winter model output Winter Model Output Discussion -12Z 07/02/13 onwards

[Roger J Smith]

On the rain-snow question, I feel that the balance is going to tilt towards snow fairly quickly once the low deepens, because there is very little mild air around to extinguish, as soon as the gradient increases, colder air lurking nearby will be drawn in and precip will begin to change phase almost everywhere except around the southwest coast.

Rapid or even explosive cyclogenesis will take place in western Atlantic and it would not be surprising if the eastern Atlantic caught some of the energy wave associated. I would basically ignore most of the model guidance beyond about 144h except to note the large-scale features, the details will change when snow cover feeds back into the boundary layer temperature field.

I hope those polar bears like take-away because I would imagine the seal population of inland Britain is rather low.

[Weathizard]

Anyone got the NAO plots?

[on the coast]

Roger and his Dutch friend predicting the uk the new home for our furry white friends. Intetesting to see Sundays 12z to see where our winter is going.

[SMU]

On the rain-snow question, I feel that the balance is going to tilt towards snow fairly quickly once the low deepens, because there is very little mild air around to extinguish, as soon as the gradient increases, colder air lurking nearby will be drawn in and precip will begin to change phase almost everywhere except around the southwest coast.

Rapid or even explosive cyclogenesis will take place in western Atlantic and it would not be surprising if the eastern Atlantic caught some of the energy wave associated. I would basically ignore most of the model guidance beyond about 144h except to note the large-scale features, the details will change when snow cover feeds back into the boundary layer temperature field.

I hope those polar bears like take-away because I would imagine the seal population of inland Britain is rather low.

Roger Im afriad I have to disagree there - the deeper & more cirular low will advect warmer & warmer air ahead of the front from around the SW quadrent, it will also mix out the cold air at the surface, remember the deeper the low the less likely the forward winds will be SE, more Southerly or Swesterly.

We need a shallow low with acute angles for a decent snow event- less mixing & advection of mild air.

S

[Allseasons-Si]

Anyone got the NAO plots?

here you go W

still neutral i am afraid

link

http://www.cpc.ncep....nnections.shtml

[london-snow]

Theres 2 ways of looking at tonight-

* 1) T72 sees a potential snowfall event heading in from the west & with minimal increments sw required to make it a widespread snowfall event,with sustained frontal snowfall-

Cold uppers then feed in from the East with the potential of snow showers & cold nights. After this, we have the potential to see some sort of undercut arriving from the NE with heights remaining high over scandi we could get locked into a cold spell from the east for some time. these tend to have a habit of upgrading nearer the time.

*2) T72 sees a low approaching from the west & the profile of the jet is about 20 degrees more east than originally progged so the low is going to sweep a bit further east -

The low is also deeper than first thought- which are the 2 key ingredients for rain instead of snow.

As the low is deep, even when it moves away SE the trailing cold air will be mixed with warmer uppers circulating around the low rather than the very cold uppers that were there a few days ago.

Post this once again a rather annoying shortwave develops over SW Norway which inhibits the retrogression

pattern & allows a low pressure to get east of iceland - & that sliding angle is a bit to steep for the UK.

After which we see a continental flow develop- however this flow is very slack with weak low anomalies over France, the upper air is also only just below freezing. Assuming we are approaching day 10 this is the 17th of feb. Winter is nearly over.

I think we are very much as a half full juncture now, you could be very optimistic or pessimistic about things depending on your vantage point, im going to sit on the fence & say my glass is 45% full, yes thats right- option 2 favoured as its been a gradual trending disapointment over the last 48 hours with a downgrade of the severity of every aspect of the cold & of course a slight hint of IMBY.

I would love to be able to swing back to those + 60's & 70%s however after looking at the ECM & studying the UKMO + ECM tonight whilst im not be silly & saying the atlantic will come roaring in it could be a case of a slow boat to the inevitable as opposed to all roads lead to rome.....

S

Did you get a chance to view the ec ensembles which from what i heard on here was rather positive & disagreeing with its op run keeping things colder. I haven't viewed the output as i been in hospital most of the day up north so sorry if i have the wrong end of the stick.

[mountain shadow]

Roger Im afriad I have to disagree there - the deeper & more cirular low will advect warmer & warmer air ahead of the front from around the SW quadrent, it will also mix out the cold air at the surface, remember the deeper the low the less likely the forward winds will be SE, more Southerly or Swesterly.

We need a shallow low with acute angles for a decent snow event- less mixing & advection of mild air.

S

Would a deeper low not pull in the colder air more quickly though?

[fergieweather]

Roger Im afriad I have to disagree there - the deeper & more cirular low will advect warmer & warmer air ahead of the front from around the SW quadrent, it will also mix out the cold air at the surface, remember the deeper the low the less likely the forward winds will be SE, more Southerly or Swesterly.

