40/70 Benchmark

I love the December pattern for us....mid Atlantic not as much.

My primary concern with respect to the first third of the winter season is that I rushed the recovery of the PV due to a dearth of appreciation for just how protracted the relaxation of the zonal winds would be. I went with something falling short of a full reversal, but even if we do see a reversal, I think that is largely immaterial to how the forecast will evolve. Another factor that played into my hasty recovery of the PV is that I operated on the assumption of a reflection event in mid January, which occurs during a stronger PV and entails a hasty recovery from the early season disruption. My guess is that my timing may be off, but I'll have the right idea in the seasonal mean. Reflection event may just be more towards the back end of my range, in latter January or early February. I also wish I had paid more attention to the behavior of the MJO in my primary analog of December 2000, which hit phase 8 mid month. I focused so much on using it as an analog for the disruption of the PV that I totally brain-cramped on the behavior of the MJO. *********************************************************************** Balance of November-December 2025 Outlook December Analogs: 2021, 2017,2008, 2007, 2000, 1981, 1970 Here is how the analog composite fared versus reality for the month of October: And versus current seasonal guidance for November (CFS): The MJO is currently crossing from phase 5 into phase 6, which is triggering the abrupt transition to a much blockier and colder weather pattern that is currently underway. The progress of the wave is then forecast to stagnate as it approaches phase 7 and essentially decays around mid-month. While the cold will moderate for a time, however, the pattern will remain stormier than average, with the first snowfall likely across the higher terrain of northern New England next weekend. High latitude blocking will be consistent as the polar vortex remains weak throughout the balance of November into early December, as the arctic high regime will be prevalent. Arctic High Regime Courtesy Lee et al (2019) Here is the December composite data versus the latest CFS temperature anomaly forecast: Expect this sale style of deviation from the MC forcing during the early portion of the season. December 2017-January 2018: December 2024-January 2025: An official SSW as defined by the reversal of the mean H85mb zonal westerly winds is unlikely, however, there is a low probability of one as suggested both by the minority of guidance, and the December 4, 1981 analog. Should there be a reversal of the H85mb zonal winds, December 4th is a good estimation of the when the PV will reach its weakest point before the split, and subsequent recovery begins. As per the expectation of a significantly weakened vortex resulting from bonafide warming that will fall short of a full zonal wind reversal, the December 2000 event is the preferred analog. This is appropriate since 2000-2001 is also the top analog from the seasonal composite. Accordingly, the polar vortex should have consolidated back to at least something approaching climatological levels prior to Christmas (18th-24th). It will then continue to strengthen with moderating temperatures in the east, per research conducted Lee et al (2019), which indicates that the arctic high regime is favored for less than 20 days following a weak PV. This is also consistent with the onset of a reflection event on December 21,1981, just over two weeks after the December 4th SSW. There is a slight chance of Ku event between the first and 15th of the month. The PNA should average negative during what will be active month, with numerous SWFE and overrunning events commencing by approximately the 10th. The probability for a White Christmas is above climatological levels given the active pattern and at least seasonable cold throughout the first half of the month, but it remains dubious for the coastal plane given the milder turn prior to the holidays. In fact, the proverbial "Grinch Storm" is more likely than not. A Pacific trough should begin to evolve towards the end of the month in conjunction with a rather stout PV, as a mild pattern ensues to ring in 2026. The month of December will average anywhere near normal, anywhere to -1F to +2F, except near normal to +2F across the middle Atlantic.

Basin-wide, but east-based, nonetheless.

I expect December significant snows to largely be relegated to 40N along the east coast.

Yea, 2007 I had one 4" event before the second week, but 2008 was only a T prior.

Absolutely. A couple of my best months of December (2007 and 2008), which I incorporated into the Outlook as analogs, didn't get going until the second week.

Good news is there are no holidays the first week of December lol

You are at almost 4,000', and not on the east coast. I mean literally on the east coast.

No one on the east coast below the latitude of NH should have been expecting any snow prior to the second week of December IMHO.

Yes, I do expect a +PNA month of January.

If they were all the same, Outlooks would be easy.

I think it will be better than January.

I think it sounds like him.

Yea, I had, 1970, 2007 and 2008 as December analogs.

I would be stunned to see that.