Model Output Discussion

[Glacier Point]

Couple of observations...

Deep convection remains east of the Dateline. As the atmosphere - ocean coupling is now fully enganged with this Nino, there is a strong probability that tropical forcing will continue to be situated in this part of the Pacific with associated downstream Rossby Wave Dispersal in phase 7/8/1.

http://cawcr.gov.au/...m/OLR/m.3d.html

The outlook for March features a strong blocking structure to the north Atlantic and supports the idea of the focus for tropical convection being more or less where it is. In other words, the hemispheric circulation is likely to become very stagnant. The signal for a continuation of a deep upper mean trough over Scandinavia is very strong.

Once the cold front clears through early part of next week, a cold pool with 850 values between -3C and -8C will become established across NW europe.

So, cold but not exceptionally cold pool established. Nothing in the longer term modelling even remotely offering a knock-out blow for mild. Any forward intertia developing in the Atlantic will, ironically, bring the cold uppers southwards.

Could well be an interesting 4-6 week period. Cold zonal ? Obviously the sun gains strength by the day making snowfall progressively less likely but average or below average looks a strong probability for the period.

Edited February 11, 2010 by Glacier Point

[johnholmes]

There are certainly some rather large differences by T+72 with the 3 main models.

Having spent time looking closely at them, that is the basic 1000-500mb its hard to see why they have this and even harder why they then diverge even more.

So I turned to the upper air pattern, it is that after all which largely governs the movement of features at the surface in this time scale.

The 300mb flow develops a 'minor' jet from about NNW from Iceland and as this elongates and develops it obviously has a marked influence on the lows movement.

What seems odd to me is just how much of a 'right hand turn' both EC and Met takes. GFS does appear to follow a more usual path given the position etc of the jet.

How we end up with one model showing the low in the North Sea and the other in the southern end of the Celtic Sea I am at a loss to explain. Nick S comments that the latest T+72 Fax chart has changed the lows position and its true. That is even more mystifying as to where its T+96 and T+120 this evening is likely to be. For it to achieve the previous Met idea then its got to do an almost sharp right hand turn over the 48 hours.

At least, come Saturday, we can, or I can, go back to first principles, as we will have its position and the upper air pattern as actuals to try and work out from first principles just where its likely to go.

Fascinating it certainly is.

just noticed the post by GP above and yes I'd agree with that idea of cold zonal with possibly temporary N maybe even NE flows at times in the longer term.

As they say, spring may be a little late this year!

Edited February 11, 2010 by johnholmes

[tinybill]

There are certainly some rather large differences by T+72 with the 3 main models.

Having spent time looking closely at them, that is the basic 1000-500mb its hard to see why they have this and even harder why they then diverge even more.

So I turned to the upper air pattern, it is that after all which largely governs the movement of features at the surface in this time scale.

The 300mb flow develops a 'minor' jet from about NNW from Iceland and as this elongates and develops it obviously has a marked influence on the lows movement.

What seems odd to me is just how much of a 'right hand turn' both EC and Met takes. GFS does appear to follow a more usual path given the position etc of the jet.

How we end up with one model showing the low in the North Sea and the other in the southern end of the Celtic Sea I am at a loss to explain. Nick S comments that the latest T+72 Fax chart has changed the lows position and its true. That is even more mystifying as to where its T+96 and T+120 this evening is likely to be. For it to achieve the previous Met idea then its got to do an almost sharp right hand turn over the 48 hours.

At least, come Saturday, we can, or I can, go back to first principles, as we will have its position and the upper air pattern as actuals to try and work out from first principles just where its likely to go.

Fascinating it certainly is.

evening john just looked at the gfs from 15 feb to at least 25 feb its mouth watering cold is this event going to be better then the cold snap just before xmas we had!!

[Jason T]

Really hard to know whats going to happen with this low pressure. The UKMO has a much weaker low, the ECM and GFS a deeper feature. I think it depends which area of the country you're in as to what you want the low to do.

Looking at the fax chart at 72hrs it looks like the UKMO have modified this towards the ECM which is very unusual as they normally only modify their 96 and 120 hrs output. If this is the case then they're also likely to do the same with the later fax charts.

However as to any snow hard to say until we see agreement on exactly what this low is going to do. If any continental air gets pulled nw as the low heads south then that would favour areas on the eastern flank of the low as theres still a very good pool of cold surface air in Europe.

As to the extended outlook the ECM drops a little tease into its later output.

Nick as i am recent newbie, always look at your posts do i take it at present its no mans land as to which model is correct and whats going to happen. Been watching and reading all posts all week, most were saying do not look past 4 days out,Now it seems we cant get an agreement past 3 days.

Confused.

