Model Output Discussion 19th January 2014-18z onwards.

[Barking_Mad]

Hold the line people! Snow is measured in inches, not potential.

[snowking]

Need some advice, I always was led to believe that we need -10 850's and not -5 which people keep going on about. Especially in streamers as it's the difference between sleet and snow. So why are so many excited with the models? Where is the interest?

 

 

Hi Victor

 

The 850 temps are given as a broad-brush guideline to be honest and I think sometimes they are taken too much as the be all and end all of snow potential.

 

Essentially it all comes down to lapse rates - how quickly the temperature increases between the 850mb layer and the surface. This is dependant upon a whole host of different variables but in short the lower the heights are (the 'blue' colours shown on the 500mb charts) the shallower the system is and therefore the lower the lapse rate is.

 

What does all that mean? Well it means that under certain circumstances the atmosphere will be at a temperature supportive of snow at the surface even with the 850mb at say only -2c. This generally occurs when we get a shallow flow from the East, and so in the upcoming scenario the only requirement we really need to look for is for the 850mb temperatures to be below 0c - as long as the air between the 850mb layer and the surface is cool enough all the way down snow will fall.

 

What happens when we get Northerlies/North-Westerlies is we tend to get higher lapse rates, hence the temperature differential between the 850mb layer and the surface is higher, so whilst we might see -5c at the 850mb layer, it could be +5c or greater at the surface.

 

But as a general rule of thumb under a shallow flow (blue colours on the 500mb charts) from the East, you simply look for sub zero 850mb temperatures.

 

A much better guide to rain/snow discrimination (and one I tend to use more prominently) is dew points. The general requirement here is for the dew points to be at or below 0c (occasionally we can see wet snow with a dew point up to 1c, but generally use 0c as a guide), as without this is doesn't matter what the 850mb temperature is, it won't be falling as snow at the surface. Of course the problem there is that dew points even as far away as 36 hours can be incorrect and this is why snowfall is always so complicated to forecast.

 

The Met Office tend to use the Wet Bulb freezing level, but this in itself can cause some issues with a rather liquid requirement (if you'll pardon the pun) for the height. It can snow with a WBFL at 400m depending upon evaporative cooling, but generally you want it at 200m or below.

 

Hope this helps,

 

Regards

SK

[SMU]

Thanks Snowking- so around 15 cm along the spine all the way to the coast ( take note catacol)

http://f1.nwstatic.co.uk/forum/uploads/monthly_01_2014/xpost-1038-0-68700600-1390566286_thumb.png.pagespeed.ic.bo8iTE0zsB.jpg

 

DRAZTIK- this must be about the 7th reply to your ECM 10-15 day mean- you keep misinterpreting the image it doesn't show the atlantic returning.

 

 

The black lines are for Geopotential heights not windflow !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Edited January 24, 2014 by Steve Murr

[Frosty.]

Refresh mate its updated now.........

Hope it's better than yesterday.

[Panayiotis]

To get snowfall you don't just need uppers of or close to Minus 5.

 

You need dew points lower than 0, you need to look at the ''wet bulb'' and the higher the humidity the better.

 

 

 

Im sure its lower the humidity the better....

[DTHFCJ]

Hope it's better than yesterday.

im saying nothing Frosty...

[Troubleatmill]

Hi Victor

 

The 850 temps are given as a broad-brush guideline to be honest and I think sometimes they are taken too much as the be all and end all of snow potential.

 

Essentially it all comes down to lapse rates - how quickly the temperature increases between the 850mb layer and the surface. This is dependant upon a whole host of different variables but in short the lower the heights are (the 'blue' colours shown on the 500mb charts) the shallower the system is and therefore the lower the lapse rate is.

 

What does all that mean? Well it means that under certain circumstances the atmosphere will be at a temperature supportive of snow at the surface even with the 850mb at say only -2c. This generally occurs when we get a shallow flow from the East, and so in the upcoming scenario the only requirement we really need to look for is for the 850mb temperatures to be below 0c - as long as the air between the 850mb layer and the surface is cool enough all the way down snow will fall.

 

What happens when we get Northerlies/North-Westerlies is we tend to get higher lapse rates, hence the temperature differential between the 850mb layer and the surface is higher, so whilst we might see -5c at the 850mb layer, it could be +5c or greater at the surface.

 

But as a general rule of thumb under a shallow flow (blue colours on the 500mb charts) from the East, you simply look for sub zero 850mb temperatures.

