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BrickFielder

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BrickFielder last won the day on July 11 2010

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  1. Upper low near south Wales will slowly trundle across southern parts of England enhancing mid level lapse rates and instability as cooler air overrides moist surface air. Satellite imagery suggests a vorticity lobe coming in to the south of the upper low which may begin to wrap slightly around the upper low. Approaching vorticity is a good trigger for storms and may indicate a little more wind shear than currently forecast. Forecast SkewTs suggest a moist profile all the way up to the tropopause which tends to lead to messy convection and weak storms. Wind shear is forecast (need to monitor this) to be low and this would tend to lead to pulse like storms which tend to have limited intensity. Locally veering low level winds could lead to spout type very weak tornadoes but weak updraft strengths may limit this. Storms possible today and generally speaking the biggest risk is localised flooding due to slow moving storms. Risk of severe storms looks low but the vorticity pattern needs watching.
  2. Nothing really to add to what others have said apart from the risk of over shooting tops in the south east in the early hours (increased lightning risk). Precipitation patterns similar to yesterdays suggestions but a little delayed. Localized flooding risk for the south east possible looking at the modelling. Satellite imagery currently consistent with vorticity waves being ejected northwards from the cut off low near Spain. First indicators likely to be on the Spain France border early evening. Current suggested precipitation patterns.
  3. Thursday early hours is a different prospect in terms of storms compared to Wednesday afternoon. Not sure that I would look at the early Thursday morning risk as your typical Kent clipper. The atmosphere looks moist all the way up and it is not clear that instability will be surface based. It has more of a nocturnal squall line look of it (wind convergence at the 700hpa level). Risk of lightning , heavy downpours and even gusty winds, but possibly limited in severity due to being elevated. So we have an non event Wednesday afternoon that might surprise us and a Thursday morning Kent clipper which isn't which also may surprise for different reasons (localized flooding). I guess I need a few more model runs to get a better handle on things.
  4. If only Wednesday conditions were a month ago then we might be looking forward to some stormy weather just south of the front in the afternoon. What we have is cloud clearing south of the front giving showery weather and rising temperatures. We have low level wind convergence and moisture pooling raising dewpoints. A little instability is forecast and the models suggest some convection is possible. Wind speed shear is present and hints of low level wind veering, which would give some ingredients towards low topped super cells. The problem is that it is not June or early July and temperatures look like they need to be up close to 30C for cloud tops to get above mid levels and a slight shower. If the models are a little bit out with there satellite imagery mapping then a surprise storm might be possible. Most likely this is another near miss but worth keeping an eye on the models in case they change.
  5. I was looking at a few parameters and thought storms might be possible today. Forecast models though are having non of it. The first thing I noticed was a thread of colder air moving across southern England. Colder air high up usually increases lapse rates and instability. It does show up as a band of vorticity on satellite images but is not as marked as the above charts suggest. The next thing I noticed was low level wind convergence zones which should pool surface moisture. Yet forecast models do not think there is much chance of storms. Even low resolution models are not keen. So it is back to forecast SkewTs for some clues. So forecast SkewTs suggest a strong CAP due to a dry warm layer which prevents air rising beyond that layer. There some unanswered questions for me with that forecast SkewT. The first is temperatures of 18C and dewpoints of 10C when they are already 20C and 14C. Where is that sliver of very cold air aloft. Is the CAP consistent with overnight weather soundings. On balance I think the forecast is probably correct but it looks a lot closer than you might expect. Temperatures hitting 25C might just. spring a surprise storm.
