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11-02-2020, 10:22   #1
Nqp15hhu
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What happened to the magnitude of this cold spell?

I was looking forward to getting some snow out of this cold spell. I analysed the charts and it was looking quite good for Ulster.

I honestly expected a covering out of this.

Tuesday, today was progged to have 516dam, -8c 850pha and 300m freezing levels which should surely yield snow at sea level, even at the coast.

Instead, what we have got is 2/3c at sea level, slightly negative dew points but positive wet bulb. So showers are sleety and barely of snow.

The only lying snow of note is above 300m, everything below that level is of a slushy mess.

The cold spell a few weeks ago, which was of -6c 850pha produced sea level snow cover with warmer atmospheric conditions. So why is this cold spell so marginal?

I study all of this and I am fairly interested in it. I can’t establish why colder atmospheric conditions would yield more marginal conditions for snow. Surely a freezing level at 300m would guarantee sea level snow?

Is the atmosphere warmer than predicted?

One thing I notice is that the dew points rise within the shower and drop afterwards. Could this be limiting snowfall?

Last edited by Nqp15hhu; 11-02-2020 at 10:27.
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11-02-2020, 10:26   #2
pad199207
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That’s a Polar Maritime for ya
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11-02-2020, 10:28   #3
Nqp15hhu
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That’s a Polar Maritime for ya
The airmass we had two weeks ago was polar Maritime too though.

Most places here got a covering.
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11-02-2020, 22:12   #4
ZX7R
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Possible reasons
The hit and miss of the showers.
Sea temperature modifying.
The length of fetch the showers took.
Another factor I see this time the east had colder temperature's less wind clamer conditions the showers produced more snow.
I believe the gsf may have been relied upon to much again
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11-02-2020, 22:26   #5
Nqp15hhu
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Originally Posted by ZX7R View Post
Possible reasons
The hit and miss of the showers.
Sea temperature modifying.
The length of fetch the showers took.
Another factor I see this time the east had colder temperature's less wind clamer conditions the showers produced more snow.
I believe the gsf may have been relied upon to much again
So you think the wind had an impact? I notice tonight temps are not dropping due to the wind, so you may be right.
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11-02-2020, 23:18   #6
Oneiric 3
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It was never forecast to be anything other than it was.

Perhaps one factor that contributed to the lack of any meaningful snow was that lack of any notable troughing in mP flow, which would have helped organise the showers and build up their intensity. Even today, I noticed on the 'cloud top temp' satellite that the 'coldest' of the showers, which weren't really that cold, only really ran across the north of Ireland.
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12-02-2020, 00:15   #7
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I didn't see a single model all week other than the hopelessly inaccurate Euro4 forecasting any worthwhile lying snow

It's always the same old crap with long fetch westerlies, definition of insanity comes to mind on here sometimes
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12-02-2020, 00:39   #8
Nqp15hhu
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Originally Posted by Oneiric 3 View Post
It was never forecast to be anything other than it was.

Perhaps one factor that contributed to the lack of any meaningful snow was that lack of any notable troughing in mP flow, which would have helped organise the showers and build up their intensity. Even today, I noticed on the 'cloud top temp' satellite that the 'coldest' of the showers, which weren't really that cold, only really ran across the north of Ireland.
I ask this because the previous cold spell had milder atmospheric conditions and brought sea level lying snow.
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12-02-2020, 11:51   #9
Gaoth Laidir
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That 516 dam line forecast by the GFS to be in over us never reached close to the west coast in the end and the 850 hPa temperatures never made it to the -8 °C it had forecast, making it to only around -6.5 °C according to both Valentia and Castor Bay soundings. The GFS, despite its updates, still has a cold bias in northerly outbreaks, and this case was no different. Caveat emptor still applies to the GFS.

What caused the sloppiness in the actual fallen snow was probably due to two factors, the saturated and hence warmer ground and a high level of sea salt in the lowest layers caused by the strong winds the whole way across the Atlantic. Lower winds would have meant less sea spray but then again more time for thermal modification, which just proves the point again that the Labrador Sea is no place to hope for decent snow.
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12-02-2020, 12:20   #10
Nqp15hhu
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Originally Posted by Gaoth Laidir View Post
That 516 dam line forecast by the GFS to be in over us never reached close to the west coast in the end and the 850 hPa temperatures never made it to the -8 °C it had forecast, making it to only around -6.5 °C according to both Valentia and Castor Bay soundings. The GFS, despite its updates, still has a cold bias in northerly outbreaks, and this case was no different. Caveat emptor still applies to the GFS.

What caused the sloppiness in the actual fallen snow was probably due to two factors, the saturated and hence warmer ground and a high level of sea salt in the lowest layers caused by the strong winds the whole way across the Atlantic. Lower winds would have meant less sea spray but then again more time for thermal modification, which just proves the point again that the Labrador Sea is no place to hope for decent snow.
I was wondering if the winds had an influence! That’s interesting about the 850pha temps though.
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12-02-2020, 12:43   #11
Gaoth Laidir
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I was wondering if the winds had an influence! That’s interesting about the 850pha temps though.
I went against my instincts a bit and got sucked into the vortex thinking that this event might prove to be better than normal but in the end I shouldn't have trusted the GFS. Nine times out of 10 it will have a cold bias in the 850 temps. Its 700 hPa forecast of -21 °C was around 3 degrees colder than the ECM and others were forecasting (and what it turned out to be).
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12-02-2020, 13:28   #12
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If I may respectfully disagree GL, but of the global models I looked before the 'event', the ECMWF was the biggest ramper of snow. The GFS, as you rightfully say, does have a well known cold bias, but comparatively speaking, it proved to be the more conservative model regarding actual snow amounts. I'll post charts later when I have more time.
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12-02-2020, 13:30   #13
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You need a direct airflow from Greenland to give proper snow here like late December 2000 and middle February 1994 (IIRC).
These events gave series of squall dumping several cms of snow in minutes.
Anything else from west of north is too moist and marginal.
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12-02-2020, 14:00   #14
Gaoth Laidir
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Originally Posted by Oneiric 3 View Post
If I may respectfully disagree GL, but of the global models I looked before the 'event', the ECMWF was the biggest ramper of snow. The GFS, as you rightfully say, does have a well known cold bias, but comparatively speaking, it proved to be the more conservative model regarding actual snow amounts. I'll post charts later when I have more time.
I'm not talking about those pink/blue/green snow/rain precip. type charts, which are very unreliable, including the ECMWF and I never trust them. I'm talking about actual brass tax measurable parameters, such as temperatures and thicknesses. When verified by actual radiosonde data the GFS turned out to be several degrees too cold (and hence thicknesses too low) than the ECM and others (ARPEGE and ICON).
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12-02-2020, 14:27   #15
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There has been lying snow on the hills in Kilkenny since early Monday morning, all the showers since were snow showers and every night the snow got topped up, so there is snow here for the third day in a row. It must have been about 6-7cm or so.
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