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2016-03-21_19-02-57

The Atlantic is showing off some classic visible cloud features of cyclone birth and decay today.  Systems labelled 1-4 on the satellite photo above show different features including stages of cyclone / mid-latitude depression formation and decaying high pressure ridge all on one satellite picture.  The chart below shows the same view with fronts.

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Starting with LOW number #2 (why not?!): the spectacular classic cloud spiral of LOW #2 indicates a mature low occluding and filling.

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This maturing occluding LOW has a couple of interesting extra vortices near the low core.

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Despite their angry look, classic cloud spirals like this on satellite photos are actually decaying and filling lows, losing their strength as pressure rises in the low core.  This particular LOW has a spectacular cold front of over 1500 miles stretching from 60N to the sub-tropics. The red colours on the RGB false colour eumetsat image below shows the cold continental polar air surging in behind the cold front.

Low #1 is a rapidly intensifying LOW off the coast of Labrador.  It looks harmless as a smudge of cloud but this shape … a so called “baroclinic leaf” indicates the birth of an angry developing storm: rapid cyclogenesis.

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This will deepen and pressure will fall rapidly in the next 24 hours as frigid continental air collides with humid maritime air under the influence of an active 200mph jetstream.

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LOW #1 is expected to form a big storm in the Labrador Sea by Wednesday. The fronts on this storm are then forecast to stretch clean across the Atlantic and bring the SE our first frontal rain for over a week by later Thursday.

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System #3  on the top satellite photo shows the HIGH pressure lingering over the South of the UK but regressing into the Atlantic.  This ridge has dominated mid to late March weather in the UK but brought a lot of anticyclonic gloom to the SE.  The deflating ridge will allow a more unsettled Atlantic westerly regime to dominate late March and early April weather.

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Low pressure #4 is an interesting developing depression in the Mediterranean, courtesy of a southward limb of the jetstream. Currently a disturbance dumping snow over the N Atlas in Morocco, this LOW is set to deepen across the Mediterranean through the week.  It will track directly ENE through the Med and bring snow to the Atlas mountains, rain to N Algeria and foul wet, windy and wintry conditions to Italy and then more snow and wintry weather to the Balkans.

Finally, for the UK our weak ridge is deflating to the SW and this will open the door to zonal westerlies and frontal systems bringing rain and wind from later Thursday and into the weekend.  Ensembles below show the dry spell ending this week and some notable rainfall spikes in the days to come, especially over the weekend.

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How high can we expect temperatures to go and how Spring-like is this weekend set to be in Reigate? Find out below!

This weekend and possibly for much of March, the weather is set to be dominated by HIGH pressure nearby to the south and low pressure to the NW which will bring in mild southerly or SW winds for Reigate.  Cooler and cloudy easterly wind directions are also possible later next week if the high pressure slips NE to Scandinavia which models are suggesting.  Whilst this weekend is expected to be pleasantly mild and spring-like and initially sunny on Saturday, temperatures are not going to break any Spring records because cloud cover is gradually going to spill from the north on a weakening cold front.

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synoptic chart Fri 6 March

The synoptic chart above shows the HIGH over the continent to the south of the UK and a deep low to the NW between Iceland and Greenland.  This is dragging in, with the help of a lively SW jetstream, a SW moist airflow over Scotland.  In fact NW Scotland has an amber warning for huge rainfall totals above 140mm over the next few days assoicated with the stalled cold front that will sit near or over Scotland for much of the time.  Warm air flow ahead of this cold front is advecting large amounts of moisture in a plume over the mountains which is causing the high totals over the NW. The charts below show the unsettled NW compared to the calm, mild and dry SE of the UK under the influence of the HIGH pressure.

The charts below show the story for this weekend.  Initially a dry airmass sits over the SE on Saturday morning but this is gradually replaced as cloudy conditions slip SE.  Temperatures through the weekend are looking mild, with Tmax 15C possible on Saturday, but anything higher is less likely on Sunday as cloud further thickens with the arrival of a weak cold front.  Saturday is probably the most pleasant day with brighter sunnier conditions especially in the morning.  The cold front in Scotland slowly migrates SE during the weekend but weakens as it does so.  By the time it reaches the SE on Sunday evening it is probably only going to bring low cloud and some drizzle.  Throughout the weekend wind in the SE is set to be light, especially on Sunday.  Misty conditions might occur overnight into Sunday and later Sunday evening in light winds.

Next week is looking generally mild and with HIGH pressure not far away to the south mostly dry.  A couple of LOW pressure systems are forecast to pass across the NW of the country and their trailing fronts will be weak in the SE but could bring cloud and some light rain.

