Will It Snow??

[johnholmes]

hi all

I promised to give this guide, so here it is.

Its not complete and it is for those who are new to trying to work out this very difficult question.

What to look for to get snow at sea level.

(1) in showers

(2) frontal weather

(1)

1) Dry bulb temperature below 5C, often 3C is a better mark

2) Dewpoint at or below zero

3) wet bulb temperature, if you have a weather station, no more than about 2C

4) 1000-500mb thickness (DAM) less than 522dm, lower if you are on the coast, but as high as 540dm it is possible in a heavy shower, but unlikely.

5) 850mb temperature of -7C or below, -5C it can occur but not often.

6) on the 850mb chart if the value shown on the contour line is below about 1290dm or 1300dm, then there is a high chance that ppn will be of snow.

(The Met Office use 1293dm for a 50% and 1281dm as a 90% chance of snow)

7) zero degree isotherm or freezing level of 1,000ft or less to give a 50% or higher chance of snow at sea level. At 2,000 ft above sea level the chance is reduced considerably.

(2)

Two types of front

(1) warm front

(2) cold front

In (1) warmer air is flowing over the top of cold air, and in the case of (2) cold air is undercutting the warm air ahead of it.

(1) is, I suppose the classic heavy snow situation which with strong winds can give blizzard conditions, even on relatively low ground, assuming all the factors are in its favour. This assumes they are, namely that very cold air lies near the surface but is not being moved away by an approaching frontal system. This stagnates and eventually retreats away again.

In this instance then the above requirements need to be met along with

(a) If prolonged and fairly heavy ppn occurs then what we call the wet bulb temperature will start to lower and this can enhance the probability of snow falling. In this instance if the wet bulb temperature is at 3C or below then the ppn can turn to snow from rain. Also, and this applies to showery conditions also, if the wet bulb freezing level(not easy to find on any chart!) is 2,000 ft or below, then ppn can readily turn to snow.

In light ppn then, often, regardless of any of the above factors being favourable drizzle or light rain will fall not snow.

(2) In this instance then if the air is not all that mild in the so called warm air and the cold air is very cold, with near negative values close behind the front, then even moderate ppn will readily turn to snow as the cold air undercuts the mild air. Use the values above for a guide.

This has only touched the surface of trying to forecast will it snow or not but I hope gives, the less experienced, a guide of what to look for.

With reference to the wet bulb temperature then this site can help you find that value if you have values of dry bulb, dewpoint and surface pressure

http://homepage.ntlworld.com/booty.weather...fo/thdia_hd.htm

On the topic of wet bulb temperature. I have still not worked out how to give Paul or Karl sufficient data that they can set up an algorithm to produce charts of wet bulb potential temperature and also wet bulb freezing level charts. If anyone knows how to do this please pm me or Paul or Karl as it would be another big step for Net Weather to be able to produce such charts from the GFS run.

Please ask questions and I will try and answer them as best as I can.

I hope you have lots of opportunity this winter to try this guide out.

Mods can you pin this and open another below it for comments please?

[noggin]

John, do you have any signs for simpletons like myself to look out for, please?

regards

noggin

[johnholmes]

sorry I thought that was fairly simple I had posted!

Dry bulb temp

dewpoint

look on gfs for where ppn is

if the 3 above coincide then you may get snow

then have a look at the zero C chart, if its below 1000ft then pretty sure

take a look at the 850mb chart, use the value I've suggested

then you should have a good idea.

is that okay?

John

[TEITS]

A superb guide there John thanks for taking the time to post this, im sure we shall all find this guide very helpful and useful.

[Nick F]

Thanks John for the guide. Seems to clarify certain things I've pondered, often people will get excited over the 528 DAM line as regards to whether it will snow or not - but it seems rather a marginal indicator - often I've been under 528 in rain - better is 522 DAM for snow - though FAX charts don't show this.

On the subject of Wet Bulb Potential temperature (Theta-W it seems to be abbreviated to) for predicting snow, although I know nothing about algorithms, Meteoblue displays clear Theta-W charts here like the one below:

... others 3 hour intervals can be found here:

http://pages.unibas.ch/geo/mcr/3d/meteo/dt/index.htm

[johnholmes]

tks Nick

yes I've seen them, what I'm trying to get is something we had in the Met Office which showed 850mb theta-w charts superimposed on something else, I think surface isobars, but have no idea how to get that.

tks though, glad some of it helps

regards

John

[SP1986]

thanks John great help.

Myself the 528 dam line has always produced here but that maybe conducive to physical height. always best to give leighway.

