National Hydrologic Assessment
March 20, 2014
Figure 1: National Spring Flood Risk defined by risk of exceeding Minor, Moderate, and Major Flood Levels
An unusually cold and wet winter across the Upper Mississippi Basin, Great Lakes region, Ohio River Valley, northern Middle Atlantic, New York and New England has produced an above normal amount of water in the current snowpack and a deep layer of frozen ground much further south than typical. With significant frozen ground in these areas, the flood risk is highly dependent on the amount of future rainfall and the rate of snowmelt this spring. Recent snowmelt has increased the near surface soil moisture and elevated the potential for rapid runoff from rain events. In addition, significant river ice increases the risk of flooding related to ice jams and ice jam breakups.
Moderate flooding is expected in southern Wisconsin, southern Michigan and portions of Iowa, Illinois and Indiana, as a result of water in the current snowpack and the deep layer of frozen ground coupled with expected seasonal temperatures and rainfall. Specific rivers at risk include the Mississippi River between Davenport, Iowa and Burlington, Iowa, the Illinois River between Beardstown, Illinois and Henry, Illinois and many smaller rivers in the area. In addition, a potential for exceeding minor river flood levels exists across the upper Midwest and east into New England.
Small streams and rivers in the lower Missouri basin in Missouri and Eastern Kansas have already experienced minor flooding this year and the threat of moderate flooding will persist through spring. This flood potential will be driven by individual convective rain storms typical in the spring.
The Red River of the North between eastern North Dakota and northwest Minnesota, and the Souris River in North Dakota have a potential for moderate flooding. For the Souris River the threat is primarily downstream of Minot, North Dakota while the Red River threat is focused on the mainstem. River ice and significant frozen ground are factors in the flood risk for this area.
There is a heightened risk of moderate flooding for western South Dakota due to saturated soil conditions. There is limited snow cover in this area at this time, however additional rainfall would increase the flood risk.
A potential for exceeding minor flood levels exists for the northern Rockies and northern Great Plains in portions of Montana, and Wyoming. Wet soils and high river levels prior to the winter freeze combined with above average mountain snowpack are the drivers for this flood risk. Localized flooding due to ice jams has already occurred, and will remain a threat through the spring thaw.
Minor flooding also is possible for the lower Mississippi River basin and in the Southeast, including east Texas, southern Arkansas, Louisiana, Mississippi, Alabama, Georgia, northern Florida, South Carolina, North Carolina, eastern Tennessee, and southern Virginia. This flood potential will be driven by individual convective rain storms typical in the spring.
The flood potential from snowmelt and ice jams throughout Alaska this spring is currently rated as normal. This forecast is based on current ice thickness, observed snowpack, and long range weather forecasts. Typically Alaska snowmelt and ice jams occur in the late April to early June time frame. Bi-weekly updates to the flood potential from snowmelt and ice jams are provided by the Alaska Pacific River Forecast Center and can be obtained here.
Heavy rainfall at any time can lead to flooding, even in areas where overall risk is considered low. The latest information for your specific area, including official watches and warnings should be obtained at: http://water.weather.gov
Current water supply forecasts and outlooks in the western United States range from near normal in the Pacific Northwest, Northern Rockies, and Upper Colorado, to much below normal in the Southern Rockies, portions of the Great Basin and in California.
Analysis of flood risk and water supply integrates late summer and fall precipitation, frost depth, soil saturation levels, stream flow levels, snowpack, temperatures and rate of snowmelt. A network of 122 weather forecast offices and 13 river forecast centers nationwide assess this risk, summarized here at the national scale.
Heavy Rainfall and Flooding
The information presented in this report focuses on spring flood potential, using evaluation methods analyzed on the time scale of weeks to months, not days. Heavy rainfall at any time can lead to flooding, even in areas where overall risk is considered low. Rainfall intensity and location can only be accurately forecast days in the future, therefore flood risk can change rapidly.
