Flood Risk Above Average in the Northeast, Ohio Valley, Midwest, middle and lower Mississippi Valley, Idaho, Mountains of western Wyoming, western Colorado and New Mexico

(March 14, 2008)

Copious precipitation, areas of substantial water held in the snow pack, wet soils, and above normal to much above normal streamflows have produced a risk of minor to moderate flooding across a swath from eastern Kansas and Nebraska through much of  the Mississippi Valley and its tributaries, Indiana through to much of New York, New England, and the northern mid Atlantic. Heavy winter snow combined with recent rain indicates parts of Wisconsin and Illinois should see minor to moderate flooding, with as much as a 20-30% chance of major flooding on some rivers in southern Wisconsin and northern Illinois. An above average risk of flooding exists as well for portions of middle Rocky Mountains, Colorado and the headwaters of the Snake River basin in Wyoming. There is substantial snowpack in parts of the West, with water content of the snow reaching over 150% of normal in some areas of Utah and Colorado. Wisconsin received record amounts of snowfall this year, and Vermont has up to a foot of water held in the snow. Parts of northern New York and New England have 2 to 4 feet of snow on the ground. The potential for ice jams persists in parts of northern New England. River ice in New York is showing signs of weakening, while the ice is firmer farther north. River ice is still very thick in Maine, with thickness up to 2.5 feet in northern Maine. A potent storm system dropped up to 20 inches of snow across Ohio March 7^th and 8^th and up to 3 inches of rain across southern New England, contributing to the elevated risk.  Recent temperatures, particularly in the daytime, across the Midwest and Ohio Valley have been above normal, leading to the beginning of the spring snowmelt season. Snow melt plus recent rainfall is causing rising river levels and some minor to moderate flooding.  Flooding is occurring from northeastern Texas through parts of Arkansas into the Midwest and Ohio Valley.

The flood potential from snowmelt and ice jams this spring breakup season throughout Alaska is currently rated as average. The March 1 ice thickness data are available for a limited number of observing sites in Alaska. Measurements indicate that ice thickness range from well below normal in southwest and southcentral Alaska to above normal in the eastern interior. Accumulated freezing degree days are now between 78 and 112 percent of normal with the highest percentage in southeast Alaska. Generally ice thicknesses are greater in areas of the state with less snowpack. An analysis of the March 1 snowpack by the Natural Resources Conservation Service (NRCS) indicates normal to above normal snowpack throughout much of the western half of the state, the Kenai Peninsula, and southeast Alaska. Some basins in southern southeast Alaska have greater than 150% of normal snowpack. The eastern half of the state including the Susitna, Copper, Tanana, the Yukon above Tanana, upper Koyukuk and north slope drainages all show below to well below normal snowpack. Regardless of climate averages there is enough snow in most areas to produce significant snowmelt runoff peaks if subjected to a rapid warming pattern.

Drought conditions have contributed to the below average flood risk across the Southeast and northern Plains. The continuing drought across the West has mitigated the flood potential from the above average snow pack as much of the runoff will either be absorbed by the dry soils or be collected in area reservoirs.  Flood risk is gauged by analyzing hydrometeorological conditions such as precipitation, groundwater conditions, streamflow, snow conditions, river ice, and reservoir storage.

Rainfall across the U.S. has varied from less than 50% of normal during the past 90 days in parts of the northern and southern Plains to over 150% of normal over much of the West and from the central Plains to New England. An active storm pattern across the northeastern and western U.S. brought repeated storms across these regions through December and February, soaking the soil and priming the areas for later flooding. Copious rainfall over this same period mitigated the effects of long standing drought in parts of the Southeast. Recent rainfall across the drought stricken Southeast has continued to ease some of the dryness, but has not elevated the flood risk.

Temperatures varied this winter, with unusual warmth across much of the eastern U.S. and northern Plains in December which continued into January across the northern states. This warmth contributed to much of the snowpack in the lower Midwest melting through the winter, leaving little on the ground as of March 7. Colder than normal temperatures spread across the Plains and Midwest in February, allowed an enhanced snowpack to return to the Midwest. Frost depths across the north central U.S. range from 1 to 3 feet in the most northern areas. The frost depths are as deep as they are because there has been less than normal snowfall over much of Minnesota and North Dakota this winter.  Chances of flooding are low in these areas because of the lack of snow.  However, if they were to receive late season snows and rain during snowmelt then the frozen ground will enhance runoff rates and lead to flooding.  The recent March storms across the eastern U.S. added substantial snow across the Midwest, with up to 20 inches in Columbus, Ohio. Where the precipitation fell as rain, it helped saturate the soil, increase streamflow, and add water to the water content of the snow pack.