  6. Usual warnings on this information that I simplify things and don't exactly follow the rules so this will be a flavour of things rather than some of the very good technical explanations on the internet from professional meteorologists (My interpretation should be taken with a pinch of salt). Maybe somebody will do a proper pinned post of this but in the meantime check Nick's very good pinned guide to thunderstorms in the UK which covers many of the ideas which about SkewTs. SkewT Diagrams are usually seen with respect to soundings. Here a weather balloon is released to rise through the atmosphere and the temperature, dewpoint, wind speed and wind direction are recorded at different levels as the balloon rises and then plotted on the SkewT diagram. From this information you get a snapshot of the air conditions and this is useful for predicting weather conditions downwind. For example the Nottingham soundings are sometimes useful for weather prediction later in the Netherlands or Germany. The only upstream sounding for the UK is Castor Bay in Ireland which tends to be useful for Scotland and Norway. UK weather models rely heavily on Satellite monitoring which give a good idea of the air conditions upwind but has some limitations. What I tend to have a quick look at is the forecast (predicted) SkewTs and because they are not based on real measured conditions I often take certain liberties just to get a rough feel of conditions. The types of things I can see from the diagrams is how humid air (dewpoint too close to temperature at any height may mean messy convection), is convection possible, is there cooler air aloft making things more unstable, is there wind shear which could enhance storms or make them long lived, is there CIN or a CAP stopping convection and how low cloud bases are and how high cloud tops are. Looking at a forecast SkewT I have numbered some features and lines and will try to explain what they are and how I interpret things (shortcuts included) 1. Winds going up through the atmosphere giving direction and strength (number of barbs). The more winds turn with height and more speed difference there is between the surface and upper levels (Wind shear) the more severe a storm might be. 2. Isotherm lines (blue) and saturation mixing ratio lines (yellow dotted) are used to calculate the lcl and ccl(cloud base). Everywhere along an isotherm has the same temperature. 3. Saturated adiabatics (green dashed lines) (dry adiabatics are the red lines but I don't really use them but should). As moist air rises it condenses out into cloud and becomes cooler. If the air is still warmer than the surrounding air after the cooling it will continue to rise. So this line is the rate at which air cools as it goes up and produces cloud. 4. The actual or forecast temperature of air at a particular height above the ground. If the temperature drops rapidly (lapse rate) between the surface and high up at the tropopause due to cooler air aloft and drops quicker than the saturated adiabatic rate then air will rise and produce cloud (convection) up to a point where warmer air is encountered. There other mechanisms for example fronts that produce cloud so it is not the complete story. 5. Dewpoint temperature which is a measure of humidity of the air. If the dewpoint and temperature are very close together then humidity is likely close to 100% and cloud will form at that level which might affect temperatures at the surface and lapse rates. Humidity at the surface will tend to dictate cloud base levels and whether any moisture is available to form clouds. 6. This is a line I add myself to guess at the cloud base level. It should be parallel to the saturation mixing ratio but I tend to use the isotherm line. To do this properly you should draw a line from the surface temperature parallel to the dry adiabatic (red lines) and draw a line from the surface dew point parallel to the saturated adiabatic (yellow dashed lines). This is the place where a surface parcel of air will begin condense out into cloud provided the meeting point of the two lines is above the temperature line.(This is where I take a bit of a rough shortcut) 7. This a line also added by myself which estimates the level of free convection (cloud top height). This should be a curved line from the intersection point determined by 6 (cloud base level) following the saturated adiabatic until it intersects with the temperature line. The larger the area between this line and the temperature line the more unstable the atmosphere is. 8. Things like elevated convection (The lines I have drawn don't work) and other scenarios may show up here. The higher the cape number the more unstable and stronger storms will happen. Li represents the lapse rate and the higher it is the stronger storms will be. CIN is a rating of prevention of convection and the higher the number the less likely storms will occur. Pressure 1000mb tells you convection should be from the surface and not elevated. Strictly speaking the forecast SkewT shown may not lead to a storm due to the CIN level, but considering it is estimated from satellite images certain liberties when taken in conjunction with precipitation charts don't seem unreasonable. OK lets test out the theory on a forecast SkewT from tomorrow. (I will try do it properly for once - if I am interpreting the lines correctly) I can see the dewpoint line close to the temperature line at higher levels. This suggests convection might be embedded or very messy. There are very strong winds up near the tropopause which would suggest a jet stream (high fast winds) and cloud tops being spread quickly at higher levels. Wind speed and direction differences are not in the key areas to affect storm development much (lower and mid levels are more important). CIN of zero shows no prevention of convection and the 200 of Cape suggest weak instability and hence weak storms. Looking at precipitation charts I would take that SkewT with a pinch of salt. Forecast SkewTs can be useful but need to be looked at in conjunction with other charts and the latest satellite imagery. Looks like multiple low level convergence zones occurring which will tend to cause moisture pooling. Moisture pooling will increase the surface dewpoint and alter any forecast SkewT which does not take that into account. Wales looks like a good spot for storms again tomorrow but any cloud clearance may spark storms so keep an eye on the radar. Hopefully this might start to clear a little fog around SkewT's but it is not easy to explain and after a while it becomes instinctive for some storm enthusiasts and you begin to take liberties.
