Cold Spell - Feb 7th to 13th 2021 - Chat

froog

Cork Snowshield. Talk to me.

Gaoth Laidir

JanuarySnowstor wrote: »

-10 uppers across the south, warmer seas only means beefier showers. Thanks now I'm even more convinced lol

I'm talking about 700 HPA temperatures, which will be 10 degrees colder in the north than the south. That would allow for beefier showers up there. And looking at 850 hPa doesn't tell you what the surface layer will be like. Modification across that sea fetch is a strong possibility

JCX BXC

froog wrote: »

Cork Snowshield. Talk to me.

If you think Cork is bad, try the west Coast!

igCorcaigh

From the tech thread.

Kermit.de.frog wrote: »

We have finally crossed the rubicon. The GFS control run does not let go of the cold continental air in the country and the main event is being primed





My instinct, given the trends, is that there won't be a mild spell at all. Quite the opposite.

I have about given up on interpreting geopotential. It seems like an abstract metric to me, I can't turn it into weather in my head

lucernarian

Calibos wrote: »

The overall point I am making is that when some of us see the latest output and have a little moan that it doesn't look like we'll get much if anything, we are told to relax and that the Precipitation models can't be trusted anyway. My point is that surely at the very least the orientation/direction of the Streamers on the Precipitation models can be trusted, in which case we can make educated guesses based on experience which of the various Irish Sea Precipitation Shadows are going to affect us.

Two things to remember - the Wales shadow is at a knife edge near Dublin in an easterly - a difference of even 1 or 2 degrees in the upper wind direction would make a difference between 10cm accumulations and 2cm accumulations. Somewhere like Louth is much more susceptible to long sea fetches and shower trains in general.

The other is that the wind direction is going to vary somewhat throughout Sunday to Thursday. So I'm hesitant to write off potentials for Dublin, and besides I think 5cm is plenty of snow in the capital region - it just won't be Storm Emma or 2010 territory.

With streamers, my go-to model has been the Euro4 for some time. It's what I'll be using over the days ahead. Weatheronline.co.uk has crashed in the meantime, I see.

Gaoth Laidir

Calibos wrote: »

Its constantly said that the models under estimate streamers.

I assume this means that they are not programmed to take into account every single little variable that can affect their formation and then their intensity.

However, one has to assume that no matter how many variables and conditions are met, THE most important one with the strongest effect is Sea Fetch. So it doesn't matter if every other condition is met, if you don't have the Sea Fetch, none of it really matters.

This is why knowing what direction the wind is blowing at the required level (700-850mb) is so important in order to work out if you have any chance at all of anything decent.

Now even if the precipitation models are not programmed to take into account all the variables (Which is why we can't 100% trust them) surely the one variable guaranteed to be programmed into them right is the Wind direction at the relevant height.

ERGO, if the precipitation models are showing high intensity into Meath and Louth from due East, surely we can at least trust that it got the wind direction at 700-850mb right. What we can't trust is the intensity. Might be overblown, might be under-cooked, the most intense region of precipitation might be a bit further south etc, but broadly speaking the model will have got the general area right.

If the only thing you can trust about the precipitation model is the track/direction of the Streamers than surely experience tells us that most of Dublin and Wicklow will be in the Angelsey/Wales shadow and end up getting SFA.

The overall point I am making is that when some of us see the latest output and have a little moan that it doesn't look like we'll get much if anything, we are told to relax and that the Precipitation models can't be trusted anyway. My point is that surely at the very least the orientation/direction of the Streamers on the Precipitation models can be trusted, in which case we can make educated guesses based on experience which of the various Irish Sea Precipitation Shadows are going to affect us.

100% this.

highdef

Gaoth Laidir wrote: »

I'm talking about 700 HPA temperatures, which will be 10 degrees colder in the north than the south. That would allow for beefier showers up there. And looking at 850 hPa doesn't tell you what the surface layer will be like. Modification across that sea fetch is a strong possibility

In very simple terms, I think that what you are saying is that whilst a relatively warm body of water combined with a long enough fetch is required to generate enough convection, too long of a sea fetch can result in the air at the required height/level for showers to form could be warmed to the point where not enough convection can be generated to generate showers and/or what falls at ground level is not snow...... And that's just looking at one aspect of the scenario.

