March 15, 2012

An above normal risk for spring flooding exists over the Lower Ohio Valley and parts of the Central Gulf Coast in Southern Louisiana and Mississippi. Above-normal precipitation this winter in parts of the Ohio River Valley has caused high soil moisture and above-normal river levels. The forecast through April calls for above-normal precipitation in the Ohio River Valley which could lead to flooding.

For the first time in 4 years, the Northern Plains are not expecting major to record snowmelt flooding. Most of the Upper Mississippi and Middle to Upper Missouri River Basins have a below normal risk of flooding. The nation's farthest northern basins of North Dakota and Montana including the Red River of the North, Souris River and the Missouri headwater have a normal risk of flooding, and will not be significantly impacted by snowpack this year.

A below normal risk of flooding exists in most of the Southwest and Southeast. Precipitation and soil moisture deficits have led to extreme drought across much of Georgia and the Florida Panhandle, West Texas, Southeastern New Mexico, and sections of western Oklahoma and Kansas.

All other areas in this map have a normal risk of spring flooding. In this category areas prone to spring flooding are expected to experience that normal level of flooding. For example, the lower Missouri valley commonly experience minor flooding each spring with little impacts. Heavy rainfall at any time can lead to river and/or flash flooding, even in areas where overall risk is considered normal to below normal.

The flood potential from snowmelt and ice jams throughout Alaska this Spring is currently rated as above normal. This forecast is based on current ice thickness, observed snowpack, and long range weather forecasts. Anchorage, Alaska has experienced nearly double the normal winter snowfall and is expected to break snowfall records by the end of the snow season. Fortunately, this area experiences a gradual spring warm up and typically is not impacted by snowmelt flooding. The interior of Alaska has near normal snow pack and expects normal to above normal ice jam and snowmelt flooding.

Uncertainty associated with this flood outlook for the continental U.S. is greater than in previous years given the primary driver for this outlook is rainfall rather than snowmelt.

Heavy Rainfall and Flooding

The information presented in this report focuses on spring flood potential, using evaluation methods analyzed on the scale of weeks to months, not days. Heavy rainfall at any time can lead to flooding, even in areas where overall risk is considered normal to below normal. Rainfall intensity and location cannot be accurately forecasted beyond a few days in the future, therefore flood risk in these areas 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.

Above Normal Flood Risk Areas

Lower Ohio Valley

Soil moisture is above normal through the Lower Ohio Valley in response to above normal precipitation (125-200%) over the last six months. April forecasts call for above average precipitation over the Ohio Valley to continue into the spring.

Central Gulf Coast

Based on current conditions and extended range precipitation forecasts, an above normal flood potential exists along the Central Gulf Coast in Southern Louisiana and Mississippi. Streamflows in those areas are above normal in response to a series of heavy rain events over the last 30 to 45 days.

Citizens are encouraged to remain up-to-date on flood conditions by monitoring the latest forecasts from your local National Weather Service office.

Normal Flood Risk

Red River of the North, Souris, and Central Plains

The Red River of the North, the Souris River Basin, and most of the Missouri River Basin (Northeast North Dakota, Northwest Minnesota, Montana, Wyoming, Northeast Colorado, Southern South Dakota, Nebraska, Northern Kansas, Iowa and Missouri) have a normal risk of flooding.

The spring flood risk in these areas is largely driven by snowmelt flooding. With the below to near-normal winter snowfall this is the first time in 4 years these areas are not preparing for major to record snowmelt flooding. The Northern Rockies of the Missouri Basin contain near the historical normal amount of snow water content for this point in winter. The Souris and Red River basins contain about one-third the amount of water in the snow as last year, ranking this season in the lower half of the last 60 years.

Mid-Atlantic and Northeast

With the exception of extreme northern New England along the Canadian border, the winter snowpack in the northeast and mid-Atlantic regions have been well below normal this winter, and will not contribute to the spring flood risk. Rivers in these regions respond to rainfall events that evolve over the springtime, which are only predictable on the scale of a week in advance.

Pacific Northwest

The threat of flooding is near normal across most of the Pacific Northwest. Weather patterns in this part of the country are generally separated by locations west and east of the Cascade Mountain Range, which define the difference in flood threat.

Rivers west of the cascades usually reach their highest peak flows during the winter. 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, and this year is no different.

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 crests will be.

The river most at risk of spring flooding is The Naches River near Yakima Washington. In all other areas, normal spring flood risk means that some smaller streams and flood prone rivers could have minor flooding with a sudden large warm-up or the occurrence of heavy rain or thunderstorms over those watersheds. Flooding during the snow melt season can occur anywhere when heavy rain falls in a river basin if the rain is intense enough.

