Model output discussion - 5th December 2016 - Into Winter we go

[knocker]

25 minutes ago, Singularity said:

 

Also, I'm not sure how the low sea ice could force a bottling up of tropospheric cold air given the reduced equator-pole temp gradients allowing more jet meandering where and whenever the tropical or stratospheric forcing is not directing it to do otherwise - indeed there's been a lot of spilling out of cold air to the low latitudes, and import of warm air from them, in recent times - hence the record low ice extent being in place to begin with. While storms may become more intense in the Arctic basin, such strong systems also tend to draw in relatively warm air from the lower latitudes. If you could enlighten me - and others on here - as to what's behind that hypothesis, that would be much appreciated.

TIA 

The loss of sea ice and effects on the NH circulation is the subject of much ongoing research and it's not relevant to this thread but to be going on with one recent paper.

For what it's worth, which isn't much, IMO the state of the ice this year must be relevant to events that have unfolded, and will continual to unfold, but I couldn't give you  scientifically argued reasons as to why.

The Melting Arctic and Midlatitude Weather Patterns: Are They Connected?

Quote

The potential of recent Arctic changes to influence hemispheric weather is a complex and controversial topic with considerable uncertainty, as time series of potential linkages are short (<10 yr) and understanding involves the relative contribution of direct forcing by Arctic changes on a chaotic climatic system. A way forward is through further investigation of atmospheric dynamic mechanisms. During several exceptionally warm Arctic winters since 2007, sea ice loss in the Barents and Kara Seas initiated eastward-propagating wave trains of high and low pressure. Anomalous high pressure east of the Ural Mountains advected Arctic air over central and eastern Asia, resulting in persistent cold spells. Blocking near Greenland related to low-level temperature anomalies led to northerly flow into eastern North America, inducing persistent cold periods. Potential Arctic connections in Europe are less clear. Variability in the North Pacific can reinforce downstream Arctic changes, and Arctic amplification can accentuate the impact of Pacific variability. The authors emphasize multiple linkage mechanisms that are regional, episodic, and based on amplification of existing jet stream wave patterns, which are the result of a combination of internal variability, lower-tropospheric temperature anomalies, and midlatitude teleconnections. The quantitative impact of Arctic change on midlatitude weather may not be resolved within the foreseeable future, yet new studies of the changing Arctic and subarctic low-frequency dynamics, together with additional Arctic observations, can contribute to improved skill in extended-range forecasts, as planned by the WMO Polar Prediction Project (PPP).

http://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-14-00822.1

Edited December 15, 2016 by knocker

[russell k]

44 minutes ago, bluearmy said:

Each member is a different percentage chance - 5%

so you would have a 10% chance of snow on each day. Can't see how you could accumulate your overall chance to be 50% over five days. You could say that you have a 50% chance of seeing snow on any given day within that five day period - I think but that would assume that each day a different member was predicting snowfall .  If the same member predicts snowfall for five days then surely that remains a 5% chance of seeing snowfall at some point In the five days rather than a 25% chance of seeing snow at some point in the five days

i think !

 

Hmmm, sorry bluearmy.. but If we have a 10% chance on 5 days it should equate to 50% over that period of it snowing on one day of those 5, a bit like rolling a dice 3 times you have a 50% chance of rolling a six regardless of which dice I use, thats why I dont understand how the % chance is calculated,

I take it than the same member has to predict below -5 *50s and precipitation at the same time. Sometimes they add up to more than 100% but it doesnt snow over those 15 days, anyway lets hope the latest NWP shows a higher chance

 

[Yarkie]

8 minutes ago, russell k said:

Hmmm, sorry bluearmy.. but If we have a 10% chance on 5 days it should equate to 50% over that period of it snowing on one day of those 5, a bit like rolling a dice 3 times you have a 50% chance of rolling a six regardless of which dice I use, thats why I dont understand how the % chance is calculated,

I take it than the same member has to predict below -5 *50s and precipitation at the same time. Sometimes they add up to more than 100% but it doesnt snow over those 15 days, anyway lets hope the latest NWP shows a higher chance

 

In the same way that if you had a 1 in 10 chance of winning a race every day over 5 days you would have had a 50% chance of winning one of the races? 

