bluewave

We had a big divergence between the CPC PNA and the 500 mb PNA especially in February which had the classic 500 mb -PNA vortex in the PACNW even though CPC index was +PNA. I liked your old idea of creating custom 500mb indices. I think the divergence between the AO and NAO is less if we just used the 500 mb composites and didn’t use the CPC. My guess is the 500 mb NAO would be negative more often than we have been getting from the CPC. So the NAO and AO would align more at the 500 mb level.

The specific phenomenon are strongly influenced by the global temperatures at any given time. This is why teleconnections which were cold in the 70s to early 90s haven’t been as cold anymore. Plus the coldest recent month in the Northeast was during one of the strongest +NAO winters on record in February 2015. January 2021 was the most negative 2020s winter NAO month and it was mild in the Northeast. So other teleconnections and patterns have a much bigger influence than the NAO in the Northeast.

The state of the NAO whether positive or negative doesn’t cause the Northern Hemisphere cold pool to get smaller like we have been seeing resulting in the limited area of these Arctic outbreaks.

Montana has been one of the small areas in North America with 4 top 10 coldest Februaries since 2018. So these Arctic outbreaks have been focusing in smaller geographic regions. In the old days the cold dropping down the Plains would spread out to the East Coast. Now the core of the cold stays in a narrower corridor closer to the Plains and Midwest. Time Series Summary for Great Falls Area, MT (ThreadEx) - Month of Feb Click column heading to sort ascending, click again to sort descending top 10 coldest Februaries since 1892 1 1936 -5.2 0 2 2019 -0.2 0 3 1922 9.0 0 4 1989 10.3 0 5 1899 11.0 0 6 2025 11.3 0 7 2018 11.8 0 8 2021 12.4 0 9 1975 13.1 0 10 1978 14.4 0

Maybe the higher 500mb heights near the East Coast are influencing the location of the wave breaks which have been leading to the orientation of the -AO and -NAO blocks during the 2020s.

It hasn’t made a difference whether it was a -NAO or -AO during the 2020s. March 2023 was one of the most negative NAO months of the 2020s so far. We still got the Southeast Ridge link up which forced the storm on the 13 to 15th too far north for my area. So it turned into an higher elevation special. 2023 1.25 0.92 -1.11

There are probably multiple factors contributing to the more +NAO winter trend and -NAO summer trend.

That’s the point. Even when we had strong -PNA -AOs in recent years, the actual 500 mb heights weren’t as low near the Pacific NW as there were in the 1950s to 1970s. So deeper troughs digging into the West during that colder era didn’t pump the Southeast Ridge. During the 2020s all it takes are relatively weak lows coming into the West and we get a big 500mb ridge amplification in the East.

Back in the 50s to early 70s we seldom had a Southeast Ridge when there was a strong -PNA -AO pattern. So the Southeast Ridge becoming such a big player in our sensible weather is a more recent phenomenon. My guess is that it’s a function of the much warmer global SST and land temps along with the warming at 500mb.

I think the more +NAO could be a function of the increased KB blocking low sea ice feedback. We used to get a much stronger vortex in Europe when there was such strong blocking across the Arctic in the old days. Instead the KB block build down into Europe preventing the usual cold trough development. So this is also contributing to the Southeast Ridge which has been increasing over the years. But even past instances of a very deep vortex centered south of Greenland like this year didn’t have as strong of a Southeast Ridge. The vortex out there this month was among the strongest that was observed in the February when compared to the other strong years in the composite. But the Arctic blocking was more intense this February than in the composite.

We have been seeing the -AO decouple from the -NAO. We used to get both falling in tandem more often. Now we get -AO and +NAOs more often.

The overall size of the troughs has been shrinking. So the record cold in spots like Nebraska had smaller geographic footprint than in the past. These strong -EPO +PNA and -AO patterns in the past had much more extensive cold and larger troughs. If we just isolate those features, then this was our warmest winter of the group which included 02-03, 85-86, 76-77, and 62-63. So those were much colder teleconnections in the past when the 500 mb ridges weren’t crowding out the troughs.

On the day of the strong cutter back in mid-February that produced the record snows from Toronto to Montreal it was more about the -AO linking up with the Southeast Ridge than the -AO being too far north. In the old days we would have done fine with the block extending west to Baffin Island. These days with the faster Pacific flow and record Atlantic SSTs we get more of these Southeast Ridge link ups. From the 1950s into early 1970s even when we had strong -PNAs, there would be a trough over the Eastern U.S. and not a ridge when there was a -AO.

I had several friends in the airline business when I lived in Long Beach. The West End had plenty of flight crews renting apartments close to JFK. It’s a really great lifestyle if you really enjoy traveling and seeing the world.

Yes during February 2021. So this is the first time since the late 1960s without a KU blizzard here during either a La Niña or El Niño February with a -5 -AO. We had our blizzard in February 2021 before the -AO peak. The February 2010 multiple blizzards with that -5 -AO had the amazing STJ pattern. Back during the weak El Niño in February 1978 with that -5 -AO we had the greatest 1970s blizzard. Then the famous Lindsey Snowstorm in February 1969 with that -5 - AO.

