South West/Central Southern England Regional Discussion 03/02/13 12z ------->

[AWD]

A quick glance at what we need to look for regarding any snow potential early next week for our region, in the model output, over the coming days.

What we have developing is a weak block of high pressure to our north/north east in the GIN corridor. This causes an Atlantic depression to undercut the block and "slide" SE over the UK during the later part of the weekend and into next week. Below is the ECM HP500 chart for 72 hours:

I have circled the high pressure responsible for the route the "slider low" is forecast to take. The black circle is a weak high pressure cell to the north of the UK, this demonstrated on the height charts by brighter green, yellow and orange colours. The brown circle shows the forecast route the depression, shown on the above chart over Greenland, is to take. You can tell this by the "pull" in the isobars inside the brown circle, showing the "tug" on the low pressure. Notice at this timeframe we are being influenced by winds of a "westerly" source, so the air won't be particulary cold at this timeframe:

Now if we move forward to 96 hours, here is ECM HP500 chart for 96 hours:

Inside the black circle remains our weak high pressure at northern latitudes. The depression has gone SE from the Iceland/Greenland area on the 72 hour chart to place itself on top of southern UK. On the depressions northern flank is where the colder air is being filtered west from the east, and on its southern flank we have less cold Atlatntic air. Now we want to be on the north side of the centre of the low, where the brown arrows showing a easterly wind are, as it is in this area where the PPN will start to turn to snow. You can see below the upper air temps at 96 hours:

This shows the colder air, shown as blue on the above chart, filtering west around the north of the low. On the ECM model we start to see this colder air filter into our region as early as 96 hours, giving the potential of snowfall quite early on in the process, however below is the UKMO chart for the same time period:

You can see the depression is less deep and managed to get further east compared to the ECM, which means the colder air is filtering around the depression further north and not as quickly, demonstrated by the brown arrows on the above chart, and the grey arrows showing less cold air over our area as we, at this stage are being influenced by a milder Atlantic source. The low pressure, on future UKMO runs, needs to be further south and west for us to see less rain and more snow.

The ECM model at 96 hours already showing an easterly component to our wind:

Now moving onto 120 hours, and we start seeing some very interesting sypnotics for our region. Below is the ECM HP500 chart for 120 hours:

The block, the black circle, still to the north of the UK, gaining a tad little strength, shown by some stronger yellow colours. The depression sat in the near continent to the south east of the UK, blowing strong, cold north easterly winds over the UK, as shown by the brown arrows. This brings cold upper air and cold dewpoints in drier continental air over the UK, including our region.The below chart shows the cold upper air temps, the blue colours:

and the below ECM windchart showing the colder feed of air from the north east:

If this verified, conditions would be prime enough for snowfall across our region. The grey circle on the above HP500 ECM chart shows a weak ridge of pressure in the mid Atlantic from the Azores, preventing the next depression from attempting to make inroads into the UK, and ruin any potential for us. The UKMO model also better for us at this timeframe too, but again, it does show a weaker depression, so how much PPN would be left is open to doubt, and the window of snowfall potential is shorter compared to the ECM. Below is the UKMO HP500 120 chart:

A slightly weaker block, shown by less yellow and more yellow-greeny colour in the black circle. A north-easterly feed of air as per the ECM.

Now finally onto the 144 hour charts. Below is the ECM HP500 144 hour chart:

The block again prevalant to our north. Again, we still have a north easterly wind feed bringing in colder air over our region, but by this time the trough has sunk far enough SE to take any PPN away with it and leave us predominetly dry. Notice the weak ridge in the Atlantic slowly collapsing towards the UK ( the grey circle ). This is because the pull on the depression to the south of Greenland is too strong for it to stay vertical, and this slowly makes inraods into the UK. The ECM wind chart below still shows the north easterly feed by 144 hours:

So thats a brief summary of what we are looking at currently. On future output, look out for yellow colours on the height charts over the GIN corridor, and look out for the centre of the low pressure on Sunday and Mondayy to be somewhere in the centre of the English channe. If it is here, then it has shifted further west and would be good news with regard to snowfall for us. If the centre is around the south north sea area, off the coast of the Netherlands, then it is not so good for us for snowfall. As long as the centre of the low pressure is to the south of our region, and we are on the northern flank of it, then some snowfall will happen at some time over the period in question.

I haven't included any PPN charts as the position and intensity of the PPN is irrelevant at this stage, this is the most likely thing to change a lot between now and the weekend.

Edited February 7, 2013 by AWD

[Erbyskerby]

My brain hurts just thinking about 11 surface features. What a nightmare to try and decipher all that correctly - don't fancy the meto job one bit!

[numbnutts]

Yeah I know what you meant lol, points west weather also mentioned the possibility of significant snow late Sunday info Monday morning and rush hour Monday morning could be interesting.

And before we get the usual posts about snow settling when it starts off as rain, yes it can easily settle when the temps drop, no matter how wet the ground is.

yeh but no but lol

[numbnutts]

the metoffice have weather warnings for sunday and monday now lets hope they stay how they are at the moment

[offerman]

Thanks for that AWD, appreciate your input as always.

[snowmadchrisuk]

Latest NMM Accumulated Snow Chart is below , I'll keep providing updates as the new ones come out ..

[TwisterGirl81]

If finally South Devon, Exeter and Cornwall get significant snow I will eat my words but I do not see it happening, we are not cold enough, the snow fairy has not liked us one bit this winter or last!

I'm sorry to sound negative, I really am not but this winter for me has been constant down grades from snow to a potential windstorm I have zero faith in any unofficial weather forecasts

Edited February 7, 2013 by TwisterGirl81

[Dorsetbred]

Edit: if it can snow over Dorsetbred's head then we should all take this as a huge positive.