  7. I see a few discussions about supercells in the last day and thought a simple explanation might help. If winds change direction as you go up in the atmosphere then clouds will have a tendency to twist as air rises. If winds at a low level are light and winds increase with height then louds will tilt along with the higher winds aloft. Tilting will separate updraft and downdraft regions and the twisting will rotate the cloud giving a supercell. If winds at the surface are very light and higher at the cloud base level then winds will tend to have a rolling barrel effect below the cloud. This rolling or horizontal vorticity can be picked up by a twisting updraft region to form a tornado. This is very rare in the UK but not so rare in Europe hence why Estofex alerts tend to happen more in Europe. As for today then the models would suggest storms for the west , east and north but not central areas. Upper level cooling is not really there like yesterday for Northern parts so maybe not as high a risk of severe storms although I notice some model anomalies. Looking at Satellite images then I see a frontal system moving eastward across parts of the west with embedded storms. That front seems to be moving faster than forecast so I am not totally convinced by modelling of where and when storms will occur. Tomorrow is a different scenario with a frontal system turning thundery across Wales and central areas through the afternoon. Upper level cooling is better but the atmosphere profile has a tendency to be moist all the way up to the troposphere so there is a risk of very messy convection. Think radar watching might be a better option at this point.
  8. From Sunday onwards the jetstream is forecast to dip south across the UK giving a long wave trough and colder air aloft. Typically when hot air from the continent gets replaced by maritime moist air and cooler air aloft spreads over the UK we get a thundery breakdown. There are worrying signs that the low wave trough becomes a cut off low over Spain and it becomes business as usual high pressure over the UK. Even with the long wave trough modelling suggest high pressure at the surface lingers over the south with easterly winds still prevailing until we get a rapid switch to Northerly winds. Forecast models do at the moment give a brief window where winds are from the south west (maritime moist air) but there is little time for surface dewpoints and humidity to rise in some southern areas. There are sings that some areas will not get a thundery breakdown and little sign of rain for the immediate future. Dewpoints of below 10C and low humidity are not good for thunderstorms. The problem with the modelling is that it maybe trying to handle a transition and holding on to the existing pattern too much. The confidence in what will happen Monday and Tuesday will be low at this point in my opinion so keep an open mind and don't be surprised by last minute changes in the forecasts.
  9. The shape of the upper trough is more elongated than I was expecting and moving north east across the UK slowly during the day. Forecast SkewT's suggest very weak instability but if skies clear a little then warming temperatures could initiate convection. The wind field looks very slack with very little wind shear although trough movement suggests some upper level wind flow. Low level turning winds could support weak spout like funnels. I guess focus might be low level wind convergence zones overlapping some clearing skies. Very marginal scenario but most models seem keen on breaking out a little convective rain. The more marked upper trough is interesting though.
  10. The front currently over the South west is bringing in moisture at upper levels. This will lead to some upper level clouds and storms as it slowly sweeps west to east during the day. It should be noted that the boundary layer (surface) is very dry and there will be significant resistance to convective initiation (CIN). What is not perhaps picked up by the models well is the possible second frontal boundary which seems likely to play its part tomorrow as upper level cooler air crosses the southern half of the UK. By early evening upper level frontal cloud band should reach the midlands and London areas and considering cloud top heights may give a lightning show. The UKV Model also seems to like initiating convection along a low level convergence zone. This is just one model but it does hint at a possibly of a cap going quite explosively. along that zone. Since this is just a hint on one model we should not get too excited and even here the convection is likely to be elevated. Not convinced (overnight soundings were not that positive) but worth keeping an eye on.
