Archives For weather

A strong jetstream to the SW of the UK will manufacture some tricky weather over the next 48 hours and continues to make details for the weekend uncertain.  Here’s an outline of the likely scenario for Reigate and SE but the advice must be to watch the forecast if you are doing anything weather-related!

The jetstream is located to the SW of the UK and is blowing strongly from NW to SE.  This means the UK is sitting in that vulnerable zone to the cool north-side of the jetstream where active small scale depressions can run-up rapidly and deepen off the left-exit region of the jetstream where temperature and pressure gradients are greatest.  These depressions are small scale but can produce a lot of rain and their track is sometimes rather erratic: and tiny changes in track can make a big difference to the weather experienced!

Two LOWS are predicted to spring out of the jet from Fri-Sun: the first during Friday – will track directly over SE England and drop possibly 10mm of rain onto Reigate.  As they track broadly S and E of UK, these LOWS will, in turn, swing the winds into a much cooler northerly or NE direction.  Reigate is unlikely to get any strong wind because the LOW pressures are due to pass almost overhead… meaning we are likely get mostly light wind but potential for heavy showers as the cool polar air “warms through” during any sunny periods which will cause convective uplift and showers.  There are also occluding fronts wrapped round these systems that could bring more persistent rain at times.  The second LOW emerges on Saturday and is currently due to track off in a more southerly direction through France but could bring rain across S coasts.  This one needs watching carefully as any movement north will cause a much wetter and unsettled Saturday than is currently on forecasts: which currently show Sat largely dry but with some poss showers later. Showers are possible as the cool air warms through and sparks convective showers on either Sat or Sun. Higher pressure ridges between these lows will bring cold nights, possibly frosty in places if cloud clears.

The complicating factor is that pretty cool air is wrapped up in these depressions, almost cool enough for sleet and wintry precipitation at times later on Friday for Reigate and during almost any showers that rotate around the remnants of the LOWS during Saturday and even into Sunday.  Whilst wintry ppt is unlikely for Reigate, further north and over hills in Wales and N England there will be snow, probably patchy. short lived temporary wet snow at lower levels but accumulations possible over higher ground.

The only way Reigate will see any fleeting proper snow is if we get any HEAVY rain during the passage of fronts later on Friday and any showers left behind in low pressure Sat/Sun.  All rainfall during this period will start as snow aloft, in fact almost all our rain in the UK starts as snow up high and simply melts as it descends.  However, if rainfall is heavy enough it can actually COOL the air sufficiently so that further precipitation falls as snow.  This is called evaporative cooling: some rainfall always evaporates on descending to the ground, even in cold weather.  If enough rain evaporates then this will cool the air sufficient for freezing levels to fall to the surface and then, bingo, snow will descend to the surface as well!  So, whilst rain can NEVER “turn to snow” (completely impossible!), it is possible for heavy rainfall falling through a sufficiently cool air mass (around -5c at 850hPa), to cause snow to reach the surface.

Update Friday: v unstable atmosphere passing over Reigate Friday after active warm front passes early morning: all ingredients for heavy showers and thunderstorms tomorrow increased risk hail thunder for Reigate, predicted earlier in the week @RGSweather.

Spring warmth is set to arrive big time this weekend, especially from Sunday on wards and particularly in the east of the UK: Reigate could be one of the warmest places so let’s keep our fingers crossed!  The contrast with just a week ago is remarkable with temperatures for Reigate possibly rising to over 20ºC on Sunday and remaining well in the ‘teens during the daytime throughout next week.  There will be a breeze of 15 mph so find a sheltered spot to enjoy the balmy atmosphere or take to the hills with a kite, go sailing, have a BBQ… #getoutside

The initial cause of this long awaited warming is LOW pressure in the Atlantic engaging with a HIGH pressure over Europe which will feed a brisk southerly wind from source regions in North Africa and the Mediterranean.  An upper level ridge building from the south is also critical in feeding warm air aloft (above 5000 m) to the UK.  The jet stream is migrating northwards and helping to feed in this warm upper airmass.  Upper air temperatures at 5000 feet (pic 1 above) are set to rise dramatically from well below freezing last week to +12°C or more through next week: Tropical air has arrived!  The rest of next week looks good with warm and mostly dry weather (but not always) continuing for Reigate and, whilst not always as warm as Sunday, certainly feeling pleasantly springy.  The synoptic configuration is set to remain HIGH pressure in the South and LOW in the North with a warm flow of SW or W winds over Reigate.