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LOW passes to the NW on Monday

Overall the high pressure looks set to dominate Europe during next week bringing dry and warmer than normal conditions.

The GFS and ECM both suggest that the HIGH could slip over Scandinavia by the end of the week, as the chart below suggests.  This would introduce cooler easterly winds to the UK but nothing too icy at this time of year, it would also remain mostly dry. Unfortunately, easterly winds are often cloudy as they pick up moisture from the North Sea that creates days of anticyclonic gloom under an inversion.

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Scandinavian HIGH… would bring dry, cool but gloomy March weather

The charts below summarise the weather outlook: high pressure domintating bringing mostly dry and mild conditions.  Nights next week could turn colder with possible frost returning.  The longer range models suggest March could turn out to be a very dry month especially in the south.

BBC summary

http://www.bbc.co.uk/weather/feeds/31774369?ns_mchannel=social&ns_campaign=bbc_weather&ns_source=twitter&ns_linkname=news_central

 

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Water vapour EUMETSAT rgb airmasses: Arctic red, tropical blue

Update Tues pm: system not bringing any significant snow to Reigate, Midlands north might see accumulations but even this is not likely to be disruptive..  The satellite image above shows water vapour and airmasses.  Notice how tropical (blue!) and Arctic (red/mauvey) airmasses are intruding across latitudes – plunging respectively north and south of their source regions.  Cold Arctic air is reaching right the way down to Spain.  Here’s a look at what is going on and how things might develop for Reigate and SE UK especially this week.  Overall things look cold and wintry and there is a slim possibility of snow for us but it’s worth watching the forecasts and twitter updates because it’s very marginal and things can change locally quite quickly.

The Arctic air arriving in London Monday left the Kara Sea a week ago. This Arctic air is arriving over the UK courtesy of a high pressure ridge over the Atlantic and Greenland / Iceland which blocks mild maritime air in the Atlantic from reaching the UK. A low (the remnants of storm #Rachel) over Scandinavia is dragging down cold northerlies assisted by a northerly jetstream aloft.  This set-up makes this week the coldest since March 2013 and, as some light snow fell in Reigate this morning (Sat 17) then that was the first snow fall here for 2 years.

The result of the pressure pattern is airmass temperatures lowering at 850hPa to -7C or lower over the course of the next 48 hours due to the steady invasion of Arctic air. Overnight tonight into Sunday morning a front in the Channel could bring light snow showers to southern England early Sunday morning.  The situation is “marginal” as the truly cold air has yet to arrive in the south and dew points are hovering around or above freezing tonight which can make a difference between whether it snows or rains.

Over Sunday the Arctic air will arrive and the upper air mass temperature at 850hPa (1500m up) will fall from -4C to -7C by Monday.  In such a cold airmass the surface air temperatures on Monday will struggle above freezing during the day to about 3C and wind chill will make it feel colder.

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An important threshold for snow is an upper air temp at 850hPa of -5C or lower.  So from Monday any precipitation might fall as snow, so long as other factors are in place.  The chart below shows the movement of the Shetland low into the N Sea and eventually further south which ushers in northerly winds.

LOW sinks south bringing Arctic air

LOW sinks south bringing Arctic air

There is reasonable model agreement as to what will happen into mid-week but uncertainty thereafter as to how long any cold will last.  High pressure is set to build over Scandinavia blocking the NE track of a low S Greenland.  A trough disruption is set to occur when the LOW near Iceland splits from the main trough and slides down the edge of the Atlantic high and sinks SE over the UK (called a slider low).  Trough disruptions are notorious for causing models problems with accurate forecasts!

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trough disruption

The slider low will bring attendant fronts with milder air mixed in, thus complicating chances of widespread snow and making forecasts tricky.  Tuesday is the first chance of any snow this week for the SE as a front moves in from the west to reach the SE around the afternoon, though details this far out cannot be certain.  As the front moves into colder air the rain could turn to snow, especially on the back edge as overnight temperatures fall.  It is very likely to be snow across the middle of the country and certainly over high ground but snow for the SE is less certain, it could be just sleety or rain depending on the mix and location of mild air from the south in the occluded front. In any case Tuesday looks light precipitation as the front weakens to the east.  This doesn’t help snow formation in marginal situations because less cold air is dragged down from aloft in light rain and there is less evaporative cooling in light rain.

Charts currently show that Wednesday has a better chance of snow action for the SE as another front, this time with heavier rain, clears east later on Wednesday or overnight into Thursday.  Exact timing is uncertain and indeed the development of this might all change despite there being sound model agreement as to the overall synoptic situation into the mid week period.