  11. Watching the radar with animation on shows some cloud moving east and some north. Looks almost as if there is a surface low crossing the southern part of England. Further north looks like a convergence zone or developing front which looks slow moving apart from spreading northwards. Maybe it is just my imagination. It could give a hint as to where storms will be later though. Happy watching.
  12. We have a long wave trough dipping down across the UK which is bringing colder air loft. This will mean steepening temperature lapse rates increasing instability (Cape) but also bring down the tropopause (cloud top height). Low wind speed and low level wind convergence along with orographic lifting (BWE possible) are thought likely triggers for convection. Low cloud bases and slack turning surface winds could lead to very slow moving winds and very weak spout like tornadoes. Models seem to give a signal for continued precipitation and instability into the night for some central areas. Having looked at the Cambourne sounding from last night I am not totally convinced upper levels will give the forecast instability. Satellite imagery also suggests the long wave trough is more vertically stacked that modelling suggests (stretches further East). There is also a clear vorticity lobe out over the Bay of Biscay which I think will tend to force instability ahead of it. Best guess is that many Eastern and Northern areas might see storms where modelling is not so keen. Very moist profiles and approaching vorticity from the south could lead to an MCS (Depends on whether vorticity begins to lift out or trundles further into France) . Highest risk is localised flooding due to slow moving storms and due to steepness of temperature lapse rates through the zero degree isotherm some larger hail. Storm training (In lines) could mean some places get repeated soaking and other places get nothing. Yet again not convinced by what models suggest in terms of coverage and placing of storms.
  13. Chance of storms across Northern Ireland with perhaps a low top supercell risk today. Biggest risk might be storm training and repeated heavy rainfall. Overnight an occluded front moves across the UK from the west. A second front follows on behind which may turn convective in the early hours tomorrow. The suggestion at the moment is that storms do not develop. That second front seems to fizzle out and we get some convection in the form of swift moving pop corn. Forecast SkewTs show no storms tomorrow but it looks marginally possible (models don't need to be far wrong to get storms). Next up is Wednesday night when there is the possibility of severe storms (overhead jet) developing in France moving up across the South East. Long wave trough is properly over the UK with cool air aloft on Thursday. Low winds speed and localised wind convergence means some potent slow moving storms might develop. Central South and Manchester Northwards look most at risk. Still early days and there is a good chance parts of the UK will not see any storms.
  14. Forecast models are not that keen on storms developing today apart from across Ireland towards the center of the low pressure. Models show very weak cape (instability) and a marked cap on convection. I am not convinced about Dewpoints being 4C but the forecast is consistent with the Herstmonceau sounding from last night apart from the cap being a little lower. The Cambourne sounding from last night though is different being more unstable and without the dry air in the mid levels. It is difficult to tell which is most representative of conditions 12 hours later but my hunch would be the Cambourne sounding likely shows conditions just ahead of the front. We also have a trough over the Bay of Biscay which is trundling around the low. Forecasts suggest this stops its forward motion and slowly moves off into France. At the very least a I would expect the approaching vorticity to spark heavy showers over Northern France which will be steered northward to southern counties of the UK. I guess storms are possible in an area just ahead of the front and along southern counties based on some of these observations. Second guessing models is always fraught with danger though. Modelling is really not that keen on storms apart from some embedded in the front over the borders and north wales. Northern Ireland also at risk early on before the low begins to lift out. Maybe someone will get a surprise storm so keep tuned to the radar.
  15. I would not expect wide spread thunderstorms today. Generally speaking we have the jet stream with dry warmer air roaring over head. Even if clouds managed to get up that high the jet stream would shred the cloud top. We do however have steep mid level lapse rates and plenty of wind speed shear. That means strong updrafts and updraft downdraft separation. This looks like very squally heavy showers. If there is low level wind convergence then there is a slight risk a more potent heavy shower. Even though we do not have much risk of storms there is a very slight risk of weak tornadoes due to strong updrafts and wind speed shear. High cloud bases does mean it is very slight. Over all not really much expectation for storms.
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