Stay current with flood risk in your area with the latest official watches and warnings at weather.gov. For detailed hydrologic conditions and forecasts, go to water.weather.gov.
NOAA’s Experimental Long Range River Flood Risk Assessment
Figure 2: Greater than 50% chance of exceeding minor, moderate, and major river flood levels during March – April - May
At the request of national partners including FEMA and the US Army Corps of Engineers, NOAA has improved decision support services with a new “Experimental National Long Range River Flood Risk” web page available at: http://water.weather.gov/ahps/long_range.php. Here, stakeholders can access a single, nationally consistent map depicting the 3-month risk of minor, moderate, and major river flooding. This risk information is based on NOAA Ensemble Streamflow Prediction (ESP) forecasts which are generated for thousands of river and stream forecast locations across the nation. With this new capability, a stakeholder, such as a local emergency manager, can quickly view flood risk at the levels which are known to affect their specific area of concern. These enhancements improve the value of the National Hydrologic Assessment, by clearly and objectively communicating flood risk at the local level.
The sections below quantify river flood risk based on the river location having a 50% or more likelihood of exceeding minor, moderate or major flood levels. The National Weather Service (NWS), in coordination with local officials nationwide, defines flood levels for each of its river forecast locations, based on the impact over a given area. The flood categories are defined as follows:
- Minor Flooding - minimal or no property damage, but possibly some public threat (e.g., inundation of roads).
- Moderate Flooding - some inundation of structures and roads near stream. Some evacuations of people and/or transfer of property to higher elevations.
- Major Flooding - extensive inundation of structures and roads. Significant evacuations of people and/or transfer of property to higher elevations.
For example, on the Red River of the North at Fargo, ND, Moderate Flood Stage is 25 feet. At that height, city parks and recreation areas near the river are impacted. The impacts of all floods are local and, as such, this information is unique for each forecast location. To access local flood impact information, visit water.weather.gov and click on any river service location
Risk of Exceeding Major Flooding
While there are no widespread areas with risk of exceeding major flooding, a deep layer of frozen ground and significant river ice throughout the northern tier of the United States may cause localized major flooding. With significant frozen ground in these areas, the flood risk is highly dependent on the amount of future rainfall and the rate of snowmelt this spring. In addition, significant river ice increases the risk of flooding related to ice jams and ice jam breakups.
Risk of Exceeding Moderate Flooding
Upper Mississippi/Great Lakes Basins
An unusually cold and wet winter across the Upper Mississippi Basin and Great Lakes region, has produced an above normal amount of water in the current snowpack and a deep layer of frozen ground much further south than typical. With significant frozen ground in these areas, the flood risk is highly dependent on the amount of future rainfall and the rate of snowmelt this spring. In addition, significant river ice increases the risk of flooding related to ice jams and ice jam breakups. These conditions coupled with expected seasonal temperatures and rainfall have produced the potential for moderate flooding in southern Wisconsin, southern Michigan and portions of Iowa, Illinois and Indiana where a large snowpack remains. The large quantity of water in this remaining snowpack is highly unusual for this time of year. Specific rivers at risk include the Mississippi River between Davenport, Iowa and Burlington, Iowa, the Illinois River between Beardstown, Illinois and Henry, Illinois and many smaller rivers in the area.
Great Plains in the Central and Western Dakotas
The increased potential for moderate flooding this spring for western South Dakota is due to saturated soil conditions. However with snow cover limited to the Black Hills at this time, additional snow accumulation or precipitation would be needed to cause this flooding. Currently, moderate flooding is expected along the Cheyenne River near Plainview, South Dakota.
Forecast locations across the Great Plains, specifically along tributaries of the Middle Missouri River have an increased risk of exceeding minor flood stage. Minor flooding is likely along the Cannonball, Apple Creek and Little Muddy rivers in North Dakota, and along the Little Missouri and Big Sioux River in South Dakota. The threat for minor flooding is a combination of elevated soil moisture and increased stream flow levels that resulted from above average precipitation prior to the freeze up. The increased soil moisture conditions in the fall combined with much below average minimum winter temperature has resulted in deep frozen ground conditions across much of the region.
