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

Villain

850 temps not as good at +120 on ECMWF.

Posting charts on phone is a pain

dacogawa

ECM is bring that low on top of us, seems even quicker than the GFS



Would the cold not slow it down/block it?

gabeeg

dacogawa wrote: »

ECM is bring that low on top of us, seems even quicker than the GFS
]

But the blocking seems a bit beefier

SleetAndSnow

gabeeg wrote: »

But the blocking seems a bit beefier

yep its definitely colder so that plus the low

Reckless Abandonment

Amazing watching that low get bounce back into the Atlantic. Secondary low keeping a cold feed of mainland Europe

Musicman2000

dacogawa wrote: »

ECM is bring that low on top of us, seems even quicker than the GFS


Would the cold not slow it down/block it?

Something not right about these charts , the jet stream is way south so how can such an intense storm head for Ireland .

Kermit.de.frog

Vicious ECM from a snow and wind perspective

https://weather.us/model-charts/euro/ireland/significant-weather/20180302-0000z.html

Earlier than other models. Later Thursday and Friday. The snow clears Friday morning.

gabeeg

The GEM had a similar outcome earlier

sdanseo

Kermit.de.frog wrote: »

Vicious ECM from a snow and wind perspective

https://weather.us/model-charts/euro/ireland/significant-weather/20180302-0000z.html

Earlier than other models. Later Thursday and Friday. The snow clears Friday morning.

Surely that storm would be orange warning for wind on its own, before any consideration that there could be several feet of snow in places beginning to melt as it arrives?

Scary stuff at this stage

Donegal Storm

Yeah while everyone's fixated on snow potential the wind looks increasingly like it might be an issue, coming from an unusual direction and someone mentioned earlier that there's a spring tide on Friday so definitely needs to be watched in that respect

I wouldn't read too much into run-to-run changes for the moment, give it to Tuesday night / Wednesday mornings run before we have a serious look at it. Looks almost certain it'll affect us one way or another but when even senior met office forecasters can't forecast with any confidence then there's no point dissecting every model run

Calibos

Are the Big 3 models reaching a consensus yet and if so, what is the consensus? Are we talking about lashing cold rain and a quick thaw or are we talking about a 1982 dumping of frontal snow and blizzards?

ThaitaniumM

Calibos wrote: »

Are the Big 3 models reaching a consensus yet and if so, what is the consensus? Are we talking about lashing cold rain and a quick thaw or are we talking about a 1982 dumping of frontal snow and blizzards?

Seems to be frontal snow then it quickly tapers off before it gets a chance to fall as rain.

at this stage I still think the low won't move that much north due to retrogression of the Scandi high towards Greenland. This would keep the rigid cold over Ireland and keep the low to the South of us and into the English Channel. It was mentioned earlier by the guys on netweather it seems.

Take it all with a pinch of salt until at least Tuesday/Wednesday

gabeeg

Uhoh, reload

Gaoth Laidir

SleetAndSnow wrote: »

ICON showing significant snow


ICON has the low further south again with the south being plastered in snow for quite a while, this is getting intense it seems!

A word of caution on the ICON precipitation charts. This model has a drizzle bias, so tends to show large areas of light precipitation when in fact the reality will be a lot less. Use the colours to determine precip type but don't take the actual coverage seriously.

Elmer Blooker

Calibos wrote: »

Are the Big 3 models reaching a consensus yet and if so, what is the consensus? Are we talking about lashing cold rain and a quick thaw or are we talking about a 1982 dumping of frontal snow and blizzards?

It'll take a few days before things become clear, its impossible to know the track of the low at this stage. Joanna Donnelly didn't go beyond Thursday which is an indication of the uncertainty.
I just wish we had better blocking to the north and not so far north west.

flanzer

Elmer Blooker wrote: »

It'll take a few days before things become clear, its impossible to know the track of the low at this stage. Joanna Donnelly didn't go beyond Thursday which is an indication of the uncertainty.
I just wish we had better blocking to the north and not so far north west.

