Flash flooding is one of the most dangerous hazards for hiking and canyoneering in the Southwest U.S., for areas such as Utah, Arizona, New Mexico, and Nevada.
Popular national parks and recreation areas including Zion, Grand Canyon, Capitol Reef, Grand Staircase-Escalante, and Canyonlands are all susceptible to dangerous flash flooding, and numerous fatalities have occurred in these areas due to flash flooding in the past.
Photo: Brian Moe, NPS.gov
The steep and rocky terrain common across Canyon Country is unable to absorb heavy rates of water accumulation in a short period of time, and runoff from heavy rainfall can occur very quickly as a result. The numerous dry washes and canyons act as terrain funnels for water and debris flow during heavy rain events, and flash flooding in these areas can occur in a matter of minutes.
Learn More → Flash Floods and Outdoor Recreation: The Basics
While flash floods have an inherent degree of unpredictability due to the localized nature of heavy rain events and the localized, rapid onset of flash flooding that can occur, fortunately, there are many ways for outdoor adventurers to obtain good forecasts and flash flood information to go in prepared. Understanding the risk ahead of time is the most important aspect, but having a plan of action once you're out in the field is vital as well.
Seasonality of Flash Flooding
Flash floods can occur at any time of the year in Canyon Country. Flash floods are most common and severe during the summer Monsoon Season, but they can occur during the fall, winter, and spring as well when strong frontal systems produce heavy rain.
Monsoon Season Thunderstorms (Late June to September)
The North American Monsoon is a seasonal weather pattern that results in frequent afternoon thunderstorms across the Southwest U.S. Moisture is subtropical in nature at this time of year, and as a result, thunderstorms often produce heavy rainfall rates within a short window of time. This is without a doubt the peak season of flash flooding.
The monsoon typically gets going in July, but in some years can start as early as June. The peak of the monsoon, when thunderstorms are strongest and most frequent, is in July and August. Monsoon season continues through September as well, though storms typically decrease in frequency and intensity by then.
Spring and Fall Thunderstorms (April to June, October)
Spring is typically the driest season in the Southwest, but frontal weather systems occasionally bring showers and thunderstorms to the region. Stronger systems involving high amounts of moisture and instability can result in heavy rain producing thunderstorms that can lead to flash flooding.
Also, snowmelt from higher elevations can add stress to rivers and canyons in April and May, and rain from showers and thunderstorms can combine with snowmelt to produce flash flooding.
In general, flash flooding events are infrequent at this time of year, especially compared to monsoon season, but they do occur on occasion.
Winter Storms with Prolonged Heavy Rain (November to March)
During typical winter storm systems, rainfall is lighter and more steady in nature as the atmosphere is more stable at this time of year. However, on occasion strong winter storms containing subtropical moisture will impact the Southwest, and these type of storms can result in prolonged periods of moderate to heavy rainfall that can produce flash flooding.
Rain and flood potential in winter systems tends to be more predictable since frontal systems produce rain over a larger area. These events are most dangerous when prior heavy rain has already resulted in rising streams, and in instances when there is snowmelt.
Preparing for Flash Flood Potential Ahead of Time
When planning a trip in canyon country, it's always advisable to obtain weather forecasts and information from multiple outlets to get an idea of whether or not flash flooding is a threat, and if so, how much of a threat.
Also, studying the terrain you will be traveling through is important. Will you be hiking through dry washes and narrow canyons, and will the road to the trailhead be dirt or pavement, and will it take you through canyons or stream beds?
1) OpenSummit 5-Day Forecasts
Knowing whether or not rain is in the forecast is a good step, which you can determine by checking our OpenSummit 5-day forecasts. Remember, that even a low percentage chance of rain could result in flash flooding depending on the type of terrain (i.e. canyons/deserts) and the amount of moisture in the atmosphere.
2) NWS Excessive Rainfall Outlook
Also, another great resource is NOAA's Weather Prediction Center, which produces a 3-day Excessive Rainfall Outlook. The map does an excellent job of highlighting whether or not an area will have a flash flood threat
One caveat to keep in mind with this resource is that it usually "understates" the flash flood potential for less populated mountain and canyon regions versus reality. All it takes is a green "Marginal" threat in canyon country for you to be concerned about flash flood potential.
