It's been a wild ride so far this winter, record low Arctic Oscillation (AO) values have been reached the past few weeks due to the massive polar blocking, a historic east coast storm in the second half of December, the coldest December nationwide since 2000, and the coldest opening to January across North America, Europe, and East Asia in over two decades. This is all due in large part to the negative QBO which set the ball rolling for a major stratospheric warming event in mid November - this propagated down into the troposphere causing heights to rise drastically over the pole, thus a tanking of the AO and NAO into December and early January. When coupled with our current strong el nino event, we've seen plenty of snow as well.


Normally these extreme negative AO cycles last about 30-45 days in duration, and this one will be no different. The sensible weather effects from the stratospheric warming were realized about 2 weeks later, by the beginning of December, and now we're seeing a sharp spike toward neutral in both AO and NAO values. This coupled with the shift eastward ofthe Aleutian low into a classic Gulf of Alaska vortex position, will yield a positive EPO/negative PNA, and thus troughiness in the PAC NW states. Meanwhile the rest of the nation will be flooded with Pacific air as a zonal flow dominates the pattern, eliminating any polar/arctic airmass influence from Canada. Essentially we're progressing into a mid winter thaw period or about 1-2 weeks in duration -- fairly common for cold/snowy winters actually. There is potential for a wintry event later in the weekend, as we approach MLK day; however, global oscillations will be rapidly becoming less conducive for snowfall on the east coast. Greenland blocking will diminish along with the Alaskan block; MJO phases 4-5 promote large scale ridging in the Eastern US w/positive height anomalies. Below is the January 500mb composite for phase 5 of the MJO.

Note on the following 8-10 day ECMWF and GFS 500mb depiction, we see a strong +EPO vortex emerging with very low heights near the Pacific NW coast, drastic lowering of heights in the arctic, and a large expanse of higher than normal heights across Canada. Thus, our arctic connection will be cut-off for the time being, as the 0c isotherm at 850mb is progged to retreat northward to near the US-Canadian border at the climatologically coldest week of the year.

How long will this pattern relaxation persist? Well, we're already seeing signs of the AO returning to a more negative modality by the end of January. In addition, typically in moderate to strong el nino seasons, our coldest/snowiest month relative to normal tends to be February. The persistent negative QBO will aid in keeping the stratosphere relatively warm, which further supports the idea of a continued negative AO regime.

Notice how the AO approaches neutral values around January before plummeting once again late month. Tropical forcing - MJO seems to be in no rush to propagate eastward beyond phase 5 over the next week to 10 days. Therefore, I believe this "thaw" will be in no hurry to revert to a colder pattern prior to the end of January. In fact, it's more likely that the pattern reload does not fully realize itself until the beginning of February. Until that time, interior Northeastern locations will be most favored for snow in a normal to warmer than normal temp regime. With that being said, this is the coldest time of year on average, so wintry precip cannot be ruled out for the tristate area.

Bottom lines:

• Gradual pattern change with the Plains/Mid-west warming significantly by mid week; the east coast will warm slightly over the next several days, more warming after MLK day.
• Southern stream short wave will propagate NE to the mid atlantic coast by MLK day; however, cold air will have largely exited stage right by that time w/ limiting blocking. Interior parts of the tristate area are most favored if there is wintry precip from this storm, although nothing can be set in stone at this point.
• Generally benign weather pattern through next week into the final days of January.
• Pattern reload with another -AO/-NAO regime and redevelopment of a western ridge should ensue by February 1st +/- a few days.
• Thereafter, I believe February may feature plenty of snow opportunities and a colder than normal temp pattern. Consistent with mod-strong nino climatology as well.