IN THIS MODULE
04.0 Weather & Slot Canyons
This is one of the easiest canyon-safety habits to understand and one of the hardest to argue with: check the forecast. Then check it again.
If rain is a meaningful concern for the canyon or its watershed, especially when you are a beginner, choose another day. The canyon will still be there.
A storm does not have to be directly above you to matter. Water can arrive from somewhere else in the drainage.
04.1 Start With Weather.gov
For canyoneering in the United States, I recommend starting with the National Weather Service at Weather.gov. Search the nearest location, then use the map to get the forecast point as close to the canyon or drainage as possible. A town forecast can be useful, but the canyon may sit at a very different elevation or miles away from that town.
04.2 Check It More Than Once
• About one week before the trip—to see the general pattern.
• Three days before.
• Two days before.
• The day before.
• The morning of the trip—before you commit.
For southern Utah, the National Weather Service Salt Lake City office also publishes a Southern Utah Flash Flood Potential Rating with locations that include Zion, the San Rafael Swell, Grand Staircase–Escalante, Capitol Reef, and other canyon areas.
04.3 What Does “30% Chance of Rain” Actually Mean?
One weather concept is worth getting exactly right. A 30% Probability of Precipitation does NOT mean that rain will cover exactly 30% of the forecast area. The National Weather Service defines it as a 30% probability that the forecast point will receive at least 0.01 inches of precipitation during the forecast period.
For a canyoneer, the bigger lesson is this: the percentage at your location is only one piece of the puzzle. A thunderstorm upstream or over another part of the watershed can still send water into a slot canyon even when it is not raining where you are standing.
04.4 Weather Is A Watershed Problem
• Look beyond the trailhead forecast.
• Consider the entire drainage feeding the canyon.
• Watch for storms upstream—not just clouds directly overhead.
• Keep evaluating conditions during the day instead of treating the morning forecast as the final answer.
04.5 Flash Flood Warning Signs
Flash floods can arrive with little or no usable warning. Do not wait for these signs before deciding to stay out. If a flood threatens while you are in the drainage, move immediately to reachable high ground away from the watercourse; do not try to outrun it downstream. Warning signs may include:
• Rapidly rising water.
• Heavy rainfall in the area or upstream.
• A sudden increase in water speed or volume.
• Debris—branches, leaves, foam, or muddy water—moving through the drainage.
• Flood watches, warnings, or severe-weather information for the watershed.
04.6 Remember the 5 S’s
When I think about flash-flood potential, I like using the 5 S’s as a simple framework. It forces you to look at more than just one thing on a weather app.
04.7 The 5 S’S
• SIZE — How large is the watershed feeding the canyon?
• SHAPE — Is the drainage long and narrow? Are there side drainages that can collect water far away from you?
• SURFACE — What is the rain falling on: sandstone, sand, clay, pine needles, a burn scar, or something else?
• SATURATION — Is the ground already wet? Has the area received rain recently?
• SAFETY — Where are the high points, exits, safe zones, and points of no return?
04.8 Temperature Is Weather Too
Weather planning is not only about flash floods. Desert heat, cold water, wind, snow, ice, and short winter daylight can change what gear you need and how fast your group moves.
• Winter temperatures can drop below freezing, adding snow or ice to approaches, exits, anchors, and canyon obstacles.
• Cold water can remain in shaded slots even when the air temperature above the canyon feels hot.
• Dehydration and exhaustion can affect judgment. If somebody starts acting confused or unusually poorly, take it seriously.
Notice how high this log is when this flash flood swept through the canyon. At this height, it is above 15 feet high!
04.9 Flowing Water Changes Everything
Moving water adds a completely different level of complexity. It can limit visibility, drown out communication, push people and gear, alter rappel lines, and create hazards that are not present in a dry canyon.
Do not rely on a fixed rule such as “wait 24 hours and everything will be fine.” There is no universal clock for when a canyon becomes safe after rain. Current conditions, the size of the storm, upstream runoff, the watershed, active watches or warnings, and the specific canyon all matter.
“Blue skies above you do not automatically mean the canyon is safe. Look at the whole watershed.”
04.10 *Case Study: Lethal Water — Kolob Canyon, 1993
On July 15, 1993, three adults and five teenagers entered Kolob Canyon. In a nutshell, there was unexpectedly high flow, powerful pools and water hydraulics in action, and two adult deaths. One fatality occurred during an attempted rescue; the other followed an attempt to recover a pack. Six survivors were reached on July 19.
Lesson Learned: Rappel height does not measure water danger. A short drop can lead into a deadly hydraulic. Assess current conditions before committing, and treat changed flow as a new decision. Entering the same hazard to rescue someone can create another victim. Equipment is never worth a life.
Think it through: What would make your team stop before the first committing rappel?
04.11 *Case Study: Keyhole Canyon Flash Flood — 2015
On September 14, 2015, seven canyoneers collected a permit for Keyhole Canyon in Zion National Park. The NPS timeline records a flash-flood warning and canyon closure at 2:22 p.m., followed by the group entering Keyhole later that afternoon. That timeline does not establish what warning information the group personally received.
Between about 4:30 and 5:30 p.m., Zion Canyon received 0.63 inches of rain in less than an hour. The North Fork of the Virgin River rose from about 55 cubic feet per second to 2,630 cfs in only 15 minutes. Keyhole Canyon flash flooded, and all seven members of the group were killed.
Lesson Learned: A forecast checked earlier in the day is not a permanent green light. Recheck conditions and official warnings before you commit, and treat a flash-flood warning or canyon closure as the end of the decision—not the beginning of a debate.
04.12 The Simple Beginner Standard
When you are starting out, stack the odds in your favor. Choose good weather. Choose a canyon that matches your group. Know the watershed. Check the forecast more than once. And if the weather gives you a reason to hesitate, use that hesitation.
“There will always be another canyon day.”
04.13 Additional Reading:
Source: International Canyon Accident Database — Kolob Canyon 1993-07-15; report submitted by Dave Nally.
Source: National Park Service — “Zion National Park Releases Names of Keyhole Canyon Fatalities” (Zion National Park, September 17, 2015).