December thus far has been relatively mild across much of the Mid-west, Great Lakes, and Northeast, with temperature departures generally +1 to +3 or greater. As was outlined in my prior post last week, the first half of December would see a back and forth temperature regime on the East coast w/ no pattern actually locking in. That will be changing quite drastically as I'll explain in a moment. We've also seen a couple big rain producers along the I-95 corridor over the past couple weeks, although today's system was a bit more wintry than progged for NW sections of the tristate area. Snow accumulating anywhere from 2-6" in NW NJ and SW NY state transitioned to sleet/freezing rain, then finally liquid precipitation -- a strong low level inversion kept temps close to the freezing mark all day in those locations. The snow threat last weekend came to fruition for interior NJ, where a few inches were recorded on average, while most coastal cities, PHL, TTN, NYC, BOS, had an inch or less. Moving forward - we're currently experiencing a major pattern shift, in terms of both global indices and tropical forcing. I mentioned last time that the reluctance of the MJO to propagate past phases 5-6 would be an inhibitor to any sustained trough on the east coast - that has been the case, but we're now seeing a much stronger wave developing in the western tropical pacific, which will likely progress rapidly through phases 5,6,7 and into 8 by the time we reach December 18th-20th. This is good news for us as it means tropical forcing will soon support lower height anomalies in the Northeast US as we head toward days 7-10 (late next week) through the end of December.

Notice the projections for the MJO in this graphic -- we're in the "circle of death" right now (no phase), but with the recent developing of a stronger tropical wave, we should reach phases 7-8 of the MJO within the next 7-10 days. Thus I see this upcoming week as a transition period in terms of our sensible weather and the global regime. Temperatures will be well below average the next few days via the continental polar airmass behind this storm, but we'll see a rebound/moderation to near normal temps much of next week. As the MJO propagates into the latter stages of phase 7 and into 8, we'll see the large scale trough gradually amplify in the Eastern US. Below is the 500mb composite for phase 8 MJO in December; note the increasing negative height anomaly over the eastern half of the nation.

As this is occurring, the AO will be in the process of tanking negative. In fact, latest GFS ensembles indicate the positive anomaly over the north pole will be so significant that the AO will go off the charts negative! The NAO will also be heading into the negative modality as a west-based north atlantic block orients itself around Greenland. Key words there being "west based" - this is something we have not heard much over the past several winters. West based, or classic Greenland blocks, are much more conducive to troughiness and consequently snowy patterns on the east coast. This particular block should develop and come to fruition within the next 10 days. Heights will also be rising along the west coast of the United States, suggesting a transition positive in the PNA modality. All these reversals I'm referring to in terms of global indices are good news for us (that is, if you are a fan of colder than normal weather): +PNA, -NAO, -AO, -EPO. To be quite honest, the potential pattern for the last third of December is one that really can't get much better from a winter lover's standpoint. The primary wildcard is the sub tropical jet; however, with this El Nino well into moderate intensity / borderline strong, southern stream should not be a huge concern, especially as we head deeper into the winter.

To highlight the key points over the next few weeks:

• Transitional period the next 7 days with global regime changes. AO tanks in conjunction with the development of a potential west based negative NAO.
• Colder than normal temps the next few days will give way to a moderating temp pattern next week, to near normal values.
• As we head beyond December 16th-18th, the progression of MJO into phases 7-8 will allow the large scale trough to shift into the Eastern US.
• The period December 18th through the end of the month is shaping up to be a possibly exciting one in terms of widespread below normal temperatures and snow threats.
• I'll leave you with this D8-10 500MB composite for both the ECMWF and GFS. Note the remarkable Greenland block progged.