Cold Spell Late Feb/ Early March Technical Discussion only MOD NOTE POST #1

Oneiric 3 · 23-02-2018 11:57pm

Helpful run down on the Skew thing GL.

Looking at data for your own area, while RH values at the high saturation levels you indicate are high (600 to 900 hPa) for the midweek period next week, values at 500 hPa remain low, between about 10 & 40%. Wouldn't this suggest that convection, while notable, would not in itself be particularly deep?

nacho libre · 24-02-2018 12:04am

Mullaghteelin wrote: »

J
EDIT: keep an eye on the 500hpa temps, if they get below -40C the showers will really ramp up.

They are projected to on some models runs. Going by some of the charts it's the north of east of England and Ireland that will get the most snow, with the south east of England only getting modest amounts. Some people on netweather will not be happy about this:pac: It looks like others might be looking on at Ireland in envy for change

Gaoth Laidir · 24-02-2018 12:07am

Oneiric 3 wrote: »

Helpful run down on the Skew thing GL.

Looking at data for your own area, while RH values at the high saturation levels you indicate are high (600 to 900 hPa) for the midweek period next week, values at 500 hPa remain low, between about 10 & 40%. Wouldn't this suggest that convection, while notable, would not in itself be particularly deep?

[IMG][/img]

For better convection it's good to have dry upper layers, as drier air is more dense than moist air. If the lowest layers contain enough moisture that can rise past any cap/inversion then it's free to keep on rising through the upper layers as long as it's positively buoyant. A temperature inversion at say 700 hPa can put a stop to rising parcels, which will be the case up to late Monday.

nacho libre · 24-02-2018 12:09am

Oneiric 3 wrote: »

Helpful run down on the Skew thing GL.

Looking at data for your own area, while RH values at the high saturation levels you indicate are high (600 to 900 hPa) for the midweek period next week, values at 500 hPa remain low, between about 10 & 40%. Wouldn't this suggest that convection, while notable, would not in itself be particularly deep?

Excellent post Oneric 3. You really are up there with the heavy hitters like M.T. Cranium, Gaoth Laidir, Iancar, Blizzard7 and Syran for weather knowledge

Oneiric 3 · 24-02-2018 12:11am

Cheers GL, much clearer now.

Gaoth Laidir wrote: »

positively buoyant

This would be a great name for a band.

SeaBreezes · 24-02-2018 12:13am

sdanseo wrote: »

No, just very low resolution.

Sorry, what does that mean?

Oneiric 3 · 24-02-2018 12:17am

nacho libre wrote: »

Excellent post Oneric 3. You really are up there with the heavy hitters like M.T. Cranium, Gaoth Laidir, Iancar, Blizzard7 and Syran for weather knowledge

Thanks Nacho but nah, not a patch on any of them.

nacho libre · 24-02-2018 12:21am

The GFS is showing a major snowfall for parts of England and possibly Ireland next weekend, but i think that low won't come as far north as it's showing.

adam240610 · 24-02-2018 12:25am

SeaBreezes wrote: »

Sorry, what does that mean?

Very rough, like a low resolution image. Look at how straight the edges of everything are in the chart.

nacho libre · 24-02-2018 12:32am

Oneiric 3 wrote: »

Thanks Nacho but nah, not a patch on any of them.

You are too modest Oneiric 3. It's damn true your weather knowledge is equally as impressive.

Anyway to try and get back on topic:

image?type=forecastPrecip&region=europa&timestamp=201803031200

image?type=forecastPrecip&region=europa&timestamp=201803031200

How brilliant it would be if this came off. Courtesy of the disjointed jet stream pushing a low up from the south. high risk for high reward. But having said that i think it will effectively run out of steam once it meets the entrenched cold air and stay further south- just like the March 2013 system

sdanseo · 24-02-2018 12:34am

SeaBreezes wrote: »

Sorry, what does that mean?

adam240610 wrote: »

Very rough, like a low resolution image. Look at how straight the edges of everything are in the chart.

The way I understand it (which is probably a stupidly oversimplified and possibly technically incorrect explanation):

That particular model has a resolution of 0.25 degrees - a full degree being 60 nautical miles so the data accuracy is +/- 15 miles.

