Model Output Discussion 17th Dec.2013-12z onwards.

[Snowman.]

Morning Folks. Well my my its looking like we are in for a pretty blowy Christmas holiday. That ECM chart for boxing day is actually a tad concerning, is that really a 955mb low over the north midlands? going forward I was wondering what the views were on the ECM, is it as shocking for coldies as last nights? it looks to me as there were some heights building way to the north. Cheers

The charts may not show widespread snow showers but what we're seeing is the transition of a piece of the Pv that's situatated in the Atlantic moving east hitting to the block then causing a ruckus. Battleground scenario is that if the block wings were back to sw'ly winds if the low can break through it would deepen heights into Europe. What ever happens after this is a long way away but for now we're going to need wait till the models toy with outcome of the Xmas low.PC is currently dead so can't post charts as often as I would like but thing are looking interesting with the jet moving south leaving the UK exposed to dreadfully raw cold lows it could get pretty wet around New Years

[Blessed Weather]

For Boxing Day the GFS 00z is suggesting the main chunk of PV will be slap bang over Scotland, presumably squeezed our direction by the Russian and Alaskan high pressures ridging polewards? Certainly stormy, but I can't help imagine what temps and snowfall the UK would experience in this situation if it wasn't for the moderating effect of the North Atlantic.

 

[stratty]

03 December 2013 - 19:38

 

......and Mother Nature has a way of balancing things out: so if it is to be mild for a time, then hopefully the swing will be the other way and we get our snow after that! 

 

+ the weeks of calm are now being balanced out by storm and tempest! Poor old Father Christmas is going to have a tough time delivering!

[Downpour]

The GFS always over does the lows, it'll never come off.

Indeed. As has been said time and again, the verification rates on such monster lows from GFS are so low at this range, you might as well worry about being reincarnated as as a California raisin.

[Gibby]

Good morning. Here is the morning report on the NWP output for midnight Wednesday December 18th 2013.

 

All models show a strengthening SW flow across the UK this morning as a rapidly deepening depression moves NE towards NW Scotland. This is shown to swing an active front across the UK later today with severe gale force winds and heavy rain ahead of it. In the NW storm force winds could give rise to damaging gusts for a time tonight when colder and clearer conditions with squally heavy showers on a colder Westerly gale arrives later, lasting through tomorrow and into Friday. Later on Friday all models show winds backing SW and remaining very strong as milder and wet wet weather spreads back across the UK from the SW. This then slowly clears from the North over the weekend, last from the South as a wave delays any clearance here and gives rise to more heavy rain on Saturday down here. By Sunday all areas look to be in a broad and chilly Westerly flow with sunshine and squally showers.

 

GFS then shows yet more mild, wet and windy weather at times in the run up to Christmas with further gales, especially in the North and West. Over Christmas itself the weather looks like reaching it's peak potential for storminess as very intense Low pressure crosses on Boxing Day with Christmas Day having been already wet and windy Boxing Day looks like it could be very stormy with severe gales and damaging gusts of wind across England and Wales coupled with heavy rain and blustery showers following. After Christmas and in the run up to the New Year little overall change is shown though things look likely to turn somewhat colder with some wintry showers on all hills and the chance of night frosts where winds fall light enough. By the New Year celebrations though the stormy and windy weather returns with gales and heavy rain sweeping East over the UK once more.

 

UKMO shows Christmas Eve with deep Low pressure out to the NW with a gale force SW flow over the UK delivering mild and wet weather through the day. On Christmas Even night it looks like a vigorous trough would spread East across the UK with very heavy rain followed by squally showers and colder air.

 

GEM today shows intense Low pressure near and over the UK on the approach to Christmas and Christmas itself with strong winds and heavy rain never far away from anywhere though this run does lessen the impact of damaging wind risk.

 

NAVGEM today maintains a low pressure belt all the way from Scandinavia across to Newfoundland with strong winds and rain rushing across the Atlantic and over the UK frequently with a mix of showery weather with brighter intervals in between. There is also less impact of damaging winds from this model either this morning.

 

ECM today shows a very unsettled run up to Christmas with rain and wind featuring for all. Over Christmas the weather looks very volatile with Boxing Day looking particularly troubling as a vigorous Low is shown to sweep across England and Wales with potential storm force winds bringing damage in places and coupled with heavy and persistent rain before squally wintry showers take over late in the day and in the days that follow.

 

The GFS Ensembles show a continuation of average value temperatures but accompanied by very wet and windy conditions throughout the output this morning with the peak of the worst of the weather likely over the Christmas period.

 

The Jet Stream shows the flow roaring over the Atlantic and the UK in the run up to Christmas before sinking South temporarily over the Christmas period itself before returning North over the UK towards the New Year.

