Ag Weather Forum

Lower-than-normal sea surface temperatures in the tropical Pacific Ocean have been in place for the last two months, but it is yet to become cold enough for long enough to classify as a La Nina.

According to NOAA, that happens when the average sea-surface temperatures reach or fall below minus 0.5 degrees Celsius in a small box bound by five degrees latitude north and south of the Equator between 120W and 165W longitude over the course of five consecutive three-month seasons. Atmospheric conditions also need to be met, but most meteorologists use sea surface temperatures as a proxy.

However, given this definition, you cannot be certain you are in a La Nina (or El Nino, the inverse of La Nina) until conditions have been present for a long period of time. Therefore, the strict definition is usually set aside to classify past events rather than focus on current and forecast events. Many times, forecasters will use a string of recent weekly temperatures and the forecast to approximate the overall impact of the Pacific Ocean on global weather patterns. Other considerations from ocean temperatures and currents can be used as well.

When all that is considered, meteorologists can effectively classify whether or not the atmosphere is in a La Nina state and affecting global weather patterns.

From DTN's perspective, the Pacific appears to be on the cusp of La Nina conditions. Ocean temperatures have been hovering around the minus 0.5-degree Celsius mark for the last six weeks, and the forecast moving forward is to stay near or below that mark for at least the next six weeks as well. The atmosphere has been starting to respond to those colder conditions, though that has not come with any significant force just yet.

The cooler waters in the Pacific change the atmospheric pattern, but usually have their greatest influence during the Northern Hemisphere winter. In that time frame, there is typically a split in the jet stream coming off of the Pacific Ocean. A portion of it splits north into Alaska, gathering up cold air from the Arctic, then depositing it into the middle of North America, leading to colder conditions for central Canada and the North-Central U.S.

The second piece of the jet stream usually slams into the Pacific Northwest with increased risks of heavy precipitation events. The two jet streams sometimes combine over the middle of the U.S. to create a storm track through the Ohio Valley, which increases precipitation there. But the southern tier of the U.S. is usually much warmer and drier than normal, which can lead to drought development and intensification.

We will be looking for these sorts of events to occur this winter.

Ocean temperatures in the tropical Pacific Ocean are not the only driver of weather across the globe, though they account for the largest portion of the effects. The effects of La Nina are not consistent from year to year either, but these are the general trends. Every event will be different from the one before based on where the coldest pockets of water sit, when that cold water develops, how long it lasts, and what the temperatures are in the rest of the world's oceans.

To find more weather conditions and your local forecast for free from DTN, head over to https://www.dtnpf.com/….

John Baranick can be reached at john.baranick@dtn.com

(c) Copyright 2025 DTN, LLC. All rights reserved.