Isotherm

The GFS based guidance is likely rushing the genesis/realization of the auspicious pattern; the ECMWF/EPS are more congruous with the physical drivers. z50 geopotential heights favor a positive NAO from now through circa Dec 28th; as the Canadian 'warming' occurs in the stratosphere and z50 restructures w/ a wave-2 paradigm, geopotential heights will begin to respond within a few days over the NAO domain, with diminution there in the final 1-2 days of December/beyond. The classical Siberian high descent initiates December 26th +/- a couple of days which will induce a +EAMT and jet extension event at the very end of December, and thus ameliorating the downstream Pacific/PNA/EPO domain pattern by the early days of January. Therefore, the December 28th-January 2nd period should feature expeditious amelioration of the hemispheric pattern, first, with improvement in the NAO domain at the very end of December, then, shortly followed by the Pacific. If there is a storm threat of significance in late December, it would likely favor the interior Northeast/New England, in my view, with a threat for I-95 beyond that time-frame (sometime in the first 10 days of January), but we'll see how it evolves.

Brief update with some thoughts on the storm threat & overall winter outlook. The provenance of this week's storm threat for the Northeast corridor can be traced backward in time to the robust poleward and eastward transfer of angular momentum, which is large part mediated this fairly unique (given various background indicators) episode of high latitude blocking. Most of the diminution of negativity is focused in the AO domain space, but the NAO has turned negative and will remain there for a period. It is this poleward momentum deposit and associated +GP height/50-50 anomaly which will act to largely countermand a less than optimal Pacific regime. This projected storm represents the most auspicious precondition set-up in a few years for the Northeast coast. It is highly likely that Baltimore, Philadelphia, Boston and NYC will receive at least 5-6" of snowfall, possibly DCA as well. The heaviest axis of snow - location indeterminate - will likely feature 8-16" on average. The most accumulation will occur approximately 1.5 degrees W/N of the path of the z850 low, and slightly W/N of the path of the z700 low, as evidenced by optimal uplift/DGZ confluence. This zone may occur anywhere from the northern mid Atlantic coast to north of NYC, depending upon the z500 evolution. It's nigh impossible to ascertain this zone right now. However, more importantly, from my perspective, is the storm opportunity provides a quasi-indicator for forthcoming pattern relapses. December's temperature departures thus far are according with my winter outlook: generally warm nationwide with the core of the warmth in the W-C plains, and cooler in the SE US. The z500 of +EPO, neutral-positive PNA is also verifying. The December AO and potentially NAO will be more negative than expected (hence the larger storm threat this week). However, this episode provides insight to me, that tropospheric receptivity is heightened for another pulse(s) of momentum transfer, and increased favorability, for my target winter month in the Northeast (January). If anything, the events of this week suggest to me that my January ideas are on track (for NYC: slightly colder than normal, and snowier than normal). A technical SSW is unlikely this winter, but the SPV should be weaker than normal for early-mid January. My expectation is the NAO will neutralize beyond this week, along with the AO, as the SPV reconsolidates and momentum alters. Thereupon, the Pacific should reorganize into a more conducive structure again, by the end of December. Thereafter, there will be another decline opportunity in the NAO/AO in the month of January, potentially operating contemporaneously with a +PNA and even near neutral EPO. My thoughts remain the same for beyond January. However, this week's storm will increase the probability that NYC's seasonal snowfall ends in the upper portion of my range, assuming the winter pattern continues to proceed as I anticipate. My projection at this stage for snowfall this week is 6-12" for NYC, a significant snowstorm. Again, the heaviest axis will be a function of z700/z850 proximities.

Thanks all for the comments. Ray - I just posted a full response to your above comments on your thread. Weather53, I have not seen either of their forecasts yet, though I think I read CWG was coming out with their outlook this week (or soon). Michigan - I think you're correct on that. I anticipate a fairly cold picture across Canada this year, not atypical for strong La Ninas, and so, any spasmodic perturbations w/ short wave trough slipping through the Lakes/Northeast could ephemerally plunge temperatures to very low values, before rising again. I especially anticipate this volatility later in December and January.

