Storm Risk - Technical Discussion & Forecasts

[Paul]

Hi all, by popular demand we're going to run a more technical storm discussion along side the general storms chat thread to allow for people to post more detailed forecasts and technical points in what should be a slightly slower moving thread.

As Nick has already posted in the general discussion, a convective forecast has already been issued for today with western and northern areas seeing the main risk:

http://www.netweather.tv/index.cgi?action=convective;sess=

[BrickFielder]

I pretty much agree with Nick and Estofex prime risk is from heavy rain from pulse type storms. The moisture plume has moved north although it is still pretty sticky here today at the moment.

It is interesting to note that Estofex suggest warmth and moisture up through the cloud combined with weak laspe rates mean electrical activity will be somewhat muted.

Winds are pretty slow with slow moving storms a risk and wind shear looks pretty meagre. Low level winds suggest some convergence around north wales which could pose a slight convergence type funnel risk in the early evening. Early evening because I suspect convectin will be a little less messy at that point.

Still instability is pretty impressive.

Forecast SkewT's confirm the moisture all the way up and light winds but impressive instability.

Larkhill 06 SkewT is pretty much in line with the forecast skewT's apart from the slight cap at 550hPa.

NMM gives the following suggested rainfall pattern.

One thing to note there is the suggested convection around devon and cornwall which I don't think has been highlighted much. Keep your eyes on the radar and we will see how it plays out.

Edited June 30, 2009 by BrickFielder

[Nick F]

Just a quick postie from me, 06z Larkhill ascent shows quite steep low-level lapse rates and air was quite dry at the surface, but above 800mb air was saturated up to around 600mb - indicative of elevated mositure plume which has destabilised somewhat bringing elevated thundery showers. This saturated profile above the boundary layer may hinder convection from the surface somewhat, but with temps reaching 30C locally and orographic lift and some convergence leading to uplift, we should have enough forced ascent for isolated storms across the Midlands and Wales this afternoon.

[johnholmes]

yes would agree with that last post from Nick-a mix of high level moisture with instability and some low level instability being released as well.

[Tim B]

What is your opinion of the chances for widespread activity this friday?

As it looks quite promising to me though it's early days and I'm not exactly a storm forecasting expert.

[johnholmes]

Not really looked that closely Tim but I'll come back to your question in a bit

[johnholmes]

in answer to Tim

Its hard to put a finger on why the models have backed way from what some might call a typical thundery breakdown after so many places (not all) have had such very warm/hot and humid weather. The upper trough does not change its shape, the flow is about the same round it/through it-some indication of pressure rising late Thursday into Friday from the SW which is perhaps part of the reason for less activity as the front moves into what might be expected to be a 'breeding ground' for convective activity. I would imagine the surface pressure rise emanates from higher level events, a change in the upper wind pattern, jet streaks perhaps, and the usual stabilisation after one thundery area moves away ( that affecting the west during Wed into Thur?

like I said to begin with Tim, I have no magic answer but those points do seem to perhaps hold a clue as to why its changed.

Mind you it could change back again!

Edited June 30, 2009 by johnholmes

[Tom Quintavalle]

Re. JHs post, more often than not the most dramatic thundery breakdowns, certainly for London and the SE have tended to come with a fall of pressure over N.France. The models seem to be showing a ridge building towards N.France on Friday, when that scenario has occurred in the past, down here in the SE, the change in the weather can be somewhat benign with a short spell of rain or even just some cloud, then the wind switches to the SW and the weather becomes cooler and less humid.

But when the breakdown occurs with a fall of pressure over N.France the transition can be much more dramatic with heavy thundery rain and some quite severe embedded storms before the whole lot moves away into the N.Sea and the fresher weather arrives.

It will be interesting to see how the end of the week will pan out. At the moment the models seem to want to show the more dramatic change to be further north and west. Not too long to wait before the picture becomes clearer.

Regards,

Tom.

[Harry]

Re. JHs post, more often than not the most dramatic thundery breakdowns, certainly for London and the SE have tended to come with a fall of pressure over N.France. The models seem to be showing a ridge building towards N.France on Friday, when that scenario has occurred in the past, down here in the SE, the change in the weather can be somewhat benign with a short spell of rain or even just some cloud, then the wind switches to the SW and the weather becomes cooler and less humid.