Plentiful rainfall has resulted in very wet soils from the central Plains to New England as well as portions of the West. This wetness is reflected in the Palmer Drought Index, as well as soil moisture conditions used by the National Centers for Environmental Prediction in their forecast models. These conditions have produced nearly saturated soils, and any additional rain will run off quickly and result in flooding. Below average precipitation across the northern and southern Plains as well as the Southwest combined with long term dryness and drought led to drying and very dry soils, with reduced flood risk in these areas.

Rivers in a swath from eastern Kansas, across the Midwest into the Ohio River Basin to much of southern New England and parts of the southern Appalachians are running above normal. Much above normal precipitation across the Northeast during February resulted in much above normal stream flows, with many sites across southern New England and southeastern New York setting new record daily flows for March 13. Rivers across much of the rest of the nation are running at normal to below normal levels for this time of year. River ice is widespread across the Northeast, with some softening noted in southern areas. River ice is still quite thick and widespread in Maine and the northern tier of states. Most rivers across interior Maine are nearly 100 percent ice covered with thickness ranged from 0.5 to 1.5 feet. Rivers across the northern tier of states contain ice, but few ice jams have been noted. The Rock River in Illinois had ice jams along much of the river, exacerbating flooding and pushed the river to record levels in the Moline Area. Ice jams have also been common in southern Iowa, northern and central Illinois, and southern Michigan during the past month. Ice jams are likely as frozen rivers and streams thaw through the spring.

As a result of the plentiful precipitation this winter across the Northeast, water supply reservoirs across the region are generally at or above target levels for early March. The New York City water supply was at 99.5 percent as of March 14, which is 12 percent above normal. Other reservoirs are at or above normal pool levels. The Scituate reservoir which provides the drinking water for northern Rhode Island has reached 97 percent of capacity.  

Across the Southeast, drought affected reservoirs have shown recent improvement.  In Virginia, the Carvins Cove reservoir, a major source for Roanoke drinking water, is now 15.7 feet below the spillway. By way of comparison the water level fell to 34.1 feet below the spillway at the height of the 2002 drought. In contrast, pool levels of most major reservoirs in north and central Georgia are running near target levels for this time of year except for Lake Lanier and Carters Lake. It is during the spring that many operators plan to fill their reservoirs to summer pool levels. Normal rainfall could take care of that situation. However, above normal rainfall would be needed for Lake Lanier and Carters Lake to fully recharge. In western South Carolina, the levels of Lakes Hartwell and Russell are near the record low levels.  

The water supply outlook across the West has improved this year compared to last. The majority of mountain snowpack is average or above as is the total precipitation since October 1, providing the potential for maintaining water supply levels or seeing some improvement. The water contained in the snow is also ranging general above average as well. Reservoir storage as of March 1 shows the effect of the persistent drought, with most levels below average. So the potential of somewhat above average streamflow will only provide limited improvements.  Of the four major irrigation reservoirs in Montana, Lima Reservoir had 125 percent average storage, Clark Canyon was holding 74 percent of average water, Gibson Reservoir had 47 percent of average stored water, and Fresno Reservoir had 55 percent of average stored water. The major hydroelectric reservoirs in Montana (Canyon Ferry and Fort Peck) had end of February storage in the average to below average range; 93 and 58 percent of average water, respectively. End of February reservoir storage in Wyoming was 68 percent of average in Boysen Reservoir and 111 percent of average in Buffalo Bill Reservoir. Month-end storage was 101 percent of average in Bighorn Reservoir on the Wyoming/Montana border. Most of the annual streamflow in the western U.S. originates as snowfall that accumulates in the mountains during the winter and early spring. This snow accumulation can continue into April. If March and early April have below normal precipitation, the snowpack could fall back to sub normal levels. 

In Summary; recent heavy rain over saturated soils has caused ongoing minor flooding in parts of the Midwest and southeastern New England. Long term above normal precipitation combined with areas of deep snowpack have resulted in a broad swath of above average flood risk from the central Plains and Arkansas into Wisconsin and across the Midwest and Ohio Valley to New England and the northern mid Atlantic. In southern Wisconsin there is a better than 50% chance of major flooding which contributes significantly to the better than 50% chance of moderate flooding along the Mississippi River. Above average snowpack for this time of year across much of the West should help mitigate some of the drought conditions, resulting in an average to below average flood risk. The exceptions are some portions of middle Rocky Mountains, southern Colorado, the headwaters of the Snake River basin in Wyoming where an above average risk of snowmelt flooding exists. Drought conditions, dry soils and subnormal precipitation led to a below average risk of flooding across the northern Plains, western Texas and the Southeast.

It is important to remember that heavy rainfall at any time can lead to river flooding, even when overall river flood potential is considered below average and ice jams can cause rapid water level rises on rivers with significant ice accumulation.

For current flood information:

http://www.noaawatch.gov/floods.php)