What happened to the magnitude of this cold spell?

Nqp15hhu · 11-02-2020 11:22am #1

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?

pad199207 · 11-02-2020 11:26am

That’s a Polar Maritime for ya

Nqp15hhu · 11-02-2020 11:28am

pad199207 wrote: »

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.

ZX7R · 11-02-2020 11:12pm

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

Nqp15hhu · 11-02-2020 11:26pm

ZX7R wrote: »

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.

Oneiric 3 · 12-02-2020 12:18am

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.

Donegal Storm · 12-02-2020 1:15am

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 :pac:

Nqp15hhu · 12-02-2020 1:39am

Oneiric 3 wrote: »

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.

Gaoth Laidir · 12-02-2020 12:51pm

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.

Nqp15hhu · 12-02-2020 1:20pm

Gaoth Laidir wrote: »

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.

Gaoth Laidir · 12-02-2020 1:43pm

Nqp15hhu wrote: »

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).

Oneiric 3 · 12-02-2020 2:28pm

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.

Danno · 12-02-2020 2:30pm

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.

Gaoth Laidir · 12-02-2020 3:00pm

Oneiric 3 wrote: »

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).

RobertKK · 12-02-2020 3:27pm

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.

Oneiric 3 · 12-02-2020 4:23pm

I know of and much appreciate your more technical and methodical methods (which I do not call into question) my point is more that those snow charts, as unreliable as they may be, are still reflective of what each model that produces them are showing based on all of the 'upper' conditions that they either use as a starting point or what they forecast. You are spot on about GFS having a colder bias with regards upper temps etc, but regarding conditions on the ground, it wasn't too far off as far as I can tell, and neither was the ECMWF in fairness.

Anyway, I don't want to saturate this thread with pointless charts, but here is what both the GFS and ECM forecast for Tues (yesterday) morning on the 10th with regards 850 temps over Ireland.

GFS is the colder of the two for sure on a more synoptic scale, but this may be down to its lower resolution.

Gaoth Laidir · 12-02-2020 4:57pm

I think each model generates those snow charts differently, taking into account things such as soil temperatures, etc. I do remember the ECM showing a larger "snow depth" at one point but as I said, I take those with a pinch of sea salt. I'm not sure that anywhere below 200 m had any snow lying for any appreciable length of time yesterday. Knock Airport did report hourly depths up to 7 cm this morning but it was only described as patchy wet snow covering less than half the ground.

But on a larger scale the GFS does and always has tended to get thicknesses woefully low. These were the thickness values versus the ECM 36-hr forecast (red dashed line ending at 20W). Around 700 km of a difference between the easternmost extent of the 516 dam line at 12Z yesterday. It turned out to be close to 520 dam in the end.

ECM

GFS

Oneiric 3 · 12-02-2020 5:49pm

I don't know what is wrong with my computer, but I can't see your images (which I assume you included given your headings).

But yes, I don't disagree regarding the 'thickness' forecasts, but that wasn't really my point, but no matter.

Thickness forecasts from Monday 00z for Tuesday 06z.

Gaoth Laidir · 12-02-2020 6:21pm

Much clearer charts than mine anyway so you're not missing much!

Nqp15hhu · 12-02-2020 7:09pm

Danno wrote: »

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.

Northwesterlies can deliver snow here.

Nqp15hhu · 12-02-2020 7:09pm

RobertKK wrote: »

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.

You weren’t affected by the warm low which came down from the north.

sryanbruen · 12-02-2020 8:22pm

Danno wrote: »

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.

Mid-January 1984 Danno

mid-February 1994 was an easterly.

Mortelaro · 13-02-2020 12:39pm

Nqp15hhu wrote: »

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?

More eloquently explained by GL,your issue was marine layer
The last 1000ft or so between cloud and ground
Its effects dissipate as you move inland or uphill
Better with both
Marine layer for you being too near the north Atlantic trumped uppers
Its another example of imperfect modeling I think as its affects should have shown
Thankfully that's why we have human forecasters

Danno · 13-02-2020 11:46pm

sryanbruen wrote: »

Mid-January 1984 Danno mid-February 1994 was an easterly.

It wasn't 1984 - too young to remember that!

Perhaps it was 1990:

What happened to the magnitude of this cold spell?

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