Nqp15hhu

igCorcaigh wrote: »

From the tech thread.



I have about given up on interpreting geopotential. It seems like an abstract metric to me, I can't turn it into weather in my head

Just look at the isobars and the width of them and their direction. You can see the isobars stretch west of Ireland. They are tight off the coast meaning that there is a boundary between air masses. You can see the airmass where there is a rapid transition from yellow to green.

The direction of the isobars is from the east.

now online

Following the weather Forum for a long time, never understand charts etc but I do look out for Cork and snow in the one sentence. ðŸ˜

KingJeremy

Zogabomb wrote: »

Looks like the 6z GFS brings much more of the country in for a snowy event on Thursday, could Ireland be the sweet spot where the two airmasses meet I wonder?

This is what I’m hoping for - please please please!!

Reversal

Tyrone212 wrote: »

A non emotive response, thanks . Yes we'll see but nothing supports those totals currently in the models. I hope he's right but fear he's not by a long way. We'll agree to disagree.

We won't know until it's all said and done, and yes certain areas will see very little. Personally I don't expect to see much. But I do think there is a risk of a level of snow that we only see a couple of times a decade, not a huge amount but certainly enough that warrants discussion here. And not once have I seen anyone ramp up expectations to anything more.

We all know even the hi res models struggle with Irish sea streamers, experience tells us to expect more precip than is modelled.

Even at that, let's deal with your claim there is no support for decent snow accumulations in the models. Below is the ICON, it has an odd tendency to stop showers dead at the coast, we know this won't be the case. Even the undermodelled streamers on the ICON give 20-30mm of precipitation in some areas over Monday - Wednesday.



There is a range of options on the table. I'd say MTs predictions are at the upper end if what is possible, but within the envelope for some areas.

There is a sizeable possibility, of areas seeing more snow than they have since 2018, and most since 2010 excluding 2018. The level of discussion on this forum is totally proportionate to that possibility. The only people who have spoken with total certainty on this event, are those who won't be affected by it no matter what the outcome.

igCorcaigh

Nqp15hhu wrote: »

Just look at the isobars and the width of them and their direction. You can see the isobars stretch west of Ireland and are tight there meaning that there is a boundary between air masses.

Yeah, I get the isobar bit. It's the colour (geopotential) I struggle with.

lucernarian

Gaoth Laidir wrote: »

100% this.

I replied to you earlier GL, thoughts on the 850 hPa windspeed forecast for the start of next week? Even for favoured areas like south Louth.

TTLF

TTLF wrote: »

How do we think wind direction looks like today guys? I know it's subject to change but we seem alright for a NE/ENE/E until Wednesday on most models I think for Dublin

the ICON-EU precip range has showers piling into E coast from Lower Wicklow to Louth and a small sea shadow for the direct middle of Wicklow... am I interpreting this correctly? :pac:

Makes sense
This is different to 2010 though
Winds are very strong so less time to pick up moisture for streamers
Winds lower north of Dublin =better

In 2010 there was little or no wind
One shower train on the 23rd dropped the guts of a foot on Arklow alone
It affected gorey to just north of Arklow (about Jack whites to the end of what used be the Gorey bypass)
It had a curl and was an unforecasted shallow low
It just sat there, then rapidly dried out inland
I actually have video of it here somewhere
There was thunder during it that put out the power again
The thunder was frequent in those 2010 streamers

Oscar Bravo

Just having a look at some TAF's and thats a juicy one for Heathrow from midnight onwards.snow snow and more snow in a gusty easterly wind


BECMG 0700/0703 04018G28KT 4000 -SN BKN006 TEMPO 0700/0713 1200 SN
PROB30 TEMPO 0700/0713 0400 +SN BKN003

Gaoth Laidir

lucernarian wrote: »

There's feck all people living on Mount Oriel (probably for the best with the amount of snow I expect there). From seeing the charts for the past week, I've been thinking that south Louth is the "sweet spot" for accumulations as it's less susceptible to small changes in wind direction and a lack of shadow from the Mourne mountains. I've already warned friends and family about potentially disruptive snow.

