Spring Model Discussion 18Z 6/3/13 onwards....

[iamgazza]

METO have just issued weather warnings for snow Sunday night for the whole of the south.

Accumulations of 3 - 5 cm could occur quite widely over southern counties.

Wrong thread but worth noting...

[matty40s]

well looks like the GFS was spot on for the Midlands tommorow, the Met has gone for it almost exactly.

[ITSY]

The 06z nae has moderate snowfall for the northern england the midlands for 18 hours straight startin tomorrow afternoon!whether it settles is a different matter!!

I've see that too, looks impressive, but I can't see it happening given the upper temps, surface temps and the origin of the precipitation. We'll see though, you never know!

[shaky]

I've see that too, looks impressive, but I can't see it happening given the upper temps, surface temps and the origin of the precipitation. We'll see though, you never know!

you never know because the dewpoints are just about okay and although the 850 temps are at -2 it can still be condusive to snowfall!!and also surface temps are at 1 or 2 degrees!!just gotta see how things pan out!!

[nick sussex]

METO have just issued weather warnings for snow Sunday night for the whole of the south.

Accumulations of 3 - 5 cm could occur quite widely over southern counties.

Wrong thread but worth noting...

I don't want to be accused of ramping but if everything falls into place then that's a conservative estimate. Given the steep temperature gradient and possible renewed energy from the low in nw Biscay then those estimates could change, theres also the possibility the heavy snow could be further south in the Channel and northern France.

That's why its very uncertain, could be great for snow lovers or a bit frustrating, there should be some snow though because the cold front has to clear the south coast, so I think its more how much than whether it will snow.

[No Balls Like Snow Balls]

The ECM 00Z op run upto T144 (London) shows Monday as the only very cold day (1c). Sunday has a max of 4c, i expect the very cold air isn't expected to reach this far South until Sunday evening. Tuesday sees it turning less cold (3c), Wed's continues the less cold theme 6c and Thurs see's rain forecast with a max of 5c. Rain could possibly turn to snow Sunday afternoon / evening. Mon is dry but cloudy. Tues is also dry but sunny, and Weds see's a chance of sleet / snow showers.

The ECM ensemble mean also agrees with temps becoming less cold from Tuesday.

GFS 06z upto T144 (London)

This also shows the very cold air not reaching this far South until Sunday evening at the earliest. Max for Sun 5c, Mon is very cold 1c, there is the possibility on the GFS that London keeps hold of the very cold air until Tues, max 1c. But then by Wed's it becomes less cold, max 4c, Thurs less colder still, max 6c.

Sunday also see's a chance of rain turning to snow on the GFS. Mon also sees a chance of snow for London from a front coming up from the South coast.. Tuesday looks fairly dry. Wed's could see some scattered light sleet / snow showers. And Thursday see's more general rain arrive.

Overall quite a similar theme from both ECM and GFS. GFS probably with better snow chances. but both agree turning less cold from Tuesday onwards, with the possibility of more general rain on Thurs.

Edited March 8, 2013 by No balls like Snow Balls

[Purga]

I don't want to be accused of ramping but if everything falls into place then that's a conservative estimate. Given the steep temperature gradient and possible renewed energy from the low in nw Biscay then those estimates could change, theres also the possibility the heavy snow could be further south in the Channel and northern France.

That's why its very uncertain, could be great for snow lovers or a bit frustrating, there should be some snow though because the cold front has to clear the south coast, so I think its more how much than whether it will snow.

Agree , at the moment it's looking pretty damned good for us folks on the normally snowless south coast (W Sussex in my case) and if we get plastered on Monday / Tues I will follow suit and get plastered myself in celebration - LOL

[mushymanrob]

The 06z nae has moderate snowfall for the northern england the midlands for 18 hours straight startin tomorrow afternoon!whether it settles is a different matter!!

bbc had temps of 3c under the snow area, im in it, im expecting wet snow that doesnt settle, or melts rapidly once the heaviest is over, plus the ground here is wet...

i notice this morning that by midweek the models are rather messy, with no clear, consistent, synoptic pattern emerging.

[mushymanrob]

. If you have any questions after reading this please send me a pm and I will do my best to answer you-I am away Monday-Friday next week though.

John

it says you cant receive pm's atm.... inbox full?

[SomeLikeItHot]

I have been looking at the various GFS ensemble runs over the last few days and its curious to note that while the METO and ECM have always been hinting at the possibility of later and weaker shot of cold, that the GFS op and with strong ensemble support were refusing to have a bar of it.Certainly in terms of the Op run it appears the GFS has modelled it better. However when you look at what has been going on with its ensembles you have to wonder a bit.

