Winter 2020-21 Discussion

[EasternLI]

These blasted 3 part ridges are always the kiss of death. 

 

Or are they??

 

I found that particular pattern to be interesting. So I did a little digging. Only instance of this that I've found in a nina thus far. Doesn't mean jack, just something interesting. At least I thought so. Seemed appropriate to post it here now.

[StormchaserChuck!]

2 weeks from now is different from now, arctic sea ice extent pattern. That one doesn't get to 100dm anywhere anyway. 

[psuhoffman]

Yes a negative NAO October is inversely correlated with a -NAO winter.   You know what else is inversely correlated with a -NAO winter?  EVERYTHING!!!  There have only been 3 negative NAO winters in the last 30 years.  Anything you run a correlation against a -NAO winter will come back as an inverse relationship simply because the odds of a -NAO winter are so extremely low regardless of whatever else is happening.   This is another example of correlation not being the same as causality!  

But lets look at the data to see if there IS anything meaningful to this October/Winter correlation debate.  So first of all there are 2 different ways to calculate winter NAO. Some use D/J/F and some use J/F/M (because that is when the NAO has the most impact on the sensible winter weather).  But regardless of which you use there are 3 years...just 2 of those 3 is different in each set.  But since we have so few truly negative NAO years lets use all 4.  1996 and 2010 are truly negative years regardless of the way you calculate.  2011 and 2013 flip flop depending on how.  2011 is a neg year if you use D/J/F and 2013 is if you use J/F/M.  3 of the 4 negative NAO winters in the last 30 years came following a negative NAO October.  The other came following a neutral NAO October (1996).   For what its worth 1996 turned severely negative later in October but numerically it was washed out by positives earlier.  

So purely statistically -NAO Octobers had a 21% chance of leading to a -NAO winter.

All other Octobers had a 6% chance of leading to a -NAO winter!

But those numbers are from way too small a sample size to have true statistical significance.  And besides...the problem with using the seasonal NAO, besides the fact the base state of the NAO is so positive that you get too rare an occurrence to glean any meaningful correlations, is that we don't need the NAO to be negative for 3 months (or even 1 month) to get snow.  We just need some periods of blocking.  And sometimes that blocking is bootleg and doesn't even show up in the numerical NAO.  And a period of positive can negate a period of negative.  Who cares if it snowed right!

So...lets look at what we all really care about...snowfall.

Averaging the 3 major recording stations in our region (DCA/IAD/BWI) together gives us an average snowfall of 18.1" over the last 30 years.

The avg snowfall in the 14 winters following a -NAO October is 21.0"

The avg snowfall in the other 16 winters is 15.6"

So...we have averaged 5.4" more snowfall following a -NAO October in the last 30 years.  

 

 

[psuhoffman]

21 minutes ago, StormchaserChuck! said:

2 weeks from now is different from now, arctic sea ice extent pattern. That one doesn't get to 100dm anywhere anyway. 

paging @stormtracker

[psuhoffman]

23 minutes ago, StormchaserChuck! said:

2 weeks from now is different from now, arctic sea ice extent pattern. That one doesn't get to 100dm anywhere anyway. 

so you like to compare a day 5 smoothed ensemble mean to an actual h5 verification?  

[Ji]

3 minutes ago, psuhoffman said:

paging @stormtracker

he unfollowed me on twitter

[psuhoffman]

2 minutes ago, Ji said:

he unfollowed me on twitter

I'll say a prayer for you 

[Maestrobjwa]

9 minutes ago, psuhoffman said:

paging @stormtracker

He's been busy celebrating his 250th birthday year.

[WinterWxLuvr]

17 minutes ago, psuhoffman said:

Yes a negative NAO October is inversely correlated with a -NAO winter.   You know what else is inversely correlated with a -NAO winter?  EVERYTHING!!!  There have only been 3 negative NAO winters in the last 30 years.  Anything you run a correlation against a -NAO winter will come back as an inverse relationship simply because the odds of a -NAO winter are so extremely low regardless of whatever else is happening.   This is another example of correlation not being the same as causality!  

But lets look at the data to see if there IS anything meaningful to this October/Winter correlation debate.  So first of all there are 2 different ways to calculate winter NAO. Some use D/J/F and some use J/F/M (because that is when the NAO has the most impact on the sensible winter weather).  But regardless of which you use there are 3 years...just 2 of those 3 is different in each set.  But since we have so few truly negative NAO years lets use all 4.  1996 and 2010 are truly negative years regardless of the way you calculate.  2011 and 2013 flip flop depending on how.  2011 is a neg year if you use D/J/F and 2013 is if you use J/F/M.  3 of the 4 negative NAO winters in the last 30 years came following a negative NAO October.  The other came following a neutral NAO October (1996).   For what its worth 1996 turned severely negative later in October but numerically it was washed out by positives earlier.  

