Model Output Discussion - Sunday 20:35--->

[Yeti]

I certainly understand that which is why even im remaining cautious. However in saying that I always believe that there is a possibility that some of these awesome charts will come off. If I had the attitude that all cold spells will go pearshaped based on past disappointments then I would choose a different hobby and stick with the countryfile forecast.

The big difference for me compared to previous disappointments is the UKMO. This model has really been a pain in the backside in the past but thankfully the UKMO is very similiar to the GFS. If it was the UKMO/ECM Vs GFS then I would side with the UKMO/ECM but at the moment im thinking the ECM is wrong. Hopefully tonight we shall get some consistency between all 3 models at +144.

Yes - I've been travelling back home so I didn't see the last two days' models, but just looking at them now things are a lot more interesting than I thought they would be!

As you say the UKMO very much sides with the GFS atm (i.e. the HP extending further N and creating a stronger block allowing the PV to undercut on the Eern flank:

http://www.wetterzentrale.de/pics/Rtavn1681.png

http://www.wetterzentrale.de/pics/Rtavn2401.png

Instead on the ECM the block is lower latitude and sinks SWwards as a result.

Looking at the hemispheric charts -

http://www.wetterzentrale.de/pics/Recmnh1921.gif

http://www.wetterzentrale.de/pics/Rhavn1801.gif

Clearly the PV fails to split fully on the ECM. If you look at the GEM, the PV splits well initially, but those shortwaves S of GL end up slicing the block in two and sending the cold into SE Europe.

http://www.wetterzentrale.de/pics/Rgem2161.gif

This is what I'd be more worried about because often such shortwaves can be picked up at the last minute as we know all too well. Obviously the GFS won't pick up on these too well until the time period reaches the higher resolution and the more reliable time frame (even within T96 some nasty little changes can occur!).

The outlook is looking good either way though, as things should begin to settle down a lot by the weekend, and the -5s are progged to arrive down here by T120. It's certainly a marked improvement on the awful weather we saw in November!

[shuggee]

Worth a look at model performance at the moment...

Nothing in it really - UKMO, then ECM then GFS by the smallest of recent margins.

[Guest North Sea Snow Convection]

Incorporating the model solutions we can see, as well as deciphering from overall ensemble information available it seems that the latest METO update suggests that High pressure becomes an increasing factor to our NW and the suggestion of showers in eastern areas becoming increasingly wintry with time also supports the idea of at least some of that colder air, hinted at in the t168 timeframe onwards coming this way. Have to say that is about the best we could want to see at this stage and the suggestion of any atlantic return has disappeared too.

A retrogression to Greenland in the models in the further outlook is not out of the question *she adds especially optimistically*

Even if the high spends an initial period close to the UK it seems that quite a block is going to set up and that makes the task of retrogression that much easier as the vortex is banished right back over Canada with the other branch in Siberia

Edited December 7, 2009 by North Sea Snow Convection

[chionomaniac]

Worth a look at model performance at the moment...

Nothing in it really - UKMO, then ECM then GFS by the smallest of recent margins.

It's a bit concerning that the GFS is outperforming the parallel system!

[Iceberg]

I think the ECM 00Z and GFS 06 Are pretty similar re the Arctic high, ECM is appaling to read though, but if you follow the 1040 and 1020 isobars you get a pretty similar pattern, which perhapes slightly less pressure on the ECM.

The ECM shortwaves are also identifyable as pressure is slightly lower across the pole, this is probably the worse area on the globe to correctly program short wave behaviour though.

[kold weather]

Iceberg, the ECM has the pressure about 20mbs in NW.Russia then the GFS with 1020mbs whilst the GFS extends the Arctic high a long way further SW with pressure at 1040mbs, thats a big difference for such a short time out.

Edited December 7, 2009 by kold weather

[Iceberg]

I am not sure it does Kold, the ECM chart has "L" at the arctic centre at 1025mb.

The bar going through Slavbard is 1030mb on the ECM.

[Yeti]

I am not sure it does Kold, the ECM chart has "L" at the arctic centre at 1025mb.

The bar going through Slavbard is 1030mb on the ECM.

