Archives For February 2016

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8-10 day mean upper air chart

The most wintry set-up of the “winter” has decided to arrive at the start of Spring! High pressure over the Pole is still keen to push out polar air into mid-latitudes as the meteorological Spring starts tomorrow.  The Arctic Oscillation shows this tendency as it has been dipping negative, showing relatively high pressure over the Pole and lower pressure in mid-latitudes. The belated rise in Polar pressure has been attributed to the stratospheric warming which occurred around a month ago.

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Unfortunately, for most of late winter the jetstream has remained strong enough to push a predominantly Atlantic flow into the UK and breach any blocks attempting to drag in sustained cold air. This is shown by the slight but persistently positive North Atlantic Oscillation below: this set-up spoilt any chance of proper cold this half of the winter.

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However, this week the wind is expected to turn more northerly as pressure lowers first over the North Sea and then the Channel / N France. By the weekend the UK will be in a cold Arctic flow courtesy of the same LOW lingering over Europe and a blocking Atlantic ridge. Between them and the jetstream they will do a good job of pulling down a cold Arctic flow this weekend. Spot the cold anomalies in the chart below. Lots of lying snow is not likely but some wintry precipitation is possible at times here, especially a marginal chance on Friday am.  Things change though, so check weather professionals like the MetOffice to make any weather related decisions.

Here is the story of how we get to some belated cold by this weekend.  Tuesday sees an end to the cool clear HIGH that brought Spring-like sunshine to Reigate and Tmax 9C.  A warm front will sweep in tomorrow morning and bring rain for most of the day for the SE.  It will be breezy, though relatively mild in the warm sector shown below, Tmax 10C.

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warm sector Tuesday

Things cool off into Wednesday as an active blustery cold front ushers in colder polar maritime air through the morning.  Any snow is restricted to the NW of the UK.

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The flow swings increasingly to the north west during the day with the possibility of showers later here, some quite heavy, maybe even with the outside chance of hail and thunder thrown in.  Tmax 6-7C.  Update: The video below shows how that showery trough passed through Reigate during Wednesday:

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Thursday is the crux to building a set-up capable of producing any snowfall at all for the SE.  A LOW is forecast to develop from a system off Greenland and cross the Atlantic smartly from the NW.2016-02-29_19-50-10

This low, with rapidly occluding warm sector, will deepen slightly and meet the cool pool sitting over the UK from Wednesday’s Polar attempt. The LOW is expected to arrive late Thursday and track south east overnight into Friday bringing in a cool NE flow capable of wintry precipitation for a while on Friday morning.

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The latest charts suggest the track into N France will result in NE winds which could bring snow for a time in the SE on Friday morning.

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meteoearth Friday ppt

It is all rather marginal for us in the SE and Reigate though. A rain / sleet event with a possible wintry mix at times is more likely and nothing much is expected to settle.

By Saturday and into the weekend a cold northerly / NE flow sets as the “Greenland” LOW settles over Europe. Wintry showers could develop across the SE, especially in any distrubances in the Arctic flow.  However, pressure is likely to be on the rise as the Atlantic ridge creeps in from the west under an increasingly anticyclonic jetstream.

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How long any early Spring cold lasts is not certain, the Atlantic ridge looks like toppling over to bring in warmer conditions later next week.

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The UKMet analysis chart for today shows a building ridge in the Mid-Atlantic and a complex large LOW over Scandinavia.  This is dragging down a cold Arctic airmass, which will usher in progressively colder upper air each day through to Thursday.  The low NW of the Azores is drifting ESE but filling and can be ignored for now.

Air at 850hPa, roughly 1500m up, is used to judge airmass characteristics because, at this height, the airmass is not affected by diurnal and surface changes like sea, forest, mountain, towns, which can create big temperature variations near the ground.  The 850hPa temperature by Thursday could dip as low as -8C over Reigate.  Whilst this is not extra-ordinary it is about the coldest and most sustained chilly dip we have seen so far this winter.  Skies will be mostly clear and nights will turn frosty with day Tmax struggling to 5-6C. Wind chill will make it feel more like freezing for most days this week.

High pressure nudging in from the Atlantic will keep any precipitation light and restricted to coastal areas.  Friday sees things get interesting and unusual.

