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

Gaoth Laidir · 13-02-2021 12:53am

Snow warnings also out for Derry and Belfast City airports now. Both say the same thing.

Snow
Issued at: Fri 12 Feb 2021 - 23:32 UTC
Valid from: Sat 13 Feb 2021 - 06:00 UTC
Valid to: Sat 13 Feb 2021 - 12:00 UTC
SNOW (OR RAIN AND SNOW MIXED) IS LIKELY TO OCCUR AT THE AIRFIELD.SLIGHT TO MODERATE SNOW IS FORECAST, WITH ACCUMULATIONS OF 2CM. SNOW TYPE WET. VISIBILITY IS NOT EXPECTED TO FALL BELOW 600M IN THE SNOWFALL, WITH A MINIMUM VISIBILITY OF 800M. WINDS ARE EXPECTED TO BE GREATER THAN 15KT WITH THE POSSIBILITY OF DRIFTING.

Mount Vesuvius · 13-02-2021 1:19am

https://twitter.com/MetEireann/status/1360369827127394306?s=19

Gaoth Laidir · 13-02-2021 2:09am

Met Éireann's 06Z aviation chart has just one occlusion lying NW-SE through Aran Islands - Killarney - Bandon, moving NE at 15 kts. No other front at all drawn on the chart. No sign of the warm sector present at 00Z.

Nqp15hhu · 13-02-2021 2:12am

Gaoth Laidir wrote: »

Sligo Airport TAF just issued. Light sleet overnight, with a slight chance of light snow from 2 am.

TAF EISG 122300Z 1300/1309 12020G32KT 9999 SCT030 BKN050
TEMPO 1300/1309 14025G42KT 4000 -RASN BKN012
PROB30 TEMPO 1302/1309 1800 -SN BKN008=

Casement. Similar, with slightly better chance of light snow from 4 am.

TAF EIME 122300Z 1300/1309 13015G27KT 9999 FEW025
TEMPO 1300/1306 14020G35KT
TEMPO 1303/1309 4000 -RASN BKN012
PROB40 TEMPO 1304/1308 1200 -SN BKN004
BECMG 1306/1308 14025G40KT=

Dublin Airport. Sleet, with slight chance of light snow from 5 am.

TAF EIDW 122300Z 1300/1324 14020G33KT 9999 SCT020 BKN050
PROB40 TEMPO 1300/1312 15025G40KT
BECMG 1303/1305 BKN014
TEMPO 1304/1310 4000 -RASN BKN009
PROB30 TEMPO 1305/1309 1200 -SN BKN004
BECMG 1320/1322 16022G38KT=

Belfast Aldergrove. Light snow from 10 am - 2 pm, with slight chance of moderate from 10 am - 1 pm.

TAF EGAA 122258Z 1300/1324 12017KT 9999 SCT022
TEMPO 1300/1310 13020G32KT
BECMG 1306/1309 BKN012
TEMPO 1310/1314 14022G35KT 3000 -SN
PROB30 TEMPO 1310/1313 0400 SN BKN006
PROB30 TEMPO 1314/1324 7000 -RADZ BKN008
BECMG 1314/1317 15022G35KT=

Is the front going to stall? It is already nearly at NI. So can’t imagine it snowing here tomorrow afternoon sadly.

Gaoth Laidir · 13-02-2021 11:05am

I think the reason for this ice pellets overnight was due to a layer of warm air aloft causing melt, with refreezing occuring again in the cold surface layer. The 23Z Camborne sounding (near Land's End) shows this layer nicely and a strong south-southeasterly low-level jet capable of advecting this layer up towards eastern Ireland later.

Red=temperature
Dark blue= wet-bulb temperature
Cyan=dewpoint.

Gaoth Laidir · 13-02-2021 2:39pm

Nice snow sounding from Castor Bay at 11Z today. A deep saturated layer at around -2 to -5 °C should be producing small flakes of plates or needles and columns. Not great for the more dentritic-type flakes, which need a saturation at below -10 °C.

Surface temp/wbt/dewpoint were 0.1/-0.1/-0.4
500-1000 hPa thickness: 535 dam
850-1000 hPa thickness: 129 dam

DOCARCH · 13-02-2021 2:45pm

Gaoth Laidir wrote: »

That diagram is clearly not correct as it is missing dinner plates! Ask Aunty!

