Archives For cold

2016-02-29_19-54-33

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.

2016-01-09_13-34-29

Atlantic jetstream has been powerful and mostly westerly

This coming week, Arctic air from Svalbard briefly encounters Tropical air from the Bahamas over Europe.  Remarkably, how cold it gets here in the UK and Europe might depend on the story of a sub-tropical storm over the Atlantic just as much as the Arctic air trying to push south.

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pressure is high over the Pole

Pressure has built over the Arctic, nudging cold air uncertainly south into European mid-latitudes.  The build in Polar pressure and relative fall in mid-latitude pressure is called a negative Arctic Oscillation as mentioned in previous posts.  The pattern is already cool because the jetstream is to our south.  The jetstream axis essentially divides the warm tropical air to the south from the cool polar air to the north.

zonal flow with powerful jetstream

westerly flow with powerful jetstream

However, the flow has been mostly westerly and even SW across Southern Britain as the jetstream is blowing purposefully from west to east across the Atlantic.  While pressure remains relatively high over parts of Europe (e.g. Med and Spain), the coldest air has been unable to penetrate very far south.  The jetstream chart above shows the unstable flow we have had this weekend bringing heavy thundery showers, some with hail, over Reigate. Check this pic of mammatus clouds this afternoon over Surrey, after thundery showers with hail.

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mammatus clouds over Brockham, Surrey

 

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cool LOW over UK, south westerlies in the south

It has been cool because the air has been circulating anticlockwise around a deep low over Scotland with a cold source region over Scandinavia. However, the airmass has been modified with a long track over the Atlantic. With such a strong zonal jetstream the more frigid Arctic air has not been able to penetrate far south into Europe, yet.

An unusual sub-tropical storm developed in the Bahamas last week over a very warm Gulf Stream.  This low pressure will come to hover in the Mid-Atlantic this week and it might just hold the key to unlocking some more Arctic air over Europe.

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300hpa shallow trough enter left

A part of the sub-tropical system is forecast to cross into Europe mid-week courtesy of a trough disruption.  This causes a part of the trough to break away and leave the parent “cut-off” in the mid ocean.

low rides jet coat tails into Europe

low rides jet coat tails into Europe

In this case, a small but vigorous “baby” LOW will ride off on the right entrance of the jetstream, a good location for deepening surface pressure, and enter Europe via the Bay of Biscay sometime on Wednesday.

disrupted trough enters Europe

disrupted trough enters Europe

Whilst this vigorous baby low will not impact the UK directly, it is set to lower pressure over Europe and, in its wake, will drag in Arctic air more purposefully SOUTH across more of the continent right to the Mediterranean.  This wind will be significant and create the first proper wintry feel for about three years with wind chill on occasions down to -7C or lower.

Importantly, the trough disruption will also build pressure in the Atlantic to the north of the sequestered parent LOW.  This is often the case in trough disruptions.

builds Atlantic high and lowers pressure in Europe

builds Atlantic high and lowers pressure in Europe

The increasing the pressure gradient in the Atlantic and lower pressure in Europe will push more Arctic air more purposefully south across the UK and into Europe. So the sub-tropical system has been key to unlocking the full Arctic blast!

Details about snow for Reigate will be updated this week.  At this stage heavy snow settling here looks unlikely but some snow fall and wintry showers are certainly possible, especially later mid-week.  It will get gradually colder through to the end of the week.  Thereafter it is, of course, uncertain! The ECM has been performing better as a model than the GFS so, on that basis, continuing cold is more likely than a sudden return to mild westerlies, which the gfs tends to do too quickly. So assume it’ll stay cold into next weekend.

2016-01-10_19-18-11

 

2016-01-05_20-38-55

GEFS cool dip mid January

A cold snap looks more likely next week from around 13 January. Nothing extreme, just a long-overdue “normal” wintry feel is on the cards.

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12Z gefs and ecm ensembles show more distinct dip in temps

 

2016-01-05_20-52-09

ECMWF upper air goes cold

Temperatures are due to take a dip below the seasonal norm.  It’s still a way off so details will change but here’s the current idea moving ahead.

