Archives For September 2015

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.

2015-09-18_18-50-09

outlook shows a fortunately settled weekend for Run Reigate

2015-09-12_15-31-07

Atlantic chart NOAA Sat 12/09/2015

The chart above shows an active situation developing in the Atlantic with LOWS offshore ready to bring some unsettled weather to the UK this week.  Reigate has enjoyed a splendidly bright and sunny Saturday but notable rainfall totals could add up this week.  A series of LOW pressure systems, arriving from the SW will force meetings between cool polar and warm tropical airmasses.  These systems are likely to bring wet and windy weather at times, especially Monday and possibly even more mid-week.  In between, brighter weather is also possible, especially Tuesday.  Before then, Sunday sees the chance of some heavy rain clipping into our region from the south east.. potentially heavy if it spills over from Kent. (stayed SE)

(scroll down if you just want to see RunReigate outlook!) 🙂
2015-09-12_12-09-14

rainfall totals ensemble comparisons

The general theme this coming week is for an increasingly perky autumnal jetstream to deepen Atlantic LOWS and bring them up to the UK from the SW with attendant rain and wind at times.

In Reigate, according to our longer term average climatology, we get our heaviest rainfall when winds arrive from a southerly direction, which will be the case periodically through this week as LOWS track to our NW.  The chart below shows how southerly winds will effect Reigate on Monday and Wednesday, and the rainfall chart for Wednesday shows potentially heavy rain that might be expected then.

The first LOW modelled to track SW to NE arrives Sunday night and into Monday.  Fronts attached to this LOW will bring wet weather into the SE and especially across the coast and Channel during Monday, some of this could be heavy and thundery in nature with cold upper air temperatures steepening lapse rates.  Windy weather, especially nearer the coast, is likely too with tightly packed isobars.  During Monday into early Tuesday SW convective wind gusts of 30-40mph are possible inland across the SE and 50mph+ in the Channel at times.

2015-09-12_15-57-22

September LOW England

By mid-week a more intense depression could quickly follow bringing more interesting unsettled weather to the UK.  On the charts below the LOW off Nova Scotia is the remnants of a now dissipated ex-tropical depression called Henri.  This system is not much to speak of at the moment, however, some models bring the remnant energy of Henri into Biscay by mid-week where it is forecast to merge and deepen with an Atlantic LOW under the jetstream and approach the UK from the SW possibly bringing an early taste of autumnal stormy weather through the UK.  This is only one outcome, so worth watching.

Extra-tropical storms inject tropical moisture and heat and energy into the Atlantic and are common in Autumn (Humberto, Cristobal in recent years).  These decaying tropical systems can decay and die mid-ocean without ceremony but sometimes they meet the jetstream, mix with polar air at the polar front and then, under certain conditions, they can intensify into significant mid-latitude depressions, enhanced with extra-tropical zest!  In contrast, sometimes these characters build HIGH pressure by pumping up ridges of high pressure with warm air if they track to the NW of the UK (like Humberto Sept 2015).  However, several models show ex-Henri riding a perky jetstream across the Atlantic this week, merging with an Atlantic low and intensifying in Biscay before pouncing directly into the UK from the SW mid-week. Southerly gales in the Channel and wet weather across in the SE are possible in this scenario but the exact track will make a big difference as to what we end up getting.  Watch forecasts for this one, as Reigate is in a moist southerly flow on most models.

2015-09-12_15-09-29

Wednesday-Thursday LOW for the UK

Further ahead, Run Reigate on Sunday 20 September is a major event for the town: a half marathon and 10km attended by thousands. The weather outlook for this event currently looks favourable, although it is too far off to be certain.  After a fairly dire week of weather coming up, most models favour the idea of pressure rising into the weekend of 19-20 September, at least in the south of England.  This would be good news for Run Reigate.

2015-09-12_13-54-17

pressure rising for Run Reigate event 2015

Cluster models also show more members putting the SE in reasonable conditions with a HIGH forecast on many runs building from the south or SW, however, you can spot that other clusters show short wave troughs in the broadly zonal flow that returns quite quickly with fronts (probably quite weak) potentially nudging into the SE as pressure potentially falls away somewhat through the weekend.  Anyhow, this a long way off so not confident with any forecast.

2015-09-12_23-48-52

Ensemble charts shown above and below give optimism that the unsettled weather arriving this week will broadly improve for the Run Reigate event. However, tropical elements roaming the Atlantic often cause problems for weather models and so we will have to wait until nearer the time for more certainty.

