convection can only occur as a direct result of this process. GFS look to base moderate CAPE on these values alone. MMN do not see this, or at the very least is very restricted.

Ireland's convection is derived via tightly wrapped occlusion whereas UK's is the development of a small surface trough. This is a slack regime and at best can only produce pulse type cell development. But given the potential for surface heating ahead of the trough we do have a small chance of the odd sferic on taller building cells. Prime location for the UK looks to be from a line from Oxford through to Essex late in the day.

Update later this evening

[Paul Sherman]

Many Thanks for that Tony. This is one I will be out and about Chasing tomorrow for sure, not messy in any way and some good Storm Photography should be on offer.

Paul S

[A.J]

nice spherics showing up over SE Ireland

http://www.isleofwightweather.co.uk/live_storm_data.htm

confirmed on NW Extra radar

[Tony Gilbert]

8.25pm Sat

MMN and GFS remain in stark contrast, as does the UKMO synopsis. Current thinking would suggest GFS is rather over bullish with CAPE values given the obvious seasonal negative indices!

A balanced view based on both models might continue to suggest a small risk of some convective showers SE regions late in the afternoon. I no longer hold with any sferic activity. The variation in model output and synopsis is rather jumpy to say the least.

Best way to deal is to await mornings model update IMO

[Nick F]

Yes, some differences in model guidence wrt to convective potential over England tomorrow post cold front. GFS seems rather ott with its CAPE progs given the airmass and tempertures/dew points expected.

UKMO suggests a post frontal upper trough on 0600 and 1200 fax charts following behind cold front in the morning - 12z UKMO meso shows a line of convection moving east across Sern England in the morning immediately behind cold front:

... after that, unstable SW to Werly flow establishes with steep lapse rates ... UKMO suggests most heavy showers over Ireland, western and central England and western Scotland during the afternoon with a few getting further east.

GFS indicates heavy showers breaking ou across southern and western areas of England, Wales, western Scotland and Ireland during the afternoon - in reponse to steepening lapse rates from 500mb cold pool and upper trough arriving aloft and some surface heating in the strong sunshine:

Some strong deep layer shear available in the morning across England - courtesy of a strong jet streak at 300mb moving east in the morning ... this may produce bowing line segments/squall lines with any convection ... LL shear and helicity looks lacking though - so tornado posibilities look v. small.

Main threat will be strong wind gusts and hail as drier colder air arrives aloft behing upper trough. A few isolated weakly electrified t-storms aswell no doubt

Think I might be out and about locally tomorrow for some possible photo opps, probably up on the heathlands of Ashdown Forest where there are panoramic views over the Weald.

[Tony Gilbert]

Hi Nick,

a classic example as to the limitations of the current forecasting systems models understanding!.... A long way to go me thinks!

Yes, major back track IMO. Prime mild air advection looks in place but recent models drop dew point ratio to expected temps pretty dramatically. At best rather elevated convection likely now!...I keep looking for some surface convergence but maybe this is not to be. Overal outlook based on the most recent model update is even more restrictive.

The models are in disagrement best wait till morning me thinks

Edited March 29, 2008 by Tony Gilbert

[Nick F]

Hi Tony, I agree - best to see what the morning brings.

With no apparent forcing feature such as a trough or front in the unstable SW to W flow following behind the post frontal upper trough clearing the east/SE in the morning - it is difficult to suggest any areas likely to see the best oppurtunities of seeing t-storms tomorrow afternoon - and the models will always have difficulty pintpointing areas at this range in an unstable environment with no organised forcing - hence the divergence.

[Tony Gilbert]

UPDATE 9.12am Sun

Both MMN and GFS pretty much in agreement now regarding convective potential today. UKMO synopsis no longer sees the development of any surface trough for the UK. The occlusion over Ireland is rather de coupled from the projected CAPE values.

The original outlook was based on the potential for solar heating today. Current thinking would still suggest there is a small chance of true convection occurring if surface temps can rise under clear sky's. Though rather a big IF!

Probably still worth a look in on the 06Z model update!

