Archives For April 2013

HIGH pressure is set to dominate the weather for Reigate most of this week and, quite possibly for much of May, with a few significantly wet interludes caused by a “cut-off” low early next week, possibly. This week, however, we have little or no rain forecast and pleasant spring sunshine raising daytime temperatures to 15ºC or more. Night time minimums could still fall to freezing to produce pockets of frost on some clear nights. North and North Easterly winds will prevent the temperatures reaching too high but winds should remain light. So.. very pleasant weather to come!

An interesting aside to all this nice weather is what is happening elsewhere: a feature called a “cut-off” LOW is causing cold and very wet weather over Spain and parts of France and thunderstorms over the Mediterranean.  This feature was born at the weekend when the cool plunge of showery weather over the UK (an upper trough) reached Spain but was “cut-off” by the jetstream slicing through South East England and cutting the neck off the trough.  This cut-off LOW will yo-yo back towards southern UK later this week (check the rain in France on the map) but is NOT forecast to quite get here… except possibly brush Sussex with rain. It will, however, bring some heavy rain across France. Cut-off LOWS are interesting because they barely feature on surface pressure charts but can cause utterly miserable weather nevertheless. So, whilst we enjoy some wonderful weather think of the Mediterranean and much of continental Europe in this LOW pressure regime courtesy, to some extent, of the action over Surrey with the jetstream last weekend!

Bank Holiday looks better and better: once light rain on Saturday has passed (a weak cold front) the risk of any showers reduces to almost zero through sunday and monday! the rain is staying firmly to the north of the UK and in France.  Bank Hol weather looks very pleasant with temperatures over 20ºC in Reigate.

Update Friday: showers likely but heavy showers & thunderstorms unlikely Saturday as upper air temps rise and build pressure rapidly during the day inhibiting convection.  Warming uppers means lower lapse rates, less perky convection, thermals rise less vigorously and less high and so cumulus clouds build  less height.

trough splits

A classic intensifying COLD FRONT passing over Reigate on Friday 26 April is set to bring some heavy rain and then a chillier feel to the weather after temperatures soared to over 22ºC today.  Reigate has been bathed in warm Tropical air brought by south westerly winds this week and night time temperatures have barely fallen below 10ºC. The arrival of cold polar air is set to push maximum temperatures below 12ºC during the day over the weekend, so it will feel decidedly less balmy.  Expect a cool, bright and showery weekend with cold nights and a frost risk. Sunday looks dry and bright with light winds, so this will be a pleasant day.  So Polar air is not all bad news with sparkling visibility as well as possibly some splendid cumulonimbus or cumulus congestus clouds on Saturday! 

This typical cold front, albeit initially lacking in perky thunderstorms, shows how relatively dense cold Polar Maritime air advancing into warmer Tropical Maritime air still hanging on in the SE over Reigate causes moderate to heavy rainfall.  The warmer, less dense, Tropical air mass has temperatures at 1500m (5000 feet) of +8ºC. The advancing cold Polar airmass from the North has upper air temperatures of -5ºC and this air will push the warmer air up. Rising air containing moisture condenses, forming clouds and rain. The more rapid and vigorous the lift, the more condensation takes place and the heavier the rain. Reigate can expect moderate to heavy rain in the morning as the cold air undercuts the warm. Rising air also causes atmospheric pressure to fall and this occurs in advance of COLD fronts. There is often a rapid RISE in air pressure as cold fronts move away and this often leads to skies clearing and a temporary lull in showery activity before surfaces warm, lapse rates and instability increase and cumulonimbus clouds form bringing heavy showers. Saturday has a heightened risk of heavy thundery showers arriving on the northerly wind in easterly building belts of convective clouds: some big cumulonimbus clouds are possible. Belts of moving showers moving down a trough on northerly winds are called “scoopers”.

may recovery planThis cooler snap looks fairly short lived with a gradual recovery to warmer temperatures in early May with a HIGH pressure predicted on some models, so possibly a dry and bright bank holiday!  Check back for updates.

