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2018–19 North American winter

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Through El Niño–Southern Oscillation

While there is no well-agreed-upon date used to indicate the start of winter in the Northern Hemisphere, there are two definitions of winter which may be used. Based on the astronomical definition, winter begins at the winter solstice, which in 2018 occurred on December 21, and ends at the March equinox, which in 2019 occurred on March 20. Based on the meteorological definition, the first day of winter is December 1 and the last day February 28. Both definitions involve a period of approximately three months, with some variability. Winter is often defined by meteorologists to be the three calendar months with the lowest average temperatures. Since both definitions span the calendar year, it is possible to have a winter storm in two different years.

Seasonal forecasts

On October 18, 2018, the National Oceanic and Atmospheric Administration's Climate Prediction Center released its U.S. Winter Outlook. The outlook noted a 70 to 75% chance of El Niño developing. CPC Deputy Director Mike Halpert specified that development was expected to occur by late fall to early winter. He added that while the El Niño was expected to be weak, it still had the potential to bring drier conditions to the northern United States and wetter conditions to the southern U.S. The outlook also noted the potential for the Arctic oscillation to bring colder-than-average conditions to the eastern U.S. and the possibility of the Madden–Julian oscillation contributing to heavy-precipitation events along the West Coast. The temperature outlook favored warmer-than-normal conditions across the northern and western U.S. with the highest probabilities from the Pacific Northwest to the northern Plains and in Alaska. Such conditions were also favored in Hawaii. The outlook also noted that the Tennessee and Ohio Valleys, the Mid-Atlantic region, and the U.S. Southeast had equal chances of either above-, below-, or near-average temperatures. The outlook did not delineate any areas likely to experience below-average conditions. The precipitation outlook noted an elevated probability of wetter-than-average conditions across the southern tier of the United States and along the eastern U.S. up to the Mid-Atlantic. Drier conditions were favored in parts of the northern Rockies and northern Plains, northern Ohio Valley, and Great Lakes regions. The drought outlook mentioned a high likelihood for drought conditions to persist across parts of the southwestern U.S., southern California, the central Great Basin, the central Rockies, the northern Plains, and parts of the interior Pacific Northwest. Drought conditions were favored to improve in the central Plains, the coastal Pacific Northwest, southern portions of Colorado and Utah, and in various areas in both Arizona and New Mexico.

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