Archives For high pressure

2016-03-20_09-26-16

High pressure has dominated the last week of our weather but it has turned out disappointing here due to cloud cover lingering under a persistent temperature inversion, not unusual for this time of year.  Lingering decaying fronts have caused drab stratocumulus cloud to spread out beneath an anomalously warm upper air mass causing anticyclonic gloom for much of the SE.

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Any convection has been limited to the lowest 1km and been unable to break the inversion, so cloud, unable to rise into cumuliform tufts associated with the stronger sunshine in April, simply spreads out into a boring grey blanket, especially when the flow arrives from the North Sea bringing additional moisture in the lower layers. An inversion is when temperatures increase with height through a part of the atmosphere, usually referring to a lower layer.

So, whilst upper air temperatures have been anomalously warm, the surface temps have been kept disappointingly low.  Somewhat “upside-down” weather.

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This is because the Spring sunshine has been unable to break through the cloud and warm the surface.  The exception has been the north and west of the country, especially the hilly parts of Wales and NW England, which have enjoyed more sunny days.

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anticyclonic gloom

Today it was the turn of the E/NE coast to get the sunshine as weakening fronts shifted south around the high.

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The coming week sees the HIGH slowly deflating, like a sad party balloon, into the sub-tropical Atlantic.  A couple of powerful late winter storms emerging out of the NE US and Newfoundland start the onslaught to break a westerly unsettled flow back across the Atlantic by Easter.

For us in the SE this change to unsettled conditions happens slowly but ensemble runs are showing around 20mm of rain is possible before the end of March. So we might expect some wet and windy days before the end of March, including a risk of a wet bank holiday period.

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A return to more mobile frontal conditions is not all bad news, especially in the SE.  A westerly flow with weakening Atlantic frontal systems will break the gloomy cloud cover and bring sunny intervals and showery episodes to clear the pollution phase we have experienced lately.  The risk for us is any fronts stalling over the SE in front of a European HIGH – this situation can dump fairly large amounts of rain. Chart below shows wind speed and an active cold front for Sunday, too far off to be reliable but worth watching.

 

The multi-model charts below show this change from HIGH to LOW during the next week.

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Note the model agreement below by Saturday for SW winds, bringing temperatures up possibly into the mid-teens in the SE.

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In short, we can expect the weather to turn the “right way up” again and we should enjoy more mobile, fresher, brighter conditions, albeit with breezy episodes of potentially heavy rain at times. The charts below show temperatures rising in the SE but rain returning by Friday.

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Further into April there are hints that pressure rises across the Pole which will bring the potential for further unsettled cooler conditions during the school holiday period. RGSweather is off to Iceland (East fjords) again so this could mean some nice cold conditions for our trip there as the AO is expected to turn negative and the flow northerly, at least for the N Atlantic.  The UK appears to get stuck in an unsettled trough for early-mid April. Worth watching as JMA and CFSv2 both agree on this blocked pattern.

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2015-09-18_19-00-43

run reigate weekend wins out for a great day

The second annual Run Reigate half marathon and 10km race on Sunday is a major event for the town.  Happily the weather looks PERFECT for both runners and spectators!

A ridge of HIGH pressure is building in as the trough which brought a few sharp showers this afternoon moves off NE. So we expect no rain at all this weekend.

A beautifully settled early autumn weekend is expected.  Cool nights at Tmin 10C with some autumnal mist early morning are likely.  Days will brighten up in sunshine to reach possibly Tmax 20C in the afternoon.

Light winds and little cloud will be appreciated by runners and spectators. The outlook is for unsettled cooler than average conditions to gradually return on Monday and into next week with rain at times as Atlantic fronts progress across the country with low pressure. Further ahead there is a chance of a warmer and drier end to September as a Scandinavian high builds to the east.

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outlook shows a fortunately settled weekend for Run Reigate

Soil moisture: possibly the most under-rated meteorological measurement!  Rarely do weathermen get animated about the extent of wet sod across the country.  Nevertheless, soil moisture, usually measured in centimetres of water in the top two metres of soil or as % saturation (see maps below), has been found to control continental scale weather patterns, summer maximum temperatures and even heat waves and the extent of droughts.  So we ignore soil moisture at our peril, especially as soil moisture also controls vegetation growth and death and the ability of farmers to grow food.

