Archives For autumn

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

The end of October has continued to be warmer than average for the season and Friday 31 October could be the warmest Halloween on record (current record around 19C). Hi Res models show the highest temps most likely somewhere inland away from cooler seas and an incoming cold front from the west.  If the sun comes out, which is most likely in the SE or East Anglia and under such a dry southerly air flow, temperatures could rise to over 20C, or even 23C in places.  Whilst this unseasonably freaky warmth cannot really be described as a heatwave (there are technical requirements to classify as a heatwave) the headlines might be correct in suggesting a possible record breaking temperature maximum on Friday.

slide into normal

slide into normal

Thereafter, November temps take a tumble nearer seasonal average through next week and at times even below average. So whilst we can enjoy the treat of near summer like temps tomorrow by next week it will be some 10C cooler as Autumnal temperatures and more rain take a hold, though nothing extraordinary.  Compare this with the first winter storm warning of the year on the East Coast USA where temps are set to freeze and the first substantial snow in the mountains is likely to occur this weekend.

Along with temperature, pressure is set to fall through the weekend and into early next week as a depression hovers NW of the UK over Iceland and brings Reigate some breezy SW winds on fronts with rain reaching us in the SE on occasions over the weekend and into the early part of next week.  A cold front later on Sunday looks especially likely to be the one to usher in the distinctly cooler polar air overnight into Monday as a trough pushes out the high pressure which moves east taking any remaining mild air into the continent.

During the first part of next week the LOW moves SE over the UK and brings unsettled cooler and damper conditions to Reigate.  This LOW is due to then move further to the south / SE over the continent and deepen to become a feature called a “cut-off” low.  This is modelled to move over the continent to the Alps and N Italy where especially heavy rain is possible and then to parts of the Mediterranean which could cause unsettled conditions to arrive in the NW Med later in the week.

2014-10-30_14-00-21

There is some uncertainty as to how the end of next week and next weekend plays out for the UK and us in the SE once this cut-off feature moves off.  It seems likely that another depression with fronts could sweep across the N of the UK and bring more unsettled conditions towards the end of next week with fronts and rain for the NW.  But there are also signs of higher pressure building from the south again.  Either way, temperatures are going to be more averagely Autumnal and November-like, unlike the warm temperature tricks October has played.

A quick reminder that, unless November and December are markedly cooler (and the jury is out on this) then 2014 is STILL on target to become one of the warmest years on record for the UK and certainly one of the warmest.  Every month has been well above average temperature (see below, CET means Central England Temperature) except of course August which was the coldest for 21 years!  Despite this hiccup October looks to be warmer by over 1.5C and ending on a corker.  Nevertheless, perhaps celebration of such temperatures should be tempered by remembering that this kind of anomalous heat is exactly the kind climate change expected by IPCC predictions and, although anomalous warmth is pleasant for humans, it is just as stressful to wildlife and the environment as severe conditions such as storms and cold snaps.

2014-10-30_14-47-28

Finally, it’s worth noting recent press headlines regarding a possible COLD winter.  Such scare-mongering headlines in the Express and Star and similar papers are usually based on just one or two seasonal forecasts from a few meteorologists who might be described as on the fringes of mainstream weather forecasting.

Whilst RGSweather does not  write seasonal forecasts, it is worth sharing that some of the wider expert and reputable weather community, including both professionals and amateur, is pretty animated about the possibility of a colder than average 2014-2015 winter (that is of course Dec, Jan and Feb).  This is despite several of the standard weather models indicating a rather warm winter at this stage (including our own UK Met Office).  There is a lot of discussion about it.

Making a winter forecast is a complicated process because it is based on many interacting factors in the atmosphere, the cryosphere and the oceans which are combined together by expert forecasters to assess likely winter conditions.

Winter forecast indicators include factors such as the extent and build up of early Autumn snow cover over Siberia/Russia (encouraging HIGH pressure and cold easterlies), Arctic Ice cover (low ice cover warms Poles, increasing pressure, pushing cold air out to mid latitudes), the North Atlantic Oscillation and Arctic Oscillation (based on development of Azores High and Icelandic LOW) and associated amplification of the jetstream, how strong it is and how wiggly (meridional) or straight (zonal) it is, (a weak jetstream with -ve NAO allows polar air to leak out into mid latitudes), the strength of the polar vortex in the stratosphere as indicated by the an equatorial lower stratospheric wind pattern called the Quasi-biennial oscillation (QBO).  A westerly phased QBO is correlated with possible increase in likelihood of Sudden Stratospheric Warming episodes that have been linked to downwellling of cold air into mid latitudes some weeks after a SSW).  Phew, you can see there are loads of tricky indicators to watch.

