SnowStorm(Jamie)

NAE 18Z shows the precipitation fading for tomorrow evening: http://expert-images...011218_1118.gif < 24hrs http://expert-images...011221_1118.gif < 27hrs You can see the dew points fall closer to freezing where the precipitation remains however this seems to be very fragmented. http://expert-images...011218_1118.gif < 24hrs http://expert-images...011221_1118.gif < 27hrs http://expert-images...011300_1118.gif < 30hrs So sticking snow unlikely away from high ground, however some light falling snow is possible, most likely towards inland parts of SE Wales where the precipitation is shown to be heaviest. Allot of focus will now turn to Sunday/Monday's possible snow event, one of the main difficulties with this will be how influential will the warm sector be, marginal indeed for the west as a flow off the atlantic will be present: http://expert-images...011318_1118.gif < NAE @ 48hrs (10m Wind Direction) http://cdn.nwstatic....8/48/ukwind.png < GFS for the same time Not easy to predict this by any means you see the frontal system runs into the wall of cold sitting over us: http://expert-images...011318_1118.gif More confidence in some snow towards eastern areas of Wales closer to the coldest of air, i wonder how intense this front will actually be though as we get closer to the time or will we see further reduction in precipitation amounts as the frontal system fails to force out the cold air. Some things to watch out for over the next few runs.

Will provide more analysis on Saturday's possible snow event after NAE 18z has come out later this evening. Just looking through the 12Z GFS data and you can see how the 0C Isotherm drops as we pull in an easterly feed off the continent. 0C isotherm basically represents the height at which the temperature is 0C. http://cdn.nwstatic....degisotherm.png < GFS @ 30hrs http://cdn.nwstatic....degisotherm.png < GFS @ 33hrs Precipitation charts for the same time: http://cdn.nwstatic....2/30/ukprec.png < 30hrs http://cdn.nwstatic....2/33/ukprec.png < 33hrs Now lets take a look at the dew points for the same time: http://www.wetterzen...cs/Rmgfs308.gif < 30hrs http://www.wetterzen...cs/Rmgfs338.gif < 33hrs So we can see dew points dropping close to freezing for most in S/SE Wales as we enter the later evening tomorrow, indeed precipitation distribution is uncertain and how heavy especially as we see the cold air get introduced from the east. Precipitation modelled to be over S/SE Wales for a few runs now, indeed heavier more so towards the SE. My current thinking is that as we go into tomorrow evening quite a few places in south Wales away from immediate coasts should see some light snow falling, not likely to be sticking other than the higher ground.

Very interesting model output showing up. The ECM indeed looks very cold with a flow from the east. I would try and stick to the ensemble means when viewing past t-144 these tend to be less subject to the swift changes that happen in the operational models as you go further ahead in time. Here's the ECM Ensemble mean today @144 hours: http://images.meteoc...H1-144_wge3.GIF Higher pressure to our North/North East with lower pressure around central Europe and a more general flow from the east becoming established. Indeed this points towards a more blocked & colder signal. Again i will show the chart i saved last Friday for the 7th December (168hours out at the time): Now lets compare it to what the ECM is now showing (now 48hours out) http://images.meteoc...CH1-48_zlq2.GIF Very similar looking at those charts Ridge in Mid Atlantic, Trough over West Europe/UK- colder flow from the N/NE. So very useful data to keep an eye on to see whether the trend remains blocked/colder or whether the models move away from this idea

