Model Output Discussion - 23rd Jan - 12z onwards

[snowking]

I may be missing something but it looks like it will not be snowing at sea level with those variable. The WBZ shows at 100m plus. Posting this site's snow chart for that time period:

 

uksnowrisk (1).png Over 100m in the central belt for snow needed?

 

Any snow is shown on hills and mountains. I am aware that with a higher altitude the chances of snow increases even with marginal variables but I always look at sea level so that covers nearly everyone. 

 

There's not much precipitation around on the charts I posted (hence the low snow risk shown) but I was making a wider point re: correlation between 850mb temperature and other variables under lower heights.

 

WBFL only needs to be 200m or lower in most instances for snow to fall - it can fall at up to 400m but tends to be rather hit and miss at such a height.

 

As for those snow risk charts well quite honestly if anybody is using those solely at this range to try and forecast snow well best of luck with that one, but even on the chart posted for the overwhelming majority of the UK landmass the snow level is down to the surface, so I am a little confused as to the point you are making here? Perhaps some confusion re: the intention of my initial post. 

 

SK

Edited January 25, 2015 by snowking

[The Enforcer]

Whilst SK was posting, I was coincidentally looking at GFS dewpoint data from today until next Wednesday. I use GFS, because that's the only model for which dewpoint data is available on N-W. On the 06z, after 3pm Wednesday, there are ten frames where dewpoints were above freezing in my location (during the course of Saturday and Sunday). On the 12z, this had dropped to one frame on Sunday. So in this respect, the 12z is better than the 06z. Unable to compare with UKMO or ECM, but assume those will be better as flow source is supposed to be colder.

[Mucka]

That's the same chart as SM posted 2 minutes earlier but a totally different take on it. No wonder some of us get confused.

 

Yes I can understand that but the two posts are both talking about the potential of that chart in actuality.

The only real difference is steve skips the messy evolution before that whereas I dwell on it and go off on a bit of rant which I've apologised for.

 

My comments about the wrong side of marginal are the run up to that chart which Steve did not comment on and that particular chart was posted to outline how far West the pattern had been pushed but also the potential it offered as eye candy compared to the more reliable timeframe where it delivers just about the worse possible evolution for maintaining cold air over the UK in that period IMO - hence the rant.

 

Steve, I'm sure, would agree the UKMO is colder and any snow would be less marginal than GFS out to that chart where it begins to draw the colder air back in. There is no reason to prefer GFS evolution over UKMO IMO.

I agree we shouldn't get hung up on 850's at this point but we want the trough further East rather than West as the GFS is modelling and I think it is reasonable to want that rather than hope the messy GFS is the right side of marginal and shortwaves will appear in the right places with the right track day 5+.

 

Hope that clears my position up at least.

 

Edit.

 

Just to add there is nothing marginal Wednesday night Thursday on GFS as I have stated previously it the period through into eekend that GFS is very margiinal for snow to lwo lying areas and ground temps will lift from Thursday until colder is drawn back in later in the weekend.

Edited January 25, 2015 by Mucka

[Captain Shortwave]

The GFS continues to mess around after the brief window for snow Wednesday night into Thursday (by the way the drop in 850s down here on Wednesday is quite spectacular, around +5C to -7C in less than 6 hours), shame it seems to occur behind the front.

We can see even by Thursday there are disagreements on the placement of that low

GFS

This sinks south east along the east coast of the UK and into Europe. To be honest the GFS keeps changing the track of this on each run.

 

UKMO

Over Scandinavia though it tends to barrel around between Scotland and Norway before beginning to sink into Europe, generally a cleaner north to north westerly flow.

 

GEM

Main low over Scandinavia, but a secondary feature over Scotland, it sort of sits in the middle I guess.

 

Only the GFS brings a significant feature in around day 6 from the Atlantic which pushes through southern England. The UKMO and GEM make nothing of it (weaker and simply rolls around the Azores high and never gets near the UK).

 

Not really a good time to be making bold statements as the deeply low 500mb heights are causing turmoil in the positioning of the surface pressure pattern. Certainly look cold with the risk of snow at times. But where and when is not really answerable at the moment. UKMO looks the best solution out of the three at the moment for those wanting cold and snow. GFS and GEM look messier.

[Jason M]

GEFS are out to 180 now.

All are variations on the theme. All are cold, some show rain, some show snow and some show rain but snow to come and vice versa. Hope that clears things up :-)

[DTHFCJ]

wet bulb freezing level i think i read it somewhere....

[Man With Beard]

Relevant to reading charts this week is Dec 2010 (second cold blast about 17th onwards) - a northerly in low 500hpas - check the archives and I think you'll find a lot of snow fell at -4C 850s. We didn't have embedded cold at the precise moment it started either.