Lower Missouri Basin
Small streams and rivers in the lower Missouri basin in Missouri and Eastern Kansas have already experienced minor flooding this year and the threat of moderate flooding will persist through spring. This flood potential will be driven by individual convective rain storms typical in the spring. Forecast locations with the Lower Missouri river basins that may experience moderate flooding include points along the Grand River in Northern Missouri, and South Grand River and Big Creek upstream of Harry S Truman Reservoir.
Red River of the North
The Red River between eastern North Dakota and northwest Minnesota, and the Souris River in North Dakota have a potential for moderate flooding. While the water in the snowpack in this area is below normal, significant frozen ground and isolated areas of above average snowpack are factors in the flood risk for this area. For the Souris River the threat is primarily downstream of Minot, North Dakota while the Red River threat is focused on the mainstem. Information provided by NOAA today, March 20, 2014 indicate that at Fargo, North Dakota, there is an 80% chance of exceeding moderate flooding and a 45% chance of exceeding major flooding. Additionally, significant river ice increases the risk of flooding related to ice jams and ice jam breakups.
Risk of Exceeding Minor Flooding
The active winter storm season appears to be transitioning to warmer temperatures. The melt from above normal snowpack started late this year. The melting snowpack combined with thawing soils leaves saturated conditions. The persistent minor flooding in the Maumee, Wabash and White Rivers are expected to continue this spring. The chance for exceeding minor flood stages in other areas of the basin is normal.
Northern Middle Atlantic, New York and New England
The Northern Middle Atlantic region as well as all of New England experienced prolonged periods of significant cold and stormy weather this winter. Similar to the Ohio River basin to the west, a large swath of the region has an above average amount of water in the snowpack and increased soil moisture where snowmelt has already occurred. Forecast locations in Pennsylvania, New Jersey, New York, Vermont, New Hampshire and Maine are most likely to experience minor flooding, due to snowmelt and potential spring rains. Rivers and streams at risk include the Susquehanna, Delaware, Passaic and Raritan river basins; north into portions of the Hudson basin, the Connecticut, Merrimack; as well as the Kennebec, Penobscot and Saint John basins in interior Maine. In addition, significant river ice increases the risk of flooding related to ice jams and ice jam breakups. Historically, the spring flood risk across the Northeast is driven by the combination of snow water content and the potential for significant rain events as the spring progresses.
Northern Rocky Mountains in Montana and Wyoming
There is a greater than 50% chance of exceeding minor flood stage at a few river forecast points within the Missouri headwaters in the Northern Rockies. This is attributed to above average snowpack that spans the Continental Divide in these states. Many watersheds have snow water equivalent (SWE) in the 140 to 150 percent of the 30-yr normal. As of March 1, the 2014 snowpack is similar to that of 2011. It is also important to note 20% of the snow accumulation period remains. Additionally, a series of heavy precipitation events in late February and early March elevated streamflow in this region. Major and Moderate Flooding has already occurred along the Musselshell River in Montana, and minor flooding is possible in the headwaters of the Big Hole in Montana, as well as the Wind and North Platte rivers in Wyoming. Finally, there is an increased likelihood of flooding across this region should heavy spring precipitation develop coincident with the peak of snowmelt.
Southeastern United States: Eastern Texas to Southern Virginia
Minor flooding also is possible for the lower Mississippi River basin and in the Southeast, including east Texas, southern Arkansas, Louisiana, Mississippi, Alabama, Georgia, northern Florida, South Carolina, North Carolina, eastern Tennessee, and southern Virginia. This flood potential is driven by individual convective rain storms typical in the spring falling in basins where near to above average soil moisture conditions exist.