 

A much better guide to rain/snow discrimination (and one I tend to use more prominently) is dew points. The general requirement here is for the dew points to be at or below 0c (occasionally we can see wet snow with a dew point up to 1c, but generally use 0c as a guide), as without this is doesn't matter what the 850mb temperature is, it won't be falling as snow at the surface. Of course the problem there is that dew points even as far away as 36 hours can be incorrect and this is why snowfall is always so complicated to forecast.

 

The Met Office tend to use the Wet Bulb freezing level, but this in itself can cause some issues with a rather liquid requirement (if you'll pardon the pun) for the height. It can snow with a WBFL at 400m depending upon evaporative cooling, but generally you want it at 200m or below.

 

Hope this helps,

 

Regards

SK

 

Great explanation SK, this is also a useful link Victor  http://www.sciencebits.com/SnowAboveFreezing

[legritter]

Refresh mate its updated now.........

Thanks for that info ,much appreciated .yes dont think the met office will react yet to possible changing events next thursday ish as its a good 5/6 days away so all cross fingers ,I feel a prozack coming on or a behind the settee sleep ,cheers 

[Paul]

Jo's latest blog is all about the snow risk over the next 7 days:

 

It's definitely feeling colder, so any snow hopes, anything for end of January?


View the full blog here

[Radiating Dendrite]

Yes I know that as it's happened before, but when it comes to streamers much less likely.

 

But we are not looking at streamers.

 

The lows crossing NW - SE will bring the precipitation.

 

You will not get a Thames streamer etc initially from this set up as -5c uppers will not generate strong convection.

[Purga]

Hope it's better than yesterday.

it's the same.

[chionomaniac]

I haven't time to post anything in detail, but to answer CH - I think that the strat is far more conducive now to allowing the forecast trop outlook to occur. Critically, we have lost the period of Vortex Intensification (VI) that has plagued the winter so far, and that leaves us with a vortex that is being bruised and battered by the forecast wave activity. We may not get a SSW or strong Greenland heights, however the disrupted vortex should be just enough to allow  the Scandi block to persist. Will hopefully be able to post more thoughts later in the strat thread - but the promise is good and not unexpected and could improve further still by mid Feb.

Edited January 24, 2014 by chionomaniac

[R.P. McMurphy]

 

I'd LOVE it to snow next week, god oinly knows I love snow as muxch as everybody else, but I'm not going to allow myself to be lead up the garden path by charts that are close, but not close enough IMBY and I've learn't the hard way not to get excited about a North Westerly epecially if the cold air isn't already in place before it arrives !!!

 

 

I think we are all well aware that charts in advance and never a guarantee, but I think many of times the Mods & Paul has stated, if you have a post along the lines of "I dont think it will do this" Just Because you have that gut feeling, or it's happened before, then that should be in the Banter thread, as it's clearly not reflecting what all of the models are showing.

 

Your OP of the Atlantic will power through etc - is completely false, as no where does it show this on the charts, so why post it if it's a gut feeling, it confuses other members who don't kinow how to read the charts or are struggling to read the charts.

 

I understand what you are saying, we have ALL been burnt many of times by charts showing very snowy periods and cold, because they are notoriously difficult to predict, so I am in full agreement with you there, we should not get excited until maybe T~48, even though I have seen it go belly up at T~24 before...

 

But please, please, please think before posting what your "Gut Feeling" is, because it's of no use to anyone.

 

Also this:

 

 

I've learn't the hard way not to get excited about a North Westerly epecially if the cold air isn't already in place before it arrives !!!

 

I'll have you know it snowed here yesterday at 20M ASL ! And that was from a super brief WNW flow.

[Radiating Dendrite]

Thats what i was thinking, with all the hype about the cold in here i was expecting better than that! Maye the North East seeing snow but for the most of the UK just cooler rain. Not impressed!

 

I would relax.

 

It was supposed to be raining in London by now and it is not!

 

This is a developing situation and the Met are being cautious.

 

Interesting 12z to come - hopefully we will see no back tracking from the Euros. Personally I am hoping that we start to see some cooler uppers being modeled with the sliders and importantly the sliders to be a little further west.

[Purga]

Thats what i was thinking, with all the hype about the cold in here i was expecting better than that! Maye the North East seeing snow but for the most of the UK just cooler rain. Not impressed!

Maybe Ian F will shed some more light on METO's more 'off the record' type musings at some stage?

[tcc]

Thats what i was thinking, with all the hype about the cold in here i was expecting better than that! Maye the North East seeing snow but for the most of the UK just cooler rain. Not impressed!