Below Normal Flood Risk

The following regions are outlined with a low risk of long range spring flood risk, and many are experiencing drought. It is important to remember that intense precipitation at any location, regardless of past conditions, can rapidly change risk, often on the scale of hours. Deadly localized flooding during severe weather outbreaks is possible, and common, regardless of long term risk. Always stay current with any Watches and Warnings from your local NWS Forecast Office, and follow the advice of your local emergency management officials should flash flooding effect your area. Please visit drought.gov for detailed outlooks, impacts and information related to your area.

Upper Mississippi and Upper Missouri Valleys

In stark contrast to water year 2011, precipitation this water year (since Oct 1, 2011) has been less than 50% of the normal across the Upper Mississippi and Upper Missouri Valleys. Snowpack in these regions is well below levels needed to warrant large scale flooding, and will not play a role in this year's spring flooding across the northern US. The High Plains of the Missouri Basin contain little to no snow, in stark contrast to last year when the high plains were blanketed with a wet snowpack at this time.

Texas and the Southwest

Precipitation and soil moisture deficits have led to extreme drought across West Texas, Southeastern New Mexico, and sections of western Oklahoma and Kansas. Precipitation through the Spring is expected to be below normal especially in the far west. Given current observations, trends and precipitation forecasts for the region, the number of river floods is expected to be lower than the normal, or what is typical for this time of year.

Southeast

Mid-March is too early to determine spring flooding potential across the Western U.S. Snowpack remains below normal in many regions. 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. Extremely high temperature 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 collaboratively generated by NOAA/NWS River Forecast Centers and the 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 and Northern Rockies to below normal, and much below normal in the Central and Southern Rockies, and in California.

• Columbia and Snake Rivers - Median forecast at The Dalles is 97% of average
• Missouri River - Median forecast at Toston is 80% of average
• Colorado River - Median forecast inflow to Lake Powell is 74% of average
• Great Basin of Nevada and Utah ? Median forecasts range from 24% to 65% of average
• California - Median forecasts range from 30% to near 60% of average

The wide range of forecasts reflects stark contrast in weather pattern between the northern and southern portions of the region.

Upper elevation areas in Columbia River Basin including the portions of Washington, Oregon east of the Cascades as well as all of Idaho received near average to above average seasonal precipitation. Snowpack is near to above average with soil moisture above average over much of the region. Median forecasts are near average in most basins, with below average runoff projected in the southern Snake River Basin tributaries.

Seasonal precipitation in the Upper Colorado Basin ranges from near average, to much-below average. Snowpack in the headwater range from near average to below average. Soil moisture in this area is above average to average. As a result, water supply forecasts at specific points range from average to below average and, as noted, unregulated inflow into Lake Powel is 74% of average.

Portions of the Lower Colorado Basin, including southern Utah and Arizona, received 20% to 70% of average seasonal precipitation. This same region saw minimum temperature anomalies as high as 4 to 6 degrees (F) above average from December through February. As a result, the snowpack in these basins is significantly below average. Similarly, soil moisture is much below average. Water supply forecasts range from 50% of average to less than 25% of average basin central Arizona. Reservoir storage in the Salt and Verde River systems are 70 and 23% of capacity, respectively.

Across the Great Basin, seasonal precipitation ranges from below to much below average. Snowpack ranges from below average in the eastern portion of the region to much below average in western portion of the great basin. 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, ranging from 60% to less than 20% of average over the state. Snowpack is 30% percent of average, and soil moisture analysis indicates below average soil moisture. As of March 1st, the water supply forecasts in California ranged from 30% to 70% of the April to July mean. Reservoir storage, as reported by the Natural Resources Conservation Service, remains above average as a result of much above average runoff during water year 2011. However, since last year was very wet, with a deep snowpack in the mountains and abundant runoff, the larger water supply reservoirs are currently running near the historical average for this time of year and will help buffer shortages this summer and fall.

The latest water supply forecasts are available at

http://wateroutlook.nwrfc.noaa.gov/maps/wsf

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, prolonged significant below average seasonal precipitation in Central and West Texas as well as portions of New Mexico continue to exacerbate drought conditions that have persisted since 2011. Forecasts continue to indicate much below normal streamflow conditions which will adversely impact agricultural, municipal and industrial water supplies. Reservoir and lake levels are much below normal as well. Potential impacts in these bodies of water include constraints on water intake structure, reductions in water available for hydro-electric production, and water quality issues. NWS offices in the region are developing forecast procedures with stakeholders to objectively monitor or project flows at these critical locations.

Across the Southeast, from the Tennessee/Kentucky border south, seasonal precipitation ranges from 90 to less than 50% of average. NWS volumetric river forecasts for the next three months indicate much below normal water availability. This has triggered drought planning levels in the Savannah River project that may impact recreation and certain fisheries below these structures. In the Chattahoochee Basin, if reduced surface water availability continues it will impact hydroelectric production but, at this time, minimum releases are not threatened.