The latest UKMET chart for Thursday shows the SE in a COL between HIGH pressure SW and NE and LOWS NW and SE. This looks like a cold wet day with sleet for Reigate, but snow is again possible, especially as the LOW drifts south and introduces a cold NE continental flow for a time on the northerly edge. This is most likely to restrict to inland areas or those higher up locally but it needs watching carefully as heavy rain might tip over into sleet then snow due to evaporative cooling.

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After mid week things look like turning somewhat milder for a while as westerly winds eventually break through properly, possibly by late week or the weekend.  This means snow chance reduces to nothing as milder Atlantic winds return.  Nevertheless, the long range models still show some propensity towards building further chilly Arctic incursions later too.

Reigate weather for next week to 10 days is overall set to calm down somewhat and, for us in the south, be generally drier and warmer than average for the time of year, most of the time. Temperatures are unseasonably warm over the next few days with night times barely falling below teens and daytime reaching 21C.  This is due to the southerly winds bringing warm air from Spain.

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A slack static cold front divides the really warm air over the SE from the cooler air to the NW. This cold front will bring cloud and rain at times across the SE, especially to the south coast, as it drifts SE over the weekend.  Winds on the coast will also be more noticeable over the weekend with 20mph+ possible, but staying mild over the weekend. The temps are likely to slip very slightly as the weak cold front edges SE on Sunday bringing some rain through the SE and more wind to the south coast.

Unfortunately, there is a hiccup to this generally benign warm weather. A gradual rise in pressure is set to be sharply upset temporarily by the remains of Hurricane Gonzalo, now battering Bermuda as a Cat 3 storm and due to arrive UK early next week late Mon /Tues.

Gonzalo hiccup

Gonzalo hiccup

Gonzalo is modelled to arrive early/mid next week.  Forecasts suggest Gonzalo will merge with the persistent Atlantic low near iceland and ride the jetstream across the Atlantic arriving late Monday as a 980mb low (not especially low) , bringing gales initially to the west and NW coasts and then gales to through the North Sea later the same day.  Some heavy rain is likely but the SE and Reigate looks currently likely to miss the worst.

After its passage across the north of the UK, the centre of Gonzalo appears to move SE down into the North Sea and behind a potentially vigorous cold front usher in significantly cooler NW winds of some potency, albeit briefly.  These appear to peg down temps a while, especially on NE coasts. Thereafter, indications are for a gradual improvement in the south as high pressure builds over the continent, albeit with some fronts reaching across to the south at times before a HIGH pressure seems to be suggested for the last week in October.

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If this comes off a dry and settled spell can be expected for half term.  The location of the HIGH starts in the south where mild conditions can be expected but the anticyclone could slip north and topple east.  if this happens then temps will fall as a cool easterly flow are brought in from a cooler continent creating a more foggy autumnal feel.  The charts below show the possible scenario for the last week in October and very start of November with things cooling off.

So in summary the weather for half term is generally settled, warmer and drier than average for the time of year but with a significant hiccup as Gonzalo arrive early next week bringing a plunge of cool polar maritime air down across the UK and the north sea behind this sharp active system.  Thereafter, a gradual improvement to the last week of October when an anticyclone is set to build across the UK bringing autumnal foggy end to October as temperatures drop to November.  Note that extra-tropical storms upset models so keep an eye on any changes to this forecast.

forecasters: SAC and Chris M

 

 

 

The “Spanish Plume” is a special weather set-up which can produce thunderstorms in the UK, as well as France and Benelux counties and beyond.

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A Spanish Plume is when hot dry air from the Spanish Plateau moves north towards the UK.  This occurs when LOW pressure exists in the Bay of Biscay and a HIGH pressure builds further east over Europe or N Europe / Scandinavia. An “omega block” is one such pattern but several different types of Spanish Plume are recognised.

As the warm dry Spanish air moves north from Iberia it picks up moisture from the Bay of Biscay, increasing the humidity of the lower layers but retaining dry air aloft.  The combination of these two airmasses, moist surface flow and dry upper flow, creates instability and potential thunderstorms. The unstable airmass can further lift over any relatively cooler air residing over N France and the UK (isentropic lifting). The lifting of the air column allows thunderstorms to grow even further.

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Jetstream and upper flow from the South

Thunderstorms are directed north on the upper winds and jetstream from the south where, given an approaching cold front, lapse rates can increase further and the intensity of storms can grow over France, Benelux and the UK and further into Europe.  The Spanish Plume is a complex weather pattern which needs some special ingredients to prime the atmosphere. Like all thunderstorms, Spanish Plume storms are based on convection in an unstable atmosphere which occurs when thermals of air rise uninhibited to a great height creating tall cumulonimbus clouds.

