Archives For low pressure

Imogen is the ninth named MetOffice storm this winter.  She formed in the Atlantic in an area of steep temperature gradients under control from an active jetstream.

 

Storm Imogen is deepening rapidly today to 953mb, though on arrival in the UK she will be occluding and filling gradually to above 960mb on her track over N Scotland into the North Sea on Monday. The exact track makes a big difference to where the strongest winds are.  Current trends are for the storm to pull wind fields further north so impacts could be less than expected. Keep an eye on the MetOffice forecast as things are likely to change. Below is an outline of Imogen’s likely activity:

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Unlike the previous eight named storms, Imogen has a more southerly track, guided by a more southerly tracking jetstream, and the field of strongest winds and heavy rain are possibly set to impact the densely populated southern part of the UK, including the SE. High waves are also expected on the Channel coast.

 

Strong winds on Sunday night will be associated with Imogen’s fronts running ahead of the depression.  The cold front is an active kata-front, associated with descending cold dry air from the stratosphere running ahead of the surface front and enhancing lift and potentially generating heavier rain and gusty conditions (image and info courtesy UKweatherworld).

On Monday gusts up to 80mph on the Channel coast are possible, while inland the MetOffice consider 60mph possible in exposed places.  Around Reigate and sheltered parts of Surrey, 40-50mph gusts are more likely.  The North Downs could see gusts approaching 60mph. The strongest winds for the SE are likely to be through midday and in the afternoon.

Yellow warnings apply to inland parts of Surrey and SE England while the entire Channel coast has an Amber MetOffice warning. The first impact will be frontal rain tonight.  Fronts passing through overnight into Monday could drop over 20mm of rain in places particularly linked to the occluding “triple point” forecast to cross the SE overnight.

 

Monday is likely to see showers, some heavy, appearing through the day.  Warm sea surface temps in the Channel are likely to cause more on the coast but the brisk winds could bring them inland as the day progresses.

The cause of the strong winds behind the cold front on Monday is a steep pressure gradient.  On Monday tightening isobars show the steep pressure gradient bringing gusty showery conditions in unstable Polar Maritime air behind the cold front later on Monday.

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The Wight-Wash Oscillation (WWO) measures the pressure difference between The Wash and the Isle of Wight and is designed as a guide to wind speed in the SE corner of the UK. The WWO on Monday shows a significant 16mb gradient between The Wash and the Isle of Wight on the WRF model.  The Euro4 model has a more modest 12mb WWO.  16mb would be the largest WWO pressure gradient recorded and greater than St Jude, which was 12mb.

On Tuesday models show a wave depression bringing more rain to the SE, some even show fleeting wintry precipitation on the back end of this low as colder air ingresses from the north.  This is unlikely to be significant, at least on Tuesday, as upper air temps remain mostly too high for snow in the SE.

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ECM colder flow mid-week

Colder conditions are preferred by the ECM as northerly winds bring cool polar air further into the country through mid-week.  The Arctic Oscillation is again going negative which shows pressure rising over the Poles trying to push Arctic air south into mid-latitudes.  However, the NAO remains positive so Atlantic depressions will continue to bring frontal depressions for this week.

The 8-10 day mean shows a deep trough over the UK meaning low pressure and unsettled conditions remain likely into half term.

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The ECM builds heights over southern Greenland which links with higher pressure over the Atlantic, a more northerly feed of cold polar air is likely in this scenario into half term . The ECM has been outperforming the GFS so the more Atlantic driven GFS chart would be the less favoured option.

The Sudden Stratospheric Warming going on over the Pole is another astonishing feature of weather at the moment.  Today (Sunday) temperatures in the stratosphere over Siberia has got up to an amazing +12C from a more usual -70C.  SSW events often build pressure over the Polar troposphere a few weeks later which can cause cold incursions into mid-latitudes.  This is by no means certain but is perhaps our last chance of any sustained cold this winter… if it were to happen it would be late Feb/March. One to watch!

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HIGH pressure dominates but is it all calm?

High pressure is known for calm, clear conditions, with little wind, cold frosty and foggy nights especially when there is little cloud. Pretty unexciting weather.  However, HIGH pressure is not as unexciting as all that.  Anticyclones can sometimes be surprisingly windy especially round the edges.  We spend a lot of time learning about LOW pressure, with associated storms and gales and torrential rain but understanding the inner workings of HIGH pressure is important to get the full picture of mid-latitude weather.

