FGUS66 KRSA 011748 ESGNV SPRING FLOOD POTENTIAL OUTLOOK NATIONAL WEATHER SERVICE CALIFORNIA-NEVADA RIVER FORECAST CENTER SACRAMENTO CA 1000 AM PST FRI MAR 1 2024 …Above normal threat of snowmelt flooding along the Humboldt River and tributaries. Low threat of flooding from rivers that drain from the eastern Sierra… There is an above normal threat of spring snowmelt flooding along the Humboldt River and its tributaries due to an above average snowpack throughout northern Nevada. There is a low threat of spring snowmelt flooding for areas along the Carson and Walker rivers where snowpack conditions are currently below normal. Flooding on rivers can occur from heavy rainfall at any time, or combined with snowmelt during the spring. At the time this product was issued, a major snow storm was underway in the Sierra. Observations and forecast runoff predictions reported herein may change significantly with this storm or as a result of subsequent storms. Please visit the CNRFC webpage (http://www.cnrfc.noaa.gov) for the latest information. Current basin conditions Snowpack After a dry start to the water year in the Sierra, numerous storm systems have moved across the region the past two months, greatly improving the snowpack conditions throughout California and Nevada. The majority of these storms have been on the colder side, which has helped build a solid snowpack in the upper elevations. At lower elevations, some areas in northern Nevada have near normal snowpacks, while other areas south of I-80 have little snow below 7,000ft. Low elevation snow in the Sierra has been minimal up to this point. The snowpack in the Humboldt watershed is currently above normal, with many snow pillows showing between 100-120% of average. Low elevation snow exists below the observational network in northern Nevada, though the snow is not all that deep. Soil moisture is above normal throughout the Humboldt, along with above average baseflow for this time of year. Accumulated volume on the lower Humboldt at Imlay is nearly double the to-date median. Snow pillows in the east Sierra are currently below to much below average, with most indicating 60-80% of average. With a major snow storm underway in the Sierra, the snowpack will make significant gains the next few days. Precipitation Precipitation totals across northern Nevada have been near normal in most areas, with the Ruby Mountains being above normal. Precipitation for the Carson and Walker basins has been much below normal thus far this water year. Conditions will improve with the ongoing storm system as it dumps heavy amounts of snow across the area. Precipitation outlooks from the Climate Prediction Center currently show an increased chance of above normal precipitation across California and Nevada during the 1 to 2 week timeframe. Seasonal (April – July) runoff forecasts The seasonal April-July runoff forecasts are above average throughout the Humboldt watershed. Forecasts range from near 100% to 140% of average along the mainstem of the Humboldt River. In the Carson and Walker basins, seasonal forecasts are near normal at around 75-105% of average. These seasonal forecasts may change significantly over the next few days as a result of the ongoing storm and continued potential for additional storms. Flood exceedance probabilities The risk of flooding during the spring is elevated in the Humboldt basin due to the above normal snowpack. With a shallow, low elevation snowpack in place across the northern Humboldt watershed, the flooding potential is further elevated due to the potential for a late season rain-on-snow event or an early heat wave. Models currently show a near 50% chance of minor flooding on the Marys River and along the Humboldt River at Elko and Comus. There is a slight risk of flooding on the Carson and Walker rivers, as the snowpack in these watersheds is currently below normal. Consult the AHPS long-range flood risk map for the latest point exceedance probabilities for the March through May time period: https://water.weather.gov/ahps/region_long_range.php?rfc=cnrfc Summary The risk of flooding during the spring is elevated along the Humboldt River and its tributaries due to the above normal snowpack, low elevation snow in the north, and above average baseflow and soil moisture. The flood threat is small along the Carson and Walker rivers, where the current snowpack is below average. Flooding can occur from snowmelt, rain, or a combination of both. Refer to the long range outlook product for updates to flood exceedance probabilities at specific locations throughout the spring. For short term hydrologic forecasts and general water resource information, please see the CNRFC webpage at: http://www.cnrfc.noaa.gov CNRFC/MI