Archives For February 2014

Weather is chaotic and numerical weather models are not perfect. The forecast for Reigate today went rather awry, though not completely.  It was forecast to rain heavily, perhaps on and off, but the forecast was for heavy rain more or less throughout the day. Check UKMO forecast from yesterday below.  Some models brought 24 hour totals of 20-30mm to SE at points on the lead up to the event.  The cause of the forecast deluge: a small scale low tracking NW to SE with a tightly wrapped occluded front crossing the area once, then lingering nearby to deposit more rain during the day before drifting off southeast. Once the front had passed through early am, it turned out to be a splendid day with sunshine and bright spells throughout, until rain later.  So what went wrong/right?

The front passed over as forecast during early am dropping 6mm on Reigate before 8am.  It then sat N of London most of the day while further south convection over Sussex caused significant Cb clouds and showers (some thundery) to spark off from midday.  For us in Reigate, we had a splendidly bright day with glorious sunshine by 8am and bubbly cumulus clouds thereafter, the odd spot of rain but nothing significant until early afternoon when the front migrated south east.  So for most daylight hours Reigate was dry, quite the opposite of the forecast.

The photos above and graphics below suggest a possible reason for this.  Reigate sat in a sort of “Goldilocks Gap” between the persistent frontal rain further north and convective rain nearer the LOW further south. It is notable that the convective showers built mostly over the land, showing almost April-shower tendencies to build on warmer land surfaces than the now-cooler sea. The occluded front sat close to Reigate, frontal wave clouds and cirrus were visible above and to the north for most of the day.  This may have helped suppress convection.  As warm tropical air is lofted over an occluded front it spreads out and forms a cirrus veil, this often suggests a broad inversion of warmer air aloft that effectively suppresses uplift of thermals: the cirrus acts like a lid.  So cumulus clouds over Reigate and the N Downs stayed small and harmless.  Not far south, in Sussex, thundery downpours developed as the buoyant air lofted uninhibited by any inversion.  You can see this on the radar image below.

Reigate was therefore dry for most of the day perhaps because of our location in a sort of Goldilocks Gap (our word) that was just far enough from the occluded front to avoid persistent rain and just near enough to benefit from the inversion to prevent convective showers. Met-Magic!  The graphics and photos try to explain this further.

This is just one possible reason why slight changes in the tack of a LOW will render a forecast completely wrong, even in the middle of a LOW pressure when all hope of a nice day might be thought lost.  Further ideas are most welcome to extend this.

Update for Reigate: tricky weather picture but here goes… 

Friday: wet cloudy day, mostly light winds for us, recent model runs now bringing in slightly less total rainfall accumulation tomorrow for Reigate.. around 10mm likely, more if showers perk up pm.  Rain arrives early hours Fri am with persistent frontal rain as occlusion wraps almost full circle around the SE, later in the day more convective showers are likely. All this drifts slowly off to the S later pm.

Fri-Sat some cloud and rain may persist late overnight, any showers turning briefly wintry over hills as temps cool off, gradually drying up through to early Sat am.

Saturday: Some light rain may persist from the remnants of slow moving front edging SE, though this may clear more quickly for a dry mostly cloudy day. Cool at Tmax 6c

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Overnight Sat-Sun: a new LOW crossing the north of the UK will sweep a front across our area and bring more rain overnight, due to clear Sunday am, though, again, latest UKMO run shows light rain a possibility during Sunday as this front weakens and becomes slow moving over the SE.  Any remaining rain

Yet another LOW pressure system, this one larger, is set to move in to UK quickly overnight Sun-Mon and bring more persistent and widespread rain overnight Sun-Mon and showers Monday.

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The longer term picture is rising pressure towards the end of next week so that next weekend could be much more spring-like and pleasant. Check back for details.

A strong jetstream to the SW of the UK will manufacture some tricky weather over the next 48 hours and continues to make details for the weekend uncertain.  Here’s an outline of the likely scenario for Reigate and SE but the advice must be to watch the forecast if you are doing anything weather-related!

The jetstream is located to the SW of the UK and is blowing strongly from NW to SE.  This means the UK is sitting in that vulnerable zone to the cool north-side of the jetstream where active small scale depressions can run-up rapidly and deepen off the left-exit region of the jetstream where temperature and pressure gradients are greatest.  These depressions are small scale but can produce a lot of rain and their track is sometimes rather erratic: and tiny changes in track can make a big difference to the weather experienced!