Unfortunately, thunderstorm formation is difficult to forecast, often being altered by local factors below the resolution of many forecast models.  The localised nature of storms also means that some places may see a tumultuous thunderstorm of epic proportions while other places not far away will see very little impressive action or nothing at all. Quite frequently plumes will skirt to the east of the UK and miss us entirely, storms staying on the continent, as initial forecasts of a “direct hit” are corrected east.  Cool sea surface temperatures in the English Channel can also deflate imported storms from France. So some understanding of the plume might help figure out how much we can expect from any convective action forecast with a Spanish Plume event!

For a wonderful time lapse of the plume passing over the UK watch this…

Read below to find out the ingredients for a Spanish Plume but, if you are in a rush, then read the BBC weather summary here (it’s a bit shorter!) 🙂

Setting the stage:

The large scale “synoptic” pattern required to prime the atmosphere for a Spanish Plume is for a cut-off LOW pressure to locate around the Atlantic coast of Spain, ideally moving into the Bay of Biscay. The LOW in the Atlantic therefore sits to the SW of the UK.  An upper level trough nudges this LOW east across a hot Iberian plateau and France picking up humidity across the Bay of Biscay.  Strong solar heating over Spain creates a thermal heat LOW circulation and local thunderstorms here.  The stage is set for a Spanish Plume!

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Spanish Plume set-up

Warm plume followed by cold front chaser!

How does this “plume” head towards the UK and create thunderstorms here? Prior to the arrival of the upper warm Spanish plume in the UK, a humid surface SE flow from the continent feeds heat and moisture into the UK at low levels, steepening lapse rates. SE winds in the UK in summer frequently herald thunderstorms due to the low level advection (movement) of heat and moisture from the continent, both critical to feed thunderstorm formation.

Meanwhile, the upper SW flow (jetstream) ahead of the upper level trough pumps warm air from the Spanish Plateau northwards. This plume rides a conveyor belt of air rising towards and over a warm front as it travels north in a warm humid sector. Now an approaching cold front from the west is needed to really kick off storms by destabilizing the atmosphere!

The LOW pressure system has cool moist Atlantic air wrapping behind the cold front (seen on satellite pics below as the white line of cloud extending from the mother low to the NW).  A warm surface conveyor is driven NE in the developing warm sector ahead of the cold front, guided by upper level winds and the jetstream.

The rising warm plume has cool Atlantic air moving in aloft ahead of the cold front.  This cool air aloft can override the warm plume, increasing lapse rates and destabilizing the atmosphere further.  Sometimes this scenario can cause huge thunderstorms called meso-scale convective systems (MCS) or sometimes supercells.

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Any solar heating will further rapidly destabilize the atmosphere creating surface based storms where temperatures rise. Storms are often squeezed into a narrowing band or chain ahead of the cold front.  The cold front itself often weakens as air tends to sink after the passage of the thunderstorms.  The cold front can pass over unnoticed behind the thunderstorm event, its main job is to advect cold air over the rising warm plume to increase lapse rates.

HIGH pressure over the Mediterranean can also assist the process by advecting warm / hot air out of North Africa, over Spain, into France and finally reach Britain across the English Channel.  The more heat the better.

Sounding unstable 

By itself, a warm moist wind wafting up from Spain will not necessarily create the biggest bangs.  Other, ingredients are required to cook-up a good thunderstorm, with large hail, cg (cloud to ground) lightning and thunder and even funnel clouds or a tornado. Thunderstorms need an unstable atmosphere to trigger the air to rise rapidly UP. Such triggers can be spotted on a skew-t or sounding chart.  Cooler dry air sinking from aloft ahead of the upper trough can destabilize the atmosphere by increasing lapse rates and CAPE (convective available potential energy). This occurs because dry air at mid levels (4000m-7000m; 400-600hpa) can cause evaporative cooling of the atmosphere.  Such cooling aloft will mean that rising parcels remain unstable to a greater height and rise freely.  The sounding below shows all the ingredients that caused a supercell storm (note dry slot at 500mb). (this is for Texas, but same applies!)