Gaoth Laidir · 13-02-2021 2:57pm

DOCARCH wrote: »

That diagram is clearly not correct as it is missing dinner plates! Ask Aunty!

13-02-2021 4:49pm

DOCARCH wrote: »

That diagram is clearly not correct as it is missing dinner plates! Ask Aunty!

Neddyusa · 13-02-2021 7:47pm

When a "snow event" doesn't materialise as per the forecasts of the OP, posters either disappear or these threads tend to either die or descend into childish bickering.

I think many of the forum regulars would actually appreciate a technical/model review of why the weather didn't turn out as predicted.

Purely technical anaysis and not an "I told you so" type thing.

Any takers?
Eg - why did Thursday and Fridays fronts not follow the track or intensity predicted even at T-6.
Or
Which models performed best etc

Kaybaykwah · 13-02-2021 7:56pm

Neddyusa wrote: »

When a "snow event" doesn't materialise as per the forecasts of the OP, posters either disappear or these threads tend to either die or descend into childish bickering.

I think many of the forum regulars would actually appreciate a technical/model review of why the weather didn't turn out as predicted.

Purely technical anaysis and not an "I told you so" type thing.

Any takers?
Eg - why did Thursday and Fridays fronts not follow the track or intensity predicted even at T-6.
Or
Which models performed best etc

In spite of the relatively occurence of "snow events" in Ireland, I feel safe in the thought that Irish forumers can find more words than the Inuit to intuit every type of snow imaginable.

snowgal · 13-02-2021 9:03pm

Neddyusa wrote: »

When a "snow event" doesn't materialise as per the forecasts of the OP, posters either disappear or these threads tend to either die or descend into childish bickering.

I think many of the forum regulars would actually appreciate a technical/model review of why the weather didn't turn out as predicted.

Purely technical anaysis and not an "I told you so" type thing.

Any takers?
Eg - why did Thursday and Fridays fronts not follow the track or intensity predicted even at T-6.
Or
Which models performed best etc

Agree it’s an interesting question for regular followers and I’d like to know too. As you say not in any argue way but purely out of interest. Also agree re charts...

Oneiric 3 · 13-02-2021 9:14pm

Neddyusa wrote: »

When a "snow event" doesn't materialise as per the forecasts of the OP, posters either disappear or these threads tend to either die or descend into childish bickering.

I think many of the forum regulars would actually appreciate a technical/model review of why the weather didn't turn out as predicted.

Purely technical anaysis and not an "I told you so" type thing.

Any takers?
Eg - why did Thursday and Fridays fronts not follow the track or intensity predicted even at T-6.
Or
Which models performed best etc

My humble theory is that Thursday's front, coming up against a dry and relatively cold easterly, was regularly 'enhanced' (given its near stationary status) by "orogenic uplift' as it pushed up against the eastern side of the western coastal uplands, which I think may explain why it was regions close by got the greater snow totals, given what little energy this front contained was constantly being forced into that particular corner of the country for hours on end.

Analysis map for midnight Thursday might better help explain:

The pattern, in hindsight, is actually pretty straight forward, but many of us, myself included and even Met Eireann themselves didn't see it for what it was in the lead up to it.

snowgal · 13-02-2021 9:30pm

Oneiric 3 wrote: »

My humble theory is that Thursday's front, coming up against a dry and relatively cold easterly, was regularly 'enhanced' (given its near stationary status) by "orogenic uplift' as it pushed up against the eastern side of the western coastal uplands, which I think may explain why it was regions close by got the greater snow totals, given what little energy this front contained was constantly being forced into that particular corner of the country for hours on end.

Analysis map for midnight Thursday might better help explain:

The pattern, in hindsight, is actually pretty straight forward, but many of us, myself included and even Met Eireann themselves didn't see it for what it was in the lead up to it.

Thanks for that. Do you mean the front didn’t have as much energy as we thought initially?

Oneiric 3 · 13-02-2021 9:33pm

snowgal wrote: »

Thanks for that. Do you mean the front didn’t have as much energy as we thought initially?

Not really, more that the front itself wasn't a particularly active one. If it was, then this may have gone down as something historic.

snowgal · 13-02-2021 9:40pm

But obviously something showed up initially where everyone thought it was, like Met Eireann etc. Did it just come off weaker than first predicted?