As we get closer to the cold on the 11th-13th rather than the spread in the ensembles reducing it has instead grown, suggesting that perhaps the cold plunge is not nailed on as tight as it suggested earlier.

So for my area - as that is what I saved charts for we see on the 0z on the 6th we get every single ensemble member suggesting at least -12 uppers on the 11th.

By the time we move to the 0z today, we are suddenly seeing that there is a possibility the shot will be less cold than before. With 3 ensemble members going for significantly less cold for the 11th -13th.

Come to the 6z and we now have 4 members who don't bring cold 11th to 13th.

It should be an expected property of ensemble modelling that the uncertainty decreases with time. I am wishing I had saved the equivalent for the ECM but I didn't certainly though yesterday they still showed a small cluster of slower and warmer members.Now that seems to have vanished.

EG The below - some spread but broadly in agreement there through the next few days.

The Ecm behavior is what we would have expected from an ensemble set.

To me this suggests that the GFS ensembles just aren't particularly good at giving a reasonable coverage of the range of future outcomes.

We also saw something similar in the run up to the cold spell at the end of Feb. When the ECM with good (but not universal) ensemble support went for a colder solution, the GFS with really solid ensemble support suggested something mild, only for the whole bundle to switch to a completely different grouping later.

So I am interested is there verification stats on how often the ensemble range verifies? It seems to me that this suggests that the GFS ensembles just aren't broad enough. To me its acceptable for an op run to get an evolution wrong say 5 days out, so long as the ensembles support a strong possibility of the alternative, but having an ensemble miss the alternative whatsoever at that range is more concerning.

Edited March 8, 2013 by SomeLikeItHot

[johnholmes]

it says you cant receive pm's atm.... inbox full?

ok I'llgo take a look it is a while since I cleared things out

looks ok now only 4, sorry 5 messages left in there now?

Edited March 8, 2013 by johnholmes

[swilliam]

I have been looking at the various GFS ensemble runs over the last few days and its curious to note that while the METO and ECM have always been hinting at the possibility of later and weaker shot of cold, that the GFS op and with strong ensemble support were refusing to have a bar of it.Certainly in terms of the Op run it appears the GFS has modelled it better. However when you look at what has been going on with its ensembles you have to wonder a bit.

As we get closer to the cold on the 11th-13th rather than the spread in the ensembles reducing it has instead grown, suggesting that perhaps the cold plunge is not nailed on as tight as it suggested earlier.

So for my area - as that is what I saved charts for we see on the 0z on the 6th we get every single ensemble member suggesting at least -12 uppers on the 11th.

By the time we move to the 0z today, we are suddenly seeing that there is a possibility the shot will be less cold than before. With 3 ensemble members going for significantly less cold for the 11th -13th.

Come to the 6z and we now have 4 members who don't bring cold 11th to 13th.

It should be an expected property of ensemble modelling that the uncertainty decreases with time. I am wishing I had saved the equivalent for the ECM but I didn't certainly though yesterday they still showed a small cluster of slower and warmer members.Now that seems to have vanished.

EG The below - some spread but broadly in agreement there through the next few days.

The Ecm behavior is what we would have expected from an ensemble set.

To me this suggests that the GFS ensembles just aren't particularly good at giving a reasonable coverage of the range of future outcomes.

We also saw something similar in the run up to the cold spell at the end of Feb. When the ECM with good (but not universal) ensemble support went for a colder solution, the GFS with really solid ensemble support suggested something mild, only for the whole bundle to switch to a completely different grouping later.

So I am interested is there verification stats on how often the ensemble range verifies? It seems to me that this suggests that the GFS ensembles just aren't broad enough. To me its acceptable for an op run to get an evolution wrong say 5 days out, so long as the ensembles support a strong possibility of the alternative, but having an ensemble miss the alternative whatsoever at that range is more concerning.

But if you look at why these do not deliver (the very low 850s) in your area you will see that it is because the low in NW Biscay has more energy, stops the front clearing as quickly and this then pushes N later. This is actually being shown on some of the ops - hence the snow warnings in the S - the actual position of this front is very hard to predict this far out. This actually leads to much more snow but not such itense cold -some may say that is better. The point though is that the actual difference beteen the e ensembles is quite small but has large effect on the 850s in your particular location. So the overall pattern is pretty similar and well predicted but with small local differences. This will not show up in the verifcation stats.

I like the way you say it is acceptable for the op to get it wrong - really they are just models.

