Cold Snap, T0-180Hours Only

premiercad

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???

pistolpetes11

A little look at the 12Z DPs

https://www.youtube.com/watch?v=OycYnVvtpmA

M.T. Cranium

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.

weatherfiend

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???

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

pistolpetes11

https://www.youtube.com/watch?v=GBQ4lbZuaxo

BLIZZARD7

Excellent fax at +72 hrs- That front keeps getting pushed west- major back-edge snow for the east coast a possibility??







Dan

premiercad

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

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

Musicman2000

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.

pistolpetes11

https://www.youtube.com/watch?v=FIaUBFZkc9A

I cant get the last two of your vids to work.

last one said private

Daniel2590

pistolpetes11 wrote: »

https://www.youtube.com/watch?v=FIaUBFZkc9A

Vid is private :rolleyes:

blackius

weatherfiend wrote: »

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

528 or lower,the closer to 520 the better.

pistolpetes11

Daniel2590 wrote: »

Vid is private :rolleyes:

Should be sorted now lads , but I have a new one from a better map coming up

talkabout

Best looking charts of the Winter by a mile. Trying not to get excited, hurt too many times before

Rebelbrowser

leahyl wrote: »

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

Actually Cork city does surprisingly well from real easterlies. I remember the city getting great snow in Jan 1987 and Feb 1991, the last two real easterly set ups. Also in an easterly there's a chance once there's real cold in western europe for low pressure systems to move up and over Ireland hitting cork a bulls eye.......we'll be grand if it comes off! Cork city is actually far less west than most consider. We are mid south really, not south west.

pistolpetes11

https://www.youtube.com/watch?v=JAU7mCpAS-U

yorlum11

pistolpetes11 wrote: »

https://www.youtube.com/watch?v=JAU7mCpAS-U

Love the vids pistolpetes11!

whitemocha

ok here is a stupid question

before when looking at the GFS how come it moves in an easterly direction and now the weather is moving west ??? what makes this happen??

probably the dumbest questions asked?

Musicman2000

whitemocha wrote: »

ok here is a stupid question

before when looking at the GFS how come it moves in an easterly direction and now the weather is moving west ??? what makes this happen??

probably the dumbest questions asked?

High pressure over scandi and lower heights down in the med, also you need to keep any eye on the jet taking a dive south

whitemocha

Musicman2000 wrote: »

High pressure over scandi and lower heights down in the med, also you need to keep any eye on the jet taking a dive south

so does this automatically mean colder weather??

Redsunset

For those worrying about what they would like to see as guidance from the AO (ARCTIC OSCILLATION) and NAO (NORTH ATLANTIC OSCILLATION) to give a good chance of cold in the current set up,let me remind you of My post from early Jan where I researched the actual AO and NAO indices for the cold spell of Feb 1991.


Feb 91 AO (ARCTIC OSCILLATION) Ranging from neutral to negative
YR...M .D.. Index
1991 2 1 -1.081
1991 2 2 0.440
1991 2 3 0.682
1991 2 4 0.201
1991 2 5 -0.375
1991 2 6 -0.778
1991 2 7 -1.267
1991 2 8 -2.045
1991 2 9 -2.671
1991 2 10 -2.651
1991 2 11 -2.519
1991 2 12 -2.522
1991 2 13 -2.392
1991 2 14 -1.754
1991 2 15 -1.312
1991 2 16 -1.371
1991 2 17 -1.287
1991 2 18 -0.383
1991 2 19 0.354
1991 2 20 0.617
1991 2 21 0.236
1991 2 22 0.095
1991 2 23 0.531
1991 2 24 0.933
1991 2 25 0.837
1991 2 26 -0.137
1991 2 27 -1.478
1991 2 28 -2.562

FEB 91 NAO (NORTH ATLANTIC OSCILLATION)

YR...M. D.. Index Mostly neutral to slightly positive
1991 2 1 1.343
1991 2 2 1.452
1991 2 3 1.317
1991 2 4 1.240
1991 2 5 0.870
1991 2 6 0.625
1991 2 7 0.465
1991 2 8 0.354
1991 2 9 0.440
1991 2 10 0.458
1991 2 11 0.324
1991 2 12 0.188
1991 2 13 0.101
1991 2 14 0.037
1991 2 15 0.093
1991 2 16 0.144
1991 2 17 0.285
1991 2 18 0.685
1991 2 19 0.932
1991 2 20 0.956
1991 2 21 0.735
1991 2 22 0.643
1991 2 23 0.750
1991 2 24 0.801
1991 2 25 0.938
1991 2 26 1.020
1991 2 27 1.128
1991 2 28 1.030

So looking at the data it appears that the AO played a large role in that cold spell. AO at its most negative from 8th to 13th Feb 91.
With a Scandi high type set up it looks like we need at least -2 or more AO when the NAO is not playing ball to give any real chance of cold to these parts. Just an example mind,won't always play out that way but food for thought.




We would like to see cold undercut like above image
Just so some new folks know,the AO and NAO are simply a more visual representation of what the ensembles are saying. They don't steer the cold/mild. That is why they too change so often.

If the Snow Queen has indeed decided to send out her troops and also conquer these lands too, I proclaim that we admit defeat and bow down before and hail,

THE GREAT WHITE SIBERIAN ARMY

jo06555

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

ophelia75

The Daily Express today has the dramatic headline that the "Big Freeze will stay for a month."
http://dailyexpress
I know that this kind of sensationalism sells papers but after all the excitement today with the models etc, there has to be more of a whiff of truth to it! However, usually these kind of predictive headlines jinx it though! The forecast is also just for England which isn't too helpful to us, but at least heading in our direction! :pac:

pistolpetes11

On the back of RED's bit on the AO and NAO , here is the latest guidance

jo06555

yorlum11 wrote: »

Love the vids pistolpetes11!

Ok so this video started with my hopes fading as all was heading east but then the fight over ireland started heading west and by the end of the video we were covered in severe cold am i right and if so when do we get emerged in that severe cold

pistolpetes11

jo06555 wrote: »

Ok so this video started with my hopes fading as all was heading east but then the fight over ireland started heading west and by the end of the video we were covered in severe cold am i right and if so when do we get emerged in that severe cold

Il put it to you like this , Sunday Night Tru Mon is like the appetizer , If the cold moves west some more it will be perfect , anything out of this time frame is really up in the Air , It could happen the warmer milder south westerly's settle in over us and we end up the wrong side of the colder airmass.

Really its baby steps before we can get going on this , its been touch and go on the models with swings on every run , there is too much uncertainty to nail my colors to the mast saying sever cold wil be here on such a date,

We will get through the weekend before having realistic prospects for what maybe coming further down the road.

nilhg

Guys, I know the 12z GFS is eye popping and if it validated would be surely keep even the most demanding cold lovers happy for a while, BUT looking at the ensembles the operational run is a huge cold outlier, with almost no support in the 4th-7th timeframe.



I can't see how this won't downgrade on one of the subsequent runs.

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

doccy

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.

jo06555

Met eireann on 9 news dont give nothing over the next 7 days apart from rain and more rain