There are models with a resolution of 0.1 degree and 0.05 degrees which would have an accuracy to within 6 and 3 nmi respectively.

Meteorite58 · 24-02-2018 12:45am

GFS and the ECM both showing a large area of LP in the vicinity by Sat, GFS trajectory showing more snow as you would imagine. Will be watching this one for sure to see where they end up.

The ECM showing the jet helping to feed moist S'ly air into the mix.

Donegal Storm · 24-02-2018 12:57am

sdanseo wrote: »

The way I understand it (which is probably a stupidly oversimplified and possibly technically incorrect explanation):

That particular model has a resolution of 0.25 degrees - a full degree being 60 nautical miles so the data accuracy is +/- 15 miles.

There are models with a resolution of 0.1 degree and 0.05 degrees which would have an accuracy to within 6 and 3 nmi respectively.

0.25deg is actually a fairly high resolution so I'm not sure why the chart is as low-res as it is, I assume its actually showing the full global GFS which runs at 1deg I think.

To give a quick overview of how the models work, they basically divide the globe into a grid with each grid square given a set of input parameters. The input conditions are resolved up to a time step (3 hours on the GFS) and any changes are communicated with the immediate bordering grid squares. The whole process then repeats with these new input conditions

Generally the higher the resolution, the more accurate the model though higher-res requires a lot more computing power and relies on a lot of estimation for inputs. Higher also doesn't always mean better for longer term forecasts, a smoother, less detailed run can give a more accurate view of the bigger picture as high-res models can over-complicate small scale features that wouldn't even be picked up on lower res models

Mount Vesuvius · 24-02-2018 1:23am

Ummm Navgem again quick with retrogression of Scandi high. Has it interfering with our prized easterly flow. Just hope that the strong easterly push is not too strong to undo our lake effect white gold. Probably just another blip but the fact it's showing again is a tad concerning

Meteorite58 · 24-02-2018 1:23am

The finer detail starting to show up. Would you call it a shallow arctic cold front on the Fax descending down over Northern counties by Tues?

Would assume that this would become more convective in nature as it travels over the relatively warmer seas helping to produce snow.

KiovwmM.gif?1

Rougies · 24-02-2018 1:24am

Donegal Storm wrote: »

0.25deg is actually a fairly high resolution so I'm not sure why the chart is as low-res as it is, I assume its actually showing the full global GFS which runs at 1deg I think.

To give a quick overview of how the models work, they basically divide the globe into a grid with each grid square given a set of input parameters. The input conditions are resolved up to a time step (3 hours on the GFS) and any changes are communicated with the immediate bordering grid squares. The whole process then repeats with these new input conditions

Generally the higher the resolution, the more accurate the model though higher-res requires a lot more computing power and relies on a lot of estimation for inputs. Higher also doesn't always mean better for longer term forecasts, a smoother, less detailed run can give a more accurate view of the bigger picture as high-res models can over-complicate small scale features that wouldn't even be picked up on lower res models

Most model output charts use interpolation to smooth out gradients to make them more human readable, especially on website charts. The chart in question is actually more in line with what the actual resolution looks like at 0.25 degrees (26km gridpoints).

M.T. Cranium · 24-02-2018 5:11am

By 120h the 00z GFS is a slight upgrade on 18z in terms of both gradient and depth of cold air. Differences are not enormous but sometimes showed considerably stronger winds at 6h intervals (I keep checking as they update). The southern stream low is about the same to 120h, arriving off the coast of Portugal at about the same time and intensity (marginally slower).

kod87 · 24-02-2018 5:14am

-15 uppers close to east coast by thursday, serious cold, but GFS shows very limited precipitation by +126 hours with showers only becoming widespread on thursday onwards

M.T. Cranium · 24-02-2018 5:20am

The low moves up a bit closer to Ireland on this run, with a vicious east-northeast gradient across Ireland and by 156h has the mixing line into Wexford, will have to have a closer look at all parameters but looks to me like it's indicating a heavy snow band drifting north across Ireland.

Around 168h the low is drifting west at about 51N and the GFS figures mostly snow on land even though 850s are eroded back to about -5 to -7 C. Large amounts of snow indicated (this evolution if it actually happened would be giving 20-30 mm precip which could indicate 20-30 cm snow at 10:1 which is a reasonable ratio for this kind of storm, but I suspect there would be some mixing).

