Fig. 1: GEFS mean 108 hr fcst hgt anom.

Every year, we go through the same general process beginning around the middle of the autumn season. Whether it was a parting glance of winter-like cold air, an early snowfall, or just hopes for the winter to come...something about this time of year leads many in the meteorological community to begin looking for the first signs of winter. This year, we've got ourselves a bit of a hole to climb out of. We're currently in a very unfavorable pattern across the globe for snow in the Eastern 1/3 of the US. There is a large mean trough anomaly on the west coast (-PNA) and a weakly positive NAO. The EPO is not cooperating, either, so we're in an unfavorable situation as far as cold air supply as well. Over the next few weeks, ensemble forecasts have the MJO traversing through unfavorable phases as well. Over time, however, the pattern will begin to change...beginning with the retrograde of the mean trough on the West Coast. We can try and time these changes using the roll forward method of our indexes and telleconnections.

Although we are currently propagating through Phase 8 of the MJO--this will be transient at best. Forecast ensembles have a weak lower height signal over our area beginning post-frontal passage on Wednesday and Thursday. Essentially, we will see a shot of cold air most likely beginning later this week and into this week. The pattern will then turn warmer. What's driving this? We can see the ensemble mean forecast height anomaly map in the image to the top right (Fig. 1). There is an anomalous trough on the West Coast of the United States -- it is deep and broad in many respects to past -PNA troughs. This is dominating the pattern across the CONUS as all of the cold air available is essentially being dumped into this mean trough. There is also a compact trough to the southeast of Greenland and the blocking ridge has been shifted to the north of that area..the NAO is likely neutral or slightly positive at this time. Essentially, we aren't getting help from the blocking latitudes either. In this ENSO state, this means the SE Ridge will have plenty of room to amplify in this pattern. Simple wavelengths tell us...big trough west---big ridge east. Without the blocking or any PV anomaly over SE Canada to help us, this is simple.

Fig. 2: MJO Forecast & MJO Phase 1 500mb compos.

This anomaly is supported by the MJO state. To the left is an image detailing the MJO forecast on the GEFS means. As a note of caution, we've been using the GEFS means pretty heavily in this forecast. The European essentially has the same idea--so we figured it probably doesn't hurt. The Euro is a bit faster with the progressing through the phases of the MJO, so that's something to be considered. The MJO is forecast by most models to traverse through Phase 8 quickly (as mentioned above) and begin its' trek into Phase 1 and 2. The GEFS had been forecasting a stronger impulse into Phase 1, but at this point it looks rather mundane. When we pull up the composite MJO Phase 1 500mb height anomalies, we can see that it matches up very well with the pattern forecast by the GEFS image above. Notice the southeast ridge anomaly, the toughing on the west coast (more intense this time compared to the historical composites) and the ridging east of Greenland.

Fig. 3: MJO T-Composite For NDJ (CPC, NOAA)

The problem with these averages is that as we roll forward to winter, wavelengths change. The temperature composites for these MJO phases, therefore, can be dramatically different compared to the 500mb anomalies. To the right you can see the MJO Temperature (F) Composites for MJO Phases in NDJ (November, December, January). Notice how Phase 1 can actually average slightly below normal T composites for that time period. However, once we roll forward into Phases 2-6 (with 4-6 being the most dramatic), the temperature departures are steadily above average. This likely suggests the pattern will not buckle to cold and wintry within the next 2+ weeks as the MJO impulse propagates through these stages, in collaboration with a poor EPO and cold air supply in North America.

Fig. 4 : GEFS mean 348hr fcst hgt anom

That being said--there is light at the end of the tunnel as we roll forward even further--probably somewhere between 3-4 weeks. This would land us in the first or second week of December. We can see the first signs of this on the GEFS ensemble mean forecast at 2 weeks advance. Notice the strong signal for positive height anomalies near the Davis Straight and Western Greenland--a very favorably positioned -NAO block. It is not overly strong as modeled on the height field, but the signal for volatility is there at this range with +12 to +18 anomalies at 300+hrs notice. Also, you can see the mean trough (which also is quite anomalous at this range of forecast) has retrograded from the Northwest US to the Gulf of Alaska. This is not a very favorable position, but at the very least would offer some more breathing room for the pattern. The EPO would need to cooperate further to get more significant cold air into Canada--but as depicted the door would be open for some intrusion of a more winter-like airmass, with the potential for some wintry precipitation as well with the blockiness over Greenland buckling back towards Central Canada. I would certainly favor the period around December 15th as a general "calendar area" for the pattern to become more wintry. Patience will be the key, for winter weather lovers, as we watch this pattern evolve.