The 500mb pressure anomaly chart below from the ECMWF (below left) also shows a nice High pressure building up from Europe into Southern England in the 7-10 day mean outlook, starting on 19 Sept. The GFS is less convinced about building such a significant high pressure. Let’s hope the ECM performs better this week.  Time will tell so watch this space and twitter for updates.  http://www.jellyfish.co.uk/runreigate/

2015-09-12_17-07-53

mean 500mb flow for 7-10 days: hope for Run Reigate

2015-09-05_22-11-17

Reigate August 2015

August Reigate Summary Statistics

  • Average temp 16.7C
  • Tmax 29.5C
  • Tmin 7.1C
  • Total rainfall 99mm
  • Sunshine 116.4 hours
  • Average pressure 1014mb
  • Max gust 29mph
  • average wind 16mph
  • dominant wind direction SOUTH

August in Reigate, like the rest of Southern and SE England, came out slightly cooler than average at nearly 17C with about twice as much rainfall than the long term average with a rainfall total of 100mm.  (Possibly* half of this fell in one deluge on 24 August when the town centre flooded quite notably from intense rain falling in a few hours (see post below)).

2015-09-05_22-32-13

South East England rainfall August 2015: 177% of long term average

Across SE England the MetOffice official records showed the rainfall total of 103mm was 177% of the August 1961-1990 long term average, so nearly double the usual total in some locations, especially near the south coast (100% being the average monthly total for August in this case). Don’t forget that August is often a wet month and that August 2014 had 84mm.  Nevertheless, if you think our Summers are getting cooler and wetter then, yes, you could well be right!  Read on to find out more.

The mean 500mb pressure pattern for August above shows a deep trough in the Atlantic, dug unusually far to the south for the time of year, and a continuation of the dominant high pressure over Europe from July, nudged further east maintaining the heat wave in Europe, especially Central Eastern Europe.  This looks like it should have promised a hot August for SE England with an average upper flow from the south and a surface mean flow from the SE. Unfortunately this pattern did not deliver any heat of note, but it did deliver occasional heavy showers and some thunderstorms, with notably torrential rain showers delivering big precipitation totals in a matter of hours.

2015-09-06_09-16-29

August 2015 heat stayed in Europe and away from UK

Despite the average southerly and SE flow bringing occasional thundery Spanish Plumes, the real heat stayed stubbornly on the continent and only fleetingly wafted temperatures exceeding 25C across the SE.  While France regularly baked in 30C+ daytime heat, Reigate and the SE could only struggle to Tmax temperatures of 25C and only once nudged 30C. Notably, the mean temperature was a tad below the 1981-2010 average but above the 1961-1990 average, showing how recent decades have been warmer.  So August was either above or below the long term average, depending on what LTA you choose. This is a small example of how weather statistics can be presented by the media to suit any argument regarding climate change.

The August monthly Central England Temperature (CET) came out at 0.1C above the long term average but this hides the cooler than average Tmax attained in a disappointing month.  This again illustrates how headline figures hide many subtle interpretations of weather statistics.  Overall, August was nearly average from the perspective of long term Central England Temperatures, with the East coming out slightly above average compared with a cooler West.  Maximum temperatures were widely lower than usual for August.

At 116.4 hours, sunshine was also only average or a tad below the 1961-1990 average.

2015-09-05_22-14-28

2015-09-05_23-09-16

sunshine duration August 2015

The dominant wind direction this August was from the SOUTH (i.e. a southerly wind).  The highest rainfall totals experienced in Reigate are brought, on our 4 year “long term average”, by southerly winds.  This makes sense because mean southerly winds are from warm source regions, travelling across Biscay and the Channel, and are frequently associated with humid warm sectors that precede fronts where heavy thundery showers can occur.

Our more frequent prevailing Westerly / SW winds bring our familiar frontal rain but this often peters out before reaching the South East as most rain is dumped over the western hills of the UK.  In contrast, warm southerly winds containing more water vapour arrive laden with precipitable water (PWAT) ripe for torrential convective downpours across Southern and SE England. The Downs (both the North and especially the South Downs) can also have a marked orographic effect enhancing this southerly rainfall pattern, whereas they have little impact on frontal rain from the west.  This warm humid southerly set-up was responsible for the Reigate deluge on 24 August, also known as a Spanish Plume.

Finally, research shows that UK Summers have got cooler and considerably wetter since 2000.  Cooler by just 0.4C (this despite some warm years) but total summer precipitation has increased by some 50mm over the last 15 years and the 10 year moving rainfall average is on the rise, most likely due to torrential rainfall events. This change to rainfall being delivered in torrential but sporadic events is in-line with climate change predictions.

2015-09-06_09-27-08

Reigate summers have got cooler and wetter since 2000

Rainfall challenge! *Our local rainfall totals are proving mighty tricky to verify at the moment.  We use three sites for measuring rainfall locally: an automatic tipping bucket rain gauge at RGS, a manual CoCoRaHs rain gauge at the same site and a local sister site in town. Unfortunately they rarely agree and sometimes vary quite considerably. Also, the manual rain gauge is used to measure monthly totals because emptying it everyday (during the holidays for example) is not always possible. Calibration of the AWS and regular rain gauge measurements is therefore an immediate target. Meanwhile, all rainfall figures are available on request and posted in these summaries along with official MetOffice rainfall figures for the South East.

Refs

https://xmetman.wordpress.com/2015/09/04/summers/

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

http://www.metoffice.gov.uk/climate/uk/summaries/2015/august