However, before all that, Reigate could have some significant rain: while overnight rain should dry up quickly Thursday morning after the front moves North, Friday looks potentially showery and some could be heavy.  April showers are kicked off by heat at the surface causing thermals to rise buoyantly through the atmosphere and condense creating convective clouds called cumulus.  If these grow into shower clouds they are called cumulus congestus or their bigger brother cumulonimbus.  Several of the right conditions for heavy showers are present on Friday such as uplifting air, plenty of humidity and wind shear.  Due to the cold weather thus far, there is currently a lack of heating at the surface to spark off significant thunderstorms but things can change.  Saturday could also have some showers associated with a weak warm front before the much heralded warm and dry weather arrives on Sunday.  Saturday warm front turned out much more perky than first thought… delivered lots of rain all pm! Keep posted on twitter @RGSweather for updates.

March 2013: “In like a lamb and out like a lion!”  

March 2013 in Reigate recorded an average temperature lower than January and a minimum temperature lower than February and the lowest dew point of -7.1°C so far this year. It was an exceptionally cold month and got colder and colder from the the one lovely day at the beginning.

The opposite of what should happen, did happen.  March started with rapidly rising temperatures reaching over 16°C in the first week but things deteriorated rapidly after that as a Polar blocking HIGH built to the North and pushed vigorous easterlies and north easterly winds towards the UK for much of the month.  A number of Atlantic LOWS tried to bust the BLOCK but were pushed to the south of the country where fronts between freezing polar air and milder air created exceptional blizzard-like conditions for some areas.  The Channel Blizzard created exceptional snowfall on 11 March for the Channel Islands but Reigate got away with 1cm of lying snow which melted in the town quite rapidly and left a day of severe wind chill down to -12ºC at 5am.  Mini snow drifts persisted on the North Downs and Reigate Hill for several days.  Arctic air arrived later on the 12 March as the LOW drifted SE and dragged northerly winds in creating brighter conditions with spectacular cumulus snow and sleet showers through to the 13 March.  Reigate had episodes of heavy sleety rain mid-March and even a clap of thunder (first recorded since Christmas Day!) on 18 March as milder air met colder air creating unstable conditions on complex fronts.  March 23 saw another snow event over the UK: record-breaking snow fall occurred to the NW over the Isle of Arran, Cumbria and Wales but Reigate again escaped the worst with a few cm falling but melting rapidly while a covering of snow lingered for a few days over the Downs.  Cold easterlies then dug-in and pushed dew points to -6°C with temperatures hovering around freezing through to the end of the month.  Dull blanket layers of stratocumulus caused by a persistent isothermal layer at around 900m in the easterly winds brought occasional flurries of sleet and snow grains and severe wind chill but little sunshine.  Temperatures fell throughout the month and Spring was delayed in the coldest UK March since 1962. 

Very low dew points (down to -7ºC) show that dry air persisted for much of March.  People with lots of hair to manage may have had a difficult time through March with curly hair going straight or fly-away due to drying out in the low humidity and dry, skin-cracking north-easterly winds.  Low dew points have a bad impact on curly hair, dragging out the moisture from the hair into the dry air.   Whilst dew points in the UK are usually high in our moist humid climate which is good for keeping curly hair curly, anyone with lots of hair to manage should check dew points on weather forecasts very carefully, especially in winter and avoid expensive hair-dos on days with low dew points… just a thought.  More seriously, farmers across the UK struggled with lost livestock in deep snow and delayed sowing of spring crops.  The weather sparked some debate over climate change and the possible causes of the cold which gripped the whole of the northern hemisphere.

January av 3.7ºC; HIGH 11.9ºC LOW -5.9ºC RAIN 58.4mm
February av 3ºC HIGH 11.6ºC LOW -2.4ºC RAIN 42.9mm
March av 3.3ºC HIGH 16.2ºC LOW -3.3ºC  RAIN 67.8mm

28-03-2013 14-36-39RGS weather blog is closing for Easter week.  While we are off-air please remember that the best view is often up, be inspired by the clouds and atmosphere around you and wonder at the record breaking Spring 2013 weather…continuing cold and dry through next week, at least!

In the meantime, please read our “Climate Cluedo” posts investigating “Who Killed Spring 2013?” to get more details and a more convincing explanation than our word-cloud above on what has caused the extraordinary spring weather.  Comments via this blog or @RGSweather on Twitter are, as always, most welcome.

Happy Easter!

