Cold Spell - Feb 7th to 13th 2021 - Technical Discussion

Donegal Storm

Slashermcguirk wrote: »

I recall December 2010 all the snow for Dublin was coming from the Irish Sea, I remember just watching the constant showers coming from the east into Dublin day after day

Dec 2010 was a very unusual event, I don't know off the top of my head of anything similar, basically an entire lobe of the polar vortex just dropped out of the arctic right on top of us. So the initial blast was northerly but from then on the cold air just sat in place with a light NE'erly tangent to the winds. We had about 30cm snow in the initial blast here but then 10 dry sunny days with freezing fog while the east was under streamers.

pauldry

This so called snow event tomorrow will not deliver for the Republic IMO.

I think tomorrow will get milder as the day goes on and it will be 4 to 6c everywhere at least by evening. That snow in the West and whatever falls tonight will be it. Tomorrow snow on Ulster but rain everywhere else bar mountains.

I'm such a kill joy but looking at Atlantic flexing its muscles on satellite

Tyrone212

pauldry wrote: »

This so called snow event tomorrow will not deliver for the Republic IMO.

I think tomorrow will get milder as the day goes on and it will be 4 to 6c everywhere at least by evening. That snow in the West and whatever falls tonight will be it. Tomorrow snow on Ulster but rain everywhere else bar mountains.

I'm such a kill joy but looking at Atlantic flexing its muscles on satellite

Donegal, Cavan and Monaghan enter chat

Kermit.de.frog

pauldry wrote: »

This so called snow event tomorrow will not deliver for the Republic IMO.

....Tomorrow snow on Ulster but rain everywhere else bar mountains.

but but there are 3 counties of Ulster in the republic

Mount Vesuvius

Taking Carlow town as an example, that to my eye looks like a heavy snow event for that time. 8AM Saturday morning

Luke-m

pauldry wrote: »

This so called snow event tomorrow will not deliver for the Republic IMO.

I think tomorrow will get milder as the day goes on and it will be 4 to 6c everywhere at least by evening. That snow in the West and whatever falls tonight will be it. Tomorrow snow on Ulster but rain everywhere else bar mountains.

I'm such a kill joy but looking at Atlantic flexing its muscles on satellite

100%. The Gulf Stream(abetted by the jet stream) is King in bringing mild weather in winter and summer. Even when we get extremes ( severe cold or very warm south or southeast winds ) in either season it wins 9 times out of 10. Of course extremes do occur, but they are rare and only really happen every decade or so.

Squidvicious

Gaoth Laidir wrote: »

Here is a good source. Click on the balloon icon to select sounding mode.

Click on the map at whatever time you want. The red line is the temperature, blue is dewpoint. Wet-bulb temperature is around 40% of the way between the temperature and dewpoint curve (closer to temperature) for temperatures near zero (that changes at higher temperatures).

If the blue and red curves are together in the lowest levels (below around 700 hPa) and are both below zero then that's a high-intensity snow setup. Even if the surface is a couple of degrees above zero, if the dewpoint is far enough below it to make the wbt below 0.5 C then snow should still prevail.

Thanks, much appreciated. I will certainly have a good look at that.

Gaoth Laidir

Radar animation from the last 24 hours. A lot of virga picked up in the east, as there was certainly no snow here in Celbridge this evening.

Fair play to Connacht. It's a pity there are no official snow depth reports from the stations. All the UK automated stations have snowdepth sensors and report hourly. Met Éireann, if you're reading...

Zogabomb

Gaoth Laidir wrote: »

Radar animation from the last 24 hours. A lot of virga picked up in the east, as there was certainly no snow here in Celbridge this evening.

Fair play to Connacht. It's a pity there are no official snow depth reports from the stations. All the UK automated stations have snowdepth sensors and report hourly. Met Éireann, if you're reading...

So frustrating for the East Coast looking at that radar, did the cold air in situ cause the front to break up on it's Eastern side? Met Eireann's Harmonie update earlier had the front making it to Dublin!

Nqp15hhu

The air in the South-eastern half of the island was very dry today, both at the surface but also notably at altitude. This resulted in Virga, which is basically snow that evaporates as it falls to the surface.