 

In Summary the weather will remain very unsettled over the entire period. Dry days will be at a premium with fast moving weather systems making for fast changing conditions in any one place with the basic ingredients of mild and wet weather alternating with colder and more showery conditions when some snow may fall over the hills, especially but not exclusively in the North. Of most concern is the potential for damaging winds to accompany these weather systems as they are extremely deep even by Winter standards and could give rise to some damaging gusts over the Christmas period itself. Longer term there seems little evidence of signs of a pattern shift even into the New Year with the Jet Stream crossing WSW to ENE across the UK at that point with High pressure remaining over Southern Europe and low to the North and NW.

[Chris K]

Indeed. As has been said time and again, the verification rates on such monster lows from GFS are so low at this range, you might as well worry about being reincarnated as as a California raisin.

 

Yes but as already stated there is some consistency between models showing a deep low pressure around the 950-960mb mark being situated near, or over the UK, on Xmas/Boxing Day.

 

I do agree that we need to wait and see what the models show towards the weekend though before it becomes likely to verify.

[Frosty.]

The GEFS 00z mean shows the very unsettled pattern persisting into the new year with the PFJ being forced further south than it will be during the next 7 days and with a bit more amplification during the christmas period with hints of mid-atlantic ridge trying to build northwards and the main low pressure further to the northeast enabling colder air to dig southeast between xmas / new year with an increasing risk of wintry ppn and night frosts / ice.

[johnholmes]

the copy of the 'snow' posts is in the technical thread where it should be easier to find

[Man With Beard]

ECM Ensemble mean for Christmas Day has 955mb close to the Western Isles of Scotland. To me, that suggests a stormy Christmas is now very likely. 

GFS Mean has the main low centred further west but just as intense, still close enough for subsidiary lows to cause damage to most parts

And these are mean charts - quite remarkable - and meaning some ensemble members will be even more severe.

Let's hope the storms (if they come) don't cut the power off for anyone, what a miserable Christmas that would be.

Edited December 18, 2013 by rjbw

[shaky]

Any news on what the likelihood of snow tomorrow for the south. The models still showing the risk to be there or is it all but gone?

[Cecil Hogmanay Melchett]

Any news on what the likelihood of snow tomorrow for the south. The models still showing the risk to be there or is it all but gone?

Looks reasonable on NAE for SE Wales and SWMidlands/CS England although the accumulation charts show a small area mainly SE Wales getting a transient covering.

Maybe parts of NW Scotland also.

Hopefully some will get lucky and get a tempory covering which,in this awful Dec so far will be a real bonus.

Edited December 18, 2013 by happy days

[shaky]

Looks reasonable on NAE for SE Wales and SWMidlands/CS England although the accumulation charts show a small area mainly SE Wales getting a transient covering.Maybe parts of NW Scotland also.Hopefully some will get lucky and get a tempory covering which,in this awful Dec so far will be a real bonus.

brilliant lets hope it upgrades a bit more!!

[shedhead]

Looks reasonable on NAE for SE Wales and SWMidlands/CS England although the accumulation charts show a small area mainly SE Wales getting a transient covering.

Maybe parts of NW Scotland also.

Hopefully some will get lucky and get a tempory covering which,in this awful Dec so far will be a real bonus.

The NAE does have a history of overdoing snow though HD.  I recall that before the Arctic plunge that never was earlier this month, the NAE was calling for 30cm of lying snow around here. In the end the only thing lying proved to be the NAE....

Edited December 18, 2013 by shedhead

[NickR]

GFS 06z seems to have the main risk of snow up here:

http://www.wetterzentrale.de/pics/Rmgfs393.gifhttp://www.wetterzentrale.de/pics/Rmgfs394.gifhttp://www.wetterzentrale.de/pics/Rmgfs398.gifhttp://www.wetterzentrale.de/pics/Rmgfs423.gifhttp://www.wetterzentrale.de/pics/Rmgfs424.gifhttp://www.wetterzentrale.de/pics/Rmgfs428.gif

[Cecil Hogmanay Melchett]

GFS 06z seems to have the main risk of snow up here:

http://www.wetterzentrale.de/pics/Rmgfs393.gifhttp://www.wetterzentrale.de/pics/Rmgfs394.gifhttp://www.wetterzentrale.de/pics/Rmgfs398.gifhttp://www.wetterzentrale.de/pics/Rmgfs423.gifhttp://www.wetterzentrale.de/pics/Rmgfs424.gifhttp://www.wetterzentrale.de/pics/Rmgfs428.gif

Strange looking o6z gfs looks a bit different to the 00z IMHO,it might be right of course.