How wet the soil obviously relates to how much it has rained recently.  During winter, in mid-latitudes, soils usually become increasingly saturated with a surplus of water building up as inputs of precipitation exceed evaporation which is reduced in the cooler months and shorter days.  During the summer, soils tend to become increasingly depleted of their moisture content as evaporation (output) exceeds precipitation (input).  This input and output of moisture forms an annual balance known as a soil moisture budget and is shown in the graph below.

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High rainfall during the winter builds up a lot of water in the soil.  In the spring time a high soil water content “uses up” more energy from the sun in the process of evaporation.  The more energy “used up” in evaporation, the more energy is lost from the system to produce sensible warming at the surface. Hidden energy, or latent heat, is required to change liquid water into water vapour.  So “latent cooling” reduces the amount of energy available to warm the atmosphere as long wave radiation.  So local temperatures can be depressed over areas of wet soil especially during a spring when wetter-than-usual soils might take a long time to dry out.  It might also be expected that, after a wet winter, there could be a cooler period until such time that the soil dries out locally and more energy becomes available to produce a sensible heat flux at the surface.

In fact, the effects of soil moisture go far beyond these micro-climatic changes and can have impacts that are continental in scale.  In a 2007 study (see below) it was found that 25% extra soil moisture could reduce continental Europe-wide temperatures by up to 2c from average summer maximums. Likewise, a 25% reduction in soil moisture could raise temperatures across continental Europe by 2c in the study period.  It was also discovered that higher winter and spring soil moisture could raise summer precipitation levels and change continent-wide pressure patterns.

The study from 2007 used reanalysis of computer weather models to investigate the impact of soil moisture on the European 2003 heat wave, the warmest for 500 years.  This heat wave killed over 20,000 people and caused crop damage.  The study found that by re-running computer models just with different soil moisture values, the maximum temperatures and heat wave intensity varied greatly.  Dry soils during the spring increased summer heat wave intensities while wet soils reduced the maximum temperatures.  The difference was significant, and in some localized regions the intensity of heat anomalies varied by 40% simply due to different soil moisture content at the outset of the model runs.  The largest differences were mainly located over central Europe.  It seems that differences in soil moisture have most impact across central continental Europe and progressively less impact on summer temps with increased distance further north.

Not only did soil moisture control temperatures, it also had a control over continental pressure patterns.  Dry soils built pressure through the middle troposphere, while wet soils could lower pressure.  This has numerous positive feedbacks: wetter soils reduce pressure which increases cloud formation and summer rainfall that enhance the wet soils.  Dry soils build pressure, reducing cloud formation, reducing summer rainfall, further drying out soils.

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So, in summary:

  • If the soil-moisture deficit is high, the dry soils raise the sensible heat flux, producing a deeper, warmer, drier low-level atmosphere: raising temperatures and enhancing surface heating and drying.  Increased drought risk.
  • If soil moisture is high, the latent heat flux by evaporation and transpiration dominates, enhancing cloud formation and a tendency for cooling.  lower temperatures and enhancing rainfall and further wetting of the soil.  Increased wet summer.

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The full article is here: Fischer_heat_waves_2007 (1)

High pressure persists this week over the UK as shown by the chart and the amazingly clear satellite pic showing great swathes of clear skies over a remarkably snow-free Europe today 11 March. It’s worth remembering that on this day last year we saw the lowest wind chill temps of the year in Reigate at -11.9c early on 12 March with -10c at 9:30am on 11 March during an episode of extremely cold NE winds which brought a blizzard to the Channel Islands.

Back to this week… the high pressure has brought delightfully sunny days and Tmax temps to Reigate over 19c last weekend , especially warm and spring like on Sunday. Since then a cooler NE/E breeze around the edge of the high as it slipped north and east has kept us cooler and occasionally more gloomy with anticyclone gloom and stratocumulus blanket on some days.

The outlook this week remains settled but with a distinct drop-off of temps over the weekend and especially into next week.  This is due to the HIGH slipping west and the jetstream dragging down cooler NW winds fresh from Greenland.  A LOW moving across N Scotland and over to Scandinavia this weekend will start the process of dragging down NW winds that will make next week cooler, especially further east where it looks like the N Sea will get a full blast of chilly air at times.

