Archives For cooler

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GEFS cool dip mid January

A cold snap looks more likely next week from around 13 January. Nothing extreme, just a long-overdue “normal” wintry feel is on the cards.

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12Z gefs and ecm ensembles show more distinct dip in temps

 

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ECMWF upper air goes cold

Temperatures are due to take a dip below the seasonal norm.  It’s still a way off so details will change but here’s the current idea moving ahead.

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The current run of wet Atlantic westerlies (above) that has brought flooding to many parts of the country, is due to weaken as pressure rises over the Atlantic and further north over the Pole.   As the persistent Atlantic LOW pressure gradually moves East this week it will bring more rain across the UK.  As it moves further east over the weekend it is forecast to draw down cooler northerly winds from the Arctic next week, at least for a while. (see below). (update: “Atlantic block” noted on chart below is probably overstating it a bit … as HIGH is likely to give way fairly promptly)

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Arctic Oscillation goes negative january 2016

The cause of this Arctic outbreak is indicated by the Arctic Oscillation (AO) going negative.  The AO is a measure of air pressure over the Pole relative to mid-latitudes.  It has been positive for most of the autumn and winter so far and this usually means a strong jetstream and mild wet westerlies for the UK.

When the AO goes negative it indicates building pressure over the Pole and more likelihood of Arctic air “leaking” into mid-latitudes.  The chart below shows the 500mb mean heights for 8-10 days time.  Spot the anomalously high pressure over the Pole relative to the mid-latitudes.

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8-10 day mean 500mb heights (ecm and gfs models)

The negative AO has been predicted by many expert long range forecasters for a long time partly because of a lack of sea ice in the Kara Sea (part of the Arctic Ocean near Siberia).  This is a long-term indicator for potential pressure rises in this region.

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In addition, the recent enormous pump of warm air, courtesy of the Storm Frank, will have encouraged tropospheric height rises over the Pole.  The result is an inflating balloon of relatively cold air waiting to pop into the populated mid-latitudes!

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surface temperature anomalies Jan 14 2016

As well as a push of cooler Arctic air, pressure is also due to rise over the UK.  With increased pressure we can thankfully expect a drier period.

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wintry pressure rise

A classic winter high might be expected to bring dry, bright clear days with cold nights with views of the stars.  On the other hand, frost, fog and icy conditions might also be expected too.  In the SE huge dumps of snow look most unlikely next week from current model runs but lows can move south in the Arctic flow and cause unexpected events. The duration of the cold snap doesn’t look long** as the Atlantic HIGH is swept away by more westerlies.  However, this is uncertain so stay tuned for more regular updates on twitter.  Of course, seek professional forecasts for decision making purposes.

update 06/01 **looking potentially more prolonged now. cold snap turning into a spell.2016-01-05_20-55-31

January 2016 cold snap

2015-16 winter forecasts have long considered the possibility of cold weather in the latter stages from Jan thru to feb. This was explored in a post here

https://rgsweather.com/2015/12/29/winter-is-nigh/

The weather has been mercifully quiet, with mostly warm and sunny conditions for Reigate over the last 2 weeks. This is gradually about to change but no particular drama is afoot other than some perky and windy frontal rain arriving in Reigate on Thurs night /Fri am, followed by showers and cooler breezier weather with hopefully some interesting clouds over the weekend!

Our two week dry spell (well, 13 days) ended briefly today with a light shower.  As the blocking high sinks away to the S, a cooler NW flow will swing by over the weekend ushered in by an active cold front on Thurs/Fri which will mark the arrival of some cooler days and frosty nights through to early next week.

The charts show a typical early spring flip-flop in temperatures as warm air from the SW on the back edge of HIGHs is replaced by cool NW and even N winds as LOWS sweep across the North of the UK.  The back-edge of the LOW moving over Scotland in the next few days will drag down chilly Polar air especially on Saturday and Sunday.

Daytime temps will struggle to double figures which, when last weekend hit 20c might feel a bit of a shock!  This cool airtstream, fresh from Polar regions, will also be warming through from below and therefore get rather unstable: this means thermals will have a tendency to rise, and keep rising through the chilly upper air, creating towering cumulus clouds and showers.  The chart below shows a typically unstable temperature height diagram with some ingredients for a showery day capable of building some nice cumulus clouds: steep lapse rate, saturated airmass through a large column of the atmosphere, some wind sheer (change of wind speed or direction with height).

