Archives For uncertainty

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.

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).

It’s all set for a battle right over the SE between tropical and polar air over the weekend. Models are struggling to tell which will win – some say any cold snap will be brief and milder tropical air will return next week, others predict polar air dominating for a while with the jetstream shifted south of the UK. Who knows?
Meanwhile, as we wait to find out, enjoy the contrail photos from 8am this morning over Reigate! What are contrails and why do they thicken up sometimes and disappear at others? Posts on clouds and cloud types coming soon!

Hang on a minute!
It seems the “beast from the east” will only tip-toe into the UK this week and then rapidly turn tail and scamper back off to the Steppes. So much for any real weather drama this week here in the South East: it looks like any cold temperatures will be short-lived and milder conditions from the Atlantic will break through by the end of the week: the map shows warmer & wetter SW winds by Friday 14 Dec seeing off any chilly Easterlies.
Snow probably for the North of the country only on the leading edge of fronts and not for us down here in Reigate. SO… you can put away your snow boots and Cossack hats for a while yet.
Nevertheless, the overall conditions are still nudging us towards cooler than average.