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13-02-2021, 22:24   #871
Oneiric 3
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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:

Last edited by Oneiric 3; 13-02-2021 at 22:44. Reason: posted wrong chart
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13-02-2021, 22:55   #872
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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!
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13-02-2021, 23:00   #873
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Quote:
Originally Posted by Oneiric 3 View Post
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.
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13-02-2021, 23:11   #874
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Quote:
Originally Posted by Oneiric 3 View Post
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.

Last edited by Nqp15hhu; 13-02-2021 at 23:15.
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14-02-2021, 00:03   #875
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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.

Last edited by Neddyusa; 14-02-2021 at 00:12.
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14-02-2021, 00:05   #876
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[QUOTE=Oneiric 3;116272299]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!
Attached Images
File Type: jpg Mayo snow 11th Feb 21.jpg (115.2 KB, 468 views)

Last edited by Goldfinch8; 14-02-2021 at 00:09.
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14-02-2021, 00:09   #877
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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.

Last edited by Nqp15hhu; 14-02-2021 at 00:20.
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14-02-2021, 00:57   #878
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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!
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14-02-2021, 01:13   #879
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Quote:
Originally Posted by Goldfinch8 View Post

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.
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14-02-2021, 01:19   #880
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Quote:
Originally Posted by Oneiric 3 View Post
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
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15-02-2021, 20:59   #881
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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.
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16-02-2021, 10:58   #882
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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
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16-02-2021, 11:15   #883
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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)
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16-02-2021, 18:36   #884
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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.


Last edited by Nqp15hhu; 16-02-2021 at 18:42.
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