Tutorial Snow Forecasting & Weather Knowledge Base

POW Hungry

Old n' Crusty
Moderator
Ski Pass
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We can go pretty DEEP in these Snow Weather forums, so here's place to keep all the fiddly information in one spot.

Don't know what a particular meteo topic means? Confused about a Global Model Plot? Don't know the name of that cloud?
Then post a brief intro into what you do/don't know or understand about a particular weather related topic (or resource) with a valid question about what you want to know and your answer shall be answered and/or debated to help all & sundry understand.

Some basic guidelines & etiquette:
  • This thread is all inclusive of skill/knowledge; from a chief meteorologist to an absolute noob. It doesn't matter
    • Skill level is not a hierarchy in here
  • Hot heads can leave the ego in the Bear Pit. Trolls will be handed a 'straight to jail' card and dealt with accordingly
  • All topics are to be discussed constructively
  • It helps if you're explaining something to back it up with some resources/links
If you've never stepped foot into the weather forum it's good to know some basic weather (WX) terms & phases. BoM has a good list published HERE.
 

Nidecker

One of Us
Ski Pass
Aug 5, 2009
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Alpine the dark side of snowboarding
W
  • This thread is all inclusive of skill/knowledge; from a chief meteorologist to an absolute noob. It doesn't matter
    • Skill level is not a hierarchy in here .
Not that it needs to be, but I find if I leave the comment to the skilled crew in the predictions, but as a "lesser individual" add some input into the obs threads, its a good way to go :) YMMV
 
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Jellybeans

Part of the Furniture
Ski Pass
Aug 24, 2015
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Wondering if this little basic tutorial I made with skiers in mind a few years back would help. It was made for an American audience, but I have adapted it to make it more relevant to Australia.
Of the many concepts in the science of meteorology, most are not crucial for snow forecasting. We are mainly interested in air temperature and pressure, snow level, snow accumulation, and precipitation. First, let’s talk about temperature and pressure.

A key concept is the snow level, which is the minimum elevation at which snow falls. You may find maps for the freezing level, which is the elevation where positive temperatures meet negative ones. The snow level is below the freezing level because snow doesn't melt as soon as it goes into positive temperatures; it falls for a few hundred meters until it melts.

Let's look at this surface air temperature map (“2 meter” in this context means “surface”). The temperature scale on the right-hand side lists temperatures in Fahrenheit; they are shown as both numbers and colors. In this map, blue coloring inside the red lines means below freezing temperature (0 degrees Celsius). Therefore, all areas colored in blue can receive snow if there is enough moisture.

F96954CE-3A56-42CB-B6D8-8802A37D8BBE.png


There are many other types of temperature charts, such as the 850hPa and 700hPa charts, which show the temperatures at approximately 1500m and 3000m, that can be used to forecast temperature at the resorts.

You may be wondering what hPa is. It is the measurement of air pressure, which is another important part of weather forecasting. Millibars (mb) and hectopascals (hPa) are used interchangeably as the units for air pressure. The mean pressure on Earth's surface is 1013 hPa: any area of surface pressure above that is called a high (or any airmass of pressure higher than that surrounds it) , and any area of surface pressure below it is called a low. In North America, lows are the driving force behind most snow events.

img_4321-png.46917

Above is a 500hPa heights and MSLP map. 500hPa heights are on the scale on the right. The dam (or decameters, where 1 dam = 10 m) in this situation is the measurement between 500 and 1,000 mb. You don't really need to understand the full science behind it, but the lower the 500hPa height, the colder it will be. For example, a dam of 582 (orange) is going to be much warmer than a dam of 540 (light blue). A dam of 540 or below is generally connected to snowfalls at around 1500m, depending on other factors in the mix.

The other concept in there is MSLP, or mean sea level pressure. The L’s and H’s signify lows and highs, respectively, with their MSLP numbers. The lower the pressure, the deeper the low becomes. A deep low is generally associated with cold air. The lines around the lows and highs signify the direction of the wind, with winds around lows being counterclockwise and winds around highs clockwise.


The following is a very widely used weather chart. I personally use this one the most to map out where it is going to rain and snow. It is an MSLP chart, which shows precipitation and the 500-1000mb dam level (explained in the previous article). MSLP stands for mean sea level pressure and shows surface air pressure, which you can measure using a barometer. The black lines around the lows (L) and highs (H) are called isobars and show wind direction and speed. Tight isobars mean it's windy, and isobars that are far apart mean it’s calm. The red and blue lines show the 500-1000 dam height with their corresponding numbers on the line.


5DDC8DD3-0284-4C78-B356-663B6E78C747.png

The other major part of the chart is precipitation, which is measured according to the scale on the right of the map.

Green = rain
Yellow and orange = heavy rain
Blue = snow
Darker blues = heavier snow
Pink = freezing rain
Purple = sleet

But what drives this rainfall? Lows are generally associated with the most precipitation because the winds around a low come toward the middle, bringing in moisture, much like a tropical cyclone. In fact, the technical term for a low is a cyclone.


img_4324-jpg.46920



This picture is a satellite image of a “Nor’easter," which is a deep low affecting the Eastern United States. You can see the clouds coiling around the low, which keeps the moisture together and then pushes it upward to condense into clouds. Highs are dry because the moisture is pushed away.