Oneiric 3 · 13-02-2021 9:47pm

snowgal wrote: »

But obviously something showed up initially where everyone thought it was, like Met Eireann etc. Did it just come off weaker than first predicted?

No, as I tried to explain, it's slow movement allowed totals to build up in regions where the front was subject to near continuous orogenic uplift.

snowgal · 13-02-2021 9:55pm

Ok thanks

Oneiric 3 · 13-02-2021 10:25pm

snowgal wrote: »

Ok thanks

This radar for Thursday evening will explain better than I can. Front was most active in and around the hills of western Connacht as the lower air mass from the east was continually pushing up against it:

Even still, I think the highest total (rainfall equivalent) was about 18mm in Newport, which isn't an extraordinary total for a front that lasted over 12hrs, hence why I said it wasn't an active frontal feature by any means.

Oneiric 3 · 13-02-2021 11:24pm

Regarding last night, this is what the Apegge model was showing for 3am with regards Theta-E, which I know nothing about except that I think it shows the air mass type at roughly the height of the boundary layer.

Seems a little 'warmer' than I would have thought? Compare, for example, what the same model was showing during the height of the event on the 11th:

I'll leave greater minds than I to explain, if they so wish, what might have gone on here regarding this particular parameter and its influence, if it had any at all, on last night's frontal passage.

Edit: and here at the 850hpa temp equivalence for the two charts above (as per the ICON)

3am last night:

9pm on the 11th:

Gaoth Laidir · 13-02-2021 11:55pm

I was fairly clear as to why the streamer situation earlier in the week didn't amount to much more than a dusting for the vast majority of people; capped convection, a dry boundary layer and the strong winds that minimised the modification of the air across the Irish Sea. At 30-35 knots it was above the accepted 20-knot optimum for sea-effect convection. I had highlighted this well in advance of the streamers starting, so it was no surprise to me to see that particular event not show the promise that the 850-hPa temperatures would lead one to belief was possible.

On the two frontal events (Thursday and Friday night), Oneiric is correct. The cold and dry air did put up more of a resistance than the models were showing, which is something I felt was possible as I've seen this happen so many times before. Another factor is the particular vertical temperature, moisture and wind profiles, which led to the production of smaller snow particles and hence a greater propensity to sublimate in the drier than forecast surface layer. Larger dentritic snowflakes formed at colder temperatures will ride it out much more easily in weaker winds than in the strong winds we had in both fronts. This is one big learning I'll take away from this.

The ECM model overdid the precipitation rates, especially last night, however I think the ICON Flash and other hi-res models did do a better job. Exactly how they parameterise the microphysics of snowflake sublimation, I don't know, but plenty of areas still did get more or less what was predicted closer to the time.

At least plenty of people have learned how to pronounce (and spell!) the word graupel!

Gaoth Laidir · 14-02-2021 12:00am

Oneiric 3 wrote: »

Regarding last night, this is what the Apegge model was showing for 3am with regards Theta-E, which I know nothing about except that I think it shows the air mass type at roughly the height of the boundary layer.

Seems a little 'warmer' than I would have thought? Compare, for example, what the same model was showing during the height of the event on the 11th:

I'll leave greater minds than I to explain, if they so wish, what might have gone on here regarding this particular parameter and its influence, if it had any at all, on last night's frontal passage.

Edit: and here at the 850hpa temp equivalence for the two charts above (as per the ICON)

3am last night:

9pm on the 11th:

850-hPa Theta-e is a combination of the temperature and moisture content of the air at that level. It's the temperature a parcel of air from that level would end up at if it were raised until all the moisture had been condensed out of it and then brought back down dry-adiabatically to the 1000-hPa level. For 850-hPa air at the same temperature, moister air will have a higher theta-e temperature than drier air. That's why theta-e charts are much better than temperature charts for marking frontal boundaries.

Nqp15hhu · 14-02-2021 12:11am

Oneiric 3 wrote: »

Regarding last night, this is what the Apegge model was showing for 3am with regards Theta-E, which I know nothing about except that I think it shows the air mass type at roughly the height of the boundary layer.

