La Nina is starting to fade, the Climate Prediction Center says. But then, it usually does as we move from winter toward summer. The big news with regard to La Nina — the intermittently recurring cooling of equatorial Pacific sea surface temperatures, displayed as the large patch of blue in the bottom graphic (sea surface anomalies) in this chart – is that forecast guidance indicates it will disappear entirely by June. That’s not shocking, either, but there have been a couple of instances in whichLa Nina has extended or recycled into an encore season — as it did in 1954-55/55-56 and in 1998-99/99-2000.
Meteorological spring consists of March, April and May, and we’re now about halfway through March that is, so far, running very close to normal in temperature and quite a bit above normal (half an inch) in rainfall. But I took a look at what spring held following 10 previous winters in which La Nina was either rated moderate or strong — this one appears to have peaked at 1.4 degrees Celsius below normal over 3 consecutive 3-month cycles, at the upper end of “moderate” literally a click below “strong.” I used rounded-t0-whole-number figures for quick calculation, but that should be good enough to give a thumbnail sketch on what is historically expected in our region (using Roanoke Regional Airport data) during what we call a La Nina spring.
* La Nina springs averaged at least 2 degrees above normal 3 out of 10 times and near normal or 1 degree above normal 5 out of 10 times. That left only two seasons – in 1971 and in 1989 — when temperatures averaged significantly below normal, about 2 degrees, over the 3-month spring period.
* La Nina springs were near normal or below normal in rainfall 9 out of 10 times. Only 1971 was otherwise, and it was only about an inch above normal (see next bullet point). Of the nine, five were 2 inches or more below normal.
* While La Nina springs leaned to the dry side, 6 out of 10 of them had at least one month that was an inch or more above normal. So rainy spells, like we’ve had in early March, are not unusual. The wettest was May 1971 that had 7 1/2 inches of rain, putting it about 4 inches above normal — barely offsetting two preceding dry months. So you could make the case that none of the 10 springs following a moderate or strong La Nina were wet seasons.
* Half of the La Nina springs — those in 1955, 1965, 1974, 1976 and 2000 – had at least one month that was at least 4 degrees above normal. Two of them — in 1955 and 2000 – had at least two months 4 degrees or more above normal. Not coincidentaly, those were the warmest among the strong/moderate La Nina springs, each averaging about 3 degrees above normal.
* Sharply cold months were few and far between in the 10 seasons studied. March 1965 was the coldest, about 5 degrees below normal (later offset by a May that was 5 degrees above normal), while March 1971 and March 1999 each were about 3 degrees below normal.
* Of the 30 months in the 10 moderate/strong La Nina springs, 16 were at least a degree above normal, 4 very near normal and 10 at least a degree below normal.
* Of the 30 months, 7 were at least 1 inch above normal in rainfall, 4 were very near normal, and 19 were at least an inch below normal.
* Perhaps most surprising — there has been at least some measurable snow in APRIL in 4 of the 10 seasons, which is definitely statistically above the curve. The most was 7.3 inches that fell on April 6-7, 1971 (’71 keeps showing up as the weirdo La Nina spring), apparently a very wet snow as the temperature did not get below 33 at Roanoke. (Blacksburg got 11 inches and Lynchburg 4 in the same event). This is Roanoke’s biggest April snowfall on record.
So, the overall conclusion is pretty much what we always say — a moderate to strong La Nina leans our region toward warmer than normal and drier than normal weather, with some short-term variability.