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

munsterlegend

The Eagle not saying too much about Thursday/Friday on the 11 55 forecast. When Emma meets the Beast it could make things more interesting was really all he said. No details though.

sdanseo

Villain wrote: »

Harmonie has snow as far north as Newry over the Border by midnight Thursday night.

What's it like intensity wise? Does it show the low position?

munsterlegend

I usually find this radar (think based on Hirlam) very accurate. The snows seems heavy enough late Thursday into Friday down South.

http://www.dmi.dk/vejr/til-lands/vejrkort/

Charles Babbage

munsterlegend wrote: »

The Eagle not saying too much about Thursday/Friday on the 11 55 forecast. When Emma meets the Beast it could make things more interesting was really all he said. No details though.

Bit of a nightmare call for forecasters. Possibility of a thaw on the south coast and then snow north from there, but of what extent?

piuswal

glightning wrote: »

Airmass is just too dry. This is the type of airmass you’d get over the continental US with crisp, cold, and dry weather. We need a proper battleground where moisture is injected from the south (like they do in the US).

Difference between there and here though is that they are a huge continent and where the cold air doesn’t get modified at low level and can truely win the battle out.

This spell has surprised me and brought me back down to Earth when it comes to temps to expect from cold air sources. If we were living on a continent where this air actually came from then we shouldn’t be seeing daytime maxes of +4 to +6 today with uppers as they were. Also, with the -15c uppers knocking our door tomorrow we should really be seeing maxes more like -7c and lows of -15c. The fact we are not a continent and the air is travelling over relatively warm water greatly modifies it at the lowest levels and stops true true cold at the surface. This I also believe (the fact the air is relatively warm at the surface) is part of the reason not a lot of water is evaporating from the sea into the air above. If it was -7c at the surface, the sea would readily evaporate.

Would the above imply that the various models surface algorithms are not properly taking into account the Irish surface?

TheMilkyPirate

Radar really prepping up. I've a strong feeling this is going to be well on the upper end of the scale of peoples expectations. Tomorrow will be a hard day to be in work, Exciting times

Liffey4A

Flowx app uses various models and has us firmly in the freezer until Wednesday!
No thaw!

Irish Steve

TheMilkyPirate wrote: »

Radar really prepping up. I've a strong feeling this is going to be well on the upper end of the scale of peoples expectations. Tomorrow will be a hard day to be in work, Exciting times

You must be looking at a very different radar to the one I am seeing, with the exception of 2 very narrow streamers coming into North Louth and Dublin, the Irish Sea is practically clear.

Right now I am underwhelmed by what's happening in South Meath, in that the Isle of Man shadow means that unless things change, there won't be more than a dusting, less than 1 Cm before the morning, and the latest suggestions for Thursday/Friday have very little for this area, between the Wicklow Mountain shadow and the northern track stopping before it gets here, I suspect there are going to be a lot of very disappointed people in this area, in that neither system will have delivered anything like the sort of snowfall that's been supposedly going to happen.

The Isle of Man shadow is a known issue, I suppose we should have expected it to be the killer, I had kind of hoped, based on some of the charts we saw that the flow would have been more easterly, so keeping us out of the IOM shadow.

So be it, we were prepared, but didn't go overboard, so nothing wasted except a bit of time.

lucernarian

Gaoth Laidir wrote: »

For heavy showers it's important to look at 700 and 500 hPa temperatures, which will be much lower tomorrow, allowing deeper convection and heavy showers.

In the satellite animation of the last 3 hours below the coldest upper pool (500 hPa) is that area of blue cloud tops moving westwards over the North Sea. Tops are now down to around -40 °C, and 500 hPa temperature is similar. That cold pool will be over northern England tomorrow midday, with around -42 °C over the east of Ireland. Should be seeing some snowage by then.

https://weather.us/satellite/england/top-alert-15min.html#play

That was nothing to do with my post though

sdanseo

Irish Steve wrote: »

You must be looking at a very different radar to the one I am seeing, with the exception of 2 very narrow streamers coming into North Louth and Dublin, the Irish Sea is practically clear.