We need a shallow low with acute angles for a decent snow event- less mixing & advection of mild air.

S

Don't lose sight of a key modification made by using Boyden (in this case for Sun-Mon using it at 50%) on each GM output frame to offer a better grasp of the snowfall potential versus where raw model only has rain.

[snowking]

GFS op beginning to get the idea now, and ECM ens suite backs the GEM and GFS suites with the NAO:

AO too:

Certainly tentative signs to back the recent height anomalies of something happening around the Greenland locale perhaps into the second half of February - the path from now until then remains uncertain though

SK

[Jason M]

Roger Im afriad I have to disagree there - the deeper & more cirular low will advect warmer & warmer air ahead of the front from around the SW quadrent, it will also mix out the cold air at the surface, remember the deeper the low the less likely the forward winds will be SE, more Southerly or Swesterly.

We need a shallow low with acute angles for a decent snow event- less mixing & advection of mild air.

S

Looking through the options available it might be as well to assume METO is the model of choice now. Its higher resolution upto 72 hours may just tip the balance in terms of nailing the low.

Jason

[bluearmy]

http://www.meteo24.fr/fr/home/meteo-france-et-europe/passion-meteo/ensemble-de-previsions.html?eps=london

Op on the warm side through the run. Note the cooling in week 2. Take out the wrong solutions and thats either v cold or averageish at best.

[Nick F]

Did you get a chance to view the ec ensembles which from what i heard on here was rather positive & disagreeing with its op run keeping things colder. I haven't viewed the output as i been in hospital most of the day up north so sorry if i have the wrong end of the stick.

Yes, interestingly the 12z EC op was at the milder end for the 10th to 11th for London:

Edited February 7, 2013 by Nick F

[Weathizard]

here you go W

still neutral i am afraid

link

http://www.cpc.ncep....nnections.shtml

Thanks for that AS.

Think for basically the whole of this winter its been neutral or close to it (-1 to + 1 range) shows the lack of blocking there has been around Greenland/North Atlantic throughout this winter. Been so much promise this winter anomalies/mean/long range wise but its never came to fruition, I certainly believe without the NAO mildy negative its very difficult to get severe freezes as this winter proves.

Edited February 7, 2013 by Weathizard

[Yarmy]

Don't lose sight of a key modification made by using Boyden (in this case for Sun-Mon using it at 50%) on each GM output frame to offer a better grasp of the snowfall potential versus where raw model only has rain.

I confess to having an excess of Shannon entropy when it comes to understanding what Boyden modification means? :-)

[fergieweather]

I confess to having an excess of Shannon entropy when it comes to understanding what Boyden modification means? :-)

Ah, sorry: see v good explanation by scrolling down at http://weatherfaqs.org.uk/node/152

It's very commonly used in all rain-snow modifications at UKMO and any other main Met ops centres internationally.

[Allseasons-Si]

Thanks for that AS.

Think for basically the whole of this winter its been neutral or close to it (-1 to + 1 range) shows the lack of blocking there has been around Greenland/North Atlantic throughout this winter. Been so much promise this winter anomalies/mean/long range wise but its never came to fruition, I certainly believe without the NAO mildy negative its very difficult to get severe freezes as this winter proves.

quote me if i am wrong but doesn't the NAO jump on board after the AO gets down to lower latatudes,just a thought?

[Ryan Dutch Weatherman]

Snow is really sometimes nowcast.

In Netherland now turn every precipitation yet into snow.

Dry snow , look at the radar. By T van 1 -2 celcius.

at 10 cm 0.5 .

En the farizeers told on tv in would be rain with their models.

Wel , this is example. So snow is very hard to predict the ammaunt /timing turn over ect.

But still nice we can talk about that.

Tommorow i will get from a britisch boy the kard from Brittain level to the sea en heighst.

So i personaly will join you ens guide at sunday/ monday by the probaly event snow.

I must study lot your country , especialy the SA . .

[fergieweather]

Critical part:

"In the course of this work, Boyden confirmed what others have already pointed out, that the 500-1000 hPa thickness parameter is the poorest discriminator for rain/snow. The best was height of freezing level and surface temperature, both difficult to forecast in 'critical' situations, and the next best/least worst was the 850-1000 hPa value, though Boyden devised the correction 'factor' that we all now use to take account of mean sea level pressure and local height."

This is why we never slavishly follow 850hpa values for assessing rain-snow discrimination. WBFL is most important parameter employed in charts from Exeter.

[Yarmy]

Ah, sorry: see v good explanation by scrolling down at http://weatherfaqs.org.uk/node/152

It's very commonly used in all rain-snow modifications at UKMO and any other main Met ops centres internationally.

Appreciated. Every day's a school day.