[Sawel]

You are right west they do matter and in my mind they are the main indicator we have on whether its going to snow but anyhow like you said its all been covered before. 

 

Anyway big differences in the output at 144   Which one will be right? UKMO is notoriously unreliable at 144 so i will maybe go GFS on this one. Either way it all looks a bit marginal but hopefully the right side of marginal 

 

And yes the screaming jet is a worry as well!!

 

If the UKMO is "notoriously unreliable at 144" then why do these statistics tell something different?

 

http://www.emc.ncep.noaa.gov/gmb/STATS/html/new_acz6.html

[SMU]

Indeed- ^^ UKMO Tops there better than ECM over the last 31 days however it is HN...

 

Anyway heres the blocker to the Northerly deep cold on the GFS which isn't on the euros-

 

http://modeles.meteociel.fr/modeles/gfs/runs/2015012512/gfs-0-102.png?12

 

note the 960 low over Scotland - what this does is - in its position west of Scotland it blocks the cold air from flooding south & forces it away south West into the atlantic-

 

The ECM & UKMO are cleaner, keeping it east of the UK

 

http://www.meteociel.fr/ukmo/runs/2015012512/UW120-21.GIF?25-17

 

So if your looking for why / If the deeper cold is coming look in that area!!
S

 

& yes RJBW great reference post to this chart-

 

http://modeles.meteociel.fr/modeles/reana/2010/archives-2010-12-18-12-0.png

Edited January 25, 2015 by Steve Murr

[snowking]

WBFL  Sorry for ignorance, all I came up with is- Wobbly Big Fat Lady.

Yes sorry I reverted back to acronyms!

 

Wet Bulb Freezing Level

[Harsh Climate]

A lot of talk about uppers and how they affect our our chances of snow. I find you only usually get low lying snow with uppers milder than -5hpa in a couple of scenarios. The first one being heavy slow moving precipitation with EVAPORATIVE COOLING and the second being, having a cold/dry under cut of air from the continent, with features bumping up against this.

 

Last weeks cold spell was the perfect example. I noticed low lying snow accumulations (below 100m) from that northwesterly only when the uppers were -7/-8 or colder. And yes the 500mb profile was about as cold as it gets. Even with very cold 500mb temps as soon as the uppers were -6 or milder it was mainly sleet below 100m.

 

 

WED night into THURSDAY all low lying areas that get frequent snow showers weill see snow accumulations, after that if the gfs is to be believed things become very marginal. The UKMO is better because it would most certainly bring in colder uppers. But as the pendulum has temps swing more conductive to snow, I'd bet my bottom dollar that our atmosphere would be much dryer.

 

 

If the gfs does win out, I think it is imperative that we get some sort of easterly/continental feed undercutting any approaching zonality or features inbedded in the northerly flow for low lying snow with uppers milder than -5 hpa.

Edited January 25, 2015 by Harsh Climate

[Frosty.]

WBFL  Sorry for ignorance, all I came up with is- Wobbly Big Fat Lady.

Well I know this much, the fat lady won't be singing for a good while from Wednesday onwards. Again I'm really surprised by some of the doom and gloom comments since we are on the cusp of the most wintry spell since God knows when..models are all singing from the same hymn sheet, just variations on the same theme..slow moving pattern coming up with long-wave trough to NE and high pressure much further west building north..very happy with where this is going.

[Jason M]

Looks like ptrb 12 is going cobra this time. Surely the best chart for months this one!!

http://www.meteociel.fr/modeles/gefs_cartes.php?code=12&ech=228&mode=0&carte=0

Greenland high, Italian low, Cold uppers, etc etc

Very much an outlier solution so early on in the run though.

[bluearmy]

Looks like ptrb 12 is going cobra this time. Surely the best chart for months this one!!http://www.meteociel.fr/modeles/gefs_cartes.php?code=12&ech=228&mode=0&carte=0Greenland high, Italian low, Cold uppers, etc etcVery much an outlier solution so early on in the run though.

In which case, given we have fantastic charts within a week, why post it J?

Anyway, quite astonished by some posts this afternoon. Kris has explained why black is not white and yet some still protest that it is.

Also, IDO, why use the coastal snow signal when it's damn difficult to get coastal snow due to the SST evident. (Unless you are purely looking at coasts with no possible onshore wind vector in which case they will probably not show any convective potential)

All very confusing for me and I don't consider myself a novice. God knows what some of the new members and guests are thinking. I imagine poor JH on his fifth scotch looking up at the jungfrau pulling out whatever hair he can spare! Fortunately , he won't be reading this thread today!

Edited January 25, 2015 by bluearmy

[Jason M]

In which case, given we have fantastic charts within a week, why post it J?

Anyway, quite astonished by some posts this afternoon. Kris has explained why black is not white and yet some still protest that it is.