This year, the October Pattern Index (OPI) is gaining attention as a possibly reliable predictor of winter outcomes.  Basically, the OPI is a clever measure of upper atmospheric conditions in October and particularly how amplified (or wiggly) the jetstream is looking.  Scientists have found that atmospheric conditions in October correlate well with winter outcomes.  A negative OPI, for example,  correlates amazingly well with a negative NAO (north atlantic oscillation) occurring later in the winter.  A negative NAO indicates a weak jetstream, itself possibly caused by low Arctic Sea Ice and high Siberian snow cover.  A negative NAO can, given the right synoptic pressure patterns (placing of HIGH and LOW pressure), be conducive to incursions of COLD air from the Poles reaching mid latitudes like the UK.  All this is experimental but extremely interesting and rather important given the rapid climate change going on in the Arctic.

Update!

Records were smashed today… warmest end to October on record at 24C

 

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.

Update: BIG cool off likely from mid-week as high slips west and draws down COOL Northerlies / NE polar winds for E UK: Tmax drop from 19c to 10c by mid week! Matches Northern blocking suggested by NAO (see text below) Likely to bring first snow cover for continental Europe. ( check @RGSweather)

cool off mid week

The movement of this high has changed in latest models – update coming later:

Rainfall, or lack of it, is the theme for next week in / update: cool off is the theme now for Eastern UK, SE and in Reigate. Warm upper air flowing from the South and SW is building a blocking HIGH pressure which will sit to the South of the UK and then migrate WEST keeping rainfall and LOW pressure way out west in the Atlantic for much of the coming week. Tmax temps during the day could top 20c and night time temps could even stay in double figures.  Given these high nighttime figures and low dew points it is unlikely that fog will be much of problem with low humidity in the SE, unless the cloud clears at night and ground temps fall away. Is this an Indian Summer? Read here. http://www.bbc.co.uk/news/magazine-15127159

The NAO (north atlantic oscillation) is a measure of pressure difference between the Azores and Iceland.  If the pressure difference is high then it yields a positive NAO and this usually accelerates westerly winds and the jetstream and brings fast moving wet and warm (winter) Atlantic weather to the UK.  Negative NAO usually indicates a weak pressure difference, weak westerly winds and a blocking pattern with higher pressure than usual and slow blocked weather which sticks around for ages.  The weaker westerlies in mid-winter can allow cold polar air to flow out of the Arctic depending on the location of the HIGH. Slow moving conditions can mean extreme weather if a HIGH causes dry weather for weeks (drought) or a static slow moving LOW accumulates a lot of rainfall.  So negative NAO is not necessarily “good” weather.  For us, at present, we have a very negative NAO, one of the most negative NAO’s of this year but the ridge is bringing in a warm flow from the continent.

The longer range CFS chart shows a major gear change down to cooler conditions later in October. To be expected of course, but it looks rather a harsh step function in the weather.  It’s long range so things can change.
Enjoy the warm dry weather and Autumn sunshine but think of the USA today where Winter storm Atlas is bringing blizzards to S Dakota and Wyoming, Tropical Storm Karen is making landfall this weekend over Louisiana and Mississippi and, in between, there are tornado warnings in Iowa, Kansas and Nebraska. Update: severe tornadoes breaking out near Sioux City, NE Nebraska. Thoughts to people there. Stay safe.

Despite the gloomy stratocumulus which didn’t shift much this weekend, it has been warmer than usual.  Saturday night was especially balmy with Tmin of 16c and today saw Tmax a shade off 19c.  This week warm Tropical upper air will continue to waft up from the South being generated by a deepening TROUGH of LOW pressure spinning up just NE of the Azores. This will continue to pump a warmer than usual plume of upper air at +13c 850hPa (5000ft, the height air mass characteristics are usually measured) which flirts off and on with the south of the UK almost all week. The result is warmer than average September temps by some +4 ot 5c.  It will, however, gradually get edge away as the week progresses.