Alaska Spring Ice Breakup Outlook

The flood potential from snowmelt and ice jams throughout Alaska this spring is currently rated as above normal. This forecast is based on current ice thickness, observed snowpack, and long range weather forecasts.

Ice Thickness

March 1st measurements indicate that ice thickness is variable across the state with locations generally between 75 and 125 percent of normal. Many locations in the Tanana basin reported slightly below normal thickness. Despite the cold temperatures mid-winter there was enough snow cover on the ice to provide insulation and inhibit significant ice growth. Accumulated freezing degree days are slightly above normal values over most of Alaska.

Snowpack

An analysis of the March 1st snowpack by the Natural Resources Conservation Service (NRCS) indicates a much higher than normal snowpack across southern Alaska and the panhandle with numerous locations reporting 150-200% of normal snow water equivalent. High volumes of snowmelt are expected this spring and summer across the southern coast. However the moderating influence of the Gulf of Alaska typically causes a slow release of melt runoff which limits the potential of snowmelt flooding. A higher than normal snowpack (110-150% of normal) exists over the Koyukuk and Kobuk basins in northwest Alaska. The snowpack in the Yukon territory is near to above normal. Areas with above normal snowpack may have an increased risk of breakup related flooding. The only areas of Alaska that have a near to below normal snowpack are the Tanana basin and the North Slope. Even so, there is enough snow in these areas to produce significant snowmelt runoff peaks and potential flooding if subjected to a rapid warming pattern. For more details, please refer to the various snow graphics or from the NRCS.

Weather Forecasts

The weather outlook for the next two weeks is for an increased chance of below normal temperatures statewide with the greatest chances for very cold conditions over Southwest Alaska. The predominant storm track is likely to remain south of Alaska allowing an Arctic air mass to filter southward across the state. However, the most important factor determining the severity of ice breakup remains the weather during April and May. The preliminary outlook for April suggests an increased chance of below normal temperatures for Alaska. The preliminary 90 day outlook for the months of April, May and June indicates a slight chance of below normal temperatures continuing over the southern half of Alaska. Dynamic breakups with a high potential of ice jam flooding typically require cooler than normal temperatures for most of April followed by an abrupt transition to warm summer-like temperatures in the 60s and 70s in Late April into early May. If a significant snow cover is retained during a colder than normal April, than an abrupt warm up could send a surge of snowmelt runoff into rivers that still have intact and relatively ice.

With the current snow and ice conditions in Alaska and the projected cool April, if a marked warm up occurs in late April or early May then there would be a high likelihood of ice jam flooding. For more information on the climate forecasts please refer to the Climate Prediction Center.

Please visit the Alaska-Pacific River Forecast Center for the latest Spring Breakup Outlook.

Spring Flood Outlook and Gulf Coast Ecology

Draining over 40% of the contiguous United States, precipitation patterns in the Mississippi River Basin have a impact on the delivery of nutrients to the Gulf of Mexico. High levels of these nutrients cause a large area of low-oxygen in the Gulf, known as the dead zone, to form each summer. 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 summer dead zone. The predicted above-normal flood risk in the lower Ohio Valley would serve to increase the severity of the dead zone. However, this increase may be countered by below-normal flood risk in the Upper Mississippi Valley. It is the net result of these forces that may determine whether the dead zone is above or below normal this summer. In early June, NOAA will be releasing its annual dead zone forecast based on observed river discharge rates and nutrient concentrations provided by the USGS.

NOAA's Role in Flood Awareness and Public Safety

Floods kill an average of 94 people each year in the US. The grand 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.
• 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 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.

NOAA's National Weather Service is the primary source of weather data, forecasts and warnings for the United States and its territories. It operates the most advanced weather and flood warning and forecast system in the world, helping to protect lives and property and enhance the national economy. Visit us online and on Facebook.

NOAA's mission is to understand and predict changes in the Earth's environment, from the depths of the ocean to the surface of the sun, and to conserve and manage our coastal and marine resources. Visit us online or on Facebook.

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. In general flood risk is assessed as high, above normal, normal or below normal. These risks are defined as:

• High: 90-day Ensemble Streamflow Prediction simulated flows significantly above normal and a greater than 50% chance of major flooding
• Above Normal: 90-day Ensemble Streamflow Prediction simulated flows above normal and greater than 50% chance of minor or moderate flooding
• Normal: 90-day Ensemble Streamflow Prediction simulated flows near normal; flood prone areas still at risk (e.g., Red River of the North, Missouri)
• Below Normal: 90-day Ensemble Streamflow Prediction simulated flows below normal; flooding not likely

Analysis of flood risk integrates late summer and fall precipitation, frost depth, soil saturation levels, stream flow levels, snowpack, temperatures and rate of snow melt. 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 a few 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.

Roham Abtahi - National Weather Service - Hydrologic Information Coordinator - March 15, 2012