Answered by Mike Huston, Meteorologist, National Weather Service
Research focusing on the strength of El Niño winters indicates that weak El Niño’s may produce considerably drier conditions across a broader area of the northwest states (top panel, Fig. 3 below). Strong El Niño winters on the other hand, appear to be much broader with the distribution of precipitation across California and to a lesser extent southern Idaho (lower panel, Fig. 3 below).
As a practical example, looking at yearly Snow Water Equivalent values for the Upper Snake River Basin above Jackson Lake (Fig. 4 below) we see that 4 out of the last 5 strong El Niño’s (red highlighted years) were drier than average. For now the most prudent action would be to approach the winter armed with the expectations suggestive of a strong El Niño. Updates will be provided on the second Thursday of each month and can be accessed at www.pmel.noaa.gov/tao/elnino/nino-home.html .
Snow Water Equivalent Values
Snow is mostly a combination of snowflakes and air. The amount of air that snow contains affects its volume (the amount of space it takes up). When snow melts, the trapped air is released.
Sometimes snow can have very little water (light fluffy snow) and other times it can have a high water content (heavy thick snow). It is important to scientists to measure the amount of water in snow (snow water equivalent) because that’s how we know how much water will be released in the springtime.
Make a guess about the amount of water you think a cup of snow will form when melted.
Materials: permanent marker, 10-oz clear plastic cup, measuring cup, 1 cup of snow (use shaved ice or crushed ice if snow is not available), spoon
- Put 1 cup of unpacked snow into the cup. Take care not to press the snow flakes together.
- Draw a line and mark the snow level with an S (for snow) on the cup.
- Set the cup on a table indoors and allow the snow to melt.
- When all of the snow has melted mark the water level on the cup SW (for snow water).
- Pour the water into your measuring cup and record the amount of snow water.
- Determine the ratio between the volume of the snow and the volume of the water. Do this by writing a fraction as shown below.
The “average” snow to snow water ratio is 10:1. This saying that if 10 inches of snow fell and it melted, it would equal 1 inch of snow water.