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thunderstorm sounding

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unstable sounding for Glasgow airport July 1 2015

Another important factor in thunderstorm formation especially for the UK is TIMING.  The arrival time and combination of unstable air masses, fronts and moisture is what makes or breaks storms. Any element that is premature or delayed can be the death of expected storm formation. The best example of this is whether unstable air arrives during a sunny warm day or at night, when surface based heating is absent.  In the case of the Spanish Plume event 7 June, the unstable air arrived at night over a comparatively stable boundary layer surface air mass.  The only storms that resulted were elevated thunderstorms embedded in the unstable upper air ahead of the cold front (which oddly brought warmer conditions later in the day as cloud cleared and sun came out but too late to coincide with the impressive instability of the plume).

A cap in the sounding can also enhance convection.  A temperature inversion can hold heated air near the surface building convective energy which is held to the ground until the cap is broken by daytime heating… if it is sunny enough, cloud cover will spoil the event.  Busting the cap can create explosive and dangerous thunderstorms, rarely seen in the UK.

Here is a summary of some other ingredients as it applies to the Spanish Plume like the one on 7 June 2014 over the UK:

Heat! the SE of England is due to get warm or even hot on Saturday with Tmax temps well over 25c in sunny spots.  This will warm the air at the surface and, like a hot air balloon, these air “parcels” will want to rise (called lift). Warm air is also being moved into the country from the south by a process called advection.  Charts showing the heat energy providing the potential for air to rise are called CAPE: convective available potential energy.  In the UK we are pleased with CAPE values of 200-500j/Kg for some thunderstorms.  On Saturday we might expect values of up to 3000j/Kg. The other measure used to assess thunderstorm potential is the LIFTED INDEX (LI) this measures the difference in temperature between a parcel of air lifted to 5000m and the temperature of the air around it.  Negative figures show the air is buoyant and ready to rise.  Values of -8 or -9 are unusually low and show a very unstable airmass with the potential for plenty of lift!

In a Spanish Plume event the upper air contains enough energy and moisture to produce elevated thunderstorms even in the absence of surface heating: this means moderate thunderstorms can occur at night and with extensive cloud cover. The morning moderate storms experienced in the SE on Saturday 7 June were all elevated thunderstorms because extensive cloud cover throughout the morning meant an absence of surface based heating to kick off more purposeful convective activity.  Here’s an excellent blog explaining elevated thunderstorms compared to surface thunderstorms that would occur due to surface heating.

Moisture! local SE winds are frequent precursors for thunderstorms in England. This is because, in summer, SE winds are often humid with a relatively high water content, advecting (moving) lots of moisture into the SE: high dew points illustrate this with some reaching 20c on Saturday… a muggy humid day. This moisture will be required, of course, to form clouds.  To form really big clouds you need a lot of water in the atmosphere.  Once water vapour starts to condense it releases latent heat and this heat gives additional lift to convection and feeds thunderstorm formation.  Interestingly and perhaps counter to what might be expected, a dry plume of air mid-way up through the atmosphere is also an important ingredient to the production of big thunderstorms.  Drier air at mid-levels aloft gives the atmosphere added instability for the production of thunderstorms.  The reason for this is that dry air cools more rapidly with height than moist air (because rising moist air releases latent heat when clouds inevitably form and this additional heat reduces the rate of cooling of saturated air with height).  The rate of cooling with height is called the “lapse rate” and the lapse rates or temperature gradient on Saturday is steep. So moisture is a critical ingredient to storm formation because it controls instability and cloud formation.

The skew-t charts show temperature change with altitude.  They are called Skew-t because the temperature lines are skewed off the vertical slightly.  Whilst they are initially odd to look at, focus on the red and blue and dashed lines: if the red and blue lines are close together it means the air is saturated (cloudy).  If they are far apart then the air is dry. If the red line skews to the right then this is known as an inversion where temperatures can increase or stay the same with height.  Such an inversion will prevent thermals rising and the formation of clouds from surface convection, a critical ingredient for big thunderstorms.

more analysis of Skew-t

more analysis of Skew-t

The other ingredient the skew-t chart shows above is plenty of wind shear with height, in this case speed shear.  Wind shear is the change of wind speed and/or direction with height.  Increasing wind speed with height has the effect of dragging air off the ground like a hoover. On Saturday there is a jetstream moving overhead during the day and this will cause divergence aloft and encourage more air to lift off the surface. Strong directional wind shear, when winds turn at angles through the atmosphere and when different winds directions meet around fronts, is also a trigger for tornadoes.  The UK quite regularly has tornadic conditions in lively convective thunderstorm weather but the ingredients for tornadoes are a even more fickle!  Our tornadoes are considerably less powerful than those in the USA but can be of great interest and still cause damage by uprooting trees, damaging rooves and chimneys and even tipping cars.

