Liquid equivalents to actual snowfall amount

[Tarmo Tanilsoo]

Hello.

I've noticed that weather models happen to give snowfall in liquid equivalents. Wondering, if it is possible to estimate how much snow the liquid equivalent would bring?

For example, if liquid equivalent is 1 inch, then how much would snow depth increase?

Thanks in advance.

 

[Jason Boggs]

I'm not sure about your particular area, but here in the southern plains it is usually around 10 inches of snow to 1 inch of water. If the snow is extremely dry and "powdery", it can be as much as 30:1. Hope this helped you out.

 

[rdale]

http://sanders.math.uwm.edu/cgi-bin-snowratio/sr_intro.pl


Operational guidance on the ratio of snowfall depth to liquid water is severely limited. The familiar 10:1 rule is unreliable in many circumstances and guidelines based on surface temperature alone are flawed. In a recent study of snow ratio (Roebber, Bruening, Schultz and Cortinas 2003; Weather and Forecasting), a method for producing superior forecasts of depth of snow was developed, based on artificial neural networks. For your convenience, this technique has been adapted on this website such that given a model forecast sounding (obtained from the Eta, the MesoEta and the GFS), you can determine the likelihood that the snow will fall into one of three density classes (heavy, with ratios up to 9:1; average, with ratios from 9:1 up to 15:1; light, with ratios exceeding 15:1).

This realtime system was made possible by an informal collaboration between UWM and the National Weather Service (NWS) office at Grand Forks, North Dakota (KFGF). Programming was accomplished by Richard Hozak of KFGF.

Objectives
The UWM realtime snow-ratio forecast page is designed as a quasi-operational system. The intent of the system is to provide a test bed for ongoing research into the predictability of snow ratio. In addition, the output from the system is provided on this web page as a service to the operational forecast community and other interested persons. However, usage of these products remains entirely at the discretion of the user and the responsibility for decisions made (good or bad) based upon the forecasts rests entirely with the user.

How do I use this page?
On this page, you simply click on the map to view all potential forecast sites, based on the output from the NOAA operational models. Then click on the blue triangle to obtain a forecast for the site. Alternatively, simply enter the site id in the box to go directly to the forecast. In either case, select the desired forecast model from the pull-down menu and click continue. Then, choose the sounding valid time, enter the forecast liquid equivalent QPF and surface wind speed. Click continue again. The probability of snow being in the heavy, average or light categories, given that snow occurs, is displayed.

Notes:
1) The forecasts are based upon average vertical motions (more info here). Should the vertical motion be focused in a particular layer, some adjustment of the forecast may be required. For example, if the vertical motion is particularly strong in a layer that contains supercooled liquid water, then riming may be increased, and snow densities will be higher.
2) The sounding profiles are based on the selected model forecast. Two different model forecasts for the same valid time may well produce different results. Obviously, you should choose the model forecast that you believe best represents the observed conditions at the forecast valid time.

 

[Robert Dewey]

There is no specific ratio for any specific area. Here in southern MI, I have seen as low as 4 to 1 (which really kills a snow forecast), all the way up to 20 to 1. While I don't have one specific method, I generally use a 10 to 1 ratio for events that are more than 24 hours away (I should start using an 8 to 1). As the event gets closer, I look more closely at the thermal forecast profiles and how deep the "best snow growth" layer is. The closer you are to 0C through the column, the lower your snow ratio.

 

[Jeff Snyder]

This isn't 100% aimed at liquid-to-ice ratio, but at winter snow forecasting amounts...

Here's a good link on brief explanations of traditional forecasting techniques (Cook method, Garcia method, etc): http://speedy.meteor.wisc.edu/~swetzel/winter/methods.html

An ingredients-based approach is explained at http://speedy.meteor.wisc.edu/~swetzel/winter/winter.html (click on "Forecast Ingredients" on the left frame, or download the Powerpoint presentation available on the right column towards the bottom)

If you have much interest in snow forecasting, and you are a relative beginner in this regard, it's worth checking out, IMO.

 

[Tarmo Tanilsoo]

Thanks for the replies. But I wonder if there is any formula accounting for different meteorological elements which could help to determine the ratio of liquid and snow?

 

[Marko Korosec]

Hello fellow European and welcome to the board!

Usually ratio is 10:1 here also and our forecasters often approximate their snow amount forecasts in the way that they multiply expected rainfall accumulation for 10 times. But as others already noted snow accumulations depend of air temperature, humidity...not to mention ground temperature which is often overlooked.
Probably there is no specific number which could tell you snow/rainfall ratio, but you can always take higher ratio when looking at big flakes falling against grains or snow pellets. The best snowfalls are usually from +1 to -2°C which often give ratio 10:1. Last winter I made a test during intense snowfall when we got 30cm of snow in the whole day and ratio was always between 8:1 and 12:1.

 

[John Wetter]

Here's a presentation on snow equivalency that was presented at the Northern Plains Winter Weather Conference:
http://web.stcloudstate.edu/raweisman/research/conference/snowtoliq2006.ppt

-John

 

[jladue]

The St. Cloud U presentation clearly shows the huge spread in possible snow ratios for any given surface temperature range. In fact, for lower surface temps, there's even more uncertainty in snow ratios. I suspect the greater uncertainty at colder temps is because Minneapolis is subject to strong thermal inversions in many significant snowfall regimes. Therefore the surface temperature has almost nothing to do with what's going on at the level where snow is being created. The study also has not considered the effects of blowing and drifting on snow compaction whereas the Roebber et al. technique did consider drifting snow.

There is another technique which tries to estimate what kind of snow particles will reach the surface. The premise is that snow ratios are highest with the fluffy dendritic snow flakes, and lower for heavily rimed snow particles, or very compact plates commonly formed in extremely cold conditions. Take a look at http://wdtb.noaa.gov/courses/winterawoc/lessons.html#ic6 and look for snow ratio basics.

Speaking of observing snow ratios, ASOS sites degrade quickly when the wind exceeds 8-10 kts. There are automated stations in the mountains called SNOTEL sites that are designed to measure snow ratio, and a few fire weather sites (RAWS) can be used too. But overall, snow ratio observations are pretty hard to come by. However, there's a volunteer rain/snow reporting site called http://cocorahs.org/
Cocorahs is a good site for which you could contribute, at least for the US states that contribute so far. The owners of the site are trying to help improve snow depth, ratio and precipitation observations.

regards,

Jim

 

[rdale]

BUFKIT now has an experimental algorithm, in part developed by Dan Cobb @ GRR and others, but it's just an early attempt at implementing it. I'll be automating Dan's program to take BUFKIT soundings and run it through the whole process and posting them online in a week or so...