Seems a little 'warmer' than I would have thought? Compare, for example, what the same model was showing during the height of the event on the 11th:

I'll leave greater minds than I to explain, if they so wish, what might have gone on here regarding this particular parameter and its influence, if it had any at all, on last night's frontal passage.

Edit: and here at the 850hpa temp equivalence for the two charts above (as per the ICON)

3am last night:

9pm on the 11th:

There was no marginality here in the snow. It was full on dry snow that was readily sticking including to our windows. This was up until 12pm.

So it’s odd to me that the 0c 850hPa was so close to here at 3am last night. It must’ve moved at a snails pace.

Neddyusa · 14-02-2021 1:03am

Fascinating stuff lads, much appreciated to read your post-event analysis!
While I don't follow it all, particularly the Theta-E values, I find your analysis even more interesting than looking at FI charts.

So often when forecasts fall flat there is no discussion on this forum of why, or what the models missed.
Understanding why modelled conditions don't materialize is the sort of thing that the "hive mind" should excel at on threads like this.

I've no insights into the week myself other than how ferociously dry it was and that I didn't expect any accumulating snow here for two rather unscientific but experience- based reasons:

1. Nine times out of ten, easterlies don't deliver anything bar very dry conditions and horrible wind in east Connacht.
2. The fact this was flagged as a snowy week by models from 240hrs out made me instantly suspicious!
2010 and 2018 are the only instances in my lifetime where snow was forecast reasonably accurately from more than 2-3 days out.
Decent snow here only ever seems to be the stuff that appears in a forecast for the first time 12 hours out, or Met Eireann issue a warning when there's already several inches in the ground. "ninja snow" as others call it.

Goldfinch8 · 14-02-2021 1:05am

Oneiric 3 wrote: »

My humble theory is that Thursday's front, coming up against a dry and relatively cold easterly, was regularly 'enhanced' (given its near stationary status) by "orogenic uplift' as it pushed up against the eastern side of the western coastal uplands, which I think may explain why it was regions close by got the greater snow totals, given what little energy this front contained was constantly being forced into that particular corner of the country for hours on end]

My own visual observations and experience on the day would definitely support this explanation. My own location is not too far east of Castlebar.
Like many else on this forum, I was keenly awaiting the arrival of this particular weather front as it moved up into the cold air.
Living here in the West, I like to watch the weather fronts roll in and then eventually clear. It was interesting to see this front quiet noticeably being stalled in its tracks during the early afternoon on Thursday 11th. The dividing line between the cold and dry air to the north and the frontal cloud and its associated precipitation to the south seemed to sit over us for a few hours. Reports were coming in of decent snowfall from the west of the county and from places like Westport around midday and just afterwards. It was at least another 4 hours before it settled into proper snowfall in this location despite an hour of two of flakes being blown around in that very strong and cold south easterly wind beforehand. On the ground here, it was as if you could see the front struggling to make any further inroads into this cold air that was being driven by that strong wind.
I could see what I knew was snow falling from posters reports out to the west and southwest but meanwhile the drier air held its ground further north.

The picture above was taken at 12.28pm on Thursday looking due west. The light coloured cloud with a faint orangey hue in the bottom left of the photo is actually snowfall over towards Westport. The more obvious grey precipitation to the left of the centre of the photo is snowfall out towards Newport and west Mayo. Towards the bottom right is Nephin mountain and the drier air out to the north and northwest. For a few hours here, you could see what looked to me like somewhat of a dividing line or a battleground between the respective airmasses.
It was interesting to look at but made even more interesting when the snowfall eventually got going here.
Thanks to Gaoth and Oneiric for the more technical and informed explanations as to what unfolded on that day. I know we are always looking forward as to what might unfold weather wise, but I certainly find it interesting to rake back over what was or indeed sometimes what could have been!

Nqp15hhu · 14-02-2021 1:09am

Theta E for a specific pressure level:

- Remove all latent heat, meaning cool the air parcel to its dew point temperature.
- Bring the air parcel to the 1000mb pressure level, adiabatically (along the DALR). So the temperature will change due to changes in pressure, but no heat is added from external sources.

The Theta E will increase with altitude (in a stable airmass) as there will be more distance for warming with descent of the air parcel.

Oneiric 3 · 14-02-2021 1:57am

Great photo once again Goldfinch and shows the unique dark beauty of that county nicely. Much of Mayo, and indeed Connacht missed out on Thursday's snow, but when Mayo does it big, it will always do it bigger.

