La Niña conditions have officially developed and may continue through early 2018, potentially influencing the weather conditions we see in the United States this winter, according to an update issued by NOAA on Thursday.
La Niña is the periodic cooling of the equatorial eastern and central Pacific Ocean. When sea-surface temperatures are cooler than average by at least 0.5 degrees Celsius, along with consistent atmospheric indications, a La Niña is considered to be in place.
NOAA said oceanic and atmospheric signals in October and early November are consistent with a weak La Niña. You can see the strip of cooler-than-average sea-surface temperatures near the equator as of early November in the graphic below.
The black box highlights the cooler-than-average sea-surface temperatures near the equator indicating La Niña conditions on Nov. 9, 2017.
There is a 65- to 75-percent chance of weak La Niña conditions continuing through the winter months ahead based on the latest forecast guidance, NOAA said. This would mark the second consecutive winter with weak La Niña conditions as a potential weather influence.
We may already be observing some influence from La Niña on weather conditions in the U.S. this fall with early low-elevation snow and below-average temperatures in the Northwest during October and early November. This is consistent with what is typically observed in that region of the country during La Niña events.
La Niña Is Here – What Does That Mean For Me?
First off, no single La Niña produces the exact same outcome.
La Niña, El Niño or the lack of either, known as the neutral phase, is only one large-scale forcing on the atmosphere. It is not the sole factor in determining whether a season is wet, dry, cold or warm. Other atmospheric influences are in play, including atmospheric blocking.
Nevertheless, there are some general themes to expect in a La Niña winter, according to NOAA:
- Southern U.S.: Above-average temperatures and below-average precipitation.
- Northern U.S.: Below-average temperatures (particularly northern Plains and Northwest) and above-average precipitation.
Winter outlooks issued in October by The Weather Company, an IBM Business, and NOAA both had a strong La Niña flavor in terms of the temperature and precipitation expectations.
Below-average temperatures are most likely in the Northwest and Upper Midwest, while the South is forecast to be warmer than average.
Winter Temperature Outlook
The red contour in the South corresponds to higher probabilities of above-average temperatures. The darker blue areas in the Northwest and Upper Midwest have the highest odds of below-average temperatures. The light blue and orange contours show where temperatures may be slightly below or slightly above average, respectively. (The Weather Company, an IBM Business)
Portions of the Ohio Valley, Great Lakes and northern Rockies have the highest odds of above-average precipitation this winter. Depending on temperatures at any given time this winter, we could see increased odds of snow in those regions.
Meanwhile, the South could be in for a dry winter, which is typical during La Niña. If this pans out as forecast, there is the possibility of growing drought conditions in parts of the Southeast, and it may also give a boost to wildfire danger in this region next spring.
Winter Precipitation Outlook
Dark green shaded areas in the Great Lakes, Ohio Valley and the northern Rockies have the greatest odds for above-average precipitation this winter. Locations highlighted in dark brown across the South have the highest probability of drier-than-average conditions. Areas not shaded have an equal chance of seeing precipitation below, above or near average. (NOAA)
During last winter's weak La Niña, the West and Upper Midwest had one of the wettest winters on record, while a large swath of the East, South and Midwest had one of the warmest winters since record-keeping began.
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