This occasionally happens here, where you will see snow falling on the hills and as you drive to sea level, the precipitation gradually turns to nothing.

Oneiric 3

Nqp15hhu wrote: »

The air in the South-eastern half of the island was very dry today, both at the surface but also notably at altitude. This resulted in Virga, which is basically snow that evaporates as it falls to the surface.

If I may ask, but are those model generated soundings any better (for actual information) than what the maps show? Surely, if the models can generate a forecast sounding like that, then parameter wise, whatever those soundings indicate will show on the surface or upper air maps at any level? I suppose one advantage to them is that they give a quicker 'eyeball' view of what the model is forecasting for all of these levels.

Gaoth Laidir

Zogabomb wrote: »

So frustrating for the East Coast looking at that radar, did the cold air in situ cause the front to break up on it's Eastern side? Met Eireann's Harmonie update earlier had the front making it to Dublin!

I think it just all shifted that bit west. Confluent flow at 850 hPa in the west shows the dynamics at play there, enhancing lift and squeezing as much moisture as possible out of the atmosphere. Precipitation generally falls in zones where 700 hPa relative humidity is >90%, and that can be seen to be the case below. Down lower (925 hPa / ~750 m amsl) the air is much drier, so any stray snow falling there this evening sublimed before it reached the ground.

ZeroSum76

Gaoth Laidir wrote: »

Radar animation from the last 24 hours. A lot of virga picked up in the east, as there was certainly no snow here in Celbridge this evening.

Fair play to Connacht. It's a pity there are no official snow depth reports from the stations. All the UK automated stations have snowdepth sensors and report hourly. Met Éireann, if you're reading...

It's amazing really how that band of precipitation coming towards the South East got completely decimated and no one, no one... could predict it?

Surely there can't be that many variables, scientifically, on what happens when air masses of different types collide under certain conditions, that can't be all pre-loaded and continuously fed into some kind of central database from the beginning of decent record taking, to be used for a continuously evolving machine learning model?

Seems like the historical data to fuel accurate modelling either doesn't exist or is not calibrated or centralised well enough?

hatrickpatrick

Can someone explain a possibly very obvious question? It snowed yesterday and the day before in Dun Laoghaire, quite heavily at times. During all of the recent snow events going as far back as New Year's Eve 2009, we had snow accumulations of various depths. However, despite fairly persistent snow here the night before last especially, none of it stuck - it more or less melted as soon as it hit the ground and this did not change as the wintry showers continued.

Why was this? I don't have the temperatures to hand but I would be astonished if it wasn't sub zero just based on how cold it felt alone, it was truly bitter in a way that it hasn't been for a very, very long time - and it has been for a lot of this week. Based on everything I thought I knew about the weather, at least some of this snow should have stuck - if not on concrete, then at least on grass, flower beds, the tops of peoples' cars, etc. But there was literally nothing - everything the snow touched just got wet instead.

I figure there must have been something quite unusual about how this cold spell evolved to produce such an event, considering how much it stands out from other cold spells, even just those brief day-long ones we sometimes had in February or March of the early 2010s - it seems very strange that there was no accumulation whatsoever in an official easterly with air temperatures feeling as cold as they did from an anecdotal human perspective.

Or perhaps it wasn't that cold at all and I'm just sufficiently older now than I was back then, such that I couldn't hack it?

Neddyusa

hatrickpatrick wrote: »

Can someone explain a possibly very obvious question? It snowed yesterday and the day before in Dun Laoghaire, quite heavily at times. During all of the recent snow events going as far back as New Year's Eve 2009, we had snow accumulations of various depths. However, despite fairly persistent snow here the night before last especially, none of it stuck - it more or less melted as soon as it hit the ground and this did not change as the wintry showers continued.

Why was this? I don't have the temperatures to hand but I would be astonished if it wasn't sub zero just based on how cold it felt alone, it was truly bitter in a way that it hasn't been for a very, very long time - and it has been for a lot of this week. Based on everything I thought I knew about the weather, at least some of this snow should have stuck - if not on concrete, then at least on grass, flower beds, the tops of peoples' cars, etc. But there was literally nothing - everything the snow touched just got wet instead.