Lift! like a hot air balloon, a warm bubble of air will only rise if it is warmer than the air around it.  On Saturday morning a CAP will exist in the atmosphere that will prevent air lifting far off the surface, thus preventing extensive vertical lift. The cap is known as a temperature inversion and the cap on Saturday is pretty solid, so little convection is expected early on, except possibly in unstable upper atmospheric layers which might cause elevated thunderstorms.   During the day, if there are suitable breaks in cloud cover, the sun will heat the surface and this will start to break down the cap.  A strong cap has the effect of building energy and heat below and, if the surface heats up sufficiently then the cap can be broken suddenly.  This usually happens in the afternoon when a sudden explosive thunderstorm could be produced.  In the mid-west of the USA tornado chasers call this process “busting the cap” and it produces the possibility of extreme weather.  On charts the cap is shown as convective inhibition that acts against convection but, at the same time, can be an ingredient for extreme weather.  Another mechanism that is capable of lifting air rapidly is the proximity of fronts which can mechanically lift the air as different air masses converge.  On Saturday, a slow moving cold front is forecast to be located along a north/south axis through the Midlands and S England, moving gradually east… this could be the focus of most activity if the cap remains solid further east.

Convergence of winds! winds converging at the surface are another local or regional factor that enhances convection. Converging winds at the surface tend to slow down and pile up, like lorries slowing down uphill and causing congestion.  When surface winds pile into each other (converge) they can only go one way… UP!  Convergence often occurs at the coast where winds coming from the sea slow down due to friction over the land.  This is one reason why the south coast often produces thunderstorms or enhances them as they cross the Channel (so long as the Channel sea surface temperature is warm enough – another story!). The other map below shows areas where there is a sufficient combination of wind shear, heating and energy to possibly start to rotate a thunderstorm: rotating thunderstorms are called supercells and are capable of producing tornadoes.  These are the “big daddy” of thunderstorm cells and would be an awesome site for anyone able to catch a photo before or after the inevitable heavy rain, hail, lightning, darkening skies and thunder!

So the action of winds converging at the surface and diverging aloft or doing the reverse, is important to thunderstorm activity. The charts below (website ) are a cross-section through the atmosphere from S-N and W-E across the UK before and during the plume.  The streamlines showing wind vectors illustrate the turbulent nature of the air during the plume.  There are elements of both lift and subsiding air at different elevations showing the complexity of the plume event.

Even with so many ingredients primed for thundery weather here in Reigate on Saturday it still remains a risk potential rather than a certainty.  Local factors might inhibit convection, too much cloud in the morning could reduce surface heating and the cap may not be broken, so no thunderstorms! It’s a matter of waiting to see the conditions nearer the time to make a final forecast and often it remains uncertain until the very day.  Nevertheless, there is a possibility of explosive convective thunderstorm action on Saturday, mostly in the Midlands and the East, if the CAP is BUST!  If this occurs then instability of the atmosphere will rapidly build extremely tall cumulonimbus clouds up to 10,000 metres tall.  The UKMO issued an uncharacteristically early warning of heavy rain for Saturday on the basis of this Spanish Plume event. Others will doubtless follow, but it is safe to ignore the daft scaremongering of the Daily Express but do keep a watchful eye on any dark clouds!

Although an accurate forecast is beyond the scope of this post and best left to the professionals, one broad indicator of possible storm pattern over the course of the day is set by convergence of winds and the likely position of the front as it progresses west to N/east.  The first plume of instability arrives in N France later on Friday and some might arrive S England Friday pm with low risk thunderstorm activity imported across the Channel from France (as it happened this occured only in the SW).  The charts for Saturday below show how the main location of biggest storm activity could move broadly from west to NE during Saturday and clear off into the N Sea overnight. (as it happened the front moved much quicker and clear S England and most parts of Central England by lunchtime leaving a bright afternoon with bubbly Cu).

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The animation here also shows the total rainfall expected to accumulate during the 48 hour plume event.

Whatever happens, do watch out for altocumulus castellanus in clear skies or asperatus undulatus (pic above) or shelf clouds and cumulonimbus developing ahead of storms as the plume arrives on Saturday and then watch for any cumulonimbus clouds exploding if the cap is bust later on!  Please send in your photos of any interesting weather phenomenon to RGSweather : on twitter and facebook and email.

Comments and any additional information always welcome!

sources: many thanks to these sites:

The weather has been mercifully quiet, with mostly warm and sunny conditions for Reigate over the last 2 weeks. This is gradually about to change but no particular drama is afoot other than some perky and windy frontal rain arriving in Reigate on Thurs night /Fri am, followed by showers and cooler breezier weather with hopefully some interesting clouds over the weekend!

