All About Mixed Winter Precipitation

by Jan Jackson

Winter storm systems can cover large sections of the country with precipitation, and they usually produce a range of precipitation types from rain, to snow, to sleet, to freezing rain. These precipitation types depend upon the air temperature aloft, as well as the surface temperature. Below are two diagrams showing the typical temperature profiles in the atmosphere (blue line) that produce freezing rain and sleet (also called ice pellets). Note the deep warm layer in the freezing rain sounding that completely melts the falling snow. This liquid rain then falls on surfaces below freezing and turns into ice. A shallow warm layer as shown in the sleet sounding, will only partially melt the falling snow, which then re-freezes into ice balls when it falls into the cold layer at the surface.


Freezing Rain
Sleet
Typical sounding for freezing rain
Typical Sleet Sounding

 

 

Freezing rain, (image below), falls as liquid, and then forms a coating of ice on exposed surfaces. Often, the icing only occurs on elevated surfaces, such as trees and power lines. The greatest threat from this would be power outages. However, if the ground is cold enough, the roads may become icy, and this is a particularly dangerous situation.

Glaze on a tree along the Blue Ridge Parkway

Blue Ridge Parkway, Floyd County, VA. February 14, 2007 (by Brian Sutherland)

 

Sleet, (image below), falls as small balls of ice. Sleet is often mixed with snow or freezing rain, so can leave an icy, slushy covering on roads. Winter storms of all sleet are relatively rare, but do occur in our area.

Sleet along a wooden walkway.

Montgomery County, January 22, 2007 (by Brian Sutherland)

 

The atmosphere during a 12 to 24, or even 36 hour winter storm seldom stays the same in any one location. Ordinarily, the temperature profile will vary in both space and time. The Blacksburg County Warning Area typically sees some type of transition to the precipitation in winter storms. There will often be a mixture of precipitation types, with some combination of rain, snow, sleet and freezing rain. This complicates forecasts of snow and/or ice amounts in winter storms.

 

The method for forecasting snow and/or ice amounts is to first forecast the amount of liquid precipitation expected. Then, the forecaster uses various model data, climatology and local geographical influences to make the best estimate of the surface temperature and the temperature profile of the atmosphere during the winter storm. Remember, the temperature profile will likely be changing in space and time across a forecast area that includes piedmont sections of northwest North Carolina and southwest Virginia, westward across the Blue Ridge and into the Appalachians of northwest North Carolina, southwest Virginia, and southeast West Virginia. A snow to liquid ratio, (SLR), is used to convert liquid precipitation to snow. The snow to liquid ratio depends on temperature. The colder the temperature, the higher the SLR. The average SLR for the WFO Blacksburg County Warning Area is between 10 and 12 to 1. So, if there was an inch of liquid precipitation in a winter storm, if the precipitation was all snow, and the temperature was around 30 degrees, then the snow totals would likely be in the 10 to 12 inch range. However, if some rain, sleet, or freezing rain is mixed in, then snow amounts could be significantly lower.

Sleet is measured the same as snow- basically the depth on the ground. Sleet to liquid ratios are closer to a 3 to 1 ratio, so if even a small portion of the precipitation is sleet, it will lower the total depth on the ground by several inches. In addition, sleet falling on top of snow will also compact the snow and make the overall depth lower. Add in any liquid precipitation, (rain or freezing rain), and the total depth of snow/sleet/ice on the ground will be significantly less than the 10 to 12 inches in the example above.

In the past, we issued winter storm warnings based on two criteria: any combination of snow and sleet falling, or the thickness of ice (glaze) on exposed surfaces. (Images below).

 

12 hour winter storm warning for snow criteria

12 Hour Snow/Sleet Criteria for a Warning (Blacksburg CWA in red)

 

Ice Storm Warning Criteria

Thickness for ice (glaze) for a Warning is 1/4 inch or more across the region

 

With mixed precipitation, it would be possible to have a significant winter storm with an inch or more of liquid precipitation that does not meet the winter storm criteria above. However, the impact to the area could easily be severe, with a mixture of snow, sleet and freezing rain on the roads, and/or the combined weight of the snow/sleet/freezing rain mixture on trees and power lines enough to cause power outages. This winter, the National Weather Service in Blacksburg will be using a 2 inch sleet depth criteria for a warning. Pure sleet events are rare enough for this area that a 2 inch threshold will still not be met to trigger a warning in most storms. When we get sleet in a winter storm, it is most often mixed with other winter precipitation. Now, whenever we have a winter storm with mixed precipitation, we will apply a weighted average to each precipitation type, (snow, sleet and freezing rain) to calculate the impact to the area. By using a combination of mixed precipitation as another definition of a winter storm, warnings will be issued based more on the expected liquid precipitation amount from the storm when surface temperatures are expected to remain below freezing. With this new approach to issuing winter storm warnings, accurate and detailed reports from our weather spotters will be more important than ever. In mixed precipitation events, we will need to know the maximum depth of snow accumulation, the depth of any sleet that falls, and the thickness of any icing (glaze) on exposed surfaces. Below is a review of how to measure snow, sleet and ice.