I figure there must have been something quite unusual about how this cold spell evolved to produce such an event, considering how much it stands out from other cold spells, even just those brief day-long ones we sometimes had in February or March of the early 2010s - it seems very strange that there was no accumulation whatsoever in an official easterly with air temperatures feeling as cold as they did from an anecdotal human perspective.

Or perhaps it wasn't that cold at all and I'm just sufficiently older now than I was back then, such that I couldn't hack it?

Short answer - the dry dry wind was like a cold hairdryer, literally evaporated the snow as quickly as it hit the ground.
That's how it was in Roscommon anyway.
Almost 3 hours of light to moderate snow blowing, but nothing accumulated despite both air and ground temps below freezing.

Gaoth Laidir

hatrickpatrick wrote: »

Can someone explain a possibly very obvious question? It snowed yesterday and the day before in Dun Laoghaire, quite heavily at times. During all of the recent snow events going as far back as New Year's Eve 2009, we had snow accumulations of various depths. However, despite fairly persistent snow here the night before last especially, none of it stuck - it more or less melted as soon as it hit the ground and this did not change as the wintry showers continued.

Why was this? I don't have the temperatures to hand but I would be astonished if it wasn't sub zero just based on how cold it felt alone, it was truly bitter in a way that it hasn't been for a very, very long time - and it has been for a lot of this week. Based on everything I thought I knew about the weather, at least some of this snow should have stuck - if not on concrete, then at least on grass, flower beds, the tops of peoples' cars, etc. But there was literally nothing - everything the snow touched just got wet instead.

I figure there must have been something quite unusual about how this cold spell evolved to produce such an event, considering how much it stands out from other cold spells, even just those brief day-long ones we sometimes had in February or March of the early 2010s - it seems very strange that there was no accumulation whatsoever in an official easterly with air temperatures feeling as cold as they did from an anecdotal human perspective.

Or perhaps it wasn't that cold at all and I'm just sufficiently older now than I was back then, such that I couldn't hack it?

1) Precipitation was just too light. Not enough actual water equivalent fell due to the capped convection.

2) The ground was too warm. There was a mild and wet period preceding the easterly and then cloud prevented any cooling at night. The little snow that did fall melted with this heat. That on grass less so due to the air insulating the snow from the ground.

3) The air was very dry, so sublimation also occured.

4) It's Ireland!

Oneiric 3

Oneiric 3 wrote: »

If I may ask, but are those model generated soundings any better (for actual information) than what the maps show? Surely, if the models can generate a forecast sounding like that, then parameter wise, whatever those soundings indicate will show on the surface or upper air maps at any level? I suppose one advantage to them is that they give a quicker 'eyeball' view of what the model is forecasting for all of these levels.

It looks to me like a similar phenomenon as many a scenario when wintry showers in a northwesterly at night die out in the midlands because their energy just drys out hitting the frosty conditions in the centre of the country
This is not meant as a criticism but human input experienced metorologists should have expected that instead of issuing yellow warnings where none were warranted
I suspect tonights wexwic adding to the yellow list(nature please prove me wrong) was unwarranted
The amount of snow in Connemara today merited an orange (and didn't get one) in my opinion
So what we are dealing with here is old faithful mother nature trumping the forecasters and the super computers again
Despite the supposed better technology,nothings changed in 40 years on that score
I spoke earlier in this thread about my memories of January 1982
I remember watching the forecast after the 9 news the night that event started
I remember being on the phone (in the hall) doing Irish homework with a class mate who lived on high ground and who said it was snowing like mad
When we were done I looked outside and it was snowing here
The weather man had his stick pointing at the small map and the fronts and said there was going to be sleet and snow in munster and south leinster and there was going to be some drifting on high ground
No big deal was made of it
We all know what happened
Nothing much has changed in forecasting despite the technology,especially with snow,its as difficult as ever

Neddyusa

AuntySnow wrote: »

