Cold Snap, T0-180Hours Only

Thought I'd post a few bits of info I find useful when trying to predict snow, especially in the knife edge we are currently looking at early next week!

Apologies to the more experienced posters if this is just going over old ground but I though it might be useful to some amateurs (myself included)

Bascially to predict snow forecasters look for (but not exclusively);

1. Thickness (dam) less than 528
2. Surface temperature (deg. C) 4 degrees or less
3. Dew point temp (deg. C) preferably sub zero

These plus other factors all come together in the right airmass to give the white stuff.


This table although quite old explains snow probability fairly well by comparing Thickness (Left) to surface temp (Top),


......-1 0 1 2 3 4
516 A A A A A A
522 A A B C C C
528 A B C C D D
534 A B C D D D

A = Probability >90%
B = 90% >Probability> 50%
C = 50% >Probability> 10%
D = Probability< 10%


12z Hirlam for sunday night/monday morning
Thickness... (really need the second lightest blue)


Dew point...



and finally surface temp..


Have I wrecked your friday evening head???

The overview is that the two main North American models (GFS, GEM) bring severe cold to Ireland by Thursday 2nd which would also imply a colder transitional period from Sunday to Wednesday.

The two most consulted European models are more gradual and do not bring the severe cold any further west than southeast England. This would imply a long term transitional situation that affects mainly eastern Ireland.

Because of this split in the guidance, I am going to remain cautious until I see full model consensus or more robust signs of the North American model solution prevailing. We were seeing output like this at one point in early January although not quite with the same amount of real-time high pressure in place.

As far as I can see, the difference in the two sets of models relies largely on the western North American ridge being amplified and pushing enough circulation across the polar region to nudge the Russian high further west. This is a rather fragile setup which suggests the European model solution needs to be kept in view until we get better indications.

One sign that the North American models may be correct is that the Alaskan arctic regime has shattered and there is nothing left to prevent the Pacific from going into this more classic La Nina pattern, with the inevitable result being a downstream deep trough over the east coast and a more meridional storm track towards west Greenland.

I would say that if the severe cold moves west, heavy snow is a given for eastern Ireland and large parts of central Ireland (as seen north to south). Thus the percentages that I gave a few days ago should probably be adjusted today to read 50% chance of severe cold and snow east, 25% chance west, within ten days. Since planning for this, to some extent, is independent of actual date of arrival, I would advise people with significant interests in the eastern half of the country to start detailed planning because this could be "upon you" within a week.

Thanks for this. My knowledge is that the the thickness had to be at 500 is that right?

The dam thickness value needs to be less than 528 dam (522 better still) measured at the 500-1000 hPa height typically, the first 5 km vertically of our atmostphere.

Partial thickness charts are availible too like the 850-1000 hPa which would be more accurate in shorter timeframe at lower altitudes.

The dam thickness here for snow would be 129 or lower

leahyl wrote: »

Is Cork left "out in the cold" again (pardon the pun) as regards snowfall? :-P

If the GFS 12z run came off then yes, Cork would even feel the cold but the ECM run dosent agree as usual.

premiercad wrote: »

Thought I'd post a few bits of info I find useful when trying to predict snow, especially in the knife edge we are currently looking at early next week!

Apologies to the more experienced posters if this is just going over old ground but I though it might be useful to some amateurs (myself included)

Bascially to predict snow forecasters look for (but not exclusively);

1. Thickness (dam) less than 528
2. Surface temperature (deg. C) 4 degrees or less
3. Dew point temp (deg. C) preferably sub zero

These plus other factors all come together in the right airmass to give the white stuff.


This table although quite old explains snow probability fairly well by comparing Thickness (Left) to surface temp (Top),


......-1 0 1 2 3 4
516 A A A A A A
522 A A B C C C
528 A B C C D D
534 A B C D D D

A = Probability >90%
B = 90% >Probability> 50%
C = 50% >Probability> 10%
D = Probability< 10%


12z Hirlam for sunday night/monday morning
Thickness... (really need the second lightest blue)


Dew point...



and finally surface temp..


Have I wrecked your friday evening head???

Love the way it stops just before waterford again here :rolleyes: NOT

nilhg wrote: »

Still things are looking colder and we will have a few very interesting days model watching coming up.

True, but the trend is there and a little less than perfection is still some turnaround from what was showing only a couple of days ago.