The Irish Sea is lighting up like a christmas tree now.

Also, the wind is switching to a slightly more ENE direction bringing the coast from the border to Wicklow into play.

My post from earlier in the other thread, with a new imaged added for 1am:

sdanseo wrote: »

Irish sea 10pm vs midnight:

pistolpetes11

Irish Steve wrote: »

You must be looking at a very different radar to the one I am seeing, with the exception of 2 very narrow streamers coming into North Louth and Dublin, the Irish Sea is practically clear.

Right now I am underwhelmed by what's happening in South Meath, in that the Isle of Man shadow means that unless things change, there won't be more than a dusting, less than 1 Cm before the morning, and the latest suggestions for Thursday/Friday have very little for this area, between the Wicklow Mountain shadow and the northern track stopping before it gets here, I suspect there are going to be a lot of very disappointed people in this area, in that neither system will have delivered anything like the sort of snowfall that's been supposedly going to happen.

The Isle of Man shadow is a known issue, I suppose we should have expected it to be the killer, I had kind of hoped, based on some of the charts we saw that the flow would have been more easterly, so keeping us out of the IOM shadow.

So be it, we were prepared, but didn't go overboard, so nothing wasted except a bit of time.

Not really model / tech related but it will get lost in the other thread but the sea is lit up now in my opinion

Donegal Storm

Irish Steve wrote: »

You must be looking at a very different radar to the one I am seeing, with the exception of 2 very narrow streamers coming into North Louth and Dublin, the Irish Sea is practically clear.

Right now I am underwhelmed by what's happening in South Meath, in that the Isle of Man shadow means that unless things change, there won't be more than a dusting, less than 1 Cm before the morning, and the latest suggestions for Thursday/Friday have very little for this area, between the Wicklow Mountain shadow and the northern track stopping before it gets here, I suspect there are going to be a lot of very disappointed people in this area, in that neither system will have delivered anything like the sort of snowfall that's been supposedly going to happen.

The Isle of Man shadow is a known issue, I suppose we should have expected it to be the killer, I had kind of hoped, based on some of the charts we saw that the flow would have been more easterly, so keeping us out of the IOM shadow.

So be it, we were prepared, but didn't go overboard, so nothing wasted except a bit of time.

Winds will shift more due east as the night goes on so showers will hopefully follow suit. I'm in the same boat in North Kildare, very underwhelmed at the moment as we've had close to nothing since a heavy shower around 6pm.

Oddly north Donegal looks to be amongst the best places to be tonight, I've kept an eye on the streamer there and there's been near continuous snow just north of Letterkenny all night despite the far shorter sea fetch. Odd that the Irish Sea has been so lifeless so far in comparison

MJohnston

After all my talk of model precip outcome quality earlier, I really have to say I think the ARPEGE HD model has performed as well tonight (so far) as it has in previous snow events over this season, at least IMBY anyway - a good solid B grade prediction from it within 24 hours.

Which makes it all the more interesting to me that we're finally entering the timeframe where ARPEGE has been pegging the most snowfall in the capital:

M.T. Cranium

Strong winds are being reported at most North Sea platforms, several have gusts to 80 km/hr. Expect that winds over Irish Sea and then Ireland will pick up gradually later tonight and through most of Wednesday with bitter cold upper levels, this could turn into a heavy snow conveyor belt type of streamer formation especially Dublin/Meath and inland -- Wales may limit streamer formation south of Bray but some heavy bursts are likely despite that handicap, mid-level slopes facing east could see heavy falls too. South coast also more in line for moderate to heavy streamers from south of Wales. Winds will turn slowly from current 060 deg to reach 100 or 110 deg but the conveyor belt should align about 090-100 deg. The IOM shadow will project further north and may go into partial eclipse as different wind directions cancel its effects.