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The current run of wet Atlantic westerlies (above) that has brought flooding to many parts of the country, is due to weaken as pressure rises over the Atlantic and further north over the Pole.   As the persistent Atlantic LOW pressure gradually moves East this week it will bring more rain across the UK.  As it moves further east over the weekend it is forecast to draw down cooler northerly winds from the Arctic next week, at least for a while. (see below). (update: “Atlantic block” noted on chart below is probably overstating it a bit … as HIGH is likely to give way fairly promptly)

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2016-01-05_21-04-00

Arctic Oscillation goes negative january 2016

The cause of this Arctic outbreak is indicated by the Arctic Oscillation (AO) going negative.  The AO is a measure of air pressure over the Pole relative to mid-latitudes.  It has been positive for most of the autumn and winter so far and this usually means a strong jetstream and mild wet westerlies for the UK.

When the AO goes negative it indicates building pressure over the Pole and more likelihood of Arctic air “leaking” into mid-latitudes.  The chart below shows the 500mb mean heights for 8-10 days time.  Spot the anomalously high pressure over the Pole relative to the mid-latitudes.

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8-10 day mean 500mb heights (ecm and gfs models)

The negative AO has been predicted by many expert long range forecasters for a long time partly because of a lack of sea ice in the Kara Sea (part of the Arctic Ocean near Siberia).  This is a long-term indicator for potential pressure rises in this region.

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In addition, the recent enormous pump of warm air, courtesy of the Storm Frank, will have encouraged tropospheric height rises over the Pole.  The result is an inflating balloon of relatively cold air waiting to pop into the populated mid-latitudes!

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surface temperature anomalies Jan 14 2016

As well as a push of cooler Arctic air, pressure is also due to rise over the UK.  With increased pressure we can thankfully expect a drier period.

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wintry pressure rise

A classic winter high might be expected to bring dry, bright clear days with cold nights with views of the stars.  On the other hand, frost, fog and icy conditions might also be expected too.  In the SE huge dumps of snow look most unlikely next week from current model runs but lows can move south in the Arctic flow and cause unexpected events. The duration of the cold snap doesn’t look long** as the Atlantic HIGH is swept away by more westerlies.  However, this is uncertain so stay tuned for more regular updates on twitter.  Of course, seek professional forecasts for decision making purposes.

update 06/01 **looking potentially more prolonged now. cold snap turning into a spell.2016-01-05_20-55-31

January 2016 cold snap

2015-16 winter forecasts have long considered the possibility of cold weather in the latter stages from Jan thru to feb. This was explored in a post here

https://rgsweather.com/2015/12/29/winter-is-nigh/

2015-04-27_20-21-59

cold plunge of polar air to end April

The newspapers have this colder than average week billed as a “polar plume”.  Cold air cannot really be said to “plume” like warm air (spanish plume).  Nevertheless, it is certainly cold up North with settling snow over the hills and wintry precipitation elsewhere too.

The GFS ensemble (several model runs combined at once) chart below shows that the colder than average (upper air) temperatures will last until the weekend at least.  Thereafter temperatures rise but note the rainfall spikes indicating unsettled conditions.

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upper air temperatures

For us here in the sheltered SE it is just cooler than average with patchy overnight frost and bright days with light showers Tuesday pm. A blustery cold front on Wednesday is likely to bring more purposeful rain for a while, as could further active fronts on Thursday when the trough axis moves through our region accompanied by the jetstream nearby to the south.  Friday is likely to see a transient high pressure ridge bringing settled weather and then a change in wind direction by the weekend.

The overall cool set-up is due to “northern blocking” which is when pressure builds over Greenland and the Pole and the, previously strong, Azores HIGH slackens off and nudges south. At the same time the usual Icelandic low pressure weakens or is dominated entirely by a HIGH pressure (see chart below).

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In this situation the jetstream works its way south of the UK.  The result is that cold polar air is able to leak south out of the polar regions and into the mid latitudes.

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The index shown above summarizes the overall pressure pattern in the Atlantic. It is called the North Atlantic Oscillation. The NAO measures the pressure difference between Iceland and the Azores.  It has been mostly positive this winter: meaning that pressure is LOW over Iceland and HIGH over the Azores.  Typically a positive NAO indicates a strong zonal jetstream and mild often wet conditions for Europe with relatively fast moving LOWS passing through.  The chart below shows how the NAO has gone negative recently and this indicates that pressure has built over the Pole, creating a blocking situation.