[Nick F]

Looks like some general heavy showers will pop-up by early afternoon across many areas of the UK, with clearer skies in the west now, apart from eastern areas where it looks to mostly remain dry with the odd isolateed shower. Lack of forcing may mean showers are well scattered - with the only thing aiding their development being sfc heating. However - there does appear to be a trough currently over SW Ireland, looking at satellite and current synoptic analysis, which looks to arrive across Wales and NW England later this morning - which could organise heavy showers here and perhaps produce the odd sferic or two.

Vertical wind shear is now weakening from the west as the jet streak shifts east away from the UK - so any severe weather associated with convection today looks very unlikely.

[MP-R]

There were some hefty downpours showing up in the Irish Sea earlier on this morning at around 7am and seemed to be intensifying as they moved eastwards but since then they just seem to be fizzling out as opposed to bubbling up. Is that going to the case for the rest of the day?

[richarddx7]

OK can someone fill in the situation a bit for me here, there is about up to 500j/kg CAPE and -2/-3 LI hanging over much of England right now but in the vast majority of places very small cumulus humilis at best.

Nothing seems to be developing at all, and I'm thinking there is simply no trigger available to initiate deep convection. Isn't a front such as a cold front or surface trough expected to come through the UK some time this afternoon? Pure surface heating alone is not a good initiator in this case for any proper convection so it seems so if that is all we are going on then one could be quite confident to assume nowhere in England apart from maybe one or two coastal location with a better moisture supply will get any thunder.

A wasted opportunity!

[The Eagle]

Got a nice thunderstorm here in Dublin atm.....and torrential rain.

Easing off a bit now though. 4 strikes in total but only one audible.

Edited March 30, 2008 by Darkman

[Nick F]

Looks like we will see more widespread and heavier showers tomorrow over the UK, with a greater risk of hail and thunder, even over Sern counties of England which escaped over the last few days.

Slow moving sfc low moving NE from Ireland tomorrow with a moist S to SWerly flow covering the UK - will be overspread by cold pool aloft (as seen on 500mb charts), this cold air aloft will create steep lapse rates - particularly as temps rise in any sunshine.

Troughs likely to be rotating around the low, UKMO swings a trough in from the SW tomorrow morning, which is likely to be a focus of a line of heavy showers with hail and thunder moving across many areas of England and Wales tomorrow morning reaching the east early afternoon. Another batch of more organised heavy showers moving east across southern areas Friday night too.

A little speed shear aloft suggests some organisation of convection into multicells. Not much directional shear suggested, so tornadoes are unlikely - however, some low-level wind convergence seems likely over Wales, Irish Sea areas and Northern England near the surface low ... so a few funnel sightings can't be ruled out.

Saturday again looking rather convective over the UK - with low pressure close by, more on the weekend later.

[Nick F]

Despite the reluctance of GFS to produce much in the way of instability tomorrow until the 12z run this evening, it would appear GFS is coming round to the UKMO idea of heavy showers developing tomorrow afternoon roughly south of the Midlands to East Anglia.

Occluded front lying E-W and slow moving and weakening over Ireland, N Wales and N England will see a slow moving area of cloud with light rain or drizzle through much of the day over these areas. Further south, skies look to become brighter over Sern England south of the M4 airmass here will be warmer and moister too with temps reaching 14-15C perhaps a 16C with dew points of 10-11C - and breaks appearing in this area will likely allow sunshine to produce some surface based CAPE across southern counties - and this will likely trigger some heavy thundery showers here during the afternoon:

- T+24 fax for 12z Sunday - weakening occlusion over central UK and Ireland ... more active occlusion arriving across English Channel with upper trough following to south.

- SBCAPE 15z Sun

Second occluded front and upper trough behind moving north into English Channel area in the afternoon and into Sern England in the evening looks like bringing an area of locally heavvy rain to SE England in particular with risk of embedded thunderstorms in the evening moving - though this second area of potential looks less certain - GFS suggests a little MLCAPE in the SE in the evening with the 2nd area.

300mb vorticity charts show a vort max developing and moving north over Sern England in the afternoon/evening as an upper trough lifts out of France - this will likely help forced uplift for convection:

Very weak winds aloft and little directional shear suggests severe weather is unlikely. Warm moist profiles suggests hail is unlikely too. Perhaps some local flooding with any downpours - especially with 2nd area developing across SE in the evening.