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A southerly diving jetstream gets into action on Friday and is set to amplify the 500mb trough and deepen a low off the west coast of the UK and develop it further into Biscay into Saturday and create a cut-off feature by Sunday.

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The Atlantic / Biscay LOW is unlikely to impact us in the SE much directly but, as it passes south of us through the weekend into Europe it will drag in cold easterly and then NE winds.  Our own mini-Nor-Easter!

Nor-Easters are famous powerful winter storms in the USA.  Our own version this weekend is a rather tame feature in comparison but notable because it is relatively unusual and has potential to bring a more sustained wintry feel to the SE through the weekend and maybe into next week.

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This southerly tracking LOW will move NE through the continent and will set up a cool Easterly then NE wind over the SE. Nothing outrageously cold for us because the coldest air is likely to stick further to the north east in Russia and Scandinavia.  Nevertheless, by Sunday a brisk NE wind with upper air of -8C or so will continue to make it feel chilly, after a cool week.

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The winds over the SE will therefore swing from  Northerly mid-week through to Southerly on Friday and thence to Easterly and finally Nor-Easterly / Northerly by Sunday. This anti-clockwise rotation is called backing and often ushers in cold air.  This is despite the wind turning through a seemingly mild southerly direction.  Remember we are sitting well north of the jetstream this weekend, which is somewhere in the Mediterranean, thus all airmasses are relatively cold and polar.

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Pressure stays relatively high throughout this “mini-nor-easter” episode and so this will limit chances of any snow unless the low decides to track further north nearer the UK OR we pick up sea effect snow as winds turn NE over a relatively warm North Sea.

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Snow showers are theoretically possible near the North Sea coast if Sea Effect / Lake Effect snow can be kicked off.  Such sea effect snow occurs when (very) cold air masses cross warm sea surfaces.  This can trigger lines of convective showers that dump lots of snow in places like the Great Lakes in the US.  Sea Effect snow occurs best when there is a temperature contrast of at least 13C between 850hPa (which must be well below freezing of course) and a warm sea surface.

The charts above show the North Sea is anomalously warm at more than 8C across a large area, although cooler near UK shores.  The upper air temps this weekend are around -8C, making a potential 16C contrast in temperature, theoretically sufficient to trigger showers. Unfortunately the airmass seems to be dry and, at this stage at least, rather stable. Looking ahead the cold spell could last into next week. Check our twitter account @rgsweather for local updates but always check professional weather forecast providers for decision making, of course.

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Reigate, Surrey January 2016 summary weather statistics

  • Tmax 12.8C
  • Tmin -5.1C
  • T av 5.2C
  • Total rain 137.2mm
  • sunshine hours 94.1 hours
  • mean pressure  1009mb
  • max gust 35mph

January 2016 was a modestly unsettled month in Reigate with frequent wet days (21 in total with >1mm) and total rainfall amounting to 137.2 mm, this was some 160% of normal expected rainfall totals for the SE as a whole for January.  This caused some surface water flooding and soggy fields but, whilst a few flood alerts were issued, the River Mole behaved itself as no one rainfall event occurred with sufficient intensity to cause significant flooding problems in the Mole catchment.

In comparison with recent years, January 2016 turned out moderately wet sitting between the very wet January 2014 183 mm and the drier Jan 2015 69mm.  Of note locally was a spectacular display of mammatus cloud on 10 January as a squall line brought a thundery trough that passed over the area bringing some hail and cumulonimbus cloud.

It was a mild month with average temps 5.2C, Tmax 12.8C and only 7 days with minimum temperatures dipping to or below freezing. The highest January max temp ever recorded occurred in N Ireland. Whilst no weather record was broken in Reigate it was still a mild month with about average sunshine.

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sunshine Reigate January 2016

At 5.4C the CET temperature for the UK was 1.6C above the long term average. At 1.13C the global average January temperature turned out to be another record breaking warm month.

Returning to local events, one brief marginal wet snowfall event occurred in Reigate and Surrey overnight 16-17 January.

Snow lay during the morning of 17 January and was just sufficient to allow families to enjoy sledging and snowman building in Reigate and Priory Park for a few hours before it all melted by midday.

