How much snow will accumulate?
The information presented on this page comes from a combination of professional literature and personal experience forecasting snow in states like Missouri, Kansas, Wisconsin, and Minnesota.
Forecasting snow accumulation is more complicated than just determining how much liquid will fall, then converting into snow amounts based on simple rules of thumb. Quick conversion factors can sometimes provide an initial estimate but in many cases will fail. In other words, forget the 10 to 1 rule (10 inches of snow to 1 inch of liquid water). This observer has seen snow-to-liquid ratios ranging from 5 inches of snow to 1 inch of liquid up to 50 inches of snow to 1 inch of liquid. Although rare, ratios can still be higher or a little lower. Many meteorological and microphysical processes influence how much snow accumulates given a certain amount of liquid water. Snow measuring procedures will ultimately affect what is reported. Just forecasting the liquid precipitation amounts is itself a difficult task. The following is a list of factors that influence snow to liquid ratios, some of which can be considered when trying to make a forecast.
1. Differences in Snow Measuring LocationsSubtle dips and ridges on what appears to be relatively flat ground can affect snow measurements. In this case, even moderate winds of 10 to 20 mph can move snow around enough to smooth out the visual differences. Measurements taken in the dips will be higher and measurements on the slight rises will be lower. The tactic of averaging multiple measurements will not work if the observer is not careful to make sure that all of the measurements are not taken close together. Stronger discontinuities of the surface and barriers like houses will aid drifting. Winds can blow snow off one residential property onto another adjacent property. Winds can blow snow from the top of a hill to the bottom, causing higher snow totals for snow observers who live at the bottom. The hope of the forecaster is that the snow reports in the vicinity of any particular town will average out into the range of accumulations that were forecast.
2. Differences in Snow Measuring ProceduresNational Weather Service offices add 6-hourly measurements to get storm total snowfall. The measurements that are added together are taken at 0000, 0600, 1200, and 1800 Universal Coordinated Time (UTC), also known as Zulu Time (Z) or Greenwich Mean Time (GMT). Snowfall is often measured more frequently to report updated snowfall amounts for statements issued to the general public. Some observers at official observation locations, such as airports, take extra measurements during heavy snow for the purpose of adding the Snow Increasing Rapidly (SNINCR) remark to the observations. SNINCR is reported when snow accumulates 1 inch or more in an hour. For example, 2 inches accumulates in one hour and a total of 6 inches is on the ground. The remark would be written as SNINCR 2/6. Amounts are rounded to the nearest inch so one-half inch would be reported as SNINCR 1/6.
Cooperative observers and other designated snow spotters may reset their snowboards or measuring surfaces after longer periods of time. Cooperative observers are specifically instructed to take measurements at least once a day but are encouraged to reset measurements up to four times a day at 6 hour increments. Random reports from other sources may not follow specific guidelines. Since snow compacts over time, even while a snowstorm is in progress, the inconsistent measuring procedures WILL produce differences in measured snow totals.
3. Mixed PrecipitationEven just a little mixing with freezing rain, sleet, or snow pellets will create a denser snowpack and limit potential snow accumulations.
4. Ice Crystal HabitCrystal growth habit determines the shapes of crystals. Short columns or needles will accumulate more densely than dendrites or plates. Dendrites in particular have a feathery multi-branched structure that allows them to accumulate loosely on the ground. The more the dendritic crystal habit dominates, the fluffier the snowpack will be. Anticipating the crystal growth habits of snow can help a forecaster determine how much snow will accumulate. Ice crystals usually experience multiple growth habits and other processes such as riming due to collision with water droplets as they fall through different temperatures and supersaturated conditions in the clouds. Dendrites and plates generally occur at -10 to -20 oC (14 to -4 oF) when supersaturation is high. The most optimum temperatures appear to be -12 to -16 oC (10 to 3 oF).
5. Ice Crystal SizeIce crystal size is strongly related to ice crystal habit. Large crystal size tends to correlate with decreased snow density, especially if crystals are dominantly dendrites. Other types of crystals such as needles and columns tend to be smaller. Size increases with greater time spent in the cloud but fracturing can break up larger crystals into smaller ones. Most important is how much time crystals spend in a region of clouds where high supersaturation, strong vertical motion, and temperatures favorable for dendrites all overlap.
6. Relation of Cloud Temperature to Density of Expected Snow AccumulationConventional wisdom holds that snow forming in colder temperatures results in higher snow to liquid ratios. This assumption is true to a point. If the assumption is followed as a rule it can result in a poor snow amount forecast. Ice crystals in the form of needles and columns, which accumulate in a tightly packed fashion, occur at temperatures warmer than -10 oC. As temperatures cool to the -10 to -20 oC range, plates and dendrites become dominant. As a result, the snow becomes fluffier as temperatures cool. The trick here is that as temperatures cool below -20 oC the crystal habit reverts back to columns and generally smaller size crystals. If most of the ice crystal formation takes place at temperatures colder than -20 oC, the snowpack will be denser than what you might expect.
7. Accretion (or Riming)Accretion is the growth of ice crystals by colliding with supercooled water droplets. Accretion increases the density of snow accumulation by increasing the thickness of ice crystals and consequently the water content of the snow. It occurs when temperatures are below freezing but too warm for significant ice crystal growth by deposition, such as warmer than -8 oC and especially warmer than -4 oC. See next section for a discussion of deposition. A deep layer of air with temperatures slightly below freezing and with high supersaturation favors heavy riming.
