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BrickFielder

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BrickFielder last won the day on July 11 2010

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  1. Modelling seems to have made up its mind that there will be little in the way of Storms for the UK today. Looking at the Brest Sounding from last night it suggested that dew-points of 20C and temperatures of 34C would be required to break the CAP. Dew-points that high are most likely out of question but I have noticed a marked difference in dew-points across the country. Heathrow as dew-points at 10C while Cardinham in the south west as dew-points at 18C. Across northern France a lobe of vorticity as sparked of a Storm which is likely to skirt the south east of the UK. We still have a number of low level wind convergence zones and the lobe of vorticity moving north from France. As late afternoon approaches it will still be touch and go whether the CAP goes. Think it is just a case of waiting and watching now.
  2. It still looks marginal as to whether storms occur tomorrow. Some models show a hint of surface based storms for Midlands through to Cumbria moving eastwards during the late afternoon. Some show elevated storms developing over the south east of England during the afternoon or surface based imports from the continent. Looking at the forecast SkewT's I can see the elevated mixed layer clearly and the suggestion that the CAP goes with temperatures in the low thirties. When I look at the soundings from the continent last night then the CAP goes at higher temperatures. Part of the difference is that over the UK tomorrow low level maritime air begins to undercut the elevated mixed layer and meets the dry continental air along a convergence zone. We should also remember that The UK being closer to the front and jet-stream will tend to alter the propensity for instability. Looking at Satellite imagery we can see the first shoots of storm development over Spain and Southern France today. This can be a good or bad omen because it means the air is unstable with higher temperatures but also leaves the possibility of dying storms from the previous day drifting over the UK blocking sunlight. To my eye there is also a thin band of vorticity just ahead of the front out from the coast of Spain. Weather models always seem to down grade storm potential and move it eastwards the day before as certain real soundings (continental) rather than satellite guessed soundings come into play.As other real air soundings come into play(Ireland) then there is a tendency to slightly upgrade storm potential overnight before storms and to move things slightly westwards. Based on this hunch about how models perform I am leaning towards storms still happening tomorrow.
  3. It is too early to really assess the storm potential for Friday, but I can see some potential for storms. These may be French imports or perhaps a sliver of a chance of surface based storms from the Midlands northwards. There are signs of possible high cloud keeping temperatures down which could negate the potential and the CAP looks pretty strong. Worth keeping an eye on as we lead up to Friday.
  4. Still some timing differences in the models.It is worth remembering that temperatures tomorrow will not be that high so low level lapse rates will be weak, potentially limiting updraft potential. It is starting to look like a combination of upper trough and low level wind convergence being a trigger for convective development. Low level wind convergence is tricky to model so exact placement of convection will be subject to change on the day. I should also point out lightning wizard cloud top height modelling which does not suggest very high cloud tops across the very south of the UK.Lightning wizards dynamic tropopause chart also hints that local convection conditions might lead to some over shooting tops though. It also looks like parts of Scotland may be in with a chance of storms as well. It is still tricky to pin point as this stage where and how strong storms might be.
  5. There are slight modelling timing differences for Saturday which are interesting. We have an upper level trough swinging across the UK with GFS modelling showing it arriving Saturday night while MetOffice fax charts suggest that it cross the the UK during early evening. The track record for GFS this year is to move this type of trough too slowly across the UK and the MetOffice to be more accurate but still a little slow. Equally modelling tends to keep the trough a little south of where it ends up. If things go to form and its risky to second guess how weather models might be wrong then we could see thunderstorms Saturday afternoon which may not be on the modelling radar yet. It is difficult to tell whether the trough will be tilted and we need some sort of modelling agreement to guess at potential but it is worth keeping an eye on.
  6. Upper level cold pool of air is forecast to move quite quickly across England today , probably leaving the east just after mid day. Looks like a small window of storm possibility 10am to 1pm for eastern counties. There is a small risk of storms in a small window of time between 14:00 to 16:00 across the midlands and south east as a narrow band of mild upper air cooling traverses across low level convergence zone convection. Winds look light and instability is meager at the most but because storms will be slow moving there is a risk of localized flooding. Slight risk of very weak spouts and key area to watch is probably from somewhere like Cambridge through to Suffolk.
