Archives For HIGH

Amazing clear blue skies this weekend in Reigate! Subtle changes at first this week, then more significant deterioration by the end… probably 😉

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HIGH pressure stays in charge through the first half of this week but with more cloud building and a few scattered showers possible.  We are on the cool side of the HIGH with an airflow from N/NE and occasionally brisk cool winds on the coast circulating round the HIGH.

HIGH pressure UK early June

HIGH pressure UK early June

Easterlies / NE winds are due to build on occasions this week especially on the south coast as super-geostrophic wind circulating round the HIGH draws cool air from the North East.  Cool nights are likely in a generally cool airmass when skies are clear of cloud and heat escapes.

cool nights.. frost up north?

cool nights.. frost up north?

Could even be a touch of frost up north on first few nights early this week.

The thickness chart above shows how the east side of the HIGH has a less “thick” airmass which is cooler as measured between 1000-500mb height). This means that, despite the overall high pressure, surface warming during the day can increase lapse rates lifting thermals into the cool air which can spark scattered showers as land warms through.  There have been big thunderstorms in Europe along the front separating this cooler airmass from the building heat of the Med.

Later in the week warmer thicker airmass is due to fold round the top of the HIGH but this might bring more cloud.  Things are on the change by the end of the week as a LOW forms in Biscay and threatens to edge north / NE to bring possibly wet weather to the SE by Friday.  The MetOffice fax chart shows a triple point crossing into the South by Friday and these can yield a lot of rain. Spanish Plume potential brewing … (updated Tues)

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metoffice fax chart plume

Whilst this is unlikely to be a full-on classic plume plume (update Tues… yes it could be best for some time!) it could sweep unstable air into the south and possibly cause thunderstorms. Heavy rain is possible too.

Thereafter the scene into the weekend looks more unsettled as a trough over the UK replaces the HIGH as it regresses (moves west) into the Atlantic. An Atlantic LOW from the NW looks on the cards for the weekend while pressure remains low. June has started cool (especially due to cool nights) and will continue to be overall cooler than average for the next 10 days. Note the change visible on the charts below.

There is general agreement for this deterioration amongst models.  So this week is likely to see a change to less agreeable conditions: lower pressure and a bit more rain at times and staying rather cool for summer!

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model agreement

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

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

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synoptic chart Fri 6 March

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

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

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

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LOW passes to the NW on Monday

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

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

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

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

BBC summary

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

 

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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.

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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.

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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.

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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

As promised, here is a final weather update for Run Reigate Sunday 21 Sept.

Reigate avoided any heavy showers or thunderstorms today.  These got stuck over N London, Essex and Suffolk where it really has been chucking it down for the past 24 hours.

The weather is turning out ideal for runners tomorrow as the weather has done a timely U-turn: a northerly breeze is replacing the unseasonably  humid and (potentially) showery southerly /SE airflow of recent days.  A weak cold front moving south through the UK will complete this transition overnight and into early tomorrow morning.  As it passes over Reigate through the small hours it will possibly yield some light rain.  So expect first thing to still be cloudy and a little damp and misty as the last of this cold front moves south across the Channel and into Europe.  Temperatures for early arrivals could be around 13C in Priory Park which will obviously feel cooler than the humid temperatures and nights of late, especially in a gentle 11-15mph northerly breeze (hardly notable really but certainly cooler for thinly clad runners).

From the outset the day will improve steadily… low clouds should gradually break up from the north through the morning and, as there will be no mid or high level clouds, sunny spells should break through nicely and lift temperatures to 17C by midday.  Humidity will fall from an early 90% RH to 55% later and, with a gentle drying NE breeze, this should be perfect for runners.  How about the rest of the week?

The outlook for #Reigate this week (shown in charts above) is for dry weather dominating, pressure remaining mostly high in the south and temperatures being maintained above the 30 year average.  The jet stream is kept well to the north and, as we continue to sit well to the south of the jet, we will continue to enjoy un-Autumn like conditions.  The north of the UK will be swept by storms as Icelandic lows move NE and fronts brush across the North.  Any fronts reaching the south will be weak and not amount to much on current models.  The amount of rain expected in Reigate is minimal this week, a mere 1 or 2mm.  So watering the garden will probably need to continue for gardeners.  As nights are cooling off misty mornings are likely.  The potential for any autumn storms reaching Reigate has been pushed way further back in these latest model runs and it seems Autumn is continually being held at bay by HIGH pressure dominating the south.  Dry, pleasant conditions prevail.  Enjoy!

