Winter Model Discussion 18Z 16/1/13 onwards.

[patrickshere]

Discount the GFS entirely at your peril guys. Whilst it is showing a different (unfavourable) outcome to the ECM and UKMO, it's now a couple of consecutive runs that have picked up on it, and it may have latched onto something the other models haven't yet. It's not unheard of for the GFS to pick-up changes like this first, or for things looking good to die even within T+72.

Just err on the side of caution a bit, I'd focus on Friday, and once Friday is done - then look to what is happening beyond that.

18z NAE is a downgrade on precipitation amounts, spread of significant snow cover and duration of the event friday for anyone outside of the SW. Still time for that to change though, and things to be brought back slightly west.

Edited January 16, 2013 by mituozo

[chionomaniac]

Not a bbc forecast discussion thread - please stick to models

[SnowObsessor90]

The GFS does seem to cotton on eventually, too this doesn't happen until deep FI.

[Paul T]

The way in which the GFS 18Z deals with the pressure to the south of the UK, over France and into the Biscay between 102hrs to 144hrs is nothing short of farcical.

Just have a little run quickly through this period, its shocking!

[Ice Day]

If I was mad and wanted to be run out of town I'd put a map up showing the snow areas with margin for error. As it is I'll keep quiet and wait for the morning model output!

Aw come on Nick. We're all grown ups (!), give it to us with both barrels.....we can handle it

[AGAL]

there yellow warning on the met site

Yellow warnings on the met site mean nothing, they seemed to have had almost the whole country on an orange alert since the weekend with the only colour Red left to go, being a National Emergency. So I wouldn't read too much into the gravity of any situation with UKMO as they are stark raving H&S mad nowadays

Edited January 16, 2013 by AGAL

[Severe Siberian icy blast]

Interestingly the gfs still picks up the greeny high signal post t300 , trends.

[swfc]

Is anyone really watching the 18z? I have a feeling no one believes the 06z and 18z gfs runs to the extent they are treated like nogaps!

totally agree.Its fine when it showing east to west movement of a front but the synoptics are rank app?????.wel see what tom brings but it could be a concern

Interestingly the gfs still picks up the greeny high signal post t300 , trends.

yep it certainly is and as reliable as forecasting a front coming into the uk friday

[pistolpetes11]

For People saying to Bin the 18Z or even the 6z , what do you make of this :

http://www.americanwx.com/bb/index.php/topic/32149-gfs-12z-and-0z-runs-compared-to-6z-and-18z-runs/

I debated whether or not to try and go through all this again, but I'll try. First, in terms of total volume of observations, you're first statement is dead wrong. The radiosonde network only forms a very tiny portion of the global observing system (though it is hugely important).

I have no idea what you mean when you say that the radiosonde data is "out of date" for 6z and 18z. At NCEP, we (currently) use a 3D analysis scheme with a 6 hour time window. If you use 12z as an example, we only assimilate observations that are taken between 09z and 15z, for that given analysis. There are ways to use observations within a longer time window, and actually take their time information (and propagation) into account (4DVAR), but we'll save that subject for thread.

In terms of aircraft data, your comment is sort of true if you focus only on a small portion of the globe (i.e. CONUS). I took the liberty to get a rough estimate for the number of aircraft observations that we assimilated, by cycle, from 00z-18z today, and here are the approximate numbers:

67610, 52744, 53490, and 72177.....not as much disparity as people think. This is partly because these are global numbers. However, we actually get a lot of observations from package carriers that operate aircraft (think Fedex and UPS). Here is an example of a distribution from 06z on the 24th:

This is only for the cruise altitude (150-300 mb) observations that were taken;, nor have I shown the obs for type 231 (that's why the numbers don't match up with what I provided above).

Keep in mind that for any given 06h window, we basically have global observation coverage (tons and tons of multi-channel IR, MW and visible satellite data, GPS radio occultations from space, satellite derived atmospheric motion vectors [satellite winds], surface observations, ships/buoys, aircraft, wind profiler, radar, in addition to the radiosonde network). Because of hyperspectral satellites, the number of observations we have access to for any given cycle is on the order of hundreds of millions (though there is a lot of redundant information)....and the number assimilated is on the order of several million (I'm too lazy and tired to get the exact counts right now).

Back on topic....now, I feel that the biggest differences between 06z/18z and 00z/12z can be seen for individual, high impact events....where better in-situ sampling can make a difference [example: some critical shortwave that is important for some downstream development or something entering the North American Raob network].

I personally don't think that in a time mean sense the skill between any of the cycles is statistically significant (mind you, this certainly wasn't always the case). However, every time I try to put together evidence to make my case, I can always find metrics, levels, or periods that seem to suggest one thing is better than the other.

