What does a winter weather expert do when there is no winter weather to forecast? I issue an outlook for the upcoming month even though most of the skill in a monthly forecast is found in the first week or two.
Making a monthly forecast or guess is always a tricky proposition. Daniel Kahneman in his thought provoking book “Thinking fast and slow” warns against drawing inferences from limited data sets as they can easily bias your conclusions. He also warns against putting much faith in so called experts when the predictability is relatively low.
Monthly forecasts of temperature and snowfall certainly meet the criteria. With much trepidation, I’m going to rely heavily on statistics from a data set that is too small to have a lot of confidence in. Nevertheless, I’m a winter weather expert who has had little to write about so I’m going to make a prediction about January’s temperature and snow prospects. Caveat emptor: such forecasts often are less than stellar.
I am forecasting above average temperatures and below average snowfall, but I do think we’ll see accumulating snow. What follows is a fairly detailed discussion of the pattern and my reasoning for this forecast.
January’s pattern will again most likely be dictated by La Nina which usually pushes the storm track far enough west to keep us on the warm side of most storms (irrespective of the Arctic Oscillation).
If polar vortex (a pool of cold air over the Arctic) stays strong and the Arctic Oscillation (AO) remains positive (keeping the polar vortex in place well to our north) January will most likely be warmer than normal. Remember that when the AO is positive, the cold air tends to remain locked up in the Arctic whereas it spills southward when it’s negative. I describe below why the AO is so likely to remain positive and temperatures warm. The warm temperature are likely to make it tough to reach the 5.6” monthly snowfall average for January.
A third factor to consider is the strength and the placement of the ridge (bulge in the jet stream) that usually is located over the Pacific but sometimes shifts east, strengthens, and directs some Arctic air into the U.S. Most of the global models are now forecasting a cold shot during the first week of January as the ridge shifts east and strengthens. During most La Nina years such eastward shifts in the ridge are usually short-lived. Therefore, despite the early cold shot, I’m guessing the month will average a little warmer than normal and that the snowfall will again be below normal.
A look back December’s positive Arctic oscillation, lack of snow and warm temperatures
Last month’s call for below average snow in December worked out pretty well as we got just a trace of snow. On the other hand, temperatures were even warmer than I envisioned, about 5 degrees above average. A very positive AO combined with a rather flat ridge in the eastern Pacific and western North America allowing warm Pacific origin air to flood across the country. This flow kept most areas east of the Rocky Mountains warmer than normal with little or no snow.
That the outlook worked out well was probably more luck than skill. Washington’s average snowfall during any month is low enough that one storm can easily push the monthly total above the norm, plus making a guess about the state of the AO is pretty risky as it is not very predictable beyond a week or two.
That the AO stayed so consistently positive through the month of December was unusual but not unprecedented. For example, in the winter of 1988-1989 the AO stayed strongly positive through March (see image at the bottom right of the page).
Strongly positive AOs in December typically bring positive AOs in January
Years that have averaged having a strongly positive AO during December have usually had the AO average in positive territory in January. This December’s AO averaged above 1.0 suggesting that this January the AO index may be positive more than negative.
Research by weather enthusiast, Bob Hill, found that since 1950 there have been 11 Decembers in which the AO has averaged above 1.0, and all but the one of those winters (1979-1980) had a positive AO in January. When he broadened the data set by looking at 15 Decembers in which the Arctic Oscillation index was above 0.80, the index averaged being positive during the subsequent January in thirteen of the fifteen years. The current forecast of the AO gives some support to the statistics.
Positive AOs in January typically mean warmer than normal conditions
Bob also constructed a temperature composite of those 15 years (when the AO was positive in December). For snow lovers the composite is troubling as the temperatures averaged about 2.5F above normal. While fifteen years is still a rather small sample size, Bob’s composite (right hand panel) agrees fairly well with look of the monthly January forecast prepared by the Climate Prediction Center (left hand panel) except that the CPC map puts its highest probability for warmer than normal temperatures across the south (the typical location for warmth during La Nina winters).
Bob’s work suggests that in January the AO is more likely to be positive than negative and that the temperatures are more likely to be warmer than normal. However, that does not rule out the potential that a negative AO might develop sometime during the month. Nor does it rule out a few cold or even frigid outbreaks embedded within warmer than normal periods.
What about snow?
A positive AO (and La Nina) in January typically depresses snowfall totals
Recently Jason posted two graphics that displayed how rare above normal snowfall was during La Nina seasons and that above normal snowfall was even more unusual during winters when the AO averaged positive during La Nina winters like we have experienced so far this winter
Don Sutherland, another knowledgeable enthusiast, constructed a graph, shown below, illustrating major snowstorms that produced more than 10 inches of snow for cities according to their ENSO state (whether we were in La Nina, El Nino or neutral states) and whether the AO was positive or negative. Note that there were far fewer major snowstorms during La Nina years (left side of graph) than either neutral (middle of graph) or El Nino (right side of graph) winters. And our odds of getting a major snowstorm without the AO being negative this January would be much lower than normal as historically only one major snowstorm was observed during the positive AO/La Nina combination: January 24-26, 2000.
The average January snowfall during the 15 positive AO Decembers Bob Hill identified was 3.1” compared to the normal of 5.6”. However, the average was only 2.1” for the 13 Januaries that had the AO average positive for the month. Above normal January snowfall was observed at D.C. only three times during those 15 years, while less than 2 inches of snow was observed in eight. Less than one inch was recorded in four of those Januaries.
As I’ve mentioned previously, 2 inch or greater snowstorms are almost 3 times more likely in the Washington-Baltimore area when the AO is strongly negative compared to when it’s positive. Therefore, this January I’ll be watching closely for signs of a shift in the AO and NAO from positive to negative. Such a shift would promise more high latitude blocking over the northern Atlantic across Greenland into the Baffin Bay area which would increase out chances of getting snow. However, based on La Nina climatology and the 15 strongly positive AO years statistics, snowfall probably will average less than normal for the January with the probability of getting less snowfall than normal being around 70%.
Reasons for uncertainty
Despite the statistics cited above, there are reasons for a lot of uncertainty about snowfall even if the AO averages in the positive ranges for the month as models forecast.
Sometimes the timing of various features converge to produce a major snowstorm during a positive AO period like we had in January of 2000.
As additional examples, to the right are three plots of seasons that began with a very positive AO in January after being strongly positive in December. Note how the three differed with respect to how strongly and how long the AO stayed positive.
In 1952, the pattern switched towards the middle of January when blocking developed. Several other years showed the AO staying mostly positive but with negative periods of a week to 10 days interspersed through the winter. There is no way to know which of these scenarios is the most likely.
It only takes one major snowstorm to boost our average to above normal. Even if the AO averages in the positive range, I’ll be watching closely for these possible negative AO periods. These would provide windows of opportunity during January when our probability of getting a snowstorm might be enhanced.
My guess for January is that temperatures average above normal and that we will see accumulating snow but that the snowfall will average below normal. However, I’ll be looking for times when the Pacific ridge amplifies and moves eastward into the western U.S. and when blocking develops at high latitudes (and hence a negative AO). If I see any windows of opportunity for snowfall, I’ll certainly let everyone know.
(Thanks to Bob Hill and Don Sutherland who contributed to this post)