[ArHu3]

13 minutes ago, russell k said:

Hmmm, sorry bluearmy.. but If we have a 10% chance on 5 days it should equate to 50% over that period of it snowing on one day of those 5, a bit like rolling a dice 3 times you have a 50% chance of rolling a six regardless of which dice I use, thats why I dont understand how the % chance is calculated,

I take it than the same member has to predict below -5 *50s and precipitation at the same time. Sometimes they add up to more than 100% but it doesnt snow over those 15 days, anyway lets hope the latest NWP shows a higher chance

 

you have a 0.9^5 , or about 59%, chance of it not snowing on one of those 5 days, so that gives you a 41% chance of snow

Edited December 15, 2016 by ArHu3

[Yarmy]

8 minutes ago, russell k said:

Hmmm, sorry bluearmy.. but If we have a 10% chance on 5 days it should equate to 50% over that period of it snowing on one day of those 5, a bit like rolling a dice 3 times you have a 50% chance of rolling a six regardless of which dice I use, thats why I dont understand how the % chance is calculated,

I take it than the same member has to predict below -5 *50s and precipitation at the same time. Sometimes they add up to more than 100% but it doesnt snow over those 15 days, anyway lets hope the latest NWP shows a higher chance

 

If there is 10% chance of it snowing each day (however it is calculated), then there is 90% chance each day that it won't snow. Therefore the chance that it won't snow at all in the 5 day period is 0.9*0.9*0.9*0.9*0.9 = 0.59049, i.e. a 59% chance that it won't snow and therefore a 41% chance that it will at least once. But that assumes they are independent events (like the consecutive rolls of a die) and of course that's not true. To calculate the actual chance you need to know the probability that it will snow tomorrow given that it is snowing today and then apply Bayes' Theorem.

[Dennis]

 

[mountain shadow]

13 minutes ago, Dennis said:

 

I have absolutely no idea what that means.

[bluearmy]

The chance of snow on the gefs : I dont believe simple maths works in his case

if only two specific members (say 1 and 2) are cold enough to deliver snow (the other 18 are mild ) then if those 2 members show snow on five consecutive days the chance of snow on each day is 10%. It's never more than 10%  on each day so it can't be 50 or 41 percent at any time over a five day period.

that logic can only appy if more members are showing snowfall ?? 

 

[WINTRY WALES]

1 minute ago, mountain shadow said:

I have absolutely no idea what that means.

Neither do I ....and to go with this a summary of Gp's post....why is it intriguing?? Any one can give a brief explanation to both of these points?

[Seasonality]

Lower heights through to the med on this run compared to the 06z mean colder air pushing further west across mainland Europe and further east across the UK. Lets see where the high ends up this time!

[FetchCB]

1 minute ago, WINTRY WALES said:

Neither do I ....and to go with this a summary of Gp's post....why is it intriguing?? Any one can give a brief explanation to both of these points?

Basically its forcing on the Vortex red at the top is good, GP's post refers to it.

 

2 hours ago, Glacier Point said:

One other thing to look for, and perhaps of greatest interest, will be pattern projected for at least the next 10-15 days. +ve height anomalies over Eastern Europe / Western Russia will be geo-effective for Wave 2 increase in the low to mid stratosphere, taking effect early January.

 

[russell k]

21 minutes ago, Yarmy said:

If there is 10% chance of it snowing each day (however it is calculated), then there is 90% chance each day that it won't snow. Therefore the chance that it won't snow at all in the 5 day period is 0.9*0.9*0.9*0.9*0.9 = 0.59049, i.e. a 59% chance that it won't snow and therefore a 41% chance that it will at least once. But that assumes they are independent events (like the consecutive rolls of a die) and of course that's not true. To calculate the actual chance you need to know the probability that it will snow tomorrow given that it is snowing today and then apply Bayes' Theorem.

Thanks for that Yarmy, so how is the 10% calculated for the ensemble suite and do all the 20 members (is it 20 including the control and op) have an equal weighting ? 

[Yarmy]

2 minutes ago, russell k said:

Thanks for that Yarmy, so how is the 10% calculated for the ensemble suite and do all the 20 members (is it 20 including the control and op) have an equal weighting ? 

On that, I don't know. I guess it's just the number of members showing snow divided by the total number of members. As someone mentioned above, Sylvain will know.