So… buckle up for the ride and let’s get super-geostrophic!  Wind blows from HIGH pressure to LOW pressure.  The wind speed and direction is the result of two forces: the pressure gradient force (PGF) is the difference between high and low pressure and sets up the strength of the wind and the overall direction which is for winds to blow directly from HIGH to LOW pressure.  Coriolis force (or Coriolis Effect) is a result of the spin of the Earth and deflects resultant winds to the right of their intended path in the northern hemisphere.  Here are some video links to review these forces before proceeding with super and sub-geostrophic winds. Skip below these videos if you already know about PGF and Coriolis.

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The winds do blow from high to low… but get pushed to the right by that Coriolis fellow!

The pressure difference between high and low pressure determines the speed of wind.  Winds do blow from high to low due to the pressure-gradient but are deflected to the right by another force called the Coriolis effect! Below is a chart showing upper winds at 850hPa (1500m) blowing round the same HIGH pressure shown on the synoptic chart at the top of the post.  Note the relatively high wind speeds circulating round the HIGH in the north of Scotland, the North Sea and across France and Biscay especially.  Winds obviously blow faster across the ocean but remember this is an upper wind chart so is above the boundary layer of most frictional forces upsetting the wind.  In any case, none of these locations is associated with a trough… it is all anticyclonic super-geostrophic wind.  So why is the wind blowing so strong when there is no LOW for miles?

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Given the same isobar spacing the wind speed aloft round high pressure ridges is often greater than the wind flowing around troughs and low pressure. This is surprising because we associate gales and windy weather with “storms” and low pressure systems.  The chart above illustrates super-geostrophic winds circulating around the Azores high across Europe.  These look pretty strong at 850hPa (1500m), the level above frictional effects of the surface.  The chart also shows the trough of low pressure over the Mediterranean where, given some of the locations with similarly spaced and even tighter isobars, the wind strength is not especially any greater and perhaps even less than that circulating freely around the HIGH.

hilo2015-02-08_22-26-30

Wind is a result of pressure differences across the planet surface.  Wind wants to blow from high to low pressure.  This is called the pressure gradient force.  Due to the spin of the Earth winds in the northern hemisphere are deflected to the right of their intended path.  The two forces, pressure gradient and coriolis force, actually balance out to produce a theoretical wind that flows parallel to the isobars called the geostrophic wind, shown above. Unfortunately, isobars are almost always curved so the geostrophic wind hardly ever actually blows.

subg2015-02-08_22-18-44

Assuming a constant isobar spacing.  Around troughs of LOW pressure the wind is sub-geostrophic. This means it blows less than the expected geostrophic wind.  In the chart above the wind is shown as a black arrow.  In addition to the coriolis force, the centrifugal force acts to “push” the wind away from the low centre and is acting in the same direction as the coriolis force.  Note that the resultant wind is pointing slightly away from the LOW towards the HIGH, which is of course not possible because the wind would be moving into and against increasing pressure.  As the pressure gradient force cannot change, the coriolis force must weaken to allow the wind to return parallel to the isobars.  This means that the wind flowing around troughs of LOW pressure has reduced force acting on them given the same isobar spacing of a similar HIGH. These winds therefore blow slower than geostrophic wind and are called SUB-GEOSTROPHIC.

superg2015-02-08_22-18-44

Here is the HIGH pressure situation.  This time the centrifugal force is acting with the pressure gradient force to push the wind into low pressure.  As the pressure gradient cannot change the coriolis force must INCREASE to pull the wind back parallel to the isobars.  This means that the wind flowing around ridges of HIGH pressure has GREATER forces acting upon them than winds flowing round lows with equivalent isobar spacing.  These winds therefore blow faster than geostrophic wind and are called SUPER-GEOSTROPHIC.

Usually, of course, low pressure cyclones and depressions exhibit tighter isobar spacing than HIGH pressure and so resulting wind speeds round LOWS are most frequently higher than the HIGH pressure feeding them.  Nevertheless, assuming the same pressure-gradient force, winds exiting anticyclones can produce higher wind speeds than those entering depressions.

useful reference

http://ww2010.atmos.uiuc.edu/%28Gh%29/guides/mtr/fw/grad.rxml

http://www.metoffice.gov.uk/learning/wind/what-causes-wind

http://education-portal.com/academy/lesson/factors-that-affect-wind-pressure-gradient-forces-coriolis-effect-friction.html

Quick update here focusing on SE especially: please note this applies mainly to Reigate in Surrey and is an amateur analysis for educational purposes.  For updates through the storm please see @RGSweather on twitter for the Bertha story as it unfolds for us in Reigate and SE.  This is called NOWCASTING (as opposed to “forecasting”).