Look at the difference in geopotential heights though -

http://www.wetterzentrale.de/pics/Rhavn1681.gif

The GFS shows a large area over the Arctic of 552+DAM heights and a clearly split PV, with one half over NA and the other over Siberia.

http://www.wetterzentrale.de/pics/Recmnh1681.gif

The ECM, on the other hand, has much lower heights (sub-536DAM) over the Arctic with the main block over the UK extending Nwards. They're not as well linked as in the GFS, in pressure or thicknesses, meaning they split apart and the block sinks SW.

http://www.wetterzentrale.de/pics/Recmnh2401.gif

http://www.wetterzentrale.de/pics/Rtavn2401.png

Edited December 7, 2009 by Yeti

[Iceberg]

Yes that might be where the confusion lies I am looking at 120Z for the ECM (114Z for GFS).

I agree that by 168 there is much more varied difference.

[stewfox]

The really unfortunate thing for us is that these seemingly insignificant short waves running through Iceland have a significant impact on the fate of potential cold spells. This has caused many disappoints over the years and it surprises me that so many are getting excited already. My advice is to ignore anything beyond T+120 and focus on events within T+96. It is right to be cautious - we have been here too many times before. Every evolution being shown is possible, but as usual, the devil is in the detail.

Isn't this the point ? The important developments are now and up to T72/T96 ie late Wednesday or more likely Thursday.

All this constant F1 posting of fabby charts for T240 is rather irrelevant ??.

What should we be looking for say in up to T72 , short waves to scupper the evolving trends ??

Surely the only thing 'nailed' so far is its going to get cooler after wednesday ?

[NorthNorfolkWeather]

Isn't this the point ? The important developments are now and up to T72/T96 ie late Wednesday or more likely Thursday.

All this constant F1 posting of fabby charts for T240 is rather irrelevant ??.

What should we be looking for say in up to T72 , short waves to scupper the evolving trends ??

Surely the only thing 'nailed' so far is its going to get cooler after wednesday ?

Yep. I agree completely that T96 is where FI really is at the moment.

All the charts I've seen still suggest that we only have a 30-40% chance of this happening. That being the case, I'll ramp and whinge in the relevant threads.

But on the good side, it looks like it's only 2 more days of rain

[Andy_]

Now to shortwaves. A shortwave is an upper level front or a cool pocket aloft. Just as a surface front causes the air to rise, upper level fronts can do the same. First, let's start with a general description of a shortwave:

(1) It is smaller than a longwave trough (shortwave ranges from 1 degree to about 30 degrees in longitude (the average one is about the size of two U.S. states put together (Iowa and Missouri put together is a good example)

(2) Isotherms cross the height contours (if it is a baroclinic shortwave). This creates an upper level temperature gradient and therefore an upper level front

(3) They are best examined on the 700 and 500 millibar charts

(4) They generate positive vorticity (mainly due to the counterclockwise curvature within the shortwave). This creates positive curvature and positive shear vorticity. If PVA occurs with the shortwave then the shortwave will deepen and strengthen due to lift created by upper level divergence.

(5) They can create an environment conducive to surface based convection or elevated convection due to the cooling aloft.

It is important to see how much moisture is associated with the shortwave. A shortwave moving over a warm and moist lower troposphere has a better chance of producing precipitation and strengthening than one moving over a dry lower troposphere. If the low level dewpoint depressions are low, the instability and lift associated with the shortwave can enhance cloudiness and precipitation. In summary, a pressure trough is associated with a low-level front while a shortwave is associated with an upper level front or a cool pocket aloft. Both are associated with rising air and can add instability to the atmosphere.

eventually i understand what a shortwave is lmao although i dont understand the views of some people on this forum saying we have seen more of them in the so called 'christmas pudding', i mean can more warmth in the atmospehere really affect isobars? :s

anyways onto the models, a rather similar UKMO to yesterday - i.e. the high being favourable for us at +144, and the met office outlook sounding promosing at the least - imo they must have good confidence in the output to add this informtation into their outlook

ECM is unispiring, i would like to think its just a flaw but we will have to wait for the 12Z, and of course the GFS 6Z is great

I'm hopeing to see a bit more insight into what the situation for us is going to be around the dates 16th-19th and so on - don't wanna be heading anymore deep into FI after that, accuweather seem to be for the idea of cold with snow being forecasted around these dates, i don't know which model they use for there forecasts however. And i would just like to add this little quote from the met office in their outlook, "The cold but mainly settled theme is likely to continue through next weekend and the start of the following week, although showers in the north and east are likely to turn increasingly wintry" <<< nice

[Methuselah]

But...Isn't the operational run likely to be more accurate - by dint of the fact that it's fed the 'right' data - all the other ensemble members being given (deliberately) inaccurate information??

