Snow lovers and winter weather  have longed to hear it: It's that time of year again. December 1st marks the first day of meteorological winter in the United States, a day which is recognized only throughout the meteorological community and not on the actual calendar. However, if you think back to winter storms of the past (especially the past several years), you may find that the meteorological calendar can often be more accurate. Remember the mid-2000's when New York City recorded snowstorms in early December (December 5, 7, 9 in various years throughout the mid to late 2000's) with some significant accumulations? Those snowstorms technically fell in Autumn, according to the regular calendar. So while not proclaiming the meteorological calendar is the know all-tell all, its definitely had its moments where it more accurately depicts the season. Pictured right: November 2011 finished as one of the warmer in recent memory, with much of the Central and Eastern United States experiencing above normal temperatures. Data courtesy Regional Climate Center and ACIS.

This year won't be one of them.

November 2011 finished as one of the warmer in recent memory, and it closed out strong in the "warm" department. Just this past week, temperatures soared to record levels two days in a row and nearly three out of four. Newark recorded 72 degree temperatures, Central Park reached 70, and all locations in the 5 boros soared into the mid to upper 60's. But the story of November wasn't just told in this last week with record temperatures, it was written well before that as the pattern became dominated by a large trough/vortex over Alaska. The lack of any high latitude blocking over Greenland or the Davis Straight (West Based -NAO by definition) only added to the problem. The Southeast Ridge flexed its muscles, and most of the Central and Eastern United States experienced above normal temperatures. Although the official numbers are not in, the ACIS monthly climate map posted above supports the dominant 500mb pattern for November (noted left in a RUC analysis from November 26).

For the hard data fans, the argument begins here. There have only been a handful of November's which have followed a similar path as the one we just experienced. 2006, 2001, 1994, 1975, 1948, and 1931 come up in a list of similar Novembers temperature-wise in the New York City area. If we look back at the meteorological winters that followed those Novembers, we see some fairly startling statistics. The average snowfall in all of those winters was roughly 16 inches, which is below average. In addition only one of those listed winters featured above average snowfall , 1948, which was roughly 46 inches. None of the other winters come close to that total--so if you account for the outlier, the average snowfall is actually closer to 10 inches in winters which followed a November similar to this one. Does this mean this winter will definitely play out that way? No, but it means that we're in some pretty rare territory as far as entering winter coming off such a warm November. And previous winters which have entered this territory haven't fared too well.

Rolling forward into the first few weeks of December, the major contributors for the November warmth are going to be changing. The dynamic nature of the pattern is allowing for a fairly transient (we know, that word is becoming overused, but it does apply here) set of features over both the Pacific and Atlantic. The large vortex over Alaska (highly unfavorable for any cold air transport into the Eastern United States) is modeled to be replaced by an equally anomalous ridge. This is fairly rare---to see such a huge change within a week. 500mb height departures will have gone from near -30 to near +30 or greater. In addition, the Polar Vortex is modeled to become more established Central Canada. This means we'll finally have a source of polar and arctic air on our side of the globe (and not over Alaska with direct transport from the pole).

This will allow for more frequent penetrations of cold air from Canada as troughs pass over the Northern 1/3 of the US. However, this doesn't mean that winter weather is on the way. In fact, it means that the pattern remains disjointed. The ridge over the Pacific will allow for these cold air shots (some of them could be quite severe, mainly over the Northern Plains), but they will be rather quick. The culprit for this time period will be the lack of high latitude blocking, with the polar vortex parking over the Davis Straight. In addition, the ridge over the West Coast is modeled to retrograde some with time, meaning the favorable trough axis will remain over the Central US. In periods where the ridging over the West Coast becomes less anomalous, the Southeast Ridge will make its return and above average temperatures will follow. Pictured left are the GFS Ensemble height anomalies, forecast for the first-second week of December.

The real question remains, how will the second half of December behave as far as wintry weather goes? The answer remains uncertain at this point, but it's prudent to lay out the things which we know are unlikely to happen at this time. First, we know it's unlikely that high latitude blocking will return as a major player in the pattern through at least the first three weeks of December. Most model guidance is consistent in developing a piece of the Polar Vortex over the Davis Straight and Western Greenland. This will support some shots of cold air, and the vortex may elongate and move south/southeast at times, but it will ensure that no ridging (west based -NAO) can build into these areas. The NAO may be negative for brief periods by the hard data (+1.0 to -1.0), but it should be east based and moving quickly. Second, we know that the Pacific will be generally in limbo. This doesn't mean the pattern won't be able to sustain itself for a few days, but essentially there isn't much support for the large +PNA ridge to stay fixated for all of December.