 

Patience is needed at this stage, remember the signal has been pretty weak for the end of next week for the block to stay put. So to suddenly find in the last couple of the signal for blocking becoming more prevalent requires monitoring at this stage. If the signal is consistent and grows stronger over the next 24/48 hours then you'll see a step change in the updates.

[shedhead]

Thats what i was thinking, with all the hype about the cold in here i was expecting better than that! Maye the North East seeing snow but for the most of the UK just cooler rain. Not impressed!

This is the problem and until the MO come on board with this caution will remain the watchword imo.

 

I'm sure someone will come up with an example, but I'm strugging to remember a time when they we badly wrong with their 6-15 day forecasts across any season. I guess it's more than a tad ironic that whilst this is very much the case, the short term forecast they issued for today after last nights 10pm BBC News was badly innaccurate in timing, extent and intensity of rainfall....you couldn't make it up really!

[throwoff]

The block certainly seems to be stronger than the models thought, or of course the Atlantic is not as strong, the rainfall on the radar RIGHT NOW that was due to head east is being pushed south.

 

As usual it goes to show even +24 models can be wrong. This evening and tomorrows runs taking into account the strength of the block on what happens with the PPN today will be very interesting.

[shaky]

Has anyone got a link where you can see live 850hpa temperatures?thanks

[R.P. McMurphy]

This is the problem and until the MO come on board with this caution will remain the watchword imo.

 

I'm sure someone will come up with an example, but I'm strugging to remember a time when they we badly wrong with their 6-15 day forecasts across any season. I guess it's more than a tad ironic that whilst this is very much the case, the short term forecast they issued for today after last nights 10pm BBC News was badly innaccurate in timing, extent and intensity of rainfall....you couldn't make it up really!

 

Yes, that forecast went all wrong really - But was this due to underestimating the strength of the block out East? Maybe so if you look at how the low has quickly in the wrong direction they were saying...

 

It's not often the Met Office are wrong to be honest, but with pretty much 85% agreement of a cold and potentially snowy spell next week, they don't mention anything, and until they do, I usher caution. Lets not forget, they do have access to a much broader scale of charts than we do... And they are Professionals.

[mickpips]

Maybe Ian F will shed some more light on METO's more 'off the record' type musings at some stage?

The METO will always sit on the fence , they work on probabilities and percentages. Given several factors within the model outputs that could change next weeks colder spell then they are very unlikely to paint a snowfest picture. As has been alluded to all morning in this thread , their forecast for today has been 'off the mark' so to speak and just last night their 5 day warning had the whole country on a yellow warning for Wind on Sunday/Monday which has now been refined quite a lot ... this shows they were very much unsure about this weekend , let alone next week !

Another example would be their text forecast for my region which states ' colder on Tuesday' yet their forecast temps for Tuesday are the same as Monday   We can view the output and come to our own conclusions ( which is why we are here ) but the probability of being correct is what's debatable.

 

Edited January 24, 2014 by mickpips

[weather eater]

The rain is not threatening the London area - stalling over the western fringes of the Chilterns. Block stronger?

It certainly looks that way, looking at the rainfall radar and it’s a real struggle even here in the west, we have only had a very light drizzle. However, that doesn’t change the point; yesterdays forecast for the eastern side of the country didn’t say raining all day. As I said in earlier post, weather has the habit of catching out the cleverest of people; the METO would be the first to say so. That doesn’t of course mean that the MO should start issuing alarmist calls about the prospects of colder weather arriving next week, the knife edge to me still looks 60/40 in favour of a colder but unsustained spell of weather rather than a major freeze up, let’s hope the 12z builds on this mornings output.

[Neilsouth]

Hi Victor

 

 

Hope this helps,

 

Regards

SK

 

Thankyou for that!

[Eskimo]

Yes, that forecast went all wrong really - But was this due to underestimating the strength of the block out East? Maybe so if you look at how the low has quickly in the wrong direction they were saying...

 

It's not often the Met Office are wrong to be honest, but with pretty much 85% agreement of a cold and potentially snowy spell next week, they don't mention anything, and until they do, I usher caution. Lets not forget, they do have access to a much broader scale of charts than we do... And they are Professionals.

Like that BBQ Summer that never was  

Of course I'm only joking!

To be fair, the critical point is at 96 hours from now, whether that low slips underneath or stays in situ!

[winterof79]

But we are not looking at streamers.

 

The lows crossing NW - SE will bring the precipitation.

 

You will not get a Thames streamer etc initially from this set up as -5c uppers will not generate strong convection.

Sorry, i think i logged in to the will it snow in the S/E regional thread in error.