Archives For February 2013

Not weather but this will get people looking up!  Visible with binoculars. S/Eastern horizon. Moving towards Pole Star; through Plough by 8.30pm.

http://www.heavens-above.com/2012da14.aspx?lat=0&lng=0&loc=Unspecified&alt=0&tz=UCT

The weather during half term looks drier and more settled for Reigate but temperatures could still bite back with COLD returning from mid-week onwards.  A HIGH pressure ridge is forecast to build near to the south of the country this weekend and then develop over Europe to the east where it should more or less block Atlantic westerlies which bring our usual wet weather.  The air flow over Reigate will be more from the south early in the week and then swing south easterly and even easterly from mid-week.  Milder temperatures are due to arrive in the coming days and reach 8 or 9ºC during the day, which might even feel spring-like early next week!  Temperatures will drop quite sharply at night and low enough for frost on any clear nights.  However, the position of the HIGH becomes important from mid-week onwards and here the models vary a lot: one model moves it north, with a LOW forming over Europe; if this happens then be prepared for much colder weather later in the week, possibly even snow showers for the east coast (see post below and map ECMWF) as cold easterlies make a return.

This will be no help at all … but the most widely used weather models are completely at odds over half term weather forecasts next week. The GFS (US) system shows warmer, unsettled Atlantic westerlies blowing back through on a perky jetstream and places a HIGH to the south of the UK bringing in milder SW winds for a time. The ECM model (European) has completely the opposite and fixes a HIGH over the NE and puts the UK firmly back in the fridge with a fierce easterly: snow again for the east. The GEFS model run sits on the fence and puts us somehere in between the two other scenarios. This illustrates how each model has slight in-built bias and it is up to experienced meteorologists to pick the one that works best in each type of weather flow.  It also shows how forecast models for more than 200 hours into the future are sometimes unreliable, can only be used to show broad patterns of developing weather and should be used with caution. Meanwhile, the remainder of this week looks drier (except for Thursday!) for Reigate, a warm front brings sleet then rain on Thursday and a HIGH should build in bringing drier and BRIGHTER weather Friday and into the weekend.  Bring back the sun…Reigate last saw the sun on Monday 9 Feb at 8.30am.

Take care on journeys this morning. <1cm wet slushy snow lying in Reigate but big local variations likely; just above freezing. Light snow/sleet will die out during the morning. Dull and cold. Map shows lying snow across UK. Snow boundary in SE goes right through out area: in Reigate we were, as predicted here, “on the edge”! Snow continued throughout the day, on and off (mostly on!), confounding most forecasts.  2cm of snow total fall but melting throughout the day caused reduction in depth in many places and patchy snow cover. 

Looks like it here! Check the video and see how showers seem to be hooked up over the Downs and depositing rain or snow; fewer showers are making it through to the Weald.  This looks like a rainshadow effect usually associated with bigger mountains like the Pennines, the Welsh hills and further afield the Rockies or the Himalayas. The rainfall shown caused serious flooding in Chichester with some roads 2 feet deep in flood water. At the same time it was snowing in London!

 

How much colder is it 100 metres above you, right now? We tested this today by driving up the North Downs which stand 100 metres above Reigate, Surrey town centre.  A Kestrel 3500 was used to collect the data from the hill and temperatures were given a good while to stabilize.  The RGS weather station sits at the foot of the same hill at 96 metres. So, the (rather un-scientific!) results at 2.15pm were as follows:

Reigate 96m: Temp 3.1ºC; dew point 2.5ºC
Reigate Hill 200m: Temp 2.1ºC; dew point 1.5ºC

So Reigate Hill is a whole degree colder than the town, the wind chill was -1C. This was noticeable also in the heavy sleet on the hill.
The decrease in temperature with height is called “lapse rate” and it, usually, continues to drop for another 10,000 metres, the top of the troposhere (“weather atmosphere”). The 1C drop per 100 metres today is a steep lapse rate. We would normally expect around 0.6C drop per 100m, called the environmental lapse rate. The freezing level is currently only 300 metres above Reigate town centre and, with temperatures falling tonight the freezing level will almost certainly arrive at the surface.  The clouds were interesting: thick nimbostratus formed a solid cloud base at 300 metres; amazing shreds of fractus cloud formed on the stiff SE wind blowing up the hill.  Fractus or thicker Pannus clouds are those whispy shreds which appear under rain clouds and indicate deteriorating weather.

Weather warning for Reigate: latest models agree with previous post and slightly nudge up the potential risk of some snow Sunday pm to Monday for Reigate. (Remember we are on the edge of the snow/rain boundary so small changes in the final track of the LOW may mean we see NONE at all or more than expected!) Nevetheless, depth of snow by Monday morning anywhere from 0cm to 4cm (esp on hills).  Rain and sleet will dominate the day but Sunday evening and overnight is when snow could fall.

Sunday-Monday night will freeze so any lying snow and icy surfaces could remain until later in the day. Be aware: Monday commute could potentially be slippery: certainly if you are going anywhere east or north of the region. Temperatures reach +4ºC Monday will melt any remaining snow. Remainder of week remains cold with possible snow showers mid-week.