We use AC scores too much, so bear with me. Here is a long time series showing the GFS AC at 500 hPa for all four cycles (the top panel is a three month moving average, and bottom panel shows the difference relative to 00z):

Notice how before 2008 that the red and blue curves are almost exclusively below the 0 line, but within the past few years the red/blue do poke above the zero difference line (and the spread has shrunk somewhat). I can assure you that the differences (at least from the middle of 2007 and beyond) are not statistically significant. Also note that for the last year that 12z seems to be doing a little better than the other cycles. However, to argue against myself.....and as the above plot would suggest, you can find individual periods that would seem to suggest some cycles are better than others (here is one recent example....a small sample size where the differences are not statistically significant):

I don't have time to do an extremely thorough evaluation using other metrics/levels, etc., but the bottom line (in my opinion) is this notion of 06/18z versus 00/12z is more myth than fact (at least in the recent NWP era, and in a time-mean sense).

Another thing to keep in mind, these metrics are looking at forecasts of the same length (120h forecasts in this case). In the short range, an updated forecast is always going to be more skillful than an older forecast valid at the same time because of the assimilation of observations. I'll use a 2 day forecast as an example:

The 42h forecast initialized at 6z is going to be better than the 48h forecast initialized at 0z (regardless of what you think of 6/18z in general). In fact, if no observations were assimilated at all at 6z, the two forecasts would be identical (since the assimilation scheme is an update to a short term guess, or 06hr NWP forecast)

[nick sussex]

Aw come on Nick. We're all grown ups (!), give it to us with both barrels.....we can handle it

Best wait for the morning output, its not just how far east the front will get but how far the precip advances ahead of it, these set ups are a complete nightmare to forecast.

For that reason even though we know the safeish snow zones the zone either side is still far from being sorted out.

I have everything crossed that as many people as possible can get some snow. And at least there are a few opportunities given the set up.

[Jason M]

Interestingly the gfs still picks up the greeny high signal post t300 , trends.

Yep, surprised this hasn't got more attention as a severe outbreak from the NE is a growing signal in the models. Pretty much in line with GP thoughts as well.

Jason

[patrickshere]

pistolpetes11, Good post, about time someone put that stupid myth to rest.

Edited January 16, 2013 by mituozo

[hammerb32]

Yellow warnings on the met site mean nothing, they seemed to have had almost the whole country on an orange alert since the weekend with the only colour Red left to go, being a National Emergency. So I wouldn't read too much into the gravity of any situation with UKMO as they are stark raving H&S mad nowadays

Clearly they're not, the potential is there to warrant and justify the yellow alerts.

Edited January 16, 2013 by hammerb32

[Timmytour]

The kind of short term corrections that we see on the moedls now are a lot rarer in summer....the mild v heat summer battles do not have the introduced complication of precipitation types and snow fields to contend with

That's why there's never a comfortable feel to a forecast of snow that's from three or four days out...... chances are if you are just outside the firing line at this distance you'll be well in it come the actual event.

[TEITS]

Best wait for the morning output, its not just how far east the front will get but how far the precip advances ahead of it, these set ups are a complete nightmare to forecast.

For that reason even though we know the safeish snow zones the zone either side is still far from being sorted out.

I have everything crossed that as many people as possible can get some snow. And at least there are a few opportunities given the set up.

Have to be honest Nick im not surprised the models are beginning to be less keen on moving this front E. I said in the regional discussion thread this afternoon that members in E Anglia need to be cautious and to not get their hopes up too high. Sometimes in these situations you are actually better off using experience from past events than using the models.

So im going to come off the fence and say those in E Anglia/SE might only see a dusting and only that applies to those in the W of these regions. I can see locations such as Norwich remaining dry. I reckon the extent of this front will be probably around Birmingham and even the W Midlands will see this front weaken. I would love to be wrong but I remain unconvinced of the BBC forecasts and feel the Met O warnings are OTT.

[Froze were the Days]

How much confidence is there on this board that the Euro's and a few other models are far closer to the mark than the GFS ? does make you raise an eyebrow when you similar runs and it has been shown a similar evolution to the break down for yonks (even it keeps being put back).

[Paul T]

Yes, pistolpetes and Mituozo,

Not only does this chart dispell the myth about the GFS 06 and 18z being more 'unreliable' but it actually confirms that the GFS 00Z, 06Z, 12Z and the 18z are in fact ALL more unrealiable that the ECM 00z and 12Z!

I can rest easy now

Edited January 16, 2013 by Paul T

[TEITS]

How much confidence is there on this board that the Euro's and a few other models are far closer to the mark than the GFS ? does make you raise an eyebrow when you similar runs and it has been shown a similar evolution to the break down for yonks (even it keeps being put back).

Let me put it this way I have stopped viewing the GFS past +72 which is why my posts in this thread have lessened.

The performance of the GFS prior to and since this cold spell began has been dreadful. We had the farce of the GFS wanting to take a SW NE when instead it passed off the Essex coastline. This model has continued to want to bring an end to this cold spell and yet this continues to be put back. The UKMO on the other hand has constantly kept the UK under a cold spell and only when the GFS gets in the +96/+72 timeframe does it realise its wrong.

Obviously it would be foolish to totally dismiss the GFS but as the ECM and the rest of the models disagree I feel its very unlikely the GFS is correct at the moment.

[PompeyFC]

what effect will that little shortwave have which has appeared on the 72hr fax chart in the channel?

