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Summary 3/16/99
Precipitation patterns in the western U.S. so far this Water Year (since October 1, 1998), have been consistent with expectations during a La Ni¤a year: generally above normal precipitation in the northern states (northern California, northern Nevada, Oregon, Washington, most of Idaho, much of Montana and northwestern Wyoming), and below normal precipitation to the south. Much of southern California, southern Nevada, southern Utah, Arizona, New Mexico, as well as southern and western Texas have been particularly dry. Water contained in the snowpack has followed a similar pattern, with amounts of 150% of average, or more, from the central Sierra Nevada, northward through the Cascades of Oregon and Washington. The water in the snowpack is greater than average in northern and western Nevada, most of Idaho, and the Rockies in Montana and northern Wyoming. To the south, snowfall has been much less than normal.
Since December, western Washington and Oregon have seen a near continual onslaught of precipitation resulting in much above normal amounts -- some locations have established new records. While precipitation has been frequent, for the most part, intensities have been moderate. As a result, flooding so far has not been severe.
To the east, the snow cover is as much as a foot or more across north-central North Dakota, northern portions of Minnesota and Wisconsin, and the upper peninsula of Michigan where some locations along the Lake Superior shoreline have as much as three feet of snow. Generally lesser amounts of snow are found extending southward just into most of Iowa, southern parts of Missouri and Illinois, as well as northern Illinois and Indiana and Ohio, and much of the lower peninsula of Michigan.
With the most recent storm, New York and New England, most of the Mid-Atlantic region from central Virginia-central Maryland northward, as well as the higher elevations of the Appalachians southward into North Carolina are snow covered. With warmer weather expected, the snow line at lower elevations could quickly retreat northward. For the most part, snow amounts in the East have returned to near normal. Higher elevations in New England have somewhat greater than normal snow cover and liquid water content in the snow for this time of year.
The short-term crop moisture index reflects the more recent precipitation patterns, with wet conditions along much of the West Coast, from central California northward, and in parts of Idaho. It also shows soils in many parts of the East as being moist in an area generally extending from northern Louisiana northward to Michigan and eastward through much of the Mid-Atlantic and throughout New York and New England. The moist upper layers of the soil and even the slightly drier areas east of the Appalachians from Georgia to Pennsylvania are the result of a sharp reversal in precipitation trends since the first of the year. The last half of 1998 was extremely dry in most of these areas, and only significant rainfall since the first of the year has accounted for the moistening of the upper layers of the soils. The longer-term soil moisture index ( Palmer Drought Severity Index) still reflects the drier conditions in the deeper soils in much of the East. [Note: soil moisture indexes can be misleading and subject to considerable error in areas with frozen soils and/or snow cover.]
Streamflow conditions, particularly in the East and Midwest, are generally similar to the short-term soil moisture pattern, with many streams running high and a few exceeding flood stage. Because of generally milder than normal temperatures during parts of the winter in many northern areas, significant river ice is less prevalent than in most years and is limited to northernmost rivers.
Areas at greatest risk for flooding include:
a.. The Red River of the North. Currently, snow water content is sufficient to cause concern for possible snow melt flooding along the northern parts of the Red River of the North and several of its tributaries, in particular, the Red Lake River and lower reaches of the Sheyenne River. Snow amounts are substantially lower than at this time in 1997 (especially south of Grand Forks), when the Red River Valley was ravaged by record flooding.b.. Devils Lake, North Dakota. Snow water amounts are substantial in the Devil's Lake basin in east-central North Dakota, an area that has been subject to chronic high water conditions since 1993. Devil's Lake is in a closed basin; it has no outlet. Since 1993, annual precipitation has generally exceeded evaporation and infiltration, resulting in a series of new record lake levels over the last few years. The lake is expected to rise almost two feet between now and its peak this summer. The lake is forecast to crest at 1446 feet above sea level, more than a foot higher than the record established last summer.
c.. Idaho, Oregon and Montana. The deep snow cover in northern Idaho and adjacent portions of Oregon and Montana raise concern for snow melt flooding. In most years, flooding in these areas peaks as late as June. As a result, depending on temperature and precipitation patterns in the intervening period, the magnitude and timing of any flooding is still uncertain.
d.. East Slopes of the Cascades. While snow in the Cascades is running much above average, ranging from 140 to 190% of normal, snowmelt flooding has historically been limited to rivers and streams east of the Cascade crest and seldom occurs before April or May. While a substantial part of the snow accumulation season remains to come in the mountainous West, barring a near shut off of precipitation which is deemed unlikely, snowmelt flooding will be a problem in many of these rivers. Also, the Okanogan River in north-central Washington is likely to see some spring snow melt flooding.
Other areas with generally lower risk of flooding, but that bear watching include:
a.. Northern New England. This substantial snowcover in some higher elevations in New England combined with higher than normal streamflows and generally moist soils has created a somewhat elevated potential for flooding in parts of Maine, New Hampshire and Vermont. But severe flooding will require substantial rainfall. At worst, snow melt alone results in only modest flooding in the Northeast. Northern rivers have sufficient ice depths to pose a threat for possible ice jam flooding.b.. Southern Ohio River basin. February and March precipitation has moistened soils and elevated river levels in much of the southern portions of the Ohio River valley. Although there is no significant snow present, these moist conditions make much of Kentucky and southern portions of Indiana susceptible to flooding. In the near term, even moderate rains could be of concern, as runoff would rapidly flow into rivers and streams and inundate low areas.
c.. Western Washington and Oregon. In the West, most major floods west of the Cascade Range are the result of heavy rain and almost always occur in the November-February time frame. Nevertheless, the very wet conditions in the Pacific Northwest combined with the continuing influence of La Ni¤a suggest that this area continues to be of some concern over the next several weeks. In addition to the possibility of river flooding, saturated soils in the coastal ranges and the Cascades are also more susceptible to mud slides.
d.. The Klamath basin in Oregon and California. While it is premature to draw firm conclusions about snowmelt flood risk likely during spring, at this time there are growing indications that snowmelt flooding is a possibility in parts of the Klamath basin in southwestern Oregon and northwestern California.
e.. Localized areas on the east slopes of the Sierra Nevada: A somewhat elevated potential also exists on eastern Sierra Nevada drainage basins north of Mono Lake and in northwestern Nevada and northeastern California. The severity of flooding will depend on the pattern of warming and amount of rain received. A gradual warm-up with dry conditions will lead to an orderly melt with little or no flooding, while a rapid warm-up with rain on the snowpack could lead to more serious flooding.
Should the La Ni¤a pattern continue as expected, extremely dry conditions are a distinct possibility later this year in much of the Southwest. However, while snowmelt streamflow may be severely depressed, many areas have adequate reservoir storage. Any water shortfalls would be primarily in locations that had access to neither stored water nor ground water.
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