The fact is, 2023 already is or likely could be the largest snowpack year of record. With record snowpacks comes the probability of exceptionally high streamflow. But will it cause flooding? First, one must define the term flood. In a scientific mode, a flood is determined by a probability rating such as a 1 in 100-year, 1 in 50 or even 1 in 10-year flood at a specific point on a river or stream. Given our current snowpack and in scientific vernacular, we are very likely to have a flood in the 50- to 100-year return period event or higher. A flood in common language use is more defined as damage from water such as flooded basements and infrastructure. This type of flooding can occur from sheet flow, from melting snow or rainfall, or streams and rivers overflowing their banks. This type of flooding can occur at virtually any location and any time depending on circumstances. River and stream flooding typically occur in fairly well-defined flood plains adjacent to waterways and that is specifically the focus on this narrative.

So, given a record high snowpack and recognizing that we are at the mercy of a completely indifferent Mother Nature, what would be the best weather scenario that would potentially minimize the peak flows of the season and why? It’s easy to quantify what the worst case scenario would be — have April and May continue cold and wet as in 1983. This postpones the snowmelt season and compresses it into a shorter time frame such that when it eventually does melt it produces melt from low, mid and high elevations at the same time as opposed to the normal melt period where each elevation and aspect zone melt sequentially.

So if cold and wet are the worst conditions, what would be the optimum? Warm and dry. Research done in the mid 1980s after the floods of 1983 showed that, given a high snowpack in any given year, above normal temperatures in April produced lower peak flows. Don’t be fooled — these flows were still high; the difference between high and very high is all we are talking about. Record high snowpacks will produce high streamflow. However, having a very warm April removes a great deal of snow early in the season from lower elevations and south aspects — in essence, it lengthens the snowmelt season instead of compressing it, which reduces the potential for higher peak flows by removing substantial amounts of snow early in the season while streamflows are low. Snowmelt follows a more sequential path than an abruptly compressed shortened one.

Another scenario that is often discussed is a sequential pattern of warm then cooling during peak flows. This indeed can suppress peak flows and did happen to a certain extent in 2011. However, the probability of this very specific weather pattern occurring in late May and June is fairly rare and if you look at the peak flow for the Bear River at Stateline, 2011 is the peak flow of record and it is No. 6 for the Weber at Oakley. Late May through June is where peak energy input to snowpack occurs and is far greater than anything that happens in April, when streamflow is fairly low and snowmelt is just beginning. It is far better to have 200% of normal streamflow in April than in May and June when peak flows normally occur.

So, what would happen if temperatures hit between 80 and 90 degrees for some days in April. This is the “we don’t want it to warm up too fast” scenario. The answer surprisingly is … not much. A little low elevation and south aspect snowpack would melt raising streamflow a little. Why? Because currently our snowpacks have a huge cold content. Any snow that melts right now simply goes to a lower level and refreezes. As an example, take a frozen chicken out of the freezer and put it in the microwave. Nuke that bird on high power for 1.5 minutes and take it out — it’s still frozen solid despite all that energy put into it. Why? Cold content. Nuke it another 1.5 minutes and yet another … finally it starts to (thaw) melt and actually cook at the edges but the center is still frozen. Same with our snowpack — it has to be isothermal at 32 degrees from top to bottom before substantial melt can occur.

Sensible heat input to the snowpack (what we normally call temperature) works only on the surface of the snow, the upper couple of inches. Snow is mostly not snow at all; it’s mostly air, and dead air is insulation. Snow has about the same insulative value as fiberglass insulation. You can’t pound temperature into the snowpack because of its insulation value. If temperature was the only thing melting snowpack, it would last much longer into the season. The fact is, short- and long-wave radiation provide 60% to 70% of the energy to snowmelt whereas temperature provides 20% to 25% of snowmelt energy. Short- and long-wave radiation penetrate many feet into the snowpack, thus driving energy deep into the pack instead of just on the surface like temperature does. Proof? Look out the window at the mountains and observe that south-facing aspects melt out clear to 8,000 and 9,000 feet before melt substantially begins on north-facing aspects. That is short- and long-wave radiation with the sun being nearly perpendicular to south aspects in April. In May and June/July, the sun is directly overhead radiating directly on all parts of the watershed, and this is where maximum energy input occurs.

Back to record high snowpack in April — there is this issue with mass. The larger the snowpack, the longer it takes to get to isothermal conditions. A good indication of this melt condition is snowpack density. Snowmelt in Utah begins when the snowpack reaches a density of 45% to 50%, and currently at lower elevations it is only 35% or so — not ready to melt. Cold and wet will keep it this way. One hundred degrees tomorrow won’t make it happen. Visualize an ice cube and a 10-pound block of ice on warm asphalt — which one melts first? The cube because it has much lower mass. It will take a long time for the block to melt and the same is true for large snowpacks, which is bad news because the longer it takes to get to isothermal condition at the higher elevations where the snow is 10-feet deep and start melting, the closer it puts us to maximum energy input in late May and June when it is ready to melt. Thus the imperative to remove lower-elevation and south aspect snowpacks, which are generally much smaller and easier to get to isothermal conditions — they are the ice cube not the block. April would be the best time to melt this snow.

So, in summary, the optimum weather scenario would be … we want it warm and dry right now to bring off some lower-elevation and south aspect snow, and if that doesn’t happen, a miracle warm-cold-warm-cold during the peak. Keep your eyes on April, hopefully warm and dry. Or … this year will test whether we have 1) been intelligent in where and how we build infrastructure and 2) how well we prepare for record high flows, because they are coming … either high or exceptionally high.

Randy Julander is a retired snow survey supervisor who ran the snow program in Utah, Nevada and parts of the Sierras for 27 years. He has published about 20 papers on snow and snow hydrology over the span on his career and has substantial experience in hydrologic modeling of snowmelt and streamflow in Utah. He has also taught snow hydrology at the University of Utah in the Engineering Department.

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