 

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The mean 500mb pressure anomaly pattern for January shows the UK in a mostly stormy Atlantic regime with storms rattling through especially the NW bringing high rainfall totals there. Storm Gertrude at the end of the month did not impact the sheltered SE much. As usual the SE was relatively protected sitting nearer the higher pressure over S Europe.

Imogen is the ninth named MetOffice storm this winter.  She formed in the Atlantic in an area of steep temperature gradients under control from an active jetstream.

 

Storm Imogen is deepening rapidly today to 953mb, though on arrival in the UK she will be occluding and filling gradually to above 960mb on her track over N Scotland into the North Sea on Monday. The exact track makes a big difference to where the strongest winds are.  Current trends are for the storm to pull wind fields further north so impacts could be less than expected. Keep an eye on the MetOffice forecast as things are likely to change. Below is an outline of Imogen’s likely activity:

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Unlike the previous eight named storms, Imogen has a more southerly track, guided by a more southerly tracking jetstream, and the field of strongest winds and heavy rain are possibly set to impact the densely populated southern part of the UK, including the SE. High waves are also expected on the Channel coast.

 

Strong winds on Sunday night will be associated with Imogen’s fronts running ahead of the depression.  The cold front is an active kata-front, associated with descending cold dry air from the stratosphere running ahead of the surface front and enhancing lift and potentially generating heavier rain and gusty conditions (image and info courtesy UKweatherworld).

On Monday gusts up to 80mph on the Channel coast are possible, while inland the MetOffice consider 60mph possible in exposed places.  Around Reigate and sheltered parts of Surrey, 40-50mph gusts are more likely.  The North Downs could see gusts approaching 60mph. The strongest winds for the SE are likely to be through midday and in the afternoon.

Yellow warnings apply to inland parts of Surrey and SE England while the entire Channel coast has an Amber MetOffice warning. The first impact will be frontal rain tonight.  Fronts passing through overnight into Monday could drop over 20mm of rain in places particularly linked to the occluding “triple point” forecast to cross the SE overnight.

 

Monday is likely to see showers, some heavy, appearing through the day.  Warm sea surface temps in the Channel are likely to cause more on the coast but the brisk winds could bring them inland as the day progresses.

The cause of the strong winds behind the cold front on Monday is a steep pressure gradient.  On Monday tightening isobars show the steep pressure gradient bringing gusty showery conditions in unstable Polar Maritime air behind the cold front later on Monday.

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The Wight-Wash Oscillation (WWO) measures the pressure difference between The Wash and the Isle of Wight and is designed as a guide to wind speed in the SE corner of the UK. The WWO on Monday shows a significant 16mb gradient between The Wash and the Isle of Wight on the WRF model.  The Euro4 model has a more modest 12mb WWO.  16mb would be the largest WWO pressure gradient recorded and greater than St Jude, which was 12mb.

On Tuesday models show a wave depression bringing more rain to the SE, some even show fleeting wintry precipitation on the back end of this low as colder air ingresses from the north.  This is unlikely to be significant, at least on Tuesday, as upper air temps remain mostly too high for snow in the SE.

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ECM colder flow mid-week

Colder conditions are preferred by the ECM as northerly winds bring cool polar air further into the country through mid-week.  The Arctic Oscillation is again going negative which shows pressure rising over the Poles trying to push Arctic air south into mid-latitudes.  However, the NAO remains positive so Atlantic depressions will continue to bring frontal depressions for this week.

The 8-10 day mean shows a deep trough over the UK meaning low pressure and unsettled conditions remain likely into half term.

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The ECM builds heights over southern Greenland which links with higher pressure over the Atlantic, a more northerly feed of cold polar air is likely in this scenario into half term . The ECM has been outperforming the GFS so the more Atlantic driven GFS chart would be the less favoured option.

The Sudden Stratospheric Warming going on over the Pole is another astonishing feature of weather at the moment.  Today (Sunday) temperatures in the stratosphere over Siberia has got up to an amazing +12C from a more usual -70C.  SSW events often build pressure over the Polar troposphere a few weeks later which can cause cold incursions into mid-latitudes.  This is by no means certain but is perhaps our last chance of any sustained cold this winter… if it were to happen it would be late Feb/March. One to watch!