BrickFielder

The storm threat today will most likely pale in comparison to Hurricanes Grace (hitting Mexico) and Henri (heading for long island). Temperatures are low for Good storm development and the risks are marginal. Forecast modelling differs in the detail so it is hard to identify areas at risk. South West , Wales and West Midlands look like being at risk of post frontal storms developing as a result of both steepening lapse rates and surface wind convergence. Squall line development seems less likely generally although some models develop a surface low early evening towards London ,and East Anglia. Tricky to say much more because models vary so much but the surface low suggested by one model does suggest moderate storm development which is difficult to identify the suggested causation. One of those days where radar watching will give better ideas of the storm risk than the models.

Hurricane Grace intensifying it cat 3 at the last minute before slamming Mexico shows how warm the waters are this year. Hurricane Henri is thought to have speeded up and while convection has picked up significantly the organization of the eye is a little messy. Looks to me like it has exploded on its northern side with a new eye forming in the north west sector. If the eye is more closed in it would be my guess that the intensity will make a last minute surge like Grace. Greatest risk is the storm surge at the minute and flooding due to heavy rain and the forecast slowing as the storm moves inland.

The wind shear hampering Henri's organization should relax later today, and as the storm passes over the Gulf Stream, forecasters anticipate Henri will become a Category 1 or 2 hurricane. Certainly looks like Long Island direct hit now although could still be New York, Boston or even Philadelphia. Regardless the storm surge for somewhere like Atlantic City looks like it could be quite bad. Think some of our experienced Tropical forecasters will be picking up on this soon. Extracts from Tropical Storm Henri Discussion Number 19 Extracts from Tropical Storm Henri Intermediate Advisory Number 19A

Forward speed of the low pressure bringing rain on Saturday means any Storm threat that was forecasted for Sunday moves to Saturday afternoon and will most likely be limited to the south West, Wales and the Midlands. Looks like there might be two bands of upper air vorticity which could trigger convection, but low level wind convergence below these seems to be the forecast trigger. Possible convective line developing at the rear of the front or just behind it followed by a further band of storms. These could trundle right across to the east of the country later on. Will probably get a clearer picture tomorrow but Eumetsat satellite imagery is consistent with the forecasts and shows convection in areas consitent with there being a storm threat tomorrow.

Still looks like there is a small window of storm possibility for Northern Wales through to Hull area. The area could stretch north to Cumbria or South to Central Midlands with perhaps a key area being somewhere like Manchester early afternoon. Perhaps some heavy showers from a low level wind convergence zone across the south west, perhaps getting to storm conditions as they drift across to London late afternoon. Not really convinced about cloud top heights as the window is small and the jet stream around the south of the upper low brings warmer air. Cloud cover and low surface temperatures will limit storm severity even though there is some low level wind shear and strong upper winds near to the jet streak. Estofex forecast in agreement but has there storm area a bit far south.

Whenever I see High pressure forecast directly north of the UK then I tend to adjust the forecast for past events. Typically High pressure between Iceland and Norway seems to be unstable and very transitory. Typically you get a High pressure ridge from Greenland towards Norway (Cold North wind for UK) followed by a Norway High ridging towards Iceland (Warmer east wind for the UK). Looks like the transition to Scandinavia High is an increasing possibility. With an Azores High ridging North the Scandinavian high can result in low pressure systems crossing the UK giving cloudy cool weather, so there is no guarantee for warmer weather.

Models not really in agreement about convective risk tomorrow, but Satellite imagery suggests to me that it is possible. The UKV model suggests some convective activity might be possible from Northern and Central Wales and perhaps up to Cumbria moving across the Country. Upper Level dynamics confirmed by Satellite analysis suggest upper level low ,with some convective activity suggested on satellite imagery . Still looking marginal , but forecast jetstreak in the region makes this worth watching closely. Probably need a few more model runs to get a real grip on whether storm potential will materialize tomorrow.