Well its about time to do the full update with regards to Wednesday mornings possible snow event over Wales. In this analysis i will try and factor in a few models including NAE, GFS, UKMO + FAX charts (12Z data). So lets take a look at the setup as shown on the NAE. We see how todays flow has come from the NW: http://expert-images...120415_0412.gif By Midnight tonight that area of low pressue drops southwards introducing an even colder source of air from the NE, this low is also our source of precipitation shown on the NAE by midnight: http://expert-images...120500_0412.gif < Note the colder upper 850hPa air moving in towards our north. During the early hours of the morning we see the occluded front push through so an enhanced area of precipitation likely as shown on the NAE charts below: http://expert-images...120503_0412.gif < 15h http://expert-images...120506_0412.gif < 18h http://expert-images...120509_0412.gif < 21h Notice how the front is heavier to the east over east Wales something which the NAE was showing yesturday. Is this consistent with current GFS model guidence? Lets take a look at the GFS precipitation charts (12z) for the same timeframes: http://cdn.nwstatic....2/15/ukprec.png < 15h http://cdn.nwstatic....2/18/ukprec.png < 18h http://cdn.nwstatic....2/21/ukprec.png < 21h And the UKMO 12Z precipitation chart: http://www.meteociel...8-594.GIF?04-18 Looking at the above data NAE & GFS 12Z i think theres is good enough consistency to say that the front will be heavier towards the east and less organised towards its western flank. UKMO signal not as clear as this is not run at higher resolution but it does show the general idea of a front pushing southward. Precipitation is alsways hard to pin down and can change so it is also worth watching the radar. Now what about the upper 850hPa temperatures and are they at or below -5C which is what is generally the figure we need to look for when discussing snow possibilities. Again using the NAE 12Z data for the same timeframe as precipitation moves through: http://expert-images...120503_0412.gif < 15h -5c to -4C upper temps into N & E Wales http://expert-images...120506_0412.gif < 18h lowering further with -4C temps across south Wales -6/-5C across Mid, N, E http://expert-images...120509_0412.gif < 21h By this point -5C upper air covering most of Wales. GFS and UKMO agree with the -5C upper air temps moving across Wales: GFS @ 21H : http://cdn.nwstatic..../h850t850eu.png < -5C upper air right across Wales UKMO @ 18H: http://www.meteociel...U18-7.GIF?04-18 very simular with the -5C line lying roughly across South Wales. Looking at the sub 528 Dam air during the same period on NAE: http://expert-images...120503_0412.gif http://expert-images...120503_0412.gif http://expert-images...120506_0412.gif As you can see the colder air floods more or less south and west by early morning and the whole of Wales is in sub 528 dam air by 21hours (Early wednesday morning) GFS in agreement also: http://cdn.nwstatic....hgt500-1000.png We must then look at the dew points that the NAE is modeling for the exact same period as precipitation moves through in the early hours: http://expert-images...120503_0412.gif < 15h - Dew points above freezing for Wales although notice the lowering to NE. http://expert-images...20506_0412.gif <18h - 0C down to -1C in E/NE Wales when Precipitation is present. http://expert-images...120509_0412.gif < 21h - Below freezing -1C to -4C across Wales while again precipitation is passing through - Remember heavier precipitation focus is shown towards east Wales. So what about the temperatures by early morning (21hours) on the NAE: http://expert-images...0509_2_0412.gif As you can see Sub Zero central & East Wales/ 1C around N Wales/ 1-3C around southern Wales roughly then higher towards the SW at around 4C. And of course lastly the MO fax chart for tomorrow: http://www.meteociel...ax24s.gif?04-12 By this stage you can see the occluded front has pushed south of the UK and we are left with a cold North flow with a ridge of high pressure building in from the west. Taking all of the above into consideration the greatest chance of Snow will be in the E & NE Wales probably a covering in some places, even some snow falling in inland parts of SE Wales. For South Wales i think a wintry mix more likely under what is modeled as light precipitation perhaps snow to higher ground, Simular story for West Wales where tempeartues are slightly higher so mostly rain with snow to higher ground.

I agree Wednesday morning is where snow prospects will be greatest, on the back edge of that front, as always timing will be an important factor. Front currently showing to push through during the early hours of the morning, so where temps and dew points will be lower. Dew points drop behind that front but are not all that high ahead of it to be honest (1-2C) pre- front and then falling to sub zero as the front pushes more and more south. NAE guidance does show a spell of Snow for NE/E Wales, will see what the models show the precipitation intensity to be tomorrow as it heads south, as i mentioned above heavier towards the east but that can change.

Cheers for that, going through those charts now on the NAE, Colder air covering more or less the whole of the UK by Wednesday morning. Will be interesting to see how the NAE/NMM model the intensity of this front as it pushes southwards. NAE Keeps the most intense section of the front over East Wales/West Mids before dropping into the SW. Will see what it shows tomorrow morning.