[BrickFielder]

Well I suppose we have to start looking at Sunday and I must admit things do look interesting. The problem is that at this range GFS may be over cooking the Cape and with a system forming over the UK models are likely to be a bit inaccurate in the detail. There are a number of areas and conditions to think about on Sunday and they are quite distinct.

1) Ireland and Northern England

Here we have fairly unstable air, moist all the way up the air profile We do have a low level convergence wind convergence to trigger the storms but wind shear is quite light. We should expect some heavy rain and hail, with plenty of lightning and thunder as tops look reasonably high, but there is not much to suggest particularly severe storms apart from the quite high Cape(which may be overdone).

2) Embedded in the cold front

There could be a case of for arguing that the cold front as it sweeps east will become more unstable. I am especially looking at more eastern areas and London. Low level moisture looks enough there is a little low level wind speed shear. My worry here would be low level moisture and GFS over doing Cape. Cloud base should be quite low so this could be a bit of a unknown quantity and history suggests this may not materialise.

3) Post frontal mid level trough ?

Troughs are notoriously hard to predict ahead of time but there is a hint of one on the 500hPa vorticity charts. Will it disappear on the next run, is it even enough to trigger convection.

GFS suggests the risk is there but struggles to make it materialise.

If storms do develop there is a hint that they could be severe. Looking at the wind profile we have anti clockwise turning winds, winds at 850hPa are moderate perhaps giving enough speed shear to consider the prospect of tornados. Instability is there with moderate cape. Drier air aloft means skies should not be so congested and you ought to get stronger updrafts.Anything can change in the mean time and all the instabililty in the world won't give a storm without a triggering mechanism (Front,trough or wind convergence).

4) South Coast

This seems to be fairly consistently shown with an area of instability devloping along the south coast as a lobe of vorticity moves in over night.

This all just a heads up really at the moment because the details will substantially change as we get closer to Sunday.

[BrickFielder]

Satellite and models would appear to differ a little. Specifically there is an stormy area along part of the south coast which will move across southern counties.

This I think has the risk of making things messy and may limit temperatures and storm severity. Lightning wizard charts show a slight pick up in 700hPa winds across London late afternoon which combined with surface heating could make things a bit interesting. To the north things a pretty messy again although there does appear to be some convection across cumbria.Forecast skewT's suggest a fair amount of Cape with light fairly unidirectional winds all the way up.

Moisture looks to arrive late in the day with dewpoints rising from the west. Height of the zero degree isotherm suggests hail may a risk again today. If I had to pick an area for today to see thunderstorms I think it might be the south west, through central southern parts to east anglia with a risk of localized flooding and moderate sized hail especially towards the east. Not much risk of tornadoes with perhaps the a very slight chance of weak spout type tornadoes forming along southern coasts.

[richarddx7]

Can anyone give us the 911 on tomorrows chance in England? Should be some CAPE around again

[BrickFielder]

Models seem to be breaking out heavy showers where dewpoints are forecast to be highest.

Current readings suggest this may not be accurate ,so the forecast must be taken with a pinch of salt. For any showers to develop you need some sort of trigger and here we need to see some low level wind convergence or area of changed vorticity. Firstly we have no real evidence of a surface trough on the 850hPa vortticy charts. On the 500hPa vorticty charts we have an area of vorticty mving in late on into the west.

850hPa -500hPa winds give a hint of the direction that storms will tend to move in, but notice also the upper level divergence across England during the afternoon.

The other thing to note which surprised me was that 850hPa winds are light. Surprising because I noticed low level cloud moving along quite nicely this morning and even last nights soundings suggested brisker winds at cloud base level. Things do change from morning to afternoon though so we shall see.

On the issue of convergence then I was expecting to see some convergence over the south west as sea breezes pick up but none of the models really show that. What is shown is some convergence further north towards the north midlands and northern England.

The rasp model shows more convergence but not really over the south west where I was expecting it.

Forecast SkewT's suggest that the atmosphere is pretty unstable with diurnal heating sparking convection readily. Wind shear looks fairly pittifull although notice the turning of the winds at low level on the SkewT I have chosen. This suggest their might be a source of vertical vorticity which could perhaps give rise to weak spout type tornadoes, but I remain to be convinced with slow storm speed possibly choking off the updrafts which would help these to form, and low level wind speed perhaps a touch too high. I am also waiting to see whether the low level moderate jet of early this morning will disappear cutting of a source of horizontal vorticity. Pulse type storms seem likely today and we should perhaps watch for storms forming at the outflow boundarys of previous storms (i.e daughter storms at the back of decaying storms).

