FI Charts (T120+ onwards) Winter 2018 **READ MOD NOTE IN FIRST POST**

Slightly more southerly movement to the cold air mass at t+63 hours on GFS.

12z UKMO 120 hrs

High pressure to north a little bit closer on this gfs run

GFS 12z t+141:

High Pressure is a little lower, but also acting as a somewhat better block and the cold air is still advected to us fairly well. Still ~1020 hPA on the new run. Also a reload of cold air from the artic is much closer than on the earlier run.

Compared to the last t+168, the 12z run gives a somewhat nicer big picture at the same point in the future:

There's also -40 degree air at 500 hPa over Ireland on the current run... :eek:

MJohnston wrote: »

A question about reading the Geop 500 charts - I know what to look for in the 850hpa temperature charts in terms of snow potential, but what makes a great looking chart for cold/snow on the 500hpa pressure charts?

Are we looking for high pressure systems to bring in the cold, dry air and then rely on incoming but blocked low pressure systems or lake effect convection to produce the snow?

Cold pool from the north sea with some embedded lows will induce lake effect showers. The fetch /wind direction then being key for how intense they can get for the North and East coast.

nacho libre wrote: »

could be some prolonged showers of the snow variety around so:D

Isn't - 40 air at that level conducive to polar lows forming?

But don't you need a very large stretch of ocean for them to form? I don't think one has ever formed in the north sea in an easterly, needs to be a northerly or NNE'rly.

https://twitter.com/metoffice/status/966726802998571009

She looks scared...:D That would be an utterly epic snowfall for parts of Ireland if it came off.

lucernarian wrote: »

GFS 18z should be around now...

Latest chart I can see is +99 (21hrs Monday), uppers of -6 and below covering Ireland (slightly sooner than previous run) and a slight shift south so far. No massive changes though, and as it's within the reliable range I can't see much change in the timing of the arrival.

Donegal Storm wrote: »

Any chance of a technical thread for next week mods? The other thread is a mess, impossible to keep up with and any good informative posts are immediately lost..


Not looked at the models since last night but nothing much has changed from what I can see, still expecting a big downgrade every time I have a look but so far so good. The main issue I can still see is the high could sink over us leaving us with a nondescript cold inversion but hopefully hopefully it won't come to that

Latest GFS pretty much as we were, cold from Monday with snow potential from Tuesday evening. Still pointless looking at smaller details

[IM

I would be very surprised if the high sunk over us. All the charts indicate very strongly that the polar frontal jet will be like 1000km too far north for a high to truly sink over us. We'll be south of it. My fear is mainly having the high come close enough to stifle serious precipitation, followed by a Greenland high that's not close enough to block warmer air reaching us 10 days away. Even in that case there would be inevitably an advection of cold air over us if the high builds elsewhere, as the Canadian SSW almost inevitably means. I think it's going to be hard to stop at least one day of very intense cold and snow.

Donegal Storm wrote: »

The main issue I can still see is the high could sink over us leaving us with a nondescript cold inversion but hopefully hopefully it won't come to that

The cold could get deflected further south, unlikely at this stage, but sinking over us?
I really can't see how the High will sink over us given the jet stream profile and the canadian vortex being all but dead, also once you have a low near Genoa it should help to maintain the high where it is.

1st March is a bit far away yet. Were the southern fork of the jet (what little there is) behave a bit differently, it would throw off that avenue of snowfall.

Suppose the high pushed the cold that bit further south as part of the block, then that 7-days-away LP would stick around France.

We've plenty of cold and some snow in the days prior in any case

lucernarian wrote: »

I would be very surprised if the high sunk over us. All the charts indicate very strongly that the polar frontal jet will be like 1000km too far north for a high to truly sink over us. We'll be south of it. My fear is mainly having the high come close enough to stifle serious precipitation, followed by a Greenland high that's not close enough to block warmer air reaching us 10 days away. Even in that case there would be inevitably an advection of cold air over us if the high builds elsewhere, as the Canadian SSW almost inevitably means. I think it's going to be hard to stop at least one day of very intense cold and snow.

Yeah that's what I meant, the south eastern flank of the high will always be nearby early next week so a slight shift southeastward and we'd just see a few dry and frosty days.

Still on track for now though and cold is all but certain, I reckon by Saturday evenings runs we should have a fairly accurate picture of Tuesday/Wednesday