This flood outlook is for the National Weather Service (NWS) Omaha/Valley Hydrologic Services Area. This area includes eastern Nebraska and portions of southwest Iowa. Questions regarding this outlook can be directed here. Spring Flood Outlook Highlights * The risk of flooding for the remainder of winter and into mid-spring is below normal. Locally heavy spring rains will cause flooding in localized areas as is the case every year. This outlook is directed towards flooding on a larger scale. * Current conditions are not conducive to major, large-scale flooding. * The threat for localized ice jam flooding remains above normal. * Graphics for select river gauges are available on our website at: water.weather.gov/ahps2/long_range.php?wfo=oax. * A video to help you better understand the probabilistic graphics is available here: youtube/watch?v=pSoEgvsnpv4 Overview: During the last two weeks there haven’t been any significant changes to the flood threat. The snowpack in the headwaters of the Missouri River increased, while the Platte River headwater snowpack remained about the same. Frost depths remain deep, if heavy rain were to fall in the next few weeks, this would likely lead to a significant flooding concern. Although the ice jam threat remains, it was alleviated when a significant amount of ice flushed out of the Platte and Loup Rivers. Mountain snowpack: For the headwaters of the Missouri River snow water equivalent values are 136% of normal through March 6, this is an increase of 10% in the last two weeks. For the headwaters of the Platte River snow water equivalent values are 128% of normal, this is relatively unchanged in the past two weeks. Plains snowpack: Across the Dakotas snow depths are not a concern. Over the past two weeks little if any water equivalent was added to the snowpack, if anything the water content was reduced a little. In South Dakota, west of the Missouri River, there is little, if any, snow away from the Black Hills. Toward the headwaters of the James and Big Sioux Rivers, snow water equivalent values are around one inch, and up to 2 inches in isolated areas. The majority of the Plains snowpack has less than one inch of water equivalent. Into North Dakota similar snow water equivalent values are being reported. Ice jam threat: This year the threat for localized ice jam flooding remains above normal. A significant amount of ice flushed through the Loup/Platte River system during the weekend of February 22nd. Some ice remains in the river, this combined with recent cold temperatures has allowed the ice jam threat to remain. Warm temperatures next week should end the ice jam threat. Frost depths: Due to the below normal temperatures observed this winter, frost depths are deep and are relatively unchanged in the past two weeks. In Valley, Nebraska the frost depth is 23 inches. While this is the only frost depth reporting site, based on regional frost depth values, it seems reasonable most areas of eastern Nebraska and southwest Iowa have similar frost depths, if not deeper. Frost depth can be a major contributor to flooding, specifically when rain or snowmelt occurs on frozen ground. The frost inhibits infiltration of the rain/snowmelt and flooding is more easily achieved. Ideally, we want the ground to sufficiently warmed-up prior to the spring rains. Given how deep the frost is this year, this will take much longer than years past, thus increasing the flood threat if heavy rains were to occur. Current river conditions: See sub-sections below. Where available stream flows have been compared to the long-term mean. Climate outlook for the next two weeks (March 12 through March 18): The Climate Prediction Center predicts a 33 to 40 percent chance for below normal temperatures with a 33 percent chance of precipitation being below normal. Climate outlook for March through May: The Climate Prediction Center predicts a 33 to 40 percent chance for below normal temperatures and equal chance for below, near and above normal precipitation through May.
Posted on: Fri, 07 Mar 2014 00:11:44 +0000
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