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Andy Larsen: Here’s how to keep yourself informed on the Wasatch Front’s flood forecast

Try your hand at checking streamflows and short-term runoff forecasts in major Utah watersheds.

(Trent Nelson | The Salt Lake Tribune) High water levels in Wasatch Hollow, Salt Lake City on Wednesday, April 12, 2023.

It’s damp out there.

As you know, a heck of a lot of snow fell this winter, including more this week. And with all of that wetness — the highest amount of frozen water we’ve seen in our mountains in recorded history — comes some natural risk of flooding.

But flooding isn’t a sure thing. Whether or not all that water leaves the mountains quickly or gradually is the main thing, and is based on a whole bunch of factors: how quickly that snow melts, the soil moisture levels when it does melt, how full various reservoirs are, and so on.

So I wanted to give you the tools you need to stay informed about the situation from a hyper-local point of view. How much snow that will flow down to you still has to be melted? Is the creek nearest to you running high enough to be at risk of flooding? Is it likely to be? How crazy would things have to get for your home to be at risk? Thanks to various local, state, and federal bodies, we have a bevy of scientific data and forecasts that can help you sort this out. Let’s dig in.

Step 1: Understand the land

Obviously, which data points are going to be interesting to you is going to depend on where you live. Luckily, we have maps for that.

First, I’d recommend checking out the Federal Emergency Management Agency (FEMA) flood map. Here, you can see how at risk FEMA believes each location is to flooding. Some spots aren’t very susceptible at all, others are at risk during big floods (the 1% or 1-in-100 years flood), others are at risk only during exceptionally rare flooding (0.2% or 1-in-500 years flood).

Of course, that 1-in-100 is a probability, not an exact science. Unfortunately, if something flooded during 1983, that doesn’t mean it’s ineligible for flooding in 2023 — probably the opposite.

You can see that most of the flood zones are either along one of the valley’s feeding creeks or in the lower-elevation areas of the valley, generally right in the center of that map.

Second, here’s the Jordan River Basin watershed map. Here, you can see all of the various creeks that end up flowing into the valley.

(Christopher Cherrington | The Salt Lake Tribune)

If the flood zone you’re worried about is along one of those creeks, you can probably narrow down your region of interest to the watershed above it. If, instead, your flooding is more in the lower-elevation areas, you’re going to want to be concerned about the bigger picture.

Checking the fundamentals

For many, just believing the forecasts below will be enough, and you have my permission to skip this section if you’d like. But if you want to see the fundamentals of flooding, you can through a number of different dashboards.

First, snowpack. Salt Lake County has said they’re going to release a YouTube video “approximately” every two weeks about current and forecasted snowpack levels; their last video was released April 4.

If you don’t want to wait, you can check snowpack levels yourself. United States Department of Agriculture’s National Resources Conservation Service’s National Water and Climate Center (whew, what a mouthful) has compiled a map of snowpack reporting stations here.

Map of snowpack measuring stations around parts of the Wasatch Front east and west of Salt Lake County. (USDA NRCS)

You can click on each to see how the snowpack is changing. Here’s one station at Parley’s Summit.

Snow Water Equivalent at Parley's Summit. (https://www.nrcs.usda.gov/Internet/WCIS/AWS_PLOTS/siteCharts/POR/WTEQ/UT/Parleys%20Summit.html)

Obviously, the more snow that gets melted, the less water there is remaining on the mountains to cause future flooding. At this point, the vast majority of water in our mountains still remains in snow form.

The above USDA map also allows you to select soil moisture monitoring stations, to check how wet soils are in various locations. Drier soils have more opportunity to soak up some of the moisture of melting snow and incoming rain.

Finally, reservoirs can be used to store some of the melting water; water engineers can sometimes direct streamflow to or away from those reservoirs to manage flows downstream. The USDA creates a monthly report of current reservoir water storage compared to capacity here; the National Weather Service’s Colorado River Basin Forecast Center has daily (and even hourly) reservoir height measurements at cbrfc.noaa.gov under the reservoir conditions tab.

