How much snow will accumulate?The information presented on this page is a combination of various professional references plus personal experience forecasting snow in states like Missouri, Kansas, Wisconsin, and Minnesota.
Forecasting snow accumulation is much 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). This observer has seen snow-to-liquid ratios ranging from "5 to 1" up to "50 to 1". Although rare, ratios can still be higher or a little lower. Many meteorological and microphysical processes influence how much snow ultimately accumulates given a certain amount of liquid water. 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 may appear to be relatively flat ground can affect snow measurements. In this case, even moderate winds of 10 to 20 mph will initially smooth out the differences, then on top of that, drifting can occur. If a large portion of land tends to dip or to rise, then even averaging multiple measurements may not be accurate. Also, winds 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 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 are taken at 0000, 0600, 1200, and 1800 Universal Coordinated Time (UTC), also known as Zulu Time (Z). Snowfall is often measured more frequently for purpose of supplying updated snowfall amounts for statements issued to the public but it is still the 6-hourly measurements that are used for the storm total. Some observers at designated observation locations, such as major 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 snow totals.
3. Mixed PrecipitationEven just a little mixing with freezing rain, sleet, or snow pellets will limit snow accumulations. Enough said.
4. Ice Crystal HabitCrystal growth habit affects snow amounts as different shapes and sizes of crystals result in higher or lower density of freshly fallen snow. Anticipating the crystal growth habits of snow can help a forecaster determine how much snow will accumulate. Usually ice crystals will experience several different growth habits and processes as they fall through a cloud. Crystal habit is determined by the temperature and by the water content of a cloud beyond saturation (supersaturation). Dendrites due to their feathery structure tend to produce the least dense and rapidly accumulating snow packs since they stick together with lots of space between the crystals. The more the dendritic crystal habit dominates, the “fluffier” the snowpack will be. Plates also accumulate in a looser fashion than other crystals types such as needles and columns. Dendrites and plates are dominant at -10 to -20 oC (14 to -4 oF) when supersaturation is high. Dendritic growth is focused more specifically at -16 to -12 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. Size increases with greater time spent in the cloud. Most important is how much time crystals spend in a region of the cloud where temperatures favorable for dendritic or plate-like growth, high supersaturation, and strong vertical motion all overlap.
6. Relation of 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 snow density by increasing the thickness of ice crystals. 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. 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 evaporation of ice crystals as water changes from solid to gas. The process causes the 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 when air is often very dry. If a meteorologist expects a region to remain on the fringes of a snowstorm and an inflow of dry air exists, the potentially small crystal size may need to be considered toward a more conservative snowfall forecast.
10. Melting Between Cloud Base and GroundObviously snow that melts completely cannot accumulate. Partially melted snow will accumulate in a very 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. More on Wind SpeedStrong winds produce a sifting effect as ice crystals tumble and break into smaller pieces as they collide with each other and various objects on the ground. The tumbling and sifting causes the ice crystals pack more tightly by filling in spaces between other crystals. Observers measuring in an open area exposed to the wind will measure a denser snowpack and likely report less accumulation than protected areas. If the winds are very strong as in a blizzard, then many snow reports will be estimated rather than measured.
12. Weight of the SnowSnow does have weight. The denser it is, the more it weighs per unit volume. Over time, snow at the bottom will be compressed at least a little by the weight of snow above it regardless of the above snow's density. Compaction is less if snow measuring surfaces are reset after shorter periods of time, such as 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 ground is saturated. Water has a high heat capacity so quite a bit of snow will have to fall and absorb the heat before the water on the ground cools to 32 degrees. Heavy snow can accumulate but the accumulation will be offset by melting from the bottom. Lighter snow has less of a chance to accumulate. Snow measurements taken on the ground will obviously be less than measurements taken on a snow measuring board if the air temperature is below freezing. Ground temperatures above freezing are particularly a problem if the ground is bare late in the season. Strong spring radiation from the sun will penetrate the clouds and heat the ground while the snow initially tries to accumulate.
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 more than a few degrees above freezing, the snow can continue to accumulate. Snow already on the ground serves as a cold surface on which the new 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.
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 Cooperative Observers. View
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 website. You will need to create a password to use the education materials.