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September 16, 2014

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Reigate Grammar School weather station, Surrey, SE England. School run website for local weather info and amateur weather observations and educational posts on weather. SkyWarnUK storm spotting. RiverSearch monitor for River Mole. Forecasts for educational and local information purposes. Please consult professional weather websites for decision making purposes. Town location VP2 wx updates to our own website pages every 15 minutes and “live” wind readings are updated every 5 seconds.  CoCoRaHS manual rain gauge monthly records. Data to UK Met Office weather observations website (WOW) and to US based Weather Underground (WU). 

Reigate weather for next week to 10 days is overall set to calm down somewhat and, for us in the south, be generally drier and warmer than average for the time of year, most of the time. Temperatures are unseasonably warm over the next few days with night times barely falling below teens and daytime reaching 21C.  This is due to the southerly winds bringing warm air from Spain.

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A slack static cold front divides the really warm air over the SE from the cooler air to the NW. This cold front will bring cloud and rain at times across the SE, especially to the south coast, as it drifts SE over the weekend.  Winds on the coast will also be more noticeable over the weekend with 20mph+ possible, but staying mild over the weekend. The temps are likely to slip very slightly as the weak cold front edges SE on Sunday bringing some rain through the SE and more wind to the south coast.

Unfortunately, there is a hiccup to this generally benign warm weather. A gradual rise in pressure is set to be sharply upset temporarily by the remains of Hurricane Gonzalo, now battering Bermuda as a Cat 3 storm and due to arrive UK early next week late Mon /Tues.

Gonzalo hiccup

Gonzalo hiccup

Gonzalo is modelled to arrive early/mid next week.  Forecasts suggest Gonzalo will merge with the persistent Atlantic low near iceland and ride the jetstream across the Atlantic arriving late Monday as a 980mb low (not especially low) , bringing gales initially to the west and NW coasts and then gales to through the North Sea later the same day.  Some heavy rain is likely but the SE and Reigate looks currently likely to miss the worst.

After its passage across the north of the UK, the centre of Gonzalo appears to move SE down into the North Sea and behind a potentially vigorous cold front usher in significantly cooler NW winds of some potency, albeit briefly.  These appear to peg down temps a while, especially on NE coasts. Thereafter, indications are for a gradual improvement in the south as high pressure builds over the continent, albeit with some fronts reaching across to the south at times before a HIGH pressure seems to be suggested for the last week in October.

2014-10-17_22-26-02

If this comes off a dry and settled spell can be expected for half term.  The location of the HIGH starts in the south where mild conditions can be expected but the anticyclone could slip north and topple east.  if this happens then temps will fall as a cool easterly flow are brought in from a cooler continent creating a more foggy autumnal feel.  The charts below show the possible scenario for the last week in October and very start of November with things cooling off.

So in summary the weather for half term is generally settled, warmer and drier than average for the time of year but with a significant hiccup as Gonzalo arrive early next week bringing a plunge of cool polar maritime air down across the UK and the north sea behind this sharp active system.  Thereafter, a gradual improvement to the last week of October when an anticyclone is set to build across the UK bringing autumnal foggy end to October as temperatures drop to November.  Note that extra-tropical storms upset models so keep an eye on any changes to this forecast.

forecasters: SAC and Chris M

 

 

 

Update: confirmed tornadoes today (see foot!)

Potentially interesting, albeit tricky, weather tomorrow for UK, Wednesday. A deep surface LOW sits to the SW of Ireland and is dragging through complex series of fronts associated with various airmasses.  Occluded fronts are tightly wrapped around the LOW, which is due to migrate NE away from the UK during the weekend and pressure to rise.  For the SE tomorrow it’s not so much the fronts but an unstable mass of warm southerly surface air that will be the main cause of any heavy showers tomorrow and some potentially thundery weather, especially when this warm air is forced up by anything… coast, hills or fronts.

Here’s likely scenario for us in Reigate, Surrey SE: Rain is likely, possibly exceeding 10mm, which is fairly wet for SE: higher is possible.  Most of this is likely to be convective rainfall due to unstable and moist air moving in from the south overnight.  Showers, possibly heavy and thundery, are most likely in the morning as the warm surface air moves into our area, causing lapse rates to increase moderately and this encourages lift and cumuliform clouds.  If the sun comes out then surface heating could spark heavy showers and thunderstorms as warm air rises freely through the atmosphere, encouraged by the jetstream overhead that effectively drags air off the ground.  One of the ingredients for thunderstorms, LIFT, is therefore partly in place tomorrow, although it will depend on sunshine for greatest effect.  If it stays overcast, which is possible, then little exciting weather action beyond just rain is likely.