Reigate: decently warm and very pleasantly “springy” through to mid-week but then a significant fall in temperature and showers to end April!
Cloudless, blue skies bathed Reigate in sunshine all day today and brought temperatures up to nearly 15ºC: perfect for cricket!  However, the air flow was a  cold easterly which meant that it felt cool in the wind, more like 11ºC, but in the sheltered sunshine it was pleasant. London Marathon Day 2013 will start at a chilly -2ºC and will cloud over later, reach max 13ºC and stay dry with only a slight risk of a light shower later; low humidity perfect for marathon runners.

The big temperature graph shows temperatures at 1500m above the surface (at 850hPa), used by forecasters for predicting temperatures because it is a height which smooths-out the complicating influences of surface diurnal (day/night) temperature changes and surface features like sea, land, cities, hills and vegetation, so making overall temperature trends and changes in air mass easier to spot.  This week it’s easy to spot the cool air mass currently over the region, the warming mid-week temperatures and then a fall-off to next weekend.  Three different airflow directions are responsible for these changes, so here’s some detail…

reigate midweekLOW to the north and HIGH to the south of the UK: this pressure pattern will squeeze breezy westerly winds across the UK as a result of the pressure gradient between the HIGH and LOW pressure. A lively jetstream will be directly overhead of the UK and drag an increasingly mild upper westerly / south-westerly air flow from the Atlantic which will bring the risk of the odd shower on a few weak fronts reaching Reigate now and again early in the week but nothing much.

The HIGH creeps up more from the south over Reigate on Wednesday and this should bring warmer and sunny conditions, possibly up to 17-18ºC in much lighter SW winds: so making for a “warmer” HIGH than the one we are currently experiencing which is dominated by a cool easterly airflow over Reigate.

End of next week and next weekend:

It looks like it’s all downhill after mid-week: temperatures are due to take a plunge with HIGH pressure building in the Atlantic with an upper level trough (LOW) digging down over the UK bringing a cold Polar air mass down to Reigate through into next weekend.  Unsettled April showers, some possibly thundery, may break out at the end of the week and over the weekend as this cool northerly air mass meets the warmer surface air and increases lapse rates and instability, encouraging convection.  Next weekend could well struggle to reach 10ºC daytime max , with some showers during the day and nights turning cold.

This is all early days and things could change, so do keep updated on twitter for Reigate weather @RGSweather.

Where are bluebells flowering NOW? Find out here nature’s calendar

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.

Reigate was one of the warmest places in the UK today.  The town bathed in air from North Africa at nearly 20ºC.  The strong SW wind driven by a 100mph jetstream aloft along a decaying front brought some superb mid and high level clouds across Reigate.  A variety of unusual clouds were visible.

Cirrocumulus is a high level cloud and unusually covered much of the sky from early on this morning. The light dappled mini-cumulus flecks 30,000 feet in the sky were beautifully fibrous in appearance.  Occasionally the cirrus rolled into parallel lines converging on the horizon: cirrus radiatus.

Virga is another unusual site for cloud spotters.  These are ice crystals falling from cirrus or altocumulus: this precipitation evaporates before reaching the ground so appears as a tail streaking out from underneath cirrus or altocumulus.  Several virga could be seen during the day but tend to disappear quite quickly.  

Unusual altocumulus lenticularis wave clouds rolled in at the end of the day on a decaying front and even deposited a few drops of rain momentarily.  Virga making it to the planet surface!  Altocumulus lenticularis are the clouds occasionally mistaken for UFO’s!  They are usually formed by high winds blowing over hills or mountains and creating wave like clouds in the air stream; our lack of high mountains makes them rather rare in the SE.

Although not as warm as today, mild and breezy conditions will persist for many days now. It will not always be dry but less than 10mm of rain is forecast for Reigate over the next 8 days so any fronts reaching us will be weak.