Ex-Bertha is turning out to be rather interesting meteorologically!  A convective potential has emerged today, which means there is more possibility of thunderstorms of some significance as the LOW passes across the UK, especially to the south of the system.  Convective gusts of 50-60mph could be possible and the odd tornado cannot be ruled out, though no need to panic because these are quite common and not usually powerful or damaging in the UK.  So it is still the case that the overall impact of this storm is still not likely to be extra-ordinary or wreak widespread havoc Daily Express style.  It is more likely to be underwhelming for most.  Nevertheless, rainfall totals in a short space of time for some places might be high and there could be some interesting weather phenomena associated with active fronts.

UPDATE Sunday 7:30am

Estofex and TORRO have issued severe convective weather warnings for the S UK. Estofex Level 2 storm warning is most unusual for the UK and TORRO do not issue tornado watches lightly.

(back to yesterdays update:) The UKMO fax chart below for Sunday midday shows a “triple point” of three fronts meeting near the SE (warm front, cold front and occlusion) Between the warm front and cold front the warmest humid air is wrapping into the centre of the LOW in the warm sector: this contains much of the moisture to fuel the storm as condensation releases latent heat driving up parcels of air.  On top of this a conveyor of cooler drier Polar air that flows over the cold front and warm sector and this increases lapse rates further encouraging lift throughout the system. The warm air eventually flows to the core of the storm as it occludes.

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All the time the jetstream to the south is lifting air off the ground (by a process called divergence in the upper atmosphere) and lowering the central pressure causing air to converge into the centre of the LOW… this results in the surface wind rushing into the centre.   Converging air at the surface has nowhere to go except up.  Rising air, especially where tropical air meets polar air at the fronts, creates condensation, thick cloud and potentially plenty of rain.  The potential water available in this storm is large.  In addition, cloud top temps, with the influx of cold air aloft, are likely to be as low as -50C causing ice to form in turbulent air that can create charge up thunderstorms.  Such storms are only a risk and may not happen at all.2014-08-09_20-59-21

For Reigate and the SE it seems we can expect more rain during the morning than was previously the case in earlier models and forecasts.  Latest models suggest widepsread rain in the SE of up to 20mm and discrete patches of high totals possibly exceeding 50mm in the SE.  This is about a month of rain in one day, so local flooding could be a problem.

Rain will arrive tonight, after midnight, and persist throughout the morning.  Wind speeds, probably 30-40mph max gusts inland, possibly more gusty in any thunderstorms, will increase towards the middle of the day and potentially be highest as the cold front moves away which, on current models looks like early afternoon.  Strongest gusts will be associated with any thunderstorms.  The good news is that by pm the cloud should break rather rapidly, however, scattered showers could follow in the brisk westerly. This regime will continue for much of the early part of the week.

Needless to say, apart from the rain potential, Reigate is less at risk from tstorms than further N during this episode (Reigate storm shield!)

Even now much still remains uncertain about this storm and it is causing lots of interest and headaches for both professional and amateur meteorologists.  The nature of the fronts may produce some organised squall like features and some organised thunderstorms for places but predicting these is extremely difficult.  Any such storms can have the potential to deposit a lot of rain in a short space of time.

Late this afternoon Saturday Bertha split in two: one rain system moving north, the other pushing more ENE.  This was unexpected.  Currently the rain moving north across Ireland is the more significant but things can change.

 

Let’s finish with the UKMO forecast for Reigate.  It shows lots of rain, potential for thunderstorms and some unsually strong winds for the time of year.  As leaves remain on trees this might cause loose branches to fall and peak rainfall totals, if met, could cause some local flooding.  Certainly nothing to panic about but do look out for any interesting weather features and send them in to @RGSweather!  Sadly for @ridelondon the prospects are not terriibly nice in the morning.