The above charts also show that the southerly / SW airstream is humid because, as it converges on the coast, the model shows rainfall increasing significantly. This increased rainfall on coasts is often caused by convergence which is due to air arriving onto the coast quicker than it is leaving (check the lower wind speeds inland) so the wind effectively PILES UP on the coast and is forced to rise as it has nowhere else to go except UP!  This is called convergence. It is clear from the charts above that any HILLS also encourage lift as South Wales and even the South and North Downs appear to be pushing rainfall totals up locally: this is orographic or relief rainfall.  The charts below shows another feature of the weather tomorrow: the winds are shown to be VEERING with height (left diag) which allows WARM air to advect (move into) into our region (right diag).  Read on for more about how veering winds and WARM AIR ADVECTION can encourage stormy weather.

 

Another ingredient for potentially unstable weather is that winds are VEERING tomorrow, albeit not dramatically, which means they are rotating clockwise directionally with height thus allowing warmer air from the south to move into a location: it is like opening the door to warm air: winds move through a southerly direction and therefore allow warm air to “advect” into our area.  A moderate wind veer is taking place overnight and into tomorrow morning.  Warmer air at the surface is overrun by cooler westerlies aloft that increases lapse rates: steepens the temperature difference between surface and air at altitude.  The air at altitude tends to stay the same temperature and is associated more with direction and origin of airmass than it is with surface heating or advection of warmer air at the surface. An increase in lapse rates adds to instability which encourages air parcels to LIFT off the ground, should surface heating occur if the sun comes out.  The chart above right shows how WARM air is ADVECTED into the South of the UK and migrates NORTH during the day.  This has nothing to do with solar heating… it is an 850hPa chart (1500m) and shows the airmass temperature which is largely independent of surface influences.  It’s a good example of WARM AIR ADVECTION with a moist air stream increasing instability causing showers and possible thunderstorms.

Finally, the warm relatively unstable airmass is being overridden by a NE turning jetstream that will encourages lift and wind shear.  Wind shear is the vertical change of direction and/or speed with height: rotation.  Shear is moderate tomorrow which might also add a twist to rising air that could even produce the odd tornado.  Nice :-)  After writing this Estofex issued a tornado warning Level 1.

Update 8 Oct: confirmed tornados from today 8 October 2014

http://www.bbc.co.uk/news/uk-england-derbyshire-29542740

 

2014-10-02_20-39-59

September was a warm and unusually dry month.  Central England temperature (CET) came out officially as 15.1C which is 1.5C above the 30 year average (Hadcet) and made September the 4th warmest since 1910. In Reigate our average September temp was 16.5C, not unexpected as we are in the warmer SE of the country.  In 2013 the average for September was 14.4C but the range of temperature was greater: 2013 Tmax 30.9C and Tmin 4.6C were both more “extreme” than the respective Tmax 26.7C and Tmin 5.2C of 2014.  Whilst the remainder of Autumn looks to be cooling down markedly now, 2014 remains on target to be an exceptionally warm year overall.  Despite a cool August all the other months have been above average.

The UK average rainfall for September 2014 was 19.4mm for the month, the driest since 1910.  In Reigate we had 22mm of rain recorded (CoCoRahs) compared to over 47mm in 2013.  The rainfall we got in Reigate this September was restricted to a few heavy thundery showers: many of these moved over London and missed us completely.  However, a notable thunderstorm overnight on 19 Sept delivered some cracking thunder and lightning at about 1am and brought 10mm of rain in less than 20 minutes, and woke quite a few people up!

Overall, however, the month was dry: 16 consecutive days of no rainfall at all came early in the month and mostly dry days with just odd showers characterised the rest of the month.  Sunshine hours totalled 120 hours. Pictures below are a medley of photos from an “extreme” month due to the exceptionally dry conditions, the driest in 100 years in some places.  September in Reigate.

 

September has retained some high daytime temperatures which are set to make it 1.4C or so above Central England Temperature (CET) long term average. New York city also experienced some warm Autumn days with 29C Tmax recently, followed closely by London with near 25C Tmax temps over the last weekend of September.  This month has also been exceptionally dry with Reigate recording just 20mm (tbc). The inevitable happens this coming weekend as Autumn arrives, albeit fashionably late.  The chart below shows upper air temps dipping as cooler polar air arrives from 5 October (note upper air temps are 1500m, so don’t panic about the scale!).

fall in air mass temp this weekend

fall in air mass temp this weekend

The charts below also show a defined change for Reigate and the SE over the first weekend in October and into Monday as high pressure and largely rain-free warm settled conditions this week give way to LOW pressure, frontal rain bands and cooler breezy-er conditions delivered by a lively jetstream from the Atlantic. The Icelandic LOW mentioned in previous posts will, at last, nibble through the anticylcone sat over the UK for so long.  It may not be quite curtains for HIGH pressure and warmth for the SE yet and some recovery is hinted at later, but it looks like the persistent dry and settled conditions will push off this weekend and be replaced by more mobile Atlantic action.