Sea Surface temperatures across the Central Pacific are starting to have a bit of a La Nina look (cooler) and the Madden Julien Oscillation looks a bit stuck. This can be a sign of cooler weather for the UK. CPC blocking forecasts suggest a blocking High shift from Greenland to Iceland, which can be tricky for weather models to get right. Often High pressure blocking suggested to our North slips eastwards in reality. ECMWF models differ in their interpretation of the signals , but both agree Azores High not really going anywhere. That Azores High will tend to drive a mobile low pressure system across the UK while it is not ridging towards Mid Europe. Temperatures look like having a North South split with periods of wet warmer weather for the South.

Maybe a chance of convective activity Thursday across parts of the Midlands in the afternoon. Its a bit marginal. Further convective risk showing up for Sunday as well.

Certainly starting to look like a transition of global blocking patterns as the Pacific equatorial winds begin to move things a little eastwards. Will be interesting to see how the possible warm seclusion cyclone coming in later this week plays out with the models not used to modelling this type of cyclone at this time of year. The potential development of a tropical storm in the Caspian Sea looks like playing a part in the near future as well. As always weather forecasting modeling beyond 5 days should be taken with a broad stroke and the indicators are generally for a change in the recent pattern.

The warm seclusion cyclone coming in from the Atlantic that the Estofex forecaster was excited about is still forecast and showing up on satellite images as developing a warm core. Modelling does not suggest significant impacts to the UK from this low pressure system, however this type of system usually turns up in January for Northern latitudes so it will be interesting to see how the models handle it. The risks here are tropical storm force winds when trees are still heavy with leaf and strong convective rain at the cyclone core. At the moment there is minimal risk if weather models are correct, but certainly an ominous sign for this autumn.

Chance of a storm in the North East of England, Scottish borders and Glasgow Edinburgh corridor as low level wind convergence sets up. Despite limited instability there is a chance of back building storms and spout type weak tornadoes considering the low level wind field.

So are we all excited then. I am sure you have seen the Fax chart and picked up on the Estofex Forecast. It is not obvious that a warm seclusion is really a 5 sigma event ( Extremely unusual) since many low pressure systems heading towards the UK go down this route. Let us take a step back and I will talk about cyclone types and transitions, well a very simplistic view ( somebody with better knowledge can perhaps run with this). Tropical storms tend to have warm cores and little frontal development whilst extra tropical (our neck of the woods) cyclones tend to have a cold core and have well developed frontal structures. Most tropical storms move North (Northern hemisphere) and transition from warm core to cold core. Occasionally a cyclone once transitioned will redevelop a sort of warm core or warm seclusion. These hybrid storms are usually abnormally low in pressure , can lead to sting jets and have tilt such that there is significant thunderstorm risks. Warm seclusions are also associated with some of the worst hurricanes where warm seclusion causes a rebuild and intensification of a hurricane. No we should not be expecting a hurricane or even a sting jet but this is early for this type of storm and the particular development may be driven by warm sea temperatures rather than cyclone history. It does mean we are stuck again in a blocked pattern instead of a mobile pattern and perhaps the demise of UK Thunderstorms is part of the pattern. May be somebody with better knowledge of cyclone cores and transitions will chip in here to explain why this particular environment is deviating from the normal so far that it might be considered a 5 sigma event.

Pretty much in line with Dan's forecast. Some low level wind convergence and the remnants of the occluded front bringing a pretty moist air profile and triggering some possible storms. Mid level cloud is scooting along whilst surface winds are low so perhaps some updraft downdraft separation and longer lived storms but moist profile is likely to give messy convection. Weak instability and low surface temperatures unlikely to drive severe storms. Some disk of back building storms and localized very heavy rainfall.

Low wave trough digs down south of the UK with the Jetstream sinking away south. Weak low pressure over Ireland begins to sink slowly. Instability is fairly weak due to low daytime temperatures but fairly wide spread for both today and tomorrow. Lightning wizard cloud top forecasts favour Scotland and Ireland for storms today. Key places to watch might be the Glasgow to Edinburgh corridor for slow moving heavy downpours. Tomorrow fax charts suggest an occluded front sweeps across the UK from the west bringing some storm potential in a swathe across England.