Two LOWS are predicted to spring out of the jet from Fri-Sun: the first during Friday – will track directly over SE England and drop possibly 10mm of rain onto Reigate.  As they track broadly S and E of UK, these LOWS will, in turn, swing the winds into a much cooler northerly or NE direction.  Reigate is unlikely to get any strong wind because the LOW pressures are due to pass almost overhead… meaning we are likely get mostly light wind but potential for heavy showers as the cool polar air “warms through” during any sunny periods which will cause convective uplift and showers.  There are also occluding fronts wrapped round these systems that could bring more persistent rain at times.  The second LOW emerges on Saturday and is currently due to track off in a more southerly direction through France but could bring rain across S coasts.  This one needs watching carefully as any movement north will cause a much wetter and unsettled Saturday than is currently on forecasts: which currently show Sat largely dry but with some poss showers later. Showers are possible as the cool air warms through and sparks convective showers on either Sat or Sun. Higher pressure ridges between these lows will bring cold nights, possibly frosty in places if cloud clears.

The complicating factor is that pretty cool air is wrapped up in these depressions, almost cool enough for sleet and wintry precipitation at times later on Friday for Reigate and during almost any showers that rotate around the remnants of the LOWS during Saturday and even into Sunday.  Whilst wintry ppt is unlikely for Reigate, further north and over hills in Wales and N England there will be snow, probably patchy. short lived temporary wet snow at lower levels but accumulations possible over higher ground.

The only way Reigate will see any fleeting proper snow is if we get any HEAVY rain during the passage of fronts later on Friday and any showers left behind in low pressure Sat/Sun.  All rainfall during this period will start as snow aloft, in fact almost all our rain in the UK starts as snow up high and simply melts as it descends.  However, if rainfall is heavy enough it can actually COOL the air sufficiently so that further precipitation falls as snow.  This is called evaporative cooling: some rainfall always evaporates on descending to the ground, even in cold weather.  If enough rain evaporates then this will cool the air sufficient for freezing levels to fall to the surface and then, bingo, snow will descend to the surface as well!  So, whilst rain can NEVER “turn to snow” (completely impossible!), it is possible for heavy rainfall falling through a sufficiently cool air mass (around -5c at 850hPa), to cause snow to reach the surface.

A wild meridional loop in the jet stream on 14-15 Feb injected a cool pool of air over NW Africa that created unsettled conditions, cloud, cold temperatures, rain, snow and dust storms over Morocco and parts of North-Western Sahara. RGSweather went to Morocco to investigate the effects of this African storm and bring you the experience of an unusual African cut-off low first hand! (well, we were going there on an expedition anyway, so why not?!).

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A cut-off low is a depression, usually associated with mid or high latitudes when a meridional (bendy) jetstream loops down and leaves a cool pool with a resultant low pressure system “cut-off” as the jetstream returns to a more northerly latitude.  Cut-off lows sometimes barely show up on pressure charts but are often persistent features that introduce unsettled weather: often associated with showers as convective activity warms through the atmosphere and gradually “fills” the LOW.

The Moroccan cool pool caused winds to circulate around the low and drag dust and sand over the High Atlas for a period as easterlies blew in across the Sahara as the low moved north and then east.

Snow is common across the Atlas mountains during winter but temperatures fell unusually low and snowfall across the Atlas fell below 1000m.

Gradually, as the cut-off low filled and moved NE, pressure built and skies cleared and, at low altitudes, temperatures rose.  Dew points fell as humidity fell and nights remained exceptionally chilly!

The animations below show the cut-off forming and cool-pool with showers and frontal precipitation (snow in mountains) over NW Africa and Morocco. Probable influence of sub-tropical jetstream too but information not available on this at the moment.

IMPORTANT UPDATE 11AM Fri: UKMET extended AMBER warning to include all of SE inland including Reigate, Surrey, London: gusts of 60mph possible overnight. 

Latest update: (pm Thurs): one HiRes model just HALVED rainfall totals for storm tomorrow: if others follow that’ll be good news for flooding.  This storm is not resolved perfectly yet, some models flip and flop before the event.  Nevertheless, one to watch for signs that, perhaps, the rain is being over-done. In fact, rainfall totals have rarely matched model runs in recent events… fast moving fronts have simply zipped by so fast that rain totals have been rather low.  example: 8.8mm on 12 Feb front yesterday.

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Now you see it…?! Rain totals just halved on latest HiRes model run (left). GOOD NEWS for floods!

