Name: Big snow melt hasn’t changed flooding outlook for the spring
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Big snow melt hasn’t changed flooding outlook for the spring

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AdvertisementBy Chicago TribuneFeb 26, 2021 at 11:38 amThe big melt is on.The left 34.1 inches of snow at O’Hare International Airport, the city’s official recording site, but high temperatures in the 40s have made a significant dent in the snowpack. Following 25 days of 10 or more inches of snow on the ground at O’Hare — the 4th largest stretch in Chicago’s recorded history — the snow depth dropped to 7 inches on Thursday.Tonight, a light rain/snow mix is expected though no accumulations beyond a dusting of snow are expected. This weekend looks splendid with plenty of sun and highs in the 50s, though locally cooler temperatures are expected where a deep snowpack remains. — NWS Chicago (@NWSChicago)The National Weather Service released an Thursday and these were the major takeaways, according to meteorologist Brett Borchardt:So what does the rapid snow melt mean for area waterways? The Tribune talked to experts from the National Weather Service, Metropolitan Water Reclamation District of Greater Chicago, Chicago Botanic Garden and U.S. Geological Survey to find out.W. Scott Lincoln, National Weather Service Chicago senior service hydrologist/hydrology program leader: “The rate of snowmelt depends on many different factors, including temperature, dew point, cloud cover, time of year, wind speed. It’s difficult to give an estimate for how quickly the current snowpack can melt, but we would be watching for warmer temperatures, clear skies and windier conditions to get rid of the snow faster. Also, the later in the year we get, the higher the sun angle, so more solar energy to melt snow on clear days. Getting near freezing or a little above freezing will likely cause some slowmelt, but we’d generally be looking for temperatures in the 40s and 50s before melt picks up. A recent example of quick snowmelt was back in March of 2019, when we had a similar amount of snow in northern Illinois, then three days with temperatures in the 40s and 50s occurred, which melted most of it. Also keep in mind that the current snow cover is very cold, in some cases more than 10 (degrees) F below freezing, and the snow cover must warm to the freezing point before it can melt.”Brett Borchardt, National Weather Service Chicago meteorologist: “The snowpack we have has been steadily melting at a pace that is just right for our rivers. We’ve seen a rise in area river stages but thus far our rivers are handling the increase in water well. ... Although snow cover, soil moisture, and recent river levels can give us an indication of the relative risk of spring flooding, the risk can increase with any weather system that produces heavy rainfall which we cannot assess generally more than a week out.”Lincoln, National Weather Service: “There could be some small river rises and ponding in urban areas due to blocked storm sewer drains. Temperatures would need to be warmer than currently forecasted for rapid melt to occur. Even without much melt, the snow depth may continue to decrease slightly due to compaction.”Borchardt, National Weather Service: “A worst-case scenario would be a rapid melt of the snowpack plus heavy rainfall plus deep frost depth, which would in effect introduce 4-6 inches of runoff water into our rivers all at once. Thankfully we don’t see any signs of a rapid warmup or heavy rain event in our future ... but the longer our snowpack remains through the next few weeks, the more concerned we’ll grow for flooding.”Lincoln, National Weather Service: “A slow snowmelt reduces our chances for flooding due to snowmelt and/or ice jams. Sunny days with temperatures in the 30s. If we have a rapid warmup into the 50s with the snow cover still in place, we’d be more concerned.”James Duncker, U.S. Geological Survey hydrologist: “The ground is still frozen in most places. The top layer is starting to warm up, but the water really doesn’t soak in. It’s like a sponge that’s been frozen hard — it can’t take on any more water.”Allison Fore, Metropolitan Water Reclamation District of Greater Chicago: “In anticipation of excess water, it is our goal to provide maximum capacity in the (Tunnel and Reservoir Plan) system and waterways to provide as much protection from flooding as possible. We are watching for significant precipitation and major warmup in temperatures that would cause significant amounts of snowmelt. Fortunately, there is no immediate threat.