Archives For September 2014

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.

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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.

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

 

2014-09-06_08-16-25

August 2014 weather stats for Reigate, Surrey.

Tmax 27.2c (33.5c 2013)

Tmin 5.6c (9.7c 2013)

Average temp 16c (17.4c 2013)

Total rainfall (CoCoRahs) 84.4mm (37.6mm 2013)

Max gust 36 mph (Bertha 11 Aug)

GLOBALLY, August 2014 was the warmest on record (NASA, see below).  In contrast, August 2014 for the UK was the COOLEST August for 21 years since 1993 and rainfall was 156% above long term average in some places, including parts of the SE. In Reigate the average temp was 16C compared to 17.4c in 2013.  It was also wetter than usual, for example, twice as much rain fell Aug 2014 than in Aug 2013.  This ended a run of much warmer than average months throughout 2014.  The CET (Central England Temperature) is still seeing 2014 as one of the all-time warmest years even with a cooler August.

Arguably Ex-Hurricane Bertha “destroyed” the balmy UK summer of 2014.  She emerged out of the Tropical Atlantic early in the month and took aim at the UK.  Despite weakening extra-tropical storm Bertha still dragged some cool polar air into the UK and Europe behind her.

Bertha brought little direct disruption to Reigate and the SE but an impressive squall line moved across our region during the 10 Aug.  There was no especially extreme weather associated with the squall line (regrettably for weather folk!) but the cloud-scapes were magnificent as the squall moved away further to the East. Bertha did bring some unseasonably gusty weather and 36mph was recorded at our weather station on 11 August.  Winds in excess of 40mph were recorded locally higher up.

http://www.metoffice.gov.uk/climate/uk/summaries/2014/august

http://www.metoffice.gov.uk/climate/uk/summaries/2014/summer

Interestingly, NASA have just recorded August 2014 globally as the warmest ever August on record.  This shows how discrete locations on a regional scale can buck the global trend… obviously some places have warmer and colder weather than the global average and some of these anomalies could be potentially record breaking despite being the opposite extreme of the global mean.

http://www.climatecentral.org/news/nasa-globe-warmest-august-18031