The charts above show theta-e temperature which shows cool airmasses over the UK clearly.  These are 850hPa temperature charts which represent temperatures at 1500m (1.5km) above the “boundary layer”.  This height is used to avoid disruptive temperature changes which occur nearer the surface caused by day and night, mountains and water bodies etc that upset the overall temperature pattern for analysis.  The situation shown below by this weekend is quite different, though still unsettled.  Note the warmer flow from the SW.

After the transient ridge on Friday it looks like pressure will fall into the bank holiday weekend as a low pressure nudges in from the Atlantic.

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pressure falls into weekend

The milder humid air brought from the SW by the Atlantic low could potentially cause some significant rain at times around the weekend and into early next week as this meets cold air over the country.  Despite the milder upper air arriving from the SW, it is unlikely the “milder” temperatures will be noticeable in such wetter and windier conditions. In winter this set-up could have brought big snow events but in early May it will simply bring rain. For the detail on timing and amount of weekend rainfall we will have to wait and see, but it certainly looks potentially quite wet, though models suggest pressure building briefly thereafter.

2015-04-27_20-10-05

Pleasant mild sunshine of late looks to give way to some cool drab easterly winds this weekend as high pressure builds over Scandinavia.

Easterlies this time of year are less cold than in winter as the continent warms under stronger Spring sunshine.  In any case, the continent has had a relatively mild winter so there is not much deep icy cold in our continental source regions so we cannot expect much real cold in this easterly episode.  Cool, drab cloudy skies are most likely … called anticyclonic gloom.  This is built when a large scale temperature inversion occurs: a cold cloudy air mass near the surface stays cold while above the air warms in sunshine and prevents convection going higher.  Low level convection, unable to rise, creates low cloud that spreads out as a thick low blanket of stratocumulus often with occasional drizzle. Most unimpressive!

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Further ahead there are hints of a possible colder mid/late March as some models suggest northern blocking: the Greenland high joining in with increasing pressure over the Pole and lower pressure blocked to the south.  This situation can allow polar air to escape into mid-latitudes given the right pressure pattern.   A forecast Arctic Oscillation (below)  diving steeply negative is also an indication that this might be on the cards.  Similarly there is a negative tilting North Atlantic Oscillation too.  Both these show indications of going negative.  The Madden-Julian Oscillation (below) is an indicator of the activity and location of  cyclical tropical convective wave systems that travel east along the Equator.  Different phases of the MJO have been connected with influencing different global weather patterns – including tenuously with European/Atlantic weather. The MJO is moving into Phase 7 and this has been correlated with northern blocking and a negative NAO, at least 50% of the time anyway!

Some of the models are hinting at the idea of some colder weather this month, but it’s a way off so not certain, worth watching though as a possibility of winters last gasp might catch people out.

 

What’s a Polar Low?

January 29, 2015 — 1 Comment

2015-01-29_18-19-06

Update: this feature is under discussion on met-forums as to whether it was technically a polar low or not!  Some features (e.g. satpic) suggest it was but other features, such as upper temperatures, suggest it may have been a small scale feature with some similar characteristics (referred to below). References below contain more details. This post still valid as it describes an interesting weather feature with several polar low characteristics, albeit jury is out on final definition here!

Polar lows are small but intense maritime meso-scale cyclones, a few hundred km across, that form quickly in cold polar or arctic air advected (moved) over relatively warmer water.  They are much smaller than our usual mid-latitude frontal depressions.  They usually occur in winter in Arctic or Polar airmasses streaming Equatorwards and they form characteristically beautiful swirls of cumulus cloud and a comma cloud formation visible in satellite pictures (see above VIIRS sat pic 29/01/2015).  Sometimes these swirls form an eye reminiscent of tropical cyclones, to which they have surprising things in common.  Heaviest snow / rain occurs near the “eye”.  This similarity is why they are sometimes called “Arctic Hurricanes”, though they do not always produce hurricane force winds.  Despite their scary name they are relatively common over ice-free mid-latitude waters, they can produce rain or snow (due to “warm core” see below) and one was recorded moving down the North Sea in Dec 1995, so they are uncommon but not unheard of around the UK.

early evolution spotted on MetOffice fax

early evolution spotted on MetOffice fax

Polar lows tend to go through a rapid life cycle of a day or two which previously caught Arctic explorers unawares with hurricane force winds blowing up from nowhere and creating high seas in hostile Arctic waters.  Gale force winds wrap tightly into these features.  Polar Lows were invisible to meteorologists previous to satellite pictures and only with very recent upgrades in Numerical Weather Prediction models have Polar Lows been “visible” at all on charts.  They are still hard to predict and models sometimes struggle to track their intensity and path.

