Archives For January 2013

FRIDAY SNOW LATEST as of 12:40pm Thursday: SNOW STARTING EARLIER BUT STAYING LIGHT!

Light snow starting in Reigate around 9 or 10am associated with a slow moving and weakening occluded front moving East.  Mostly light snow for the rest of the day (Friday <5cm lying snow), but possibly getting heavier later afternoon and evening.  Snow showers on Saturday & Sunday and snow possible Monday & Tuesday with a new LOW developing out west.  Temperatures staying low all weekend: at or significantly below freezing throughout: windchill -8°C Friday lunchtime for Reigate.

 

 

FRIDAY SNOW LATEST as of 12:40pm Thursday: SNOW STARTING EARLIER BUT STAYING LIGHT!

Light snow starting in Reigate around 9 or 10am.  Mostly light snow for the rest of the day (Friday <5cm lying snow), but possibly heavier later afternoon & evening.  Snow showers on Saturday & Sunday and snow possible Monday & Tuesday with a new LOW developing out west.  Temperatures staying low all weekend: at or significantly below freezing throughout: windchill -8°C Friday lunchtime for Reigate.

Predicting the onset of snowfall (or rain) several days ahead is often the least accurate part of weather forecasting. Predicting temperature, wind speed and direction are usually considerably more accurate. The UKMO (UK Met Office) achieve around 70% accuracy for their 3 hourly precipitation forecasts for the current day and over 90% accuracy for temperature forecasts. Pin-pointing exactly WHAT TIME it is going to rain or snow in a particular location several days ahead stretches even the most powerful supercomputers and taxes forecasters. For example… here is a medley of current predictions for snow in Reigate: (no longer a valid forecast but interesting nevertheless!)…

Friday snow starts in Reigate:

Meteogroup = 9am; BBC=11am; UKMO = 12:00pm; GFS = 12:00pm.

Saturday snowfall rate in Reigate:

UKMO = Light snow all day; BBC & GFS = Light snow all day; Meteogroup = none after 6am!

Variations in temperature predictions are less marked but on Sunday there are significant disagreements: the lowest minimum is down to -9°C on Sunday night whilst other models place minimums at -3°C.  Thursday evening will bring the most accurate prediction for snow and models should agree more by then. Our own forecast might go something like: Light snow starting after lunch on Friday and going overnight through to Saturday when there will be continued snow showers for most of the day. Snow accumulation: 3-8cm.

A medley of pictures to show more interesting winter weather afoot!  Much talk of heavy snow on Friday as “warmer air” from the Atlantic is predicted to push in from the west. As warmer air from the west meets the dense cold continental air we expect precipitation to initially fall as snow on the leading edge of fronts and then, as warm air takes over, turn to rain … well, that’s the usual scenario. But hang-on!! Things might not be quite so text-book this time.
Charts are predicting cold windchill and moderate but persistent snowfall for Reigate from Friday pm through much of Saturday as a SLIDER LOW slips south east round the HIGH pressure and makes a wobbly attempt to bring milder air in. However, if the LOW moves South it will keep Reigate on the cold side of the fronts with a very chilly SE wind pulling in on Sunday. Probably no mild spell yet then. Charts for early next week also look distinctly wintry! Details will change but for the time being the jet stream is keeping firmly to the south of the UK, so Reigate will stay cool!

The NOAA satellite photo from Tuesday 15-Jan afternoon.

 

The main story for us in Reigate is now COLD but some snow showers mid-week could still deliver 5mm of lying snow in Reigate with a HIGH pressure over Scandinavia bringing bitterly cold north easterlies (-6°C windchill) to the South East with snow showers moving west.  The photo shows the front on Monday finally clearing off by about 5pm.

Update: Friday and the weekend look interesting: a “slider low” undercutting the HIGH (which is loathsome to move) could cause a classic “snow-on-the-leading-edge-of-fronts situation”.  Being sheltered in Reigate on the eastern side of the country this may amount merely to some light snow followed by miserable persistent and cold rain over Saturday. Meanwhile, enjoy the mostly dry but VERY cold weather!

Sleet and you shall find” is the likely scenario for Monday in Reigate! Deep and crisp and even snow is not looking on the cards for us on Monday despite it snowing almost everywhere else right now (see map) and despite low temperatures overnight and tomorrow morning.
Light snow might lay for a while am but temperatures are due to creep above freezing for Monday as a warm front pushes “warmer” air across the region. This will deliver sleety rain on a southerly wind rather than snow through the morning. Nevertheless, snow is notoriously tricky to forecast and the cold front following closely behind might still deliver moderate snow showers as temperatures fall Monday pm and overnight to Tuesday heading in on an increasingly brisk and cold Northerly / NE.  Why is snow so difficult to forecast accurately?  There are good reasons for this… but this is a question to tackle later.  Meanwhile, latest model runs suggest cold will hang on for at least a week, snow still very much on the cards… so perhaps “ask and it will be given to you” might still hold true for snow lovers? (Matthew 7:7)