[fergieweather]

Critical part:

"In the course of this work, Boyden confirmed what others have already pointed out, that the 500-1000 hPa thickness parameter is the poorest discriminator for rain/snow. The best was height of freezing level and surface temperature, both difficult to forecast in 'critical' situations, and the next best/least worst was the 850-1000 hPa value, though Boyden devised the correction 'factor' that we all now use to take account of mean sea level pressure and local height."

This is why we never slavishly follow 850hpa values for assessing rain-snow discrimination. WBFL is most important parameter employed in charts from Exeter.

Ps it's also why the GFS ppn type discrimination charts are broadly pants.

[Zenarcher]

Before talking about Sundays low the models are obviously very messy at the moment, a very tricky time for them this coming weekend. Over the past 48 hours we have seen the models under estimate the high pressure over Scandinavia for example even the 18z GFS tonight and NOGAPS 18z runs have continued to give it more power this has once again given everything a push West this is all happening around 96 hours onwards and I expect them to continue to upgrade the high and give a push West for another while yet before they start getting it right.

So here are my views on the models tonight and some of their ensembles,

Sundays Low = Models are struggling with it and will change with time the ECM has the center of the low over Wales on Sunday afternoon and the GFS 12z had it slightly more South but on the 18z has moved it up over Wales where the ECM has placed it. The UKMO looks similar to the GFS 18z here they both have it around 995mb but the ECM goes much deeper at 990mb.

The other models like NOGAPS, JMA and GEM have it in a similar position but weaker.

Now looking through the ECM members all 50 of them about 35 that's 70% show the low to be similar to the operational run or make the low slightly deeper the rest go for a low that is weaker so the OP run has good support here.

Sunday is likely to create a wintry event for the UK but where exactly at this range is difficult to say and how much snowfall as well, the 18z shows snowfall across Scotland and Northern England, other parts of England and even Ireland see some, but for this event it won't really be before Saturday afternoon before anyone can say anything with decent confidence.

After Sunday and next week =

UKMO - Up to Wednesday it still has good strong pressure over Scandinavia, it gains good support from the ECM and other models tonight. The changes made to the UKMO today compared to yesterday is the pressure over Scandinavia being given a big upgrade, yesterday 1025mb isobar on the chart barely reached Iceland today it goes much more West nearly into Greenland.

ECM - ECM does have everything further West compared to the UKMO but the GFS doesn't agree with it completely. After you look at the models here it proves they don't have a clue what's going on at 120 hours a lot of disagreements on the little features that make large differences later on.

GFS - The GFS 18z changes show us what can happen, the image below shows the 12z and 18z side by side for the same time the low goes over the top on the 12z and the 18z goes under which gives us a more blocked pattern.

This annoyingly is one of those little features where there is big disagreement on between the models but it is a good example of what may happen.

JMA - Does send energy over our North but it sends high pressure down from Scandinavia but in no means does that say we will have mild weather, no way it says and has -8c uppers across the UK for a amount of time.

GEM - Only goes up to next Wednesday but gives the ECM and UKMO some support and comparing it to it's 00z run it adds to the pile that have made the shift West and upgraded the high pressure over Scandinavia.

NOGAPS - Talking of the 18z run agrees on a blocked pattern and is a good one overall it gives a push more West earlier on as well.

Overall - Sunday still uncertain about that one but over the coming days confidence will grow on what we may see there after that and into next week all the models show a blocked pattern and are constantly pushing things more West this is good to see as it gives the Atlantic a harder time to break through I will say looking through these models and the ensembles for the next 7 days none of them are keen on bringing an Atlantic weather setup pattern any time soon.

Edited February 7, 2013 by weathermaster

[Ryan Dutch Weatherman]

GFS op beginning to get the idea now, and ECM ens suite backs the GEM and GFS suites with the NAO:

AO too:

Certainly tentative signs to back the recent height anomalies of something happening around the Greenland locale perhaps into the second half of February - the path from now until then remains uncertain though

SK

To this is very good , Combi Ao en Nao indeze very good.Its making me now more stronger in my visions.

[SMU]

Don't lose sight of a key modification made by using Boyden (in this case for Sun-Mon using it at 50%) on each GM output frame to offer a better grasp of the snowfall potential versus where raw model only has rain.

HI Ian-

Using Boyden @ 50% for Sun / Monday still only gives us 1293 DAM available @ 850-1000 HPA

WUnderground has GFS thicknesses at-84

http://expert-images...021100_0712.gif

Whats the UKMO thickness overnight Sun into Mon>?- sub 1290 DAM?

Steve

Edited February 7, 2013 by Steve Murr

[london-snow]

Yes, interestingly the 12z EC op was at the milder end for the 10th to 11th for London:

Thanks Nick for confirming,

What can we say about today's output?

Confused.com stands out with options aplenty as we approach the weekend and beyond. However with a colder outlook out weighing a milder outlook at the moment. On to tomorrow we go.

Night all.

[svalji]

Thanks Ian for your valuable input - do the higher resolution models like nae have this correction built into their output?