Weather and climate forecasters use words and information in very particular ways that may be different from what we are accustomed to. Here is a list of terms we use in this newsletter:

Weather and Climate — The difference between weather and climate is timescale. Weather is the day-to-day interaction of factors like temperature, humidity, precipitation, cloudiness, visibility, and wind. To understand climate at a given place requires looking at weather trends over relatively long periods of time—months, years, and decades. In addition to studying weather, scientists examine climate trends or cycles of variability to understand the bigger picture of long-term changes.

Temperature and Precipitation — Throughout this newsletter, we report past temperature and precipitation data derived directly from the GridMET daily 4-km-gridded meteorological dataset from the University of Idaho. Temperature data are reported as seasonal averages; precipitation data are reported as seasonal total precipitation. Our three-month temperature and precipitation forecasts come from NOAA’s Climate Prediction Center.

Normal(s) — Climatologists use the term “normal” to compare current conditions or forecasts, such as temperature or precipitation, to the past. Here, the normal value is the statistical mean (the average) for a given measurement in a specific place during a specific period of time. Climatologists use the most recent 30-year period, rounded to the nearest decade, to define normal in North America: 1981–2010. The goal is to look far enough back in time to capture variation in weather patterns, but not so far as to be irrelevant to recent conditions. In 2021, we will start using the 1991–2020 period.

La Niña/El Niño — El Niño and La Niña are the warm and cool phases of a recurring climate pattern across the tropical Pacific, the El Niño Southern Oscillation (ENSO). When ENSO is between warm and cool phases, conditions are called ENSO Neutral. ENSO is one of several global climate phenomena that affect Montana’s weather patterns, and ENSO conditions often guide seasonal climate projections for Montana. Current ENSO conditions and up-to-date projections are available on NOAA’s ENSO website.

Snow Water Equivalent (SWE) — SWE is the amount of water contained within the snowpack. It can be thought of as the depth of water that would theoretically result if you melted the entire snowpack. SWE is measured across the West by the USDA Natural Resources Conservation Service’s SNOTEL network of snow monitoring stations. The SWE percent of normal represents the current snow water equivalent found at selected SNOTEL sites in the basin compared to the normal value for those sites from 1981–2010.

Root Zone Soil Wetness — Root Zone Soil Wetness is a measure of how much water has saturated the soil. More specifically, it’s the relative saturation between completely dry (indicated by a 0) and completely saturated (indicated by a 1) between 0 and 100 cm depth. In the maps in this newsletter, soil saturation comes from NASA’s Soil Moisture Active Passive (SMAP) satellite program “SPL4SMGP” data product. Soil moisture is mapped using a combination of radar and radiometer measurements from space and surface observations at an approximately 9-km spatial resolution.