Indian summer this week, but will it continue?

GFS Ensembles showing above normal mid-level height anomalies throughout parts of the Central US during the last week of October. Courtesy Stormvista.

Most of the area has seen the first frost/freeze of the season with the exception of the immediate coast and urban areas (NYC, JFK, LGA, EWR). Any period of warmth / above normal temperatures that occurs after the first freeze is referred to as an “Indian Summer.” We’re certainly see that today – widespread warmth with temperatures surging into the 60s after morning lows in the 30s/40s for many. High temps near 70F in land portions of New Jersey cannot be ruled out today, and tomorrow should feature 70s across the entire metro area out ahead of the next cold front.

Beyond this week, folks are beginning to get antsy for the winter, and some are excited about the prospect of more blocking this winter compared to last winter’s complete lack of any block (Blocking – the tendency for positive height anomalies to form in the northern latitudes which can result in troughiness / cold weather in the middle latitudes).  While it’s true that we have plenty of blocking right now, that doesn’t necessary mean that we’ll see it persist into winter. We’ll have to see how the pattern evolves over the next several weeks.

Moving forward, many forecast models are suggesting a generally mild regime for the Eastern US. Why is that the case given the negative NAO and AO currently in place? Doesn’t a negative NAO mean troughiness / cold in the East? Generally in the winter months that is true, yes, but the correlation between NAO values and height anomalies in the Eastern US is not as strong as other indices, like the PNA, right now.

Keep reading for all of the details, technical discussion, and forecast…

The PNA (pacific north american index) is progged to remain negative for the next couple weeks, and that basically means the tendency for a trough to develop in the WESTERN US and a ridge in the EASTERN US. The NAO is negative which teleconnects to a trough in the East, but it’s signal will be overpowered by the PNA. Think of the PNA as someone benching 500 while the NAO currently benches 350 – in the month of October. If this same pattern were to occur 2 months from now, mid December, the NAO would be benching 500 and PNA being the weaker of the two. However, at this point in time, the Pacific signalling will be outdueling the Atlantic, resulting in a bit more warmth than cool for the coming weeks.

Just to illustrate the point visually, below are some maps.

This is the October PNA correlation with 500mb heights. Notice the positive correlation in the West and negative correlation in the East. In other words, when the PNA is negative, heights are negative in the West, and positive in the East. When the PNA is positive, heights are positive in the West, and negative in the East. The correlation is about -0.4 to -0.5 in the entire Northeast, which is fairly strong.

Now, below is the map of NAO correlation in October to 500mb heights:

There is a positive correlation in the East, but only 0.3 to 0.4 rather than the higher values of -0.4 to -0.5 we saw with the PNA. So the NAO does exert some force, and there will be shots of cooling across the Northeast, but I think overall, the pattern is milder than normal through early November.

In December, here’s the NAO correlation to 500mb anomalies. Note it’s much stronger, 0.6 to 0.7 positive correlation in the Northeast:

 

When we compare that to the PNA correlation in December – notice the correlation is only -0.1 to -0.2 in our area, much less of an impact whereas the NAO correlation is stronger in December.

 

So this will be something interesting to follow as we head toward the winter season. If the negative PNA / negative NAO pattern persists, we’ll begin to see more and more cooling across the Northeast as the influence from the NAO grows stronger. The SE ridge tends to remain firmly intact throughout the winter with a negative PNA, however.

Regarding the next few weeks – other indices like the MJO (madden julian oscillation) favor a milder than normal regime in the East through Halloween, with the potential of some cold weather in early November if we can get the MJO into phase 2. MJO models indicate we’ll be swinging through phase 1 over the next 7-10 days which is a warm phase in the Eastern US. There issome divergence among the models by the day 7-15 range – some take us into phase 2, others into the circle of death (no phase), and still others keep us in phase 1. However, even those models that bring us into phase 2 have it as a very weak amplitude wave. In other words, it may be too weak of a wave to have major effects, and the cold outbreak might not be that impressive in earlyNovember.

For now, I think the safe bet is milder than normal for our area through the next 2-3 weeks with occasional shots of below normal air, but they will be transient in nature. Below are the temp composites for each MJO phase. Notice phase 1 for this time of year is warm (top left), with significant cooling in phase 2 (1st image in 2nd row). So if we can get into phase 2 by the end of October / beginning of November, we might have a stronger cold shot.

 

 

Here’s what the negative PNA / negative NAO / negative AO (artic oscillation) pattern looks like, as depicted by the GFS and ECMWF for days 7-10. Notice the positive height anomalies across the top of the globe, trough in the Western US, milder East.

 

 

Bottom line — expect a biased milder than normal pattern through early November, with occasional, transient (1-2 day) shots of colder air. The neg NAO and neg PNA is likely to continue to early November, so the mean trough axis will generally be too far west to give our area significant long lasting cold. This may change as we head beyond the first week of November as noted with correlations above. The MJO suggests the possibility of a cold outbreak in early November, but it may be transient and unimpressive if the wave is of low amplitude.

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