1st January 2025 - 8th January Snow Potential, Freezing Conditions - Discussion

I think what’s being missed, as Wolfie says, is that a southern movement is in the the interests of most of the 26 counties (not just, or even primarily Munster). Donegal is the obvious outlier. But biggest winner from southern movement are the midlands, they go from possible snow to likely snow (and possibly all snow throughout). For south Munster it increases our chances of back edge snow but no more than that. We are going to see plenty rain first no matter what.

Arome is a decent hi res model. Only goes out to 51 hours so better looking at it tomorrow as it only goes up to Saturday evening at the moment but it's already showing respectable snow accumulation. I think Arome did quite well with November's snow.

I got a smart telescope from Santy this year and it's been out the back imaging for me since sunset! Amazing skies out there tonight, even in my suburban setting.

A little white stuff fell here the last hour - probably frozen rain/hail/graupel - very cold and now beautifully clear skies in South Sligo 90m asl. I'd say the paths will be treacherous in the morning

Deleted.

JS

Basically, in this event the cold air is already in place from the north and the precipitation is coming in from a warmer low pressure from the south. Where the cold air and warmer low pressure system meet we get lots of snow. It's very fine margins though, and an easterly wind from the Irish sea will bring temps up too much for coastal regions.

In a cold easterly, the cold air is actually coming from the east, and is usually a lot colder as it is sourced from the continent which is usually already very cold before it gets to us. The "streamers" happen because the Irish sea is relatively warm and the air above is very cold so it makes showers easily. But usually it's still marginal on the immediate coast.

Now if you combine the two, very cold easterly winds and a low coming from the south and you get the holy grail, Beast from the East / Storm Emma style.

I’ll break this down into two different scenarios that cause this to occur and compare it to the typical snow streamers you’d get from the same wind direction

Scenario 1; E and NE winds cause rain

When a low pressure comes across colder air (as seen in a situation like this) you’d want to have the 850hPa temperatures in the atmosphere to be around -8 or above, to put it simply, the air temperature below this threshold isn’t cold enough (forecasted in these event to be -5), and because of this, sea breeze acts as a “hairdryer” of sorts and rises the dew point level above freezing, leading to a wintery mix or cold rain (An example of this was seen December 10th 2017), unless you have elevation in this setup, or the 850 temperatures are higher than -8, the sea breeze will rise the temperature above 2C. Rain can transition back to snow again if you remove this onshore tilt, (which occurs as the low pulls away Sunday evening from 1PM). Complicated but extremely marginal

Scenario 2: Easterlies and NE winds bring snow to the east coast. Essentially unlike above where the wind is strong and more violent, you’d want a slack, unstable flow of air off the Irish Sea in which contrasts the warmth of the Irish Sea, leading to these snow showers piling onto the coast. Normally in these situations the 850hPa temperatures are at -8 or colder and the DP is well below 0 in the slacker flow. This caused situations seen in November 2010/Dec 2010, and the BFTE.

The one notable exception was seen in snowstorm Emma, where the 850s were crazy low (-16 or something) so you don’t have to worry about any mixing or wind strength because the contrast between the upper air temperature and the Irish Sea is so extreme it’ll always fall at snow.

If you’re wondering about March 1st 2024 and its snowfall in Dublin (similar situation to Saturday/Sundays low) the wind direction went offshore for the city letting the rain change to snow. Meath (sorry gonzo 😭) and places like Swords ended up unlucky with the NE tilt and saw cold rain.

I tried my best to make things less complicated but honestly it is complicated so that’s the best I can offer from one Dub to another :)

Radio1 weather just mentioned a multi hazard event over weekend!! Big news story tomorrow I think

can’t wait for the family and friends to catch wind of this from tabloids and say to me “it’s meant to snow tonight!” and I have to rebuttal with an explanation of what onshore wind is 🥲

Here's the live view from clear East Cork.

Another frame from earlier.

The best snow imo is around 5-10cm that falls in the late evening and sticks around for a few days. Not too disruptive, looks amazing, everyone's happy.

Storm Emma actually displaced all that deepest cold air from the original easterly portion of the event.

Was still bucketing snow throughout the Friday though. But A) frontal event, so airmass can be less cold and still snow, B) so much snow on the ground for much of the East coast, that the air was conducive to snow throughout, i.e., the soundings supported snow (old GL was great at posting these as Emma encroached). Add in C) that the easterly gale was also flowing over snowfields upstream in the UK and the sea is colder in March than it is in Feb/Jan. Thus, no issues with no so super cold airmass overhead.

A more sobering comparison might be January 1982. Dublin had easterly gales, and at times the airmass overhead was not spectacular, the deepest of the cold was further north for most of the event looking at NOAA/NCEP archive charts. Dublin had the deepest snow, in spite of that. I'll take a stab that our SSTs in the Irish Sea have risen enough since 1982 to make frontal events that bid harder in early winter, when the sea is warmer than it is in Feb or March. You need all the help you can get in these setups near the coast, and record breaking SSTs (though I think I read we have exited that period of crazy warm ones) don't help a jot.

Anyway, those of us in Dublin/east coast need to take a backseat on this one most likely. Our day will come. Maybe later Sunday. The models are also still playing with the idea of secondary features and lows popping up in the wake of the main low as it moves away East, could be something there.