[Polar Continental]

Thanks John,

another good guide for us winter lovers, some very useful and educational information there for many this winter.

I wonder if someone could write a guide for the mild rampers in here, they seem to be missing out

Regards

Paul

[Stoxs]

thanks john

i think this will help

one question

what the hell is a bulb temp

wet or dry

but on a serious note it will be handy to look up later if or when the great white appears

thanks again

all the not so bright weather chasers

[Evo]

thanks john

i think this will help

one question

what the hell is a bulb temp

wet or dry

but on a serious note it will be handy to look up later if or when the great white appears

thanks again

all the not so bright weather chasers

Try here (and here)

Edited November 16, 2005 by Evo

[johnholmes]

thanks for posting those Evo.

Its extremely difficult to pitch any info at a level for all. Harder still not to appear to be talking down to someone, so thanks again.

Please ask if anything is unclear.

regards

John

[John Cox]

Great post John,

Got it all execpt the zero C chart.

What is this and where do i find this chart?.

Regards,

John

[noggin]

sorry I thought that was fairly simple I had posted!

is that okay?

John

Thanks, John. The links posted by Evo have also been useful. I am an absolute beginner as far as the technicalities of weather forecasting are concerned! However, where there's a will there's a way and I am determined to learn as much as I can.

I appreciate that many people on here are experts! I would never feel that I was being "talked down to".......just pleased that some kind soul was kind enough to take the time and effort to go through it again in terms suitable for a novice. I do not get offended!

[johnholmes]

hi N

the only way to learn is to read what is posted and if you do not understand then ask. Don't be afraid to ask. That is how we all learn. thanks

JC

Its on Net Wx Extra, and shows where the zero degree isotherm is, just an example below

the other way is to look at a skew-T diagram and see where the environmental curve for the dry bulb crosses the 0 deg C isotherm.

Is that okay?

regards

John

[John Cox]

Hi John,

Just to clarify.

You are saying that along the line of 1000ft the temp should be 0C in order to give snow at sea level.

In this map what is the temp along the 1000Ft line straddling the centre of Ireland??

The colours dont really make sense to me or the range at the side.

Regards,

John

[johnholmes]

sorry John just dashing out, will chat later today

cheers

John

[johnholmes]

Hi John,

Just to clarify.

You are saying that along the line of 1000ft the temp should be 0C in order to give snow at sea level.

In this map what is the temp along the 1000Ft line straddling the centre of Ireland??

The colours dont really make sense to me or the range at the side.

Regards,

John

to answer your query John

The lines are in METRES not feet. So the 1,000 line your refer to is 1,000 metres which is approx 3,000 feet. I would prefer the values to be in feet but there we are!

At the height you quoted it means the temperature is zero C. For snow to fall you really need it about 1,000 feet or 300 metres to be sure it is snow. Even then if the ppn is very slight then it will be drizzle or slight rain in some cases.

hope that helps

John

John says he is not too clear about what the chart below means

so what I will do is put it into ms paint and try and explain on the picc itself. May take a little while so bear with me

regards

John

here with the extra explanation of the picture

John

[johnholmes]

right so let us look at a skew-t to see if this helps a little

below is the one from Lerwick at 12z today-Thursday 17 November 2005

below is the METAR(weather report for Sumburgh about 25 miles south of where the skew-T was done.

EGPB 171420Z 36007KT 9999 FEW012 SCT045 04/M02 Q1016 NOSIG

it has no showers, a dry bulb temp of 5C and dewpoint of-1C

so let us put that in the skew-t and see what we get

in this instance, in red, I have put the dry bulb temp and dewpoint, and (refer to the Guide if necessary) I have then shown where the cloud top would get to using that temp and dewpoint. You can see it is quite a bit lower than the THEORETICAL cloud top. That suggested clouds could bubble up to about 7500ft, I think I said, but in reality with the values at Sumburgh airport only cloud of about 3,000ft thickness is being released.

Not enough with the zero C at about 1,500ft to give any showers.

I hope this explanation has helped to show how easy it is to work these things out and also how the initial appearance may be misleading as to what actually happens.

yet another bit of info

the last Net Wx VISUAL sat picc in that area. Interestingly it shows a rather more stratiform type of cloud over Shetland than further south east.

http://www.net-weather.co.uk/pages/charts/...171046-hvct.jpg

regards

John

ps, as ever pse ask questions those new to this.

j

Edited November 17, 2005 by johnholmes

[John Cox]

Hi John,

I appreciate the time and effort on this one.

It is now as clear as day, well almost.

I will study this further this evening.

Thanks again.

Regards,

John