You have to be a pretty desperate winter weather expert to attempt a monthly forecast when there are so many conflicting signs. The complexity of the pattern heading into February extinguishes any chance of a definitive forecast.
About the best I can do is a wild guess. So here goes: my guess is that February will probably again feature above average temperatures and below average snowfall. Note: the February average snowfall is 4.8, 7.6, and 8 inches for Reagan National, Dulles and BWI airports, respectively.
However, the month will probably have more windows of opportunity to get snow than the previous two months. And it only takes two middling storms to end up exceeding the average.
Role of the Arctic Oscillation (AO)
Each of the last two months, I’ve issued temperature and snowfall outlooks. When the call was made for December, it was made fairly confidently because of the combination of La Nina and, significantly, having a much stronger than normal polar vortex , i.e. the positive phase of the Arctic Oscillation (AO), which keeps the cold air to our north. These were a potent combination that strongly favored above normal temperatures and below normal snowfall for the month.
As I discussed in January’s outlook, strong positive AO events that start in late November or early December, on average, typically continue through that month.
However, by February, the historical records (based on 15 past cases) shows odds of sustaining a positive AO is little better than a coin-flip. Hence, the challenge in determining what temperatures are going to do.
Blocking can force cold air south over the mid-latitudes but only if the blocking is in the right place. Like in real estate, the significance of blocking is dictated by its location. Unfortunately, the location and configuration of any blocking on the model forecasts is still not optimal for snow lovers. Instead, the configuration favors any cold shots being short-lived. Whether that changes or not will probably dictate how February plays out.
Role of La Nina
Another factor to consider is the ongoingLa Nina in the equatorial Pacific Ocean. Below I’ve posted the typical temperature and precipitation anomaly patterns associated with a weak (top panels) and a strong La Nina (bottom panels) based on the Oceanic Nino Index (ONI).
Role of La Nina and the Arctic Oscillation together
If the AO goes further negative and then stays negative, our chances of snow would increase. The European model and its ensemble mean have not been as bullish as the GEFS ensembles on taking the AO negative. Therefore, I lean towards the index staying negative but not strongly so.
For our temperatures to average below normal, the AO usually needs to be strongly negative and I don’t think that is likely. So I’m inclined towards compromising between the two maps (above right) rather than opting whole-heartedly towards the negative AO composite. However, the Northern Plains region is likely to be quite a bit warmer than shown on either of the panels.
Note that only one snowstorm occurred with a positive AO during a La Nina that was as strong as this one. Snowstorms in La Nina years usually come during minor events of an inch or two except in an exceptional year like 1996 when the AO was negative. That year there were multiple events. The bottom line is if you want a significant snowstorm this February, hope for a negative AO. While I think the temperatures probably will average above normal, I’m not as negative about snow as during either of the previous months. Still, the month is still probably more likely to end up with less snow.
The Pacific North American pattern
The dip in the jet steam from a positive PNA pattern has two impacts: one, it delivers cooler air into the mid-Atlantic region; two, it sometimes leads to upper level systems diving to our south initiating the development of low pressure systems that then track up the east coast. The stronger the ridging out West, the more chance of strong digging in the jet stream downstream. Depending on where the frontal zone is located, the storm track can be inland (a rain track) or can be just off the coast (a snow track).
This year, despite the La Nina, more often than not, we have had a positive PNA pattern except during the past several days when it has been negative. Both the European model and GEFS ensembles suggest that the PNA pattern will start the month weakly positive. The associated ridging along the West Coast might deliver some modified cold air into the east. If that happens, it might offer a short window of opportunity for winter weather in our region sometime during the following week. Right now, that looks like a long shot as the Atlantic pattern still leaves a lot to be desired with a configuration that usually means that cold air will be transitory and high pressure will be tough to lock in to our north.
Two other models, the European weekly and CFS2 monthly forecasts continue to advertise a warm month for the CONUS except for Alaska. They provide one of the reasons for not buying completely into the typical weak La Nina and negative AO composites instead tempering them towards a warmer solution.
The CFS2 weekly forecasts suggest that the positive PNA will be temporary and that the ridge will again retrograde later in the month. If that is indeed true, the monthly temperatures are likely to end up warmer than normal even if we get a cold shot or two prior to the pattern switching back to a negative PNA look. However, the same CFS2 model also suggests that during the latter half of the month there could be a period when a block develops over Greenland which would be a plus for snowlovers if such a change occurred as it would also provide a mechanism for keeping cold high pressure to our north. That development is a big if since some of the models have erroneously advertised a similar change earlier this winter.
In summary, I have little confidence in making a definitive call about temperatures or snowfall for February. My wild guess and it is a big one is that the temperatures for February again average above normal. My guess for snowfall is that we should have several chances for getting snow this month as the pattern has improved somewhat over that of the past two months but the pattern still is less than optimal for getting more than nickel and dime type of events. Therefore, I’m guessing that snowfall will again average below normal but with little conviction about the call except that it is as likely to be wrong as right.