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South West and Central Southern England Regional Weather Discussion 22/02/2018 Onwards


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Sod the snow, Tom Baker is trying to steal your car. 

I want to see this in the morning.  

Good afternoon all ,with the passing of my beautifull wife last Autumn things have been rather busy at home but now starting to quieten down ,so today i,v been for a great walk up local woods .with th

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My outdoor sensor disagrees with the local station. I've got -5.7C. Town has -5C. Nearly a degree difference. These low dew points though.. gosh.

 

Temp -5C  Humidity 68%  Dewpoint -10C   Pressure 1014.0 

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5 minutes ago, AWD said:

My personal target by Friday afternoon is 10cm.  If I can reach this figure, I will be more than happy.

Anything less than 5cm and I will be hurt and disappointed.

Good luck measuring that level snow  in a gale force easterly wind :D

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Just now, EllyTech said:

Has anyone considered that the snow depth estimates are undercooked? The showers today, brief though they were, were dumping a cm each time. Just an hour of blizzard would create havoc. 

i was just thinking the same

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1 hour ago, ajpoolshark said:

yes, just seen that on the ECM, disruption of the parent azores/sw approaches LP causes a daughter LP to form further east along the Channel.......happy days if that verifies!

I spotted that on some of the models this morning...just about spitting out that secondary low. Interesting stuff.

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1 minute ago, EllyTech said:

Has anyone considered that the snow depth estimates are undercooked? The showers today, brief though they were, were dumping a cm each time. Just an hour of blizzard would create havoc. 

Yeah definitely. What appeared to be a light shower on the radar turned into a full blown 10 minute blizzard.

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A POSSIBLE HISTORICAL WEATHER EVENT - PART 1

Good evening everyone. Well I've finished my business activities for today and I have given myself two and half clear days for blizzard watching (hopefully). I'll really get involved during this evening and tomorrow. I still believe that no model, no forecast or no warning is going to nail this, except by luck more than judgement. This is one of the most highly complex weather pattern set-ups that I seen in all my 57 years of weather viewing (from age 8). There are some quite extreme and conflicting forces at play which just adds to the huge uncertainties.  So much can go wrong and so much can go right. I'll go through the pros and cons later in my next post around late evening. I still want to mention today's snow (because it may well be relevant ahead of the possible "big one" tomorrow) and then I'll consider the pre-conditions to tomorrow's event without making my own forecast at this stage (I will commit later this evening) :)  . 

I've had almost nothing down here in Exmouth. I've been watching the Channel streamer all day and it has been within 2 to 3 miles south of here (and varied between 5 and 10 miles all day). It has hit areas from the Isle of Wight though all southern peninsulas, all of south west Devon, most of Cornwall and on to the Scilly Isles and many of these areas have seen periods with almost continuous heavy snow showers with some substantial accumulations. The streamer had been edging further north and north-west but not as far as I had suggested. In fact, I have exchanged several PMs with a few of you (I often run out of my 5 a day sticky note allowance) and I was trying to cheer up and reassure one of two of you. Again, I apologise for raising any hopes. As i said in my post early this morning, the wind still has a fraction too much due east in it having been mostly only very slightly south of east. Such a waste of good snow out there in the Channel.

There was also an even rarer "Bristol Channel" streamer today - I can only recall one of those twice in my lifetime. Even the tiny Isle of Lundy got snow! The snow showers in the east have made it all the way to Bristol and on into South Wales. If the wind had stayed just north of east, they would have easily made it down into the south-west today. So just 2 or 3 degrees north of east or south of east would have made all the difference with either snow from the east or snow from the streamer. How unlucky is that. Got to laugh or I'll cry :D There's just a narrow swathe of the whole UK which has missing out so far.

The streamer has already lasted longer than the Met O and other weather forecasters predicted and it's still there. In fact it has expanded yet again during the last few hours and is also intensifying.  I imagine that many of you are getting very excited about this. It seems that quite a few more of us will see some snow showers tonight either moving down from the east and/or with the streamer being pushed inland by the more east-south-easterly flow and strengthening winds (see pressure chart section below).

                          1525                                                             1725                                                              1920                                                          Live radar

5a96ead9a951a_1535sr.thumb.png.a937384c5c5020e59702a31f486422bc.png     5a96eac753755_1725sr.thumb.png.90d18f8bfd7aa1a8b0cae9f0a773b546.png    5a9703b8a9127_1935sr.thumb.png.77a427a3c1777db759ab57da425e113d.png     lastsnowradar_uk.gif    

The wind "had" temporarily veered east-south-east this morning and that pushed the streamer to our side of the Channel. Just when it arrived the wind backed very slightly to barely south of due east  (almost due east now) and this explains why the streamer has struggled to move any further north. In an earlier post, I referred to low level convection and this is why the streamer has defied the forecasters and maintained much of its strength. Normal convection is caused by solar energy which heats the surface during the day and in, for example, a Polar Maritime north-westerly, we see the large shower clouds buildup during the day but they disperse rapidly around sunset. A showery air stream will rapidly loose its energy when a LP system advances. The thin high cloud reduces the solar energy and cuts off the source of the convection. What we have now is convection caused by the very cold air becoming very unstable in its surface layers as it moves out over the much warmer sea. It picks up moisture and readily forms clouds and then showers. this is why the snow showers can continue overnight as they do not rely on solar energy. The air/sea temperature contrast is maintained. The longer the sea fetch the greater and more widespread the convection activity is. All are "exposed" coasts have seen frequent snow showers during the last couple of days. The Channel streamer only forms if the air is very cold and the wind is in precisely the right direction. North-east is no good as that crosses 100 to 200 miles of England. East is fine to get it going but east-south-east is ideal. to bring it onshore. They are very rare simply because the source region of northern France is rarely cold enough to provide a sufficient temperature contrast. This week France has been even colder than much of the UK.

