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

sdanseo

pad199207 wrote: »

This has all the hallmarks of going wrong for us too and not just for Southern England. People dismissing the milder air being brought into the mix is rather premature. Could turn out to be very marginal for South and East Coasts.

Fingers crossed this northward and westward trend stops in future model runs.

JanuarySnowstor wrote: »

I often here this line about the gfs but find little justification for it....I've seen several times where the 18z picks up a new and correct trend.

I also find the gfs ppm charts very accurate while some think there a load of rubbish

Recent experience with GFS ppn charts has been the complete opposite for the east coast. Unfortunately by this I mean it has shown snow over the last few months several times only for nothing at all or much less than predicted to materialise.

Gaoth Laidir

Rougies wrote: »

I find 15cm a rather conservative estimate. Any particular reason you don't think it will be higher?

As an example lets take the 48 hours from Tuesday evening to Thursday evening and assume the liquid-equivalent ratio averages 1:18 that would equate to just 8.3mm of precip for that duration. In an unstable flow with -12 ish uppers over an 8C Irish sea surely one would expect higher totals?

Of course the hit and miss nature of the showers will cause local variations, so 15cm is quite likely somewhere on the east coast but you seem to be suggesting that the highest (lowland) totals will be about that value?

a) Total accumulated precipitation (liquid equivalent) up to midnight Wednesday is around 5-10 mm. Say 20:1 gives about max possible 20 cm. But...

b) hit and miss nature, and

c) some melting in between daytime showers.

With the subtle variations in wind direction expected all places should get a fairly even shot at showers, without any one area getting prolonged streamers for days on end. That's why I reckon lowland areas will see up to 15 cm from the showers up to Thursday.

JCX BXC

sdanseo wrote: »

Recent experience with GFS ppn charts has been the complete opposite for the east coast. Unfortunately by this I mean it has shown snow over the last few months several times only for nothing at all or much less than predicted to materialise.

Recent experience is based on Northwesterlies? If so I'd completely discard that evidence.

Kermit.de.frog

Gaoth Laidir wrote: »

c) some melting in between daytime showers.

I'd be surprised if this was much of a factor on Wednesday and Thursday given the projected temperature profile which is sub zero maxes practically everywhere.

ThaitaniumM

The only saving grace is that cold air is quite stubborn and rigid. So we are still at guess where the low will track.

Kermit.de.frog

Latest Fax chart for Friday. Not bad.

snowstorm445

GFS control run and the mean both have the low heading up towards the south coast as per the original track, a good 100 miles east of the OP. Seems like an outlier although you never know.

ZX7R

Does the G.S.F model not have a tendency to track lows further north than the other models, should we not take that into account also

NAGDEFI

I can't quite get my head around how this vigorous Atlantic depression is scooting so rapidly north with the Jet Stream going in every direction bar north east..

As the PM Dawn song Set Adrift On Memory Bliss (1991) says..

'I guess I'll leave that question to the experts. Assuming that there are some out there. They're probably alone, solitaire'.

I know the experts are out there though

ThaitaniumM

Kermit.de.frog wrote: »

Latest Fax chart for Friday. Not bad.

Surprising how far the 528 Dam line stretches into the Atlantic

esposito

ThaitaniumM wrote: »

The only saving grace is that cold air is quite stubborn and rigid. So we are still at guess where the low will track.

Am hoping the low tracks south/se of us. From a selfish point of view I want a continuation of the easterlies

Gaoth Laidir

Kermit.de.frog wrote: »

Latest Fax chart for Friday. Not bad.

That upper warm front lying from the east across to the UK may give boost to precipitation.

Gremlin

I find it hard to be pessimistic about the Friday low. I think the general worry is that it brings up milder air from the south but cold air is stubborn and hard to shift couple that with the blocking pattern and all I can see is lovely moist air riding up over stubborn cold. In the meantime I'm more interested in the convective activity between now and then. Forgetting Friday if we were offered the intervening period by the weather gods we'd have bitten their hand off.

Gaoth Laidir

Snow radar for Germany. Note the dying out of showers over land.

Note also how they get going again around the Germany/Netherlands border. That is around 100 km of a sea fetch from the land to the east.

http://www.meteociel.com/observations-meteo/radar2.php?region=de&mode=1#



By the time we get showers the airmass will have picked up more moisture and won't need such a long sea fetch over the Irish Sea.

NAGDEFI

Met Eireann 22.55 update for later in the week:

'Thursday: Bitterly cold on Thursday with scattered snow showers. Highest temperatures ranging from -1 to +2 degrees, in strong east to northeast winds. A more persistent spell of snow is expected to move up from the south during Thursday night, with significant accumulations in many areas. Fresh to strong easterly winds will lead to drifting in places.

Friday: Early indications are for further falls of snow for a time on Friday, but clearer conditions, with scattered snow showers, will extend gradually from the south. Temperatures not quite as low as on Wednesday and Thursday, but still very cold.'

No mention of rain or 'Schleet'.