Ray - saw your comment on my thread. Just had a read of your forecast - great work and write up as per usual; well reasoned. As you noted, we are essentially harmonious on the bigger picture and overall season, but mostly differ on timing. I had initially thought early winter/Dec would be favored about a month ago, but as some of the mid autumn data emerged, it drove me toward the January-focused potential more and more. Re March, agree, lower confidence expressed here, though at this juncture I think it will be fairly hostile for the Eastern US. Here's my analogs z500 composite for March, if you're interested. Will be fascinating to see how this winter evolves, and certainly from a more local perspective, whether we can exceed snowfall totals of the past two winter seasons.

My winter outlook has been posted on the main board, which links to it, for those interested:

My winter outlook for the 2020-21 season can be read on my website, linked here, for those interested: https://www.lightinthestorm.com/

A strict construction of the ENSO-oceanic analysis may reveal neutral tending toward weak nino, but inclusion of the GSDM into the analysis yields a different conclusion. The total sum of momentum additions/subtractions evinces an apparent deficit/easterly momentum dominating the tropical/sub-tropical domains, and as such, retrogressive features will be the mainstay of the short to medium term. Whether the GWO neutralizes, thereby comporting more with the oceanic appearance remains to be seen.

@IrishRob17 @40/70 Benchmark, That is correct, I am not advocating for the once per day observation -- which is much different than capturing the maximum snowfall for the 24 hour period. Ray, to be clear, I am merely presenting the guidelines as they currently stand, which states that non-airport measuring should be maximum snowfall for the 24 hour period. I am not advocating that the 6-hr clearing is necessarily a bad method. I just think congruence with historical records is important and we never should have added the option for 6hr clearing in the 1998-2012 period. Per my discussion with Matt who works with the NJ climatologist, there was extensive debate among the people on the panel for this 6hr - maximum snowfall issues. Interestingly enough, the person who pushed most for the 6-hr method was the same person who reversed his thinking and signed off on the return to the maximum snowfall method. Snow measuring is an inexact science; there are arguments for both methods.

I concur with this too, I think they should have maintained the maximum depth standard.

Agree, and I'd argue the once per day observation is the least accurate of all for sure. The clearing/vs maximum depth should be fairly similar in most storms that aren't > 32F at the surface.

It's definitely a mess now. I'm not sure why there aren't clearer instructions disseminated.

Correct. This is per Matt G who works with the State Climatologist. "One final thing regarding this past event. We did receive a 31.0" storm total snow report in Kinnelon (Morris County, NJ) that no doubt got at least a little attention, as it would represent, as far as I know, the largest event total in NJ from the storm. I checked in with the observer, and he confirmed that he cleared the board 3 times mid-storm to reduce the effects of compression. Unfortunately, that makes his inflated measurement invalid, at least in the context of the current guidelines (though even with the 6 hour rule, this must have been more like 1 hour clearings), so we've nixed the value. BTW, I'll personally be working on crafting a NJ snowfall map tomorrow based on hundreds of reports received. I post it here when complete."

Prior to 1998, it was max-depth unless instructed. I think max depth would maintain more congruence w/ historical record. I've previously provided reference to the current NWS guidelines. I checked with the NJ State Climatologist, who specializes in snow cover and was very much involved in the crafting of the guidelines. What he was able to explain to me was that traditionally, the six hour option has only been available to professional observers at airports. However, back in about 1998 the NWS rules were revised and the six hour option was added for Coop observers*.

Per the NJ state climatologist: "When things were reconsidered in 2012, the 6-hour option was removed except for "when instructed." Now, I'm not sure of the frequency of this usage, but apparently the 6-hour option is not in use, only when instructed to do so by the NWS (usually airport).

They're not supposed to as per official guidelines. They should be taking intermediate depth measurements to capture maximum depth. Unfortunately, there is a ton of miscommunication on this issue. NWS airports do use it sometimes when advised (See last post).