For Reigate, the change afoot starts gradually, with the odd light shower possible Wednesday, Thursday seeing a rise in pressure again and a continuation of warm and dry calm conditions to end the week.  However, a glance to the north west will show an active cold front descending SE during Friday and arriving over SE and Reigate during Saturday morning.  It is likely to push through by afternoon and leave brighter fresher conditions through Sunday, which looks not a bad day at all for Reigate, albeit cooler.

 

The jetstream chart for Thursday and next Monday shows a significant shift south directly over the UK. This is forecast to enhance the trough over Scotland and dig it deeper into the south of the UK during early next week.  Breezy conditions are likely into early next week with Monday having possible country-wide rainfall, especially heavy in the south as the warm air lingers CLOSE by to the south and interacts unfavourably with the colder air mass that could produce a lot of rain. The upper air charts below show how close the warm air lingers to the south of the UK.

RGS weather club produces forecasts for school events on Fridays. Here is the latest for Open Day Saturday 27 Sept and beyond.

Temperatures staying unseasonably warm this weekend and through much of next week: considerably above the September 30 year mean.  Apart from odd light showers early next week it should stay largely dry. No big breakdown yet on the charts, except potential hints later next weekend at the earliest.

Temperatures staying above average and rain below average

Weekend:

Tmax 22C, Tmin 10C+ staying in double figuresovernight, which is well above the seasonal average.

Winds light, no more than 10mph, and mostly from a warm southerly direction.

Cloud will come and go this weekend, odd scattered light shower possible Saturday, but rather unlikely.

Look ahead

After a slight threat of rain early next week from a slackening in pressure and shallow LOW with a weak front to the SW, the HIGH is expected to build back over the UK through mid week and bring in a warm southerly flow with Tmax20C, maintaining the mostly dry, settled, but occasionally cloudy conditions.

High slackens grip slightly early next week, but hangs on

A deep Atlantic storm near Iceland next week will threaten to bust the blocking HIGH pressure over the UK but it looks unlikely to succeed until next weekend or second week of October at the earliest before the inevitable breakdown into “normal” Autumn weather.

September will work out warmer and drier than this August.

Forecasters Tom Tatham and Chris Meredith and SAC

RGS weather club meets every Friday lunchtime in Room 12 to do forecasts for school

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.

2014-09-20_19-55-34

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.

2014-09-17_21-17-30

5,000 runners and thousands of spectators are expected to enjoy a fantastic day for the Run Reigate Half Marathon on Sunday 21 Sept in Priory Park. It is a long way ahead to forecast with any accuracy but the weather at the moment for Sunday 21 Sept suggest Reigate is maintaining warm temperatures (Tmax 22-25C), light winds with sunny spells but with possible scattered showers.  This week is warming up and could even reach a heat peak by Thurs/Friday of up to 25C but Sunday currently looks to cool slightly.

2014-09-14_10-54-00

Next Sunday is a long way ahead of course and things can change a lot so updates will be necessary.  In particular, Atlantic frontal systems are nearby but a re-building high pressure to the NE looks to keep them at bay for next weekend.  Do check back and follow twitter @RGSweather for updates. Also please note this is a local amateur educational weather station site so of course consult professional weather providers such as the UK Met Office for important decision-making purposes ;-) .  Here are synoptic details to explain more about what is likely to happen this week in the lead up to Run Reigate.

The current blocking omega high pressure that has brought such warm and settled weather over the last two weeks or so takes a bit of a nudge from the cut-off LOW near Iberia to the SW this week.  The chart above left shows pressure falling throughout this week to Friday and then steadying out but with a hint of a rise in pressure for Sunday.  Falling pressure in warm conditions means scattered showers become more likely. By mid-week the Fax met office chart below shows a warm easterly / SE flow across the South from a warm continent as a shallow trough extends across N France and the Channel with weak fronts to the south bringing a risk of showers that could reach S UK at times, though no drama is expected with a slack pressure pattern and the jetstream hundreds of miles to the north of the UK, well out of the way.

slack easterly flow mid-week

slack easterly flow mid-week

The charts below show the same LOW to the west of Portugal/Spain niggling away at the HIGH and moving a little closer to UK SW shores.  This is not indicative of a full breakdown, however, as the HIGH looks to build back next weekend, albeit to the NE and probably briefly before a possible collapse east thereafter as fronts and depressions eventually break through from the Atlantic, but that is all uncertain.  Next weekend could be Summer’s last stand!

