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La Nina, Part II: Northern U.S. set for wet, cold winter

Tim Hearden WFP-hearden-winter-outlook-120421-2.jpg
Interstate 5 passes over Northern California’s Shasta Lake, the federal Central Valley Project’s largest reservoir. Fog and steady rain blanket the lake, which was at 36% of capacity on Oct. 21.
NOAA’s seasonal outlook predicts drought will persist as Southwest, Southeast remain warm and dry.

La Nina oceanic and atmospheric conditions are back for a second straight winter, promising colder, wetter storms in the northern U.S. while leaving the drought-stricken Southwest warm and dry.

From December through February, storms are expected to trek through western Alaska, the Pacific Northwest and northern Rockies into the Great Lakes and Ohio Valley, while above-average temperatures are anticipated throughout the South and much of the East, according to the National Oceanic and Atmospheric Administration’s 2021 Winter Outlook.

The pattern should improve drought conditions in the Northwest and Northern Plains but won’t do much for the Southwest, where the Colorado River’s Lake Mead and Lake Powell are about one-third full.

“Our main region of concern this winter remains the Southwest,” said Jon Gottschalck, chief of the federal Climate Prediction Center’s operational prediction branch. “We do expect drought to persist in many areas of the Southwest and southern Mountain West. The monsoon season was stronger this year, but improvements were not enough to remove drought in the Southwest.”

Forecasters expect drought to develop in large swaths of Texas, Oklahoma and Kansas as well as the Carolinas, coastal Georgia and the Florida peninsula, Gottschalck told reporters in a webinar on Oct. 21.

Cold storms in north

La Nina is the colder counterpart of El Nino, and both are driven by sea surface temperatures in the equatorial Pacific Ocean that determine where the storm track sets up. With a La Nina, colder ocean temperatures push storms to the north. It’s not uncommon to have La Nina conditions in back-to-back winters.

This year, the La Nina appears to be “a little bit more established earlier than last year,” with more water and a “colder signal” that favors below-normal temperatures a little further south than in 2020-21, Gottschalck said.

Conversely, warmer-than-average conditions are most likely across the southern tier of the U.S. and much of the eastern U.S., with the greatest likelihood of above-average temperatures in the Southeast, according to the National Weather Service. The Upper Mississippi Valley and small areas of the Great Lakes have equal chances for below-, near or above-average temperatures, the weather service predicts.

NOAA explains its seasonal outlooks provide the likelihood that temperatures and precipitation will be above-, near- or below-average, and how drought conditions are anticipated to change. They don’t try to predict snow accumulation, which can vary with individual storms.

West coast drenched

The Winter Outlook comes as a series of storms are pounding the West Coast. A powerful bomb cyclone developed in the northern Pacific on Wednesday night, Oct. 20, providing a catalyst for an expected parade of storms that will unleash nearly 2 feet of rain in some areas and several feet of snow in the mountains by the middle of next week, according to Yahoo News. Gale, high-wind and flash flood warnings were in effect Thursday from Northern California through Washington, the news service reported.

The storms should help water levels in Northern California’s Shasta Lake, the centerpiece of the federal Central Valley Project, and Lake Oroville, the State Water Project’s chief reservoir. The reservoirs were at 36% and 37% of capacity, respectively, on Thursday, according to the California Data Exchange Center.

But California’s longer-range prospects will depend on the strength of the La Nina, which now is considered moderate. A lackluster La Nina could perpetuate a drought that has already depleted the state’s surface and groundwater supplies, creating acute challenges for farmers and ranchers, the California Farm Bureau Federation notes. Climatologists say the state needs 140% of average annual precipitation to produce normal water runoff.

“The state of California is often in the dividing line,” Gottschalck said. “La Nina is often more impactful in the north, with warmer and drier weather in Southern California. But there’s always a level of uncertainty.”

Forecasters are most confident that drought conditions will improve in the Pacific Northwest, said Brad Pugh, the CPC’s operational drought lead.

“The next two weeks should be quite wet, with 5 to 10 inches of precipitation in some areas,” Pugh said. “We’re going into the wettest time of the year – November, December and January. It should help significantly in the Pacific Northwest and Northern California.”

TAGS: Water
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