So purely statistically -NAO Octobers had a 21% chance of leading to a -NAO winter.

All other Octobers had a 6% chance of leading to a -NAO winter!

But those numbers are from way too small a sample size to have true statistical significance.  And besides...the problem with using the seasonal NAO, besides the fact the base state of the NAO is so positive that you get too rare an occurrence to glean any meaningful correlations, is that we don't need the NAO to be negative for 3 months (or even 1 month) to get snow.  We just need some periods of blocking.  And sometimes that blocking is bootleg and doesn't even show up in the numerical NAO.  And a period of positive can negate a period of negative.  Who cares if it snowed right!

So...lets look at what we all really care about...snowfall.

Averaging the 3 major recording stations in our region (DCA/IAD/BWI) together gives us an average snowfall of 18.1" over the last 30 years.

The avg snowfall in the 14 winters following a -NAO October is 21.0"

The avg snowfall in the other 16 winters is 15.6"

So...we have averaged 5.4" more snowfall following a -NAO October in the last 30 years.  

 

 

Well that was certainly putting down a hammer. Loved it

[raindancewx]

I posted the graphs in the New England thread for anyone who wants to see, but there actually is a real relationship between the Oct NAO and the DJF NAO if you are in a winter following an El Nino. It is a weak relationship, and it vanishes since 1990. But it is there if you use the 24 winters following an El Nino from 1950 to 2019. In the winters not following an El Nino, the relationship is very near 0 in a correlation sense. Long-term, in an October after an El Nino, the correlation (r-squared) to winter is close to 0.2. Probably real, but not precise enough to be useful. Believe it or not, the Spring months have stronger r-squared relationships to the DJF NAO, which is why it is interesting we had the first -NAO April in forever in 2020.

[psuhoffman]

1 hour ago, raindancewx said:

I posted the graphs in the New England thread for anyone who wants to see, but there actually is a real relationship between the Oct NAO and the DJF NAO if you are in a winter following an El Nino. It is a weak relationship, and it vanishes since 1990. But it is there if you use the 24 winters following an El Nino from 1950 to 2019. In the winters not following an El Nino, the relationship is very near 0 in a correlation sense. Long-term, in an October after an El Nino, the correlation (r-squared) to winter is close to 0.2. Probably real, but not precise enough to be useful. Believe it or not, the Spring months have stronger r-squared relationships to the DJF NAO, which is why it is interesting we had the first -NAO April in forever in 2020.

So....

 

[raindancewx]

I'm not sure that the lack of relationship since 1990 is a climate change thing though - I think it could conceivably revert back. But the Typhoon Tip guy had asked about if it changed in more recent years. I don't use October to try to predict the NAO. I've had much better success using the change from April to May and March to September as a blend to predict the NAO in winter. March to September is probably indirectly an indicator of the total sea ice extent change, and April to May is just a decent indicator for whatever reason. I'm pretty sure there is academic literature supporting May as a predictive period for the NAO though, I think that's where I got that part of it.

 

[ravensrule]

3 hours ago, Ji said:

he unfollowed me on twitter

Why he followed you in the first place is a better question. 

[Maestrobjwa]

3 hours ago, psuhoffman said:

Averaging the 3 major recording stations in our region (DCA/IAD/BWI) together gives us an average snowfall of 18.1" over the last 30 years.

The avg snowfall in the 14 winters following a -NAO October is 21.0"

The avg snowfall in the other 16 winters is 15.6"

So...we have averaged 5.4" more snowfall following a -NAO October in the last 30 years.  

 

Hey I'd take that  I wonder why the average went up following the October -NAOs? Do we just kinda chalk that up to chaos and it being, as you said, too small of a sample size to say there's a definite correlation? 

[CAPE]

2 hours ago, Maestrobjwa said:

Hey I'd take that  I wonder why the average went up following the October -NAOs? Do we just kinda chalk that up to chaos and it being, as you said, too small of a sample size to say there's a definite correlation? 

There are other factors/indices that influence snowfall here in a given winter, as you well know. 

[StormchaserChuck!]