A few higher topped clouds with increased rainfall rates showing up in the South West with perhaps a rear frontal squall line. No Lightning showing up apart from Ireland. Currently affecting Swansea but speed estimate would bring it to Cardiff within the hour and towards Bristol stretching up to the west midlands within maybe two hours. Not exactly what was forecast but interesting development and typically these lines have low cloud bases, have gusty rear downdrafts and can create tail end storms at out flow boundaries which can benefit from a little insolation (rise in surface temperature) and low level wind speed shear. In other words you can get very weak spout like tornadoes from tail end convection. At the moment cloud tops don't look high enough to give much more than a swift deluge of rain and gusty wind conditions, but worth watching to see how things develop (beginning to show up on EUMETSAT Images along the tail edge of vorticity band).

Main focus of storms today likely to be over Ireland with slow moving heavy downpours. Possible storm developments for Wales, South West, West Midlands, maybe further across the south. Not all the models agree on timing so for some models surface based storms are limited to Wales and the South West while others move the instability further East I to the early evening perhaps even getting to East Anglia or London. Fairly weak instability across the board but some low base cloud possible in post frontal low level convergence zones which when combined with low level wind shear suggests roll up vorticity ingestion possible. Mid to upper level wind shear is declining at this point so only the leading edge storms likely to have sufficient updraft and downdraft separation to utilize vorticity ingestion. Overall temperatures are low for High enough instability for severe storms and timing of various components will be critical for storm initiation. Gut feeling looking at Satellite then I am not impressed with storm potential, yet something tells me we could have a surprise moderately severe storm. Over to radar watching now.

Pretty much agree with Dan's forecast. Timing of upper air warmth limiting cloud tops looks to be slightly wrong on modelling so perhaps better chance of storms than originally forecast. Key areas north Midlands , Yorkshire, Merseyside along with parts of Scotland and Northern Ireland. Eyes are then drawn to Friday. Temperatures are limited so severe storm possibility is limited but an interesting scenario. Scenario is likely to change as we get closer to Friday. I did notice some instability for Iceland , Norway today which is likely to enrage the no storms club.

Not really a classic thunderstorm day. Upper level disturbance with jetstreak over running the southwest. Complicated by Eumetsat images suggesting modelling slightly off. Favourable areas looking like Wales and Midlands late afternoon and perhaps towards London early evening. Low level wind convergence suggested with South West and South Coast areas most likely to see convergence. Instability looks tied to humidity and highest in the Bristol area. Yes that is pretty weak instability and when you look at cloud cover coming in from Ireland even what little is suggested might not occur. From those charts looks like heaviest convection would be in the southwest, but forecast skewTs are telling a modified story affecting depth of convection (cloud height). Cloud height potential for the Midlands looks slightly better than for South West. All needs to be taken with a pinch of salt especially when seeing todays satellite images but worth keeping an eye on. My take would be instability is too weak for much more than an isolated storm, but it is interesting modelling agrees on precipitation for London area mid evening (May have missed something about the upper dynamics here).

Yes risk of a few thunderstorms in the Eastern Scottish borders and may be the north east of England. My eyes are also drawn to North Devon late afternoon where a low level wind convergence takes place. There is a risk of convection occurring here by the looks of it although detail models don't agree. I would struggle to see how deep convection occurs but if it did then the wind parameters might be favourable for a supercell or tornado. At face value looking at all the models you would have to say No way, you are having a laugh, but there is just this niggle that makes you want to double check and keep an eye on the radar. Difficult to assess so very low risk. Overnight intense little low pressure heads towards the UK bringing some unseasonal gusty winds to parts of the southern UK. Models are still wobbling about the exact track but potentially gusts up to sixty miles an hour might be possible. Equally there may be some thunderstorm risk from even the early morning.