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Photo mosaic of the squall line that passed over Reigate on 10 Aug afternoon: quite a feature!

and tornado reports of damage from various locations including Hull, plus other news here:

http://metofficenews.wordpress.com/2014/08/10/sundays-rain-and-wind-data/

http://www.bbc.co.uk/news/world-europe-jersey-28746723

 

 

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 weekend weather will be windy and wet first thing Saturday with showers and bright intervals following after a well defined front moves away early. Typically, no deluge is forecast for Reigate and the East but early rain on Saturday could be heavy for a time around breakfast. Most rain will fall further west. The most significant feature of weekend weather for Reigate will be wind: gusting from the west over 35mph at times on exposed hills on Saturday, less on Sunday.  

A summer depression with a LOW central pressure for the summer (994mb) is passing straight across Scotland over the weekend. This has winds spiralling anti-clockwise around the centre of the LOW, for Reigate this means gusts over 30mph on the hills and generally widespread 20mph winds. Quite a breezy day for mid-summer. As the cloud breaks in the afternoon it will be good kite flying weather. More rain and cloud could be widespread on Sunday, some of it heavy and showery in the afternoon.
For next week things look much better as a large HIGH pressure builds across the entire Atlantic and pushes any more depressions well to the North over Iceland. This HIGH will settle over the west of the UK and build across the East as the weekend LOW moves out into the North Sea. We can look forward to some pleasant dry summer weather well into next week, reaching over 20C. But always with the threat of a shower in the east where winds will be from the NW or west.
So…total rainfall for the next week in Reigate is forecast to be a mere 10mm or so, hardly very much. At least 8mm of this is forecast to fall this weekend, making next week very dry indeed!

During the recent last spell of warm humid sticky weather with the warm southerly plume of potentially thundery downpours, Reigate got no showers at all! The capped inversion layer prevented any significant convection.  Despite immensely high forecast CAPES and lifted index, each forecast would predict heavy showers and potential thunderstorms but none would arrive as the last model runs would lower capes and LI.  Below the inversion, with no significant convection to break the cap, cloud simply spread out into dull strato-cumulus formations covering the sky and reducing the warming by the sun and the instability of the lower layer of the atmosphere was therefore reduced.  No cap was busted, no spectacular cumulonimbus were seen round Reigate.  Some occured nearby in Kent and the Channel and running up the N Sea.  Several models (NMM, GFS, NAE) consistently predicted showery rain almost everyday, some of it heavy, yet none arrived.  Other models were nearer the mark and picked up better on the lack of powerful convection beneath the inversion (UKMO, HIRLAM).  So, in Reigate, being so sheltered and pleasant, it never rains but it pause!

Update Sunday: EURO-HEAT is winning out briefly this week! Possible for 30C mid-week with thunderstorms. More later mini heat wave!?

Forecasts for this week are swinging all over the place… from heat licking in from Europe over the SE and bringing attendant thunderstorms to a ridge of the Azores High building into the South and calming things down for a time, but keeping things in a westerly airflow and generally Atlantic and unsettled.

So, after possibly very heavy rain / thunderstorms for us in SE on Monday / Tues what will it be for the rest of this week… Euro-heat and thunderstorms or Atlantic cool and breezy (i.e. a continuation of what we have now)?

The forecast essentially depends on three things: the movement of the deep LOW shown hovering off the SW coast on the Sunday chart above, the strenght of the Azores High and the location of our good friend the jetstream (overhead at the moment, further south than it ought to be!).

The LOW is forecast to form a trough over Spain and sink south over the Med producing showers there.  This will leave a complex of fronts in N France with heavy rain moving north over the South of UK on Monday and Tuesday (heavy showers, poss thundery Monday pm).  If the Mediterranean LOW yo-yos back to the UK later in the week it will bring a plume of heat with it but latest model runs show a ridge extending from the Azores HIGH shutting out any Euro-heat and keeping the UK in a westerly airflow pretty much under the jetstream… meaning continued unsettled weather especially in the North, for the coming week.  Essentially this means the jetstream (below) stays in control of UK weather and brings a succession of LOWS with rain and wind and brief calm dry spells, especially in the south of the UK which fairs better.  If the Azores HIGH can build into the South this will calm things down here for a longer period.  This looks most likely at the moment from midweek onward until another LOW pressure comes in on Friday – the longest day. 

jetstream in control

Mid summer model mayhem! Update Saturday am: Rapidly changing situation… recent runs (2 charts below) show much reduced risk of any thundery activity later this week; perhaps some Monday pm and eve but ridge building over South should calm things down but shut door on any big heat from Europe. Keeping air flow westerly and unsettled. Things are changing on every run so check back for updates!

