bluewave

Unfortunately, the arrival of spring warmth is coming earlier while the date of the last freeze has held more steady over time.

The Euro was fairly close this month. It had forecast a global mean average temperature of 1.67 C in September. The September average came in a +1.63 C.

The original paper that RONI is based on discusses it mostly being developed to prevent weak La Nina’s from getting lost in the mix rather than some deficiency in measuring super El Niño events. Plus the weaker El Niños not being able to fully couple. Both instances were also a function of the record WPAC warm pool. So the ONI and the absolute SSTs are a good measure of how intense this event is becoming. My original ONI post was correct as the original ONIs we relied on for many years into the 2010s had the ONI with the 1972-1973 event peaking at the +2.0 to +2.1 range. So a lower end super El Niño event.

The planet doesn’t care about squabbles based on how humans define things. It just responds to the actual temperatures. So the headline story for the planet right now is that we are exceeding past super El Niño events by a wide margin and it’s still only early October. All-time SST records are increasing from Nino 3.4 to 1.2.

The old ONI that we used before the recent updates was also +2.1. It’s perfectly fine to use the ONI as a measurement of the actual intensity of the event. With an event as extreme as this RONI can make the event seem weaker than it actually is. https://x.com/BenNollWeather/status/2105084471750037987 From a scientific perspective, part of the challenge is that weather patterns in different parts of the world are not equally sensitive to the same El Niño index. In South America, for example, Niño 1+2 is particularly important, while patterns across the U.S., Australia, Singapore and many other regions are more closely associated with conditions in Niño 3.4. Some rely on a blend of ocean-atmosphere indicators (e.g., ONI, RONI, SOI, EQSOI, OLR, MEI, etc.) to capture coupling. For the public, the minutiae of those indices matter much less. What's record-breaking in one index may not be record-breaking in another, and I think it's incumbent on the communicator to make that clear. At the very least, if a record is being claimed, then a reference to the index/data set being used is essential (see attached). Not everyone will read the fine print, but it's duty of care. I think there's an opportunity for a larger body to step in (e.g., WMO) to classify a record El Niño event, if there's sufficient evidence, like they would a global temperature record. Lastly, I think the RONI-ONI debate will go on for years. RONI is more closely tied to atmospheric responses. ONI informs absolute extremes. In the ONI space, every tenth of a degree matters to global water vapor levels and temperatures, and therefore, the weather. The best approach is up for debate, but I think showing both is a fair balance. One El Niño, many ways to measure it! https://x.com/WeatherProf/status/2105064342441939064 On the other hand, if you are trying to communicate the absolute strength of this El Niño vs other historic El Nino’s, how global warming has impacted it, or how it may impact global temperatures in the coming months - then use the traditional ONI or raw sea surface temperature anomalies. Certainly it can be confusing to the public, but I’d argue the relative index inadvertently obscures the larger climate signal - it can be misleading because it basically subtracts the warming that has occurred in recent decades. So it’s unintentionally deceptive.

Warm spots from Philly to Newark could be closer to the AI in the mid to upper 80s with enough sun today. It’s possible some of the locally cooler spots across the region could dip under 40° next week with some frost. But looks like a quick warm up a few days later. Not much cold to work with so much warmth across the CONUS.

It was a+2.1 event up until a few years ago before the CPC dropped it to +1.8. So until very recently it was rated as a lower end super.

There has been a deep upper low across the Arctic since the spring with record 500 mb ridging across the mid-latitudes. So this looks like a continuation of the same theme. Both the AO and NAO have been mostly positive over this period. Almost looks like some type of rebound following the strong winter blocking with the early SSW back in November. The last time we had a +AO October with a super El Niño was 1972. But even October 1972 managed to get the more common -EPO.

Yeah, they could reach 80° today with enough sun ahead of the main warm push tomorrow. So a warmer start to the month before we cool down next week. The interior areas have a shot at their first frost next Tuesday into Wednesday.

Another 80°+ day coming up on Friday. That will get Newark to 115 days this year. It will leave 2026 only 3 days behind 2015. Also 6 consecutive years in the top 10 for most 80° days. Time Series Summary for NEWARK LIBERTY INTL AP, NJ - Jan through Dec Top 10 for most 80° days Click column heading to sort ascending, click again to sort descending. 1 2015 118 0 2 2024 117 0 3 2021 116 0 4 2026 114 92 - 1994 114 0 5 2016 113 0 - 1993 113 0 - 1991 113 0 6 2011 111 0 - 2010 111 0 7 2023 110 0 8 2022 109 0 9 1959 108 0 10 2025 106 0 - 2007 106 0

Much different October El Niño pattern than we have seen in the past. The ridges are significantly further south than usual. One ridge east of Japan and another over the CONUS. Similar to the pattern we have been seeing since last May. This stronger mid-latitude ridge is the same pattern driving the -PDO +AMO. The strongest October El Niño ridge is typically in the EPO region of Alaska and NW Canada. Also different from 1997 with the strong -WPO and 2015 with the -EPO. So a further south 500 mb ridge axis from those years also. While the early September model forecast correctly got the ridge east of Japan, the ridge over North America is setting up much further south.