6 hours ago, raindancewx said:

I'm not sure that the lack of relationship since 1990 is a climate change thing though - I think it could conceivably revert back. But the Typhoon Tip guy had asked about if it changed in more recent years. I don't use October to try to predict the NAO. I've had much better success using the change from April to May and March to September as a blend to predict the NAO in winter. March to September is probably indirectly an indicator of the total sea ice extent change, and April to May is just a decent indicator for whatever reason. I'm pretty sure there is academic literature supporting May as a predictive period for the NAO though, I think that's where I got that part of it.

 

I found a better correlation to May-Sept SSTs in the NOrth Atlantic, vs actual NAO. It was like 0.75 vs 0.58. It hasn't worked the last Winter though so... 

[nj2va]

On 10/13/2020 at 8:56 AM, pasnownut said:

Then ignore them and look at typical tellies,incidies,base state, catapillars, squirrels, nuts and all of the stuff we used to and go from there.  

Thats not a dig, thats just trying to keep it fun.  

I think you missed the point of my post.

[frd]

 

Simon made this cool presentation.

 

 

Also, looking from his site, there are some minor changes with regard to to the development of the early season PV. 

CFSv2   

 

 

     GEFS   

 

 

 

[40/70 Benchmark]

1 hour ago, StormchaserChuck! said:

I found a better correlation to May-Sept SSTs in the NOrth Atlantic, vs actual NAO. It was like 0.75 vs 0.58. It hasn't worked the last Winter though so... 

 

What are the result for it this year?

I remember your correlation began very promising, but has struggled of late. No offense, as that if often the case in this field and doesn't mean that there is not value there. I do agree that summer has more predictive utility with respect to the winter time NAO, than does October.

[40/70 Benchmark]

7 hours ago, ravensrule said:

Why he followed you in the first place is a better question. 

[frd]

 

A good read -  posted by bluewave yesterday from a recent study on the winter  + NAO and the near record + IOD last November. 

 

<

A recent study linked  the very strong +NAO last winter to near record +IOD in November. The +IOD was associated with the standing wave in the MJO 1-2 which was observed as early as last October and November. Those October MJO phases were similar to the other La Niña and Niña-like below normal snowfall seasons in 19-20, 18-19, 11-12, and 07-08. They mention a stratospheric pathway linking the flat ridge north of Hawaii and the more +NAO in the Atlantic. The normal to above normal La Niña snowfall seasons since 2000 had stronger MJO 5 forcing in October. Those seasons were followed by intervals of -EPO and -NAO blocking. So this may potentially be how we saw the October to winter MJO relationship since 2000. But as always, there are no guarantees this relationship will work every season. 

https://rmets.onlinelibrary.wiley.com/doi/full/10.1002/asl.1005

Predictability of European winter 2019/20: Indian Ocean dipole impacts on the NAO

3.3 Stratospheric pathway

As alluded to in the previous section, the stratospheric teleconnection pathway of the IOD to the Atlantic involves the Aleutian cyclone, and is similar to that already documented for ENSO (Manzini et al., 2006; Ineson and Scaife, 2009). Demonstrated best by the IOD experiment in the month of December, Figure 3cshows a Rossby wave train emerging from the Indian Ocean that leads to poleward flow in the North Pacific near the dateline, and equatorward flow to the east of this. These two responses combine to give anomalously positive MSLP just south of Alaska and the Aleutian Islands. Figure 4a shows that this MSLP anomaly in DJF is in a very similar location to that due to ENSO (shown also in fig. 1 of Ineson and Scaife, 2009). For the single case of winter 2019/20, this signal is evident in early winter, as discussed below, and again in the DJF mean (Figure 3b) due to a particularly strong signal in January and February.

As described in Ineson and Scaife (2009), the positive MSLP anomaly (Figure 4a) acts to reduce the strength of the climatological Aleutian cyclone and, thereby, reduces the amplitude of planetary waves propagating upwards into the stratosphere. Figure 4b demonstrates this reduction in planetary wavenumber 1 amplitude (diagnosed using geopotential height at 100 hPa, area averaged 40–80°N, and then Fourier decomposed; Hardiman et al., 2008) in the IOD experiment, and is consistent with fig. 10b of Fletcher and Cassou (2015).

Reduced planetary wave driving in the stratosphere leads to an anomalously strong stratospheric polar vortex (defined by U(60°N, 10 hPa) in Figure 4c, and see also fig. 10 of Fletcher and Cassou, 2015). Anomalously strong vortex signals propagate downwards into the troposphere, resulting in a positive NAO at the surface approximately 1 month later (Baldwin and Dunkerton, 1999; Kidston et al., 2015).