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By the afternoon especially on Saturday cumulus congestus clouds might even be tall enough for some sharp showers, possible hail in the cold upper air and the odd clap of thunder is not impossible.  Nevertheless, total rain predicted is low and there will be sunny intervals. April showers, but a month early!  Next week sees a continuation of flip-flop spring weather.  April has the possibility of some high pressure returning with sunny and dry weather but the exact timing of this remains uncertain.  There is also some indication of unsettled pattern in April too, so take your pick!

LOW over Scandinavia

LOW over Scandinavia

On 5th Sept 2013 Reigate was, unofficially at least, the hottest place in the UK! (source weather online)top 10 hottest 5 sept

Polar air quickly swept aside the heat of the continent in less than 24 hours. Whilst by no means cold, the 13c reduction in temperature in 24hrs combined with the first rain for 11 days in Reigate made it feel quite Autumnal.  As models suggested the 5mm rain was less dramatic than originally thought… which is surprising considering the sharp change in air mass.

The cool whirlpool of polar air brought down by the jetstream from Iceland will be held in place by a meridional loop in the jetstream for several days.  It’ll gradually warm through but this could spark off a few showers.  So no true Autumnal stormy drama yet, just a gradual edging out of summer heat.

This weekend a big LOOP in the jetstream is set to rip up the summer heat still lingering over the southern part of the UK. In a dramatic weather battle in the upper atmosphere above our heads the southerly blowing limb of the jetstream will bring POLAR air to west of the UK to fight it out with the continental tropical heat lingering over the south and east. Heavy rain and strong winds will be the result of this battle but the exact location of the heaviest downpours is tricky to be precise about. Broadly speaking, the heaviest rain will start in the South of the UK on Friday and move North, rotating over to the NW and falling heaviest over the northern hills as the LOW pressure drifts NW during Saturday and Sunday. The South could have comparatively drier days after any heavy rain on Friday, but stay tuned for details on that.  Friday rainfall for Reigate could exceed 10mm starting in the afternoon – so possibly very heavy rain for a few hours Friday pm, with lighter rain through Saturday and possibly none at all on Sunday as the LOW moves away to the NW.

Very warm air drifting up from the continent over England by a northward blowing jetstream will be forced to rise over the markedly colder polar air invading from the NW through Friday and Saturday. The difference between these air masses is very marked: the warm 28ºC surface air currently over Reigate equates to -12ºC at 5000m while the invading cold Polar air mass is -28ºC at 5000m, which will push down surface temperatures on Friday to a Tmax of only 16ºC! It is this contrast between the air masses which is a hallmark of autumnal weather and the key to creating lots of rain: polar air meets tropical air, forcing it skyward, forming rain with gusty winds spiraling round LOW pressure: typical autumnal scenario (except this weekend’s scenario is actually quite unusual: called a trough disruption with the surface low drifting off in an unusual direction: from SE to NW, unlike usual LOWS which track west to east across the UK along a zonal jetstream).  Wind speeds for Reigate this weekend could gust at 25mph at times on Saturday. No thunder is now forecast this weekend as the polar air is relatively stable, being on a return leg to the Poles.  

Thereafter, as you can see from the ensemble below, next week looks to remain cool and unsettled but with some improvement in the south possible later in the week as pressure could rise by next weekend, but no return to hot conditions is expected.

ensemble sept 4

The “something and nothing” weather in the South East of the past week and the uncertainties in the forecast are set to continue for a while. Some met-people call these conditions “unforecast-able”. Models seem to be unreliable beyond a few days and even hours. Rainfall has been especially hit and miss to forecast in the SE: predictions have been varying wildly for specific days between torrential, heavy, some and then no rain arrives at all! The reason is possibly the lack of the usual “zonal flow” in the jet stream: i.e. west to east flowing jet.  The jetstream is meandering north-south and weather systems are more or less STATIC: the UK has been stuck in a low pressure trough for over a week.  The normal procession of low pressure systems (depressions) and brief sunny HIGH pressure ridges seems a distant memory: it simply hasn’t been a feature of our weather for ages. Forecast models seem to struggle with this.

meridional flow jetstreamThe overall synoptic weather situation remains the same. That is: a big blocking HIGH over the Atlantic and very weak westerlies with the jetstream in a North-South pattern (meridional) bringing down cool northerly winds direct from the Arctic which “pool-up” across Northern France and Southern UK creating a LOW pressure trough.  LOW pressure in Spring with a stronger sun can mean pleasant warm sunny spells but showers: it is the showers which, fortunately, have barely troubled Reigate.  However, (and this does look more certain!), a significant little LOW is set to spiral down the N Sea Thursday – Friday, deepen along the way and strengthen Northerly winds and bring rain, especially to the SE: some frontal rain and showers are predicted to accompany this LOW but again – it could be rather hit and miss depending on how close the LOW gets to the SE England and the strength of accompanying fronts.

