Cold Spell Late Feb/ Early March Technical Discussion only MOD NOTE POST #1

nacho libre · 23-02-2018 5:43am

The UKMO is very snowy:)

nacho libre · 23-02-2018 5:53am

Kermit.de.frog wrote: »

Temperatures on Wednesday

Max temperatures on Thursday

!

Incredible for the time of year, even if what the GFS shows end up being couple of degree wide of the mark - which is likely given its the GFS. To have temperatures struggling to get above zero at this time of year is exceptional.

Kermit.de.frog · 23-02-2018 6:02am

GFS ensemble average now sub -12c by Wednesday...which is extraordinary

The control run is awesome

M.T. Cranium · 23-02-2018 6:27am

The GFS and GEM have an extremely cold look at 850 to 500 mbs, and both seem to relax the block after about 2-3 March rather quickly, which makes me suspicious because it seems plausible that the block would maintain itself longer over snow covered western Europe (as we should see by then).

Not too concerned about ICON, GEM did something similar last night and recovered, and Arpege does not follow it. Betting the ECM will be another great run.

M.T. Cranium · 23-02-2018 7:26am

ECM a touch faster than twelve hours ago (has some parameters a bit further west at 00z than modelled by 12z by 28th) otherwise out to 120h fairly similar looking, not following the ICON in any way. North Atlantic blocking also looks slightly further north by 1-2 deg. Money charts are coming up now.

Kermit.de.frog · 23-02-2018 7:54am

ICON has support from NAVGEM now for that quick evolution to a more traditional Greenland high setup that would quickly displace the easterly. Still cold but not the same cold for us.

I think real caution needs to be applied at this point.

However, ECM is excellent. UKMO excellent. GFS excellent.

M.T. Cranium · 23-02-2018 8:29am

You might be more into orange than yellow if there's any red alert and the red could spread just to your south, some output showing heaviest snowfalls well inland like Laois-Kilkenny.

The way that the polar high is building into northern Scandinavia, I can't see this very fast evolution to only a Greenland high without the 2-4 day interval of cold easterly winds that form the core of the rest of the model output. And even if it did happen that way, it might not be a death blow to the cold/snow potential.

500 mb winds over Sweden at 00z were showing NNE 70 knots. That sounds more like a slow retrograde sequence. But it shows that the high latitude blocking is beginning to push into the mix. Quite exceptional to see widespread -20 to -30 C near the eastern Baltic but a driving mechanism still building further north.

The Baltic is still fairly warm too, this is not like 1947 where the entire Baltic had frozen over by late February, and so northern Germany and Denmark might see some locally heavy streamers starting later today or certainly by Saturday.

TheMilkyPirate · 23-02-2018 8:47am

Fantastic runs again this morning, If the ICON and NAVGEM were the only models showing the easterly and the GFS UKMO and ECM weren't I don't think we would be too confident so I wouldn't worry too much but it would be nice to see them come back on board before the day is out.

PokeHerKing · 23-02-2018 8:58am

Kermit.de.frog wrote:

ICON has support from NAVGEM now for that quick evolution to a more traditional Greenland high setup that would quickly displace the easterly. Still cold but not the same cold for us.

Can you post the icon & navgem please Kermit?

BLIZZARD7 · 23-02-2018 9:54am

EC Snow accumulation chart in inches by Friday 00z - From Nick L on netweather - Dublin in the firing line

2D761940-D7FB-4112-BB2F-B131F24A0772.png.9a229935601739a7dada799093291bf5.png

2D761940-D7FB-4112-BB2F-B131F24A0772.png.9a229935601739a7dada799093291bf5.png

kod87 · 23-02-2018 9:58am

BLIZZARD7 wrote: »

EC Snow accumulation chart in inches by Friday 00z - From Nick L on netweather - Dublin in the firing line

Snow accumulation chart??? How does snow accumulate on water?

littlema · 23-02-2018 9:59am

Gfs for next Friday supporting ????

leahyl · 23-02-2018 10:02am

What's the pink off the south coast?

BLIZZARD7 · 23-02-2018 10:04am

kod87 wrote: »

Snow accumulation chart??? How does snow accumulate on water?

His words, not mine- I suspect it's a total snowfall chart and the assumption is all that falls will stick in these temperatures.

@leahyl 10-16 inches of snow by the looks of it, that's why Cork does better off a South easterly tilt rather than Northern Easterly.

Gaoth Laidir · 23-02-2018 10:07am

kod87 wrote: »

Snow accumulation chart??? How does snow accumulate on water?

It's totalling all of the precipitation falling as snow, not the accumulated depth on the ground.

I would add too that the ECM may slightly underplay the amount of snow showers extending inland as it tends to kill them off too quickly over cold land.

Gaoth Laidir · 23-02-2018 10:41am

On the subject of solar heating at this time of the year, it's not negligible compared to say the end of November 2010.

The maximum possible midday solar irradiation at 53N on Feb 28th is 780 W/m², while on Nov 30th it's around 533 W/m². That means that the sun will be up to 46% stronger next week than it was at the start of the cold spell in 2010. For reference, the June 21st value is 1023 W/m², and the Dec 21st one is around 485 W/m².

Also, at around 10h45min, daylight will be almost 3 hours longer next Wednesday than on Nov 30th (7h56 min).