  7. I don't have a lot of confidence in the modelling today as it looks to me as if the modelling is about 3 to 4 hours to slow in moving things north and east. This moves the storm risk further towards the north east in my view. It does however perhaps bring other factors into play which wont be modeled. Here I am looking towards the south west initially. Last time I questioned the modelling though, the models were correct and I was wrong so take this with a pinch of salt.
  8. I have been watching the models for Friday with interest and struggled to get a handle on whats going on. Models still don't exactly agree and the modelling has moved instability about across the UK over the various runs. I think my difficulty lies in the fact that there might be more than one thunderstorm potential here. Looking at the modelling we have an elevated mixed layer with significant instability above the boundary layer. Looking at some of the forecast SkewT's I can see a CAP on low level convection as a result. Where we have low level wind convergence below this then there is potential for enough moisture pooling to break the CAP.The most likely place for this is probably the north east and shows up as a trough on the UK Met Office Fax Charts.Whether thunder storms are elevated or surfaced based the instability is high enough to produce large hail and lightning. We also have occluded cold fronts which will introduce colder drier air aloft which will pass over the still moist low level boundary air and replace much of the elevated mixed layer.UK Met Office fax charts and other modelling do not exactly agree on where they will be.This change to the environment looks capable of breaking the CAP and initiating surface based convection. Models don't quite agree on whether temperatures and dew points will be high enough or how near to the surface that drier air gets. Potentially you have some directional wind sheer in the boundary layer with speed sheer aloft and moderate instability. This is a scenario which can produce an isolated moderately severe Thunderstorm. Some of the charts I am looking at.
  9. Watching the clouds around the storms yesterday I noticed some low level clouds heading in a different direction to the clouds above. This suggests quite a strong inflow and up draft environment that can moderate low level winds. This is something that can happen when low level winds are light and will not show up well on forecast charts. It does mean that you will have some low level vorticity (wind shear) that is very localized. Today is slightly different to yesterday in that we have upper level divergence from an incoming small jet streak. This high level jet should enhance convective height, but also should provide greater up draft and down draft separation. In simple terms we have slow moving, strong and long lived storms. Forecast low level wind charts show wind convergence zones but also hint at localized wind fields. Forecast SkewT's show moderate instability driven in large part by moist low level air (dew points). They also show light winds except at the highest level which we might get high level cloud from early storms streaming out ahead of early storms dampening down storm development. There is also very little in the way of dry air going up through the atmosphere which means convection could be a bit messy. There is also the suggestion that storms may become detached from the boundary layer as we go into the evening. The wind field is also not conducive to super cells (rotating storms) so a clear tornado threat is off the cards (Any splitting storms can change this though) Possible risks are localized flooding, lightning damage (mainly sheet lightning) , possible weak short lived tornadoes. The conditions are there for some severe storms (more so than yesterday)and localized wind winds always make me nervous.On the other hand there are reasons why convection is messy and temperatures are pegged down to limit storms.
  10. Interesting question. It has been suggested that during warm phases of the AMO that tropical storms are more likely to develop into mature hurricanes.However the correlation between AMO and a particular hurricane season is pretty weak and the frequency of storms is largely unaffected.I think during 1971 to 1994 (25 years of AMO cool phase) there were only 36 major hurricanes compared to 1945 to 1970 when there were about 67. It has been suggested that we might just be entering another AMO cool phase, although any affects will be muted by climate change. When we look at hurricane landfalls then there is a marked increase in the warm phase and a shift to the east coast during the cool phase. Personally I believe the AMO is a reflection of the default position of the Bermuda Azores High Pressure. During a warm phase it is closer to Bermuda and is more south west in the Atlantic pushing tropical storms into the gulf of Mexico. During the warm phase high pressure is more North east towards the Azores resulting in tropical storms curving northwards out to sea. The jury is out on what exactly causes the AMO or even if it really exists. The theory which seems to ring most true with me is that cloud fractions and dust formation across northern Africa changes. This possibly being driven by Indian Ocean changes which link back to North Pacfic ocean changes. There has been modelling which shows that the PDO leads the AMO by 1 year and when the AMO leads the PDO by 11–12 years. This seesaw motion being reflected in the location of deep water formation at polar ice edges. This is all complicated by whether the AMO cycle actual is 25 years the same as the PDO or 60 - 80 years as most literature suggests.Perhaps the cycle is disrupted by volcanic aerosols (major volcano eruptions) as some have suggested or CFC emissions. It could also be linked to the Wolf-Gleissberg cycle Solar cycle with a 5 year lag (This is very contentious but supported by a least one paper -Knudsen, M. F. et al.) There seem to be some indications that the effects of the AMO may not behave quite as previous ones have done with some marked implications for North America and Europe. Warmer seas in the Mediterranean and around the northern coast of Africa affect the eastern extent of the Azores. We might also see a weaker gulf stream with a propensity to turn east out into the Atlantic slightly further south along the American east coast leading to a more marked boundary between warmer and cooler waters. This all complicated by droughts and dust as weather patterns change due to the pattern change, which affects inbound short wave radiation and outbound long wave radiation and possibly global temperatures (Although climate change alters this). There are all sorts of conflicting ideas surrounding the AMO. This means I cannot guarantee any of the information here and perhaps somebody with more knowledge will correct a few misconceptions I have. Hopefully yourself and others will explore further and provide more concrete analysis. I hope this gives a starting point as I don't think there is a clear answer.