Stratocumulus clouds

January 13, 2013 — Leave a comment

The best view is often up (or down if you are in the International Space Station)… so we are starting a “cloud collection”!
Stratocumulus clouds over Earth Jan 4 2013 and over Reigate January 13 2013 9am: spread-out cloud masses which can cover the sky: fairly flat cloud bases, often “rolling” (undulatus billows): low altitude clouds, usually occur at heights above 1500 feet to 6500 feet. Indicate stable atmosphere i.e. not much change in temperature with height (sometimes even indicate a temperature inversion where temperatures increase with height at a certain altitude), so not much upward convection, so the clouds don’t rise in height, so rarely cause much precipitation. Stratocumulus often tell you that the weather is not going to change much for some hours, watch out for them thickening up or rising in height.  The photo shows “stratocumulus perlucidus” which are slightly broken up, allowing sunshine between the gaps. They differ from cumulus clouds by usually being clumps joined together, whereas cumulus clouds are separate and detached (caused by convection).  It might also be true to say that any low altitude cloud that is clumpy, joined together and has a fairly bubbly top … is usually stratocumulus!!
Weather: 1.9°C, cloud height approx 2000 feet; wind direction north-east 5-10 mph.
Stratocumulus clouds are frequently ignored or considered boring. We think they are worth a second look!

Please join in our cloud collection by sending your own photos of clouds:try and name the cloud type if you can and tell us something about the location and weather at the time.

Arctic air travelling courtesy of the jet stream will blow in from Greenland and deliver frigid temperatures and snow to Reigate for the first part of this week. Precise details of snowfall are still changeable but temperatures are certainly due to take a fall during this week but getting there will not be straightforward!

13-01-2013 08-21-08Light snow on Monday (see map left) currently forecast to start falling in Reigate early am (updated sun07:30), could reach 6cm (update 8pm sunday:unlikely – looks much less now) but, oddly, temperatures may rise considerably above freezing Monday afternoon (+4°C?) and bring rain for a while probably melting any fallen snow before cooling down Monday night with more snow possible: a v complex situation which will change hourly! A developing LOW along an occluded front later on could bring more snow to the SE on Tuesday when air temperatures will probably stay below freezing all day and wind chill at midday could be as low as -9°C. The reason on Monday & Tuesday … a strong jet stream blowing direct from Greenland at over 100mph, 9km above our head, will bring Arctic air (-50°C at 9km) direct to Reigate in less than 24 hours journey time from “ice sheet to high street”!  A BLOCKING HIGH sitting out west in the Atlantic is preventing milder air from making progress further east.  The cold weather will continue so long as the HIGH stays put.  The situation is likely to remain much the same for a while, though at the moment forecasts are uncertain beyond a few days ahead due to the “sudden stratospheric warming” which is taxing every super-computer weather forecasting model including the UK Met Office, let alone your friendly RGSweather station enthusiasts!

Update! by mid-week the HIGH will slip over to the north of the UK and drag in bitterly cold easterlies from the continent (see map wednesday thursday) Snow showers likely for eastern England.

SEE UPDATE! Ready for winter? This weekend sees cold air pushing in from the North East but not much snow for Reigate: icy, sleety rain Saturday with maybe some snow over the hills later and possibly a dusting to lower levels over night to Sunday; but mostly the rain clears off south on Saturday before the very coldest air arrives behind it. However, the chart shows fronts tracking south across the UK from the NW on Monday afternoon through to Tuesday which could bring heavier snow to Reigate and the SE. Watch this carefully! Further ahead, several long range forecasts agree that the cold weather could stay with us for 2 or 3 weeks caused by LOW pressure holding over Central Europe and a HIGH pressure sticking to the North over Iceland blocking our usual warm westerlies and feeding COLD polar winds from the North and North East across the UK for the whole of January.

The weekend sees an atmospheric battle taking place overhead between tropical and polar air masses. The video below shows the stage being set for this battle: watch the westerly winds pushing against the cold polar North-Easterlies sweeping down from Scandinavia and meeting over… well, Reigate!! Anyhow, this weekend temperatures will fall throughout Saturday from 3°C to freezing by late afternoon. Rain could turn sleety from mid-afternoon onwards as the coldest air pushes in and light snow is possible, dying out early Sunday morning. Sunday looks dry and cold, feeling like -7°C in a chilly northerly wind. Monday is set to be even colder, staying near freezing all day and feeling like -8°C in the wind, but dry (though latest is for snowy front moving south according to UKMO). The rest of the week is unclear but likely to stay cold until at least Wednesday with more significant snow likely Tuesday. Longer term predictions suggest January will stay cool, with a few milder interludes.

The weather map shows weather for Sunday 13 Jan. A LOW pressure sweeping SE across the southern part of Britain and a HIGH to the north of the UK will usher in very cold Easterly and North Easterly air during this weekend. Snow is looking more likely for us in SE England. A front moving south east over Saturday and Sunday will divide the warmer air to the south from the cold invading easterly winds and this front is where snow is most likely to fall. For Reigate, some forecasts put a 50% chance of moderate snowfalls starting Saturday evening and increasing to 75% chance of several cm of snow falling on Sunday. Wind chill on Sunday will make it feel -7°C: so wrap up! Importantly, the rest of next week looks cold so snow could fall and linger at least through to Wednesday when milder air might push in. More detail from Alex Deakin here…

http://www.bbc.co.uk/weather/feeds/20964928