The best outcome would be something almost like this without the erosion of the 850 mb temperature field over Ireland but it would be a wild ride for sure (Thursday-Friday would be the snowstorm tapering off on Saturday although maybe not if colder air comes back around from northeast England).

patneve2 · 24-02-2018 5:23am

GFS 00z has us buried under a blizzard at 150 hrs...very far out but amazing chart

M.T. Cranium · 24-02-2018 5:40am

The low circles back around to northwestern France by 198h and the cold slowly deepens again over Ireland in a continuing northeast flow. Would suggest further snow on 4 and 5 March at least in north and central Leinster.

Any forecast made off this model alone would have to incorporate a range of uncertainty because it's a bit of a close call with marginality at least south of a Dublin to Shannon line, but maybe a risk people would prefer to take given the upside.

However, until we see the full model output, hard to assess how realistic this new loop track would be (it is considerably further west than the previous run but it results in colder conditions once the loop is finished).

(added later _ cold spell gradually ends 5 to 8 March on GFS with another looping low making a broader sweep and aligning with a strong Greenland high to return things to the cold westerly theme of earlier parts of the winter ... not a very convincing set of maps though).

BLIZZARD7 · 24-02-2018 5:45am

Red warnings looking likely mid week - these charts are astonishing in both how snowy they are and the depth of Cold they show. Beyond +120hrs the track of that low coming up from the south is far from decided- The GFS does not deal with these things well at this range (great at picking them out initially though) expect lots more variations on this - The potential is there for historic amounts of snow, I do feel bad for farmers and the vulnerable here, this would likely be some of the most extreme weather anyone is likely to experience in this country. I for one can't wait to see how it unfolds - The Situation needs monitoring though.

UKMO is stunning -

UW120-21.GIF?24-05

M.T. Cranium · 24-02-2018 5:52am

GEM takes the same low a few degrees further east ending up in southeast England at 156h, 850s never fall below -8 C for Ireland to that point. Low then loops about how GFS had it at 18z, passing southeast of Wexford and uppers remain fairly cold throughout.

Verbatim could be a big snowfall producer for several days.

BLIZZARD7 · 24-02-2018 5:56am

M.T. Cranium wrote: »

GEM takes the same low a few degrees further east ending up in southeast England at 156h, 850s never fall below -8 C for Ireland to that point. (model still coming along)

Indeed it looks better at retaining the cold over Ireland while still bringing the low into the equation - actually it still mixes out eventually but it takes the second Atlantic incursion to really clear out the cold

gem-0-150.png?00

BLIZZARD7 · 24-02-2018 6:13am

ICON goes from hero to zero and back again to hero in the last few days, most sustained cold run this morning, relentless north easterly with embedded lows and troughs, uppers below -8c from +60hrs til the end of the run at +180hrs, No let up in sight. (often uppers are -10 to -13c)

M.T. Cranium · 24-02-2018 7:39am

ECM brings the deep low north about the same track as GFS, 975 mb off Brittany by day 6, then only a small loop by day 7 with the wall of cold holding it back, 850 mb temps never fall below -9 C in Ireland after reaching a minimum of -14 C in the southeast. To day seven this is quite a blockbuster run with heavy snow potential by the end of the month and first three days of March.

kod87 · 24-02-2018 7:58am

No end to the cold in sight according to latest ECM, another very good run. Looking increasing likely the cold will last longer than just next week.

Meteorite58 · 24-02-2018 9:14am

It's looking like a perfect storm situation developing for cold /snow towards the weekend. Strong winds showing up fri also giving possible blizzard like conditions you would imagine. Snowfields possibly keeping the temps below freezing for large areas. Cold temps good chance of maintaining into the following week it would seem.

Meteorite58 · 24-02-2018 9:29am

Temperatures across Europe last 24 hrs

Rebelbrowser · 24-02-2018 9:33am

Meteorite58 wrote: »

It's looking like a perfect storm situation developing for cold /snow towards the weekend. Strong winds showing up fri also giving possible blizzard like conditions you would imagine. Snowfields possibly keeping the temps below freezing for large areas....]

