UNITED STATES: INTENSE WATER SURPLUSES TO PERSIST IN IDAHO AND MONTANA

22 May 2018

THE BIG PICTURE
The 12-month forecast indicates deficits in the US West, Southwest, Southern Plains States, and South Atlantic States. Deficits are expected to reach exceptional intensity – a return period of over 40 years – in pockets of northeastern Utah and southwestern Colorado, with deficits of varying severity in Oregon, California, Nevada, and New Mexico. Primarily moderate deficits are forecast for Kansas, western Oklahoma, and Texas, though conditions may be severe the Panhandles of Texas and Oklahoma. In the East, deficits will be severe in South Carolina and eastern Georgia. Areas of moderate deficit also include northeastern North Dakota through the western half of Michigan’s Upper Peninsula, and Maryland, Delaware, and southern Maine.

Moderate to severe surpluses are forecast along the Missouri and Yellowstone Rivers leading to a block of extreme to exceptional surpluses in western Montana and central Idaho. Pockets of surplus are also forecast for eastern Washington. Moderate to severe surpluses are expected in Michigan’s Lower Peninsula, the Ohio River Valley, and in parts of the Lower Mississippi where conditions of both deficit and surplus may appear as transitions occur. Moderate to severe surpluses are also forecast in a broad path along the borders of Oklahoma and Arkansas, and Texas into Louisiana.

Outside the contiguous US surpluses are forecast for northwestern Alaska and the northern half of the Alaskan Peninsula trailing well into the interior; deficits are forecast in patches of the eastern half of the state. Intense surpluses are forecast for Hawaii, and severe deficits are expected in western Puerto Rico.

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

The near-term forecast through July indicates that widespread surpluses stretching from Michigan’s Lower Peninsula through the Ohio River Valley and the Lower Mississippi Basin will retreat considerably, leaving nearly normal conditions. Surpluses will persist on the Missouri and Yellowstone Rivers, and surpluses reaching exceptional intensity will persist in western Montana, northern Idaho, northwestern Wyoming, and into eastern Washington.

Deficits observed in prior months are expected to downgrade. In the East, deficits in the South Atlantic states will retreat, though moderate to severe deficits remain in the forecast for eastern South Carolina and southern Georgia. Southern Florida will transition from deficit to moderate surplus. In the West, much of California’s northern half will transition to relatively normal conditions; moderate to severe deficits are forecast for the state’s southern half. Deficits will increase in Oregon, though primarily of moderate intensity. Exceptional deficits will persist in Utah from the northeast though the southwest, and will emerge in greater extent in southwest Colorado, but deficits in surrounding states will moderate, leaving some isolated pockets of intense deficit. Moderate deficits are forecast for Nebraska, eastern North Dakota, Minnesota, northern Wisconsin, and pockets of other Upper Mississippi Basin states. The US Northeast will transition from surplus to normal conditions with some moderate deficits.

From August through October water anomalies are expected to downgrade overall. Moderate to severe deficits are forecast along the Arkansas and Colorado Rivers and in western Colorado and northern Utah, with some isolated pockets of greater intensity. Moderate to severe deficits are also forecast for western Oregon and western Washington. In the East, deficits in South Carolina and southern Georgia will downgrade to primarily moderate intensity though some pockets of extreme deficit will emerge. Conditions of moderate deficit are forecast for Minnesota and Wisconsin. Prior surpluses along the Missouri and Yellowstone Rivers will return to normal conditions. In western Montana surpluses will downgrade to primarily moderate; Idaho and eastern Washington will transition from surplus to relatively normal conditions.

The forecast for the final months – November through January – indicates the emergence of widespread moderate to severe surpluses from Michigan’s Lower Peninsula southwest through Indiana, Illinois, Missouri, eastern Kansas, Oklahoma, and northern Arkansas.

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

IMPACTS
Drought conditions in the Southwest have claimed the lives of over 100 wild horses in a remote region of Arizona. As of mid-May, the entire state was in severe to exceptional drought, according to the US Drought Monitor.

Much of Kansas is experiencing some level of drought as well and conditions have cut the yield potential for hard red winter wheat in the state's southwest and in northwestern Oklahoma to half that of last year, forcing some farmers to consider abandoning their wheat fields. Kansas is the nation's top winter wheat producer but this year's drought in the US breadbasket, along with a global surplus of wheat, has triggered the lowest expectation for America's winter wheat harvest in over a decade. Similarly, roughly 60 percent of this year’s Texas winter wheat is likely to be of unusable quality. West Texas has had less than two inches of rain since last October, down eight inches from the same time period last year.

Warm April temperatures reaching 30 degrees above average caused rapid melting of above-average snow pack in the Rocky Mountains, swelling rivers to emergency levels in eastern Washington and western Montana. High water washed out a culvert running under Interstate 15 between Helena and Butte, forcing the highway’s closure in both directions. In Washingon, the Okanogan River reached its highest flood stage in over 40 years. Evacuations are ongoing as of mid-May, as rivers in some areas are expected to remain above flood stage for days or weeks.

Homes, businesses and roads in northern Maine were inundated with up to eight inches of water along the Fish River chain of lakes, which swelled beyond their banks following heavy rain and snow melt late last month.

A recent study revealed that record ocean heat content of the Gulf of Mexico during the early summer of 2017 contributed to the strength and size of the Category 4 Hurricane Harvey which devastated Houston, Texas, last August.

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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