I'm thinking 5cm is a much more realistic target for the the likes of Dublin though. One thing I didn't account for was the forecast 850 hPa wind speed chart, and 50 kts is definitely on the high side for allowing convective potential. Any thoughts on that? On one hand, slower upper air allows more time for saturation and cloud formation, on the other the faster speeds might allow precipitation to simply cross Britain outright and regain momentum over the Irish Sea (this happened in 2010 with showers reforming only 50-60 km off the English coast).

I think you mean 50 kph winds at 850hPa. That's just above the general limit for well-organised streamer bands but that's nitpicking a bit. With stronger wind well need a longer fetch, which is why Wales is in the way in an easterly.

sjb25

I took a break from boards for a while have to say I missed the absolute drama in here anybody around boards for a long time will know the posters in here to listen to and what to block out everybody relax something is going to happen

Tyrone212

Reversal wrote: »

Even the undermodelled streamers on the ICON give 20-30mm of precipitation in some areas over Monday - Wednesday.

A pocket of 5 to 9mm is the highest total shown over land. No 20 to 30mm over land.

Kermit.de.frog

A snow shower train Monday night for north Dublin...

lucernarian

Gaoth Laidir wrote: »

I think you mean 50 kph winds at 850hPa. That's just above the general limit for well-organised streamer bands but that's nitpicking a bit. With stronger wind well need a longer fetch, which is why Wales is in the way in an easterly.

Tbh I'm better at remembering the overall map and colour of it, when comparing to previous events that I remember the charts of. I didn't pay attention to the scale used, more that upper wind speed was rather higher than past events that brought a lot of snow.

As regards Cork, I remember the synoptics were kinda similar for there as pre-Storm Emma in 2018. The reports from Kinsale of 30-50 cm dumpings there makes me thing that low level modification won't be that big a deal for there and it's gonna be simply too cold even with the coastal influence to stop the snow from affecting the south coast. I hear what you're saying about 700 hPa temperatures being on the weaker side though.

Calibos

AuntySnow wrote: »

Makes sense
This is different to 2010 though
Winds are very strong so less time to pick up moisture for streamers
Winds lower north of Dublin =better

In 2010 there was little or no wind
One shower train on the 23rd dropped the guts of a foot on Arklow alone
It affected gorey to just north of Arklow (about Jack whites to the end of what used be the Gorey bypass)
It had a curl and was an unforecasted shallow low
It just sat there, then rapidly dried out inland
I actually have video of it here somewhere
There was thunder during it that put out the power again
The thunder was frequent in those 2010 streamers

That unforecasted Shallow low is what I made my Frown face graphic from. Arklow got a foot from it. Dublin got a foot from it. Here in Bray.....

Gaoth Laidir

Gaoth Laidir wrote: »

I think you mean 50 kph winds at 850hPa. That's just above the general limit for well-organised streamer bands but that's nitpicking a bit. With stronger wind well need a longer fetch, which is why Wales is in the way in an easterly.

It was in the way in February 18,never-mind next week
Ergo I'm expecting nothing here unless wind direction eases and veers
South wexford into waterford and North Dublin look good
Those should also give Gonzo a good dump
Caveat minor wind changes when we reach T-00 make huge differences

Nqp15hhu

Depends on the intensity of the showers and the temperatures staying low. In 2010 we had streamers here that pilled up inch by inch.

But that only occurred because the air was colder and produced more snow per mm of water vapour.

Gaoth Laidir

AuntySnow wrote: »

It was in the way in February 18,never-mind next week
Ergo I'm expecting nothing here unless wind direction eases and veers
South wexford into waterford and North Dublin look good
Those should also give Gonzo a good dump
Caveat minor wind changes when we reach T-00 make huge differences

Plenty of fibre in the diet should help.