[tinybill]

Confused??? You wait until the 18z starts rolling out! My guess would be a split in the LP around Birmingham, with the eastern flank dropping blue salamanders from the skies and the western flank whisking small mammals up into the clouds. OT I am sure, but seems as plausible as any other forecast at this stage. Sorry.... I'll get my coat...

all i say just looking at the gfs from monday 16 feb till F1 fantasy land

feb 25 its mouth watering all eyes on the update to-night!!!

[Duane S.]

Really hard to know whats going to happen with this low pressure. The UKMO has a much weaker low, the ECM and GFS a deeper feature. I think it depends which area of the country you're in as to what you want the low to do.

Indeed. I really have hit a brick wall trying to work out what the final evolution will be here. The whole scenario of a low droping south over the UK with a very southerly based jetstream (as I suppose it would have to be by definition) is certainly not exactly a common one.

As I posted earlier on today, the problem is, I just don't know which way I want to see the charts trending!

Edited February 11, 2010 by s4lancia

[nick sussex]

Nick as i am recent newbie, always look at your posts do i take it at present its no mans land as to which model is correct and whats going to happen. Been watching and reading all posts all week, most were saying do not look past 4 days out,Now it seems we cant get an agreement past 3 days.

Confused.

The main point of uncertainty isn't the general pattern but the smaller scale detail which is very important in determining snow potential. Generally its better if the low doesn't develop too much. On top of this its the track which is causing further uncertainty, its not just the initial snow possibilities but down the line in how the pattern might evolve that this effects, the further south the first low gets the better as shortwave energy is likely to head into this causing it to pivot back, so its like a domino effect here where the initial track and intensity of the low at 96hrs can effect further snow chances and how much the flow becomes modified.

At the moment the UKMO looks the odd one out of the major global models and I would expect some modification of the UKMO raw output when the next fax charts come out.

Indeed. I really have hit a brick wall trying to work out what the final evolution will be here. The whole scenario of a low droping south over the UK with a very southerly based jetstream (as I suppose it would have to be by definition) is certainly not exactly a common one.

As I posted earlier on today, the problem is, I just don't know which way I want to see the charts trending!

Well the only trend that I think everyone can agree on is as south as possible! Regardless of whether the low goes further west or east.

Edited February 11, 2010 by nick sussex

[Frosty.]

All I know is I would rather see an Arctic Low dropping down from the north than the usual sw / ne progression. There is no sign of mild weather in the outlook and the jet looks like remaining well to the south which will keep the uk at least in rather cold air and much colder than that at times, especially further north, it's still unclear how this arctic low will impact on the BI next week but there is a recipe there for snow due to the potent mixture of cold air and very unsettled conditions.

[R.P. McMurphy]

Couple of observations...

Deep convection remains east of the Dateline. As the atmosphere - ocean coupling is now fully enganged with this Nino, there is a strong probability that tropical forcing will continue to be situated in this part of the Pacific with associated downstream Rossby Wave Dispersal in phase 7/8/1.

http://cawcr.gov.au/bmrc/clfor/cfstaff/matw/maproom/OLR/m.3d.html

The outlook for March features a strong blocking structure over Canada and western Greenland, and supports the idea of the focus for tropical convection being more or less where it is. In other words, the hemispheric circulation is likely to become very stagnant. The signal for a continuation of a deep upper mean trough over Scandinavia is very strong.

Once the cold front clears through early part of next week, a cold pool with 850 values between -3C and -8C will become established across NW europe.

So, cold but not exceptionally cold pool established. Nothing in the longer term modelling even remotely offering a knock-out blow for mild. Any forward intertia developing in the Atlantic will, ironically, bring the cold uppers southwards.

Could well be an interesting 4-6 week period. Cold zonal ? Obviously the sun gains strength by the day making snowfall progressively less likely but average or below average looks a strong probability for the period.

Excellent summary Glacier Point. Thank you for your input. Why you don't be Chief Officer at the MetOffce I don't know!

[bluearmy]

http://www.knmi.nl/exp/pluim/vijftiendaagse/index.html

the trend back to proper cold after the trough continues ...................

[Cheese Rice]

What I don't understand is, if the low is from the Arctic wouldn't it be a polar low? There don't seem to be any mild air in the northern hemisphere and the source of the low isn't cold either and yet the GFS shows milder uppers circulating around the low, why is that?

Edited February 11, 2010 by 10123

[len]

What I don't understand is, if the low is from the Arctic wouldn't it be a polar low? There don't seem to be any mild air in the northern hemisphere and the source of the low isn't cold either and yet the GFS shows milder uppers circulating around the low, why is that?

Go through or animate this sequence http://en.vedur.is/weather/forecasts/atlantic/#eg=temp&type=temp

[dubmuffin]

So, my very basic understanding reads, that as the high pressure moves away west, warmer air from the atlantic gets dragged round the top clockwise, and then as the low comes down, the warmer air gets dragged back anticlockwise from the west?

[nick sussex]

What I don't understand is, if the low is from the Arctic wouldn't it be a polar low? There don't seem to be any mild air in the northern hemisphere and the source of the low isn't cold either and yet the GFS shows milder uppers circulating around the low, why is that?