[Dean E]

Yellow warnings on the met site mean nothing, they seemed to have had almost the whole country on an orange alert since the weekend with the only colour Red left to go, being a National Emergency. So I wouldn't read too much into the gravity of any situation with UKMO as they are stark raving H&S mad nowadays

This is a very misleading statement. The Met Offices' job is to give the UK a reliable and accurate forecast as a standard. They have more information than anyone else and are reconised as one of the best weather services in the World.

They have given most of the country a Yellow warning, as this is going to be a 'widespread' snow event. But there are many disagreements between models and predictions in what will happen, they have 'warned' people to be aware. Its their job and they have a duty to inform the public.

Sorry for going of on one mods. Its unfair for new-commers to be bombared to disbelieve the Met.

Anyway...

GFS is a upgrade for the SW in terms of the Snow line, with Parts of Dorset and now Devon coming into play for once. Less progress westwards in FI (in my mind is around +120hrs!)

Sending shortwave into Biscay at +138hrs and then trying to find a place for the Azores high... Crazy!

SM

Edited January 16, 2013 by SnowMania

[ukpaul]

Have to be honest Nick im not surprised the models are beginning to be less keen on moving this front E. I said in the regional discussion thread this afternoon that members in E Anglia need to be cautious and to not get their hopes up too high. Sometimes in these situations you are actually better off using experience from past events than using the models.

So im going to come off the fence and say those in E Anglia/SE might only see a dusting and only that applies to those in the W of these regions. I can see locations such as Norwich remaining dry. I reckon the extent of this front will be probably around Birmingham and even the W Midlands will see this front weaken. I would love to be wrong but I remain unconvinced of the BBC forecasts and feel the Met O warnings are OTT.

I think it's more complex, the front is splitting and the south coast does seem as though it will benefit a while longer from any snow, through to Saturday evening. You can see things pepping up on the NAE at 48 hours through the channel. You can see a pivot from around the Staffs/Shropshire area beyond 48 hours (on WRF), taking it further west than now progged up north but further east down south. Being where I am I've been very eagle eyed in watching that transpire!

[Thundery wintry showers]

Certainly some classic snowy synoptics showing up on tonight's model outputs with Atlantic weather systems having a real struggle making progress against a Scandinavian blocking high which is forecast to increasingly ridge across to the north of the UK. Many parts of the country are thus in with a major shot at frontal snow while the easterly component to the wind direction should bring a fair number of snow showers to the east of high ground to the north of the fronts. Chances are some areas, most likely the western half of Scotland and the far south-west, will miss out, but the majority of the country should get at least some lying snow by the end of the weekend.

As others have noted the models are increasingly placing the main frontal belts further south and west as we get nearer the time- this means that the chance of frontal snow over the north and east of England is diminishing, but this does also mean increased chance of those same areas getting a significant supply of convective snowfalls off the North Sea, while the chance of snowfalls penetrating to low ground over much of Wales and south-west England is increasing with each run. At present a belt extending from Merseyside SSE-wards to central southern England appears to be in the firing line on Friday.

There is no definite end in sight for this cold spell- the Atlantic will probably get in eventually but it looks unlikely to happen before the middle of next week now, at least away from the south-west. I think it is unwise for snow lovers to worry too much about any breakdown as we have a lot of cold and potentially snowy weather to get through before then, and the UKMO and ECMWF both keep the UK in the freezer beyond then.

Edited January 16, 2013 by Thundery wintry showers

[PolarWarsaw]

Can someone link me to the NAE and NMM models and the times they update? Sorry on my phone.

[bradymk]

GFS Ensembles also showing a westward shift of precip extent, past experience showed us to expect this but a bitterly cold weekend to come for sure, snow potential increasing once again next week for all and as TWS says above, the front further S & W could result in more convection for north eastern areas.

[Bonjangle]

Certainly some classic snowy synoptics showing up on tonight's model outputs with Atlantic weather systems having a real struggle making progress against a Scandinavian blocking high which is forecast to increasingly ridge across to the north of the UK. Many parts of the country are thus in with a major shot at frontal snow while the easterly component to the wind direction should bring a fair number of snow showers to the east of high ground to the north of the fronts. Chances are some areas, most likely the western half of Scotland and the far south-west, will miss out, but the majority of the country should get at least some lying snow by the end of the weekend.

As others have noted the models are increasingly placing the main frontal belts further south and west as we get nearer the time- this means that the chance of frontal snow over the north and east of England is diminishing, but this does also mean increased chance of those same areas getting a significant supply of convective snowfalls off the North Sea, while the chance of snowfalls penetrating to low ground over much of Wales and south-west England is increasing with each run. At present a belt extending from Merseyside SSE-wards to central southern England appears to be in the firing line on Friday.

There is no definite end in sight for this cold spell- the Atlantic will probably get in eventually but it looks unlikely to happen before the middle of next week now, at least away from the south-west. I think it is unwise for snow lovers to worry too much about any breakdown as we have a lot of cold and potentially snowy weather to get through before then, and the UKMO and ECMWF both keep the UK in the freezer beyond then.

Cracking post and well said and understanding for us Newbies , keep them coming like this .. love reading posts that i can understand without the complicated words that i cant get my head into , thanks for sharing