[weather eater]

Just viewed the 06z run, this has a different take from the 00Zs on how the HP develops. What this demonstrates IMO is the futility of getting agitated about future projections. Once we get model agreement on this crucial factor then we can focus on how the synoptic's pan out.

[Paul]

But...Isn't the operational run likely to be more accurate - by dint of the fact that it's fed the 'right' data - all the other ensemble members being given (deliberately) inaccurate information??

Not really in truth, as the reason that the ensembles are run is because you will never have the perfect dataset to start a run with, so although there are deliberate adjustments/ 'errors' in the ensembles members, it's reasonable to assume there are also imperfections in the operational dataset too.

[Thundery wintry showers]

Also- the GFS operational out to T+144 supports an odds-on evolution to a cold easterly or north-easterly. The most important bit, I feel, is not how things evolve from there, but whether we get there in the first place. T+144 is still a bit too far out to be certain, but I suggest that the building blocks for this easterly actually arise within the higher resolution part of the GFS.

As is often the case the 12Z runs will be quite important. We really need to see the ECMWF show a similar scenario out at T+144 to have much confidence in this cold easterly coming off, as opposed to a moderately cool cloudy one or a stagnant high pressure scenario.

[Methuselah]

Not really in truth, as the reason that the ensembles are run is because you will never have the perfect dataset to start a run with, so although there are deliberate adjustments/ 'errors' in the ensembles members, it's reasonable to assume there are also imperfections in the operational dataset too.

Thanks Paul...I've never been too sure??? So, I guess that that makes the ensembles no more than a measure of the operational's uncertainty???

[Paul]

Thanks Paul...I've never been too sure??? So, I guess that that makes the ensembles no more than a measure of the operational's uncertainty???

Yep, although all this following of each member is fun at times and can come up with some amazing charts which show what may be possible, the purpose of the ensembles is really to give a confidence level in the operational and control runs.

It's things like the spread charts which aren't used so often by people which I think can give the most info, for instance the couple of charts here show the mean sea level pressure from the ensembles + the spread which is the number of hpa between the highest and lowest member across all parts of the map. So you can see on this chart at fairly close range that the only area of real uncertainty is around the low pressure system to the southwest of the UK

Whereas we jump ahead a few days and the confidence drops considerably - with a large spread in many areas and particularly low confidence around the low pressure off the east coast of the USA

[Duane S.]

Isn't it nice between the hours of about 11:30am and 3:30pm in situations like this?

No need to worry about watching every chart of a run slowly shatter your dreams or a set of ensembles confirm what you were already dreading!

Oh well, here we go again, 12z about to roll out soon...

[karyo]

Isn't it nice between the hours of about 11:30am and 3:30pm in situations like this?

No need to worry about watching every chart of a run slowly shatter your dreams or a set of ensembles confirm what you were already dreading!

Oh well, here we go again, 12z about to roll out soon...

It is a time when the anticipation starts to rise and the biting of the nails begins, in the quest for biting cold!

Karyo

[cooling climate]

If the UKMO and GFS 06Z are correct at t144 then its definitely game on.

The BBC weather man at 13.30 said high pressure could be around for some

considerable time to come from this weekend onwards.

Everything further west at t72 very good signs.

[chionomaniac]

There are worse places for the MJO to skid to a halt. Let's hope it doesn't get bogged down in phase 5.

Latest GWO update showed that we are in phase 4 but with an upwards tendency for added momentum.

[mountain shadow]

There are worse places for the MJO to skid to a halt. Let's hope it doesn't get bogged down in phase 5.

Latest GWO update showed that we are in phase 4 but with an upwards tendency for added momentum.

What would a 'neutral' MJO suggest? Anything can happen?

[cooling climate]

Who would have thought out to t96 and even better ridging than on the

06z run.

Could be another belter on the way.

[chionomaniac]

What would a 'neutral' MJO suggest? Anything can happen?

Tends to be no clear signal for enhanced tropical convection to send the MJO into a particular phase.