A persistent Pacific pattern is back again, putting us in line for a rare triple La Niña this winter. But with the past two winters playing out very differently in North Carolina, what can we expect this year?
In our eleventh annual winter outlook, we will check current ENSO conditionrarity of a Third La Niña in a rowhow our weather may look how has this winter been and recently snow events have formed statewide.
The recent development of ENSO
Since first appearing in late summer 2020, the La Niña pattern has stabilized considerably. Sea surface temperatures over the equatorial Pacific Ocean have been at least half a degree Celsius lower than normal for 24 months of the past 26 months, according to the agency’s assessment. Ocean Nino Indexusing a three-month running average.
Even this summer, when ENSO conditions typically decline or transition between periods, there is no doubt that La Niña is taking place, as sea surface temperatures are still at least zero below zero. 8°C above normal.
It is easy to spot the current La Niña pattern on maps of sea surface temperature anomalies, as a cooler body of water extending westward from the coast of South America.
Its atmospheric signature is also present, with long wave radiation comes out above normal over the western Pacific – an indication of a decrease in cloud cover related to the high pressure over that area – and strengthen the east trade wind along the equator.
All of which tells us that La Niña has matured and solidified into the winter, which raises confidence that it will affect our weather for at least part of the season. Usually, that means warmer and drier conditions over much of the Southeastern United States as the jet streams weaken and retreat northward. But as we have seen, that is not the case every year.
While our current La Niña model starts with winter 2020-21, you’ll never know it by our weather in North Carolina. That winter ranks as ours 12nd-wettest on profileand it is characterized by an active hurricane path from the Gulf of Mexico through the Carolinas – a more typical pattern of El Nino.
Last winter brought More typical La Niña impactranked as our 10th warmest and 45th driest place since 1895. However, the large-scale pattern changed in January, with cooler and wetter weather in North Carolina, including including some snowfall.
Each of these is a moderate La Niña, with an Oceanic Nino Index in winter of 1.0°C below normal. Climate Prediction Center consolidated forecast are predicting an identical one-degree anomaly this winter.
However, there are reasons to believe that this winter’s La Niña may not be as strong. Like NOAA’s Latest ENSO Blog Post Note that the Pacific trade winds have weakened for about a week and seasonal forecast models are showing a the decline of this La Niña through the winter, with the possibility of a return to ENSO neutral conditions in the spring. That would also be in line with a typical ENSO development, which typically sees peak intensity in the October-December timeframe.
NOAA’s Emily Becker correctly noted that “La Niña and its effects on rain, snow and temperature will most likely continue throughout winter, regardless of the exact timing of the anomaly occurrence. usually Niño-3.4 minimum.” But if recent trends continue, it could at least make this La Niña the weakest of our recent trilogy.
Three times in a row?!
While it is not uncommon for La Niñas to dip twice In previous years, it was rare to see them for three consecutive winters. Since 1950, it has only happened two other times: from 1974 to 1976, and from 1999 to 2001.
With such a small sample size, it’s hard to tell if the third La Nina in the line tends to share any common characteristics. However, some evidence suggests that when La Niña lasts longer than a year, its effects tend to be more pronounced, especially our winter rainfall.
That’s especially true during the last three La Niña episodes. Drought covers North Carolina began in late 1998, when La Niña first appeared, and it persisted through the summer of 2002 in the hardest hit areas including western Piedmont.
The cumulative impact of those three La Niñas, along with a number of unusually local hurricane seasons, is staggering. In the four years from 1999 to 2002, Hickory received 61.65 inches of rainfall – which is more than a year of rainfall – below its normal rainfall.
Fortunately, we do not see such deficits increase during the present long-lived La Niña period. Over the past 2 years, Wilmington has been about 16 inches shorter than normal, and most of Piedmont has been roughly equal or slightly above normally.
But that does not mean that drought is not a concern at the moment. Especially after our dry Octobermuch of the state was already in an unusually dry or drought-prone state, and even after Nicole, seasonal rainfall deficits remain in some areas. That means the drought and its evolution will continue to be a storyline in the coming months.
This Winter in North Carolina
Given our history with La Niña events, it’s reasonable to expect Overall conditions are warmer and drier this winter. NOAA’s Winter Outlook which suggests an increased chance for above-normal temperatures and below-normal rainfall over much of North Carolina.
Of the 25 La Niña winters since 1950, 18 have been warmer than the 20th-century average in North Carolina, including seven of the past eight. Of those 25 La Niña winters, 19 were drier than usual.
However, that doesn’t mean the whole season will be warm and dry. ENSO’s grip on jet streams tends to loosen at some point each winter and models closer to home – including cold air sneaking in from Canada and tracking storms along our coastline – could offer some diversity. While La Niña winter tends to less favorable for winter eventsNor can we completely rule them out, because january ago showed.
Of our past eight La Niña winters, snow has fallen as early as November 21 (2008) and as late as March 21 (2018), but snowfall is most common in January. In Raleigh, 10 of the last 21 measurable snow days in the past 8 La Niña winters occurred in January. That makes it an important month to watch this winter, both in terms of snow potential and drought resilience.
Although generally drier than usual, Drought conditions faded last winter, from more than half of the states classified as Severe Drought (D2) in early December to just 9% of states classified as Moderate Drought (D1) at the end of February. The improvement we get is reduced evaporation and cool season water demand, and timely moisture replenishment from the slow-soaking rain events in January.
A less favorable scenario would be an unrelenting La Niña pattern like the one in 2010-11, which saw three consecutive months of drought statewide and generally. drought recessionincluding unusually early fire activity during long windy days in mid-February.
Even if it is not accompanied by worsening droughts or wildfires, these early start always something to watch for. During the past 11 La Niña winters, February has been warmer than the 20th century average, including record warmth in February 2018 including a temperature of 80 degrees in the middle of the month.
Snow drought update
At this time last year, areas south of Piedmont had gone almost four years without seeing even an inch of snow on the ground. We recorded the “snow drought” in a blog post previous November.
Just two months later, we it snows in January almost every week There was a little something for most people in the state. As the map below shows, less than a year has passed since the last 1-inch snowfall in most areas, except for the far south and central coasts, where there is more ice than snow in winter. last year.
Last winter, thicker snow was harder to come by, but much of the Ranges and far northeast corner of the state picked up more than 6 inches in one of our winter storms.
In Elizabeth City, it was only the second 6-inch blizzard in the past 20 years, with a 7-inch accumulation in end of January 2014.
For most states, several decades have passed since the last 12-inch snow event. Much of eastern North Carolina last saw a footstep on the ground in the 1980s, and parts of the Triad including downtown Greensboro had to head back further, to early March 1969.
Several western locations saw such total mass during the mid-January storm earlier this year. Brevard had a total of 12 inches in the two days above January 16-17it was the first snow there since December 9-10, 2018.
Snow is never a certainty in North Carolina, especially during La Niña winter. But as January of last year showed, opportunities can still open up.
With a shift in patterns and enough offshore cool air and humidity converging at the right times, heavy snow events are still possible, even in a generally warm and dry winter.