Tyrone212

Reversal wrote: »

So did you intentionally ignore the part of my post where I addressed the ICONs incorrect tendency to stop showers dead at the coast. Of course you did.

You said it gives 20 to 30mm in some areas. I pointed out those areas are at sea. No one lives there. The highest level over land is 5 to 9mm.

Slashermcguirk

Yes but you are ignoring the fact that these don’t factor in the showers coming onto land, you think all of these large streamers and snow showers they are showing in the Irish Sea will just lie idle in the sea for 3-4 days and won’t make landfall ???

Tyrone212 wrote: »

A pocket of 5 to 9mm is the highest total shown over land. No 20 to 30mm over land.

Gaoth Laidir

highdef wrote: »

I'm in agreement. Whilst 30cm of lying snow is very unlikely as a result of the snow streamers (the possible frontal event is a different kettle of fish), there is of course the chance depths somewhat approaching it could occur in some localised spots that happen to get stuck in snow trains for lengthy periods of time and are also on high ground. Gaoth Laidir mentioned Mount Oriel (@251m ASL) and going by the current forecast that would be one of the prime spots for greater depths. There are some houses above 220m on that hill and the people living there may well see some very decent snow depths.

Whilst I don't think anywhere will reach 30cm, if your location happens to get stuck in a snow train for a couple of days, it could happen. It's unlikely but as it is plausible, I can't disagree with MTs forecast.

Look at it this way, if I say that some parts of the east that are located in the areas most likely to get the snow streamers are forecast to get a dusting of snow at most, this at first appears to be a very inaccurate forecast. However, it's very possible for some of these areas to avoid most or even all of the snow showers, just as it is that other (possible nearby) areas get 20cm+ of lying snow by midweek.

My above forecast and MTs forecast are completely at odds with each other but both are possible, depending on exactly where you are located.

I don't get the animosity shown towards Tyrone earlier. He made a valid point in my opinion and didn't make it in a bad way. Maybe a few sore heads on this Saturday morning.

My aunt lives up on Mount Oriel and it is a prime spot. From her front window there's a perfect view of the flat land descending down to the sea near Anagassan and Clogherhead. With such flat ground rising to her high terrain, an easterly or northeasterly flow would bring her the goods. Not much else in the way of high ground around that area, though, unless you look at the Cooley mountains.

An interesting week ahead.

lucernarian

Reversal wrote: »

So did you intentionally ignore the part of my post where I addressed the ICONs incorrect tendency to stop showers dead at the coast. Of course you did.

You'd want to provide some evidence of how ICON behaves in past scenarios to show this tendency to stop at the coast.

The issue with ICON is that it's showing a lot of convective potential - even in areas that are most affected by being downwind of Wales - yet the showers mostly stop on the coast, while other models show more limited shower activity but they do progress e.g. in North Leinster and south Ulster. The problem is that ICON seems to be getting something wrong and that doesn't help the forecast's credibility.

The other issue with your claim is the very chart from the very model that has this tendency to stop showers at the coast. Right beside it, in Britain, it shows snowfall accumulations up to 100km inland from the coast...

Tyrone212

Slashermcguirk wrote: »

Yes but you are ignoring the fact that these don’t factor in the showers coming onto land, you think all of these large streamers and snow showers they are showing in the Irish Sea will just lie idle in the sea for 3-4 days and won’t make landfall ???

They lose intensity quickly overland and die out.

Zogabomb

Tyrone212 wrote: »

A pocket of 5 to 9mm is the highest total shown over land. No 20 to 30mm over land.

Yes but look at the constant train out in the Irish Sea, they have to make landfall somewhere.

You said earlier that all we would get is light convective showers on the East Coast and Bone Dry everywhere else, that is as misleading a forecast as someone predicting 30cm of snow, the reality is probably going to be somewhere in between both forecasts

Bottom line, by Ireland's standards this week is going to be interesting and it's been a super time for weather fans