Flood Risk High for the Red River of the North Basin; Above Average Risk for Iowa; Missouri; Ohio River Basin; and across much of the South and Mid Atlantic (February 19, 2010) General Assessment Copious precipitation during the fall and winter, wet soils before freeze-up, enhanced streamflows before freeze-up and areas of substantial water in the snow pack have produced an imminent risk of major flooding along the Red River of the North and a risk of minor to moderate flooding across much of the upper Midwest and Midwest. During the previous 6 months a broad area has received above normal precipitation. Heavy rainfall and an active southern steam for storms created saturated soils from the Gulf Coast states through the Southeast and the mid Atlantic. Many locations continue to have high flows. The active weather pattern is expected to continue, creating an enhanced risk of flooding. Across the Ohio Valley, saturated soils and a snowpack holding up to 6 inches of water results in a greater than average chance of flooding along the Ohio River. Water from the northern Plains combined with output from the Ohio River will help push locations along the Mississippi River above flood stage. These conditions make future flooding more likely in this area. The enhanced precipitation has helped erase much of the drought that has plagued the Nation. The remaining areas are confined to the West, where the drought conditions combined with sub normal snowfall to date have resulted in a lower than average risk of flooding. The risk of flooding is gauged by assessing the cumulative impact of hydrometeorological factors such as precipitation, groundwater conditions, streamflow, snow conditions, river ice, and reservoir storage. Areas of the West, with sub normal snowpack and dry soils, are expected to have below normal streamflows, negatively affecting the water supply across the region. 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. Specific Areas of Above Average Flood Risk Upper Midwest and Red River of the North Based upon forecasts produced by National Weather Service field offices using the Advanced Hydrologic Prediction Service (AHPS), significant spring flooding is expected across the upper Midwest, Missouri and Mississippi Valleys, with a high risk of flooding in the Red River of the North Basin; the James, Minnesota, Des Moines and Crow River Basins. A very wet Fall, with areas of 200 to 400% of normal precipitation, produced exceptionally wet soils in advance of freeze up across the region. This pattern has continued, with Winter precipitation continuing above normal. December precipitation ranged from 150 to as much as 400% of normal, while January surplus precipitation had a lesser extent, there was a substantial area of above normal precipitation. One notable feature of this winter is its constancy. There has been no period of thaw to reduce the amount or water available for runoff. North Dakota, South Dakota and most of Minnesota had minimal snow cover through much of December with periods of sub-zero temperatures. This combined with high soil moisture contributed to deeper, harder frost for many areas. These conditions may increase runoff potential during the Spring thaw. Frost depths range from 1 to 3 feet in the Upper Midwest, some soil water equivalent measurements have noted a layer of ice at ground level, creating a difficulty in accurate remote measurements. Streamflows were also running above to much above normal. The streamflows before the freeze-up this year were significantly higher and over a broader area than last year, setting the stage for an even greater risk of flooding in the areas of high risk. The snowpack across the Upper Midwest and Midwest is more extensive this year than last. The water content of the snow pack in the Upper Midwest ranges from 2 to as much as 10 inches while across the Midwest the water content of the snow ranges from nothing in the southern portions to as much as 6 inches in the northwestern section. With rivers and streams beginning the spring thaw season at high levels, combined with a significant amount of water held in the snow pack, conditions are ripe for flooding this Spring. Extended-range AHPS-based forecasts indicate that major flooding is likely for most locations on the mainstem of the Red River of the North and many locations on its North Dakota tributaries across southeastern North Dakota. Locations that have a greater than 90% risk of reaching or exceeding major flood stage are Fargo, Abercrombie, Lisbon, Harwood and West Fargo. More comprehensive outlooks for this region can be found at: http://www.crh.noaa.gov/ahps2/index.php?wfo=fgf The extended-range AHPS-based forecasts also indicate that Devils Lake has a nearly 100% chance of establishing a new record high water level, exceeding the record of 1,450.7 feet set in June, 2009. These outlooks are subject to change based on the timing of the spring melt combined with future precipitation and any river ice-jam flooding. Mississippi River Flooding is anticipated along the Mississippi River this Spring. Melting from the abundant snowpack upstream both along the Mississippi River basin and tributaries as well as the Ohio River basin will provide sufficient flow to create some flooding along the river this Spring. Ohio River Basin Wet soils combined with substantial water held in the snowpack have produced an area with an above average risk of Spring flooding in the Ohio River basin. The extensive high amounts of water held in the snowpack along the Allegheny Mountains puts western headwater streams of the Ohio River such as the Cheat and Monogahela and eastern