Weather is always interesting, of course, but this week is extra-fascinating! 3 weather things to watch this week in Reigate:
1. Watch out for… highly unstable cool air moving off the Channel onto warming surfaces during the daytime leading to convection, cumulonimbus clouds and heavy showers, maybe risk of thunder and hail over Reigate: especially Monday and Tuesday.  April showers in a cold March?
2. Keep an eye on … the big show-down between the powerful Polar North Easterlies spilling out from huge blocking HIGH pressure over the Arctic and the, so far, rather feeble South Westerly maritime air that has made little impression recently.  They are due to have a face-off on Thursday. The leading edge of fronts might be snow…but will the warm air reach Reigate or will the LOW retreat out into the Atlantic?
3. You can’t see this in the sky but… note the off-the-scale negative Arctic Oscillation graph that indicates a weak jetstream and high pressure over the Pole which allows Polar air to flood out over the UK easily. Will this mean another fake start to Spring this weekend (check the temperature graph leaping up at the weekend!) and then a return to cold next?
Reigate will mostly stay on the “warm” side of this LOW and avoid the very coldest easterlies this week which will stay up north (except mid-week which will be cooler down here) but the cold air will never be far away so frosty nights are certain, cool temps in slacker winds are definite, heavy showers hit and miss and a minor risk of snow anytime are still worth watching for!
Hope you enjoy the week weather watchers!

quick update Monday: cold weather continues to next week… polar easterly winds return Monday-Tues. after warmer spell this weekend but poss SNOW preceding warm front Saturday- update later.  Heavy rain with snowfall/sleet poss in between for SE: on weds, thurs & Fri. Very active, very odd, potentially extreme weather…watch this space for updates. So, for the moment the Easterlies win! some are calling this the MIA: Mini-Ice Age!

quick update Weds: snow threat Fri/Sat as Atlantic fronts on a vigorous depression move in from the west and move up against the cold air.  This may dump snow on Reigate Saturday.  Next week looks COLD … back in the freezer high threat: snowy Easter possible.  NOT the start of Spring by any stretch…someone tell those birds.

21 March UK north south divideThe low pressure bringing rain and cool conditions this weekend looks like dragging in a return of north easterly winds as it moves south over the UK.  This means continued cool/cold weather for Reigate Mon – Wednesday next week.  Temperatures, whilst not as cold as the last spell of Easterlies, will bring some rain and maybe even sleet or snow at times through mid-week.  There will be a big battle between milder air to the south and cold air to the north of the UK next week and significant snowfall is a risk where these air masses meet.  Currently, it looks like the front will be further north than Reigate and in the slightly milder air so we will escape the worst of any snow threat poss Tues/Weds and may miss it completely but cool damp unsettled weather is certainly sticking around next week so keep watching for updates!

A cool LOW pressure tracking across from Greenland and Iceland will move over the UK on Friday and sit over Reigate for the weekend and into much of next week. Atlantic depressions usually have some warm tropical air circulating with cold polar air but not this one!  With high pressure to the west any warm SW air flow is being blocked out, leaving Reigate on a “cold-washing-cycle” with mostly swirling cold polar air circulating around the low. Cool air coupled with low pressure even in this cool Spring weather causes unstable airmasses.  Instability means that big showers can develop as thermals rise from warming surfaces even in the weak Spring sunshine we are experiencing.  Convective thunderstorms are a remote but interesting potential risk for Reigate over the weekend.
Various fronts and troughs will circulate around the LOW pressure which is blocked by a HIGH over Scandinavia and E.Europe and won’t move much. The LOW will bring showery rain, some of it heavy with the possibility of hail and even thunderstorms, especially over the weekend (50% chance of convective thunderstorms on Sunday over Reigate).  Temperatures will never climb much above 7°C and, with gusty winds of over 30mph, it will feel cool, but nothing like what we had earlier this week. Some models predict a return of cold Easterly winds as the LOW slips south over the UK and drags in cold continental air again. There is still uncertainty about this but it will certainly be an unsettled week. Other models see glimmers of spring after around the 23 March as winds turn more SW and a ridge builds over the country bringing drier conditions.  More on this later, keep your fingers crossed!

Here is a summary of the causes of the March 11 2013 “Channel Blizzard” which brought extra-ordinary “Spring” weather to SE England and the Channel Islands and N France. At RGS we had record low wind chill temperatures of -10°C at 9.30am, 36mph gusts and sub-zero temperatures all day.  Follow the numbers on the map to get a quick view of why it all happened!

1. Cold source region for Polar Air: the NE winds originated from the Polar regions with temperatures below -15°C and traveled across an extremely cold continent to reach the UK.

2. On their journey, the North Easterlies warmed a little over the North Sea (by now +4-5°C) which caused showers to form in unstable air (warming causes air to rise, clouds form and it snows). These showers formed lines called “snow streamers” which fed snow to the SE for most of the day and into Tuesday morning.