Update for Reigate and SE England on UKstorm Fri 14 – Sat 15 Feb:  the storm tomorrow is yet another serious weather event, this time focusing most intensely on SW, Southern coasts and SE England and Wales overnight Fri-Sat.  Below are some satellite pictures from today showing the rapidly intensifying storm in the Atlantic to the SW of the UK.

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The wind and rain forecast on NWP models for inland SE areas are severe: often the forecast winds and rain do not transpire, coming in at a good deal less than the modeled projections especially for sheltered towns where most people live.  However, the magnitude of figures consistently being predicted for gusts and total rain associated with this storm in our area are consistent enough to be of some concern, especially for places exposed to southerly winds (on hills like North Downs, for example) and at risk of flooding, either surface water flooding or in the flood plain for the River Mole or tributaries.

Rainfall will be heavy throughout Friday and could total over 20-30mm in 24 hours in places.  If this exceptional rainfall does occur then extensive local flooding is likely to result rapidly, especially along the River Mole catchment which has a flashy response to rain within 10hours.  Winds are due to possibly exceed those experienced this winter so far, forecast to peak at over 60mph gusts inland early Sat am.  Heavy rain is also likely to add to the problems of flooding.

UKMO warnings stand at Yellow alerts for Reigate and Surrey from Fri through to Sat 15 Feb and AMBER alerts for all places nearer the coast.  The storm is rapidly intensifying and will bring heavy rain all day to Reigate and Surrey and the SE as a whole through Friday, totals could be over 30mm, which will cause the River Mole to flood in places and plenty of surface water on the roads. Strong winds build gradually through the day and will peak overnight in the small hours of Saturday morning.  The winds, even inland, could even gust at 60-70mph and for southern coasts 75-80mph.

These are exceptionally strong gales for inland and, if these NWP modeled magnitudes transpire, it would be exceptionally bad news for the region already suffering from flooding. Remember our strongest gust for YEARS in Reigate was 52mph in January.  So any wind speeds over 60 mph will be exceptional. The UKMO also forecast wind gusts over 60mph around midnight.  ESTOFEX have issued a Level 1 tornado warning and for isolated gusts from afternoon onwards due to warm air advection and presence of sub-tropical air wrapped in the system.  This will add lift and energy to frontal rain bands which might cause thunderstorm activity, the odd clap of thunder embedded in fronts might be a sign of tornadic potential activity so keep an eye our for funnel clouds or drops in cloud base or tornados.  If you see any of this please tweet or email a picture!

The saving grace is that it passes over relatively quickly and winds will abate on Saturday leaving a bright possibly showery and blustery day. The remainder of half term week looks less dramatic and with a ridge of high pressure flirting with the South it should be more settled, albeit with another LOW early in the week, but this is much less intense.

RGS weather is on holiday for the remaining week, inspecting the other side of the jetstream in the Atlas Mountains of Morocco. Intending to fly out from LGW at 4am on Sat morning! Close up and personal with this storm but sadly no tweet updates are likely.

By the way, press details can be hopelessly inaccurate, especially certain papers!  The pic below is wrong: indicating several storms that were not a special problem for the UK… handle with care!

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Four days of UKMO weather warnings and this hasn’t been unusual this winter!

Unsettled is an understatement, the weather is still on over-drive! Here’s a quick update on rain and storm events upcoming this week for Reigate, as the weekend summary didn’t quite do it justice!  The jetstream is still blasting across the Atlantic and giving birth to storm after storm.  Causes of this extraordinary jetstream are complex, but most “local” to the UK is the extreme contrast in temperature across North America with a buckling jetstream dragging frigid conditions across much of that continent.  This creates a steep temperature and pressure gradient that kick-starts a powerful jet across the Atlantic.  Storm force lows are being created amazingly quickly and most of them are being directed at the UK.

So, here’s a summary of storms on offer this week for Reigate and Surrey…

Tuesday: active front with cold air behind sweeps across the UK and arrives in Reigate am for heavy rain at lunchtime with gusty winds for a time. 10mm is possible in a short space of time which could lead to local FLOODING.  The front will whizz through quickly by the afternoon and leave a brighter but cooler feel pm even with possible wintry showers on a brisk cool W/NW wind.