[pyrotech]

Can i just clear up he 10% of snow in 5 days

Its like  a 10% chance of winning a race on Monday

You are 1 of ten runners so 10% chance of winning

On the Tuesday your also 1 of 10 running, so still 10% chance

Your chance of winning has not changed ( still 10%)

If not after 10 days it would mean your 100%  guaranteed of winning within the ten days

Your not, you could lose every day

However 

If a model predicts a 10% chance of snow over 8 days in january

70% chance over 3 days

and 0.1% chance for all other days you can see that for 3 days its predicting a fairly good chance

8 days its a low chance but possible,  

and just about forget the other days

What ever it says thou means very little beyond 5 days but its rolled on not added up

If a forecaster said that the storm will hit London, Tuesday 5%, Wednesday 80%  or Thursday 15% it means yes 100% 

a storm will hit but uncertainty of when but you can evaluate most likely Wednesday but if a change that change means

likely to be later (Thursday 15%) than earlier (Tuesday 5%) 

If that all makes sense

[northwestsnow]

UKmo-

GFS-

Edited December 15, 2016 by northwestsnow

[Seasonality]

Anyone else see 'The scream' by Edvard Munch. Hint, the right eye is centred over Iceland.

 

[warrenb]

 see the Xmas Eve storm is back

[Summer Sun]

GFS 06z and 12z both going for a 950mb storm on Xmas Eve

[jvenge]

If you are of a cold persuasion and live in the UK, the 12z is not your friend.

However, after a rather toasty Christmas eve, temps returning to just around normal for the UK by the 25th. 

[Summer Sun]

GEM going for a stormy Christmas

Edited December 15, 2016 by Summer Sun

[SMU]

46 minutes ago, mountain shadow said:

I have absolutely no idea what that means.

It means that the vertical eddy flux that is currently in a sedated state has the inverse effect on the zonal wind @60N-

As mentioned earlier & highlighted as a period of post xmas to watch for a deceleration of the zonal wind - you can see on the model plots above a significant increase in vertical wave flux indicating a slow down in the wind- period from xmas up to the start of new year

s 

[swfc]

pretty horrific xmas on the 12z.cant think of a worse looking chart on the day itself!!!!

[Geordiesnow]

3 minutes ago, Summer Sun said:

GFS 06z and 12z both going for a 950mb storm on Xmas Eve

Chances for such deep lows is definately a possibility and one to keep an eye on, ignoring any potential impacts regarding xmas, as a weather enthusiast, I would love if a deep low does occur, it is winter after all, deep lows are common and lets be frank, snow impacts looks very unlikely.

Got to say though, I am very dissapointed what the models are hinting at regarding the strengthening of the Northern arm of the jet which in turn looks like it will push away that cold PV over Western Russia and we get mild SW'lies blasting right through there. What we are seeing is by far the worse possible weather pattern you can get if your hoping for cold because its going to be well above average well to both our North and East and you get a situation where there is barely anything significant cold air on our side of the Arctic.

So its a case of looking North and West for any cold air to head our way and have some more in the way of moderated chilly airmass whereever we are on the cold side of the jet. In summery, significant snow looks a very long way from heading this way.

[oasis]

9 minutes ago, Geordiesnow said:

Chances for such deep lows is definately a possibility and one to keep an eye on, ignoring any potential impacts regarding xmas, as a weather enthusiast, I would love if a deep low does occur, it is winter after all, deep lows are common and lets be frank, snow impacts looks very unlikely.

Got to say though, I am very dissapointed what the models are hinting at regarding the strengthening of the Northern arm of the jet which in turn looks like it will push away that cold PV over Western Russia and we get mild SW'lies blasting right through there. What we are seeing is by far the worse possible weather pattern you can get if your hoping for cold because its going to be well above average well to both our North and East and you get a situation where there is barely anything significant cold air on our side of the Arctic.

So its a case of looking North and West for any cold air to head our way and have some more in the way of moderated chilly airmass whereever we are on the cold side of the jet. In summery, significant snow looks a very long way from heading this way.

Though some may disagree, if the russian high can cling on a little bit, warm 850s can get pushed into the arctic, then that can be a trigger for the wave 2 action, as mentioned by GP in strat thread. Hopefully such wave 2 action could push back against the downward propagation of insanely high zonal winds. Also of note Anthony Masiello on twitter mentioned that we're in a - EPO state? Which apparently couples well with wave 2 action when it comes to impacts. Seeing the insane push of "green" 850s into the arctic could lead to colder impacts down the line. 

Edit: The more directly that warm air can go into the arctic, the less the siberian cold pool will be pushed back - I think.

Edited December 15, 2016 by oasis
More info

[Summer Sun]

Some big old storms on the 12z

12z

06z