storm risk thursThere is a 30% chance of thunderstorms over Reigate area through Thurs and Friday afternoons (as the sun heats the surface which creates bubbles of warm air through the day which convect upwards through the cool Polar airmass creating tall cumulonimbus clouds).  With upper air temperatures at 5000 feet as low as -12ºC later this week any vigorous showers may fall as hail.  Frontal rain attached to the LOW will certainly feel chilly in the wind. Night time temperatures could fall as low as 4ºC and any wind will make it feel distinctly cool.  A weak ridge over Sat and Sun may bring pleasantly warmer and drier weather but some models are showing a return to LOW pressure in a storm arriving from the NW on Monday bank 

Meanwhile, sincere sympathy and thoughts to those caught by the terrible EF5 tornado which caused such terrible damage in Moore, Oklahoma yesterday.  Following events on Twitter and news reports on destroyed schools was very sad and upsetting indeed.  The tension between following exciting weather and the potential for witnessing terrible disasters unfold in front of their eyes was palpable for the storm chasers and met-enthusiasts involved.  Unfortunately, the weather in MidWest continues to threaten areas with tornadic conditions: lately in New York state too.  Take care out there.  Our UK weather is usually mercifully benign in comparison.  

Update Weds 20:25hrs: Gale warning for South coast and exposed hills: SW wind 30mph+ GUSTING 40-50mph Thursday pm. Channel will experience v strong winds.

A deepening LOW pressure is winding up off the west coast of the British Isles and will cross the UK this week bringing cloud, rain and strong winds and cooler temperatures.  Reigate and the SE, however, will remain comparatively sheltered from the worst of the rainfall, probably only amounting to 10mm over the next 10 days or so, which isn’t much compared with the deluge elsewhere.  The first fronts make an entrance tomorrow with moderately heavy rain for Wednesday, slowly clearing off in the afternoon. The most interesting bit of weather looks like Thursday overnight into Friday when strong south westerly winds circulating around the LOW cross our region, gusts around 50mph are possible in exposed areas like the North Downs around Reigate.  The BIG map shows wind arrows circulating around the centre of the LOW as it moves across Northern England and Southern Scotland (wind speed in mph). Friday looks pretty wet all day and Saturday could bring a threat of thundery showers.  The unsettled regime continues with blustery westerly winds throughout the weekend keeping temperatures around the mid teens. Weekend looks blustery and unsettled with showers Saturday, some thundery possible.
The longer term May forecast looks rather unsettled especially for Reigate with LOW pressure likely to sit over Europe bringing rain to the SE, while higher pressure sits over the North and West of the UK bringing better conditions to those regions. The graph shows more rain through the rest of May showing this sort of set up. Lovely!

A dry start to May but how long will it hang on?!

Before Saturday’s shower, it last rained in Reigate 5 days ago but rain is forecast this week.  Nevertheless, the first 10 days of May are due to be below average rainfall (see map below), despite wet weather mid-week.  Much of the rainfall in Reigate during early May is likely to be showery, some heavy but rather short duration so not amounting to much.

europe wet and dry may

May Bank Holiday in Reigate is likely to be a pleasant 20ºC, but not quite the sizzling 26ºC claimed in some newspapers! The warm weather we have been enjoying has been courtesy of an upper level ridge to the south pumping a warm air mass from a warm source region to the south west around the Azores. This warm upper air has built a surface HIGH pressure to the south of the UK which has kept us largely dry, warm and cloud free.  The tropical airmass this week has had temperatures at 1500m (850hPA) of +10C over Reigate; (quick reminder that in March we had uppers of -12ºC!). This week could see upper air mass temperatures drop to nearer 2ºC over Reigate by mid-week. Surface temperatures will still be OK, reaching mid-teens in the stronger May sunhsine but night time temperatures could be quite chilly and even risk a touch of frost if skies clear later in the week when a HIGH is due to build through again at the end of the week. High pressure will build back in the south and, though a long way off to be certain, next weekend looks staying dry with temperatures probably recovering nicely in any sunshine.