The pressure pattern and direction of our weather will be the “reverse” of those cold days of March: south westerly and westerly winds from the Atlantic will prevail most of the time over the next few weeks.  LOW pressure systems are set to cross to the north of the UK and HIGH pressure will sit to the south: this will keep the jetstream more or less persistently over the UK and drag in milder but moist air, especially for the west. Reigate and the east of the UK will be drier and can expect daytime temps around 15C, cooling towards the end of this week.

What makes a thunderstorm?

The conditions for lightning occur when powerful updrafts in cumulonimbus clouds force water droplets and ice crystals to rub against one another, creating massive amounts of positive- and negative-charged particles. The updrafts cause these two types of charged particles to separate, with the top of the thundercloud usually becoming positively charged as the lower part becomes negatively charged.

Here are the ingredients which formed the heavy “April showers” and first thunderstorm (TS) of 2013 over Reigate on Friday afternoon, 12 April 2013.  Whilst only a minor storm, it possibly still contributed to a multi-vehicle non-fatal accident on the M25, closure of the motorway for several hours, poor visibility, local flooding and hail across the area. Convective isolated rainfall events like these are important but tricky to forecast accurately: predicting exactly how much rain will fall, what type and precisely where and when isolated showers and thunderstorms will take place has a lower success rate than other elements of forecasting, like temperature predictions, for example.  Here is a round-up of the key indicators that enabled @RGSweather to issue a forecast for possible thundery activity more than 4 days before and monitor it’s development thereafter and issue a local forecast warning of a thunderstorm risk with very marginal low level tornado risk on the morning of 12 April.

A single relatively minor thunderstorm developed mid-afternoon with hail and lightning crossing north of Reigate on April 12, following a line roughly along the M25 between Leatherhead and Reigate.  The morning saw scattered and heavy showers but little organised severe weather.  Skies darkened over Reigate by 3pm under thicker cumulonimbus cloud and this thunderstorm caused some hazardous driving conditions on the M25 and a non-fatal multi-vehicle accident coincided exactly with the time the storm passed over the M25 which was closed clockwise for several hours thereafter.

Synoptic Situation: An upper trough over the UK and surface low (pictured) across southern UK moved slowly east during Friday, with an unstable airmass building cumulonimbus and increasingly heavy showers during the day as April sunshine heated the surface and increased instability. Here are the figures for yesterday and brief analysis…

LAPSE RATES: +29°C: Cold temperatures at 500mb heights and warming at the surface in the April sunshine caused steep LAPSE RATES of 29ºC. Lapse rates are the drop in temperature with height usually measured between 850hPa (1500m) and 500hPa (5000m). Steeper lapse rates indicate an unstable airmass where parcels of warm air heated at the surface in spring sunshine will rise rapidly and remain warmer than the environmental air surrounding them. Such air parcels will condense, releasing latent heat, which causes further rapid uplift and potential for the formation of cumulonimbus clouds given the absence of any inhibiting factors, like a cap (inversion or isothermal layer… see link below).

CAPE and Lifted Index: 378j/kg; LI -1: Convective Available Potential Energy (CAPE) is a measure of the energy in the atmosphere for convection (j/kg).  Figures in MidWest USA approaching 6000 j/kg cause tornadoes. Here in the UK, CAPES above 300j/kg can cause thunderstorms.  Lifted index is the difference in temperature between the environmental air at 500hPa and a parcel of air lifted to that height: a negative LI indicates buoyancy in rising air parcels and instability and significant convection.

Vorticity at 700hPa: upper air velocity at mid levels means that air is rising. April 12 has UVV: upwardly mobile air at mid-levels.

PWAT: 20mm: Precipitable water is the amount of water that would fall to the surface if all the moisture in the atmopshere rained or hailed out.  Relative humidity is a measure of how saturated the air is at various levels in the atmosphere.  100% means saturated: most levels were at least 80% RH.

