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Flood Loss Data Fatality Data Spring Flood Outlook Loss Reports Flood Safety Info

National Hydrologic Assessment

March 19, 2015

spring flood outlook map

Figure 1: National Spring Flood Risk defined by risk of exceeding Minor, Moderate, and Major River Flood Levels  

Minor river flooding is expected through New York and New England, with localized moderate flooding in western New York and eastern New England, this spring. Late January through February featured record cold temperatures throughout New York and New England, and record to near record snowfall across eastern New England. As a result, 3 to 9 inches of snow water equivalent remain locked in the snowpack over eastern New England and western New York. This represents a much above normal snowpack for east coastal New England and western New York, but a normal to below normal snowpack for parts of interior New England.  The charged snowpack has raised flood concerns for the spring melt. As the warmer temperatures of springtime near, rivers and streams will begin to swell as the snowpack melts. Fortunately, below normal seasonal soil moisture, limited frost depths, and below normal ground water levels will allow snowmelt to infiltrate into the ground alleviating flood risk. However, significant river ice across northern New York and northern New England increase the risk of flooding related to ice jams and ice jam breakups. A significant warm up coupled with heavy rainfall during spring snow melt would exacerbate the flood risk.   

There is a 50 percent chance of exceeding moderate flood levels in small streams and rivers in the lower Missouri River basin in Missouri and eastern Kansas which typically experience minor to moderate flooding during the spring. This flood potential will be driven by rain and thunderstorms. The Missouri River downstream of Nebraska City, Nebraska may also experience minor flooding due to convective activity.   

Moderate flooding is also expected in the lower Ohio River basin including portions of southern Illinois, southwestern Indiana, and western Kentucky.  Several recent winter storms brought rain and snow to the Ohio River Valley and its tributaries, including the Tennessee and Cumberland Rivers. Melting snow and rain caused recent minor to moderate flooding. This has primed soils and streams for flooding to persist as the springtime typically brings heavy rains to this region.  

Minor flooding is possible from the Gulf Coast through the Ohio River Valley and into the Southeast, including east Texas, Louisiana, Arkansas, Missouri, southern Iowa, Illinois, Indiana, Ohio, Kentucky, Mississippi, southern and western Alabama, southern Georgia, northern Florida, the coastal Carolinas and coastal Virginia. This flood potential will be driven by individual convective rain storms typical in the spring.  

In Alaska, the flood potential from snowmelt and ice jams this spring is currently rated as below 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.

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 timescale 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 continues improving its decision support services with the “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, North Dakota, 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 Flood Levels  

While there are no widespread areas with risk of exceeding major flood levels, significant river ice across the interior Northeastern United States increases the possibility of localized major flooding.  With significant snowpacks 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 Flood Levels

New England and Western New York

A record cold and snowy period during the second half of winter over much of the Northeastern United states has left deep snowpacks and significant river ice. Snow water contents of 3 to 7 inches cover southern New England, with up to 10 inches in the higher terrain of northern New York, Down East Maine, and the mountains of northern New England. River ice thicknesses are running 1 to 2 feet across interior and northern New York and northern New England. Should a prolonged warm up occur coupled with heavy rains, exceeding moderate flood levels is expected.

Lower Missouri and Lower Ohio Basins

The lower Missouri River basin in Missouri and eastern Kansas has a threat of moderate flooding 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 smaller streams in the vicinity of Kansas City. The Missouri River downstream of Nebraska City, Nebraska may also experience minor flooding due to convective activity.  Moderate flooding is also expected in the lower Ohio River roughly from Evansville, Indiana to the confluence with the Mississippi River.  Several recent winter storms brought rain and snow to the lower Ohio River Valley and its tributaries including the Tennessee and Cumberland Rivers. This has primed soils and streams for the typical heavy spring rains which impact these areas with flooding every year.    

Risk of Exceeding Minor Flood Levels  

Ohio Valley, Tennessee and Cumberland Valleys

These areas experienced prolonged periods of significant cold and stormy weather this winter. Forecast locations in Illinois, Indiana, Ohio, Kentucky, Tennessee and West Virginia are likely to experience minor flooding, due to river ice breakup in northern regions and potential spring rains.  Historically, the spring flood risk across the northern basins, including the White River, the Wabash, and northern sections feeding the Ohio River are driven by the combination of snow water content and the potential for significant rain events as the spring progresses. These minor spring floods are typical, and generally occur every few years. Further south along the Tennessee, Cumberland and drainages feeding the lower Ohio River, spring flood risk typically concerns primarily heavy rainfall. However, with an abnormally cold and snowy winter in these areas, melt has already contributed to flooding. Additional spring rains may quickly change river levels and increase flood risk in these areas.

Southern Plains and Southeastern United States

Minor flooding also is possible across the Southeast, including Missouri, Arkansas, Louisiana, Mississippi, Alabama, Georgia and northern Florida. In addition, minor flooding continues to be possible across the eastern Texas, including the Sabine and Neches Rivers that have already experienced minor to moderate flood levels recently. This flood potential is driven by individual convective rain storms typical in the springtime, where near to above average soil moisture conditions exist.  