Polar Low circulations do not last long and, like their tropical cyclone cousin, they tend to decay rapidly once they move from warm sea over cooler land, because the convective energy and steep lapse rates driving the system are lost.  Cold upper air temperatures and a warm sea assist steep lapse rates that can cause thunderstorms with active convection.   Lightning strikes were recorded in this 29/01/2015 low as it came ashore over Northern Ireland, typical of polar lows.

Charts modelling the evolution of this low, albeit rapidly in the last few hours, have predicted tracks moving the system SE across the UK overnight.  Areas especially at risk from snow include Northern Ireland, N Wales and Midlands and possibly parts of the Southern England into early Friday.  However, the system is likely to fill rapidly overland as the sensible heat flux available for convection is lost over the colder land.  Additionally, the land is rougher than the ocean and this increased roughness increases surface convergence (air arriving faster than it lifts) and this causes the central pressure to rise and the system to decay.

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decaying polar low

Like hurricanes, Polar Lows form over oceans and gain much of their energy from them.  Polar Lows usually form in places where there is a rapid change in temperature and/or pressure horizontally, this is known as a baroclinic zone.  Edges of ice sheets or simply where warm and cold air meet are prime locations.

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baroclinic zone west of Scotland: breeding ground for this polar low

Warmer ocean water is another essential trigger for polar lows.  The warm surface water provides essential mositure and lift that creates convection, condensation in frigid air and the release of latent heat that develops cumulus and cumulonimbus clouds. The clouds then wrap into a tight circulation around a rapidly deepening low core not dissimilar to a hurricane.  Polar lows are much smaller and more transient than a regular mid-latitude depression.  Polar lows tend to form on the eastern side of a high pressure ridge and to the east of a decaying occluding mother low.  Both these features can be seen on charts.

An important feature of a polar low is the formation a warm core.  Charts below show this as milder surface temps at 950hPa and the theta e chart which appears to show warm air too.  Internal evolution of this system seem to suggest it has a warm core perhaps comprising Arctic flow chasing down a long-track of milder airmass originally sequestered by the extremely active cold front that swept through this location yesterday secluding a warm pool in the core of the mother low sat over Scandinavia.  This warm pool appears to have advected east to meet incoming Arctic air.  An area of positive vorticity (spin) contributed to the evolution by adding spin to the air that caused the low to form.

The polar low that developed today NW of Scotland has many of the hallmark characteristics of a Polar Low and seems to be generally accepted as such mainly because of the defining characteristic swirl of cumulus clouds round the low pressure core.

Note:  met community not all agreed whether this was a Polar Low or not : has many characteristics but some consider it too warm with rain in some areas rendering it a meso-scale slide low or similar. 

some links on Polar Lows

http://www.weatheronline.co.uk/reports/wxfacts/The-Polar-low—the-arctic-hurricane.htm

http://rammb.cira.colostate.edu/wmovl/VRL/Tutorials/SatManu-eumetsat/SatManu/CMs/PL/backgr.htm

https://polarlows.wordpress.com/tag/climate-change/

http://www.keesfloor.nl/wolken/sat/polarlow.htm

Reigate and the UK has a cold spell arriving imminently, but nothing as dramatic as the US NE snow storm Juno. Our cold spell has little to do with the US storm except that Juno is doing a good job of pumping SW warm air on its backside into building the mid-Atlantic high pressure ridge that will encourage Arctic air to plunge south over the UK this weekend, in conjunction with a LOW over Scandinavia.  So storm Juno is not arriving in the UK, and never will, but it has indirectly impacted our weather along with other storms in the US NE seaboard.  Theta E charts are good for looking at airmasses, the different air masses stand out clearly as different colours below. The metoffice fax charts are for comparison.

theta e chart: good for airmass spotting

theta e chart: good for airmass spotting

 

UK metoffice fax chart

UK metoffice fax chart

The active cold front sweeps across the UK tomorrow Wednesday bringing rain to the SE but more importantly ushering in a blustery NW polar maritime wind carrying air from a source region over the Greenland ice cap which will push temperatures from near double figures in the morning in Reigate to freezing overnight with wind chill making it feel more like it should at this time of year.  Reigate is unlikely to see snow during the cold front passage as the coldest air takes a while to arrive behind the frontal rain.