While the Euro is doing better with +1.2, it’s still too low also. Probably related to the models underestimating the strength of the WWBs in the longer range. The next Euro update on the 5th should be interesting when the warmer current conditions are incorporated into the new forecast. The EPS weeklies have been closer.

The September departures across the region match the pattern of strong onshore flow with plenty of clouds and rain. The coolest sites are BDR and LGA with persistent ENW flow off the LI Sound keeping the highs cooler. NYC shows the wet foliage too close to the ASOS lowering the departures artificially. JFK and EWR further from the LI Sound are probably closer to what NYC would show if the site was properly maintained. MPO was the warmest part of the area away from the marine influence. It recorded its 4th warmest September average minimum temperature since the new site was established in 2000. Marches the theme of the warmer minimums leading the departures across the area. September 2026 BDR…-0.7 LGA….-0.6 NYC….-0.9 JFK…..+0.8 ISP…...+0.5 EWR….+0.5 NBW….+1.7 SMQ…..+1.5 MPO…..+1.8 Time Series Summary for POCONO MOUNTAINS MUNICIPAL AIRPORT, PA - Month of Sep Warmest average low temperature Click column heading to sort ascending, click again to sort descending. 1 2011 55.3 0 2 2018 54.8 0 3 2015 53.3 0 4 2026 52.8 1 5 2021 52.1 0

The Euro issues an extended forecast every two months. These really long range forecasts aren’t always the most accurate. But Nino 1.2 has reached a new all-rime high of +5.345. This is even beyond what the Euro was forecasting. So the warm pool in the EPAC is stronger than anything that we have seen. This makes it plausible that this event could linger at least into next June. Even models like the CFS which had this peaking in +1.2 at only +4.5 are showing it going into at least next June in 1+2. The record breaking WWBs just keep pushing the warm pool further east than model forecasts. So such an extreme heat build up in the EPAC will take to wind down.

The seasonal forecast models continue to underestimate the mid-latitude 500mb ridging pattern leading to this persistent -PDO.

Yeah. But since there are so many missing days some years, it’s not an accurate annual average temperature. The average only includes the days with actual data. Lake Harmony only has snowfall starting with 2020-2021 and no temperatures. Data for October 1, 2020 through May 31, 2021 Click column heading to sort ascending, click again to sort descending. LAKE HARMONY 2.4 WNW CoCoRaHS 77.4 PORTLAND 2.7 WNW CoCoRaHS 63.8 PALMERTON 5.8 ENE CoCoRaHS 59.3 ALLENTOWN LEHIGH VALLEY INTERNATIONAL AIRPORT WBAN 58.1 MACUNGIE 1 SW COOP 57.1 SCHNECKSVILLE 2.7 SW CoCoRaHS 51.7 SCHNECKSVILLE 1.7 ENE CoCoRaHS 51.3 JIM THORPE 1.1 NNE CoCoRaHS 50.8 LEHIGHTON 1SSW COOP 49.2

Wettest September on record for ISP. This was the 3rd consecutive month with part of the area recording 10”+ of rain. So plenty of clouds and onshore flow kept the minimum temperatures up and the maximums down. 10 Wettest Time Series Summary for ISLIP-LI MACARTHUR AP, NY - Month of Sep Click column heading to sort ascending, click again to sort descending. 1 2026 8.76 2 2 1966 7.50 0 3 2008 7.46 0 4 2023 6.78 0 5 1975 6.57 0 6 1996 6.41 0 7 2004 6.37 0 8 1974 5.95 0 9 1999 5.65 0 10 1978 5.56 0 Monthly Data for September 2026 for Upton NY NWS CWA Click column heading to sort ascending, click again to sort descending. CT NORTHFORD 0.8 SW CoCoRaHS 12.33 NY CENTEREACH 1.3 NE CoCoRaHS 10.70 NY SELDEN 1.6 ESE CoCoRaHS 10.67 CT DARIEN 2.4 NW CoCoRaHS 10.35 NY PORT JEFFERSON STATION 0.3 SSW CoCoRaHS 10.27 NY RIDGE 1.8 SE CoCoRaHS 10.18 CT JEWETT CITY 3.0 ESE CoCoRaHS 9.98 NY RIDGE 1.5 SE CoCoRaHS 9.97 CT TRUMBULL 2.6 SSE CoCoRaHS 9.67 NY SETAUKET-EAST SETAUKET 0.7 SSW CoCoRaHS 9.52 CT DURHAM 1.2 W CoCoRaHS 9.48 NY MOUNT SINAI COOP 9.37 CT STAMFORD 1.0 S CoCoRaHS 9.35 NY RIVERHEAD RESEARCH FARM COOP 9.32 CT DARIEN 3.6 N CoCoRaHS 9.31 CT NORWALK 2.9 NNW CoCoRaHS 9.30 CT WALLINGFORD CENTER 1.9 WNW CoCoRaHS 9.30 CT NORWALK 1.4 ENE CoCoRaHS 9.15 NY SHIRLEY 2.9 N CoCoRaHS 9.15 CT NORWICH 2.5 NNE CoCoRaHS 9.14 CT CHESTER CENTER 2.7 WNW CoCoRaHS 9.11 NY ISLIP TERRACE 1.1 NNE CoCoRaHS 9.05 NY MANORVILLE 1.8 SSE CoCoRaHS 8.99 CT NORWICH 5.4 SE CoCoRaHS 8.95 CT LEDYARD 0.5 NE CoCoRaHS 8.85 NJ WAYNE TWP 2.3 ESE CoCoRaHS 8.76 NY ISLIP-LI MACARTHUR AP WBAN 8.76

