Model Output Discussion - 28th November - 4th December

[nick sussex]

Just when you think you've overcome the spoiler shortwave up it pops!

As you can see here the ECM does hold the trough and low further back than the GFS but that small shortwave causes problems by stopping the high from ridging north, so the big three outputs disagree at 120hrs.

We'll have to wait till tomorrow to see which one is correct.

[ronan]

You all mention the potential for Scotland and the North West. Does this include N.Ireland as we haven't been mentioned to much. In past years we have done well from this type of setup.

[tinybill]

well things might be interesting on the east coast between 9 th and 10 i know its a long way off but its nice to see !!!

[Zakos]

As Michael fish has said "Snow as far south as the midlands on Monday"

And yes he quite rightly says it wont settle unless you have a bit of high ground. Its possible that there could be some accumulations in the midlands, but even if this happened they would only be temporary, and would have to occur at night.

Fri/Sat/Sunday and possibly beyond is a different story. The air is much much colder and snow would fall and settle no doubt.

Theres no reason to necessarily say that the Second cold blast will be downgraded.

I dont recall the GFS showing anything as severe this for the monday cold snap -

The GFS never showed real potential for low-level snow in the south for monday. It does for the end of next week...

[johnholmes]

http://forum.netweather.tv/topic/27989-how-to-try-and-forecast-snow/

posting again for those who may not know about it or not yet used it-it does work

remember to factor in your height and distance from the sea, one increases your chance of snow falling (height=approx 1C fall for every 1000ft) the other sea proximity decreases it, that is due to the sea in the winter half of the year being comparitively warm.

there are at least 7 parameters to deal with assuming that precipitation is shown over your location to start with.

Get all 7 and you are in for a white world, a couple not meet the criteria and its unlikely you will see more than wet snow or sleet.

[Frosty.]

You all mention the potential for Scotland and the North West. Does this include N.Ireland as we haven't been mentioned to much. In past years we have done well from this type of setup.

[I remember Atlantic 252]

I have -6 uppers on monday, both control and 0perational go below -5C for most of monday, so surely snow possible for the south?

next fri, sat is a long way out, but hopefully snow then for a time before toppling

[jethro]

Any chance we culd witness a pv split nick?

Or is there nt enuf energy 4 that 2 happen?

There's no limit on characters per post...... for the benefit of miserable old sods like me, please no text speak.

[nick sussex]

Given the differences as early as 120hrs between the models then nothing can be ruled out or in this evening in terms of a possible northerly, theres so much uncertainty with shortwaves and the phasing upstream which makes a big difference to what happens in the UK.

Unfortunately the model you really want on side, the ECM decides to throw a major spanner in the works, but it is better to have the UKMO as the odd one out rather than the GFS, keep an eye on tonights fax charts, if the UKMO think the ECM is the more likelier option they will modify their raw output towards it on the 120hrs chart.

[J10]

If we look at criteria at 6am, 850Hpa is at -5c. Thickness around 527 dam and 1300m. Dewpoints below zero

This meets the minimum criteria but this is extremely marginal.

You would be wanting around -7c, thickness around 520 dam and 1280m to be more confident.

As a result in the south, if there are any showers, a wintry mix is possible, mostly rain or hail, but with some wet snow/sleet, in heavy showers early am. During the daytime, probably the best expected is hail. IMO higher hills are required for any settling snow, possibly 300-400m.

The local BBC forecaster just said for Monday hail, sleet/snow showers with most of the snow on the hills and mountains.

Using John's excellent analysis at 6-9am.

1) Dry bulb temperature below 5C, often 3C is a better mark. Check for most inland

2) Dewpoint at or below zero Check for most

3) wet bulb temperature, if you have a weather station, no more than about 2C - Not certain on this one.

4) 1000-500mb thickness (DAM) less than 522dm, lower if you are on the coast, but as high as 540dm it is possible in a heavy shower, but unlikely. Marginal at 528dam

5) 850mb temperature of -7C or below, -5C it can occur but not often. At -5, Marginal

6) on the 850mb chart if the value shown on the contour line is below about 1290dm or 1300dm, then there is a high chance that ppn will be of snow. Around 1300 dam. Marginal

(The Met Office use 1293dm for a 50% and 1281dm as a 90% chance of snow)

7) zero degree isotherm or freezing level of 1,000ft or less to give a 50% or higher chance of snow at sea level. Not met

The higher you are the higher the chance of you getting snow. Thus if you live at 2000ft this increases your chances of snow considerably to someone at sea level.

So on this basis, it not ideal for snow, but by meeting some criteria, there maybe the odd flake of snow, but not that much, a wintry mix is definitely possible.

[Cheshire Freeze]

Given the differences as early as 120hrs between the models then nothing can be ruled out or in this evening in terms of a possible northerly, theres so much uncertainty with shortwaves and the phasing upstream which makes a big difference to what happens in the UK.