Amazing charts. Just a slightly depressing thought though. A lot of us pass our winters on here looking for snow to show up on day 10 of some model. Presuming this upcoming spell is anything like as now modelled, isnt it going to be hard to muster up the enthusiasm in future to post on threads regarding the possibility of a nw incursion having -7 uppers which might, just might give wet snow for a time if you are above 150m?

DOCARCH · 24-02-2018 9:39am

Rebelbrowser wrote: »

Amazing charts. Just a slightly depressing thought though. A lot of us pass our winters on here looking for snow to show up on day 10 of some model. Presuming this upcoming spell is anything like as now modelled, isnt it going to be hard to muster up the enthusiasm in future to post on threads regarding the possibility of a nw incursion having -7 uppers which might, just might give wet snow for a time if you are above 150m?

People have short memories!

Meteorite58 · 24-02-2018 9:43am

As a guide looking like Tues evening for the first chance of substantial snow amounts down over the country. Models not in agreement yet as you would imagine, track and intensity nearer the time.

Cloudio9 · 24-02-2018 9:53am

Meteorite58 wrote: »

Temperatures across Europe last 24 hrs

Love watching that chart. It’s like someone is pouring a big jug of blue ink over Eastern Europe.

Reckless Abandonment · 24-02-2018 9:53am

Question. The flow looks to be east north east for most of the week with is fantastic for streamer development .But Could the little front/disturbances in the flow reduce overall snow amounts for some?? as they would reduce convection by increasing cloud cover. I know they would bring snow to more people but at the risk of redusing streamer activity.

Meteorite58 · 24-02-2018 9:55am

Rebelbrowser wrote: »

Amazing charts. Just a slightly depressing thought though. A lot of us pass our winters on here looking for snow to show up on day 10 of some model. Presuming this upcoming spell is anything like as now modelled, isnt it going to be hard to muster up the enthusiasm in future to post on threads regarding the possibility of a nw incursion having -7 uppers which might, just might give wet snow for a time if you are above 150m?

Indeed but the cold looks so entrenched by this stage and the models atm in good agreement of a deep area of LP close to Ireland coming up from the S laden with moisture would give rise to the probabilities of substantial snowfalls. Track and intensity more near the time. No doubt early stages and evolving but that is the aim to try to determine what is going to happen.

Meteorite58 · 24-02-2018 10:06am

https://twitter.com/severeweatherEU/status/967311172553854977

https://twitter.com/severeweatherEU/status/967323567896612864

https://twitter.com/severeweatherEU/status/967321841080635392

Gaoth Laidir · 24-02-2018 10:20am

Reckless Abandonment wrote: »

Question. The flow looks to be east north east for most of the week with is fantastic for streamer development .But Could the little front/disturbances in the flow reduce overall snow amounts for some?? as they would reduce convection by increasing cloud cover. I know they would bring snow to more people but at the risk of redusing streamer activity.

There's very little land-based convection at this time of the year, only sea-based, so cloud cover won't really have much effect. A kink in the isobars is a signal of an upper level forcing which will actually help boost convection. That's the kind of thing were looking for.

typhoony · 24-02-2018 11:19am

Wednesday is an ice day for the whole country, no doubt somewhere will get above freezing but some parts of the country could struggle to get above -2c

Donegal Storm · 24-02-2018 11:58am

Not far off 40cm of snow in Cork city centre on this mornings ECMWF from the potential storm on Thursday/Friday, GFS has more of a nationwide event but with a quick thaw afterwards. I suspect it'll be Wednesday before we have a clearer idea of how that low is going to behave but potential definitely there for something big

With thestreamers from Tuesday night to Thursday, while the charts look great at face value for snow, wouldn't the strong winds be a problem in that the airmass will have very little time over the Irish Sea to pick up moisture?

pad199207 · 24-02-2018 12:07pm

Donegal Storm wrote: »

Not far off 40cm of snow in Cork city centre on this mornings ECMWF from the potential storm on Thursday/Friday, GFS has more of a nationwide event but with a quick thaw afterwards. I suspect it'll be Wednesday before we have a clearer idea of how that low is going to behave but potential definitely there for something big

With thestreamers from Tuesday night to Thursday, while the charts look great at face value for snow, wouldn't the strong winds be a problem in that the airmass will have very little time over the Irish Sea to pick up moisture?

