Precipitation Type
Precipitation type refers to the form that precipitation takes when it reaches the ground—or in some cases, when it evaporates before making contact. While many people are familiar with basic forms like rain or snow, each type has different implications for flash flooding, field conditions, and interpreting weather forecasts. For canyoneers operating in terrain highly sensitive to water input, understanding these distinctions is not just useful—it’s essential.
Types of Precipitation[edit]
- Rain: Rain is the most familiar and immediate concern. It forms when air temperatures stay above freezing from the cloud base to the surface. Because rain runs off quickly into drainages, it can produce flash floods with very little warning. In canyon environments, even light or moderate rainfall upstream can create serious hazards downstream.
- Freezing Rain: Freezing rain occurs when supercooled droplets fall through a shallow sub-freezing layer near the surface and freeze on contact. Though uncommon in desert canyons, it can make rock surfaces, anchors, and gear dangerously slick. It doesn’t typically contribute to flash flooding in the same way liquid rain does, but it can severely affect mobility and safety.
- Snow: Snow forms when the entire air column is below freezing, allowing ice crystals to fall all the way to the ground. While snow doesn't typically cause immediate runoff, it is a significant and regular factor in the high desert regions of the Colorado Plateau and other canyoneering areas. Snowfall can accumulate in higher elevations or north-facing terrain, particularly during winter and early spring. Although it may melt relatively quickly depending on conditions, the volume of water stored in accumulated snow can later become a major contributor to runoff. Its presence must be factored into both route planning and flash flood risk assessment, especially in transition seasons where temperatures fluctuate rapidly. But that accumulation becomes important later, especially during spring melt events or when rapid warming occurs.
- Sleet: Sleet refers to small, frozen pellets that form when raindrops pass through a deep freezing layer before reaching the ground. Like snow, it accumulates and can lead to flooding only after melting. In canyon terrain, sleet mainly affects travel conditions by making surfaces slippery.
- Hail: Hail forms in strong thunderstorm updrafts, where raindrops are carried into very cold layers of the atmosphere and freeze. The result is solid balls or lumps of ice. Large hail can cause injuries and damage, but more importantly, hail-producing storms often bring heavy rainfall capable of triggering flash floods.
- Graupel: Graupel, sometimes called "soft hail," occurs when snowflakes are coated in supercooled water droplets, forming small, soft pellets. Graupel indicates atmospheric instability and often precedes convective storm activity. While it doesn’t add much direct runoff, it signals that more intense precipitation may follow.
- Virga: Virga is a unique desert phenomenon in which rain appears to fall from clouds but evaporates before reaching the ground. It can often be seen as wispy streaks trailing downward from cloud bases, fading into nothing. Virga doesn’t contribute to runoff or flooding, but it’s worth paying attention to: it’s often associated with dry, unstable air and can signal conditions favorable for sudden downdrafts or the eventual arrival of moisture-bearing air masses.
SWE and Delayed Runoff[edit]
In areas that receive snow, especially higher elevations feeding into canyons, the Snow Water Equivalent (SWE) is a more useful metric than snow depth alone. SWE tells you how much liquid water is stored in the snowpack. A shallow layer of dense, wet snow can contain more water than a thick layer of powder. Forecasts and hydrologic models use SWE to estimate how much water will be released when snow melts—either gradually or all at once. For canyoneers, this matters greatly during spring or during warm spells following storms.
The type of precipitation is just as important as the amount when assessing flash flood potential. Liquid forms like rain and freezing rain produce immediate runoff. Solid forms like snow, sleet, or graupel delay runoff but can store large amounts of water for later. When rapid warming follows snowfall—especially if accompanied by rain-on-snow—the result can be sudden and severe runoff. Freeze–thaw cycles complicate matters further by creating unpredictable melt rates and unstable surface conditions.
Understanding Forecasts and Mixed Precipitation[edit]
Forecasts may specify not just the chance of precipitation, but the type—such as rain, snow, sleet, or hail. More detailed forecasts, such as those from the National Weather Service (NWS), provide hourly or graphical breakdowns of expected precipitation type and amount. These are often separated by category—rain, snow, sleet, freezing rain—and may include different accumulation predictions for each. It’s not uncommon for both rain and snow to appear in the same forecast window.
This occurs because weather forecasts are not tied to a single elevation or pinpoint location—they describe conditions across a forecast area, which can include a variety of microclimates. Rain may fall in the lower sections of a canyon while snow accumulates on a higher pass. Even within a single drainage, shifting temperatures and layered atmospheric profiles can produce mixed precipitation at the same time. These nuances are especially common during transitional seasons or frontal events.
Many NWS point forecast pages now offer interactive elements such as hourly forecast tables and visual graphs. These tools often show separate values for different precipitation types, and some even include predicted snow water equivalent or ice accumulation values. This can help canyoneers assess not just how much precipitation is expected, but what form it will take—and when.
Precipitation Type and Flash Flood Risk[edit]
In addition to the likelihood and amount of precipitation, the type of precipitation also affects flash flood risk. Liquid forms—such as rain, freezing rain, and hail—present the most immediate threat, as they generate runoff quickly and directly contribute to rising flows in drainages. Even modest rainfall upstream can accumulate in slot canyons and create dangerous downstream conditions with little notice.
Snow behaves differently. While it rarely causes immediate flooding, it introduces delayed and sometimes prolonged risks depending on how and when it melts. There are two primary ways snow contributes to flash flooding:
- A fresh layer of snow on higher terrain can melt rapidly if temperatures rise suddenly. This is particularly dangerous when followed by warm rain or a sharp warm front, which accelerates the rate of melting. In these situations, a seemingly stable snow layer can release large volumes of runoff in a matter of hours.
- In spring, deeper and more persistent snowpack at higher elevations melts gradually as temperatures warm. While this often results in steady flows, freeze–thaw cycles can disrupt that stability. Warm days followed by freezing nights create meltwater surges that are irregular, and this instability can increase the risk of flash floods even without rainfall.
A key factor in evaluating these risks is the Snow Water Equivalent (SWE)—the amount of liquid water stored within the snow. A few inches of wet snow can release more runoff than a foot of dry powder. Forecasts that include SWE, or tools that provide gridded SWE estimates, can be extremely useful for anticipating delayed flooding due to snowmelt.