Unfortunately the model you really want on side, the ECM decides to throw a major spanner in the works, but it is better to have the UKMO as the odd one out rather than the GFS, keep an eye on tonights fax charts, if the UKMO think the ECM is the more likelier option they will modify their raw output towards it on the 120hrs chart.

I have a feeling they'll go with their own model for now Nick. Best we can hope for tonight now is an 18z GFS which backs the UKMO. If it does then I suspect the ECM may back-pedal on the 00z run.

FWIW the uppers look decidedly dodgy on the ECM as close as 48 hours- by far the model with the highest uppers during Sunday and Monday.

Edited December 2, 2011 by CreweCold

[January Snowstorm]

Trouble with the whole pattern is if it doesn't deliver we could be back to a Euro high

This would seriously damage any potential for the Christmas period.

Still I sense upgrades are on the way

[nick sussex]

I have a feeling they'll go with their own model for now Nick. Best we can hope for tonight now is an 18z GFS which backs the UKMO. If it does then I suspect the ECM may back-pedal on the 00z run.

Everyone knows the ECM is my favourite model so I'm particularly annoyed at it for throwing out that shortwave!

The UKMO isnt alone this evening globally, the GEM and NOGAPS do back its upstream pattern in terms of phasing though the latter does throw out a shortwave but further to the se.

Because of the timeframes involved we won't have to wait long for model agreement, I expect the mornings runs to finally agree on the upstream pattern.

[Gibby]

Hi folks. Here's a summarisation of the 12z operational outputs of GFS, UKMO and ECM.

GFS shows a strong West or NW flow for the next week or so with increasingly cold uppers moving across the UK from the NW early next week. A breezy weekend is shown with rain tonight moving away South tomorrow with showery Westerly winds over Scotland with snow from these falling over the mountains. Over Sunday and Monday this weather steadily extends SE to all areas with showers becoming widespread and wintry on ever lower ground as we move into next week. For Southern areas a lot of dry weather would occur with showers mostly to the West. After a brief less cold spell midweek a further surge of cold showery North to Northwest winds return late next week with the risk of some substantial snow on Northern high ground, blown about in the wind. In FI pressure builds to the South of the UK with dry and milder conditions likely in the South with changeable and windy SW winds affecting the North at times. Under any clear skies though overnight frost and fog could develop if winds fall light enough making it rather cold at the surface.

The GFS Ensembles for London show a period of below normal 850's before levels return to the long term average later in the run. The operational was one of the milder options in the latter stages of the run. In Aberdeen fluctuations occur widely later in the run with 850's becoming near the long term average or a fraction above by the runs end.

UKMO tonight also shows a windy and increasingly colder scenario between now and midweek with West or NW winds pushing rain bearing troughs SE tonight followed by clearer weather for all Northern regions by tomorrow. A showery NW flow then develops for the following three days with snow on all Northern hills and lower ground too in the far North with perhaps some accumulation there. Later in the week it looks like milder air will push back across the UK albeit a temporary feature as indicated by the 144hr chart.

ECM follows a similar route to UKMO but brings developments midweek more to the front as a trough crosses East with slightly milder air being chased away by a further surge of cold air by next weekend. The run ends with further deep Low pressure out to the NW throwing troughs across the UK followed by yet another cold and showery NW flow soon after the term of the run.

In Summary the next two weeks continue a fairly mobile west or NW flow over the UK with Low pressure maintained well to the North for most of the run. Although winds are mostly from the West some rather cold air will frequently be fed across Britain with Northern regions running the risk of seeing snow at times, though accumulations away from the hills should be minimal. Frosts and fog will be quite limited due to the strength of the wind.

[J10]

My latest Alpine slog blog is up, this contains synoptic charts for the UK and the whole of Europe for the next 7 days. While they may be a little snow here, there WILL be plenty of snow there.

[shedhead]

Just when you think you've overcome the spoiler shortwave up it pops!

As you can see here the ECM does hold the trough and low further back than the GFS but that small shortwave causes problems by stopping the high from ridging north, so the big three outputs disagree at 120hrs.

We'll have to wait till tomorrow to see which one is correct.

Was always likely to happen though Nick wasn't it? We've seen the kinds of N and NW'erlies modelled time and time again at T+120hrs and beyond, but what we've also seen time and time again is shortwaves developing in the shorter range and putting a very different complexion on the reality.

[nick sussex]

Was always likely to happen though Nick wasn't it? We've seen the kinds of N and NW'erlies modelled time and time again at T+120hrs and beyond, but what we've also seen time and time again is shortwaves developing in the shorter range and putting a very different complexion on the reality.

The spoiler shortwaves normally form near Iceland especially in PM flows, in this instance our spoiler decides to pop up off the eastern USA, this area seems to be causing alot of model variability.

The ECM ensembles will be out soon so it will be interesting to see what they suggest.

[Snowmadsam]

The spoiler shortwaves normally form near Iceland especially in PM flows, in this instance our spoiler decides to pop up off the eastern USA, this area seems to be causing alot of model variability.

The ECM ensembles will be out soon so it will be interesting to see what they suggest.

