bluewave

That was a very cold -NAO/+PNA pattern for the first week of December 2003. It was the snowiest first week of December and the 11th coldest for NYC.

The state of the NAO/AO/PNA isn’t as important to interior sections of the Northeast as it is to us this time of year. We needed extreme blocking for a Boxing Day Blizzard December high end outcome at the coast.

Sure. I just saw this report from Albany. They can pick up very heavy events this time of year even with a +AO and -PNA. The coast is limited to what it can get this time of year with the lack of good blocking. But even a light event is a really nice way to start December.

Nothing prettier than a fresh snowfall at the beach.

Officially 2 inches of snow on the colder surfaces here in SW Suffolk.

Indications were for a cooler start to December. That much has been obvious for a while. The make or break period for December departures has been mid to late month in recent times. That portion of the forecast is still outside reliable range. Plus long range modeling has been very poor with the raging Pacific split flow.

Another recent paper highlighted how unusual the initiation of the March 2015 unprecedented MJO event in phase 4 was. This helped trigger the 2015-2016 super El Niño event. https://www.nature.com/articles/srep46692 The emergence of this convection–circulation pattern signaled the onset of an MJO that later strengthened to an unprecedented amplitude (MJO real-time multivariate index24 of 4.62 on March 16, 2015 compared with the previous record of 4.02 on February 14, 1985; http://www.bom.gov.au/climate/mjo/) during its eastward propagation over the tropical Pacific in March (Fig. 2c)13. In contrast to the canonical MJOs, which typically originate in the tropical IO, the MJO in spring 2015 initiated in the western Pacific. Our analysis revealed that only 3 out of the 75 large-amplitude MJOs (occurring in 1975, 2013, and 2015; approximately 4%) during 1974–2015 initiated in the western Pacific (i.e., phase 4 of the MJO index24, Supplementary Figure 2). Notably, only in 2015, the MJO preceded the onset of an El Niño event. No eastward-propagating tropical signals from the west but strong extratropical perturbations were observed preceding the 2015 MJO event; this lead–lag relationship implies the potential extratropical effect on triggering the MJO. Furthermore, the percentile values reported in Figs 1 and 2 show the unprecedented amplitudes of the extratropical SLP, northerly, WWB, tropical SLP anomaly crossing the central and eastern equatorial Pacific (Fig. 1c), and the MJO strength. This observation revealed the distinct characteristics associated with this MJO: the effect of extratropical forcing, atypical genesis location and timing, and the extremity of amplitudes in many aspects. The warm ocean surface in the equatorial western Pacific might be another favorable condition conducive to MJO occurrence. A vertical cross section of the monthly ocean temperature along the equator (Supplementary Figure 3) revealed that the upper ocean in the central Pacific was approximately 1.5 K warmer in February–April 2015 than the long-term mean. This warming in the central Pacific was markedly higher than that during the onset of the 1997–98 El Niño. A previous study13 reported the effect of this warm water on MJO development in early spring 2015. An analysis of the SST evolution since early 2014 indicated that the warm SST in the central Pacific was primarily the remaining positive SST anomaly from the aborted 2014 El Niño (data not shown). The extratropical forcing in late February likely triggered the anomalous convection over this warm water and initiated the rigorous atmosphere–ocean interaction in the tropical western and central Pacific and onset of a strong MJO event. Conclusion and Discussion An atypical MJO initiated to the west of the dateline in early March 2015 and rapidly amplified to an unprecedented magnitude over the warm SST in the central and eastern Pacific on March 16. Following the MJO, the SST in the equatorial central–eastern Pacific encountered rapid growth and ultimately evolved to a strong El Niño comparable with the 1982–83 and 1997–98 events. Before the MJO onset, we observed a persisting high-pressure system accompanied by strong cold northerly in the extratropical western North Pacific. On the basis of data diagnostics and numerical experiments, we identified an atypical effect of extratropical perturbations in the western North Pacific on triggering the onset of the MJO in March 2015 and indirectly contributing to the onset of the 2015–16 El Niño. The main results are summarized as follows: 1 Observational analysis indicated that the strong cold northerly, which was associated with a persisting high-pressure system in the extratropical western North Pacific, penetrated southward to the tropical western Pacific and triggered the tropical convective instability that led to the onset of the MJO at an atypical location, namely west of the dateline. The critical effect of the extratropical disturbances on the MJO onset was confirmed by numerical experiments by using an atmospheric general circulation model coupled with an ocean mixed layer model. 2 The MJO developed rapidly to an extreme magnitude because of the favorable ocean conditions: a warm upper ocean temperature in the equatorial central and western Pacific remaining from the aborted 2014 El Niño. 3 Both data diagnostics and numerical experiments revealed that the strong WWB associated with the MJO triggered the first pulse of downwelling Kelvin wave-like perturbations that later induced the onset of the 2015–16 El Niño. Extratropical forcing was the unique characteristic of the reported MJO–El Niño event. The onset of El Niño by an MJO has been observed often. However, according to our review of relevant literature, the present study is the first to report the onset of an El-Niño-inducing MJO in the western Pacific triggered by extratropical perturbations. Extremity was another unique characteristic. Several aspects of perturbations, such as extratropical and tropical SLP, northerly, and the MJO reached unprecedented amplitudes. The reasons for these unique characteristics remain unknown. However, our study revealed the possible effect of extratropical forcing, which has not been considered previously, on the onset of MJO and El Niño. Such a mechanism, although it might not occur frequently, warrants further attention and may elucidate the onset of an MJO and its potential effect on El Niño.

I am hearing the static on the AM radio from the heavy thunderstorms south of Long Island. There may be some small hail or groupel with the heaviest cells.

Yeah, lightning east of Atlantic City.

That’s usually how it is during the warm season. But forecasters don’t need to worry about snowfall then.

Divergence between the NAM and HRRR. Just like we see with warm season convection.

The latest HRRR has lightning potential as the band really intensifies.

The HRRR seems to have the best handle on that band right now. It should really blossom as the day goes on.

Into nowcast time now as the band is beginning to form near Trenton.

This storm evolution and various records will make a good topic for a research paper.

That is an impressive convective snow band in the NAM. Very steep midlevel lapse rates and elevated TT’s. It will be interesting to see where it actually sets up.

Yeah, this was the easy part of the forecast.

I am close to that bright band on the radar. Just enough of a warm tongue aloft so I am sleet.

Moderate sleet now in SW Suffolk and 33.6 degrees.

Yeah, we’ll see what the mesos like once they get into their better range tonight and tommorow.

The Euro wouldn’t be bad for the first week of December.

Yeah, the sleet just changed to rain here in SW Suffolk.

The 12z NAM has the best chance for snow at LGA Monday night. Starts to changeover later in the afternoon.

That feature almost looks like a hybrid IVT and CCB in the soundings. Notice how steep the midlevel lapse rates are with the elevated TT’s. Could be some intense snowfall rates where that sets up.

Root for the stronger CCB on the 12z NAM to pivot through NYC. The details with that feature will make or break this forecast.