As a final thought, IF Bardarbunga caldera in Iceland erupts explosively then the winds this week are mostly favourable for keeping ash away from European airspace.  With LOW pressure over Iceland and HIGH over the south of the UK it is likely that upper atmospheric winds will be kept blowing strongly to the North / NW over Iceland for much of the week (i.e. S/SW winds prevailing).  There are hints of occasional westerlies or SW and these might send any ash to Scandinavia.  In any case, it is purely speculative and the caldera seems happy to continue its collapse right now without any imminent sign of explosive subglacial eruption. Worth watching though.

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Things have changed a bit since the previous post so a quick update is needed for race day.  Expect a final update Saturday night!

The unseasonable humidity and thundery showers for the end of the week and Saturday are set to be “blown away” on Sunday and replaced by ideal weather conditions for runners.  The charts below show a big difference in Tmax temps between 26C 22C (updated) for parts of the SE on Saturday (itself depending on cloud cover and showers) and a cooler 16C on Sunday.  The reason for this marked fall in temperature is a re-building HIGH pressure to the north which is introducing cooler air from a northerly direction to replace the humid continental air flow of the previous week. This change was expected but has arrived a day earlier than modeled previously.  So Sunday is drying up and cooling down, perfect weather for the day! More details below:

Race day is likely to start with lingering cloud but it will brighten up slowly, albeit remaining cloudy through the morning.  Some models keep light rain for a while but any remaining lingering showers should move away early.  Humidity will also fall, from 90% RH at race start to around 70% RH by midday,as a drier cooler clearer northerly air mass quickly replaces the humid continental air of previous days. The fresher upper air will be wrapping around the HIGH pressure building to the north and arriving in Reigate as a Northeasterly breeze.

Early on, temperatures of 13C will make for a fresher start for runners.  Sedentary early-bird spectators might begin to feel a little cool at the start after the relative heat of the previous days, especially in places open to the light NE breeze.  An extra layer might be a good idea for spectators. The day is set to warm up steadily to a pleasant Tmax 16-17C, ideal running conditions.

Though never a real nuisance inland, a gentle breeze is set to pick up to 11-12mph from the NE by mid-morning and there might be low “gusts” reaching 15-20mph for open parts of the route away from trees and exposed to the NE.  Reigate is sheltered from northerly / NE winds by the North Downs and any drying breeze is more likely to be welcome by sweaty runners! Another benefit is the air quality will be excellent compared with previous days of high pollution before the northerly moved in.  So a healthy run too!

Overall, the day is set to be great weather conditions for runners: a cool start, cloudy but drying out and warming up pleasantly with a drying breeze.

A great way of illustrating the air mass change this weekend is to look at Theta e and CAPE graphs below.  Theta E basically measures air mass instability (the higher the temp value the more “unstable” showery the air mass can be, similar with CAPE (convective potential energy)… spot how these two dramatically change on Sunday: from relatively unstable, energetic and showery conditions, to much more stable cooler air mass in a short space of time.

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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.

Update Friday: end of this week looking like a cooler shot from North: cold front to bring wet conditions across UK during unsettled Friday with jetstream dragging in colder northerly winds from Pole behind for weekend (after a mild-ish period mid-week as warm air incorporated into system).  Not snowy or outrageous, just cooler for Reigate. SE could win out at weekend as a bright, showery regime is possible on this NW airstream. Also, a possible “chilly” snap early/mid-dec appearing on charts now but not a major winter event likely at present, worth checking back for updates. No US winter storm for us, yet!

If this was July it would be all fine. The HIGH building from the SW sitting over the UK this week will do its best to deliver some nice weather but, for the SE especially, it “could do better” with some cloud and drizzle sometimes spoiling the potential for a glorious end to Autumn.  Nevertheless, there will be predominantly calm light winds and dry weather this week so nothing to complain about and no wintry weather on the cards for us in the SE at least.  The south-east and Reigate sits in between a cold plunge over the East of Europe driven by a LOW drifting over Scandinavia and a HIGH to the SW of the UK driving upper air on a long circuitous route from the SW Atlantic, over Iceland and then down the N Sea.