As said, during mid-week the HIGH is forecast to edge further away to the NE causing pressure to fall in the SW and South, but not enough to allow a complete breakdown.  Lower pressure will increase the threat of showers that could push into the SW and south of the UK, especially Thurs/Fri, though details are sketchy on how far they reach east, especially in light winds and rainfall totals look small and patchy.  Reigate could stay dry the whole time in this scenario.  On the other hand, any showers that do perk up could potentially be thundery and quite heavy (apologies if this appears to be weather obfuscation!) . The most noticeable effect of the niggling Biscay located LOW is to build a slack warm flow through the week from the SE / S / E bringing in potentially warm / very warm temperatures from a hot continent for the time of year.  These could even reach mid to high 20’s at end of next week according to current charts, though this is the max likely and the weekend looks to cool down somewhat which is good news for runners.  France is set to hit 30C in places later this week if this pattern comes off.

Despite the scary looking rainfall charts above, the HIGH pressure should keep fronts at bay at least on current model runs for next weekend.  Note the rain spikes in the black chart above show that MEAN rainfall values (the white line at the bottom) are generally much less than 5mm which is hardly very much rain at all and certainly no wash out.  Also, take note of the overall conditions for the forthcoming week in the summary ensemble charts below: warmer than average temperatures and drier than usual, showing that any showers are likely to be scattered and not add up to significant totals.  The high CAPE values (convective available potential energy) give an indication of potential instability which could trigger sharp showers or thunderstorms but these are often over-done this far out and settle down much lower nearer the time.  Needs watching tho.

SO… for Run Reigate this is a good outlook for the time of year.  It is especially good as a breakdown in the weather has been repeatedly threatened for sometime but is still being effectively held off by the blocking High which models currently suggest will build back for a while next weekend.  Here’s hoping they stay on target with this idea!  Good luck runners… RGS will be out assisting, running and spectating.

 

HIGH sits over UK

HIGH sits over UK

High pressure continues to dominate September weather in the UK. In Reigate it hasn’t rained for 8 days and temperatures during these sunny days with broken cloud have regularly reached the low 20’s and even Tmax 23c on Tuesday.  The HIGH is edging to the NE and this will encourage light easterly winds over the SE which often brings more cloud off the North Sea and a risk of more dreary anticyclonic gloom at times perhaps building up by this weekend.  Cloud at night will maintain temps easily into double figures for Reigate but depress them somewhat into high teens in the day.  Nevertheless, it will remain dry into the weekend and any sunny spells will feel delightfully warm for late summer.

 The blocking anticyclone is called an omega block which is named as it looks like the Greek letter.  Traditionally these are persistent blocks which can last for a long time maintaining dry settled conditions, as it is now.  The jetstream remains far to the north taking stormy weather over Greenland and Iceland (where the Holuhraun fissure eruption is taking place and the larger Bardarbunga volcano threatens to blow as the caldera sinks by a remarkable 20 metres).  Winds over Iceland are generally blowing away from Europe at the moment and taking the SO2 eruption cloud over East Iceland and Norway.  Last weekend there was a brief spike of SO2 over England and Wales which was brought down by a cold front that carried some sulphur dioxide from Iceland over Ireland and into Scotland, Wales and England.  The spike short lived and unlikely to cause any health impacts. 

The models (GFS and ECM) have been predicting the end of this HIGH pressure for a while but it has persisted and bucked the computer models.  Currently, models see the HP (high pressure) drifting slowly further to the NE next week and pressure will fall first across southern UK creating conditions for showers to drift up on a slack Southerly flow: some models bring the first light showers as early as Sunday and Monday, however this is probably too early. 

2014-09-10_21-54-15

Another factor that is worth watching is a developing tropical disturbance off Cape Verde in the Atlantic which model runs are predicting will arrive in the Mid-Atlantic in the middle or end of next week.  The track is not certain yet, of course, but it could have an impact on how this high pressure eventually either falls apart or even builds back in again after a possible unsettled spell from mid-week next week. 

2014-09-08_21-40-07

worst case scenario is only one possible outcome of many… direct hit of extra-trop on UK