Other Regions/Low Flood Risk Areas
Weather patterns in this part of the country are markedly different west and east of the Cascade Mountain Range, which define the difference in flood threat.
West of the Cascades - Rivers west of the Cascade crest usually reach their highest peak flows during the winter. Due to orographically induced precipitation, the vast majority of river flooding in western Washington, and almost all major floods, occur between November and March. Spring snowmelt comes too late to add to this threat, this year is no different.
East of the Cascades - Rivers east of the Cascades reach their annual peak in late spring or early summer when the mountain snowpack melts and runs off. The snowpack usually reaches its annual maximum in April and rivers typically crest between mid-May and mid-July. As a general rule the larger the snowpack is at the end of the season, the higher the river crests will be.
Even with a lower spring flood risk, some smaller streams and flood prone rivers may experience minor flooding with a sudden large warm-up or the occurrence of heavy rain or thunderstorms over those watersheds. Flooding during the snowmelt season can occur anywhere when heavy rain falls in a river basin if the rain is intense enough.
Western Texas and the Southwest including California
There is very low chance of flooding over the southwestern United States, as drought continues to impact the region. The primary factor in development of significant river flooding over most of the region is the occurrence of excessive rainfall in relatively short periods of time, even for areas where drought conditions persist or have developed.
Please visit drought.gov for detailed outlooks, impacts and information related to your area.
Western U.S. - Regarding Spring Flood Prediction
Mid-March is too early to determine spring flooding potential across the Western United States. Snowpack remains below normal in many regions outside of the far northern U.S Rocky Mountains. However, there is still ample time left in the accumulation period for the spring flood potential to change. Weather conditions preceding and during the melt period determine the threat of flooding. Rapid warming can lead to elevated melt rates. During the melt, when rivers and streams are flowing at or near capacity, any precipitation can increase the risk of flooding. As always, citizens are encouraged to monitor the forecasts from their local Weather Forecast Offices.
Western Water Supply
Water supply forecasts are produced for mountainous basins in the western United States that supply water for agriculture, municipalities, and industrial uses. Forecasts reflect current hydrologic conditions including snow pack, soil moisture, and weather and climate outlooks. As these conditions change, especially over the next couple months, forecasts will be updated to reflect these changes. Water supply forecasts are generated by NOAA/NWS River Forecast Centers and the Natural Resources Conservation Service (NRCS) National Water and Climate Center.
Current water supply forecasts and outlooks in the western United States range from near normal in the Pacific Northwest, Northern Rockies, and Upper Colorado, to much below normal in the Southern Rockies, portions of the Great Basin and in California:
- Columbia and Snake Rivers - Median forecast at The Dalles is 100% of average
- Missouri River - Median forecast at Toston is 116% of average
- Colorado River - Median forecast inflow to Lake Powell is 116% of average
- Great Basin – Median forecasts range from 50% to 90% of average for most locations
- California - Median forecasts range from less than 20% to 50% of average
These wide ranging water supply forecasts reflect the stark contrast in weather patterns between the northern and southern portions of the region.
Upper elevation areas in the headwaters of Columbia River Basin, including portions of Washington and Oregon, east of the Cascades, received above average seasonal precipitation. Snowpack is above average over much of Washington, Idaho and northwest Montana. Median forecasts are above average to average in most basins, with below average runoff projected in the southern Snake River Basin tributaries.
Seasonal precipitation in the Upper Colorado Basin is above average. Snowpack in the headwaters range from above-average to average. Soil moisture in this area is above average. As a result, water supply forecasts at specific points range from above average to average and, as noted, inflow into Lake Powell is 116% of average.
Portions of the Lower Colorado Basin, including southern Utah and Arizona, generally received 50% of average seasonal precipitation. As a result, the snowpack in these basins is well below average. Similarly, soil moisture is average to below average. Streamflow forecasts range from 40% of long term median for the Gila River system to 46% of long term median for the Verde River system. Reservoir storage in the Salt and Verde River systems are 55 and 67% of capacity, respectively.