Thank you for your support of the area.

AWD's analysis as usual paints the various scenario's and presents a good read.

Looking at the fax charts, It looks more opportunistic however I still see the ground being very wet before any white stuff begins to fall, and temps are not that cold even the WBFL charts show great height over the weekend, and then dropping later on Monday.

As we know it's all about position position position, I'd like to see the low slightly more west initially, before it begins it's slide, then to take a slight east curve over northern france, keeping all the colder air to it's north and east waiting for the drop.

Edited February 7, 2013 by Dorsetbred

[gottolovethisweather]

So thats a brief summary of what we are looking at currently. On future output, look out for yellow colours on the height charts over the GIN corridor, and look out for the centre of the low pressure on Sunday and Mondayy to be somewhere in the centre of the English channel. If it is here, then it has shifted further west and would be good news with regard to snowfall for us. If the centre is around the south north sea area, off the coast of the Netherlands, then it is not so good for us for snowfall. As long as the centre of the low pressure is to the south of our region, and we are on the northern flank of it, then some snowfall will happen at some time over the period in question.

Very good AWD, not a bad resume from just a brief summary.

The section I have highlighted from your post above explains things very well indeed IMHO. As to where we ideally want to see this depression placed, the margin of error is little more than a couple of hundred miles either East or West and (North or South) as you suggest.

If folks now think of where our little island is situated in regards to all of this, you can truly see why things will chop and change to within probably t+12 hours. Also note that all the matters to which AWD mentions have to fall in place before this yet undeveloped surface feature even comes our way. Then there's the pertinent matter of who gets SNOW and how much does or doesn't fall.

Edited February 7, 2013 by gottolovethisweather

[gottolovethisweather]

If finally South Devon, Exeter and Cornwall get significant snow I will eat my words but I do not see it happening, we are not cold enough, the snow fairy has not liked us one bit this winter or last!

I'm sorry to sound negative, I really am not but this winter for me has been constant down grades from snow to a potential windstorm I have zero faith in any unofficial weather forecasts

[Dorsetbred]

If finally South Devon, Exeter and Cornwall get significant snow I will eat my words but I do not see it happening, we are not cold enough, the snow fairy has not liked us one bit this winter or last!

I'm sorry to sound negative, I really am not but this winter for me has been constant down grades from snow to a potential windstorm I have zero faith in any unofficial weather forecasts

Twister there's always a line between negativity & realism. You are being realistic, because that's what's happened. Can't see that being negative by any means. Like all of us, you understand the fine line that's drawn for such events to take place. Keep with it...

So back to the meaning of the thread, now patiently waiting for today's rainfall to begin

Edited February 7, 2013 by Dorsetbred

[gottolovethisweather]

I have zero faith in any unofficial weather forecasts

To be fair, I have little faith in the official forecasts on the odd occasion too. Please remember we are dealing with something undeveloped at t+96 hours or so which is another reason to keep informed of the progress being made.

Furthermore, each and everyone of us knows their own location climatic traits better than any forecaster will ever do. The professional forecasters are often looking at things from a global perspective right down to nearly t+0 and that's way the difficulty lies for location specific forecasts.

Edited February 7, 2013 by gottolovethisweather

[Coast]

Please remember we are dealing with something undeveloped at t+96 hours or so which is another reason to keep informed of the progress being made.

Everyone is struggling with a forecast. Some think they have it nailed and are being very rash with issuing forecasts of anything from baking sun through cold rain and 10ft snow drifts. I'm not sure when we will get consensus, but it's got to be a minimum - 48hrs before hand until we can even start to think about the possible areas and amounts. It's difficult for everyone, it's just that some are being bolder than others!

[stevofunnelcl]

click the more details link on the weather warnings, and you get chief forecasters assessment

Huge uncertainty, and warnings will get updated

So, they start off covering a large area, but i expect the warning areas could shrink, as they firm up on details

Would be great if they stayed like though, perhaps an amber upgrade for some

[Beast of Plympton]

Nice heavy snow at the moment in Princetown. get the dew point down and could be white again tonight!!

[jessandjon]

Nice heavy snow at the moment in Princetown. get the dew point down and could be white again tonight!!

Grrrrr :-)

[Beast of Plympton]

Grrrrr :-)

Sorry

[Coast]

Nice heavy snow at the moment in Princetown. get the dew point down and could be white again tonight!!

To be fair, you are almost in the troposphere up there!! :lol:

[jessandjon]

To be fair, you are almost in the troposphere up there!! :lol:

I must say my ears do pop when going to princetown!!

[Beast of Plympton]

To be fair, you are almost in the troposphere up there!!

I know, your right!! Hard to breath sometimes up here.

I must say my ears do pop when going to princetown!!

Im sure I have gone a bit deaf from that since living here...

[jessandjon]

I know, your right!! Hard to breath sometimes up here.

Im sure I have gone a bit deaf from that since living here...

Pardon???

(Sorry:-). )

[cornish snow]

Nice heavy snow at the moment in Princetown. get the dew point down and could be white again tonight!!

Should build yourselves a ski resort, you would make millions!

[Beast of Plympton]

Pardon???

(Sorry:-). )

Heard that one before.... just...

Should build yourselves a ski resort, you would make millions!

Bleeding good plan!!

[snowmadchrisuk]

Well latest gfs looks pants for our area , by the time it get's Cold enough the precipitation has cleared .. Let's hope for some greater shower potential on Monday.

[Dave Kightley]

Well latest gfs looks pants for our area , by the time it get's Cold enough the precipitation has cleared .. Let's hope for some greater shower potential on Monday.

shows snow by Sunday evening ate