Just as an FYI, the latest NWS/COOP guidelines for measuring snowfall are to take the maximum accumulation of new snow in a 24 hour period. The 6 hour clearing method is no longer in effect, unless specifically instructed to by a NWS office (airports sometimes). See the following: section 3.1 and onward http://www.nws.noaa.gov/os/coop/reference/Snow_Measurement_Guidelines.pdf The following is courtesy of Matt G. who operates with the New Jersey State Climatologist, regarding the official guidelines: I've previously provided reference to the current NWS guidelines. I checked with the NJ State Climatologist, who specializes in snow cover and was very much involved in the crafting of the guidelines. What he was able to explain to me was that traditionally, the six hour option has only been available to professional observers at airports. However, back in about 1998 the NWS rules were revised and the six hour option was added for Coop observers*. This was despite the protest of several people on the committee, as a higher up (I won't name names, but any snow weenie will know the name well) overrode them. When things were reconsidered in 2012, the 6-hour option was removed except for "when instructed." The same fellow who brought about the 6-hour allowance, oddly enough, signed off on the removal of the 6-hour option. All that said, the biggest debate on the committee related to the snow->melt-> snow in a single day situation, the same occurrence that has been argued about here. Ultimately, the compromise was made to simplify the guidelines and call daily snowfall the maximum accumulation achieved during a day, period. There's arguments to made both ways (and it sounds like there were among the experts on the committee), but ultimately, the guideline is objective, consistent, and most in line with historical practices. All that said, from what I've seen as a heavy user of Coop data and a state coordinator for CoCoRaHS, far and away the most common "problem" with snow measurements is that many observers (and from what I've seen, perhaps the majority of non-airport Coop) take their measurements once daily at ob time without making any attempt to capture the maximum accumulation. This is probably not a new issue, as I bet it's been relatively common through the years. But sometimes the differences between maximum accumulation and ob time measurement can be significant, so this is something I gently remind volunteers about, with the understanding that many are unable (e,g., at work or sleeping) or unwilling to take these extra measurements (some Coop sites, for instance, are businesses or municipal operations centers, thus not manned all day). It sucks, but we live with it. As for frequent board clearing, I suspect it's a relatively rare occurrence, though people still assert that the 6-hour clearing rule is the standard. I would not be surprised if even some NWS employees are not aware of the current guidelines (hopefully most are). It just won't die. One final thing regarding this past event. We did receive a 31.0" storm total snow report in Kinnelon (Morris County, NJ) that no doubt got at least a little attention, as it would represent, as far as I know, the largest event total in NJ from the storm. I checked in with the observer, and he confirmed that he cleared the board 3 times mid-storm to reduce the effects of compression. Unfortunately, that makes his inflated measurement invalid, at least in the context of the current guidelines (though even with the 6 hour rule, this must have been more like 1 hour clearings), so we've nixed the value. BTW, I'll personally be working on crafting a NJ snowfall map tomorrow based on hundreds of reports received. I post it here when complete. This is a good debate to have! Thanks for sharing your thoughts. * A note about Coop stations. There seems to be some confusion about what constitutes an "official" report, which is subjective in the first place. The airports are first-order stations that also double as Coop stations. The remainder of Coop sites are manned by volunteer observers, and their data also constitute "official" records in my mind, in that they make up the permanent climate record of the US and are quality controlled and archived in NOAA's NCEI GHCN-Daily dataset. For that matter, so now are CoCoRaHS data (if you are a CoCoRaHS observer, your data are a part of the weather/climate records archived at NCEI!).

Turned out much better than it seemed it would a couple weeks ago. Maples are mostly past peak here now, but still holding great color, and oaks are now peak. I think we'll see major leaf drop with the CAA event at the end of this week.

Still going to end up a bit high on NYC/LGA/TTN, pretty close on PHL/EWR. Happy with the call overall.

Cooling degree days have been the lowest probably since 2009 for our area. I only have 766 CDD's for year thus far, quite low comparatively after the past couple summers.

Agreed on all counts. Those numbers will almost certainly be too high. And yeah, definitely feeling much more fall like in terms of sun intensity now. Really noticing the lengthening nights.

16 here now. We're pretty much done given the upcoming pattern. Might be able to make the average of 18 for around here.

I like where I stand with my pre-season predictions; might even end up a tad aggressive for some areas: PHL: 28 EWR: 25 NYC: 16 LGA: 19 TTN: 21

3, 5, and 7 here for May, June, and July respectively. Total 15. So far nothing in August, and it looks like nothing for awhile.

94F and day 14 for the season here.