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

Oneiric 3

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

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

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?

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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

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

Cheers GL, much clearer now.

Gaoth Laidir wrote: »

positively buoyant

This would be a great name for a band.

SeaBreezes

sdanseo wrote: »

No, just very low resolution.

Sorry, what does that mean?

Oneiric 3

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

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

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

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:






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

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

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

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

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

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.

Rougies

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

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

-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

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

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

M.T. Cranium

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

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 -

M.T. Cranium

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

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

BLIZZARD7

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

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

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

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

Temperatures across Europe last 24 hrs

Rebelbrowser

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?