theta e

theta e

This cold front is interesting because it roars in so fast across the Atlantic, swinging around a low pressure between Iceland and Norway, that parts of the warm and more humid sub-tropical maritime air ahead of it is effectively chased up into the low core where it is secluded and trapped / sequestered by the advancing polar air.  This has implications for later when warm pools of air form occlusions that could enhance snow risk as they are dragged south by the advancing Arctic airmass through to the weekend.

arctic air with mixed up mT air

arctic air with mixed up mT air

UK metoffice fax chart

UK metoffice fax chart

The polar air from Greenland is a comparatively dry airmass but extremely cold aloft and becomes unstable as it passes over a comparatively warm Atlantic.  This combination is likely to bring considerable snow to high ground over the N/NW after the initial front has passed.  Usually snow showers in the NW would be it with polar maritime air but this wind will be strong enough to push showers across more of the country and even reach us in the SE during Thursday.

 

Expect many of these showers to be wintry in air that is below -5C at 850hPa (1500m).  Often snow showers form streamers / lines of showers that could accumulate reasonable snow cover for some places whilst others see nothing.

The secluded warm air trapped aloft will form occluded fronts that are due to move south later Thursday and into Friday as the isobars squeeze together between the building Atlantic ridge and the Scandinavian low pressure.  This will actually raise the temperature a tad for a while on Friday before the colder Arctic airmass arrives, direct from the north, into the weekend.   The warm secluded air will oddly make the advance of Arctic air initially warmer than the polar maritime, thus temperatures on Thursday are likely to be colder than those on Friday.

The coldest Arctic air is due to arrive through Sunday into Monday when some truly chilly air will make it down through the UK.

2015-01-27_23-16-05

 

A “potentially historic and crippling snow storm” is imminent for the USA NE seaboard especially New York State with cities from New York to Boston being warned by weather agencies of a potentially life threatening blizzard lasting some 48 hours.  Update Tues 27: whilst the worst of the blizzard missed New York heavy snow occurred further North.  The system tracked further east than modelled.

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New York escaped worst, but Boston hit as expected with record snow in New England

 

Watch unfolding live pictures on webcams here:

http://player.theplatform.com/p/2E2eJC/NBCNewsIE8?guid=nbcnewslive_nyc1

http://www.earthcam.com/usa/newyork/timessquare/?cam=tsstreet

Potentially 36 inches (3 feet) of snow could fall in places (that’s waist deep!) as a Nor’Easter (NorthEasterly) wind buckles round a rapidly deepening low tracking NW up the coast over the next 48 hours. All weather agencies are busy getting the news out to people in the regions likely to be impacted.

Official warnings from NOAA and all regional weather agencies are giving these urban populations warning after the system has caught computer models napping.  The storm has rather unexpectedly “blown up” recently to become a potential threat only in the last 24 hours or so.

The charts below show the synoptic set-up that develops this system, named Juno, currently a rather benign low tracking west out of continental US.   This LOW is forecast to deepen rapidly due to it’s location in relation to a fast meridional jetstream looping round a HIGH pressure ridge in the US west.  The LOW is dragging cold air from the continent interior and, as this cold air interacts with humid air and plenty of moisture over the Atlantic, it will turn into a snow-making machine that could last 48 hours and dump feet of snow widely across many places indicated on maps above.

Here are some synoptic charts showing the development.  Note the track sliding up the NE coast.  The Nor’Easter wind itself can be seen developing on the northern edge of the low which, as it is forecast to slow and stall while it deepens (a common feature of bomb depressions) the strong NorthEasterly gales, some up to 70mph, will continue to pump snow across parts of the NE building up depths to 2 – 3 feet in places.

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Note the warm sea surface temperatures off shore which will provide enormous quantities of moisture for snow production on the NE wind.

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Here is a chart showing minimum temperatures and possible snow cover by Wednesday.

The US is more used to heavy snow and more able to cope with it than the UK because the USA has a continental climate which experiences colder winters with more regular and significant snow events.  Earlier this winter Buffalo recorded feet of lake-effect snow.  Even so, this snow storm is serious and a state of emergency has been issued for New York.  The difference this time is that this storm will dump a lot of snow in a very short space of time and more people will be impacted in a densely populated and urbanised part of the USA.