All this means stronger melting between the showers now compared to 2010. Surfaces such as pavements, cars, etc., will heat up much quicker once a small part becomes exposed, melting that edge of snow cover, increasing the exposed area, further melting, and so on.

Of course these values are on a perfectly clear day. With extensive cloud cover around the difference is greatly reduced, though in scattered cloud conditions spot irradiation can actually be greater (due to reflection of sunlight from surrounding clouds).

Anyway, just one thing to note when comparing this to 2010. If we look at 8th January 1982, the solar figure is also only around 500 W/m². Every week makes a big difference at this time of the year.

pad199207 · 23-02-2018 10:44am

Gaoth Laidir wrote: »

On the subject of solar heating at this time of the year, it's not negligible compared to say the end of November 2010.

The maximum possible midday solar irradiation at 53N on Feb 28th is 780 W/m², while on Nov 30th it's around 533 W/m². That means that the sun will be up to 46% stronger next week than it was at the start of the cold spell in 2010. For reference, the June 21st value is 1023 W/m², and the Dec 21st one is around 485 W/m².

Also, at around 10h45min, daylight will be almost 3 hours longer next Wednesday than on Nov 30th (7h56 min).

All this means stronger melting between the showers now compared to 2010. Surfaces such as pavements, cars, etc., will heat up much quicker once a small part becomes exposed, melting that edge of snow cover, increasing the exposed area, further melting, and so on.

Of course these values are on a perfectly clear day. With extensive cloud cover around the difference is greatly reduced, though in scattered cloud conditions spot irradiation can actually be greater (due to reflection of sunlight from surrounding clouds).

Anyway, just one thing to note when comparing this to 2010. If we look at 8th January 1982, the solar figure is also only around 500 W/m². Every week makes a big difference at this time of the year.

So plenty of lethal icicles dangling from roofs and gutters I’d imagine next week!

BLIZZARD7 · 23-02-2018 11:14am

Oh my god, -15c 850 temps out to Galway Bay on the 06z for Wednesday into Thursday - 1987 levels matched there

06_144_ukthickness850.png?cb=13

Top temperatures of -3 to -5c under heavy snowfall - for the 1st of March!!

Daniel2590 · 23-02-2018 11:32am

GFS 06z

Nearly have a blob of -18 uppers reaching the east coast :eek:

Oneiric 3 · 23-02-2018 11:36am

Donegal Storm wrote: »

Nice to see you back Wolfe

Seconded.

For some reason, his return reminded me of this classic...

https://www.youtube.com/watch?v=7YoY1rGFeVs

Can't for the life of me think why...

BLIZZARD7 · 23-02-2018 11:37am

Slack low pressure by the end of the week and deep cold over snowfields - lows of -8c to -15c possible under those conditions. I await the 12z's with Interest.

Rebelbrowser · 23-02-2018 11:53am

The evolution keeps getting quicker and in fairness MT flagged that this might happen as did an Austrian meteorologist who posts on NW. I reckon the cold could arrive on Sunday evening ultimately.

Larbre34 · 23-02-2018 1:13pm

BLIZZARD7 wrote: »

Slack low pressure by the end of the week and deep cold over snowfields - lows of -8c to -15c possible under those conditions. I await the 12z's with Interest.

My personal record was -15.4C over snow on Christmas morning 2010 at around 8.30am, semi-rural site on Dublin/Meath/Kildare border. Frankly I'd expect to see that broken at my loc by Sunday 4th March

Oscar Bravo · 23-02-2018 1:36pm

STATUS YELLOW

Weather Advisory for Ireland
Exceptionally cold weather is forecast for next week with significant wind chill and severe frosts. Disruptive snow showers are expected from Tuesday onwards, particularly in the east and southeast.

Issued:Friday 23 February 2018 12:00
Valid:Friday 23 February 2018 12:07 to Friday 02 March 2018 12:00

patneve2 · 23-02-2018 1:51pm

Gaoth Laidir wrote: »

It's totalling all of the precipitation falling as snow, not the accumulated depth on the ground.

I would add too that the ECM may slightly underplay the amount of snow showers extending inland as it tends to kill them off too quickly over cold land.

This. Winds are looking strong so I wouldn't be surprised if ppn peaks may occur somewhat inland, like the first spell of 2010 (the one at the end of Nov). Could be seriously nasty on the coast next week, 50 km/h winds and snow squalls coming in off the Irish sea

patneve2 · 23-02-2018 2:09pm

Great, 4/5 days of excellent snow potential. Should be some windows when sst to 850 hpha differences go over 20 degrees C. Hoping for more runs backing all of this up

Mullaghteelin · 23-02-2018 2:23pm

BLIZZARD7 wrote: »

EC Snow accumulation chart in inches by Friday 00z - From Nick L on netweather - Dublin in the firing line

That gap in the heaviest precipitation downwind of the Isle of Man is aiming straight at me. Hopefully wind direction varies about enough so that all of us get a decent share.
Nothing worse than having to endure the high temperatures from the sea, but without the compensation of decent shower activity.

Gonzo · 23-02-2018 2:55pm

Mullaghteelin wrote: »

That gap in the heaviest precipitation downwind of the Isle of Man is aiming straight at me. Hopefully wind direction varies about enough so that all of us get a decent share.
Nothing worse than having to endure the high temperatures from the sea, but without the compensation of decent shower activity.

going by that I could be in the firing line for the IOM shadow.