ThaitaniumM

Large Funeral wrote: »

Met Eireann 22.55 update for later in the week:

'Thursday: Bitterly cold on Thursday with scattered snow showers. Highest temperatures ranging from -1 to +2 degrees, in strong east to northeast winds. A more persistent spell of snow is expected to move up from the south during Thursday night, with significant accumulations in many areas. Fresh to strong easterly winds will lead to drifting in places.

Friday: Early indications are for further falls of snow for a time on Friday, but clearer conditions, with scattered snow showers, will extend gradually from the south. Temperatures not quite as low as on Wednesday and Thursday, but still very cold.'

No mention of rain or 'Schleet'.

My guess is that they are going by the ECMWF

Gaoth Laidir

Large Funeral wrote: »

I can't quite get my head around how this vigorous Atlantic depression is scooting so rapidly north with the Jet Stream going in every direction bar north east..

As the PM Dawn song Set Adrift On Memory Bliss (1991) says..

'I guess I'll leave that question to the experts. Assuming that there are some out there. They're probably alone, solitaire'.

I know the experts are out there though

The Jetstream doesn't direct these large upper lows, it's the other way around. The Jetstream flows around them. The problem with Friday is that we're talking about a cut-off low approaching an easterly jet from the south. As I said lastnight, models have a tendency to join up cut-off lows to the jet to early, so this could be what we're seeing here and hence why it gets shifted to the west. If the low doesn't catch the jet so early then we could see the low stay more southeast than predicted. Still a few more runs to go. I'd be interested in tonight's 00Z and tomorrow's 12Z.

Villain

ThaitaniumM wrote: »

My guess is that they are going by the ECMWF

Yes but the also use their own experience and knowledge which in scenarios like these can be very important.

While the charts for Thursday night into Friday are spectacular if we stay on right side of it I think some people may be jumping past the snow showers to come before that which are going to leave their mark!

sdanseo

Gaoth Laidir wrote: »

Snow radar for Germany. Note the dying out of showers over land.

Note also how they get going again around the Germany/Netherlands border. That is around 100 km of a sea fetch from the land to the east.

By the time we get showers the airmass will have picked up more moisture and won't need such a long sea fetch over the Irish Sea.

Some distances for reference:

Gaoth Laidir

Kermit.de.frog wrote: »

I'd be surprised if this was much of a factor on Wednesday and Thursday given the projected temperature profile which is sub zero maxes practically everywhere.

I'm thinking both sublimation and solar melting, independent of air temperature.

Gaoth Laidir

Thickness analysis for 12Z today, showing the 510 dam line splitting Germany down the middle, but the 504 dam line hasn't made any further progress further west than the same time yesterday, still straddled across Poland.



Yesterday's 12Z chart.

NAGDEFI

Villain wrote: »

Yes but the also use their own experience and knowledge which in scenarios like these can be very important.

While the charts for Thursday night into Friday are spectacular if we stay on right side of it I think some people may be jumping past the snow showers to come before that which are going to leave their mark!

That's it. And as we all know they wouldn't mention snow Thurs/Friday without a lot of thought put into it.

Yes let's enjoy what we have coming.

lucernarian

sdanseo wrote: »

Correct I think. Essentially a small chance of least / most column occuring, 25% chance of either the 25% or 75% colums occuring, and 50% chance column most probable outcome.

Considering there is not a defined upper bound, a histogram-like way of communicating probability like MT Cranium's is not a good or clear way to do it.

Thanks for the explanation, I was thinking that's what he meant

Rougies

Gaoth Laidir wrote: »

Thickness analysis for 12Z today, showing the 510 dam line splitting Germany down the middle, but the 504 dam line hasn't made any further progress further west than the same time yesterday, still straddled across Poland.

Exactly as was modeled, and the the 504 line will still be over Poland at 12z tomorrow.



By 12z Wednesday though :eek:

nacho libre

I think no matters what happens after Wednesday it is likely to be the peak of the cold between Wednesday and Thursday. Even if we get a reload courtesy of the Scandi High moving to Greenland, I don't think we'll see the depth of cold with it that we will have by Wednesday.

By the way if parts of the country do get 15 - 20cms of snow by Wednesday night, how much would be lost to sublimation. Also does cloud cover decrease the amount of snow loss to sublimation?

BLIZZARD7

UKMO has the all out Blizzard option still on the table this morning, serious conditions if this comes to pass- A once in a generation snowfall for sure. GFS is quite different at a close timeframe though so confidence still very low in what happens at the end of the week. It's still looking likely that we are going to see very big disruption later in the week anyway.

ThaitaniumM

Seems on the GFS the low pulls away to the west just as it makes an impact

kod87

it might be several days before we have model consensus on the general track of that low, it's track could make a huge difference, if it goes favourably for us it could lead to an extraordinary event but could also end up in a miserable pile of wet sh!te.

JCX BXC

Low moving even further westwards in the 00Z run. However, it doesn't look as bad as it sounds.

ThaitaniumM

The models seem to be all over the place regards the low and cold. The cold uppers seem to dissipate quite fast when that low reaches us. Think we have to wait and see where the low goes. The models are certainly not in agreement with what's forecasted!