headwater streams feeding ht Potomac River at an above average risk of flooding. These amounts range from 1 to nearly 8 inches over much of western Pennsylvania, West Virginia, Ohio and northern Kentucky. If the snowmelt is slow with few storm systems, the flooding might be minor. However, if snowmelt is rapid in combination with significant rainfall, the potential for significant widespread flooding in the Ohio River basin could be high. South and Mid Atlantic Well above average precipitation across the South and Mid Atlantic from October to current ranging above 150% across a broad area has led to nearly saturated soil. A very active southern stream in the atmosphere has produced repeated moisture laden storm systems which have left the soils nearly saturated and eliminated much of the lingering drought that has persisted since 2007 in parts of Texas. Throughout the fall and winter, repeated storms have caused flooding across the southern states. Even though this area is not impacted by snow melt, it is primed for continued flooding. This presents a significant danger to lives and property across the region. An average of about 100 people perish annually in flooding related incidents, with over half of those being vehicle related. The combination of wet soils and anticipated continuation of an active weather pattern bringing abundant rainfall has produced this area of heightened risk across the South. The same active weather pattern brought heavy amounts of precipitation and in some cases record snowfall to portions of the mid Atlantic. The snow cover ranges to as much as 20 to 30 inches deep, with nearly 6 inches of water held in the snow. The current flooding in the coastal plains of the Carolinas has been ongoing throughout the winter, and will continue well into spring. Any additional rainfall will exacerbate this situation and ensure elevated streamflows for several more months. A sudden warm-up with rain can cause a rapid melt and enhance flooding potential. These factors combine to produce an above average risk of flooding this Spring. Across this region heavy rainfall is the primary factor which leads to river flooding. Heavy rainfall can cause river flooding any time of the year. Arizona Several locations in Arizona saw flooding in January due to heavy rains. Soils are saturated in areas that saw these heavy rains. Additionally, there is a significant snowpack in higher elevations and anticipated above average rainfall due to the presence of El Nino climate conditions. Snow melt alone is not expected to cause flooding, but heavy rains in combination has led to this area of heightened risk of flooding. Status of Key Hydrologic Variables Temperature and Frost Depth Temperatures this winter have been variable across the United States. November saw much of the U.S. with above average temperatures, with the north central states seeing the highest departures from normal. A strong reversal occurred in December, when much of the western two-thirds of the nation was plunged into a deep freeze, and most of the rest was below normal. The see-saw continued in January, with above average temperatures in the West , the northern tier of states and Northeast, and below average temps elsewhere. The cold in Florida led to substantial crop danger due to freeze. The colder than normal temperatures across the South led to lower than normal amounts of evapotranspiration and allowed the soils to hold more of the moisture than it usually would have. It is important to note that while the northern tier was above normal, the average temperature is still quite cold and no significant periods of 'January thaw' were observed. Frost depths across the north central U.S. range from 2 to more than 3 feet deep across parts of the Dakotas into Minnesota as a result of saturated soils preceding the Winter freeze up. Depths run from 1 to 3 feet in the Great Lakes states. On northern tributaries to the Mississippi River, frost depths are around 1 to 2 feet, while in Iowa tributaries the depths are shallower due to early snow cover. Precipitation and Snow Conditions Precipitation across the U.S. last fall was plentiful across much of the central and southeastern quarter of the nation while the West saw wide areas of 70% or less of normal precipitation. The abundant precipitation continued across the north central section of the country in December and January, while the Southwest also saw anomalously high precipitation totals. For the nearly 6 month period of October 1 to February 19, there are broad areas through the central U.S., the Southwest and the southeastern U.S. with over 150% of normal precipitation. In San Antonio, TX the 2nd wettest 6 month period on record was reported (30.7 inches since September 1st). Across the southern U.S., the heavy precipitation has led to several rounds of river flooding this Winter, with many locations either near or at flood stage currently. Snowpack as of February 1st across the West is above the long term average over much of Arizona and southern Rockies as well as basins in the Sierra Nevada. Much of the rest of the West remains below the average for this time of year. It is important to note that there are still 4 to 6 weeks until the peak of the snow accumulation season across the West, and conditions could change significantly during this time. A substantial snowpack covers the northern Plains, with lesser amounts through the Great Lakes and Midwest. Significant amounts of snow also remain on the ground in the Ohio Valley and portions of the mid Atlantic. Snowpack across New England is well below normal. Water held in the snowpack is a key element in