United States: Widespread water surpluses to continue

29 May 2019

THE BIG PICTURE
The 12-month forecast ending January 2020 indicates that water surpluses of varying intensity will affect many parts of the U.S. but will be especially widespread in a vast path at least two states deep on either side of the Mississippi River reaching from eastern South Dakota through Michigan, and south nearly to the Gulf. Extreme surpluses are forecast along much of the Mississippi River and exceptional surpluses along stretches on either side of Memphis, Tennessee. Surpluses will be extreme on much of the Missouri River as well. Other areas where surpluses are expected to be particularly intense include Sioux Falls, Omaha, and Cincinnati.

In the West, moderate to severe surpluses are forecast for much of California and more intense surpluses in the Rockies, including southwestern Montana, Idaho, southern Wyoming, eastern Nevada into Utah, and western Colorado. Surpluses are also forecast along many rivers. In the Northeast, primarily moderate surpluses are forecast in eastern New York, Vermont, and pockets of surrounding states.

Moderate to severe deficits are expected in the Pacific Northwest, and moderate deficits in northwestern Minnesota, South Carolina, eastern Georgia, and southern Florida.

Outside the contiguous U.S., surpluses are forecast for much of Hawaii. In Alaska, surpluses are forecast southeast of Barrow in the far north, and along the Upper Koyukuk and central Yukon Rivers and will include exceptional anomalies. Moderate to severe surpluses will reach south to Bristol Bay. Moderate deficits are expected in the Seward Peninsula and into western Alaska; along the Tanana River east of Fairbanks; around Anchorage and Valdez in the south; and at the tip of the Alaska Panhandle. Severe deficits are forecast for Puerto Rico.

FORECAST BREAKDOWN
The 3-month maps (below) show the evolving conditions in more detail.

From May through July, widespread surpluses of varying intensity are forecast in a vast area on either side of the Mississippi River. Surpluses will moderate in the Ohio River Valley and downgrade somewhat in South Dakota, Nebraska, and Kansas, but will intensify in Wisconsin and increase in Michigan, Oklahoma, eastern Texas, and the Gulf States. Exceptional anomalies are forecast around Sioux Falls, and along the Mississippi River on either side of Memphis and from Louisiana to the Gulf. Surpluses are also expected along the Missouri, North Platte, Arkansas, and Red Rivers. Farther west, surpluses will increase in the Rockies, transitioning from deficits in northern Idaho and Montana. Surpluses in California will decrease somewhat and moderate. In the Pacific Northwest, deficits will shrink in Washington, and Oregon will transition from deficit to moderate surplus.

In the East, surpluses are forecast for New York, Vermont, and New Hampshire, including a small pocket of exceptional intensity in Upstate New York. Moderate deficits are forecast in the Southeast in South Carolina, Georgia, and Florida.

From August through October, surpluses will shrink and downgrade in states on the eastern side of the Mississippi, with nearly normal conditions returning to parts of the Ohio River Valley. Surpluses will downgrade in states on the western side of the Mississippi but will remain widespread. Anomalies will be most intense in eastern South Dakota. Surpluses will shrink considerably in California and the Rockies, but many pockets of surplus are forecast for southwestern Montana, eastern Nevada into Utah, southern Wyoming, and western Colorado. Moderate surpluses will persist across central Arizona. Moderate to severe deficits will increase somewhat in the Pacific Northwest. Some moderate deficits are forecast during this period for South Carolina, Georgia, and pockets of Florida.

The forecast for the final months – November 2019 through January 2020 – indicates surpluses in the Upper Mississippi Basin, the Rockies, and in southern Georgia, and moderate deficits in western Oregon.

(It should be noted that forecast skill declines with longer lead times.)

IMPACTS
This season’s Mississippi River flooding, having lasted in some areas for over 95 days, will likely supercede the Great Flood of 1927 as the worst flooding in modern history. Roughly 250,000 of the 540,000 flooded acres are agricultural fields, preventing many farmers from planting this year. At the end of last month, the governor of Mississippi sought a federal disaster declaration in response to the flooding in March.

A flood barrier along the swollen Mississippi River failed late last month, sending a deluge of floodwaters into downtown Davenport, Iowa. The flooding submerged vehicles and overtook the first floors of some buildings along the riverfront, prompting water rescues of stranded people.

The governor of Louisiana declared a state of emergency due to the storm, after which authorities opened the Bonnet Carre Spillway for the second time this year, and only the 14th time since 1927. The spillway is opened when the Mississippi is high enough to threaten the Greater New Orleans area by releasing tons of Mississippi River water, and all the pollution it carries, into Lake Pontchartrain. Charter fisherman have been reporting dolphin deaths in numbers they’ve never seen, likely due to the onslaught of nutrient and chemical pollution. The flooding also causes local salinity levels to plummet, threatening oyster leases along Louisiana’s coastline. The President of St. Bernard Parish declared a state of emergency due to the likely impacts on the local seafood industry.

A storm system dropped heavy rains on Houston, Texas, prompting high water rescues of stranded motorists and closing part of Interstate 10 in mid-May. Heavy rains elsewhere in Texas caused flash flooding, killing at least one person.

NOTE ON ADMINISTRATIVE BOUNDARIES
There are numerous regions around the world where country borders are contested. ISciences depicts country boundaries on these maps solely to provide some geographic context. The boundaries are nominal, not legal, descriptions of each entity. The use of these boundaries does not imply any judgement on the legal status of any territory, or any endorsement or acceptance of disputed boundaries on the part of ISciences or our data providers.

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