HIGH pressure sitting to the S and SW will build as a warm flow of SW upper air flows to the NW of the UK. This will, oddly, bring warmer conditions to the North of the UK initially than the South in the first part of the week. Initially a cool flow from the N and NE will keep the SE cool and with possible frost early in the week where temps will struggle to Tmax 6c and fall to freezing at night. Conditions warm through a little from weds onward and frosts become less likely as the warm air is stirred into the system and brings up temps day and night.  It also delivers more humid air which usually means more cloud for SE rolling in off NSea. 

Later in the week the jetstream is predicted to pull down cooler air flow from the north again, especially for E and SE England and, whilst there is no risk of wintry weather for us, this will cause another dip in temps with a spell of wet weather spilling down on a cold front towards Fri / next weekend, though nothing out of the ordinary.  It remains edgy but snow lovers will have to be patient, the air we are getting simply isn’t cold enough… yet!

When you are out and about this week: consider a global weather picture: a long haul jet from Deep South of USA delivers a warm Tropical airmass over the North Atlantic to build the HIGH this week which is making our UK weather, albeit it may not feel like it!…

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Reigate will see a wet, miserable start to the week, albeit a warmer day than the cool weekend due to the warm sector bringing in tropical air.  Rain is due to arrive for Monday morning journey to work and thereafter remain pretty drizzly and on and off all day with some heavier showers poss around midday and pm, including rain clearing on Tuesday morning.  The rest of the week looks drier and, whilst not bone dry, a high pressure edging into the SW from the Atlantic will inhibit the sort of rain and storms we have seen recently and push them way up north to Iceland.  

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For us it will be much calmer and drier with Tmax around 9c and cool nights down to 3 or 4c air temp, staying mostly frost-free unless cloud cover clears overnight. Further ahead models want to push down November temps from mid November onward, including a precursor of something briefly colder towards the end of the week as the HIGH shifts west and north of the UK and a LOW slips south down the North Sea: this could drag in some chilly winds.

A few wet days this week and then things looking warmer, brighter and more beautiful from the weekend!  Whilst not a heatwave it will be dry and pleasantly warm, even building to 23c possibly next week.

Reigate and SE is in for some heavy rain from lunchtime onwards Tuesday as a frontal wave forms under a brisk NW jetstream lying across the country. A wedge of warm tropical air will surge up from the SW and blast briskly up the Channel with gale force winds up to 50mph, and meet the Polar air blowing in from the NW lying over the country. The difference in the temperature of these air masses is pronounced: with places to the North of the front seeing Tmax 10c and those well to the south coast possibly hitting the high teens 17c, though in a brisk wind it won’t feel like that!  In between, the tropical air will lift, creating thick cloud and rain. Reigate can expect around 11-12mm of rain, possibly more, starting lunchtime and getting heaviest in the afternoon and getting lighter through the night.

After a drier Wednesday and another wet day on Thursday, things are still looking up for a pleasant weekend as a HIGH builds giving us a much drier run through maybe even to the end of the month.

Confidence now high for the HEAT WAVE starting weekend then continuing thru early next week and possibly beyond.  Updates later.

Early days still, but there is some indication of July warming up considerably from next weekend after a breezy mid-week. Models are coming together showing the Azores High building right across the UK from next weekend (7 July) bringing anticyclonic sunny and warm weather and a warm upper air flow. Some charts show temperatures in the south east exceeding 25°C and some even show 30°C! Whilst the UK is set to be much warmer than normal (positive anomaly), much of Europe is cooler than usual, still stuck in the trough of cool air.

warm UK 7 July

 It is worth remembering that the sun is very strong this time of year and, although a weather advisory seems tedious, it is worth slapping on the sun cream and adding a hat: slip,slap, slop!  Of course, our own heat wave will be nothing compared to the temperatures the SW USA is experiencing right now.  Record breaking heat is pushing temps up to the hottest ever recorded 54ºC.  That beats previously discredited records in Libya. ttp://www.nypost.com/p/news/national/western_issues_vegas_could_treated_Z7IWo9K7JwgXdDcqNOJ7jO

UV levels