Chance of storms this afternoon for Western Wales and Ireland. Later into the night we have what looks like an occluded front bringing elevated imports into the southern half of the UK. High cloud tops outflow dominant storms are likely to bring a Good lightening show and localized flooding. Mainly south west and south coast but perhaps beginning to become a little surface based early Saturday morning across London and M4 corridor. A word of caution about the forecasts. You need to pay close attention to where the instability is and where the steep lapse rates are. So the area to look at in the 8am to 10am period is the South East. Its perhaps mid afternoon in the South West that conditions come together for surface based storms. Weak wind shear , bit of a cap , low cloud bases and moderately High tops with slight jet over running. Looking at Satellite images then this forecast could be significantly off in terms of severity and location but caution is needed to prevent over hyping the storm potential. Some Good photo opportunities are likely though if you get the location right.

Models suggest that humidity will be down today compared to yesterday and any upper level support will be moving out of the UK. Personally I am not seeing dewpoints drop off too much and the overnight soundings do not look much drier than than the day before. Similar pattern of low level wind convergence as previous days with Cumbria and London being areas of possible storm development. Not totally convinced about the lack humidity and dry air compared to yesterday but the modelling has been pretty good in recent days. Satellite images confirm upper level disturbance is mostly on its way out of the UK so storms like yesterday are unlikely.

Very similar to yesterday with local heating and low level wind convergence zones. I say the same as yesterday but late afternoon early evening there may be a different storm opportunity. I did think maybe it was driven by upper level vorticity but looking at the forecast skewTs it may be driven by a bit more moisture at the 700 -800 hpa level. These may not be surface based storms by 9pm. Looking at overnight soundings suggests that 700-800 hpa moisture is prevalent across the UK apart from the Cambourne sounding in the South West. Comparing the EUMETSAT satellite images and forecast upper level vorticity raises some questions about upper level dynamics as well. Best guess we have some London and South East storms during the afternoon and the early evening storms for East Anglia with a chance those early evening storms might be a little more widespread. t

Same as Yesterday except it is a little less dry aloft. Models seem to suggest south east corner of the UK might get a storm and fax chart hints at a surface low. Forecast SkewTs suggest instability is available and seem a bit wetter aloft that yesterday. Possibility of an upper air disturbance crossing the UK as well. Does not seem to overlap with low level convergence but will be interesting to see how things play out. Think it is likely to be similar to yesterday with a few more mid level clouds. Nottingham sounding from overnight is consistent with forecast SkewT's except the warm nose at 500 hpa is marginally still there. In the channel south of the the south west looks like a few showers could develop from Satellite images. As yesterday not convinced storms will develop and looks like the south east is likely location if they do. Looking further out to the end of the week sees a Rossby wave push towards northern France and as it comes in it should create air divergence aloft at the jetstreak exit which may create conditions for thunderstorms. The models do keep changing on this with one run keeping things south and into France only and the next moving the wave eastwards too fast for it to really come into play for the UK. Too far put to put any detail on later in the week at the moment. For today I am watching the showers in the channel to see if they develop.

Tricky to say whether storms might be on the cards today and generally speaking anticyclonic environments are not conducive to storm development due to dry sinking upper air. The other problem with anticyclonic conditions is that although there is instability there is no trigger for storm initiation. Low level wind convergence, orographic lift (hills and mountains) become your only triggers. Today modeling is not really in agreement about where low level wind convergence is. There is no agreement on whether convection occurs at all. Looking at forecast Skewt's I can see instability with moister air coming aloft from the West. Comparing to soundings and current conditions I can see a tendency to underestimate dewpoints (17C already in some places) but also that humidity is very shallow (Nottingham sounding from last night) and with a bit of a warm nose at 500hpa. Pulling it altogether is a bit tricky and convection is likely to be isolated and I think late afternoon when moisture comes in aloft would be the likely time for storms. Lightning wizard charts show some risks of supercells and spouts in places so an isolated storm could be severe. My gut feeling is there will be no storms, but I am watching an upper level disturbance on the Eumetsat images (satellite RGB images) currently across northern England which is forecast to slip southwards over the Midlands later to see if it over runs a low level convergence zone (Chance of a surprise storm). Not sure what other forecaster's think?