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phil nw.

Model Output Discussion 4th Dec.2013-12z onwards.

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for newbies this may help to see just how mild or cold it may feel.

Look at the dewpoint charts, say the Extra and msl I mentioned in an earlier post. Low values say 5C or below, especially if coupled with any marked wind speed and it will 'feel' cold. 5c probably represents what we call returning Polar Maritime air (rPM), that is air that started life way north, probably the far north of N America or Greenland but has then been brought way south before moving NE again. Characterised by sunny spells and showers, rain, hail, possibly sleet but with snow over higher ground especially in the north. At or below 0C then it is much more likely to be direct Polar Maritime (PM) or possibly with the isobars from the north then Arctic Maritime (AM). Values of Td above about 8C or so in winter usually indicate air from a southern region Tropical Maritime and usually characterised by fairly cloudy skies especially on windward coasts and hills, hill fog, it can brighten up east of the main high ground in the UK, sometimes aided by the Fohn effect.

This is not a totally fixed set of values, more to give new folk an idea of the numbers to look for with dewpoints.

 

hope that helps and thanks Jo for your comment, nice to see the modern generation of Met forecasters using the old ideas!

 

Finally, although it may take all the fun out of wondering what the weather will do, the 6-15 day outlooks and the 16-30 day variant, from UK Met is by far the most objective reading of what the models are showing than anything and I mean anything you will read either here or any other web site. Yes of course they do change, be a bit daft if they did not as the professionals analyse and decide which model(s) is the most likely to verify. Yes of course they get it wrong at times but not that often which is how it should be from them. No doubt folk on here will disagree and quote those occasions when they did get it wrong but take my advice they are correct some 80% of the time out to about 7 days, beyond that not sure what the statistics show, maybe Jo can help out with that?

I vaguely remember 85%. for out to day 5. being talked about. Would be very tricky to 'mark' the accuracy beyond the 5 day deterministic forecasts, all I've got from Met Office is http://www.metoffice.gov.uk/about-us/who/accuracy/forecasts  and that is all very short term, No other data is available to you and me

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The paths of the Atlantic lows at the weekend are jumping about all over the place. The second one for later Sunday, slightly further on this run. Still Lewis, Harris and Skye in the line for 80mph + gusts

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The paths of the Atlantic lows at the weekend are jumping about all over the place. The second one for later Sunday, slightly further on this run. Still Lewis, Harris and Skye in the line for 80mph + gusts

Indeed, I think becoming very unsettled, locally stormy at times in the north and west is about as much as can be confidently said for now. The track and intensity of each individual system will continue to chop and change on every run, but the odds are many places will get something of a battering between now and the first Christmas morning sherry...Posted Image

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So far, even down South it doesn't look that mild a run after Monday, with Tuesday being somewhat chilly I bet, so far with the GFS 12z. Maybe the implications of uncertainty regarding the first two weekend depressions (as stated by Jo) are effecting things as we move further along the output. FI still at a short timeframe right now so may well be further surprises along the way. No really cold uppers yet but certainly a bit of a chill will be felt in Scotland more often than not.

 

That sure is a ferocious depression out in mid-Atlantic, waiting in the wings even at t+6 hours on the UKMO output. Posted Image Ah, thankfully for Scotland its NOT the actual surface feature which only appears some 36 hours later or so, nevertheless a succession of deep lows are on their way. Posted Image Is it part of the Polar Vortex out in mid-Atlantic at t+6, I'm thinking it must be? Maybe just a trough, afterall.

 

post-7183-0-38445500-1386865890_thumb.gi

 

Sorry chaps for these edits, I'm having a few funny moments. Posted Image

Edited by gottolovethisweather

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Feb 1983 had a CET of 1.7C so there must have been a few wintry events that month?  My memory is rather vague for that month believe it or not!

 

Was a high pressure based month so in Feb was probably Frost and Fog that caused the low CET . will be interesting though to see how this year pans out compared to that winter . 

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I vaguely remember 85%. for out to day 5. being talked about. Would be very tricky to 'mark' the accuracy beyond the 5 day deterministic forecasts, all I've got from Met Office is http://www.metoffice.gov.uk/about-us/who/accuracy/forecasts  and that is all very short term, No other data is available to you and me

 

yhanks for that Joe-the pressure one is interesting. Once again not too difficult to get that correct 3 days out but not the same accuracy with other parameters.

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GFS extreme FI at least offers hope of a pattern change leading up to new year.Posted Image

 

Posted Image

Edited by Mucka

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Well certainly becoming very unsettled on current output and stormy for some .but the high on continent still wants to hang on for dear life .some realy deep cold air being pulled south in time lets hope some of it reaches our shores .certainly a very interesting time ahead ,catch up later Posted Image

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Overall though, I believe the prevailling direction of the Jet Stream as it nears the UK is from SW to NE, hence why they are following this rather 'classic storm track'.

 

 

 

Sounds about right, based on this (NCEP reanalysis of 300mb scalar wind* mean in December from 1948):

 

Posted Image

 

The jet not only steers depressions, but sections of the jet are favourable for upper divergence and convergence, with enforce particular surface patterns. The poleward exit region of a jet favours surface low pressure, so you can see why climatologically we often see depressions in the region around Iceland and Greenland and NE Canada. 

 

The area around the Azores (roughly) lies near the equatorward exit of the polar jet and the poleward entrance of the sub-tropical jet. Both these regions favour surface high pressure, hence the semi-permanent Azores High.