The cause of the forecast #slightly# cooler weather arriving in Reigate this week is a possible significant switch in the airflow from mostly southerly winds to a more westerly/north westerly wind direction: a cooler direction across a cool Atlantic ocean and from a cooler source region around Iceland. The cause of the change in the wind direction is an Atlantic LOW forecast to cross the UK mid-week. This LOW will bring Atlantic fronts and rain, possibly heavy at times mid-week, before clearing to showers.  A HIGH is forecast to build back into the south by next weekend bringing dry weather back and temperatures back to possibly hit 20ºC again. As usual, this is an early forecast and things will change so do keep in touch on twitter @RGSweather.

high and low pressure cross section

Why is HIGH pressure usually dry and cloud free?  Air sinks in HIGH pressure: as it sinks it warms and dries out. Warm air can contain more water vapour than cool air, so any moisture tends to evaporate in HIGH pressure (anticyclones).

In LOW pressure (cyclones / depressions) air is LIFTED and expands, cools and any water vapour will condense, forming clouds and eventually rain.

Air flows from HIGH to LOW (but not straight! it is deflected to the right in the northern hemisphere by the coriolis force… more on this later).  This gives us the familiar surface pressure charts with winds apparently spiraling round HIGHS and LOWS.

air flows from high to low!

Reigate: decently warm and very pleasantly “springy” through to mid-week but then a significant fall in temperature and showers to end April!
Cloudless, blue skies bathed Reigate in sunshine all day today and brought temperatures up to nearly 15ºC: perfect for cricket!  However, the air flow was a  cold easterly which meant that it felt cool in the wind, more like 11ºC, but in the sheltered sunshine it was pleasant. London Marathon Day 2013 will start at a chilly -2ºC and will cloud over later, reach max 13ºC and stay dry with only a slight risk of a light shower later; low humidity perfect for marathon runners.

The big temperature graph shows temperatures at 1500m above the surface (at 850hPa), used by forecasters for predicting temperatures because it is a height which smooths-out the complicating influences of surface diurnal (day/night) temperature changes and surface features like sea, land, cities, hills and vegetation, so making overall temperature trends and changes in air mass easier to spot.  This week it’s easy to spot the cool air mass currently over the region, the warming mid-week temperatures and then a fall-off to next weekend.  Three different airflow directions are responsible for these changes, so here’s some detail…

Mon-Tues-Weds
reigate midweekLOW to the north and HIGH to the south of the UK: this pressure pattern will squeeze breezy westerly winds across the UK as a result of the pressure gradient between the HIGH and LOW pressure. A lively jetstream will be directly overhead of the UK and drag an increasingly mild upper westerly / south-westerly air flow from the Atlantic which will bring the risk of the odd shower on a few weak fronts reaching Reigate now and again early in the week but nothing much.


The HIGH creeps up more from the south over Reigate on Wednesday and this should bring warmer and sunny conditions, possibly up to 17-18ºC in much lighter SW winds: so making for a “warmer” HIGH than the one we are currently experiencing which is dominated by a cool easterly airflow over Reigate.

End of next week and next weekend:

It looks like it’s all downhill after mid-week: temperatures are due to take a plunge with HIGH pressure building in the Atlantic with an upper level trough (LOW) digging down over the UK bringing a cold Polar air mass down to Reigate through into next weekend.  Unsettled April showers, some possibly thundery, may break out at the end of the week and over the weekend as this cool northerly air mass meets the warmer surface air and increases lapse rates and instability, encouraging convection.  Next weekend could well struggle to reach 10ºC daytime max , with some showers during the day and nights turning cold.

This is all early days and things could change, so do keep updated on twitter for Reigate weather @RGSweather.

Where are bluebells flowering NOW? Find out here nature’s calendar


Spring 2013 is likely to make some false starts in Reigate and the SE of England.  This week starting 4 March certainly looks warmer but more unsettled with some rain returning as Atlantic influences and LOW pressure take over as the HIGH drifts off south east over the continent.  It could reach pleasant max temperatures of 13ºC mid-week in a warm bath of southerly winds but these will be accompanied by some rain. The sketch map shows what models are predicting for Spring after this warm spell ends by this coming weekend.  The medium range charts see LOW pressure moving down over the continent and a ridge of HIGH pressure building back up to the north of the UK over the coming weekend.

march slide downThis could mean cool easterly winds returning from 9/10 March dragging temperatures back to rather wintry like figures by next weekend and beyond. Temperatures could take a significant fall, not as harsh as mid-winter, but certainly down to daytime temperatures of 5ºC again or perhaps even lower; in any case, much cooler than expected for March and possibly even cold enough to bring SNOW back on the agenda!  The north of the UK will fair better if this scenario pulls off update: cold plunge arrives from NE – which will be hit worst … anyhow, expect some ups and downs in the weather and a big false start to Spring.