I won't be too surprised if they go to red alert with the morning forecasts, not so much for Emma but for this current set-up.

pistolpetes11

M.T. Cranium wrote: »

Strong winds are being reported at most North Sea platforms, several have gusts to 80 km/hr. Expect that winds over Irish Sea and then Ireland will pick up gradually later tonight and through most of Wednesday with bitter cold upper levels, this could turn into a heavy snow conveyor belt type of streamer formation especially Dublin/Meath and inland -- Wales may limit streamer formation south of Bray but some heavy bursts are likely despite that handicap, mid-level slopes facing east could see heavy falls too. South coast also more in line for moderate to heavy streamers from south of Wales. Winds will turn slowly from current 060 deg to reach 100 or 110 deg but the conveyor belt should align about 090-100 deg. The IOM shadow will project further north and may go into partial eclipse as different wind directions cancel its effects.

I won't be too surprised if they go to red alert with the morning forecasts, not so much for Emma but for this current set-up.

At 5CM fall now in Newbridge Central Kildare , and still coming down strong , appear to be under a streamer for the next while also , so could potentially be looking at a red warning coming the morning

sdanseo

All the streamers seem to be merging as was predicted a day or so ago (I can't remember by whom)

It's just a big wall/converyor belt of snow out there now.

Gremlin

In Raheny. The intensity of the action has really started to up over the last hour. There seems to be no end to the streamers. Hoping that as the flow shifts more from 090deg we'll get the max fetch of the Irish Sea to really give those showers more time to convect.

M.T. Cranium

M.T. Cranium wrote: »

Winds will turn slowly from current 060 deg to reach 100 or 110 deg but the conveyor belt should align about 090-100 deg. The IOM shadow will project further north and may go into partial eclipse as different wind directions cancel its effects.

I won't be too surprised if they go to red alert with the morning forecasts, not so much for Emma but for this current set-up.

A shift to a 090-100 deg wind direction would be good news for the Dundalk/north Louth area.

Mutant z

You obviously have it much worse over there in the east here towards the south west its been mostly calm apart from a short spell of sleet earlier in the afternoon but nothing too destructive. It cleared fairly quickly although Met Eireann have given warnings that there is worse ahead on Wednesday and Thursday so we're not out of the woods yet but hopefully we can avoid the very worst of it fingers crossed.

Irish Steve

Just been looking at the isobar chart, and it's become clearer why things were so different here this evening. For a number of days, we've been getting South East winds, (we could hear departing traffic at the airport), but there's a huge kink in the isobars over the Dublin area, which has pushed the winds significantly North Easterly, hence the effects of the Isle of Man Shadow on this area. That's set to change over the next 2 or 3 hours, and become the Easterly flow that was being shown on just about all of the models over the last few runs, and that looks set to change what's happening here.

I don't know where that ripple came from, but it was a significant disturbance in the flow that's been in place for a number of days, and it's had a significant effect on what's been happening here this evening.

sdanseo

Can anyone confirm what timeframe this chart shows precip for? Obviously it's valid in 3 hours from time of publication, but is it precip in 3hrs or precip per hour

Anyway, showing around 3-6, locally 8mm. Probably 8 to 16cm of snow smack bang over the four main Dublin commuter counties.

Status Red inevitable now. Not sure when, but tomorrow morning most likely.

lucernarian

Irish Steve wrote: »

Just been looking at the isobar chart, and it's become clearer why things were so different here this evening. For a number of days, we've been getting South East winds, (we could hear departing traffic at the airport), but there's a huge kink in the isobars over the Dublin area, which has pushed the winds significantly North Easterly, hence the effects of the Isle of Man Shadow on this area. That's set to change over the next 2 or 3 hours, and become the Easterly flow that was being shown on just about all of the models over the last few runs, and that looks set to change what's happening here.

I don't know where that ripple came from, but it was a significant disturbance in the flow that's been in place for a number of days, and it's had a significant effect on what's been happening here this evening.

There's some truth in that. The wind direction is changing now and for a while the IoM shadow won't have so much of an effect. There's about 4 cm already in parts of north Co. Dublin. The shallow low today was cold enough for snow but at the same time still under high pressure (like 1030 hPa or so).