I noticed that. She went as far as Thursday, until you could see the low pressure tracking north from the Bay of Biscay, then cut back to the more immediate forecast

mickger844posts

What we can gather from todays runs in FI is at 96hrs. Everything is too messy and not so plausible beyond that. Lets enjoy what we get Tuesday and Wednesday and see how the rest pans out.

tbayers

Quick one. With the mention and ongoing threat of streamers coming off the Irish sea, is there a difference in night time and day day streamers. The obvious statement would be that it would be worse at night, colder air temps, better conditions etc. Is this true though?? Once the easterlies kick in will it just be a conveyor belt??

highdef

tbayers wrote: »

Quick one. With the mention and ongoing threat of streamers coming off the Irish sea, is there a difference in night time and day day streamers. The obvious statement would be that it would be worse at night, colder air temps, better conditions etc. Is this true though?? Once the easterlies kick in will it just be a conveyor belt??

Yes, pretty much conveyor belt stuff, day and night. Upper air temperatures will dictate how much convection is possible, with air pressure having some part. Needless to say, we will have issues with either of those. Over sea, the difference between day and night time temperatures will be fairly insignificant.

lucernarian

Gaoth Laidir wrote: »

A word of caution on the ICON precipitation charts. This model has a drizzle bias, so tends to show large areas of light precipitation when in fact the reality will be a lot less. Use the colours to determine precip type but don't take the actual coverage seriously.

Why would the ICON be taken seriously at this medium range to begin with?

It was very flakey at times about predicting the arrival and intensity of cold air tomorrow in the first place, compared to the big 3.

munsterlegend

mickger844posts wrote: »

What we can gather from todays runs in FI is at 96hrs. Everything is too messy and not so plausible beyond that. Lets enjoy what we get Tuesday and Wednesday and see how the rest pans out.

I suppose for a lot of people Tuesday/ Wednesday isn't as good for those away from the east coast. Thursday/Friday has the potential to be epic if on
a knife edge.

Meteorite58

https://twitter.com/wanstead_meteo/status/967753191562588160

https://twitter.com/greenlandicesmb/status/967729677988220928

https://twitter.com/RyanMaue/status/967828521191124992


https://twitter.com/Netweather/status/967806177282330624

Near current temps across Europe

igCorcaigh

lucernarian wrote: »

Why would the ICON be taken seriously at this medium range to begin with?

It was very flakey at times about predicting the arrival and intensity of cold air tomorrow in the first place, compared to the big 3.

Wasn't ICON the first to pick up on the Easterlies? Ppt is a different matter though.

coffee to go

BBC weather from this evening's Countryfile. Not holding back on Thursday into Friday!

Snowbie

highdef wrote: »

Yes, pretty much conveyor belt stuff, day and night. Upper air temperatures will dictate how much convection is possible, with air pressure having some part. Needless to say, we will have issues with either of those. Over sea, the difference between day and night time temperatures will be fairly insignificant.

Most significant is the Irish Sea is much cooler at this time of year than in Novemeber or December. Deep convection occurs when there is a greater difference in temperature between the atmosphere and sea surface, this is obviously better in late nov and dec like we saw in 2010 and Oct 2003. But the saving grace here is that upper air temps (850mb) are significantly cold too so it sorts of balances out so convection over the sea should be decent with trains of cumulus congestus rolling onshore and heading inland but Ive low confidence of explosive convection at this time occuring until perhaps wednesday when 850mb could be -13 or -14C, we shall see.

A cooler sea is also good news for those of us that live near the coast where it is usually more marginal in these events.

ninebeanrows

I must admit I am a little surprised by the EC EPS, they are quite progressive and have really left the UKM out on its own as regards to the evolution over the weekend.

However still sig. spread from Friday but definitely a move to introduce less cold/ more marginal airmass over the weekend.