 

Guide to Measuring New Snow/Sleet / Guide to Measuring Ice

New snow or sleet is the amount of snow or sleet that has fallen since your last observation or since the snow or sleet began.

Recommended equipment:

  1. -         Snowboard
  2. -         Ruler or yard stick

 

It is recommended that the snowboard be a flat piece of plywood as close to a square as possible and at least 144 square inches (1 square foot). It is probably best to have a 2 or 3-foot square (or nearly square) snowboard. A larger snowboard is easier to find after several inches of snow. A wooden deck away from buildings and trees is a suitable substitute.

 

The snowboard should be placed in a grassy area where snow accumulates uniformly. Do not place the snowboard under tree branches or wires or near buildings, homes, automobiles, roads, driveways or sidewalks. It you live in a heavily wooded area, try to locate your snowboard in an exposed clearing or in an area where there are less trees. Do not place the snowboard where snow from a neighbor's snow blower might land. The snowboard does not have to be on the ground all winter, just when snow is expected. (I don't want you to kill your grass by leaving the snowboard out all winter.)

 

When you make an observation, use a ruler or yard stick and measure the depth of the snow or sleet on the snowboard to the nearest half-inch at several places. The average depth is the new snow or sleet since the last observation reported to the nearest inch. When you are finished making measurements, wipe all the snow or sleet off the snowboard. Try to make new measurements whenever there is a precipitation change. It is especially important to get the maximum snow depth when there is a changeover to mixed precipitation.

 

It is fairly easy to measure new snow if the wind is light and there is no blowing and drifting snow. When there is a lot of wind causing blowing and drifting, there may be no snow on your snowboard. In this case, or if you feel the amount on the snowboard is not representative of how much snow fell, make a good guess on how much new snow fell.

 

Guide to Estimating Ice Accumulation

 

Recommended equipment: - Ruler

 

Ice accumulation occurs when rain falls with surface temperatures below freezing. This is commonly called freezing rain.

 

The best way to measure ice accumulation is to break off a very small branch/twig from a tree. Use a ruler to measure the thickness of ice in tenths of an inch. (See image below showing approximately 1/4 inch of ice accumulation.) There may be one side of the branch or twig with a greater thickness of ice than the other side. In this instance, please report the average thickness of ice buildup.

how to measure ice

A half inch of ice buildup will usually start to bring down tree limbs and power lines.

 

Guide to Measuring Total Depth of Old and New Snow and/or Sleet

Snow and/or sleet depth is simply the depth of snow and/or sleet on the ground, both old snow and/or sleet and new snow and/or sleet. Since snow tends to compact and may melt from underneath, your last total snow/sleet depth plus your new snowfall usually will be more than your most recent total snow/sleet depth. In addition, sleet or freezing rain falling on a fresh snow cover will cause a lot of compaction.

 

Recommended equipment: - Ruler or yardstick.

 

After making a new snowfall measurement, measure the total snow depth of both old and new snow. To measure snow depth, simply go around an area where there is an average snowfall and make 5 to 10 measurements. Make sure all measurements are at least 5 feet apart. The average of these measurements is the snow depth. Be sure to avoid making measurements in snowdrifts, in areas where most of the snow has been blown away, under trees, or near houses. If there has been any wind at all, you will likely find significant differences in depth.


Please take an observation and report it to the National Weather Service by phone 1-800-221-2856, or email at rnk.skywarn@noaa.gov, when any one of the following criteria are met:

1. New snow accumulation (if equal to or greater than an inch) since the last observation or since the snow began. Also include duration of time in which the new snow fell.

2. New sleet accumulation (if equal to or greater than a half inch) since the last observation or since the sleet began. Also include duration of time in which the new sleet fell.

3. A changeover to sleet after an inch or more of snow accumulation.

4. Any ice accumulation.

 

If you are calling us to report one of the above mentioned items, please include the following information as well:

1. Your location.

2. Depth of old and new snow and/or sleet.

3. Current weather (related to precipitation type and intensity, if any) including any blowing or drifting snow. This would also include current temperature, if you have a thermometer.

 

During an active winter weather event we will make sure that additional people are here to take your reports. In addition, you may phone in observations at other times, if you feel the information would be significant.