But when the breakdown occurs with a fall of pressure over N.France the transition can be much more dramatic with heavy thundery rain and some quite severe embedded storms before the whole lot moves away into the N.Sea and the fresher weather arrives.

It will be interesting to see how the end of the week will pan out. At the moment the models seem to want to show the more dramatic change to be further north and west. Not too long to wait before the picture becomes clearer.

Regards,

Tom.

The charts at the just keep changing - sometimes suggesting the pressure is going to rise, though some suggesting it will fall - rise/fall, heads/tails! lol

Edited July 1, 2009 by Harry

[BrickFielder]

Pretty much a repeat of yesterday with storms being a little more north.

Slight risk of convergence type funnels for northern scotland and north wales and perhaps north cornwall where convergence occurs.

The atmosphere will be pretty saturated so convection could be messy which would tend to suggest southern most storms are likely to be the most active.

Biggest risk is from pulse type slow moving storms with heavy flooding leading to a risk of localized flooding.

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Quick heads up for tomorrow for those in the midlands when things change slightly as a system approaches from the south west. Wind shear does start to pick during the day and as well as embedded storms there may be a few ahead of the front with a trough preceeding it. Theres not much dry air aloft so I am not expecting any spectacular at the moment but there may be some ingredients for more severe storms if some of the conditions change from the forecast.

[BrickFielder]

Tricky forecast today with some model disagreements. Metoffice output does not really fancy much in the way of storms today with perhaps a few over scotland and some embedded in the front. NMM seems to be suggesting apart from scotland cloud will move in from teh west dampening down activity.

There are a couple of things that we should draw attention to though. The first is that there may be some pronouced directional shear for storms developing in Scotland. Enough perhaps for a short lived tornado to develop although deep layer sheer is poor and cloud bases do not look that low which rather limits things. It is also a somewhat drier profile than elsewhere in the UK which should help more descreet cells to develop. Take note though that some models don't really buy into much of a storm at all in the area.

The second thing to draw your attention to is moisture convergence towards the midlands late on in the day.

Low level winds do tend to converge and to be honest I am not sure which way things will go.

GFS and NMM not really agreeing on cloud cover and the metoffice output having none of it, mean its a bit of a wait and see scenario.

Couple of extra charts, the first showing sbcape which highlights where the main risk might be today and lightning wizard's super cell chart although mid levels winds are not strong enough for super cell development in my view.

Larkhill 06 sounding shows some knee high moisture but lower levels look surprisingly dry when compared to forecast skewT's even taking into account it is early morning.

[VillagePlank]

Possibilities, in far SE, today, although GFS doesn't forecast t/storms this far east:

Here's the skew-t for 18z tonight ...

Some CAPE, but mucho limited by CIN. However, there's some shear at the same level, and if surface heating is around about 30C at 18z, this would break the cap releasing large amounts of CAPE, and the cap may be broken, anyway, because of the shear, even though it looks slight at the moment.

12z Herstmonceaux sounding vitally important for SE chances later on in the day, but, as always, and rather like snow-forecasting, if one parameter is out then, usually, it's not going to happen.

Edited July 2, 2009 by VillagePlank

[Harry]

Sorry if this is a stupid question, but I noticed on the windflow charts for 2100 what looks like a Low (thundery?) developing along N France through the evening period. The 0000 and 0300 charts has it moving over the SE.

I am aware that when big thunderstorms develop, as are likely to do along N France today, they can develop their own path, independent of prevailing winds etc.

However, is there a possibility that as this L moves NW towards the SE, that storms could come with it?

Edited July 2, 2009 by Harry

[johnholmes]

it is a possibility but only that at the moment

storm forecasting is a bit like snow forecasting-much as VP has suggested above-ALL the parameters need to be showing a yes.

you will have to wait until early evening before any of the forecasters or those with a reasonable understanding of the requirements can give an honest answer

[BrickFielder]

Looks like it weakens as it crosses the channel. With surface temperatures begining to cool it may not be surface based by then also. Could give a good electrical show as it approaches the coast. As ever with storms developing over France they can develop there own local wind fields and conditions so prediction only starts to be reliable once the storms have started to form. One to watch though if you are in the SE.