15-06-2013 10-26-50 westerly stream

This week could see a warm plume of air from southern Europe causing temperatures to rise significantly in the South especially.  This may also bring thunderstorms. A real mix this week of wet, windy, dry and sunny, warm, humid and thundery weather could be on the cards. Check back for updates as things get clearer! 

The North and Midlands saw some very thundery downpours today, whilst the south experienced unseasonably high winds over 50 mph in places.  Reigate had peak gusts of 33 mph being sheltered by the Downs and Priory hills surrounding the town here!  Anyhow, it could potentially be the turn of the SE to see some (more?) severe weather later next week, but it’s a long way off so stay tuned for updates and on @RGSweather on twitter.

The weather really is not going to play ball next week despite being on the run-up to the longest day of the year.  Some decent warm weather is possible mid-week but much of the rest looks unsettled, showery at times and occasionally windy too: it’s hardly looking summery except the potentially warm maximum temps mid-week prior to any thundery interlude later on.  Whilst Reigate temperatures could be set to rise to 23ºC by middle-late in the week as air from a heating Europe is fed our way, any potential temperature “spike” is likely to be regrettably brief and spark off showers and even thunderstorms as pressure stays low.  This remains uncertain and will change but models are suggesting a potential “heat leak” from the continent over the SE for a short time later next week but, as suggested, if this happens it will be accompanied by very heavy thundery downpours for the SE.

Before all that potential excitement, the start of the weekend will be dominated by the passage of a classic depression with a perky cold front carrying some moderate to heavy rain crossing the entire country overnight Friday and clearing the east coast on Saturday morning.  A strong jetstream will push this through quickly.

rain clearing Saturday

rain clearing Saturday

This will leave a breezy day of sunshine and isolated showers hanging around for most of Saturday, though the UKMO plays these showers down. Sunday looks cloudy and mostly dry until later in the day when another low, this time approaching from further south west, brings potentially heavy thundery showers to Reigate on Monday afternoon as it lingers near the SW.  Rain could linger, lighter, through Tuesday.  This low is not due to cross the UK, instead it will sink south and then yo-yo back up later in the week.

yo-yo low

yo-yo low

Now, there is a chance of some big thunderstorms getting into SE England around mid-week.  Most models agree that a trough will develop as the LOW sinks south.  The jet stream will cut off this low as it floats around Spain for a day or two basking in heat and warming through.  The story thereafter becomes uncertain but the cut-off low could yo-yo back north and feed some continental heat and energy back up north to the UK by Weds/ or Thursday. Heat advecting into a cool UK will increase lapse rates (decrease in temps with height) and could set off whopping convective showers and thunderstorms.  Another measure of thunderstorm potential is called lifted index (shown below) The Lifted Index (LI) is defined as the temperature of a rising air bubble when it reaches about 5,500m subtracted from the actual temperature of the environmental air at 5,500m. If the Lifted Index is a large negative number, then the parcel will be much warmer than its surroundings, and will continue to rise = convection! Thunderstorms are fueled by strong rising air which remains much warmer than the air surrounding it (like a rising hot air balloon) thus the Lifted Index is a good measurement of the atmosphere’s potential to produce severe thunderstorms. The map below is the latest GFS run showing LI across Europe on Thursday.  Note the salmon red colours in UK!

lifted index thurs #

The movement and location of this YO-YO low is critical in determining whether this happens at all.  If it matches the GFS it could cause significant thundery weather by dragging in a “lick” of very warm air over the south east and east of the UK, if it runs with the ECMWF it will probably stay south, moving further east and fill over the continent and be potentially less interesting.  Worth watching the yo-yo low!

monday 17 june

Heads up, Reigate!  Or rather… wrap up, stay in and keep warm: SE weather for the end of this week looks awful and, whilst nothing approaching the US Mid-West, it could be described as potentially severe for the SE.

Pick words from this list and repeat and then remind yourself it’s nearly June! “COLD, heavy persistent rain, windy, wet snow, sleet, hail, heavy thundery showers, freezing wind chill, 35 mph northerly gusts… at the end of May!

Thursday: bright start but with cold northerly winds +20mph and heavy showers building in the afternoon.  Feeling chilly at only 10ºC max.  Overnight temperatures fall to 3ºC and any showers could even fall as icy sleet or even wet snow on the Downs. 