Main risk today I think will be localised flooding from slow moving pulse type storms mainly across the central south area.

[johnholmes]

00z ascents suggest, where convection does get going, for England and Wales, would give tops between 25-about 28,000ft.

most of it is marginal which will be why the values of CAPE and LI are not very high.

[BrickFielder]

Might as well get Sunday's storm discussion going although confidence in the outcome must be low at the moment as timing of the frontal system and a number of other factors must come together. Lets start with the moisture plume from the south.

We know that if the winds are from over the bay of biscay that moisture is likely to be realised,but if it is from over the continent then moisture, instability and the risk of thunderstorms are rapidly reduced.Models at the moment show that the moisture will be present.

Meteoblue Charts for Sunday show a number of interesting points. Low level wind convergence across the UK and instability building from the south. Notice also how 700hPa winds are stronger than GFS suggests and how east of the convergence winds should almost perform a 180 degree turn in the lower atmosphere. At 500hPa there is also a suggestion of a mid level trough which is not seen on GFS 00.

GFS is a lot more woolly about any convergence over the UK.

Looking at the complete wind profile for GFS shows fairly weak winds all the way up with stronger winds aloft coming in from the west. What is noticeable is the divergence of the winds at upper levels.

Forecast SkewT's show a very moist profile all the way up with a noticeable sagging above 700hPa. The risk here is that low cloud will prevent temperatures rising to 18 degrees and to much messy convection will limit any individual clouds building. Forecasts show dewpoints around 15 Degrees which is unlikely to any depth and 12-13 seems a more realistic amount to use for the SkewT, which reduces instability by a fair amount but must be balanced against GFS under estimates of inland temperatures during spring. Cloud tops look to be quite high so there ought to be an increased risk of electrical activity. Zero degree isotherm looks quite high so hail looks less likely. The slack wind profile,with low level convergence ought to be an environment favourable for weak tornadoes forming from vertical vorticity sources. MeteoBlues stronger low level jet does interest me because it could indicate a source of low level horizontal vorticity. Upper wind profiles suggest supecells are unlikely without help from storm splitting or outflow boundary conditions.

Slow moving Heavy downbursts may be the order of the day with perhaps electrical activity being higher than we have seen from storms so far this year.This does all of course depend on timing and a number of factors coming together which the models may not have an accurate handle on yet. Biggest risk would be localized flooding due to the slow moving nature of any storms and the amount of moisture.

[BrickFielder]

Still quite tricky to forecast for tomorow. Notice however how the dewpoints of 16C are now forecast at just over 13C. This seems about right given sea temperatures at this time of year although satellite imagery shows some very tropical air moving north. I was worried about cloud limiting temperatures but they seemed to rise OK today with a fair amount of upper cloud cover. Plenty of moisture.

Interesting that there might be a bit of a cap at lower levels which breaks as 850hPa temperatures begin to drop off.

Forecast SkewT's for midday show elevated instability so we might expect a kind of hazy humid start.

Then temperatures rise and it looks like we may get some boundary layer initiated convection.

Low level convergence is still forecast.

Late afternoon 850hPa winds drop off across the area (always quite low to the east). There would appear to be a short window not long after midday with convection around or to the east of the convergence with a moderate low level jet which could provide a source of vorticity. The prospect of supercells forming seems low looking at the wind profile but cloud tops should be quite high.So very slight risk of weak short lived tornado.

Biggest risk I think will be some localised flooding.Hail will probably be a lesser threat this time but can not be ruled out.

Edited May 3, 2008 by BrickFielder

[BrickFielder]

Quick update.

Less instability forecast now than yesterday, but this is not much of a surprise. Surface base instability is really limited but increases during the afternoon. Most of the activity of the moment looks elevated with rain tending to evaporate to some extent on the way down.

A few thunderstorms developing over north France but they are likely to be weakened by crossing the channel.

I do like the look of the forecast low level wind convergence as I think this will begin to focus convection.

Forecast SkewT's show weak instability but I notice how dry air at lower levels becomes moister as the day progresses.