Our two week dry spell (well, 13 days) ended briefly today with a light shower.  As the blocking high sinks away to the S, a cooler NW flow will swing by over the weekend ushered in by an active cold front on Thurs/Fri which will mark the arrival of some cooler days and frosty nights through to early next week.

The charts show a typical early spring flip-flop in temperatures as warm air from the SW on the back edge of HIGHs is replaced by cool NW and even N winds as LOWS sweep across the North of the UK.  The back-edge of the LOW moving over Scotland in the next few days will drag down chilly Polar air especially on Saturday and Sunday.

Daytime temps will struggle to double figures which, when last weekend hit 20c might feel a bit of a shock!  This cool airtstream, fresh from Polar regions, will also be warming through from below and therefore get rather unstable: this means thermals will have a tendency to rise, and keep rising through the chilly upper air, creating towering cumulus clouds and showers.  The chart below shows a typically unstable temperature height diagram with some ingredients for a showery day capable of building some nice cumulus clouds: steep lapse rate, saturated airmass through a large column of the atmosphere, some wind sheer (change of wind speed or direction with height).

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By the afternoon especially on Saturday cumulus congestus clouds might even be tall enough for some sharp showers, possible hail in the cold upper air and the odd clap of thunder is not impossible.  Nevertheless, total rain predicted is low and there will be sunny intervals. April showers, but a month early!  Next week sees a continuation of flip-flop spring weather.  April has the possibility of some high pressure returning with sunny and dry weather but the exact timing of this remains uncertain.  There is also some indication of unsettled pattern in April too, so take your pick!

LOW over Scandinavia

LOW over Scandinavia

Weather is chaotic and numerical weather models are not perfect. The forecast for Reigate today went rather awry, though not completely.  It was forecast to rain heavily, perhaps on and off, but the forecast was for heavy rain more or less throughout the day. Check UKMO forecast from yesterday below.  Some models brought 24 hour totals of 20-30mm to SE at points on the lead up to the event.  The cause of the forecast deluge: a small scale low tracking NW to SE with a tightly wrapped occluded front crossing the area once, then lingering nearby to deposit more rain during the day before drifting off southeast. Once the front had passed through early am, it turned out to be a splendid day with sunshine and bright spells throughout, until rain later.  So what went wrong/right?

The front passed over as forecast during early am dropping 6mm on Reigate before 8am.  It then sat N of London most of the day while further south convection over Sussex caused significant Cb clouds and showers (some thundery) to spark off from midday.  For us in Reigate, we had a splendidly bright day with glorious sunshine by 8am and bubbly cumulus clouds thereafter, the odd spot of rain but nothing significant until early afternoon when the front migrated south east.  So for most daylight hours Reigate was dry, quite the opposite of the forecast.

The photos above and graphics below suggest a possible reason for this.  Reigate sat in a sort of “Goldilocks Gap” between the persistent frontal rain further north and convective rain nearer the LOW further south. It is notable that the convective showers built mostly over the land, showing almost April-shower tendencies to build on warmer land surfaces than the now-cooler sea. The occluded front sat close to Reigate, frontal wave clouds and cirrus were visible above and to the north for most of the day.  This may have helped suppress convection.  As warm tropical air is lofted over an occluded front it spreads out and forms a cirrus veil, this often suggests a broad inversion of warmer air aloft that effectively suppresses uplift of thermals: the cirrus acts like a lid.  So cumulus clouds over Reigate and the N Downs stayed small and harmless.  Not far south, in Sussex, thundery downpours developed as the buoyant air lofted uninhibited by any inversion.  You can see this on the radar image below.

Reigate was therefore dry for most of the day perhaps because of our location in a sort of Goldilocks Gap (our word) that was just far enough from the occluded front to avoid persistent rain and just near enough to benefit from the inversion to prevent convective showers. Met-Magic!  The graphics and photos try to explain this further.

This is just one possible reason why slight changes in the tack of a LOW will render a forecast completely wrong, even in the middle of a LOW pressure when all hope of a nice day might be thought lost.  Further ideas are most welcome to extend this.