M.T. Cranium

I think we are probably at a reliable orange alert situation and a probable red alert situation now (met are probably constrained by policy on lead times for these, but feel certain we will go to orange by Tuesday).

Radar upstream showing very good snow streamer production in north Germany and south of Sweden, even some streamers off the two larger lakes in Sweden. This bodes well for widespread snow streamer development by Tuesday in eastern Ireland.

My overall impression is that the models have all shifted somewhat colder for Friday into weekend with the Biscay low either looping near Cornwall or making a less extensive incursion into Ireland.

Verbatim, the ECM solution would bring prolonged heavy wet snow in Munster, I think the reality with a development like they show would be slow and incomplete phase changes with precip tapering to drizzle or snow grains higher inland, but not until a large amount of snow had accumulated over much of the country (20-40 cm with local 60 cm on higher terrain).

Either the streamers at maximum on Wednesday or this Thursday-Friday storm could qualify (or appear to the public) as a blizzard. Hence my belief that a red warning is probable at some stage.

I have tweaked my most likely snowfall values in 25% intervals and added Waterford, Galway, Belfast and Limerick.

TOTAL SNOWFALLS PREDICTED OVER PERIOD 27 February to 3 March 2018 (cms)

LOCATION ______ Least __ 25% __ 50% __ 75% __ Most

Dublin ___________ 5 ____ 20 ____ 37 ____ 55 ____ 70

Cork _____________3 ____ 18 ____ 30 ____ 45 ____ 60

Waterford ________ 2 ____ 15 ____ 25 ____ 35 ____ 48

Galway __________ 1 ____ 10 ____ 20 ____ 30 ____ 40

Belfast ___________5 ____ 20 ____ 35 ____ 50 ____ 65

Limerick/Shannon __ 2 ____ 12 ____ 23 ____ 35 ____ 50

If you are interested in your own location, in most cases you could interpolate from these point data except would extend the Dublin values with little change southwest into Laois before tapering down to values shown further south and west. Also add up to 15 cms for higher elevations near cited locations, for example, highest parts of Wicklow might have a min of 20, 50% value of 60 and max of 100 cms.

The lowest set of numbers would probably apply to far northwest coastal locations but few would be much lower than Galway as shown here.

As to maximum snow depth, that would be less than these values as these are totals of daily values. Snow depth from previous days would be likely to settle and so the maximum snow depth achieved might be three quarters of the ongoing total snowfalls.

Melt potential on 2nd-3rd could be assessed as follows:

Risk of no significant melt of lying snow (5 cm a day ablation in cold wx) __ 20%

Risk of partial removal of snow cover at 10 cm a day in 2-5 C temps ______ 50%

Risk of rapid removal of snow cover within 24-36h ____________________ 20%

Other situations (not much snow to melt, or further accumulating snow) ___ 10%

highdef

Snowbie wrote: »

Most significant is the Irish Sea is much cooler at this time of year than in Novemeber or December. Deep convection occurs when there is a greater difference in temperature between the atmosphere and sea surface, this is obviously better in late nov and dec like we saw in 2010 and Oct 2003. But the saving grace here is that upper air temps (850mb) are significantly cold too so it sorts of balances out so convection over the sea should be decent with trains of cumulus congestus rolling onshore and heading inland but Ive low confidence of explosive convection at this time occuring until perhaps wednesday when 850mb could be -13 or -14C, we shall see.

A cooler sea is also good news for those of us that live near the coast where it is usually more marginal in these events.

We should be able to get some forewarning of what to expect in advance, based on how intense showers are when they land in Eastern England. The North Sea is a degree or two cooler than the Irish Sea so in theory, showers for be more intense for Ireland, assuming similar upper air temperatures.

Meteorite58

What a day Fri is shaping up to be by the ECM, these charts showing high end warnings more then likely if they verify. Bit of chopping and changing to go though.

sdanseo

MT / sryan - based on those potential values of snowfall, where does this fall vs. historic events

e.g. how much snow fell in the space of a week in 47, 63, 82, 91, 10?