Ok well the 18z model output for the GFS and NAE is now out so lets take a look at the snow prospects for Wales over the next few days. If we start with tomorrow we have a few things a cold NW Flow with upper air temps of around -3C down to -5C filtering through in a few spots show on the NAE here: We have sub 528 dam air over Wales more or less all day apart for later in the evening where it tends to wrap itself around the area of low pressure so slight mixing out of colder air during this point before the low pressure drops south allowing a more northerly flow once more re-introducing the colder air and sub 528dam: < NAE @ Midday tomorrow showing sub 528dam < Slight rising of sub 528 dam air as warmer air gets pulled in more or less from the west over Irish sea. < NAE Showing colder air being re-introduced by Wednesday morning As that low drops south Tuesday night into Wednesday morning some precipitation will move southward along an occluded front as shown by the Met Office fax chat: http://images.meteoc...fax36s_htl6.gif Dew Points generally around 3C down to 0C across Wales (Higher to the west coast colder to east Wales). So showers tomorrow which should get going quite fast and get pushed inland on a fairly strong NW flow will be of rain and hail on immediate western coasts turning more wintry in nature inland especially over high ground. Showers that get pushed into central Wales may well contain some snow more so in the heavier showers. With regards to the front pushing down from the North i think some snow is certainly possible on the back edge as the colder air catches up with the front. Something to perhaps consider here is you usually find the colder air undercutting evaporates the precipitation as it heads south & also factor in as the front goes over the higher ground of mid Wales. A spell of sleet and snow is possible i would say again in central and eastern Wales where the precipitation is shown to be heavier on the NAE and the dew points generally lower. A wintry mix in low ground southern Wales on the back edge of the front. So an interesting few days at least with regards to weather prospect's Looking further ahead here is the ECM 0z Ensemble mean i saved on 30th Nov (Friday) for 7th december: Now here is the ECM Operational run today at 96 Hours (for same time): Trough over Western Europe and UK, Ridge over Mid Atlantic the ECM Mean also picked up on the general NE Flow which is now showing on the ECM operational model for 7th December. Giving the fact i have watched this over the last few days i would place high confidence in this outcome with a general NE Flow by the end of the week as the low pressure clears eastwards. Indeed Colder rather than milder then that ridge to the west perhaps having some more influence on our weather as it builds closer towards the UK.

Thanks for posting the above charts, I agree with your current thoughts and will do the full NAE update this evening for Wales with regards to snow and what might come after this period.

Just to add some thoughts on Sundays incoming front from the Atlantic. Looking at the MO fax chart (0Z) a warm front will move in from the west, behind it slightly warmer air moving in: NAE Shows how the upper air temperatures rise fairly quickly to positive values as the precipitation moves in: < 850 hPa Upper temps on the rise < Precipitation advancing from the West. Dew Points also above 0C apart from NE Wales where perhaps these vales will hover around freezing maybe just below before gradually rising: Rain the main focus of this event however some Sleet/Snow initially over NE Wales especially the higher areas. But as we now enter December the question is will there be a return to cold for the festive season or will milder weather take over. Lets take a look at the ECM Ensemble means for 7th December now (144hours out) As posted yesterday ECM 0Z for 7th Dec: And today ECM Ensemble mean 0z also for the same timeframe (7th dec): Still looks similar to me with heights in the mid atlantic and a trough over W Europe, so a colder flow from a northern quarter. What perhaps is more noticeable now is the slight eastward shift in the heights the atlantic towards the UK. Will be keeping an eye on this over the next few days.

Indeed it seems like things are going to stay rather more on the colder side as opposed to mild. Lots of interesting discussion going on in the Technical Model & Stratospheric threads with regards to the polar vortex being allot weaker and not as organised. Here are the Ensemble means from both the GFS & ECM 06Z & 0Z @168hours (Later next week) http://images.meteoc...H1-168_rre8.GIF < ECM 0Z http://images.meteoc...H0-168_gzo4.GIF < ECM 850hPa Upper air temps http://images.meteoc...-1-168_sda6.png < GFS 06Z http://images.meteoc...-0-168_uzl5.png < GFS 850hPa Upper air temps Looking at these charts and one thing really does stand out is that ridge in the mid Atlantic and with no large vortex or more simply no intense area's of low pressure over Southern Greenland. So the idea of this ridge going further northwards up into Greenland is pretty feasible judging by these current ensemble mean charts. Trough over western Europe & UK so generally colder conditions from the North, these are just ensemble means so no good picking out any finer details other than theres a higher chance of it being colder than milder if this signal remains consistent.

So it's getting to that time of year again when the prospect of cold gets greater and we are certainly seeing that in the models now and over the last few days. The models are fluctuating with certain features such as positioning of troughing/low pressure which in turn will determine where exactly the colder air gets directed. Just to show using both the ECM and UKMO 0Z models as an example you can see how heights/high pressure builds just to our west and also the far NE just above Northern Europe with a trough digging down into Western Europe introducing a much colder feed of air. ECM @ 72hours: http://images.meteoc...CH1-72_jnx9.GIF UKMO @ 72hours: http://images.meteoc...N72-21_lob0.GIF At this stage i wouldn't be to focused on whether it will be snowy here in Wales. A flow from the North East/East will be good for introducing colder temperatures once the cold is in place then we can watch out for surface features such as troughs and fronts which could bring wintry precipitation if its cold enough. Back to whats going on now and a lot of rainfall to come again over the next couple of hours- precipitation accumulation shown by NAE: Gradually this will move North Eastwards by tomorrow morning along with its associated area of low pressure: Will be very interesting to watch the models over the next few days and see how it all works out.