The National Weather Service and regional weather agencies and the New York Mayor have all issued weather warnings for the blizzard urging people not to underestimate the possible impacts of 2-3 feet of snowfall over the next 48 hours.

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http://www.nbcnewyork.com/news/local/NYC-New-York-New-Jersey-Connecticut-Blizzard-2015-NorEaster-Snow-Wind-Whiteout-Record-289770081.html#

Here are some tweets and information emerging on the eve of this storm:

 

More info:

Excellent write up and explanation of the evolution of this Nor’Easter storm http://www.washingtonpost.com/blogs/capital-weather-gang/wp/2015/01/28/raging-snow-howling-wind-the-meteorological-evolution-of-the-blizzard-of-2015/

http://forecast.weather.gov/product.php?site=NWS&issuedby=OKX&product=WSW&format=CI&version=1&glossary=1&highlight=off

http://www.wunderground.com/blog/JeffMasters/comment.html?entrynum=2902

http://edition.cnn.com/2015/01/25/us/weather-storm/

http://www.bbc.co.uk/weather/features/30989760?ns_mchannel=social&ns_campaign=bbc_weather&ns_source=twitter&ns_linkname=news_central

http://www.climatecentral.org/news/what-warming-world-means-for-major-snowstorms-18594

http://meteorologicalmusings.blogspot.co.uk/2015/01/a-meteorologists-thoughts-on-blizzard.html

http://www.bbc.co.uk/news/world-us-canada-30996010

The ensemble charts above show that we can expect a week of change ahead.  At the start of the week fronts will bring modest episodes of rain and swings in temperature to the SE followed by a notable change mid-week as both temperature and pressure fall (see charts above) with the possibility of snow for the SE.   By next weekend there is a risk of some proper cold into the start of February.  As usual Reigate and the SE will be sheltered from most of the action but the weather will take interesting and notable swings in a predominantly downward direction nonetheless.  Models agree on how this transition will happen and it is largely based on the evolution of a LOW starting life off Canada near Newfoundland …. here are charts from the ECM that shows the story of our Canadian LOW and how it is likely to bring another taste of winter to the UK.  (Update: worth explaining that the Canada LOW mentioned here is not the same as the Nor’Easter LOW that caused blizzards in NE USA Monday-Tuesday this week: it is a LP preceding it.  If you look below at Chart 2 for 28 Jan you can spot the infamous Nor’Easter bashing the NE coast well modelled on this ECM chart from back at the weekend. The US Nor-Easter Blizzard2015 storm is responsible for building the Atlantic BLOCK helping to push Arctic winds our way but it is not travelling to the UK. The NYC storm looks to travel North up the Canadian coast, filling near S Greenland, unable to break through the Atlantic high pressure ridge extending north that it, in part, helped to build. Hope that helps!)  In any case, the weather set-up gives Reigate another flaky chance of some snow.

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1. The story starts now with the Azores HIGH pressure giving a dry and pleasant weekend for the SE, Saturday has seen brilliant blue skies as a result.  The Azores HIGH has been dominating the Atlantic recently and our Canada LOW, deepening rapidly off Newfoundland on Monday, will be forced round the HIGH to the north west to Greenland. This LOW will deepen rapidly because of the great temperature difference between frigid air pouring off Canada and humid sub-tropical air fed up from the south courtesy of the Azores high.

For us in Reigate this period sees a ridge of the Azores high pressure ebb away slightly during Sunday and this will nudge a mild SW flow to raise temperatures temporarily high into the UK and SE overnight into Monday morning.  This minor weather episode (marked by the ups and downs in temperature on the 850hPa ensemble chart top of page) will be heralded by increasing cloud on Sunday as fronts bring some patchy rain in on a warm front overnight into Monday.  Polar air will follow a cold front later on Monday and a ridge of high pressure will build quickly overnight turning the winds into the NW with a dry chilly night in store for us in Reigate and SE into Tuesday.

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2. Later on Tuesday the Canada LOW is forecast to move NW to near southern Greenland where it will feed on a brisk northerly wind of freezing cold air from the Greenland ice cap.  This freezing cold air will create a vigorous cold front that will reach the UK early on Wednesday.  This active cold front will usher in an unusually cold Polar Maritime NW’ly wind across the country reaching the SE late Wednesday with the possibility of heavier rain or even snow. One to watch carefully.