Also, IDO, why use the coastal snow signal when it's damn difficult to get coastal snow due to the SST evident. (Unless you are purely looking at coasts with no possible onshore wind vector in which case they will probably not show any convective potential)

All very confusing for me and I don't consider myself a novice. God knows what some of the new members and guests are thinking. I imagine poor JH on his fifth scotch looking up at the jungfrau pulling out whatever hair he can spare! Fortunately , he won't be reading this thread today!

Erm, because its the model discussion thread? Also its not exactly to T3000 hours either and is also perfectly possible.

Edited January 25, 2015 by Jason M

[Eugene]

Looks like ptrb 12 is going cobra this time. Surely the best chart for months this one!!

http://www.meteociel.fr/modeles/gefs_cartes.php?code=12&ech=228&mode=0&carte=0

Greenland high, Italian low, Cold uppers, etc etc

Very much an outlier solution so early on in the run though.

 

 

No there are several runs similar but yes not got major support on the GFS ensembles, wonder if the dreaded MLB high just centred to our west could occur around the 4th onwards.

Edited January 25, 2015 by Eugene

[inghams85]

Agreed! If anything this spell will be a snow forecasting education for some on the forum BA! Some really odd posts today!

In which case, given we have fantastic charts within a week, why post it J?

Anyway, quite astonished by some posts this afternoon. Kris has explained why black is not white and yet some still protest that it is.

Also, IDO, why use the coastal snow signal when it's damn difficult to get coastal snow due to the SST evident. (Unless you are purely looking at coasts with no possible onshore wind vector in which case they will probably not show any convective potential)

All very confusing for me and I don't consider myself a novice. God knows what some of the new members and guests are thinking. I imagine poor JH on his fifth scotch looking up at the jungfrau pulling out whatever hair he can spare! Fortunately , he won't be reading this thread today!

[Cloud 10]

I think i'm developing a fixation for these purple charts.

 

ECM at 96hrs is loaded with "weather"

 

[chris55]

Ian F's tweets should allay some peoples concerns regarding 850s etc. The met would not be signalling snowfall if they were concerned it would be to warm. Last week when things looked marginal the Met were confident it would just be rain/sleet for most, and they were right. Now they are warning of snowfall.

 

So the initial colder spell - Weds to Friday looks like a good bet. Where any snow falls will almost be a nowcasting event, though obviously the prone areas to the north and west are favourable, though troughs popping up in the flow are very possible.

 

From this point into the weekend we need more runs though to me a continued northerly with even colder air setting in looks about right.

[inghams85]

It's not criticism. It's been an interesting debate in regards to snow parameters but several experienced posters have already proved why the marginality of 850's in this set up aren't critical yet posters still post the same things in regards to uppers being too high blah blah. It's tiresome to go round in circles

[MR EXTREMES]

Erm, because its the model discussion thread? Also its not exactly to T3000 hours either and is also perfectly possible.

Yeah and most of it has been arguing over snow where when and dew points.

But to be fair I've not seen forecasts as confident as is coming out tonight even the recent cold spell proved difficult but this time there's more urgency to the forecast.

Using the chart Lorenzo posted up working from that there's a clear indication that most of the uk could well be suprised.

Then the models continue to drag colder and colder air into the uk so any mild sectors become mixed out by colder uppers and the added bonus of enriched surface cold.

Other that a slightly milder break on Thursday afternoon for no less than 12hrs with the next pulse of cold being Arctic sourced.

Even the polar air being shown Tuesday showing snow for the North.

Ecm is going to show us a downgrade because honestly you couldn't upgrade the ecm and ukmo charts we been seeing over the last couple of days.

And as ba suggests posting charts with coastal snow being marginal well of coarse always is but at this point in winter February is a month that snowfall can be at its best.

[Captain Shortwave]

ECM pretty much duplicates its morning run out to day 5.

850s similar too. The pressure pattern suggest a front moving southwards later on Thursday and into Friday, this could produce an organised band of precipitation. (850s along the band look to be around -5/6C)

 

Northerly at day 6

850s on the fall again, Friday looks a little less cold with temperatures around 4-6C (this just going by the BBC forecast from earlier, this looks like the high point for temperatures). Turning cold or very cold from Saturday.

Edited January 25, 2015 by Captain shortwave

[I remember Atlantic 252]

I think i'm developing a fixation for these purple charts.

 

ECM at 96hrs is loaded with "weather"

 

ECU1-96.GIFECU0-96.GIF

 

very marginal, rain low levels?, to newbies, purples do not mean cold

[shaky]

very marginal, rain low levels?, to newbies, purples do not mean cold

no mate that looks pretty good with snow!! Especially across england! !

[Day 10]

Cold digging back in at 144