TTI index

Total Totals Index (TTI): 60: this is a forecasting index used to measure potential storm strength.  It is calculated using the difference in dew point and temperature between 850hPa (1500m) and 500hPa (5000m).  TTI’s in the MidWest of >60 can yield severe tornadic supercells.

Wind shear: this means change in speed and direction of winds with height.  April 12 saw little deep layer wind shear: winds were blowing at similar strength and direction throughout the atmosphere so little rotation or organisation into severe storms could develop.  Nevertheless, slow moving storms deposited a lot of rain locally and caused minor localised flooding.

WAA: warm air advection: introduction of warm air at the surface increases lapse rates and can increase likelihood of severe TS: 12 April saw little WAA and this inhibited the development of any organised severe weather.

Towards the evening warmer air moved in aloft and, along with the removal of surface heating as the sun set, rapidly reduced lapse rates and inhibited convection causing towering cumulus clouds to melt away leaving a clear night.

So, several ingredients were present to create a marginal storm risk but the absence of some other critical factors like WAA and wind shear kept a lid on the severity and distribution of thundery activity yesterday.  Hopefully, this brief round-up of key storm indices relevant for SE England should help in predicting more severe weather in our region in the future.

Tornado Titans posted this on the CAP and skew-t charts. v helpful. If this is all too much then watch this instead…!

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

Polar Continental (Pc) air is most common in winter as HIGH pressure forms over cold northern continental interiors and pushes out freezing air to mid-latitudes.  In summer, when it does occur, Pc brings dry stable and warm conditions to the UK as the continents warm up.  Pc has been an unusually frequent visitor this March and effectively reversed our usual south westerly prevailing wind. As Spring sunshine warms the surface and Atlantic LOW pressure systems edge closer to the UK next week dragged by a more northerly migrating jetstream, we can be assured that moist maritime air will be making a return and any remaining incursions of polar continental air will increasingly lose their frequency and ferocity, Russia has to warm up sometime!
air masses UKMOBefore we bid “farewell” to the freezing Polar Continental air until next winter it is worth remembering the good times.  Pc has occasionally brought crystal clear skies with excellent visibility and dramatic views of the sky both day and night (as anyone staying up to see the ISS will testify). The long picture series shows Cumulus Congestus building over Stratford on Avon last week and an unusual Pileus Altocumulus Lenticularis veil forming over the dramatic rising thermals. Pileus is a fleeting, ephemeral cloud type and forms as convective up-draughts in the cumulus force upper winds over the rising congestus, just like air being forced to rise over a mountain range.  Moisture in the air condenses, or sublimes into ice, and forms a beautiful veil called Pileus.  The photos were taken over just two minutes and then the Pileus melted away.  Pileus is a beautiful cloud but has a darker side because it sometimes forms above rising nuclear mushroom clouds and volcanic eruptions.

Cloud streets, lines of Stratocumulus, were also a feature of the easterly winds: where an isothermal “cap” (temperatures staying the same with increasing height) kept a lid on rising thermals and clouds remained flat and formed lines in the airstream.  Cloud streets seem to urge us to follow them, pointing the way to something important over the horizon.  Finally, the “sundog” (mock sun) was another fleeting feature of polar continental air, though not exclusive to it: apparently only 5/100 people have ever seen a sundog, so here is a picture of one in case you haven’t caught one yet.  They occur as low-angled sunlight refracts through hexagonal ice crystals.

Pc air wasn’t all as beautiful as this of course: freezing grey blankets of dull stratocumulus dominated the weather for days in the south east and deposited icy snow grains right through to Friday.  Nevertheless, I do hope you had the time to look up and admire the best of the Polar air show this March.  So, Polar Continental may crack our cheeks and rage and blow but we’ll kind-of miss it… won’t we?  “Adieu, adieu, adieu… remember me.” Exit Ghost of Pc! (The photos above were all taken along the Stratford canal last week, the statue is William Shakespeare in Bancroft Basin).

n.b. March summary for Reigate coming soon!