Edited March 8, 2013 by swilliam

[Chris W]

Looking like we're still pretty locked into a very blocked pattern from the GFS 06z (a cold run throughout) with a strong jet way south over Africa/Spain, which albeit not great for the mild fan does at least promise to keep the Atlantic at bay and therefore keep us much drier than last year - I'm sure this at least is welcomed by all of us. Could be quite some March if we end up with something close to this in the later part of the month:

[Timmytour]

.

Come to the 6z and we now have 4 members who don't bring cold 11th to 13th..

just one observation on this....are those four members reflecting the position from a few day ago.....ie bringing in the cold later and for longer?

I'd imagine some members might not have found the "trip wire" that most of the others did that accelerated the cold reaching us...and then leaving us.

[SomeLikeItHot]

But if you look at why these do not deliver (the very low 850s) in your area you will see that it is because the low in NW Biscay has more energy, stops the front clearing as quickly and this then pushes N later. This is actually being shown on some of the ops - hence the snow warnings in the S - the actual position of this front is very hard to predict this far out. This actually leads to much more snow but not such itense cold -some may say that is better. The point though is that the actual difference beteen the e ensembles is quite small but has large effect on the 850s in your particular location. So the overall pattern is pretty similar and well predicted but with small local differences. This will not show up in the verifcation stats.

I like the way you say it is acceptable for the op to get it wrong - really they are just models.

I am not particularly interested in "better" or "worse" in this comment. Personally I would prefer just cold enough for lots of snow to extreme cold and no snow sp I agree its an upgrade. What I am concern with in the post though was the apparently falsely stated accuracy of the GFS ensembles.

I do agree though that examining a single point T850 could be misleading, especially given the gradient that occurs on the 10th -11th, but at the same time I anecdotaly observed similar for other locations in the midlands and south. The north has stayed with a similar pattern. Still though, it is saying that the GFS's ensembles modelling of the low was too limited in its range of how strong it would be and now its had to increase that.

just one observation on this....are those four members reflecting the position from a few day ago.....ie bringing in the cold later and for longer?

I'd imagine some members might not have found the "trip wire" that most of the others did that accelerated the cold reaching us...and then leaving us.

It varies one brings in mild cold later then gets quite warm. Others just delay it.

[m1chaels]

I have been looking at the various GFS ensemble runs over the last few days and its curious to note that while the METO and ECM have always been hinting at the possibility of later and weaker shot of cold, that the GFS op and with strong ensemble support were refusing to have a bar of it.Certainly in terms of the Op run it appears the GFS has modelled it better. However when you look at what has been going on with its ensembles you have to wonder a bit.

As we get closer to the cold on the 11th-13th rather than the spread in the ensembles reducing it has instead grown, suggesting that perhaps the cold plunge is not nailed on as tight as it suggested earlier.

So for my area - as that is what I saved charts for we see on the 0z on the 6th we get every single ensemble member suggesting at least -12 uppers on the 11th.

By the time we move to the 0z today, we are suddenly seeing that there is a possibility the shot will be less cold than before. With 3 ensemble members going for significantly less cold for the 11th -13th.

Come to the 6z and we now have 4 members who don't bring cold 11th to 13th.

It should be an expected property of ensemble modelling that the uncertainty decreases with time. I am wishing I had saved the equivalent for the ECM but I didn't certainly though yesterday they still showed a small cluster of slower and warmer members.Now that seems to have vanished.

EG The below - some spread but broadly in agreement there through the next few days.

The Ecm behavior is what we would have expected from an ensemble set.

To me this suggests that the GFS ensembles just aren't particularly good at giving a reasonable coverage of the range of future outcomes.

We also saw something similar in the run up to the cold spell at the end of Feb. When the ECM with good (but not universal) ensemble support went for a colder solution, the GFS with really solid ensemble support suggested something mild, only for the whole bundle to switch to a completely different grouping later.

So I am interested is there verification stats on how often the ensemble range verifies? It seems to me that this suggests that the GFS ensembles just aren't broad enough. To me its acceptable for an op run to get an evolution wrong say 5 days out, so long as the ensembles support a strong possibility of the alternative, but having an ensemble miss the alternative whatsoever at that range is more concerning.

I may be wrong but I think the issue here is that the model ensembles are 'initial condition pertubations' with exactly the same deterministic model applied to each perturbation so if the model is wrong for one it can be wrong for all, in some situations a small divergence in initial conditions has little short term impact on the modelled future and this results in 'good ensemble agreement', in other situations small differences in inital conditions quickly result in divergent solutions.