Using the MJO can be very useful as we roll forward in forecasting a few weeks in advance. In this case, forecast models and their ensembles have been extremely inconsistent in forecasting the strength and phase of the MJO over the last several weeks. However, a more significant MJO impulse has emerged over the last several days, and the forecast ensembles have adjusted accordingly. This is supported in the latest MJO phase diagram (pictured left). Using the ECMWF plumes (with higher resolution and more members than the GFS), we can see the mean MJO forecast takes us through a moderate impulse in Phase 3-4 and then to a weaker Phase 5...followed by a much weaker impulse by Mid-December. The phase diagram supports a turn away from the more favorable phases 7 and 8 (for winter weather), and a weaker and less impactful mjo impulse by that time. This is not to say that the pattern can't become more favorable by then, but if current forecasts are correct..we won't have much help from the MJO at that time as it will be too weak. Note that this MJO forecast only runs through December 14th, still leaving half of the month unforecast.

Using these MJO phases, we can analyze a historical dataset of surface temperature anomalies for each phase. These charts allow us to analyze how each phase has affected the surface temperature compared to average, historically. Looking back at the phase diagram forecast above once again, we can see that much of December (through the 14th) is spent in Phases 3, 4 and 5. Not surprisingly, the surface temperature anomalies historically match up to the forecast pattern over the next two to three weeks. Additionally, looking back to Phases 2 and 3 (where we have spent the last few weeks) we can see that they match up very well with what was observed in November throughout the country. Pictured to the right are the surface temperature anomalies discussed above, for each phase of the MJO historically during the months of November, December, and January.

The main storm mode, aside from any potential cutoff lows (which could, admittedly, throw a large wrench into this forecast),  looks to fall under the category of "Southwest Flow Event". These events feature shortwaves moving west to east, or southwest to northeast from the Central to Eastern US. The term "Southwest Flow" comes from the southwest flow of air which establishes itself in the mid and sometimes lower levels of the atmosphere during these events. You can see a very recent post on southwest flow events by clicking here and scrolling to the bottom half of the post. These events tend to be most common in the pattern being advertised on the latest models, where some troughiness is modeled in the Southwest United States, and the Southeast Ridge is not completely dead. In this case, the Southeast Ridge combined with the forecast Polar Vortex over Eastern Canada, as well as the troughiness over the Southwest US could allow for a gradient to develop over the Mid-Atlantic and Northeast...potentially as far north as our area. This would serve as a favorable setup for the development of low pressures to our west and northwest--eventually redeveloping to our east-- as shortwaves eject northeast from the Southwest US. Although the cold air to our north (noted with the vortex over Greenland/Davis Straight) wouldn't typically allow for complete torch/heavy rain...the presence of the southeast ridge essentially assures us that the potential for moderate or greater winter weather impacts from these systems would be confined to New England.

Using all of this information, it appears that through December 15th-20th, a sustained winter pattern (with cold air and chances for snow) will not establish itself over the Eastern United States, with the potential exception of Northern New England, and the potential inclusion of Southern New England if any events are timed well with cold air intrusions. During this time period, temperatures could run generally a degree or two above seasonal averages. What happens after mid-December is what will decide how December plays out in the history books--and much of that will be decided by the eventual path of the MJO as well as solar activity. In order to see the pattern revert to a more favorable one for significant cold and snow, we will need changes in both the Atlantic and Pacific. Forecast models are still well out of their useful range towards the end of December--but rolling the MJO forward, we can assume that an eventual weakening of the vortex over Greenland will occur. What happens thereafter will depend on the MJO impulse and other effects on the Pacific--if we lose cooperation from the Pacific again, it will be another case of wash-rinse repeat.

So this year, we can say with a good amount of confidence that meteorological winter will not mark the beginning of snowy and wintry conditions as they sometimes have in the past. That being said, this isn't meant to insinuate that winter is doomed.In this case, for the snow lovers, it will take continued patience as we try and flush out an unfavorable pattern over the next few weeks. When exactly will the pattern change? We don't have an answer at the moment. These things are a gradual process. Sometimes they can suddenly jump to be more favorable, and sometimes they can take several weeks. Other times, they don't happen at all. The atmosphere can be incredibly stubborn. The state of the QBO suggests that we should not be locked in this pattern for the entire winter--and that eventually we should see the return of some high latitude blocking. The great thing about meteorology and weather forecast is that things change each day, moment even, as the atmosphere remains in motion. What we can say for sure is that winter is here for the meteorological community. Forecasting its physical arrival, in the form of bitter cold air and snowflakes flying, will be a challenge for us all over the next few weeks. We're looking forward to it.

Article written by JH. Published December 1st, 2011 at 12:00am. Looking for a forecast? Use our new Forecast Beta tool to see the latest forecast in your specific area for the upcoming week, or view our Technical Forecast Discussion for the more serious weather enthusiasts. Also, check out our new Forecast Overview tab. For up to the minute details on forecasts, watches, warnings, and alerts, follow us on Twitter and Facebook.