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

Artane2002 wrote: »

Here is what is shown for Saturday morning by the GFS 06Z. Correct me if I'm wrong but this looks like an all snow event in Ireland to me? The South is in -4c uppers while the far north has -6c uppers.

I think you'd usually want upper -8s but the ground and sea temp might move that around, I don't trust the GFS anyway in these situations, think the ECM does do bad weather a lot better. The GFS percip chart is showing snow on land and rain at sea looks marginal there. I'm still going with the ECM and not even thinking about it properly till 96 hours out, worth keeping in mind as the charts progress.

Artane2002 wrote: »

You need uppers that low for showers. You can have uppers between 0 and -2 and it can still snow during frontal events. Some of our greatest snow events have come from relatively high/marginal uppers. Also dew points are arguably more important, they must be below zero for snow to fall.

Sorry, forgot about the dew points, Sunday morning head

My personal opinion is we will see changes in models in the next few run regarding that low on Friday Saturday. I expect to see the low a little future south keeping us in an northeasterly flow

Here is the NAVGEM on Saturday morning, at the same time as the GFS above. It is having none of this blizzard, with the South and West in -8 uppers while the east and north are in -6 uppers. Just goes to show that this is far from nailed. (Yes I know the NAVGEM is a poor model but still! )

Artane2002 wrote: »

Here is the NAVGEM on Saturday morning, at the same time as the GFS above. It is having none of this blizzard, with the South and West in -8 uppers while the east and north are in -6 uppers. Just goes to show that this is far from nailed. (Yes I know the NAVGEM is a poor model but still! )

I usually trust the ECM and UKMO a bit more than the rest.

This UK Met Office surface chart for Wednesday needs to be archived, so archive it I shall:



Beautiful.

Today's 850 hPa temp anomaly analysis (from the GFS) for the 36 hours up to 06z.



I am trying to archive as many weather charts from this upcoming spell as possible just to keep them on record, because it is a pattern we sadly don't see very often these days, and may not again for a very long time.

UKMO Early warning for Thursday's potential. However they note the high uncertainty at this range.

https://www.metoffice.gov.uk/public/weather/warnings#?region=sw&date=2018-03-01&regionType=area

Chief Forecaster's assessment
A weather system is expected to move slowly north through Thursday into Friday and has potential to produce widespread snow, accompanied by strong to gale force winds. As less cold air follows from the south, there is a small chance of snow turning to freezing rain bringing an additional ice risk. There is still uncertainty in how this system will develop, but there is a small chance of the combined effects of snow, strong winds and ice leading to severe impacts.

Well if they are starting to move south at this stage it might show some hope, if we could keep the ppn factor of the low by having it not go to south but also keep the cold that would be the best thing to happen, which hopefully the next model runs will start to show rather then pointing the low towards the north

ECM looks promising well into Saturday, then the low seems to split into two seperate lows. While the low has moved southeast on the GFS.

All seems to support the thought that models are simply struggling with all this madness?

MJohnston wrote: »

Well I figure as they have their own snow depth charts, they should be able to use those in any feedback loop. I'm just wondering what effects lying snow does have, because if we're going to get a fair amount of unmodelled streamers...well you can see where I'm going with that.

Lying snow causes lower temperatures. During the day because it blocks the sun heating the ground, and at night because it blocks heat from the ground warming the air.

The ICON 12Z looks fun during Wednesday night if snow is your thing into Thursday.

MJohnston wrote: »

How much can lying snow affect other conditions such as air temperatures, and are models good at being able to take this into account?

Yes. Snow fields lead to lower temperatures.

Do you remember january the 10th 2009. If you do, it was time a radio forecaster was memorably heard to utter the following: you can never turn your back on snow? He said that because Met Eireann had forecasted an all rain event everywhere- it wasn't( at least not in the north west), and why it was not was due to wind direction, the angle of attack, and the snowfields, which despite much warmer upper air still had enough cold air lower down to ensure what hit the ground was snow. We had neary 12 hours of blizzard like condition.

I am sure some models depicted what happened accurately, but Met Eireanns did not. So to answer your question, it is possible, if the wind direction is favourable, areas well inland could see all snow even if the uppers are very marginal.

ps i might have got the date wrong, but the point remains accumulated snow, as well as other factors i mentioned can sway things in favour of snow