Across the Great Basin, seasonal precipitation has been below average. Snowpack across the basin is 70% of average. Water supply forecasts range from below average, to much below average over all watersheds in the Great Basin.
In California, precipitation has been significantly below average for much of the state for this water year. Most areas of the state have received 25-50% of average for the water year, while the Northern Sierra and portions of the North Coast mountains have received 50-70% of average. State-wide snow water content is 27% percent of the April 1st average. The April through July streamflow volume forecasts in California range from 15-25% of average in the Southern San Joaquin and Tulare basins to 25-50% in the Central and Northern Sierra as well as the region above Lake Shasta and much of the Klamath Basin. Storage for the major reservoirs in the northern part of California are generally at 50-75% of average for this time of year, or about 40-50% of capacity. Storage in the Southern San Joaquin and Tulare basin reservoirs are a little lower at 40-50% of average or about 20-30% of capacity. This year’s reduced snowpack in the mountains will continue to result in diminished reservoir storage throughout the summer and fall.
Water Resources East of the Rockies
Projections of surface water availability provided by the National Weather Service play a crucial role in water resource decision making in other regions of the country. To date, normal to below normal precipitation across much of Eastern New Mexico, Central and West Texas and Oklahoma continue to reinforce drought conditions. U.S. Drought Monitor indicates that moderate to severe drought persists across much of the region with isolated pockets of extreme drought. Water supply forecasts from the NWS and NRCS continue to indicate much below normal streamflow in the Arkansas and Rio Grande Basins. The US Seasonal Drought Outlook indicates that drought conditions will persist or intensify across New Mexico and West Texas, while conditions may improve in central Oklahoma.
Alaska Spring Ice Breakup Outlook
The flood potential from snowmelt and ice jams throughout Alaska this spring is currently rated as normal. This forecast is based on current ice thickness, observed snowpack, and long range weather forecasts.
March ice thickness data are available for a limited number of observing sites in Alaska. March 1st measurements indicate that ice thickness is variable across the state with locations generally between 75 and 120 percent of normal. Many locations in the Tanana basin reported slightly below normal thickness. Ice thicknesses on the Alaskan North Slope and in south central Alaska are near normal, while thicknesses in western Alaska are above normal. Accumulated freezing degree days are near normal over most of Alaska except in south central Alaska where they are below normal.
An analysis of the March 1st snowpack by the Natural Resources Conservation Service (NRCS) indicates normal and above normal snowpack north of the Alaska Range and in Copper River basin. Snow monitoring sites in the Canadian Yukon are reporting 100-130% of normal. South central and western Alaska are reporting below normal snowpack with less than 50% of normal in the Kuskokwim basin. Snowpack is near normal on the lower Yukon basin. For more details, please refer to the various snow graphics from APRFC or from the NRCS.
The weather outlook for the next two weeks is for an increased chance of above normal temperatures and precipitation over the majority of the state. The most important factor determining the severity of ice breakup remains the weather during April and May. The preliminary outlook for the next 90-days suggests an increased chance of above normal temperatures for western and northern Alaska.
The overall conditions have above normal snowpack in the eastern interior and Canadian Yukon with lower snowpack and ice thickness on the Kuskokwim and lower Yukon. South central Alaska is currently poised for a mild breakup due to relatively warm temperatures, and below normal snowpack. For more information on the climate forecasts please refer to the Climate Prediction Center.
Spring Flood Outlook and Implications for Gulf of Mexico and Chesapeake Bay Hypoxia
The predicted elevated flood risk in the Upper Midwest and Ohio valley may result in a larger hypoxic zone in the northern Gulf of Mexico than the average of recent years. Flood conditions may lead to higher than normal springtime discharges of nutrients and freshwater from the Mississippi River into the Gulf of Mexico, conditions that promote hypoxia formation and spread. This cause and effect relationship, however, can be confounded by weather events such as tropical storms and hurricanes, which can disrupt hypoxia formation and maintenance.
