National Oceanic and Atmospheric Administration's
National Weather Service
National Hydrologic Assessment
March 19, 2015
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
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
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.
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
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.
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.
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.
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.
Exceeding Minor Flood Levels
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
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.
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
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.
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.
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.
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
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.
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About this Product
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 19, 2015
Page last Modified: 19 March, 2015 10:48 AM