It looks to me like a similar phenomenon as many a scenario when wintry showers in a northwesterly at night die out in the midlands because their energy just drys out hitting the frosty conditions in the centre of the country
This is not meant as a criticism but human input experienced metorologists should have expected that instead of issuing yellow warnings where none were warranted
I suspect tonights wexwic adding to the yellow list(nature please prove me wrong) was unwarranted
The amount of snow in Connemara today merited an orange (and didn't get one) in my opinion
So what we are dealing with here is old faithful mother nature trumping the forecasters and the super computers again
Despite the supposed better technology,nothings changed in 40 years on that score
I spoke earlier in this thread about my memories of January 1982
I remember watching the forecast after the 9 news the night that event started
I remember being on the phone (in the hall) doing Irish homework with a class mate who lived on high ground and who said it was snowing like mad
When we were done I looked outside and it was snowing here
The weather man had his stick pointing at the small map and the fronts and said there was going to be sleet and snow in munster and south leinster and there was going to be some drifting on high ground
No big deal was made of it
We all know what happened
Nothing much has changed in forecasting despite the technology,especially with snow,its as difficult as ever

Yeah it's why I'm always very suspicious of any snow forecast from more than 2-3 days in advance.
Here in East Connacht at least, the most significant falls of snow over the last 40 years were forecast no more than a day in advance and sometimes not forecast at all.
December 2000 springs to mind as a prime example but there were many others.
Today once again showed forecasting snow in Ireland is a total mystery - which is the attraction of these threads of course

Nqp15hhu

If you want to know if the ground is frozen for snow. Go out and walk on your grass, it should feel hard and almost like it’s concrete. Perhaps with some raised bumps.

The grass here is frozen, and even though frosts melts during the day the topsoil does not. So snow will lie on that.

Whereas it will have a harder time lying on ground that is mucky and soft.

Neddyusa

Neddyusa wrote: »

Today once again showed forecasting snow in Ireland is a total mystery - which is the attraction of these threads of course

An invitation to the nuthouse more like

Donegal Ken

AuntySnow wrote: »

It looks to me like a similar phenomenon as many a scenario when wintry showers in a northwesterly at night die out in the midlands because their energy just drys out hitting the frosty conditions in the centre of the country
This is not meant as a criticism but human input experienced metorologists should have expected that instead of issuing yellow warnings where none were warranted
I suspect tonights wexwic adding to the yellow list(nature please prove me wrong) was unwarranted
The amount of snow in Connemara today merited an orange (and didn't get one) in my opinion
So what we are dealing with here is old faithful mother nature trumping the forecasters and the super computers again
Despite the supposed better technology,nothings changed in 40 years on that score
I spoke earlier in this thread about my memories of January 1982
I remember watching the forecast after the 9 news the night that event started
I remember being on the phone (in the hall) doing Irish homework with a class mate who lived on high ground and who said it was snowing like mad
When we were done I looked outside and it was snowing here
The weather man had his stick pointing at the small map and the fronts and said there was going to be sleet and snow in munster and south leinster and there was going to be some drifting on high ground
No big deal was made of it
We all know what happened
Nothing much has changed in forecasting despite the technology,especially with snow,its as difficult as ever

The only thing I don't get with the warnings is the have started forecasting amounts again something they have not been doing for a while.

A orange snow ice warning is issued when the cretria is met below
3cm or greater in 6 hrs
5cm or greater in 12 hrs
10cm or greater in 24 hrs

Most certainly a orange warning could of been issued for Mayo and Galway earlier but I think there was alot of uncertainty on how long it would last.

As for Saturday I think the criteria for a orange warning will be met across parts of the northwest and north. Models are in better agreement for Saturday than they where for Thursdays event. We may see one issued as early as tomorrow.

Danno

The low across SW France I think is after deepening more quickly to my untrained eye looking at the Sat24 loop. The GFS has this low set to curve southeastwards into the Med near Ibiza... If only it would track east towards southern Germany...

Gaoth Laidir

Thirty-one stations all over Ireland, north and south, and every one of them is reporting easterly or southeasterly winds. The most southerly component is 140 ° at Belmullet and Donegal Airport, with everywhere else generally 100-120 °.

Even despite the precipitation forecast in the Dublin TAF overnight tonight, the surface wind still doesn't deviate (at least up to 6 am). It's the same Knock, Shannon and Cork. All keeping southeasterly winds until at least then. That surface wedge of easterly air just does not want to budge, so any precip tonight will be from upper frontal zones and will again be falling through this cold dry layer. Not one airport has any overcast periods forecast. That tells you a lot about ultimate precip totals that will be registered at the surface.