“MWRD operators continuously monitor the weather forecasts and monitor real-time radar and rain gauges to analyze the severity and duration of storms. The MWRD pumps down tunnels and reservoirs to maximize the capacity of the system to handle snowmelt and potential storms. If a storm is significant, operators also lower the levels in the Chicago Area Waterway System to create additional capacity in the waterways to handle the additional flow.”Duncker, USGS: “We see these typically late February events can cause an increase in the hydrograph (a chart that displays the change in water level and/or flow over time), combine it with the National Weather Service’s snowfall and water equivalent data, and that helps to give us a better understanding as to how each little watershed responds.”Water content of snowThe map below shows the amount of water in the snow on the ground.Source:Fore, MWRD: “Our Tunnel and Reservoir Plan (TARP) captures and stores combined stormwater and sewage that would otherwise overflow from sewers into waterways in rainy weather. This stored water is pumped from TARP to water reclamation plants (WRPs) to be cleaned before being released to waterways. TARP protects a 375-square-mile area for Chicago and 51 suburbs that rely on combined sewer systems, and they are designed to collect both sanitary and stormwater.”Duncker, USGS: “As the snow melts, salt that was spread on roadways dissolves and moves with the runoff. We tend to see a spike in the chloride content of water quality samples from local streams.”Borchardt, National Weather Service: “A deep snowpack also means there’s literally inches of water frozen on the ground. That can spell trouble if it melts really fast. And, if we also have a deep frost depth, the water can’t penetrate into the ground, thereby becoming 100% runoff. So, the water content of the snowpack is actually an important variable when forecasting spring flooding.”Duncker, USGS: “This is an unusually high snowfall winter for us in Chicago. We’re looking right now at anywhere from 16-18 inches of snow on the ground across the city and suburbs. … The water equivalent ranges from just under 2 inches to just over 4 inches of water (measured as melted snow in gauges maintained by the National Weather Service). That’s the same as a 2- or 4-inch rainfall.”Lincoln, National Weather Service: “Snow water content information is measured by taking a snow core and then melting the snow to come up with a liquid depth equivalent. There are also planes with special equipment that estimate the water content of snow on the ground below. The (National Operational Hydrologic Remote Sensing Center) estimates the snow cover using a combination of information from a model and also from observations.”Borchardt, National Weather Service: “When there is a deep snowpack in place, the frost depth (or depth where the soil is 32 degrees or colder) struggles to grow downward since the snow acts like a nice ‘blanket,’ insulating the ground. Keeping the frost depth in check is important to prevent pipes well below ground from freezing. Thankfully, this is rarely an issue in our neck of the woods as we often have a snowpack in place when the air temperatures take the dive. Our frost depth right now is about 4 inches (as of Feb. 18). In parts of the country where a deep snowpack is less common, the frost depth can get as low as 60+ inches below ground, leading to big problems!”Lincoln, National Weather Service: “In some ways, the deep snow cover has helped us, because it has insulated the soil during our recent very cold stretch of weather, preventing the soil from freezing to a deep level.Tim Johnson, Chicago Botanic Garden senior director of horticulture: “The snow cover is good for gardens, as it insulates against intense cold and fluctuating winter temperatures. The snow cover helps mitigate the temperature extremes and reduces the penetration of cold on days and nights when temperatures are in the teens and below. Without snow, the ground can freeze more deeply.Lincoln, National Weather Service: “Frozen soil can prevent rain or snow melt from soaking in, increasing the amount of water headed for area rivers (and possibly causing flooding).”Johnson, Chicago Botanic Garden: “Plants that were produced in containers and installed last year can heave out of the ground because of the freeze/thaw cycles that typically occur in late winter and early spring. If newly installed plants were not mulched last fall, heaving is more likely. Think of the snow as mulch for your garden in addition to creating a beautiful winter scene.”Fore, MWRD: “The MWRD encourages the public to minimize their use of water before and during storm events to reduce the amount of water flowing into the sewer system. This will provide maximum capacity for combined stormwater flows. Check out our for more tips.” Advertisement

AdvertisementBy Chicago TribuneFeb 26, 2021 at 11:38 amThe big melt is on.The left 34.1 inches of snow at O’Hare International Airport, the city’s official recording site, but high temperatures in the 40s have made a significant dent in the snowpack. Following 25 days of 10 or more inches of snow on the ground at O’Hare — the 4th largest stretch in Chicago’s recorded history — the snow depth dropped to 7 inches on Thursday.Tonight, a light rain/snow mix is expected though no accumulations beyond a dusting of snow are expected. This weekend looks splendid with plenty of sun and highs in the...

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Jonathon Berlin

Kori Rumore

The Chicago Tribune is a daily newspaper based in Chicago, Illinois, United States, owned by Tribune Publishing. Founded in 1847, and formerly self-styled as the "World's Greatest Newspaper" (a slogan for which WGN radio and television are named), it remains the most-read daily newspaper of the Chicago metropolitan area and the Great Lakes region. It had the 6th highest circulation for American newspapers in 2017.

Big snow melt hasn’t changed flooding outlook for the spring

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