The trough's that you see drawn on the Met Office fax charts as solid black lines are associated with an increased area of cloud and precipitation for example 06z Fax Chart for Sunday Morning: http://www.meteociel.../fax/fax24s.gif You can see a trough moving through on the NW flow just off the coast of N Ireland, so increased shower activity likely there. However the main problem for shower activity as we go through the night into tomorrow will be the ridge of high pressure building in as shown here on the NAE: http://expert-images...021815_1812.gif Then you can see the ridge build in from the west: http://expert-images...021903_1812.gif So the shower activity we have now across Wales will become limited away from NW Wales i think.

I can also confirm it is snowing in Swansea with some slightly bigger flakes blowing about. Not sticking here though but thats most likely because i am on lower ground.

I agree with you looking at the dew points it shows them rise quite quickly as the front moves in from the west: http://expert-images...020412_0318.gif http://expert-images...020415_0318.gif http://expert-images...020418_0318.gif In this type of setup the further east you are the better, closer to the cold continental air. So Central/ East Wales will be most likely to have the snow, although i think even SE Wales will see a spell of snow away from the immediate coast before turning back to rain as the milder air gradually pushes east.

Just went up to the Clase in Morriston and it was snowing and starting to stick pretty well to the roads, cars and grass. Although below this level there was next to nothing with just rain and sleet. I took a picture sorry for the poor quality but its something:

Good to see some reports of snow, just rain here in Swansea at the moment, sleet likely later as dew points continue to fall. Front making very slow progress Eastwards looks like allot of its energy is also getting absorbed by the high ground of west Wales.

Sorry for the confusion there here is his tweet, its a few tweets down: "doubt there will be much in Abertawe" http://twitter.com/#!/DerekTheWeather The part in my post about snow on higher ground & North Swansea above 100m is just my current thinking

Still looking good for snow in C/E/SE Wales where those dew points are forecast to be lowest: http://expert-images...2918_2_2906.gif http://expert-images...2921_2_2906.gif http://expert-images...3000_2_2906.gif Some charts above from the NAE 06z showing the dew points as we head into the evening. Coastal parts of the SW of Wales are least likely to get snow, including Swansea take a look at Derek's tweets: http://twitter.com/derektheweather More or less saying not much for Swansea/Pembrokeshire and cant argue with him on this looking at the dew points maybe some falling snow in Northern and higher parts of Swansea above 100m or so i would say. Lets see where it goes from here then, and for those who have snow dont forget to take and post your pictures!

Here's the Met Office fax chart for Mid day tomorrow: http://www.meteociel.../fax/fax24s.gif So we have some Atlantic (warm) fronts attempting to make progress eastwards against a slack easterly flow with colder air moving off the continent, so what we have is the milder Atlantic air interacting with colder continental air with potential for snow somewhere along this point. As to where exactly will get the snow its difficult to tell the models have shifted the snow risk eastwards and back westwards today. One thing i did notice was the 0C Isotherm level: http://cdn.nwstatic....degisotherm.png GFS showing it around 500-600m at first rising from the SW thereafter, this is a better level we have seen so far this winter at least. Like what has already been mentioned Central/East Wales will have the greatest chance for falling and sticking snow, this is because of the colder temps, dew points and upper air. As for the West i think some falling snow is possible away from the direct coastline with mainly sleet and rain in the far SW of Wales where the dew points will remain above freezing: http://expert-images...2918_2_2818.gif http://expert-images...2921_2_2818.gif http://expert-images...3000_2_2818.gif

Some differences in the models now with how far east that front gets, the GFS is showing it getting further east while the NAE at the moment holds it out further west. NAE & GFS 06z precipitation charts for same time: http://expert-images...012921_2806.gif < NAE http://cdn.nwstatic....6/39/ukprec.png < GFS UKMO 06z @ 1900hrs (t+42) tomorrow: http://www.meteociel...2-594.GIF?28-06 As you can see also further east. Now look to the Met Office warnings guidence: http://www.metoffice...ings.html?day=2 You will see they have now highlighted the risk of snowfall for western parts of England also which further supports the eastward progression of the front. The further east the front gets the more marginal it will become for those in W/SW Wales and allong southern coastlines. Although for central and east Wales dew points seem to remain low enough to support precipitation falling as snow so a solid chance for these areas in my opinion: http://www.wetterzen...cs/Rmgfs368.gif http://www.wetterzen...cs/Rmgfs398.gif