I believe that the streamer may still be there (perhaps rather less intense) until the frontal systems from the approaching LP start to dominate. This means that we could see something interesting well in advance of any impact from the LP itself. Furthermore, the wind is set to veer to the south-east as it gets closer. Any remaining showers "should" then be swept onshore. I might be wrong about this but I believe it'll be worth watching out for. The same goes for all the snow showers right down the eastern side of Britain - they are forecasting them to be a lot weaker tomorrow. I feel that as long as the flow continues from an easterly quarter (as it will for all central and northern areas tomorrow) that they'll continue to see strong shower activity. With winds veering into the east-south-east more generally, the flow from East Anglia southwards will be coming over a much shorter sea crossing  In fact with an even stronger wind, conditions might be even more dangerous with any lying snow likely to drift quite severely. Let's look at some more charts.

              Live Standard Satelitte                               Live Infra Red Satelitte                             Live "all France" radar

anim_ir.gif       anim_ir_color.gif      lastsnowradar.gif      

These satellite images will be so important for watching tomorrow's event unfold and they reveal so much. I'll explain them in more detail now to set the scene. In the first image we can see all the clouds over the UK moving from east to west. This is the very cold dense air in place over the entire country. You can also see the cloud associated with the LP pushing steadily north-eastwards. Now, something very interesting. Note that you can see the surface flow in the south-west approaches is still easterly (or just north of east). You can also see it disappearing under the veil of cloud ahead of the LP. Unusually, the surface flow remains in place a long way out to the south-west. All this is even clearer when you view the infra red imagery. Now this is showing us that the flow associated with the LP is riding well over the top of the  surface flow. This is not uncommon with an incoming LP with the warm front extending high cirrus clouds well ahead of it. Eventually the flow usually reaches the surface and displaces the flow in front of it. Occasionally the incoming flow does not reach the ground and the leading front stalls. Then there can be a further push with another front or successive fronts gradually making more and more progress. More rarely, the whole system stalls and either fills up and/or is deflected away north-westwards or south-eastwards. With our huge deep cold block in place, it is not surprising to see such an extreme example of the surface flow forcing the incoming flow well off the surface. I will pick up on this in my next post later this evening as it might be one of the crucial factors going forward.   

              Pressure as at 1750                             Pressure 24 hours to 1820                               Pressure Live                                            Live Isobars                               Fronts & Troughs 1300 Feb 28th

pression2_eur2-17.png   tempresult_bak5.gif   pression2_eur2.png  pression.png   analyse-2018-02-28-12.png

Our faithful Scandi HP has started to decline but it has managed to maintain its position. This may be important over the next few days as it may help to hold the cold block in place for somewhat longer and fend off the attack from the incoming LP. it will also maintain the very cold continental flow. You can see that the flow has been veering more east-south-easterly during this afternoon. The LP is moving inot the frame now too. At this stage it is still on an easterly to east-north-easterly trajectory. This trajectory will be one of the key factors in deciding the outcome through tomorrow and Friday. I will be comparing the live charts with some of those modelled. I'll be posting them up on here regularly.

Right, I'll break for dinner now and carry on in "part 2" with much more later this evening. I'll look at surface temps, dew points, wind chills, 850s and assess the strength of the block. I haven't even looked at the latest model output this afternoon/evening as there are so many solutions on the table. We need to ignore all the hype and try to evaluate what's in front of us. Believe that and you'll believe anything :D I'm as excited as anyone and I'm struggling to manage my own expectations let alone anyone else's!  There is a great chance of a huge blizzard down here tomorrow but there is extraordinary uncertainty. For those of us who have missed out so far,  some might be getting very concerned now. Perhaps Toysarus should have delayed going into administration for several days as they and Mothercare would have sold out of toys and prams if tomorrow ends in another failure :D 

David

 

Edited by Guest
Correct typos and check charts
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2 minutes ago, MidnightSnow said:

Yeah definitely. What appeared to be a light shower on the radar turned into a full blown 10 minute blizzard.

So are we saying 5-6cm per hour? We might be able to judge who is getting huge totals by putting a clock on the blizzard time.

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4 minutes ago, MidnightSnow said:

Yeah definitely. What appeared to be a light shower on the radar turned into a full blown 10 minute blizzard.

I don't think many of us have seen a proper blizzard, when you cant see more than a few feet ahead then your close. I have seen really heavy snow on mendip over years but never a true blizzard. I have only experienced them on ski holidays a few times.

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1 minute ago, Nights King said:

I don't think many of you have seen a proper blizzard, when you cant see more than a few feet ahead then your close. I have seen really heavy snow on mendip over years but never a true blizzard. I have only experienced them on ski holidays a few times.

I have actually. That's why I'm slightly terrified. 

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2 minutes ago, Nights King said:

I don't think many of you have seen a proper blizzard, when you cant see more than a few feet ahead then your close. I have seen really heavy snow on mendip over years but never a true blizzard. I have only experienced them on ski holidays a few times.

Yeah maybe a blizzard was an exaggeration, my bad.

What's the word for snow coming down hard and windy? Basically it was that.

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