Nevertheless, it is unlikely to become terribly unsettled down here in the SE as the longer term picture looks like high pressure hangs on in the south keeping us mostly dry.  The north of the UK might see more frontal action as LOWS bring fronts across more northerly parts.  Models seem to be keeping the south relatively dry for another week to 10 days.

After the wettest winter on record, groundwater levels in the chalk aquifers are the highest EVER in the SE and the Mole Valley particularly.  There are still 50 flood warnings in the SE and locally flood water continues to pour across the A25 between Reigate and Dorking near Brockham.  This groundwater flooding, though of course nothing like as damaging as the fluvial river flooding during the winter along the R Mole and the Thames, could last for some months yet as the winter rain continues to percolate into the chalk water table and flows out along geological boundaries at inconvenient locations such as normally dry river beds and bournes and across impermeable surfaces such as roads. The map showing soil moisture below shows how wet the soil remains after this extremely wet winter.

Temperatures through this week look increasingly summery with dry weather courtesy of our summer friend the “Azores High”.  Coincidentally, the meteorological summer starts today, June 1st.  Temperatures are due to mostly increase this week in Reigate possibly topping out at 25°C by next Saturday but the building heat comes with a twist!  The HIGH pressure we are enjoying over the UK right now is due to slip North and then West. UPDATE: this will take its time… breakdown looks like Mon/Tues next week, weekend fine!
later next week

This will expose us increasingly to LOW pressure over Europe and a feed of warm air from a LOW in Biscay.  Summer heat combined with LOW pressure can yield thunderstorms as thermals lift into the atmosphere creating big cumulonimbus clouds.  A feed of warm air (warm air advection) will also encourage this thundery activity.  

early june rainfall europeThe continental LOW over Europe has been a breeding ground for thunderstorms and heavy rain which has plagued the continent recently.  With high pressure possibly leaking away from the SE UK by next weekend, models are suggesting that, with a stronger sun and lower pressure, heavy thundery rain could invade from the LOW pressure to the South.  So, enjoy our summer friend this week, the Azores HIGH, because it could break down and allow convective storms and fronts to move across the UK from the continent by next weekend.  

Increasing storm risk

Increasing storm risk

The above graphs show rainfall and CAPE.  CAPE is a measure of convective available potential energy and is a predictor of showery and thundery activity. CAPES so far this year around Reigate have been max of around 300 j/kg: giving a few thundery showery days during May.  Some model runs predict CAPES of 800j/kg later in June which could yield some bigger thunderstorms.  Nevertheless, recent tornadic supercells in Oklahoma have readily built CAPES in excess of 6000 j/kg so ours are small-fry by comparison.  Also, this is along way off and models do get things spectacularly wrong, so storms are certainly not a certainty but worth considering as an idea. 

Good news! HIGH pressure is building across the UK for early June and should last well into next week. The “Azores High” is moving our way and this will bring settled, calmer, warmer, drier, brighter and occasionally sunny weather to Reigate and much of the UK. Some rain is possible as fronts brush across the west and a trough over Europe brings a low risk of thundery showers later next week for the SE but this is not certain at this stage. The weather is looking mostly good for early June with temps reaching Tmax 20ºC.  

SSTs late May 2013Despite this the SE will remain average or cooler than average for the time of year due to being on the cool side of the HIGH: winds will be from a cool east or NE crossing the North Sea (still cool 8-9C) as the HIGH migrates to the North and East of the country.  

high and low pressure diagram

High pressure is where air sinks over a wide area. Sinking air is caused by cooling either from Tropical air chilling over comparatively cool oceans, like the “Azores High”, or air cooling as heat is lost from frigid continents in winter, like the continental HIGH over Asia./ Siberia.  Subsiding air inhibits convective updrafts of air (thermals) which create cloud and rain, so HIGHS are usually dry. HIGH pressure systems, also called anticyclones, also BLOCK the passage of low pressure cells pushing frontal systems away to the edges.  So, HIGHS are more or less the “opposite” of LOW pressure which has dominated our weather for most of May. In LOW pressure systems (cyclones or depressions) air rises, cools and condenses to form clouds and rain.  

air flows from high to low!Air flows from HIGH to LOW: that is, winds blow from areas of HIGH pressure to LOW pressure.  Due to the rotation of the Earth wind is deflected to the right in the Northern Hemisphere.  This is called the coriolis force and creates the familiar pattern of spiraling winds circulating anti-clockwise into LOWS and clockwise out of HIGHS in the Northern Hemisphere (it’s the opposite in the Southern Hemisphere).