 

Reigate is in an interesting location regarding the much anticipated snow tomorrow!  Whilst we will not experience anything like the metres of snow winter storm Nemo has delivered to Boston today, by Sunday evening Reigate is forecast to be on the edge of some exciting snowy weather to the north and east and in a superb position to watch events unfold as we sit on the battle front between warm (rain) and cold (snow) air!  It is due to rain most of the day on Sunday and turn to snow in the evening, initially and most significantly, over the North Downs.  Small gains in altitude could make a difference in whether you see snow or sleet or rain tomorrow: it’s that close! 

The LOW bringing all the uncertainty in the forecast is due to slip SE during tomorrow. It has warm air wedged between fronts to the south and west where precipitation will certainly fall as rain, some of it heavy.  On the leading edge of the advancing warm front, winds will turn increasingly Easterly during Sunday and drag in progressively colder air, especially over night into Monday morning.  This drop in temperature on Sunday afternoon is what will change any rain into snow /sleet.  By Sunday evening the warm front is due to be near Reigate but forecasts suggest it will stall and then slip south enhancing those cold easterlies but, at the same time, moving away the heaviest precipitation overnight which will peter out early Monday morning.  Temperatures will be above freezing throughout the day but only around 2 or 3°C, cold enough for evaporative cooling to create some local snowfall in heavy rain (see post yesterday).  Winds increasing to 30mph gusts in the afternoon will make it all feel thoroughly unpleasant.  Whilst rain and sleet are MOST likely all day, the zero-degree / freezing altitude decreases dramatically further north and east from Reigate.  Snow is therefore possible on the North Downs as even small increases in altitude locally could potentially add significant accumulations if we are positioned on the leading edge of the front for a period of time.  There is also a risk of snow settling lower down during Sunday afternoon and evening but temperatures are such that sleet seems most likely.  Total rainfall for the day is forecast to be up to 10mm so it will be a wet day regardless of how much snow falls.  Any snow falling in the evening could accumulate 2 or 3cm of snow, especially on the Downs.  So, to summarise, Reigate tomorrow is due to be increasingly wet and breezy as the day progresses, temperatures 3ºC, falling to 1ºC overnight to Monday.  Heaviest rain will be in the late afternoon / evening (6pm onwards) and this might turn sleety / snowy, especially over the Downs.  The forecast is full of uncertainty but, whatever turns out, do enjoy being in a pivotal meteological location tomorrow!  Meanwhile, scattered wintry showers are developing over our region this evening (Saturday).

The impending LOW approaching from the NW this weekend is a real teaser!  It will track SE across the country and the centre of the LOW will pass to to the SW of Reigate sometime over Sunday night. It will then “stall” for a day or two and drag in cool winds from the East.  Fronts associated with this LOW will bring every type of precipitation known…probably!  A weakening warm front slowly drifting east across the country on Saturday is not likely to reach Reigate until the evening. Saturday will be cloudy, calm and cool but hopefully remain dry. The leading edge of the warm front could bring some light snow by Saturday evening.  Sunday will be the BIG ONE… wet, cold and miserable, all day. North of London there could be heavy snow.  Reigate is on the VERY EDGE of the forecasted snow area (see map). Temperatures will remain above freezing at up to 3-4C, too warm for snow. However, despite this, heavy rain could turn to snow any time.  From Sunday afternoon onwards a process called evaporative cooling could cause snowfall. Some raindrops always evaporate on their descent from clouds, even in really cold air. Evaporation (turning a liquid into a gas) uses energy / heat (think of how chilly you feel stepping out the shower; the cause = evaporation!). Now imagine millions of tiny raindrops evaporating, for every evaporating raindrop some energy (heat) is taken out of the atmosphere and lowers the air temperature. In marginal snow conditions this temperature drop can be sufficient to turn HEAVY RAIN (the more raindrops the more cooling) into SNOW, even if surface temperatures are above freezing.  So, the question for Reigate this weekend is not “will it snow on Sunday?” but “will it rain hard enough to snow on Sunday?”  Enjoy the precipitation anticipation!

Snow is very fussy. It requires particular conditions throughout the depth of the atmosphere to form. Any one of these conditions not met, and it will fall as sleet or rain. For example, snow requires particularly cold temperatures (obviously!) extending right the way up through the atmosphere to allow deposition of ice crystals instead of water droplets: indicator temperatures are lower than -27°C at 5500m, lower than at least -5°C at 1500m and surface temperatures at least below 2°C, so it doesn’t melt on the way down.  Also, a dew point below freezing is a pre-requisite for snow to fall. On Sunday and overnight to Monday not one of these conditions is met over Reigate, so snow in the current model forecast is not likely: 5000m temperatures are too warm (-25°C), at 1500m the temperature is a balmy +1°C and the dew point is +5°C through most of that period.  The only period when all the snow-forming conditions are met over Reigate (as things stand currently) is on Monday afternoon / evening.  Unfortunately, at that stage the LOW will have drifted off and taken much of the precipitation with it.  So… does this discount snowfall for Reigate over the weekend? Not at all, snow is still very possible because of nocturnal cooling and evaporative cooling and any slight change in the track of the LOW, but this is another story for later. Stay tuned!