Yes I’d be weary of those strong winds in off the sea during the week if I was on the immediate coast

Snow angel · 24-02-2018 12:12pm

I posted this in chat but I might get better clarification here.

Please excuse my ignorance on this but people have mention the gfs shows blizzard like conditions in the FI next weekend. I was wondering how this can happen when the 850 temps are only -4 at best in the south? Can someone explain to which chart shows these conditions as I only understand 2-3 of the charts personally. Is it the low pressure that signals blizzard conditions. Just want to learn, not doubting anyone.

JanuarySnowstor · 24-02-2018 12:14pm

Snow angel wrote: »

I posted this in chat but I might get better clarification here.

Please excuse my ignorance on this but people have mention the gfs shows blizzard like conditions in the FI next weekend. I was wondering how this can happen when the 850 temps are only -4 at best in the south? Can someone explain to which chart shows these conditions as I only understand 2-3 of the charts personally. Is it the low pressure that signals blizzard conditions. Just want to learn, not doubting anyone.

-4 from the Atlantic is rain or sleet but -4 with a cold frigid airmass already in place would mean snow. Air is much dryer from the East so you don't need such cold upper temps

kod87 · 24-02-2018 12:18pm

JanuarySnowstor wrote: »

-4 from the Atlantic is rain or sleet but -4 with a cold frigid airmass already in place would mean snow. Air is much dryer from the East so you don't need such cold upper temps

But with those higher uppers you will need surface temps to be at 0 or below surely, that won't be the case next weekend which is why I feel snow would be marginal in the GFS set up (which I feel is an unlikely setup anyway)

Oneiric 3 · 24-02-2018 12:29pm

850 hPa temp anomaly analysis (GFS) for the last 30 hrs up to 6z. Western edge of that cold air over NE Europa now pushing west

Kermit.de.frog · 24-02-2018 12:39pm

Temperatures for Wednesday, Wednesday night, Thursday

102-580UK.GIF?24-6

km79 · 24-02-2018 12:40pm

Kermit.de.frog wrote: »

Temperatures for Wednesday, Wednesday night, Thursday

That wouldn’t be TOO bad s regards frozen pipes

Kermit.de.frog · 24-02-2018 2:33pm

The distribution of the snow showers across the models (low and high res) is gradually getting more widespread and with intensity being upgraded as we get closer...ECM going for isolated thunderstorms now in the east. The models never handle streamer action well to be honest.

108-574UK.GIF?24-6

snowstorm445 · 24-02-2018 2:58pm

Looking further ahead, the GFS seems to be firming up on a breakdown of the cold spell by next weekend as Atlantic lows moving up from the south beginning to break down the block (but not without dumping a load of snow over us beforehand).

The ECM keeps us in frigid air right through to the end of the run, as the lows are kept further away and a northeasterly flow is established.

Gaoth Laidir · 24-02-2018 3:19pm

Snow angel wrote: »

I posted this in chat but I might get better clarification here.

Please excuse my ignorance on this but people have mention the gfs shows blizzard like conditions in the FI next weekend. I was wondering how this can happen when the 850 temps are only -4 at best in the south? Can someone explain to which chart shows these conditions as I only understand 2-3 of the charts personally. Is it the low pressure that signals blizzard conditions. Just want to learn, not doubting anyone.

That's why looking at forecast sounding is important. At that time of your chart, the forecast for the vertical profile of the atmosphere over Dublin is shown below. The temperature curve (red) and dewpoint curve (green) are together through a deep layer of the lower atmosphere, and they're all pretty much to the left of the 0-degree isotherm (blue). Just the very lowest surface layer is around or just above zero, and that's where the marginality comes into play. In that sounding it looks like very wet snow or sleet.

But if you look at the 850 hPa temperature (3-4 degrees, as you say), on its own sometimes doesn't show the whole picture. It's below it that's important.

Mickeroo · 24-02-2018 3:21pm

So the east is getting the worst/best of it with a decent but more manageable covering in the west if I'm interpreting the models right?

Temps not as crazy low as 2010 but the amount of snow coming is what makes this exceptional?

Cold Spell Late Feb/ Early March Technical Discussion only MOD NOTE POST #1

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