@NQP, are you sure it was dry snow that you experienced last night? Dry snow normally doesn't stick to windows and has trouble even sticking to surfaces (roofs, roads etc) The last dry snow event here for example was back in 2018, and rather than sticking on anything, the snow just blew around the place and piled up along walls and buildings. and that's just the snow that didn't blow back up into the sky!

Oneiric 3 · 14-02-2021 2:13am

Goldfinch8 wrote: »

The picture above was taken at 12.28pm on Thursday looking due west. The light coloured cloud with a faint orangey hue in the bottom left of the photo is actually snowfall over towards Westport. The more obvious grey precipitation to the left of the centre of the photo is snowfall out towards Newport and west Mayo. Towards the bottom right is Nephin mountain and the drier air out to the north and northwest. For a few hours here, you could see what looked to me like somewhat of a dividing line or a battleground between the respective airmasses.

This is such a great description and photo capture of that one moment in time. This is the radar image around the time you took that photo:

As others described at the time, the frontal edge was constantly being nibbled at from the north and east as it tried to push further inland. The clouds in your photo is are the same sort (altostratus) that the snow started falling from here. Normally when I see prep, whether it be snow or rain, falling from such clouds, it leads me to believe that the approaching front is not particularly strong.

Nqp15hhu · 14-02-2021 2:19am

Oneiric 3 wrote: »

Great photo once again Goldfinch and shows the unique dark beauty of that county nicely. Much of Mayo, and indeed Connacht missed out on Thursday's snow, but when Mayo does it big, it will always do it bigger.

@NQP, are you sure it was dry snow that you experienced last night? Dry snow normally doesn't stick to windows and has trouble even sticking to surfaces (roofs, roads etc) The last dry snow event here for example was back in 2018, and rather than sticking on anything, the snow just blew around the place and piled up along walls and buildings. and that's just the snow that didn't blow back up into the sky!

It was dry snow. Hence why it melted so quickly!

https://streamable.com/4r5wam

Oneiric 3 · 15-02-2021 9:59pm

Meant to post this the other night. Short video showing the 850hPa temp and pressure pattern for this now infamous spell of weather, just to keep it archived before this thread is closed off.

https://streamable.com/6frhhv

Never at any stage was this air mass sourced from the continental interior/Russia as vexingly, some UK forecasters and media commentators claimed. It was purely an Arctic air mass that at times came at us via western Scandinavia.

Images are from the ICON analysis + 5 hour forecast via Wetterzentralie and there are a few frames missing, hence the odd jump.

16-02-2021 11:58am

FYI I MT who had mentioned Eastern slopes in South wicklow
From a number of sources, including my milk lorry driver this morning, there was close to a level foot on the high ground on the Eastern side of croghan mtn West of Arklow
Where exposed this had drifted 4 or 5 feet deep
Worst in the Ballycoog area where the public roads going around croghan were blocked for a day
The drifts are still in the woods this morning
That area would have been exposed to the south easterly,on the Eastern slope of croghan about 7 kms inland
The village of ballycoog is about 200 metres asl ,whilst the public road around and down to a place called thomastown rises to about 350 metres
The mountain is 1997ft,whatever that is in metres and slopes to that public road at roughly 350 metres
That road was impassable, cutting off several houses

lucernarian · 16-02-2021 12:15pm

Is there any reasons why the ICON model is of late being favoured so much by posters here? If you're looking for high resolution for mesoscale events, there are better models. If you're looking for longer term synoptic forecasts, there are better models. If you want to see global patterns and influence on upper level movements like WAA and the PFJ, there are better models. I've never seen this fascination with it in the last 15 years (at least including the DWD's previous effort which could be described as trash by many)

Nqp15hhu · 16-02-2021 7:36pm

Just an example of how 528dam can produce rain. (As discussed last week). Tonight the 528 decametres line comes into the western half of the island.

Freezing levels are still 800 metres and the rain/snow line is 500 metres.

So this goes to show that 528 decametres is pretty useless in maritime air flows. And you would be better concentrating on the freezing level and wet bulb 0c level as a measure of snow chances.

The wet bulb zero level is around 500 metres in Ulster and NW Connaught which ties up with same snow-rain line.

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

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