Reigate recorded a max gust of 36 mph last night as a vigorous ana-cold front produced a SQUALL LINE of extra heavy rain and gusty winds that rapidly crossed the country from west to east.  Needless to say, the squall line reduced in strength as it arrived in Reigate and the SE but we still experienced some gusty winds early on.  Nothing like as strong as the 80mph+ winds further west in Wales with associated hail.  An ana-front is where the warm sector air is forced to rise more vigorously than “usual”, in this case due to a strong jetstream encouraging uplift of air from the surface due to divergence and rapidly falling pressure aloft.  This literally “hoovers” air off the ground and produces lots of condensation, thick cumulonimbus clouds and plenty of heavy rain. Wind max during this storm for UK: S Uist 90mph; Plymouth 85mph. Other high winds include:

max gusts assoc with cold front

max gusts assoc with cold front

The culprit was the LOW pressure shown below in the UKMO analysis chart for 18 Dec.  A cause of this extra vigorous cold front was cold Polar air originating from Canada.  Spot the brisk W/NW winds following closely behind the front.

18-12-2013 20-24-36

The front produced 7.6mm of rain in a few hours. In total, since Saturday, Reigate has had 30mm of rain, so expect river levels to be high.  The River Mole has a flood warning in place from the Environment Agency. The Daily Express made a good call and got their prediction spot on a week ago with this article http://www.express.co.uk/news/nature/448402/Freak-storm-to-batter-Britain-100mph-winds-and-downpours-to-cause-chaos

What next?  There’s a calm period with a bright pleasant cool day coming up before more windy weather hits late Fri and heavy rain early this weekend.  A powerful jetstream is making for some stormy weather in the run up to Christmas.  There is the threat of a Channel storm around Christmas Day with possible heavy rain and gales for the south. Or the ECM if bringing in a very intense storm for the NW of the UK on Xmas Eve. Take your pick!

19-12-2013 07-05-43

19-12-2013 06-55-10

More technical details on ana fronts here http://www.zamg.ac.at/docu/Manual/SatManu/main.htm?/docu/Manual/SatManu/CMs/Cf/backgr.htm

and LEWP line echo wave pattern/squall lines http://www.estofex.org/guide/1_4_3.html

Weather Watch: update: heavy rain overnight Friday, moving away am; even HEAVIER rain overnight Fri – Sat, moving away am. Some possibly torrential and totals poss exceeding 30mm in next 48 hrs. 

The second significant Autumnal gear change is looking more certain for early next week as a big LOW sweeps between Iceland and Norway across the North of Scotland and a brisk active northerly jet combines to drag in a breezy and cool Arctic airmass from the Poles all the way across the UK.  The isobars on UKMO charts can be traced all the way back to the Poles, so expect chilly weather… possibly creeping into the low teens and feeling like 10c in the wind.

fri fronts

The process starts on Friday as fronts bring potentially heavy rain across the South, including Reigate, especially Friday pm and overnight into Saturday morning. The situation is complicated with fronts lingering over or near the south throughout the weekend so expect rain, some heavy, almost anytime during Friday through to Saturday. Saturday will be cooler than Friday.

The real news, though is the temperature plunge next week. Keep posted for more on this Autumnal development.

There might be a glimmer of some warmer temperatures returning by 21 Sept as a high builds from the SW … temps of low 20c’s might be expected to make a return but certainly not a heat wave!

Reigate should see 30ºC Tmax for the first day of August and high 20’s on Friday. The cause is a warm tropical plume of air flooding over the south of the UK from Iberia (Spain) and Africa brought in as the jetstream moves north and a LOW to our west and HIGH to the east builds a brisk Southerly airstream. Thursday will be dry and bright and the night will feel very muggy with high dew points and lows of over 20ºC at midnight: warm and damp, the air will feel close! Friday will be warm too but slightly less so for Reigate, dawning bright but with cloud spilling in associated with a weakening cold front pushing east across the country. Little frontal rain is expected but the cooler upper temps could release the huge heat energy built up in the lower atmosphere and kick off heavy / thundery showers for a while, especially Friday morning. Confidence is low currently but this might occur, the potential is there with all that heat.  

Every thunderstorm indicator is giving green lights to thunderstorms in the SE sometime in the next few days BUT the airstream is capped with warm uppers: this appears to be the only factor stopping convective action. This means heat building in the lower atmosphere will stay put until forced to rise into the unstable layers above (see sounding graph above). The approaching cold front might be sufficient to produce something of convective interest on Friday, morning especially… before the warm Euro air retreats back to east. So far Reigate has seen little of the impressive thundery activity elsewhere in the UK this summer. You might be forgiven for thinking it must be our turn soon! It’s unlikely to be in the next few days as the risk is small for any major thundery outbreak.
The synoptic situation remains similar over the next few days with HIGH pressure over the continent and LOWS edging close to the NW of the UK bringing rain and wind to the NW. The SE should escape with any trailing fronts weakening and Reigate should remain largely dry but for any threat of isolated thunderstorms in building heat.