3) NWS Southern Utah Flash Flood Potential Rating
If you have plans in Southern Utah, then the Southern Utah Flash Flood Outlook produced by the National Weather Service in Salt Lake City should be your go-to source. This excellent graphic predicts flash flood potential for numerous popular canyoneering areas out to 2 or 3 days.
4) Flash Flood Watches and Warnings
Flash Flood Watches are issued for a larger geographic area when conditions are favorable for flash flooding to occur. Typically, watches are issued 6-48 hours in advance, so if you're lucky you may be alerted to the fact that your intended backcountry destination is under a Flash Flood Watch before you head out of cell service.
A Flash Flood Warning is issued for a more localized area when flash flooding is imminent or occurring. Warnings indicate immediate danger, and if you receive a warning for your current location then it's imperative to head to safer ground immediately.
Unfortunately, if you are in the backcountry when a Flash Flood Warning is issued, there is a good chance you will not receive the alert. It's a good idea to carry a NOAA Weather Radio with you, which will significantly increase your odds of picking up on a signal that could alert you to a warning.
5) Check in with Park Rangers at Visitor Centers
If you are in a park and have not obtained a recent weather forecast, or would just like more timely information to make sure you aren't missing something, then you can always check in with park rangers who will have the latest weather forecast.
Flash Flood Safety When You're Already Out Adventuring
Once you're outside, it's important to monitor terrain and weather conditions as you go, and be prepared to make a quick retreat in case the weather starts to turn threatening and/or water levels begin to rise.
The most important consideration is that flash flooding in canyon country happens very quickly, and you may only have a matter of minutes to retreat to higher ground once a flash flood beings. Remember, no one can outrun a flash flood.
1) Study the Terrain and Mentally Plan "Escape Routes" as You Hike
If a thunderstorm develops and/or a flash flood begins, you have to act very quickly. Therefore, it's possible to scout out escape routes for getting to higher terrain safely if conditions turn on you.
If you are traveling through deep slot canyons, then your margin for error decreases due to the amount of time it could take to exit and reach higher ground, so take this into consideration when planning and consider saving it for a sunny day with no threat of rain. Remember, you cannot outrun a flash flood.
2) Choose Your Campsites Wisely
If you are camping, then make sure you choose a location that is not vulnerable to flooding. Higher terrain well above stream beds is advisable, and plateaus or areas above canyons are even better. Avoid setting up camp in dry washes at all costs unless you are 100% certain of a dry forecast.
3) Plan Your Outings in the Morning During the Summer
During Monsoon Season, thunderstorms typically happen during the afternoon hours, so planning your outings in the mornings before storms fire up is a wise decision. Plus, it's cooler and more comfortable in the mornings anyways. Still, you'll need to be weather aware while you're out, as occasionally storms can develop in the morning hours during active monsoonal patterns.
4) Be on the Lookout for Thunderstorm Development
Flash floods during thunderstorm season are a response to localized storms that produce heavy rain, but flash flooding can occur well away from a storm in areas that are still sunny and have not received rainfall. Therefore, it's important to keep your eyes on the skies for vertically growing cumulonimbus clouds that are a sign of developing thunderstorms.
Also, if you hear thunder and/or see lightning, that means a storm is close enough to produce heavy rainfall and flash flooding, and therefore you should seek higher ground immediately.
5) Signs of a Flash Flood to Look Out For
Zion National Park recommends watching for these signs of imminent flash flooding (and this applies to all canyon country areas):
- Surge in water
- Change in water color
- Roaring water sound
- Increased debris in the water
If you encounter any of these signs, then head to higher ground immediately and do not attempt to cross the water. It only takes six inches of fast-moving water to knock an adult off of their feet!
These tips are also valid for driving to and from trailheads, or on any jeep roads. Being in your car does not add any extra degree of safety during a flash flood event, and can be a death trap when drivers attempt to cross a flooded stream while underestimating the power of the water.
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