A polar low is different from a normal low formed north of the polar front. A polar low is formed generally in an unstable northerly flow when the upper atmosphere is generally below -35, these aren't well forecast further out than 24hrs. Interestingly they're also called Arctic Hurricanes as they often have an eye in the centre!

The low the models are currently forecasting actually forms near Iceland, the deeper this becomes the more likely we'll see milder air sucked into its circulation and a larger milder sector.

[Cloud 10]

What I don't understand is, if the low is from the Arctic wouldn't it be a polar low? There don't seem to be any mild air in the northern hemisphere and the source of the low isn't cold either and yet the GFS shows milder uppers circulating around the low, why is that?

The low actually forms in between greenland and iceland as shown on the +48hrs above,although it does draw in air from the arctic as it develops and heads SE S SW wherever it decides to go!

[Cheese Rice]

Thanks for the replies

[MKN]

A polar low is different from a normal low formed north of the polar front. A polar low is formed generally in an unstable northerly flow when the upper atmosphere is generally below -35, these aren't well forecast further out than 24hrs. Interestingly they're also called Arctic Hurricanes as they often have an eye in the centre!

The low the models are currently forecasting actually forms near Iceland, the deeper this becomes the more likely we'll see milder air sucked into its circulation and a larger milder sector.

could be a good thing though if the uppers remain cold enough as it would result in more precip and stronger winds, which would certainly be interesting. Of course to much mild air sucked into the system and its rain. Fine balance between the two.

[damianslaw]

Won't be until Saturday before we can all have some confidence in terms of where this low pressure is likely to track, until then, its pure speculation, hence the difference in the main models this evening in terms of where the low is likely to track.

I'll stick my neck out though and say I don't believe it will deepen anymore than is currently being shown and if anything I think it will slacken a bit. I also think it is unlikely to track any further west than is currently being shown by ECM and to a lesser extent UKMO. Consequently the likely trend is a slacker low on a more south easterly position, perhaps not as far east as the North Sea, but importantly the track will be enough to increase chances of low level snow for many central, northern and eastern parts.

Longer term - it is a superb synoptical evolution for unsettled cold conditions, its been a long long time since we have seen such a synoptical pattern develop, a very southerly tracking jet, a major block over Greenland and the Scandi trough on a SW path- a pattern like this is very difficult to shift, the usual westerly flow is turned on its head, we could be in for a lengthy cold and hopefully snowy spell. Very high chances for heavy snow later next week as we see those cold NE winds dig in..

[nick sussex]

Just to confuse things further this evening the UKMO have gone with their own output in the fax charts for 96hrs and 120hrs. Which is really strange considering they very unusually modified their 72hrs output to the ECM! something they rarely do.

Edited February 11, 2010 by nick sussex

[MKN]

The 18z at 114 looks ever so slightly milder. rain turning to snow most likely for southern half of britain. At least initially. Lets see how the rest of the week looks.

[January Snowstorm]

Much better 18z as the low dives further Southwest

Can tomorrow give us all a decent upgrade

[Liam J]

gfs 18z showing quite a snow event monday onwards imo, better the further north you are, slightly milder air at first with the low then uppers are freezing and below to -5c and 528 thickness over most with surface temps low single figures for many with 2/3c over cumbria.

i expect evaporative cooling would come into it + intensity of ppn which looks heavy but 4 days to go so many more chopping and changing until the models get a grasp of the track and intensity.

Edited February 11, 2010 by james12

[billy]

Just to confuse things further this evening the UKMO have gone with their own output in the fax charts for 96hrs and 120hrs. Which is really strange considering they very unusually modified their 72hrs output to the ECM! something they rarely do.

I like the look of that 120hr Fax chart as it looks to be dragging in some of that cold air from the south east from over france. Could be quite interesting over the south east corner of the country . Looks like pressure rising to the north east too on that chart. Possible easterly setting up later next week .

Edited February 11, 2010 by billy

[mountain shadow]

Certainly I would be very surprised to see any lying snow for low altitude areas during daylight hours. Uppers and dew points no where near good enough.Sleetmageddon at best.

[smich]

A polar low is different from a normal low formed north of the polar front. A polar low is formed generally in an unstable northerly flow when the upper atmosphere is generally below -35, these aren't well forecast further out than 24hrs. Interestingly they're also called Arctic Hurricanes as they often have an eye in the centre!

The low the models are currently forecasting actually forms near Iceland, the deeper this becomes the more likely we'll see milder air sucked into its circulation and a larger milder sector.

"Arctic Hurricanes" ?! You scaremongerer you! LOL You should know better!

Polar Lows are associated with a distinct "hook" or "comma" feature but not an eye surely!!!!

And of course Polar Lows are usually much smaller features than the depression puzzling the models currently.

smich