Other Regions/Low Flood Risk Areas  

Pacific Northwest

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, and 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 last six months has brought above normal precipitation to southern New Mexico and West Texas, but drought conditions still persist in this area. The US Seasonal Drought Outlook indicates that drought conditions will persist or intensify across portions of New Mexico and West Texas. Further east, Oklahoma and east Texas are in the grip of an extreme drought, although long term forecasts show some relief to the area. 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.

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 82% of average

·      Missouri River - Median forecast at Toston is 86% of average

·      Colorado River - Median forecast inflow to Lake Powell is 71% of average

·      Rio Grande and Pecos River – Median forecasts generally range from 60% to 105% of average

·      Great Basin – Median forecasts range from 35% to 75% 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 near average in sections of Idaho and western 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 generally below average. Exceptions include the upper Green River Basin in Wyoming and Colorado River headwaters with near average precipitation.  Snowpack is above average in the upper Green River Basin of Wyoming and near average in the Colorado River Headwaters. Elsewhere snowpack is below average.  Soil moisture is near to above average over most of the upper Colorado Basin with the exception of the San Juan Basin where below average conditions exist. Water supply forecasts at specific points range from much below to near average. Inflow into Lake Powell is forecast to be 71% of average.

Seasonal precipitation in the lower Colorado Basin, including southern Utah and Arizona, varies significantly. Above average precipitation has occurred in the Little Colorado River Basin, with near to below average precipitation elsewhere in Arizona. In southern Utah much below average precipitation has been observed. The snowpack has all but been depleted in most of the Arizona Basins in part due to above average winter temperatures. In southern Utah snowpack is much below average. Soil moisture conditions are above average in the Gila River Basin and parts of the Virgin River Basin and range from near to below average elsewhere. Streamflow forecasts include 35% of median for the Little Colorado River Basin, 85% of median for the Salt River Basin, and 55% of median for the Gila River Basin. The Virgin River Basin runoff is forecast at 45% of average. Reservoir storage in the Salt River system is 50% of capacity.

For the upper Rio Grande in southern Colorado and New Mexico that drain portions of the San Juan and Sangre de Cristo mountain ranges, the current basin wide snow conditions are generally below average, but improving recently with regular storm system patterns.  The Pecos River basin within New Mexico received some unusually heavy rainfall amounts last fall, such that along with additional snow/rain events, water year precipitation totals are well above average.  Snowpack conditions across the Rio Grande and Pecos mountainous areas vary widely ranging from 60 to 90 percent of normal with localized above and below outliers.  Along the northern and middle Sangres in Colorado and New Mexico, the snowpack improves with a general range from 90 to 110-plus percent.  As a result, seasonal water supply forecasts are generally below normal, with the exception of near to above normal along the middle to upper Sangres.

Across the eastern Great Basin, seasonal precipitation has been below to much below average.  Snowpack varies dramatically and ranges from 10% to 65% of average at most locations. Higher elevation headwater locations in the extreme northern Great Basin are closer to average. Several sites in the eastern Great Basin have their lowest snow on record. Soil moisture conditions are below average in all areas with exception of the highest elevation headwater areas in the Bear River, Weber River, and Provo River Basins where near to above average conditions exist.  Water supply forecasts generally range from 35% to 75% of average in the eastern 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 17% percent of the April 1st average.  The April through July streamflow volume forecast in California is below average everywhere, but wide ranging. Forecast volumes for the Trinity River are 30-40%, upper Sacramento (northern Sierra) 30-65%, San Joaquin River 25-40% and for the Tulare Lake Basin 15-35% of average. The April through September upper Klamath Basin streamflow volume forecast range from 30-50% of average. Storage capacity for the major reservoir on the upper Klamath is at 83% and on the Trinity is at 47%. Reservoir capacities in the upper Sacramento basin range from 50-60%, 10-45% in the San Joaquin basin and only 10-25% of capacity in the Tulare Lake Basin. This year’s reduced snowpack in the mountains will continue to result in diminished reservoir storage throughout thesummer and fall.

Alaska Spring Ice Breakup Outlook  

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

Ice Thickness

March ice thickness data are available for a limited number of observing sites in Alaska. March 1st measurements indicate that ice thickness is generally below normal across the state with a few exceptions being the North Slope and a scattering of locations in the interior where ice thickness is near normal or slightly above.  Areas below normal to well below normal are the upper Kuskokwim basin and south central Alaska. Accumulated freezing degree days are below normal across the state.  

Snowpack

An analysis of the March 1st snowpack by the Natural Resources Conservation Service (NRCS) indicates near normal snowpack in the upper Yukon while the rest of the state is below normal to well below. Southcentral and southeast Alaska including the Kenai are well below normal. For more details, please refer to the various snow graphics from APRFC or from the NRCS.  

Weather Forecasts

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 Alaska.  If daily temperatures are consistently above normal over the next two months, interior and southcentral Alaska are likely poised for a mild breakup as what little low elevation snow exists gradually melts off. 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 spring flood risk in the upper Midwest and Ohio valley is anticipated to lead to average hypoxic zone conditions in the northern Gulf of Mexico this summer.  Flood risk is slightly higher over portions of the Ohio River valley but is also predicted to be normal over much of the upper Midwest resulting in a net average condition. Flood conditions, should they occur, 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 locally 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 5,212 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 2015. 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.

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.

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. 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.  

Roham Abtahi

National Weather Service

Hydrologic Information Coordinator

March 19, 2015


 

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     Page last Modified: 19 March, 2015 10:48 AM