A point to note from the charts above is that the central Atlantic is anomalously COLD at the moment and so the brisk NW’ly wind will not warm up as much as usual on its journey over the Atlantic to the UK, increasing the likelihood that it will bring wintry stormy gales to the NW, with some models even showing wintry precipitation for us in the SE too, perhaps reaching us by Thursday, though these are likely to be sparse unless the cold front stalls in which case more significant falls are possible. (Update: GFS 18Z suggests this cold front could be active and bring snow across entire UK in its wake…)

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NW airflow 2m temperature anomaly

Check the temperature anomaly chart above which shows how unusually cold this NW’ly wind is going to be.  Usually NW winds do not impact SE England with snow unless fronts stall or there is an especially active undercut of unstable cold polar air.

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3. Our Canada LOW that started life just off shore from Newfoundland is forecast to cross Iceland mid-week and then slide SE into the North Sea, around the blocking high extending north through the Atlantic to meet rising pressure in Greenland, a good scenario for a COLD Europe!  As the LOW transits SE into the North Sea it will bring down Polar and then Arctic northerly winds on its back and snow for the North, NW and NE coasts.  Arctic air rarely brings snow to Surrey or Reigate … it usually runs out of moisture and lift on its transit across the cool land and instead the south and SE usually gets azure blue skies with frosty nights.  NE and E coasts can get snow showers.  Daytime temperatures everywhere may well stay near freezing if this comes off.

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It’s too far ahead for any detail but some model runs show Polar LOWS sliding round the edge of the trough on the left exit side of the jetstream (where lows develop rapidly) as the trough moves east .  These daughter lows can bring snow to the SE but this is too far off to be certain.

So how long might this cold snap last?

There are indications both for and against a more prolonged cold snap but, on balance, the peak of any Arctic cold looks likely to be shortlived as the high topples east, ebbs south and allows gradually more westerly winds back across the UK.  For enduring cold we really need the LOW to move south into Europe and pressure to build to the north bringing in a blocked situation allowing cold easterlies into the UK (beast from the east).   However, this scenario looks unlikely because the Azores HIGH is likely to remain relatively dominant.  This is shown by the generally positive North Atlantic Oscillation chart above.

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North Atlantic Oscillation

The NAO is a forecast measure of the sea level pressure difference between Iceland and the Azores.  When the NAO is positive it usually indicates a strong pressure difference with a big HIGH pressure over the Azores and a LOW over Iceland.  A positive NAO correlates with a fast zonal westerly jetstream and mild wet winters for the UK.  A negative NAO indicates colder winters in which pressure rises to the north (Iceland) and allows easterly or NE winds to bring cold airmasses from the Arctic or more commonly Siberia into the UK.

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Arctic Oscillation

The Arctic Oscillation (AO) is a related cousin of the NAO.   The Arctic Oscillation is a bigger scale comparison of pressure at the Pole compared with that in the mid-latitudes.  A positive AO usually correlates with lower than normal pressure over the Poles compared to the higher pressure further south in the Atlantic.   This brings mild wet conditions to Europe (zonal westerly winds).  A negative AO sees pressure rise higher over the North Pole and this acts like a balloon to push polar air out into mid-latitudes… a cold winter scenario.  The AO forecast shows it going negative by early February and this correlates with the cold snap.  Unfortunately it looks like the AO will go positive again thereafter but this is too far off to be certain.  On the other hand…. !!

The charts above show a Polar view of the Northern Hemisphere.  They show that pressure is forecast to rise over the Pole disturbing the zonal westerly flow of the polar vortex by a displacement of the polar vortex away from the Pole (see below).  This might increase the likelihood of a prolonged cold plunge of Arctic air reaching the mid latitudes including the UK  This is good for cold weather enthusiasts in Europe!

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The other longer-term chart shown above in favour of a cold late winter is a sudden stratospheric warming forecast in early February.  A SSW can lead to pressure rising over the Polar regions a few weeks later, disrupting the upper westerly winds, potentially upsetting the jetstream and bringing cold to mid-latitudes.  This is well correlated and was significant in bringing a late winter in 2013.  So… much of interest at the moment as we enter the final third of winter 2014-2015.

note: for all decision making purposes and forecasts please consult professional agencies: e.g. MetOffice at http://www.metoffice.gov.uk/public/weather/forecast/gcpg7rs0t#?tab=fiveDay&fcTime=1422057600

http://www.nc-climate.ncsu.edu/climate/patterns/NAO.html