  11. Potential convective risk Thursday from the occluded front moving up from the south west. While modelling suggests some potential for convective development we should consider that air aloft looks moist perhaps producing a high thin layer of cloud which could dampen down temperatures.Low level wind convergence looks to be in place triggering development which could perhaps create storms behind the upper level disturbance. Combining this with perhaps a warm air inflow from the south and perhaps we could see stronger development. Looking at the skewT forecast profiles then we can see strong wind speed shear in the lower levels, combined with very weak surface winds and convergence which can help with strong inflows. Upper winds look unidirectional and not increasing with height which might limit updraft longevity. Personally I think surface temperatures are too low for strong convection and squally heavy showers might be the order of the day.
  12. Tomorrow is a bit tricky to forecast due to model variances. What we should expect is elevated showers for the south west and parts of Wales overnight. Then it gets tricky due to a cap ,cloudiness due to elevated convection, low level convergence not lining up with upper level dynamics. Dew points of around 17 or 18 C which matches today's dewpoints in some places could release a significant amount of instability. Think we need to take a closer look in the morning.
  13. Chance of a few thunderstorms today where weak low level convergence takes place. Instability is rather meager and winds look to be straight line but increasing with height. This should give some updraft downdraft separation with upper cloud pushing out ahead of the storm. So storms might be long lived but fairly zipping along. Convergence zone look to feed moisture from the north rather than the south and with relatively low dew points and temperatures we should not expect really strong updrafts. There is however just a hint of winds at the 700hPa to 800hPa level being a little stronger towards the East mid afternoon. Storm risks look fairly low with perhaps the greatest risk from convective gusts. Worth keeping an eye on but unlikely to be a classic storm day.
  14. I am looking closely at tomorrow afternoon mainly for an area from Reading through to east Anglia for thunderstorms. Satellite imagery shows the low that has brought the bad weather over Friday beginning to elongate with a vorticity lobe working its way around it. This lobe should head towards northern France but might encourage some divergence aloft. Current modelling suggests a number of convergence zones setting up across the south of the UK. At the same time the upper jet begins to move of into the continent. Some modelling suggests the remnants of the lower level jet may remain over eastern parts. Instability looks meager but lowering cloud bases combined with some low level sheer could produce some localized enhanced conditions. Limiting conditions are upper cloud and weak temperatures which would limit updrafts. Urban heating might just make the difference. Something to watch out for even the modelling suggests very limited possibilities for thunderstorms.
  15. Picking out a particular area for thunderstorms is quite tricky today as Models vary somewhat. The general trend looks to be Ireland , Wales , Northern England and Scotland are favored for thunderstorms. Instability looks to be fairly low and storms should be moving fairly fast. However looking at Satellite Imagery I can see a slight kink in the vorticity pattern with some potential storm development associated with it. This seems to be suggested in the GFS Lightning wizard troposphere charts, however the timing is likely to be different. This is something which should be watched closely and might suggest parts of England in the south west , through the midlands and possibly East Anglia could see a storm during the afternoon. It is tricky to predict and will probably need now casting.
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