the flooding potential picture this year. The water in the snowpack across portions of the Dakotas, Minnesota, Nebraska and Iowa ranges from 3 to 5 inches while across portions of the upper Ohio River basin snow water equivalents range from 1 to 8 inches. This stored water represents a substantial flooding potential when the spring thaw begins. Soil Moisture and Drought Conditions Plentiful rainfall has resulted in very wet soils across much of the eastern two-thirds of the nation. The wetness is reflected in the Palmer Drought Severity Index as well as soil moisture conditions. This wetness can enhance runoff, especially in areas with ample snow cover. Dry soil conditions across areas west of the Rockies combined with above normal temperatures and substantially below normal precipitation to create a reduced flood risk. River Conditions Rivers in a swath across the East coast states, the Southeast into the Gulf Coast states as well as across the Plains states are all running above normal as of February 19th. The active southern stream bringing numerous storm systems across much of this area has created these elevated conditions as well as storm systems crossing the middle of the country. The persistence of this pattern can be seen in the series of 28 day average streamflow maps of late fall and winter. November, December, January and February maps all depict substantially above normal streamflows. River ice is a potential problem across much of the northern U.S. In the North Central U.S., the elevated frozen rivers create an enhanced probability of ice jams during the spring melt. This will lead to increased chances of ice jam flooding on the Red River of the North and its tributaries. River ice also persists across northern New York and northern New England, creating the potential for ice jam flooding in the region. Water Supply The water supply outlook as of February 1st across the West is variable, with only southern sections in Arizona and, southern Utah and Colorado and New Mexico having an above average streamflow forecast. Mountain snowpack is variable as well, with significant surpluses in parts of Arizona and near normal amounts in California and New Mexico and some other scattered basins. The rest of the West is showing sub normal snowpacks. Precipitation during fall and winter was generally below normal. Reservoir storage as of February 1st shows the effect of persistent drought, with most levels below average but some states showing some recovery. The anticipated sub-average streamflows will not improve the drought situation in those affected areas. 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. The eventual water supply outlook will depend on the outcome of the remainder of the snow accumulation season. 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 mid February. Groundwater monitoring wells across New England and New York state show levels near to above normal. The notable exception is seen in the groundwater monitoring wells from the Genesee River valley westward into the Finger and Oneida Lake regions in west central and central New York State where levels are below normal. Without consistent rainfall, water supply could become an issue in this region later this spring and summer. Lakes and reservoirs are near or above target levels for this time of year. As of February 16, the New York City water supply system was at 92 percent of capacity, which is 9.3 percent above normal. Across Maine, reservoirs in the Kennebec and Androscoggin River basins were running 27.3 and 50.2 percent above normal levels respectively. The Scituate Reservoir, which is the main water supply system for northern Rhode Island including the city of Providence, was at 103.7 percent of capacity as of February 17 and spilling excess water into the mainstem Pawtuxet River. With below normal amounts of water in eh snowpack available for melt runoff, water storage is not a concern going into Spring. Across the Mid Atlantic, water supply conditions are generally close to average. In the Southeast, reservoir conditions have also returned to a plentiful condition as a result of the substantial rainfall during the fall and winter. Summary Early season heavy rain saturated soils which froze deeply before snow fell across the northern Plains, combined with substantial snowpack, has created an area of above average flood risk. The area of snow cover is more extensive than last year, creating the potential for a more widespread flooding event. There is a 100% chance that the Red River at Fargo ND will exceed the major flood level. Deeply frozen rivers which froze at a high level in the region have created an above average risk of ice jam flooding. Major flooding also remains likely along the James River in northern South Dakota. Heavy rains and saturated soils has caused flooding on rivers across the South and East Coast states. Additional heavy Spring rainfall is likely to cause renewed flooding. In areas of the mid Atlantic and Ohio Valley that have very substantial amounts of water held in the snow pack, flooding is a concern when the melt commences. In Arizona rainfall has caused flooding in some locations during the winter. The nearly saturated soils combined with above average snow pack has led to an above average risk of flooding. Ongoing drought conditions and below average snowpack led to a below average risk of flooding across portions of the West. In addition, generally dry conditions have yielded a lowered risk of flooding across parts of the Great Lakes and New England. For current flood information: http://www.weather.gov/ahps http://www.noaawatch.gov/floods.php | 