Also of interest will be the fact that the cold front towards cornwall and west wales is begining to destabilise.

From now on its watching the radar and satellite for details.

[BrickFielder]

Chance of a storm today coming up from the south west. Not particularly impressed with the model output with GFS showing Dewpoints around 15C.

10-11C seems reasonable, but most of the instability indicated relates to the low level dewpoints. The point here is that low level dewpoints may reach 15C but are unlikely to be more than a few meters deep which can not be utilised for convection.

We appear to have both a Cap and some convective inhibition today which will likely suppress convection. The Cap being a band of warmer air which stops cloud bubbling up beyond it. Notice that the air could be faily unstable above if it was moist but is actually rather dry. Convective inhibition is there because low level lapse rates are not steep enough with the dewpoint today to initiate convection. This is shown fairly roughly on the forecast SkewT below.

As the day goes on and temperatures reach about 20C inland away from eastern coasts (where it will be cooler in the east wind) and as temperatures relax at about 750hPa due to a trough moving in then some convection could start.

All the instability in the world is no good without a trigger. Certainly orographic lift over Wales could be a focus but there are indications on Satellite of an upper level trough swinging north east across the UK. On the Satellite picture below you can see some redish areas over the UK which denote dry stratospheric air being pulled down and indicate an advection jet.

We can see this on the 300hPa wind forecasts. This is likely to a focus and trigger for activity.

Western areas are most likely to see convection because of higher temperatures rather than any extra moisture.

Isolation and city heat islands may play a part today.

General lack of moisture, should mean only isolated storms and it is so marginal that it is feasible that nothing will happen.Favoured area has to be northern parts of Wales with tornado risk being low due to weak upper level winds although some low level speed and directional wind shear may be enough for an isolated weak funnel being reported although it seems unlikely at the moment.

Edited May 7, 2008 by BrickFielder

[BrickFielder]

Tricky to forecast today with models not really getting a very good handle on conditions. Lets take GFS's suggestion of dewpoints at 17C which I am afraid is not realistic. During the height of summer dewpoints may just peak at 16C but I doubt whether they will get much above 13C today.

Most of todays instability is forecast on the back of quite high dewpoints and moderately high low level temperatures.

What is noticeable from GFS is how storms break out ahead of the cloud.

All this should in theory give a storm risk as on the following chart.

Unfortunately cloud cover looks thicker than forecast and the sun is struggling to break through. 6am Larkhill sounding tells its tale.

Warmth at 900hPa and 550hPa with some drier air aloft. My guess would be temperatures would need to reach around 21C for deep layer convection to take place, but some mid layer murk (850- 600) is likely to take place before that. The weather front is pretty weak but on the latest satellite pictures there is just a hint of sharping in the upper trough. There may just be some convergence across the pennines during the afternoon which may be a focus.

I can't help feeling cloud will limit convection and any storms may be further east than forecast although perhaps we should not rule out imports from the south. Convergence late on may even play a part.

Main risk I think will be localized flooding from slow moving storms. There is some low level veering of winds with a little speed shear which may be enough for a weak tornado to develop but I am not even sure we we get any storms at the moment. Forcing is pretty weak as is instability despite what the charts say. Zero degree isotherm may be a little high for hail unless some very strong updrafts develop which again may be limited by cloud cover.

[BrickFielder]

Warm front swinging across from late evening through the night will be wet with gusting winds at low level. Its not the warm front that has peaked my interest but the subsequent cold front and bent occlusion.See MetOffice Fax charts for 00z tomorow and 12z tomorrow

Warm front looks pretty moist all the way up, despite low level directional wind shear the profile is not really convective.

The cold front is however convective if low level dewpoints are to be believed.

This gives us a potentially severe thunderstorm outlook for a short period between about 10am - 2pm for the midlands through to north england.

There is lots to go wrong with this, but I would be looking at that cold front and occlusion very carefully.Will low level dewpoints really reach the levels suggested, will there be enough insolation to get low level temperatures to rise? I suspect timing means temperatures will be too low in southern counties with cloud bases being elevated, but just maybe further north a surface based storm may develop. Forecast confidence in any severe storm activity is currently low but some potential remains.

Edited May 27, 2008 by BrickFielder