A weakening HIGH pressure over the UK will gradually melt away and drift over the continent and allow cooler air into Northern Britain. Reigate will remain in the warmer air most of the week by the looks of things.
Most models agree that the Azores LOW will fill gradually and drift NE over Spain and France during the latter half of the week and bring a nasty rash of showers to those areas. These might encroach on Southern England by the end of the week and weekend bringing potential for thundery outbreaks but this is too far off to be certain.
The distribution of rainfall is somewhat unusual, with a large accumulation of rainfall accumulating in a great spiral to the west of Spain associated with the Azores “Low” and lots of rain in Eastern Europe associated with the large Russian LOW. Note that the SE of England and Reigate in particular might just be one of the driest places in Europe by the end of next weekend as we seem to be sandwiched between these two great weather systems… well, models permitting! *one of driest places in Europe, probably! The GFS shows no rain at all for Reigate this week, but that’s quite a long shot! 

Another observation to match the LOW over the Azores is the negative forecast for the NAO: the North Atlantic Oscillation measure.  The NAO shows the pressure difference between Iceland and the Azores: when NAO is positive there is a big difference / pressure gradient and a strong jetstream and unsettled, fast moving weather across the Atlantic.  When negative, like this week, there is a meridional jetstream, a weak flow west to east and weather systems can get stuck… so not much change is expected.  This matches the forecast this week… not much change and no fast moving weather.

negative NAO sept

The ensemble model below shows that there will be a gradual decline in temperature during the course of next week and certainly beyond. More unsettled weather is also likely to arrive sometime next weekend but models still look rather uncertain so… keep checking back!

gefs 22 sept


Chart on left shows rain forecast for tonight (Fri-Sat).  Chart on right shows some interesting synoptic situation in Atlantic early next week.

Reigate and SE could see some heavy and even torrential rain from late afternoon / evening today and overnight clearing Saturday am. Totals could exceed 30mm in places, but more widely totals between 10-20mm. This LOW is moving from SW in the Channel and proceeding overhead across the SE with a complex of associated fronts. This system heralds the mother of all LOWS which is moving in on Sunday… sweeping an unusually cold Arctic blast across the whole country by early next week. Autumn is here and the Atlantic shows signs of greater activity with a more active jetstream and the appearance of the first ex-hurricane / extra-tropical storm Humberto on the Atlantic Charts next week too.  Humberto is delivering Tropical air to the Pole, while our cold Arctic blast does the opposite.

The big news this weekend is the start of the temperature drop-off which looks to last most of next week.  Saturday will start this process off as cooler air is set to sweep down across the country, mainly from Sunday onwards.  Tmax might reach 12c but in the wind it’ll feel 10c or so and with rain as well it’ll all look thoroughly Autumnal!  Later next week there are signs of high pressure building back in with better weather and a recovery of temps, but let’s wait and see on that one!

850hPa drop this weekend

Back to school,  a new term begins and Autumn is knocking on the door… but not quite yet as summer will hang on by its fingernails in Reigate! A flirt with cooler Autumnal conditions, a last blast of summer heat and a possible thundery breakdown are all packed into next week … so read on as the weather gets more interesting.

So, no surprises of course but summer is on the wane as cooler air from Iceland threatens to sweep away our benign August warm calm conditions at the end of next week. This weekend is a rather weak “dry-run” of future Autumn storms with cooler air briefly sweeping a slightly cooler airmass from NW across the UK, but Reigate should stay dry throughout the weekend, albeit a little cooler than recently.

However, Summer is not going quietly and while cooler temps will inevitably win-out, mid-week looks like returning warm temps as high as 27ºC with a plume of warm air from Spain. Unfortunately, this last blast of summer heat (probably!) will be rudely and rapidly swept aside by brisk NW cool air flow by the end of next week associated with a LOW pressure system to the north of Scotland. There could be a brief thundery breakdown associated with this change as fronts lift away any warm air still lingering over the UK, especially the south. Most of the instability bringing any threat of storms is located over the near continent on current models but that could change and the SE might see some storms as cooler air undercuts the warm. One to watch!

spanish plume poss

 

 

Apart from stating the obvious, that it’s got colder, Autumn 2012 has had some notable highs and lows, some of which buck the trend towards a feeling of worsening winter weather:

The average temperature for Reigate this autumn has been exactly 10C with the warmest temperature of 27.9C on September 11th and the coldest temperature of -2.4C recorded on November 30th.  Despite feeling chilly, the first frost in Reigate only occured in the last week of October.

October was by far the wettest Autumn month here in Reigate with 121.8mm of rain and only 7 dry days, while November had 78.8mm, 9 dry days and September just 64.4mm and a whopping 2 weeks of dry days!

Finally, as expected, November has been the windiest month with a highest gust of 41mph, followed by gusts in October of 31mph and September 27mph.  The average wind speed recorded for each month are very similar, all around 8-9 mph.