M.T. Cranium

500 mb analysis at 00z shows 90 knot NE winds over Norway and the polar vortex over the North Sea heading straight for Ireland ... I would toss a lot of the numerical snowfall guidance at this point, it is clearly underdone for today (which we suspected around these parts).

I think the Thursday situation will just be a continuation with breaks as the boundary layer becomes stable ahead of convection associated with Emma. Guidance seems to be converging on a Thursday night and Friday snowfall event that only changes phase on outer southwest coast if anywhere, snow tapering to flurries as the low loses energy later Friday. That's because the polar vortex feature continues on southwest until it makes better contact with the next low in the series and gives it some energy, robbing Emma of remaining dynamics.

Model depictions past five days are probably so speculative as to be fairly worthless for operational forecasting. It seems equally likely that the cold will dig in and resist all advances for quite some time, as compared with the slow and irregular breakdown scenario on some models.

Shorter term, a dry slot of 2-3 hours may open up in the flow across the Irish Sea but I wouldn't count on a full break from snow streamers, then a more vigorous complex already affecting northeast England (NW posters speak of blizzard conditions near Newcastle and Middlesboro) will move into position for late morning hours.

OneEightSeven

M.T. Cranium wrote: »

Shorter term, a dry slot of 2-3 hours may open up in the flow across the Irish Sea but I wouldn't count on a full break from snow streamers, then a more vigorous complex already affecting northeast England (NW posters speak of blizzard conditions near Newcastle and Middlesboro) will move into position for late morning hours.

What caused this dry slot to form? I'm sitting here in Meath wondering why the easterly didn't deliver like the northeasterly did for South Dublin and Kildare, the Isle of Man shadow was a real hindrance to decent snow fall here tonight.

JanuarySnowstor

It's really a red warning in Dublin already MT don't you think? I said it yesterday Wednesday could actually deliver even more than storm Emma.
I personally think it's becoming a dangerous situation with people stuck at home. The government are wondering about schools - it's a wonder if anyone in East and South will get out

M.T. Cranium

I would guess they are discussing the timing of their red alert now, but I think it has reached that level in some places, yes.

As to the Isle of Man, I think there should be enough variation in wind direction to spread the wealth today (too bad it's not the wealth of the Isle of Man but I digress).

Looks like Cork may be getting some snow soon too as winds veer more easterly on south coast. That will be a come and go situation probably.

JCX BXC

Nobody took a look at the 00Z GFS I take it? Has the system moving west quicker on Friday night/Saturday. Less cold after it moves than it was last night. Subject to change.

M.T. Cranium

When the core of cold air gets out over the Irish Sea during daytime convection period, all hell is going to break loose, expect blizzard conditions in many parts of Leinster and near the south coast sometimes spreading from those regions further west. Eastern and central Ulster will also have heavy snow at times.

There are some remarkably cold temperatures at the surface now in England and Wales, for example -7 C at B'ham, -8 C at Bala, Wales, and a stunning -9 C at Southend-on-Sea. Have seen it as low as -11 C in Oxfordshire during the night.

With a stronger gradient developing, conditions will be ripe for maximum sea effect production rates perhaps even surpassing the best we saw in late 2010 which is saying something. It goes without saying that these streamers will be discharging lightning at a prodigious rate.

If these conditions were over the Great Lakes I would be thinking 50 to 80 cm potential, not sure if salt water reacts less vigorously and the Irish Sea is a bit less than ideally sized or shaped so let's say 30 to 60 cm potential in these streamers later today. That won't happen all over Leinster but some places will get amounts that may further complicate what's left of road travel.

Mount Vesuvius

I've seen enough of the 00z ECM to say Red Alert is a must for Emma
GFS says same. Before that Red for all of Leinster could be required between now the her arrival. The Irish sea will be a snow machine again.

munsterlegend

JCX BXC wrote: »

Nobody took a look at the 00Z GFS I take it? Has the system moving west quicker on Friday night/Saturday. Less cold after it moves than it was last night. Subject to change.

I note Met Eireann in their update have the dreaded rain word from Storm Emma. The harmonie model must be showing that closer to the coast.