I can remember living back in Long Beach in the old days when the Concorde would fly over just after 9am every morning heading into JFK. Now that was really loud. But you got used to it in Long Beach and it was pretty cool.

But not when we have had the -AO dip below -5 in the past.

We had a very strong Southeast Ridge this February which pushed the storm track well to the north. This was the first time a -5 -AO linked up with the Southeast Ridge. So another case of the -AO not delivering for coastal sections like we saw in December 2022 and numerous other times this decade.

I know that. But with these further north storm tracks, even places outside the traditional snow belts can do very well. While past -5 -AOs favored DC to Boston, this was another one that linked up with the Southeast Ridge which has become a regular occurrence in the 2020s. So the heaviest snows got pushed up into Canada instead with this much warmer storm track. But the most impressive records continue to be in the favored lake effect zones like we just saw in Gaylord.

The EPS is correcting warmer now closer to St.Patricks Day as the blocking is linking up with the Southeast Ridge. So we are finally shifting into more of a spring pattern. As the cool downs behind the fronts become weaker and the warm ups stronger. New run Old run

Good to see the Euro and GFS continuing with the first 70° potential next week for the usual warm spots.

The Euro has a strong El Niño Costero next few months but it isn’t able to transition to full basin warming. Still too early to tell what happens due to the spring forecast barrier. Sometimes these Costero events go full basin like we saw in 2023-2024 and other times the signal fades out like in the spring of 2017. Getting a strong enough WWB and OKW is usually the determining factor as to whether the whole basin shifts into El Niño or not.

The lake effect zones have had numerous snowfall records in recent years. So maybe you mean synoptic snows? This 7 year data conforms to what has been happening over the last 30 to 60 years. So you make an error when saying it’s only 7 years. It’s not singling anything out but rather it’s matching the longer term existing trend. From the 60s into the early 90s my area had a cold and stable temperature pattern. So the areas around NYC had mostly mid range snowfall seasons. Very few much above normal and much below normal snowfall. Since the 90s we have shifted to an all or nothing snowfall distribution pattern with hardly any snowfall seasons near the middle of the distribution. So our snowfall has become much more extreme with well below and well above dominating. This 30 year shift has occurred against the backdrop of rapidly warming temperatures. The record snowfall highs from 09-10 to 17-18 and the record lows since 18-19 perfectly conform to this 30 year snowfall distribution shift. In the colder era before the mid 90s, the NYC area could reach the middle range of 19” to 31” seasons without historic KU benchmark NESIS storms. So a colder climate provided more options of reaching this mid range.Obviously, the KU seasons were the ones which finished well above the mean. So NYC Metro and other locations have fewer options of reaching this middle range. Many seasons jumped above this range from 09-10 to 17-18 relying exclusively on a 1 in 100 year phenomenal concentration of Benchmark storms with a significantly higher concentration of NESIS events experienced in the past. So my guess is that relying on a another 9 year run like that is probably a low probability chance of reoccurrence. Now we are left with the current all or nothing pattern where the higher and middle range distributions seeing smaller probabilities. So it’s no surprise when we lost the benchmark tracks for much of the time since 18-19 ,that the region is at the all-time record lows for snowfall. Past lows near this range over 7 year periods were primarily a result of drier conditions and not record warmth. These were followed by numerous 50” + seasons which were easier to come by in a colder climate. So as we continue to warm, both storm tracks and the seasonal background temperature warming provide less opportunities for snow. Having to rely so heavily on just one type of storm track has left us open to repeated well below normal snowfall seasons last 7 years. Since the Great Lakes cutters, I-78 to I-80 hugger tracks, and suppressed Southern Stream lows with the faster Northern Stream of the Pacific had lead to record low snowfall. The data suggests a few ways this could go in the coming years. First, my guess is that the longer term snowfall peaked in 17-18 around NYC .Then 18-19 began the decline that has been expected in a warmer climate reducing snowfall chances. Option one is that the last 7 years continues and represents the new normal for snowfall. Option two is a return to occasional benchmark storm tracks with an uptick in snowfall next several seasons but coming up well short of 09-10 to 17-18. This would be followed by another downtown in the 2030s if it did occur as temperatures continue to warm. Option 3 is a return to the 09-10 to 17-18 benchmark era which I give the smallest chances to.

We will need steady rains in order to come out of this drought which has been going since late last summer. There is higher water demand in the warm season so drought and heat would lower the reservoir levels and lead to water restrictions which haven’t been in effect since 2002. Most people probably don’t want watering restrictions for their lawns. So hopefully we can see a relaxation and not intensification of this drought for much of the US.

It would be good news if the new Euro seasonal wetter pattern into the summer verifies on the update today. But who really knows with these long range rainfall forecasts. One wild card could be how much warming we get in the ENSO regions. https://charts.ecmwf.int/products/seasonal_system5_standard_rain?area=NAME&base_time=202503010000&stats=ensm&valid_time=202504010000