Watching the streamflow

But if you want to get direct information on just how much water is flowing now, and how much is going to flow in the future, there are numerous sources of that more immediate information.

Salt Lake County has installed an impressive list of water flow sensors in its streams to measure just how fast they’re moving in real time in cubic feet of water per second. You can find them at the Realtime Streamflows link at rain-flow.slco.org.

An example screenshot of Salt Lake County's realtime streamflow sensor map. Current data can be found at https://rain-flow.slco.org/.

It’s an at-a-glance look at where streams are at this very second. Yellow stations are currently at “ordinary high water” conditions, meaning they’re carrying a lot of water but still operating normally. Red stations are operating at “estimated flood flow” conditions, which could mean flooding somewhere nearby.

Clicking on each station also allows you to see a graph of the data over the last few days, so you can see how things are trending. You can also see each creek’s estimated water depth.

Short-term forecasts of flows

What’s going to happen moving forward? The Colorado River Basin Forecast Center — at cbrfc.noaa.gov — has short-term, 10-day forecasts of the flow of each of the major watershed creeks. These monitors are a bit more sparse than Salt Lake County’s.

Map of Colorado River Basin Forecast Center river stations. (https://www.cbrfc.noaa.gov/lmap/lmap.php)

These CRBFC forecasts for each location take into account future precipitation and soil moisture. That’s pretty neat!

Take this example: here’s the graph for the streamflows for the station at Emigration Creek, just above Hogle Zoo.

As an example, here's Emigration Creek hydrograph showing past creek flows, along with a forecast based on upcoming weather. (https://www.cbrfc.noaa.gov/dbdata/station/peakgraph/rank/rankoverview.html?dashid=CCSU1)

Yep, it flooded on April 12 down Emigration Creek, largely in the area around Wasatch Hollow Park. But for the next week, we should be good with no flooding in that area, barring a surprise.

The cool thing is that you can do this with any of the creeks flowing into the valley.

Long-term flow forecasts

The CRBFC also gives longer-term forecasts at each river monitoring station — along with an estimated date of when those flows might peak. Because we’re less sure about what future weather will be like more than a week out, these forecasts have relatively wide ranges of surety. They’re not going to be able to put them on a graph like the 10-day forecasts above, but instead put them in more probabilistic format:

Here’s the CRBFC’s forecast for its Big Cottonwood Creek station, for example:

Colorado River Basin Forecast Center's forecast of the how high Big Cottonwood Creek's mean daily flow might get — and when we might see peak flows. Note that the two forecasts are independent of each other. (https://www.cbrfc.noaa.gov/dbdata/station/peakgraph/rank/rankoverview.html?dashid=BCTU1)

The CRBFC says that Big Cottonwood Creek’s flood watch level is at 798 cubic feet per second, and so it looks like we have a greater than 50% chance at hitting at least that mean daily flow level. But the peak, according to its latest forecast, isn’t likely to happen until late May or June. That being said, that could change by as much as a month in either direction, depending on the weather.

When peak runoff is expected really depends on the creek involved. The forecast indicates Emigration Creek’s flows have likely (but not certainly) already peaked, but the creeks with snow from higher elevations take longer to see their highest streamflows.

Regardless, these are really high flow numbers. Remember that really long-named governmental body from the second section? The aforementioned USDA NRCS NWCC creates monthly water supply forecasts for three month periods — on April 1, they released the forecast for the April-July period. And they forecast huge amounts of water coming through Salt Lake County’s creeks.

(Christopher Cherrington | The Salt Lake Tribune)

In four of those seven forecasts, the most likely outcome involves actually meeting or exceeding 1983 or 1984 water supply records. In the others, reaching 1983 heights is considered to be at least possible. This is pretty remarkable, but let’s remember to check in on May 1 to see what the new forecasts look like.

Hopefully, that list of tools can keep you informed on what’s going on. More hopefully, nature ends up working to move the water from high elevations to low elevations safely, and no flooding occurs where you are.

Andy Larsen is a data columnist for The Salt Lake Tribune. You can reach him at alarsen@sltrib.com.