 

* I'm assuming they take the mean wind vector and take the magnitude of that. It looks like a sensible result anyway.

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GFS extreme FI at least offers hope of a pattern change leading up to new year.Posted Image

 

Posted Image

 

Yep continued hints in deep FI of a pattern change to potentially something much colder as we head towards the New Year period.

Edited by CreweCold

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Yes very very fi I know , but with what's going on in the stratosphere in 9/10 days time could this be the beginning of the vortex been shaken to the point of falling apart ?

post-9095-0-63653500-1386867635_thumb.jp

If we can get the vortex to drain away from the polar regions then northerly blocking will take hold , but we have a very disturbed week next wk to come , and maybe longer . But the are just tentative signs now of a major pattern change after Xmas . Which in reality is when the uk's snow and cold normally comes.

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Yes very very fi I know , but with what's going on in the stratosphere in 9/10 days time could this be the beginning of the vortex been shaken to the point of falling apart ?

Posted Imageimage.jpg

If we can get the vortex to drain away from the polar regions then northerly blocking will take hold , but we have a very disturbed week next wk to come , and maybe longer . But the are just tentative signs now of a major pattern change after Xmas . Which in reality is when the uk's snow and cold normally comes.

 

We saw these signs popping up in the models in early November prior to the mid-November brief cold snap, but they came to nothing. Will be interesting if repeated failures form a pattern this year, or if they manage to get it right?

 

Can't find the exact run I was thinking of, but some examples in this thread of promising output which turned out very mundane in the end:

http://forum.netweather.tv/topic/78340-model-output-discussion-1st-nov2013/page-75

Edited by forecaster

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We saw these signs popping up in the models in early November prior to the mid-November brief cold snap, but they came to nothing. Will be interesting if repeated failures form a pattern this year, or if they manage to get it right?

O I agree.I can count many of times when a cold spell looked likely only to be taken away from us inside t120 , which is why without stratosphere backing cold spells are very unlikely . If we are after really wintry weather , we all need to look at the stratosphere forecasts , this time wave 2 activity does look the strongest it's looked so far , but it's only a forecast so it needs to watching . But IF it comes in to fruition then that can wobble the vortex , and open the door to a more realistically cold pattern . But even then many things depend on things falling into place ,take last year for example the winter was loaded with potential , but even with the vortex destroyed we had a segment sitting over Greenland which in turn fed the Atlantic . We don't need things to be as favourable as last winter . But we do need the vortex to disrupt and wobble , then hopefully it can spill energy down to mid latitudes and give areas further north a chance for high pressure to form !!

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Following on from the ECM 00hrs run which began to improve the profile over the Arctic the GFS does give a small crumb of hope for coldies. I think moving forward the slow sinking of the jet helped on its way by the AO trending negative looks a plausible scenario.

 

However we're a long way from anything major in terms of a spell of below average temps, its a start at least. The issue has always been whether we would see a limpet PV blob refusing to move, whether this was a temporary situation or if it was going to be one of those awful stretches of mild zonal dirge.

 

We still have to see this better Arctic profile move in to the more reliable timeframe but the GFS 12hrs and trend of the ECM 00hrs are similar. The ECM could do with salvaging some of its reputation post T144hrs!

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6z ens for here paint a wet picture, even if it's mild (at times) it's hardly going to be pleasant when it's soaking wet and blowing a gale. 

 

Posted Image

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Most Decembers are the same, think most apart from 2009 and 2010 have been on the mild side and generally very dominated by low pressure as the PV is at its most active phase. I'd even put March up as being a better month for snow and cold than December. Let's just wait until at least February has passed before we can name and fame/shame this winter period.

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Most Decembers are the same, think most apart from 2009 and 2010 have been on the mild side and generally very dominated by low pressure as the PV is at its most active phase. I'd even put March up as being a better month for snow and cold than December. Let's just wait until at least February has passed before we can name and fame/shame this winter period.

I read a statistic which had been validated and checked, but I cannot find the source now, that stated that we are more likely to see a white Easter than a white Christmas! So the model output at the moment is far from surprising, we have just been spoilt by recent years where significant cold weather prospects have emerged earlier on in the year than we had grown to expect.

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 sit well S of the PFJ. ??

Polar Front Jet! isn't that normal tech speak, was it called something different before?

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 sit well S of the PFJ. ??

 

I meant North! My bad!

 

I've edited.

Polar Front Jet! isn't that normal tech speak, was it called something different before?

 

I think it's because I put S rather than N that he pulled me up. Though he knew what I meant!

Edited by CreweCold

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Yes very very fi I know , but with what's going on in the stratosphere in 9/10 days time could this be the beginning of the vortex been shaken to the point of falling apart ?

Posted Imageimage.jpg

If we can get the vortex to drain away from the polar regions then northerly blocking will take hold , but we have a very disturbed week next wk to come , and maybe longer . But the are just tentative signs now of a major pattern change after Xmas . Which in reality is when the uk's snow and cold normally comes.

Yup thats what we need for some sustained cold. PV break up into 3 parts will do nicely.

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The last couple of runs of the ECM 32 have shown really cold outliers on the Glasgow Bishopton EPS - I discounted with the way that the models were looking but now, with the final few frames of the 12Z GFS (on NH view), maybe time to pay attention - we'll see if the outliers show more trend tomorrow.

 

Posted ImagePosted Image

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