September 2026 was the 3rd consecutive month with parts of the area going over 10” of rain. Monthly Data for September 2026 for Upton NY NWS CWA Click column heading to sort ascending, click again to sort descending. CT NORTHFORD 0.8 SW CoCoRaHS 12.33 NY CENTEREACH 1.3 NE CoCoRaHS 10.70 NY SELDEN 1.6 ESE CoCoRaHS 10.67 CT DARIEN 2.4 NW CoCoRaHS 10.35 NY PORT JEFFERSON STATION 0.3 SSW CoCoRaHS 10.27 NY RIDGE 1.8 SE CoCoRaHS 10.18

You can see a the continuous 500mb mid-latitude ridge from Japan across to the CONUS drove another big PDO drop in September. This has been a repeating theme during the 2020s. PDO attempts to go neutral than makes another big decline. The record warm subsurface east of Japan to south of the Aleutians tells the story.

One of the bigger beach erosion events of the 2020s so far.

The big story as we head into October is that the 500mb ridge is setting up further south than the recent seasonal models were forecasting. This has been the case each month going back to May and June. The October El Nino ridge is typically located over the EPO region of NW Canada and Alaska. The forecasts drop this ridge down into the CONUS at least for early October. We’ll need to see what happens mid to late month for a complete picture. So if this model error pattern continues into the winter and spring, then the typical Nino ridge north of the Great Lakes would adjust further south than previous super El Niño events. The December ridge can typically be further south anyway to some extent. So the big test will probably come JFM. It will be interesting to see how things work out. This stronger mid-latitude ridge axis since the late spring is causing the PDO to remain negative and the AMO positive. It’s also evident to the AAM disparity that you noted. New forecasts for early October Seasonal forecast from early September Typical October El Niño pattern

The actual SSTs late this fall into winter may continue to be near +30C in spots along the West Coast of Southern Mexico and Central America. During a much weaker El Niño in 2023-2024 they were at +29C. Plus the magnitude of the warmth in Nino 1-2 and 3 are forecast to pull the forcing east. This happened in 1997-1998 with much cooler SSTs.

The stratospheric interactions are also related to the actual tropical forcing in addition to the state of the QBO. During the later portion of the winter in 1983, it was more of a full-basin SST configuration compared to 1997-1998. All the guidance has a warm pool extending up along the West Coast of Mexico. This along with the record warmth in 1-2 and 3 lead to the forecast for split forcing leading to a strong eastward lean. During many recent seasons warm pools off the equator had had areas of forcing west of Mexico and to the east of Hawaii.

RONI is a relative metric to SST outside the actual ENSO regions. But the BOM RONI for Nino 1 and 2 are higher close to the actual ONI readings. So this is why we maintain the ONI alongside the RONI. Plus the agencies in South America use both since the record streak of temperatures there are related to the actual ONI readings. Remember Nino 1+2 has been one of the few ocean regions of the planet which didn’t warm prior to the 2023-2024 super El Niño. https://www.cpc.ncep.noaa.gov/data/indices/ 23SEP2026 25.4 4.7 28.8 3.9 29.7 3.1 29.8 1.1

The east based nature of 1982-1983 backed off during the winter relative to 1997-1998. The stratospheric conditions were related to the different nature of the ENSO forcing between both those seasons. The Central Pacific warm pool has already shifted the farthest to the east it has ever been. This is only expected to increase in the coming months.