3. Much warmer SW winds at +10°C met the frigid cold Polar air mass somewhere over the Bay of Biscay but they didn’t mix well and they certainly didn’t get on!  In fact, the cold, dense polar air pushed the lighter, less dense tropical air right up off the ground, to over 4000m, where all the moisture condensed, formed cloud and snowed!  The high winds experienced across SE England were “squeezed” like toothpaste between the HIGH over Iceland the LOW over France (a high “pressure gradient”: look how close together the isobars are over SE England and the Channel!).

4. Next? Things will stay cold during mid-week as the UK remains firmly in Polar air and enjoys some dry weather courtesy of a HIGH over Iceland (unusual).  Clearer skies and frosts.  However, a Polar Low forming SE of Greenland at the moment is due to break through the Icelandic HIGH later in the week and bring unsettled conditions to the end of the week / weekend: as Polar air is still involved this may bring further snow and sleet.

Greenwich Lightship is a UK Met Office weather station in the middle of the English Channel. 3 metre waves, 50mph winds, sow and 10 foot waves were recorded.  Unbelievably horrendous conditions which the Channel Islands experienced as a blizzard.

Watch the satellite animation below and spot the storm winding up in the Bay of Biscay before it hits the Channel and says “Hello” to France and the UK!

A severe blizzard in the English Channel will brush perilously close to Reigate today and overnight. Sub-zero temperatures for the next 48 hours and strong gusty easterly winds of >20mph will make it feel like -10ºC and potentially much lower (see wind chill charts posted earlier).  Whilst the UKMet-Office model refuses to bring any significant snow to Reigate other weather models certainly do!  In fact, some forecast as much as 10cm of snow to our region falling later Monday and overnight especially. With cold temperatures and strong winds this snow will blow around and be very unpleasant indeed, and could amount to 10cm, especially south and east, less west and north.  Updates @RGSweather on twitter.

Sunday final analysis update: crossing the Channel Monday?…be warned of v cold blizzard conditions!  LOW centre set to track thru N France most likely bringing only light snow to Reigate Monday morning but increasing risk of significant accumulation later in the day and overnight into Tuesday.  Any shift further north in the fronts could bring heavy snow to S counties pm.  Latest hi-resolution NMM model (below) shows light snow showers all day but risk increasing to heavy snow pm extending into southern counties of England, reaching Reigate late pm or evening….; UKMet Office predicts the least snow of all the models.V V tricky forecast.

So: Reigate forecast for Monday-Tuesday: emphasis on bitterly COLD, snowy day with extreme wind chill for (any) time of year with a low risk of heavier snow later and overnight to Tuesday, especially for places further South. so…accumulations 0 – 10cm by Tuesday am.

sunday 06Z NMM model

Cold weather alert! Reigate should prepare for an impressive weather change this weekend into next week.  The temperature will drop like a stone over Sunday night: shown by these Reigate maximum temperatures at midday on Saturday max +11ºC, Sunday max +5ºC, Monday max -1ºC with wind chill even as low as -18ºC (yes) possible in forecast 40 mph gusts in exposed places.  Winds will average 20 mph but even this will cause wind chill of -8ºC.  It will feel cold on Monday!  Light snowfall could start Sunday pm and continue overnight through to Monday. It may seem odd, but this snow will be caused by warm air from the South over-riding the frigid COLD Polar air blasting in from the NE: meeting at a front.  Check the temperature height diagrams to see how this works: warm, moist air circulating around the LOW in the Channel will be undercut by the dense freezing cold polar air from NE. The warm air forced to rise causes cooling, condensation, cloud formation and, as upper air temperatures are so cold… SNOW is likely to fall. Possible accumulations Sunday – Monday around Reigate could be 0-3cm (higher figures on hills) and more possible to the south of the region where precipitation could be more intense nearer the fronts.  So…positioning of the LOW is critical.  The snow maps give an idea of potential snow areas in the south on Monday.  There could be light snow showers on and off in Reigate and another moderate fall is possible Monday pm as the LOW tracks nearer to the south east of our region.  It will feel cold in strong gusts from the East: wind chills down to an outrageous -18ºC are theoretically possible if you are caught in strong easterly gusts with air temps below 0ºC all day. Temperatures will actually fall through the day on Monday as the Polar air takes a grip.  Lowest temperatures all winter are possible overnight Monday-Tuesday. Hopefully, this weather change will not take anyone by surprise: it has been on the cards for over a week and flagged up here on 2 March. The rest of the week will remain cold, but feel less cold in light winds, and drier with pressure building from the north, but more snow might precede rain later in the week as things warm a little next weekend.  More updates later as details emerge for Monday which is still v changeable!