Just taking a look at the NAE data regarding the possible snow event on Sunday/Monday. One thing that i noticed was the westward shift of colder (upper) air for the same time as seen here: http://expert-images...2912_2_2712.gif < 12z http://expert-images...2912_2_2718.gif < 18z If that shift west continues to happen the frontal systems will also be further west which means little precipitation for Wales. However if that isn't the case i think its quite likely some snow will fall especially over eastern/central parts of Wales, wintry mix in the south gradually turning to snow Sunday night into Monday possibly as the dew points continue to fall as the flow turns more to the east off the cold continent: http://expert-images...2918_2_2718.gif < Temperatures http://expert-images...2918_2_2718.gif < surface flow

I agree was quite surprised to hear it had snowed in Swansea earlier as the conditions were certainly not all that great for snow. NAE 18z still going with that theme of dew points generally above freezing until later in the evening. I think there is a good chance tomorrow of a streamer setting up as the steering winds (@850hpa) turn to the NW and perhaps to the NNW for a short period of time later in the evening: http://expert-images...012718_2618.gif http://expert-images...012721_2618.gif Like i said earlier though marginal conditions still but a wintry mix seems likely with snow over the hillier areas

Didn't have any snow today where i was in Bridgend although we had hail & thunder. Higher parts of Swansea had some wet snow so i have heard. Looking at the NAE 12z it is likely that some wintry showers will continue to effect western/north western parts of Wales tonight. As for tomorrow if we look at the Met Office fax chart for Mid day Friday we can see that their is a trough moving through Wales in the NW flow with an occluded front lying across northern parts of the country associated with the low pressure system seen here: http://www.meteociel.../fax/fax24s.gif The NAE 12z for the same time shows sub 528 DAM air: http://expert-images...012712_2612.gif Upper air temperatures around -4C lowering to -5C: http://expert-images...012712_2612.gif http://expert-images...012715_2612.gif Precipitation moving through most likely in response to that trough swinging through as we have seen above on the FAX chart: http://expert-images...012712_2612.gif http://expert-images...012715_2612.gif http://expert-images...012718_2612.gif Again i think it looks fairly marginal for snow away from hillier areas with most likely rain/sleet/hail to low ground, the reason i think this is because of the dew points which are shown on the NAE to be above freezing around 1-3C as the precipitation moves through as seen here: http://expert-images...012700_2612.gif http://expert-images...012715_2612.gif Although by evening those dew points are at or just above freezing so if any precipitation is still around some wet snow will be possible, especially in central and northern parts of Wales: http://expert-images...012718_2612.gif http://expert-images...012718_2612.gif

Very much an interesting period upcoming, the Met Office 16-30day outlook has spoke about the two possible scenarios for a few days now as we go into Feb both of which are possible as mentioned on their outlook. Just looking at the GFS & UKMO 12z data and the diffrences are clear. The UKMO 12z wants to build the Azores high eastwards centering it over south UK and west Europe. This limits any continental feed: http://images.meteoc...144-21_lgm9.GIF GFS wants to link the Azores high with the Russian high and with some lower pressure in Europe we begin to pick up a colder continental flow : http://cdn.nwstatic....airpressure.png Difficult one to call, again the UKMO 12z today looks allot like the CFS output did back in late Autumn in terms of building higher pressure towards the south of the UK and into parts of west Europe. A cold dry easterly would be beneficial to us in Wales for lowering surface temperatures and also allowing further cooling of the Irish sea. Thus if we see pressure rise towards Greenland later on and we get Northerly/NW flows the moderating effect of the Irish sea will have less of a role than it has had, allowing for a greater chance of snow rather than rain/sleet showers.

Indeed looking at the models we do have a toned down North Westerly blast for the end of the week with high pressure just to our west. Will check the updated Met Office fax charts later but id imagine nothing particularly wintry from this NW shot. Pressure is higher over Wales and the coldest air is further east as shown on the ECM 12z @ 96hrs: http://images.meteoc...CM0-96_aka5.GIF Cold day here today and the temperature is currently 0.8C.