 
fohn effect #1

fohn effect #1

The HIGH is forecast to move to the NE of the UK and this will bring in cooler NE and E winds to the SE: but light winds always. Remarkably, on Tuesday the relative location of HIGH pressure to the East and LOW to the SW of Iceland will mean Northern Iceland could be warmer than Reigate! The reason is an unusual weather phenomenon called the Föhn Effect.  Iceland has big ice sheets and mountains in the south and central region,including Vatnajokull, which is the biggest ice cap in Europe and has the highest mountains in Iceland.  The southerly / SW wind building over Iceland, created by the movement of the HIGH to the east and movement of a LOW to the South, will roll up and over these mountains and ice sheets and create LOTS of rain and snow!  On the windward slopes, in the teeth of these SW winds, saturated air cools as it is forced to rise over mountains.  However, as it it cools there is a release of latent heat due to condensation.  On the leeward side of the mountains and ice sheets the comparatively dry air sinks and warms up at a faster rate than it cooled. This is because the descending drier air warms more rapidly than the ascending cool air cooled (due to that latent heat being released)! (Still with me?!)  The result is a warm, dry wind streaming down from the mountains of central Iceland and bathing Northern Iceland in temperatures over 18ºC. It is rather like an “extreme” rainshadow effect experienced to the east of mountains in the UK, for example.  Föhn winds are associated with most high mountain ranges which experience stable saturated airmasses being forced to rise over them, such as the Alps and the Rockies.  In the Rocky Mountains, the Fohn wind is called the Chinook or “snow-eater” which melts snow rapidly across the plains at this time of year.

A dry start to May but how long will it hang on?!

Before Saturday’s shower, it last rained in Reigate 5 days ago but rain is forecast this week.  Nevertheless, the first 10 days of May are due to be below average rainfall (see map below), despite wet weather mid-week.  Much of the rainfall in Reigate during early May is likely to be showery, some heavy but rather short duration so not amounting to much.

europe wet and dry may

May Bank Holiday in Reigate is likely to be a pleasant 20ºC, but not quite the sizzling 26ºC claimed in some newspapers! The warm weather we have been enjoying has been courtesy of an upper level ridge to the south pumping a warm air mass from a warm source region to the south west around the Azores. This warm upper air has built a surface HIGH pressure to the south of the UK which has kept us largely dry, warm and cloud free.  The tropical airmass this week has had temperatures at 1500m (850hPA) of +10C over Reigate; (quick reminder that in March we had uppers of -12ºC!). This week could see upper air mass temperatures drop to nearer 2ºC over Reigate by mid-week. Surface temperatures will still be OK, reaching mid-teens in the stronger May sunhsine but night time temperatures could be quite chilly and even risk a touch of frost if skies clear later in the week when a HIGH is due to build through again at the end of the week. High pressure will build back in the south and, though a long way off to be certain, next weekend looks staying dry with temperatures probably recovering nicely in any sunshine.

The cause of the forecast #slightly# cooler weather arriving in Reigate this week is a possible significant switch in the airflow from mostly southerly winds to a more westerly/north westerly wind direction: a cooler direction across a cool Atlantic ocean and from a cooler source region around Iceland. The cause of the change in the wind direction is an Atlantic LOW forecast to cross the UK mid-week. This LOW will bring Atlantic fronts and rain, possibly heavy at times mid-week, before clearing to showers.  A HIGH is forecast to build back into the south by next weekend bringing dry weather back and temperatures back to possibly hit 20ºC again. As usual, this is an early forecast and things will change so do keep in touch on twitter @RGSweather.

high and low pressure cross section

Why is HIGH pressure usually dry and cloud free?  Air sinks in HIGH pressure: as it sinks it warms and dries out. Warm air can contain more water vapour than cool air, so any moisture tends to evaporate in HIGH pressure (anticyclones).

In LOW pressure (cyclones / depressions) air is LIFTED and expands, cools and any water vapour will condense, forming clouds and eventually rain.

Air flows from HIGH to LOW (but not straight! it is deflected to the right in the northern hemisphere by the coriolis force… more on this later).  This gives us the familiar surface pressure charts with winds apparently spiraling round HIGHS and LOWS.

air flows from high to low!

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.

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…

Mon-Tues-Weds
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.