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Posted at 08:30 PM ET, 01/26/2012

February outlook: Another warm month with less snow than normal? That’s my guess

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.

TECHNICAL DISCUSSION

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.


Strength of polar vortex (top panel) and Arctic Oscillation (AO) index (bottom panel) over time. Blue indicates strong polar vortex and positive AO. Red indicates weak polar vortex and negative AO.
The strong polar vortex event that kept most of December and much of January on the warm side has indeed weakened. And thus, the positive AO regime has at least temporarily switched to negative. All the blue colors on the chart to the right are indicative of a colder, stronger than normal polar vortex snd positive AO. Note that by around mid-January (right side of figure) all the blues had switched to red indicating that the vortex has become warmer and weaker than normal. A weaker vortex usually makes it easier for blocking to develop.

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).


Winter precipitation (left) and temperature (right) anomalies in a strong La Nina (bottom) and weak La Nina (top)
Note that for a weak La Nina, our temperatures tend to average near normal while during stronger La Nina winters our temperatures usually average above normal. The ONI index for the three months ending in December was -0.8 and looks to be running around -0.9 for January, essentially right at the border between the two classifications. A compromise between the two would suggest that temperatures might average a little above normal for the month based on La Nina climatology alone.

Role of La Nina and the Arctic Oscillation together


La Nina temperature anomlies (blue indicates colder than normal, orange indicates warmer than normal) during different phases of the Arctic Oscillation.
La Nina acts differently depending on what is happening across the polar regions. Usually, if the AO is strongly negative during a La Nina, temperatures across our region stay close to normal (see figure to right). Sometimes if the AO is negative enough, like last year, our temperatures can even average below normal. However, if the AO stays neutral or hovers in weakly negative territory, the temperature pattern across the U.S. is much warmer.

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.


Scatterplot showing four inch snow storms for El Nino and La Nina and positive and negative phase of the AO.
La Nina winters do not favor major snow storms, even 4 inch storms are tough to achieve. To the right, I show a scatter diagram showing the ONI index (an indicator of El Nino, positive, and La Nina, negative) and AO for all 4 inch or greater storms. All the dots below the horizontal blue line are storms that occurred during La Nina winters. Neutral and El Nino years are more likely to produce a 4 inch or greater storm than a La Nina year. The dots to the left of the vertical blue line fell when the AO was negative while those that occurred when it was positive are on the right side of that line.

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


Height (or pressure) anomalies at 18,000 feet (500 mb) during a positive PNA pattern.
The other important feature that is likely to play a role in determining our weather in February is the location of the Pacific North American pattern ridge. To the right is a map showing the correlation between the 500 mb heights and the PNA index. Note that a positive index has above normal heights (warm colors) or a northward bulge in the heights and jet stream across the western U.S. into Canada. That ridging forces a trough (dip in the jet stream) across the eastern U.S. where you have below normal heights (cool colors) especially across the Southeast.

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).


Forecast generally favor a positive PNA pattern into early February (NOAA)
During La Nina years, the PNA pattern discussed above is typically reversed, or negative, leading to a ridge in the Southeast which forces most low pressure systems to track to our north and west leaving us on the warm side of the storm and rain. That’s the primary reason for the lack of a 4 inch or greater storms during such seasons.

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.

Additional considerations

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.


The CFS2 monthly forecast indicates above average temperatures over the entire Lower 48 during February
The CFS2 monthly forecast (to the right) is the big fly in the ointment. For our region it has done a credible job in forecasting the monthly temperatures over the last two months. The model argues for keeping February temperatures warmer than normal. However, its weekly forecasts suggest that there may be shots of cold air and that the location of blocking could become the most favorable for snow towards mid-month than it has been all season so for snow lovers the month may not be a complete disaster.

Summary thoughts

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.

By  |  08:30 PM ET, 01/26/2012

Categories:  Latest, Winter Storms

 
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