What do shortwaves do?

[IDO]

What do shortwaves do?

"A shortwave trough can be defined in several ways. The following are characteristics that most shortwave troughs possess:

(1) Shortwaves are smaller than longwave troughs

(2) Shortwaves have a counterclockwise kink to the height contours

(3) They are associated with an upper level front or a cold pool aloft

(4) Shortwaves generate positive curvature vorticity and positive shear vorticity

(5) Shortwaves often represent baroclinicity in the troposphere (WAA and CAA)

(6) Shortwaves are imbedded within the longwave trough / ridge pattern

(7) Shortwaves are best located on the 700 and 500 mb chart / prog

(8 ) Rising motion occurs within the exit sector of a shortwave

(9) Their size and influence ranges from the mesoscale to the synoptic scale

(10) Shortwaves move faster than longwaves (usually more than twice as fast)."

If that helps you are a better man than me!

[feb1991blizzard]

I know this is FI because there could be massive variations depending on the models handling of shortwaves but as per my december forecast, i really dont see any Euro High lasting very long before some sort of cold zonality returns for xmas. Even supposing the ECM is modelling the SW correctly at 120-144 the Euro gets shunted out of the way quicker and granted its still zonal but it looks like at least cold zonality in its later output, as ive said though all FI at the moment.

[shedhead]

"A shortwave trough can be defined in several ways. The following are characteristics that most shortwave troughs possess:

(1) Shortwaves are smaller than longwave troughs

(2) Shortwaves have a counterclockwise kink to the height contours

(3) They are associated with an upper level front or a cold pool aloft

(4) Shortwaves generate positive curvature vorticity and positive shear vorticity

(5) Shortwaves often represent baroclinicity in the troposphere (WAA and CAA)

(6) Shortwaves are imbedded within the longwave trough / ridge pattern

(7) Shortwaves are best located on the 700 and 500 mb chart / prog

(8 ) Rising motion occurs within the exit sector of a shortwave

(9) Their size and influence ranges from the mesoscale to the synoptic scale

(10) Shortwaves move faster than longwaves (usually more than twice as fast)."

If that helps you are a better man than me!

Thanks for that Nick. I'm surprised in this overly PC day and age some shortwave hasn't complained their title is derogatory and from now on would like to be called 'height challenged waves'

[Stratocumulus perlucidus]

As Michael fish has said "Snow as far south as the midlands on Monday"

And yes he quite rightly says it wont settle unless you have a bit of high ground. Its possible that there could be some accumulations in the midlands, but even if this happened they would only be temporary, and would have to occur at night.

Fri/Sat/Sunday and possibly beyond is a different story. The air is much much colder and snow would fall and settle no doubt.

Theres no reason to necessarily say that the Second cold blast will be downgraded.

I dont recall the GFS showing anything as severe this for the monday cold snap -

The GFS never showed real potential for low-level snow in the south for monday. It does for the end of next week...

I might be wrong as im a noob myself but that is only the oc isotherm level. The HGT, 500 ang 850 HPA are different to what this chart suggests. I think its an outlier??

[Zakos]

I might be wrong as im a noob myself but that is only the oc isotherm level. The HGT, 500 ang 850 HPA are different to what this chart suggests. I think its an outlier??

No, You are correct in saying that the Isotherm level does not necessarily indicate conditions cold enough for snow, however, in this case, they are -

http://hw.nwstatic.co.uk/gfsimages/gfs.20111202/12/177/h850t850eu.png

it is also not an outlier, as it supported strongly by the ensembles, and backed up by the previous run.

Im not forecasting a "snowmaggedon" next Friday, I'm just suggesting that snow is more likely in the south in the second cold blast than the initial one.

And don't worry, I'm a noob myself aswell

[k19]

I might be wrong as im a noob myself but that is only the oc isotherm level. The HGT, 500 ang 850 HPA are different to what this chart suggests. I think its an outlier??

I believe this is the correct chart for the time you are thinking of, but the high pressure is too near to us effectively shifting the precipitation eastwards at the moment, so although it would be bitterly cold, the chance of snow is low at this time, at least thats what i believe is the case, i may be wrong.

[BLAST FROM THE PAST]

Was always likely to happen though Nick wasn't it? We've seen the kinds of N and NW'erlies modelled time and time again at T+120hrs and beyond, but what we've also seen time and time again is shortwaves developing in the shorter range and putting a very different complexion on the reality.

Like this week Shed? Some cold temps have been had and it looks decidedly chilly from weekend into next week. The only difference i see is that the reality is that it is and looks like being a lot chillier than was thought? [just thinking of general discussions on the board]. This spell ahead hasn't been scuppered at all I would suggest but is a tas better than many had thought.

BBC SE News forecasting daytime maxima of 5 and 6c Mon to Wed.....that is decidedly chilly in my books.

Just looked at the UKMO model.....has to be watched I think as it is not normally a 'cold generous' model.

BFTP

Edited December 2, 2011 by BLAST FROM THE PAST