8. Deposition vs. AccretionDeposition is the process by which ice crystals grow at the expense of water droplets as water vapor moves from the surface of the droplets to the surface of the ice crystals. The vapor pressure at the surface of the ice crystals is lower than that of the droplets. Crystal growth by deposition results in crystals that are less thick since they are growing structurally rather than by collecting additional layers of water by accretion. The more time ice crystals spend growing in clouds cold enough for deposition to dominate over accretion, the lower the water content of the snow.
9. SublimationSublimation is the process of ice crystals changing phase from solid to water vapor without an intermediate change to liquid. The process causes ice crystals to shrink in size, ultimately resulting in tightly packed accumulation on the ground. Sublimation is a consideration mainly at the start of a snowstorm where cold, dry air is already in place.
10. Melting of Snow Before Reaching the GroundObviously snow that melts completely cannot accumulate. Partially melted snow will accumulate in a soggy or slushy fashion. Very difficult to anticipate how dense snow will accumulate in this case. Odds are it will be less than a 10 to 1 snow to liquid ratio.
11. Wind SpeedStrong winds produce a sifting effect as ice crystals tumble and break into smaller pieces by colliding with each other and various objects on the ground. The sifting causes the ice crystals to pack more tightly by filling in spaces between other crystals. Observers measuring in an area more exposed to the wind may measure less accumulation than protected areas. If the winds are very strong, such as in a blizzard, then so much snow may be blown away or drifted so severely that some snow reports may need to be estimated rather than measured.
12. Weight of the SnowSnow does have weight. Over time, snow at the bottom will be compressed at least a little by the weight of snow above it regardless of the snow's density. Compaction affects snow measurements less if snow measuring surfaces are reset after shorter periods of time, such as each hour or every three hours. Compaction is greater if measuring surfaces are reset after longer periods of time, such as every 6 or 12 hours.
13. Ground Temperature Above FreezingWarm ground inhibits snow from accumulating, especially if the surface of the ground is near saturated. Water has a high heat capacity so quite a bit of snow will have to fall and melt to absorb the heat before the ground cools close enough to water's freezing point for snow to accumulate. Heavy snow can overwhelm warm ground and accumulate faster than the snow melts from the bottom. Lighter snow has less of a chance to accumulate. Different types of surfaces have different heat capacities and will have different temperatures due to whatever solar radiation penetrates the clouds. This variation in temperature due to the sun is especially true from late March into May when radiation is stronger. Highly variable snow accumulations will result with snow accumulating on grassy areas easier than roads and dirt. A white colored or light wood colored snowboard will likely accumulate more snow than the surrounding ground.
14. Ground Temperature Below FreezingIn some cases the ground is frozen but the air is a little above freezing. Snow will likely accumulate at least initially. If the temperature does not rise much during the event, the snow can continue to accumulate, especially if the intensity increases. Snow already on the ground also serves as a cold surface on which falling snow can accumulate.
15. Snow MetamorphismMetamorphism of ice crystals is a process "in which water sublimates from the sharper edges of the ice crystals and deposits on the more rounded edges, making the snow crystals more rounded and dense. Such rounding is accelerated at temperatures approaching freezing" (Paul J. Roebber, Sara L. Bruening, David M. Schultz, and John V. Cortinas Jr. 2006, p. 266). Snow metamorphism begins within a short time after snow reaches the ground, even within the standard 6 hour time between successive snow measurements. Extended periods of time between snow snow measurements can result in lower recorded amounts.
16. A Question to PonderOn January 25 to 27, 2004, a snowstorm dumped 27.1 inches of snow and 1.06 inches of liquid at the National Weather Service in Duluth, MN. A storm on December 14th to 15th, 2005, produced 15.8 inches of snow and 1.30 inches of liquid at the same location. The question: Which storm produced more snow?
Formal ReferencesBaxter, M.A., C.E. Graves, and J.T. Moore, 2005: A Climatology of Snow to Liquid Ratio for the Contiguous United States. Wea. Forecasting, 20, 729-744.
Bruening, S.L., J.V. Cortinas, P.J. Roebber, and D.M. Schultz, 2003: Improving Snowfall Forecasting by Diagnosing Snow Density. Wea. Forecasting, 18, 264-287.
Doesken, N. J., Judson, A., 1997: The Snow Booklet: A Guide to the Sciences, Climatology, and Measurement of Snow in the United States. 2d ed. Colorado State University Department of Atmospheric Science, 86 pp.
Thut, A.S., 2006: Snow Density and its Underlying Variables. University of Wisconsin Madison, Department of Atmospheric and Oceanic Science, AOS 401, Prof. G. Tripoli.
Ware, E.C., D.M. Schultz, H.E. Brooks, P.J. Roebber, and S.L. Bruening, 2006: Notes and Correspondence. Improving Snowfall Forecasting by Accounting for the Climatological Variability of Snow Density. Wea. Forecasting, 21, 94-103.
Other ReferencesSnow Measurement Guidelines for National Weather Service Surface Observing Programs
Federal Meteorological Handbook No. 1 - Surface Weather Observations and Reports.
[ Specifically reference Chapter 12, sections 12.7.1.y and 12.7.2a(3)(b) ]
"The Ice Crystal Process" section of "Mesoscale Aspects of Winter Weather Forecasting: Topics in Winter Wx Forecasting" by COMET's Meteorological Education and Training (MetEd) website. You need to create a password to use the education materials.