In the northern Gulf of Mexico each year a large area of low-oxygen forms in the bottom waters during the summer months, often times reaching in excess of 5,000 square miles (the average area since 1985 is 5,200 square miles). This area of low-oxygen, otherwise known as the “dead zone”, is strongly influenced by precipitation patterns in the Mississippi-Atchafalaya River Basin (MARB) which drains over 41% of the contiguous United States. Changes in precipitation will influence river discharges into the Gulf which carry the majority of nutrients helping to fuel the annual dead zone. The Upper Mississippi and Ohio Valleys supply the majority of the nutrients to the Gulf, so examining spring flood risk in these basins can provide a useful indicator of the possible size of the dead zone.
Another system with recurring summer hypoxia is the Chesapeake Bay. Hypoxia in the Chesapeake Bay has also been linked to nutrient loadings and river discharge, especially from the Susquehanna and Potomac Rivers. The spring flood outlook for portions of these basins indicates a slight risk for minor flooding but with the absence of any predicted moderate flooding we anticipate the size of hypoxia in the Chesapeake Bay will be average for 2014. This assumes typical summer conditions in the Bay region and the absence of major disruptive events such as tropical storms and hurricanes.
The spring flood outlook provides an important first look at some of the major drivers influencing summer hypoxia in the Gulf of Mexico and Chesapeake Bay. In early June, the actual river discharge rates and corresponding nutrient concentrations will be available from USGS. This information will be used by NOAA’s Ocean Service to release its annual dead zone forecast for the Gulf of Mexico and Chesapeake Bay which will provide an actual forecasted dead zone size based on the available data. In June and July, the dead zone sizes will be measured and compared against the predictions. Ongoing efforts are aimed at extending out the quantitative June forecast by one to three months and eventually out to a quantitative forecast based on the spring outlook data and predicted water discharge rates developed at key sites within each watershed.
NOAA’s Role in Flood Awareness and Public Safety
Floods kill an average of 89 people each year in the US. The majority of these cases could have been easily prevented by staying informed of flood threat, and following the direction of local emergency management officials.
To help people and communities prepare, NOAA offers the following flood safety tips:
- Determine whether your community is in a flood-risk area and continue monitoring local flood conditions at http://water.weather.gov.
- Learn what actions to take to stay safe before, during and after a flood at www.floodsafety.noaa.gov.
- Visit http://www.floodsmart.gov to learn about FEMA’s National Flood Insurance Program and for flood preparedness advice to safeguard your family, home and possessions.
- Purchase a NOAA Weather Radio All-Hazards receiver with battery power option to stay apprised of quickly changing weather information.
- Study evacuation routes in advance and heed evacuation orders.
- Turn Around, Don’t Drown – never cross flooded roads, no matter how well you know the area or how shallow you believe the water to be.
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About this Product
The National Hydrologic Assessment is a report issued each spring by the NWS that provides an outlook on U.S. Spring flood potential, river ice jam flood potential, and water supply. Analysis of flood risk integrates late summer and fall precipitation, frost depth, soil saturation levels, stream flow levels, snowpack, temperatures and rate of snowmelt. A network of 122 weather forecast offices and 13 river forecast centers nationwide assess this risk, summarized here at the national scale. The National Hydrologic Assessment depicts flood risk on the scale of weeks to months over large areas, and is not intended to be used for any specific location. Moreover, this assessment displays river and overland flood threat on the scale of weeks or months. Flash flooding, which accounts for the majority of flood deaths, is a different phenomenon associated with weather patterns that are only predictable days in advance. To stay current on flood risk in your area, go to http://water.weather.gov for the latest local forecasts, warnings, and weather information 24 hours a day.
National Weather Service
Hydrologic Information Coordinator
March 20, 2014