[Paul Sherman]

As Brick says it looks iffy but certainly a Chance.

I am going to bite the bullet tonight as I have tomorrow off and the kids are on a Teacher Training day so dont have to get up at Stupid O'Clock in the Morning and if parameters are still looking good will be heading down towards a good vantage point along the South Coast as I dont believe the Storms and Lightning will penetrate much further than 10-15 Miles once they hit the Land.

Paul S

[BrickFielder]

Lapse rates look pretty poor over most of the UK today with the exceptions tending to be northern scotland and northern ireland.

There may be some embedded storms moving north but cloud cover may limit surface based activity.

Perhaps a slight possibility for east anglia suggested by cloud top temperatures.

Convergence zone in the glasgow edinburgh corridor late afternoon increases low level wind shear so this may be an area to watch.

Chances for northern ireland to catch a storm tomorrow and perhaps more widespread storms on Sunday.

[BrickFielder]

Some marked convergence zones are forecast for the glasgow edinburgh corridor late afternoon so this may be worth keeping an eye on for convergence type funnels.

Mid level lapse rates are forcast to be steeper than they have been for some time.

There will be a marked area of drier air through mid levels across parts of the midlands while precipitatable water concentrates away from this area. This suggests less storms across the midlands a marked line of storms developing from dorset to east anglia through the key period. The few storms that do develop in the midlands may be enhanced by the drier air.

Low level vorticity charts give a good indication of developments as well.

Along the south coast there will be much stronger directional sheer, but surface wind speeds could be a little strong for tornado development. Mid level wind sheer seems to be greates towards wales and the midlands which could enhance super cell possibilities in the case where storms split. Note directional sheer is not that great.

There will most likely be a place todays where the conditions are favourable for tornado formation probably on the boudary of highest PWAT,dry air through the mid levels and sufficient sheer at both mid and low levels. Southern East Anglia comes to mind as somwhere with potential.

Looking at the satelite pictures shows some storms developing over wales which seem likely to head towards the liverpool manchester area. These might be slow moving and pose a significant risk of localized flooding.

Keep an eye on low level jet developments as this will be critical.

Edited July 6, 2009 by BrickFielder

[Neilsouth]

Thankyou for that indepth write up

[johnholmes]

yes along with the convective ouput from Nick F on the Front Page, as far as NW goes they are the two to read for a constructive idea of what to expect on any convective day.

tks Brick-even I can understnad it!

[BrickFielder]

There could be a problem with cloud cover today and surface temperatures not reaching those forecast. 20C is possible in a northerly wind, but it might be stretching it a bit under cloud cover. Currently there is a band of rain moving down across the midlands which should reach southern areas later. This should in theory pep up with some surface heating.

Attention is drawn to east anglia where some surface convergence could give rise to some spout/convergence type funnels early afternoon especially with the low cloud base.

From the forecast SkewT's I also notice a little directional sheer towards the south coast which might bear watching.

Also I notice some strong low level cape across the midlands with a touch drier air aloft which could produce some stronger updrafts.

I guess this is reflected in the following lightning wizard chart which should be taken with a pinch of salt.

Biggest risk will be from localized flooding from slow moving storms particularly towards the north east anglian coast. I guess actually events will over take the modelling again today and cloud cover will play its part.

[Methuselah]

Thanks for all the hard work, BF. :winky:

[Neilsouth]

Thanks for that :winky:

Just a question why is it the low countries get so many storms in summer?

[johnholmes]

tks Brick

My own take on it is that heavy showers are going to occur for much of England and Wales, less widely than yesterday, and much less so for N Ireland and Scotland(Aviemore and other places yesterday got torrential downpours).

For England and Wales then I would suggest its roughly s/e of a line from the Humber to Isle of Wight where the heaviest are likely to be. In this area then parts of northern East Anglia could be the area for the torrential downpours.

Watch the surface charts by early afternoon and the radar. This should give clues as to whether any convergence zones are being set up-these will be the main trigger along with the upper trough, roughly in the same area I indicated above.

Edited July 7, 2009 by johnholmes

[BrickFielder]

The low countries tend to have warmer temperatures due to the effect of the continent where high pressure has a tendency to form during summer. The boundary between atlantic weather and continental weather tends to be closer to the low countries as well.