Friday: things actually look worse than earlier this week: COLDER northerly winds will drag temperatures right down to 3 or 4ºC in heavy showers and a maximum of  8 – 10°C with temperatures falling lowest in showers.  The centre of a nasty deepening LOW pressure will track directly across the SE on Friday and we can expect persistent rain, some heavy at times and accompanied by really cold winds (for the time of year).  Later in the day heavy thundery showers have the potential to bring hail and sleet.  Significant instability is forecast (*CAPE >350 j/kg; before the Oklahoma tornado CAPE measured 5000 j/kg), currently the highest this year in the SE, which means there is a fair chance of severe weather events in the form of thunderstorms.  Watch out for mountainous cumulonimbus cloud formations extending to the height of Mount Everest and possible funnel or mamatus clouds under dark bases.
Freezing levels will be down to 600m and during any heavy showers evaporative cooling could potentially cause heavy rain to turn to sleet as snow forms above 1000m and heavy rain draws down colder air.  Evaporative cooling happens when enough of the heavy rain evaporates on the descent to take heat out of the air and freeze supercooled droplets or sleet into proper wet snow.  Strong winds gusting 35mph will make it feel truly nasty.  This all sounds ridiculous for May… but the potential is there..let’s hope it’s not quite this bad!

Friday night will be a chilly and wet experience outside.  Temperatures will fall to 4ºC but feel near freezing in northerly winds. Not good for camping.

The weekend recovers reasonably well with a weak high pressure ridge bringing bright and dry spell Saturday behind the Friday storm. The fine weather  could hang on through Sunday but showers could threaten as pressure falls away as a deep LOW approaches from the NW to threaten Monday with rain.  The rest of half term looks unsettled but a bit warmer and drier at the end and best in the NW of the UK where HIGH pressure is likely to ridge across.

*CAPE = convective available potential energy: how buoyant a parcel of air is and this determines speed of vertical extent of convective cloud formation.

In the end: Max 8.4C; Min 4.3C; Rainfall 8mm; gust 24mph NW; So, in the end, pretty foul and unpleasant but fortunately no observed wet snow or sleet anywhere.  Timings of light rain onset were good; heaviest rain pm and cold temps made it feel more like January. No thunderstorms heard but lightning was recorded in SE on ATD lightning detector; CAPES fell to just over 100j/kg in the last run of GFS in the morning, which is a little disappointing for extreme weather enthusiasts!

Thursday morning will see winds in excess of 100mph for Reigate… fortunately well above the town at 20,000 feet or so! This is the jetstream. These winds are pivotal in producing strong surface winds which will “touch-down” later in the day in Reigate. Surface gusts of over 40mph are possible with +30mph average winds. These winds are due to a deepening area of LOW pressure crossing the north of the UK over the next few days. The Irish Sea, English Channel, south coast and hilly areas like the South and North Downs will see the highest gusts. With trees in leaf there may be the odd branch falling. Batten down the hatches and secure loose bins! (Video: you can see the LOW crossing Northern England.  Reds 60mph; yellows 40mph; greens 30mph).

Quick update for NEXT week: LOW pressure dominates as a cyclone sinks down from the NW and an upper TROUGH sits over the UK while HIGH pressure builds to the NORTH: this will bring a distinctly AUTUMNAL feel to Reigate next week: temperatures struggling to get into the mid-teens, with periods of showery and breezy weather: so, NOT hot at all: cool, unsettled and rather wet for the time being.  More details later! Check @RGSweather for regular updates.

This Atlantic chart for Wednesday shows a classic run of LOW pressure systems (depressions) charging in from west to east towards the UK associated with an active jetstream. This week LOW pressure will bring rain and wind to western parts but most of these LOWS will skirt to the NE and hit Scotland and Northern England hardest whilst any trailing fronts reaching Reigate will have weakened considerably.  In Reigate we can expect occasional light rain and some breezy weather (30 mph gusts) most days this week, particularly on Wednesday, but nothing dramatic and rain only amounting to 6 mm or so for the whole week.  HIGH pressure ridges separate each depression bringing a few days of calm, dry and brighter weather. The next significant anticyclone (HIGH) is expected to build across the UK just in time for the weekend and London Marathon, hopefully bringing dry and pleasant weather.  For Marathon day the weather currently looks like being 11ºC with light southerly winds and no rain.