99C32D1B-F712-414F-80D9-4A39AE8A6A1F.png


When all the factors add up, you get snow. This map shows how much snowfall in inches, according to the scale to the right, will occur over the next seven days (168 hours).

Here is what to look out for when forecasting snow:

  • A snow level below the height of the resort you ski,
  • Lots of precipitation, and
  • A deep low and a 1000-500mb dam height level of 540 and below.

You might have noticed “GFS” in the map titles. The Global Forecast System is the particular weather model that produced the chart; the most widely available weather model, the GFS is run by the American NOAA. Other weather models of note are the Euro model, produced by the European Center for Medium-Range Weather Forecasts (ECMWF), and the Canadian model known as GEM (Global Environmental Multiscale), which comes from the Canadian Meteorological Center.

[All weather forecast charts are created by Levi Cowan, a graduate PhD student at Florida State University: http://www.tropicaltidbits.com/analysis/models/.]
 

teleroo

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Ski Pass
Jun 19, 2019
1,563
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Illawarra
Just sharing my own internet tootling on the Madden Julien Oscillation. Here is a MJO plot from:
https://www.cpc.ncep.noaa.gov/products/precip/CWlink/MJO/CLIVAR/ecmf.shtml

ECMF_phase_51m_full.gif


You all can read up on the general behaviour of the MJO on the internet - BOM has a good youtube vid. What I was lacking was knowledge on how to interpret the graph above. So here goes. From the title it is a forecast of MJO values (using the ECMWF model) for 23 August to 06 Sept 2020. These are the dots within the thick green line (each day is a dot). Red line is last 40 days. Now, the MJO is basically an eastwards moving zone of tropical convection and moisture generation that circulates around the equator. The entire loop around the equator has been divided into eight "phases" (sections of longitude). Australia lies in the middle of phases 4 and 5. So on the above graph, the MJO was both strong and positioned roughly north of Australia between 5th and 8th August. The convection to Australia's north provides a good source of moisture and this fed into the Hand of Faith system discussed here:
https://www.ski.com.au/xf/threads/august-2-8th-the-hand-of-faith.88101/

The MJO then weakened and tracked eastwards across the Pacific Ocean (phases 6 and 7) and currently is over Africa. The strength of the MJO is indicated by how far each day (dot) is away from the inner black ring - the further out the stronger the MJO. So the current MJO forecast has it strengthening a little further over next four days whilst in phase 1, before progressively moving east and being over Australia again around 06 Sept. But next time it returns over Oz, it is currently forecast to be within the inner circle, so therefore not very strong.

Legend from website above: "Phase diagram showing the evolution of the last 40 days of observations along with the 15 day ensemble ECMWF forecast. The yellow lines are the 51 ensemble members and the green line is the ensemble mean (thick-week 1, thin-week 2). The dark gray shading depicts 50% of the members fall in this area and the light gray shading indicates 90% of the members. "
 

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GobrinIce

One of Us
Ski Pass
Jul 9, 2019
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Thank you @Jellybeans, can't believe how readable they all are now, I'll be coming back to your explanations a lot. And @teleroo I was just going to ask that very question about the MJO plot and you've explained it beautifully.

So all I have now is for someone to please explain the '500hPa Longwave'. Prior tutorial threads had links to a video that is no longer up.

Thank you.
 

Swiss

One of Us
Ski Pass
Jun 29, 2015
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434
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Canberra
I'd be keen to see a post explaining all the different types of snow (falling and on the ground) described in some of these threads.

We all know what things like sleet and powder are, but a little glossary of terms like graupel, sastrugi, sago, spring corn and the rest would be great.
 

POW Hungry

Old n' Crusty
Moderator
Ski Pass
May 28, 2000
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Eastern Burbs of Sydney
I'd be keen to see a post explaining all the different types of snow (falling and on the ground) described in some of these threads.

We all know what things like sleet and powder are, but a little glossary of terms like graupel, sastrugi, sago, spring corn and the rest would be great.
I'll get to this in the coming weeks. Good topic.
 

Jellybeans

Part of the Furniture
Ski Pass
Aug 24, 2015
13,048
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813
Canberra
I'd be keen to see a post explaining all the different types of snow (falling and on the ground) described in some of these threads.