[Trom]

ECM Ensemble mean for Christmas Day has 955mb close to the Western Isles of Scotland. To me, that suggests a stormy Christmas is now very likely. 

GFS Mean has the main low centred further west but just as intense, still close enough for subsidiary lows to cause damage to most parts

And these are mean charts - quite remarkable - and meaning some ensemble members will be even more severe.

Let's hope the storms (if they come) don't cut the power off for anyone, what a miserable Christmas that would be.

And conversely 50% of the deviations will be less severe than the mean  It would be nice to see a +/-1.96 std dev band around mean on the ensembles 

[Nick F]

Looks reasonable on NAE for SE Wales and SWMidlands/CS England although the accumulation charts show a small area mainly SE Wales getting a transient covering.Maybe parts of NW Scotland also.Hopefully some will get lucky and get a tempory covering which,in this awful Dec so far will be a real bonus.

GFS has dew points and wet bulb temps too high for snow to fall generally south of Scotland away from higher ground, let alone settling. Though downdrafts in heavier showers may bring some wet snow flakes down. Main signal is for rain or hail in showers over England and Wales, though snow likely over NW fells, Pennines and Welsh mountains. Northern and western parts of Scotland most at risk of snow settling at lower levels inland away from the coasts. Edited December 18, 2013 by Nick F

[Nick L]

And conversely 50% of the deviations will be less severe than the mean  It would be nice to see a +/-1.96 std dev band around mean on the ensembles 

 

Not necessarily true. If it were the median, yes. But one particularly severe or weak ensemble member could skew the mean.

[Cecil Hogmanay Melchett]

GFS has dew points and wet bulb temps too high for snow to fall generally south of Scotland away from higher ground, let alone settling. Though downdrafts in heavier showers may bring some wet snow flakes down. Main signal is for rain or hail in showers over England and Wales, though snow likely over NW fells, Pennines and Welsh mountains. Northern and western parts of Scotland most at risk of snow settling at lower levels inland away from the coasts.

Are we talking about the 06z Nick?

That run is slightly different to the 0z ..

[shaky]

Are we talking about the 06z Nick?That run is slightly different to the 0z ..

in what way happy days?is the 06z better than the 00z for snow?

[Cecil Hogmanay Melchett]

in what way happy days?is the 06z better than the 00z for snow?

The 06z drags in some less cold uppers around the 36 hr mark,the 0z didn't do this,all to do with the behaviour of the low i guess.

NAE 06z seems keen to introduce some snow to the areas i mentioned not sure gfs would but its all very marginal no doubt evaporative cooling wet bulbs etc all very marginal.Maybe elevation will be a key player.

Basically its a nowcast ..

[shaky]

The 06z drags in some less cold uppers around the 36 hr mark,the 0z didn't do this,all to do with the behaviour of the low i guess.NAE 06z seems keen to introduce some snow to the areas i mentioned not sure gfs would but its all very marginal no doubt evaporative cooling wet bulbs etc all very marginal.Maybe elevation will be a key player.Basically its a nowcast ..

good stuff adds to the excitement. And we all know a lot of the best snowfalls have fallen in marginal events in the uk.

[Frosty.]

I can't see there being much mild weather during the upcoming xmas / new year period, if we used the Ecm 00z op run as a template for the current / upcoming pattern, it shows rather cold and very windy conditions for most of the time with just very short milder interludes and given the strength of the winds, any mildness would be blunted. It's a rather cold feeling outlook with 528 dam covering the uk at times and continuing very disturbed with potentially stormy conditions with heavy driving rain alternating with squally wintry showers and frosts / ice during the quieter interludes.

[shaky]

The 06z nae looks pretty good for snow across a vast swathe of wales and the midlands. Hopefully the 12z gfs will come in line this afternoon. Dew points look good aswell throughout. I hope ian ferguson can shed a bit more light on this later.

[Trom]

Not necessarily true. If it were the median, yes. But one particularly severe or weak ensemble member could skew the mean.

 

 

Not necessarily true. If it were the median, yes. But one particularly severe or weak ensemble member could skew the mean.

It is true - it's why I said deviations (meaning difference between observation and mean) rather than 50% of the outcomes.  Skew would affect the median, and mode in relation to the mean but not the arithmetic mean itself which is just a mathematical fact.  Skew does have the biggest distortion on mean and least on mode with median in the middle.  When we say skew has a big effect on mean it's really in comparison to normal distribution.  In a normal distribution mean = median = mode.  +ve skew mean>median>mode and reverse for negative skew.  

 

I do agree that if I had said 50% of the outcomes then I would have been assuming a normal distribution, anyway splitting hairs.  Out of interest does anyone know if the ensembles are normally distributed around their mean?

Edited December 18, 2013 by Trom