UNITED STATES: SEVERE WATER DEFICITS FORECAST TO PERSIST IN THE SOUTHWEST, SURPLUSES IN IDAHO

26 April 2018

THE BIG PICTURE
The 12-month forecast indicates significant 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 Utah, Arizona, and Colorado, with deficits of varying severity blanketing Oregon, California, Nevada, and New Mexico. Primarily moderate deficits are forecast for Kansas, Oklahoma, and Texas, though conditions may be severe from the Texas Panhandle into southwestern Oklahoma. In the East, deficits will be severe in the Carolinas and in Georgia’s southern half. Some primarily moderate deficits are also forecast for northeastern North Dakota, northern Minnesota, and Maine.

Surpluses are forecast along the Missouri and Yellowstone Rivers, and in Idaho, Montana, and northeastern Nebraska into southeastern South Dakota, including exceptional surpluses in northern Idaho and western Montana. Primarily moderate surpluses are expected in southern Minnesota, Iowa, southern Michigan, and in the Ohio River Valley, along with conditions of both deficit and surplus in the Valley (pink/purple) as transitions occur.

Outside the contiguous US, surpluses are forecast for northwestern Alaska and the northern half of the Alaskan Peninsula trailing into the interior; deficits are forecast for the eastern half of the state. Intense surpluses are forecast for Hawaii, and moderate 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 – April through June – indicates that deficits will diminish and downgrade in the South Atlantic States, retreat considerably in the Gulf States, downgrade to moderate in the Southern Plains States, persist with intensity in the Southwest, and shrink in central California. Exceptional surpluses are forecast for Idaho, western Montana, and western Wyoming, and surpluses nearly as intense are expected along the Missouri and Yellowstone Rivers. Moderate to extreme surpluses will increase in northeastern Nebraska and emerge in southeastern South Dakota. Iowa, Missouri, and western Illinois will transition from deficit to moderate surplus. Surpluses in the Ohio River Valley will shrink and downgrade to moderate. Moderate surpluses will emerge in southern Michigan.

From July through September water anomalies are expected to downgrade overall. However, severe to extreme deficits are forecast to emerge along the Arkansas and Colorado Rivers, and moderate to occasionally severe deficits will emerge throughout California and the Pacific Northwest. Moderate to extreme deficits will persist in the Carolinas and Georgia. Conditions in the Ohio River Valley will transition from surplus to near-normal. Surpluses will persist in southern Michigan, Iowa, northeastern Nebraska, southeastern South Dakota, and western Montana. Some areas of moderate surplus will emerge in northern Maine and southern Florida.

The forecast for the final months – October through December – indicates a pattern of anomalies similar to the prior three months though deficits will be mild. Moderate surplus is forecast along the Mississippi River.

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

IMPACTS
A feud has erupted between states served by waters of the Colorado River. Four US states - Colorado, New Mexico, Utah, and Wyoming - have accused Arizona's largest water provider of manipulating supplies and exceeding its water allotment while other states work to conserve and reduce consumption. A prolonged drought as well as climate change is reducing the flow of the Colorado while population growth in the regions it serves is expected to increase water demand in the coming years. The accusation involves purported manipulation of water levels in Lake Mead reservoir which supplies water to Arizona, California, and Nevada. The river provides water to 40 million people in the US and Mexico.

Record rainfall on Hawaii’s island of Kauai caused major flooding and a series of landslides in mid-April. Exceeding all previous records, 685 mm (27 in) of rain fell in a 24-hour period. The US Coast Guard assisted with evacuations by helicopter as some regions were cut off by landslides blocking highways.

Snowpacks in New Mexico’s mountains are substantially below normal levels as spring arrives. The Rio Chama Basin is in the best condition at 45 percent of normal, while the Jemez Basin at 16 percent and the Pecos at 3 percent are only marginally better than the Gila and Rio Hondo at zero. With those numbers, 99 percent of New Mexico is in some stage of drought and there's little prospect of relief from spring snow melt.

Citizens of Louisiana’s Isle de Jean Charles are about to become the first Americans to experience climate resettlement. An island that once was 22,000 acres in size has now been reduced to 320 acres due to land subsidence (sinking), erosion, and sea-level rise. A site for a new town has been identified an hour’s drive north.

Over the past 500 years, Mississippi River flooding has gotten worse due to too much engineering, asserts a recent study by a Northeastern University researcher. There has been an increase in the frequency and severity of floods along the river amounting to a 20 percent increase in risk from flooding. According to the research, only a small fraction of that increase can be attributed to climate change, while the bulk is the result of human engineering for navigation and flood control.

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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