We all know what things like sleet and powder are, but a little glossary of terms like graupel, sastrugi, sago, spring corn and the rest would be great.
Would this work, someone wrote this on Wikiski (which is technically part of this site) some time ago?
  • Ankle Breaker: See Crust.
  • Breakable: See Crust.
  • Boilerplate: Ice that is so hard it is difficult to get an edge in... and is sometimes see thru (like ice in your freezer tray).
  • Corduroy: The surface left after a grooming machine has finished with snow. Corduroy describes the appearance oof this snow before boarders and skiers sink their edges into it.
  • Corn Snow: Usually found in the Spring, this snow is characterized by it's large, corn kernel size granules. Also known as Ego or Hero snow as it is very easy to ski on and very forgiving.
  • Crud: Heavy, wet snow (although other opinions exist). The US generally refer to this as 'tracked out' powder.
  • Crust: Snow that has a crust on the surface. Generally formed by light rain/mist freezing on the surface, trapping in the dry snow underneath or by wind compacting the surface layer.
  • Death Cookies: Large frozen lumps of snow created by poor grooming, avalanche debris, crumbled cornices etc. Deadly when hit, and even worse when covered by a layer of new snow so they are invisible. Easily confused with dead wombats.
  • Dust on Crust: Crust with a big of fresh powder on top (so you can't see what you are skiing on, but you're skiing on junky snow).
  • Dry Snow: Falls at temperatures of -4 or below. Will generally squeak when walked on (like sand on a beach), and will be 'grippy' when packed.
  • Ego Snow: See Corn Snow.
  • Elephant Snot: Similar to wet snow but characteristic of Aussie conditions when you get nice dry snow and then it warms up and turns to the consistency of porridge.
  • Freeze Dried: Snow that has a lot of moisture sucked out of it by a heavy frost. This type of snow can sometimes has similar properties to powder.
  • Firm: See Ice
  • Hard Pack: When natural snow becomes firmly packed by repeated grooming or continuous wind exposure. Often snow that has never melted or recrystalized.
  • Heavy Snow: Normally warm fresh snow (falling at near zero degrees).
  • Hero Snow: See Corn Snow.
  • Horizontal Snow being blasted horizontally by a blizzard.
  • Icy: Icy is a hard, glazed surface created by one or a combination of the following: freezing rain, rapid freezing temperatures or saturation from ground water seeping up into the snow and then freezing. This type of snow often has a translucent appearance.
  • Machine Groomed: Loose granular snow that has been repeatedly groomed by power tillers.
  • New Over: This snow is used to describe any accumulation of snow over an existing surface, for example "New over packed powder" or "New over machine groomed".
  • Packed Powder: Powder snow that has been packed down up the above mentioned forces. It is no longer fluffy, but not hard snow, either.
  • Powder: The product of fresh, natural snow. Cold, new, loose, fluffy dry snow that has not been compacted by skier traffic or grooming.
  • Porridge: See Elephant Snot.
  • Sago: Minature hail like snow that falls at temps close to freezing. Immature snow that has gone through many freeze thaw cycles in the cloud before falling. Excellent at ripping the face off poor unsuspecting skiers and boarders as they ride the chairlift with a 80km/h gale at the top.
  • Sastrugi Firm (if not icy) windblown ridges.
  • Sierra Cement: See Elephant Snot - term used mostly in the Sierra Nevadas in California as their snow isn't as dry as places like Colorado
  • Slurpee Park: See Elephant Snot.
  • Slush: Typically Spring snow that has suffered the effects of high temperatures and becomes very wet.
  • Softening: Typical Spring conditions, started out rock hard, turning to slop by lunchtime.
  • Spring Snow: Generally used in the same context as corn snow, but can be used to mean the full cycle of snow that occurs in Spring, ie: firm in the morning, softening to corn snow, then possibly slush.
  • Wet Snow (Wet Pack): Snow that has become moist due to thaw or rainfall. Snow with a high moisture content when it fell.
  • Wind Blown Pockets: Dry snow that accumulates in valleys/depressions from being blown by the wind.
  • Wind Packed: Fresh snow that has been packed by the wind. Generally, this snow is dry.
  • Wind Polished: Ice, in resort snow report speak.
  • Yellow You don't want to know. Not to be eaten.

http://www.wikiski.com/wiki/index.php/Category:About_Snow
 

Phil Hart

Hard Yards
Sep 15, 2014
13
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www.philhart.com
What's the primary data source/model chart people look at when estimating the elevation that snow will settle at when the big systems come through?

Do you start with an assumption say that 5400 thickness equals snow settling at ~1300m (?) and then make adjustments in your head for what the actual forecast thickness is? e.g. each 40 increase in thickness equals snow level up by 150m (?)?

Or do you look at 850hPa Temp and assume that in areas of T=0 snow is settling at ~1500m and then mentally add 140m (1/0.7 SALR) for each extra degree in 850 Temp?

Or are you looking at forecast soundings or some other (simpler!) spot forecast?
 

Jellybeans

Part of the Furniture
Ski Pass
Aug 24, 2015
13,048
21,520
813
Canberra
What's the primary data source/model chart people look at when estimating the elevation that snow will settle at when the big systems come through?

Do you start with an assumption say that 5400 thickness equals snow settling at ~1300m (?) and then make adjustments in your head for what the actual forecast thickness is? e.g. each 40 increase in thickness equals snow level up by 150m (?)?

Or do you look at 850hPa Temp and assume that in areas of T=0 snow is settling at ~1500m and then mentally add 140m (1/0.7 SALR) for each extra degree in 850 Temp?

Or are you looking at forecast soundings or some other (simpler!) spot forecast?
You can look at freezing level forecasts, or even better snow level forecasts. It does the maths for you.
Try this for EC
And this for GFS

But yes, you can also look at soundings for a look at the wider picture (all the layers in the atmosphere).
 
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