Villain

The poor harmonie model has taken a beaten this week! The arome model has been better but they have all struggled really. Will be interesting to watch what happens tomorrow morning but I’m not buying that harmonie for tomorrow but we shall see.

Gaoth Laidir

Dublin 11Z TAF: Intermittent light sleet, with a chance of intermittent moderate sleet, overnight from 3 am. I'm going to pull an all-nighter...

TAF EIDW 121100Z 1212/1312 13020KT 9999 FEW020 SCT040
TEMPO 1212/1303 14022G35KT
TEMPO 1303/1310 15025G40KT 4000 -RASN BKN012
PROB40 TEMPO 1305/1310 1200 RASN BKN005=

highdef

Gaoth Laidir wrote: »

Dublin 11Z TAF: Intermittent light sleet, with a chance of intermittent moderate sleet, overnight from 3 am. I'm going to pull an all-nighter...

TAF EIDW 121100Z 1212/1312 13020KT 9999 FEW020 SCT040
TEMPO 1212/1303 14022G35KT
TEMPO 1303/1310 15025G40KT 4000 -RASN BKN012
PROB40 TEMPO 1305/1310 1200 RASN BKN005=

That's a very annoying TAF in the sense that it shows that there is potential showing!

Reversal

Gaoth Laidir wrote: »

Dublin 11Z TAF: Intermittent light sleet, with a chance of intermittent moderate sleet, overnight from 3 am. I'm going to pull an all-nighter...

TAF EIDW 121100Z 1212/1312 13020KT 9999 FEW020 SCT040
TEMPO 1212/1303 14022G35KT
TEMPO 1303/1310 15025G40KT 4000 -RASN BKN012
PROB40 TEMPO 1305/1310 1200 RASN BKN005=

GL, quick question on these aviation forecasts if you don't mind?

Whenever I look at them the reductions in visibility seem to have standard values, 4000m, 1200m etc. Similarly CBs always seem to have a base of 1800ft? Do those values have some significance in aviation?

Gaoth Laidir

Gaoth Laidir wrote: »

Dublin 11Z TAF: Intermittent light sleet, with a chance of intermittent moderate sleet, overnight from 3 am. I'm going to pull an all-nighter...

TAF EIDW 121100Z 1212/1312 13020KT 9999 FEW020 SCT040
TEMPO 1212/1303 14022G35KT
TEMPO 1303/1310 15025G40KT 4000 -RASN BKN012
PROB40 TEMPO 1305/1310 1200 RASN BKN005=

They'll probably change it to +sn during an observation overnight

Gaoth Laidir

Reversal wrote: »

GL, quick question on these aviation forecasts if you don't mind?

Whenever I look at them the reductions in visibility seem to have standard values, 4000m, 1200m etc. Similarly CBs always seem to have a base of 1800ft? Do those values have some significance in aviation?

I haven't noticed such a pattern before and I've never seen it mentioned anywhere. Maybe it has something to do with statute miles, which is what is used for all visibilities in the USA METARs. 4000 and 1200 m would be 2.5 and 0.75 statute miles, respectively, which seem round numbers that would be reported in US METARs. Of course, aviation uses nautical miles for everything else, so here you have the confusing mix of units again.

Or else it's just coincidence. I see Aldergrove's TAF reports 5000 and 2000 metres (3.1 and 1.25 S.M.).

TAF EGAA 121104Z 1212/1312 12016KT 9999 FEW030
TEMPO 1212/1312 13020G32KT
BECMG 1308/1311 SCT007 BKN012
TEMPO 1308/1312 5000 -SN
PROB30 TEMPO 1309/1312 2000 SN BKN006=

I've seen Cb bases forecast at different heights than just 1800 ft, but given our climate, 1800 ft is probably the most common. That would be a 4.5-degree dewpoint depression (multiply dewpoint depression x 400 to estimate cumulus base height agl).

Nqp15hhu

We have had a massive jump in temperature here. What happened? I thought we were under the cold air until late Saturday?