Wednesday: looks grim… another tight storm approaches from the SW.  This one brings more gales to the SW, Wales but also pushes them further inland across the Midlands.  The south coast and the NE coast (on departure) will also see gales.  Wind for the SE will be pushing in off the Channel at possibly 50-60mph with 40mph gusts inland for inland places Reigate, peaking pm.  Rain is due to be heavy, especially pm and early evening and it will be cloudy all day.

Thursday… looks like a brighter showery day with possible heavy thundery showers at times. Friday / Saturday brings in the next storm with high rainfall totals adding to already saturated ground.  The likelihood of more flooding seems high at this stage which is terrible news for those places along the Thames that have been so badly hit today.

After another big “50 year storm” battered the SW and south coast today, the weather next week unfortunately looks to continue very unsettled with the possibility of stormy conditions at times, especially for the South and west.  Reigate, though more sheltered than most, will not escape the action. Mid-week Tues/Weds sees a likely cold snap with a chance of snow mainly falling over high ground to the NW but also snow showers possibly falling anywhere, albeit briefly and probably not reaching the SE. Before all that, let’s do a quick review of the interesting weather today…

In Reigate there was some interesting weather to report today as an unstable brisk SW airflow picked up moisture from the Channel and built cumulonimbus cells that rolled up to Reigate in the morning… see below:

In Reigate gusts peaked at 39mph, more widely gusts reached a measured 43.5mph on N Downs and exceeded that elsewhere to reach 50mph in places. The SW and coastal areas had gusts well over 70mph. In the morning some well developed cumulonimbus thunderstorms rolled across the area and produced hail and some lightning around Reigate, Dorking and Guildford. During these storms downdrafts caused temperatures to drop sharply but also the pressure to rise locally.  Now, let’s look at some charts for next week… 

It’s worth noting that the Northern Hemisphere is exceptionally cold and snowy this year… except for UK where it is exceptionally stormy and wet!  There are signs that things will calm down after this coming week as pressure creeps up.  This week, however, looks a little rough, windy, wet and unsettled.

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Models are still battling with the precise intensity and track of storms next week. ECM wants to bring in a southerly tracking rather intense low with some big gales through the South and the Channel prior to opening the door to a cold plunge of NW winds fresh from Greenland on Tuesday / Weds. The NW is not the usual direction for country-wide snow, at least not in the sense of prolonged bitter temperatures like 2013 but this particularly unstable polar maritime airmass is being delivered direct from Greenland/Canada and has some chilly upper air wrapped up in a low pressure behind an active front that will cross the UK through Tuesday and drop plenty of rain / sleet /snow. It’s the air behind this front that may bring snow.  Whether it reaches SE is not certain, but the fridge door will not stay open for long as a bigger Atlantic storm sets to arrive at the end of the week.

Finally, at the end of the week there is a sting in the tale… another almost identical storm to this weekend appears on the scene on Thursday and runs through Fri/Sat.  Some models (GFS) show extraordinary wind speeds for inland at the moment but often they over-do things at this early stage and they usually calm down nearer the time. Lots of interest this week so watch this space for developments.  As weather is so fast moving our twitter feed is the place to get best up-to-date info. @RGSweather

January 2014 summary

February 2, 2014 — 1 Comment

Brief summary of weather this January in Reigate. January 2014 has been one of the wettest on record for SE England.  UKMO say that January 2014 in Southern England and SE has been the wettest for 250 years (since “records began 1766”).  Records specific to Reigate do not go back that far (c4 years) but the town had over 180mm of rain in January, more than 3 times the rainfall in January last year.   Kenley, our nearest official met office wx station, recorded just over 200mm of rain.  Kenley is on top of the North Downs so is likely to experience more rain due to orographic uplift, especially in the convective downpours on squall lines and thunderstorms that occurred this month (posts here).

A significant proportion of Reigate rain arrived in the form of intensive downpours brought by humid unstable Southerly / SSW air masses bringing thundery cells from the Channel Coast, notably on 28 January when Reigate had thunderstorm activity and spectacular cumulonimbus clouds (one big clap over the town at 1:00pm).  This was during the cut-off low warm-through at the end of the month.

On 23-24 Jan a squall line arriving from the NW brought the highest gust of wind recorded in Reigate for at least 4 years: 52mph.  The squall line brought more trees down locally and recorded gusts over 60mph on the Downs. Posts on that event here.

The month began wet with the Mole running high and ended with the river still in a flood state.  Here are some pictures illustrating January 2014 around Reigate.

http://www.metoffice.gov.uk/news/releases/archive/2014/Early-January-Stats