However I think there is a tendancy for people to think of the ensemble runs and their outer bounds as some sort of probability distribution. When the BoE forecast inflation they actually use a fixed start point but the model is not completely deterministic - it is instead subject to random fluctuation at each time period. A large number (10s of thousands in a typical Monte-carlo simulation, compare this to 20/51 ensembles) of model runs are made and with a probability distribution applied to whether more or less likely random changes take place in each time step there is generated a 'most likely' central path with divergent paths becoming less and less frequently observed the further they are from the central forecast.

Edited March 8, 2013 by m1chaels

[SomeLikeItHot]

I may be wrong but I think the issue here is that the model ensembles are 'initial condition pertubations' with exactly the same deterministic model applied to each perturbation so if the model is wrong for one it can be wrong for all, in some situations a small divergence in initial conditions has little short term impact on the modelled future and this results in 'good ensemble agreement', in other situations small differences in inital conditions quickly result in divergent solutions.

However I think there is a tendancy for people to think of the ensemble runs and their outer bounds as some sort of probability distribution. When the BoE forecast inflation they actually use a fixed start point but the model is not completely deterministic - it is instead subject to random fluctuation at each time period. A large number (10s of thousands in a typical Monte-carlo simulation, compare this to 20/51 ensembles) of model runs are made and with a probability distribution applied to whether more or less likely random changes take place in each time step there is generated a 'most likely' central path with divergent paths becoming less and less frequently observed the further they are from the central forecast.

You are right the ensembles vary the starting conditions to produce their results, and also correct that they don't reflect any error in the model itself. However the Met Office regard the ensembles as probability distribution according to their website.

I quote.

"We design the ensemble forecast system so that each member should be equally likely, so that the ensemble can be used to forecast the probabilities of different possible outcomes."

Edited March 8, 2013 by SomeLikeItHot

[WillinGlossop]

New and improved nae!

http://www.weatheronline.co.uk/cgi-bin/expertcharts?LANG=en&MENU=0000000000&CONT=ukuk&MODELL=nae&MODELLTYP=1&BASE=-&VAR=prty&HH=42&ZOOM=0&ARCHIV=0&RES=0&WMO=&PERIOD=

[Captain Shortwave]

Another weather development has been a slow moving shortwave developing in the Southern north sea which drifts very slowly eastwards. This looks likely to bring a lot of rain to East Anglia and Kent. 1-2 inches possibly coming from pretty much 48 hours non-stop precipitation. The rain also looks like turning to snow Saturday afternoon as reiterated by the met forecast for the region. Doubtful it will settle but looks like a miserable Saturday down here.

Edited March 8, 2013 by Captain shortwave

[SomeLikeItHot]

The NAE snow accumulation shows it falling as snow in Nth East Anglia. Or more likely slush given how fast it departs.

[Captain Shortwave]

The NAE snow accumulation shows it falling as snow in Nth East Anglia. Or more likely slush given how fast it departs.

Given it's quick development. Only showed up on last nights charts (this little system) with a sudden increase of rainfall for the South East corner, I wouldn't be surprised if someone in say West Norfolk/Cambridgeshire/Northamptonshire could get a few centimetres of snow tomorrow afternoon. TEITS would be very happy I think

Edited March 8, 2013 by Captain shortwave

[keithlucky]

Another weather development has been a slow moving shortwave developing in the Southern north sea which drifts very slowly eastwards. This looks likely to bring a lot of rain to East Anglia and Kent. 1-2 inches possibly coming from pretty much 48 hours non-stop precipitation. The rain also looks like turning to snow Saturday afternoon as reiterated by the met forecast for the region. Doubtful it will settle but looks like a miserable Saturday down here.

More snow than was first forecast with the front developing because of the meeting of the two Air masses meeting

[legritter]

Yes a very very interesting wintry spell on its way ,and not right out in FI .there will be winners and losers but lets grab it whilst we can .its going to be a forecasters nightmare at the moment untill we get a bit nearer especially for the south .OK it would have been better a couple of weeks back but looking at charts etc and we have all the ingredients there the rest will be Location and some luck .Quite possibly a Classic set up come late sunday /monday we dont get many but perhaps this could be the one ,better now than come May from a coldies perspective .well off to catch GFS ,and tonights FAX could be a peach ,cheers

[Radiating Dendrite]

+60.................warning channel low!! It is literally moving along the south coast!

Amazing, in December we had over a 1000 members on here watching for a snow event at +144, now we have one (and a possible biggy) at just +60 and only 133 members are interested!

Oh well, their loss!

Edited March 8, 2013 by Radiating Dendrite

[Cheshire Freeze]

I'd be very surprised if there wasn't a significant risk of a snowy trough swinging in from the east with these sort of thicknesses and uppers.

Somewhere near the south coast has potential for 20cm of snow IMO. Strong winds too.

Edited March 8, 2013 by CreweCold