In fact, this positive MSLP anomaly in the Aleutian region occurs also in November, so reduced wave driving (Figure 4b) and an anomalously strong stratospheric polar vortex (Figure 4c) are already apparent in November. Indeed, the November polar vortex strength is anomalously positive in IOD composites, the IOD experiment, ERA‐5, and all forecast systems (not shown). Fig. 4 of Nie et al. (2019) demonstrated that an early winter preconditioning of the stratospheric polar vortex in November descends through the stratosphere and troposphere in the following winter months, projecting onto an anomalously positive NAO in DJF.

The winter of 2019/20 was anomalously warm and wet across the UK and Northern Europe, due to a strongly positive NAO. The winter was well forecast by the C3S and the Met Office DP3 seasonal forecast systems. Even the details of individual months, such as the transition from the negative pressure anomaly west of the UK in December to a positive NAO in January/February, were well forecast by all seasonal systems. Such remarkable agreement amongst systems is suggestive of the positive NAO being strongly driven by global influences, and predictable in this case. In this paper, composite analysis and numerical experiments are used to identify the very strong positive IOD event at the start of the winter as the key driver.

Two teleconnection pathways are identified using an experiment in which two ensemble forecasts, one with the observed November 2019 Indian Ocean SST anomalies, and one with the negative of these anomalies, are produced using DP3. The difference in the ensemble mean response, shows a Rossby wave train originating in the Indian Ocean and propagating across the Pacific and Atlantic Oceans. In the Atlantic, this wave train projects directly onto the observed Atlantic MSLP anomalies. In the Pacific, the wave train acts to reduce the amplitude of the Aleutian cyclone and therefore the amplitude of planetary waves propagating into the stratosphere. This results in an anomalously strong stratospheric polar vortex, projecting onto an anomalously positive NAO. This numerical experiment shows good agreement with both the ERA‐5 reanalysis data and the C3S multi‐model seasonal forecasts in terms of the details of both teleconnection pathways. Furthermore, both pathways are very similar to the well documented tropospheric and stratospheric teleconnection pathways whereby ENSO impacts the north Atlantic MSLP (Hardiman et al., 2019).

The impact of the IOD on the Atlantic jet stream and associated precipitation anomalies is a northward shift in the jet latitude, a slight increase in the jet strength, and anomalously high precipitation over the UK and northern Europe, as shown in Figure 5. This is consistent with an anomalously positive NAO and agrees well with the features observed in winter 2019/20. There is a remarkable agreement between the IOD experiment and the C3S multi‐model mean forecast. The signal in the December observations is noisier (Figures 3a and 5a), but this is expected, being only a single realisation of a single month.

A knowledge of the teleconnection pathways between the IOD and the North Atlantic gives greater confidence in the seasonal forecast skill they offer. The frequency of positive IOD events has doubled in the 20th century, and their intensity has also increased, with this trend projected to continue (Abram et al., 2020). It is likely, therefore, that such connections will become increasingly important for seasonal forecasting of European winters during the rest of the 21st century.

>

•  

 

[StormchaserChuck!]

It actually warmed up significantly to positive

[psuhoffman]

@frd that would bode well for this winter. Still not great as we typically struggle even in a better Nina but it would suggest we might avoid total pac ridge hell like last year. I’m still not sold one way or another. Lots of conflicting tea leaves. 

[frd]

 

I imagine this might be useful in winter, or anytime. Great resource material from Simon. 

Check it out. 

 

 

 

[frd]

 

Nina update 

 

 

[StormchaserChuck!]

meh, I'm not really impressed by this

[yoda]

https://www.noaa.gov/media-release/us-winter-outlook-cooler-north-warmer-south-with-ongoing-la-nina

 

 

[BristowWx]

It seems like whatever condition...Nina, Nino, neutral..the temps always show some shade of orange for winter outlooks in our region.  It’s baked into the cake. 

[WinterWxLuvr]

1 hour ago, yoda said:

https://www.noaa.gov/media-release/us-winter-outlook-cooler-north-warmer-south-with-ongoing-la-nina

 

 

Isn’t that a typical Nina look?

[CAPE]

6 hours ago, WinterWxLuvr said:

Isn’t that a typical Nina look?

Boilerplate. I guess it's as good as anything else for a general winter outlook.