40/70 Benchmark

https://easternmassweather.blogspot.com/2025/09/guidance-aligning-with-eastern-mass.html Preliminary composite out..subject to changes next 6 weeks. DJFM 2025-2026 Per 1991-2020 Climate Period

Guidance Aligning with Eastern Mass Weather Expectation for Weak La Niña Confidence in ENSO Analogs Growing September ENSO Update The updated consensus from the International Research Institute (IRI) for the month of September has finally reflected the significant cooling trend that Eastern Mass Weather has been advertising since the month of July. Indeed, after supporting an ENSO neutral peak that was warmer than the La Niña threshold (-0.5C) in region 3.4 all throughout this past summer, anywhere between -.31C and -.42C, the September suite now matches the preliminary Eastern Mass Weather forecast within a peak range of between -0.5C and -0.7C. When viewing the latest subsurface data, it is easy to see why this latest suite of guidance finally followed the relatively aggressive trend towards cool ENSO that had been anticipated. Note the recent burst of easterly trades during much of the month of August in the annotation below, which triggered the upwelling of the rather extensive subsurface cold pool over the eastern half of the ENSO regions to the surface. The ramifications of this expected development are two-fold. Firstly, the notion of this particular cool ENSO event meeting the technical definition of La Niña per CPC cannot yet entirely be discounted. While the concept of an official La Niña designation remains dubious considering the next round of trades is expected to be weaker, they had been relatively impressive throughout the summer, and were even prevalent enough to compare favorably to those that normally precede the development of weak La Niña. Additionally, the pressure dipole, which consists of lower pressure across the western Pacific and higher pressures to the east in a developing cool ENSO event, has also been slightly more impressive than the weak La Niña group. This is unsurprising considering that the MJJA mean SOI value for 2025 is 2.96, which is actually slightly higher (more La Niña like) than the weak La Nina composite MJJA value of 2.71. What is being observed is a continuation of the predominate trend of the past decade for the atmosphere to be more representative of La Niña than the ONI would imply due to the western Pacific warm pool. Warmer West/Cooler East Pacific SST Anomaly Dipole Characteristic of Cool ENSO This is why the Relative Ocean Nino Index (RONI), which in a simplistic sense de-trends the general background warming of the tropics that is attributed to climate change from ENSO, is of greater importance. The current tri-monthly JJA RONI value of -.46 is already bordering on La Niña territory, and is likely to finish anywhere between. -0.9 and -1.1 based on analog seasons. The continued descent of both the IOD (left) and the AAM (right) is also reflective of an atmosphere that is entering La Niña mode more aggressively than the ONI would imply, and thus rendering an official designation per ONI, which is still considered unlikely due to the fairly late start of the more concerted cooling, moot. The second consequence of the stronger interval of easterly trades triggering the surfacing of the subsurface cold pool in the eastern half of the ENSO region is that it reaffirms the idea that this particular cool ENSO event will remain at least somewhat eastern biased throughout the fall and into boreal winter season. Expectation for Eastern-Biased Cool ENSO Remains The rapid cooling recorded in the Eastern region of 1.2 from 0.8C above average on August 6th to -0.3 below average by August 27 is very evident in the CPC data below. Date 1.2 3 3.4 4 06AUG2025 0.8 -0.1 -0.3 0.0 13AUG2025 0.6 0.0 -0.3 0.1 20AUG2025 -0.1 -0.3 -0.4 -0.1 27AUG2025 -0.3 -0.6 -0.4 -0.2 03SEP2025 -0.2 -0.4 0.3 -0.2 10SEP2025 -0.2 -0.5 -0.5 -0.3 17SEP2025 0.0 -0.4 -0.4 -0.3 Note that although this was followed by a slight recovery with the abatement and translation westward of the trades in September, the event currently remains consistent with the east-based composite in which the max anomalies are centered between about 120 and 140 degrees longitude. Current SST Profile Matches East-Based Composite (Max Anomaly 120-140W) East-Based La Niña Composite (Max Anomaly 120W) The persistence of this regime is generally supported by the consensus of guidance, which is perhaps slightly more supportive of an eastward leaning basin-wide event by winter, as opposed to a bonafide east-base event. This is perceived to be a rather trivial discrepancy, especially given the modest strength of the ENSO event. The newest El Niño Modoki Index (EMI) guidance from JAMSTEC has also remained fairly consisted in it's depiction of an EMI value of around -.20 to -.30 during the boreal winter season. This range is similar to the EMI analogs of 1995-1996, 2017-2018 and 2021-2022, which is supported by the similarity in both the placement (near 140 longitude) and the intensity of the respective subsurface cold pool analogs relative to 2025. One final point of interest in closing is that the only slight change from the EMI August guidance is that it delayed the onset of the eastward translation, presumably owed to the fact that it took until latter August for the subsurface cold pool to surface in region 1.2. This will be a trend to watch moving though the fall in the event that the recent cooling in region 1.2 becomes a trend; however, this particular region is prone to volatility and the subsurface is currently not conducive to any such trend. Now that enough information has been gleaned throughout the summer and into the onset of boreal autumn, a preliminary seasonal composite can be introduced. Preliminary DJFM Seasonal 500mb/Temperature Anomaly Composite The operative word here is "preliminary", as this these forecast products will be subject to changes over the course of the next six weeks, prior to the issuance of the final products in November. That being said, without further ado: DJFM 2025-2026 Per 1991-2020 Climate Period The above composite is the product of an integration of polar, extra tropical Pacific, ENSO and summer-fall temperature analogs. One final ENSO update will be issued in latter October before the complete Winter Outlook 2025-2026 is published in November.

I don't do tropical seasonal calls. Show me the forcast that called for a hyper active season....JB maybe? I guess there is an inherent level of subjetivity at play with respec to what one considers "very" active, but to me, this isn't it. Forecast for 2025 Hurricane Activity Forecast Parameters CSU Forecast for 2025* Average for 1991-2020 *CSU released its first seasonal forecast for 2025 on Thursday, April 3th, with updated forecasts on June 11, July 9th, and Aug 6. Named Storms 16 14.4 Named Storm Days 80 69.4 Hurricanes 8 7.2 Hurricane Days 30 27.0 Major Hurricanes 3 3.2 Major Hurricane Days 8 7.4 Accumulated Cyclone Energy (ACE)+ 140 123 ACE West of 60 degrees longitude 87 73

Interesting....some of my heavy-hitter analogs on that list.

No, it wasn't...it was supposed to slightly more active than average.

This is the most useless tropical season that I can recall....yet to even acknowledge the season the on blog. Nothing to see here, either.....except the same 2 or 3 "doing it"....

There are some posters you just know are never going to deliver good news, and if they ever do, it will be measured and delivered with ample caveats. Seem them post....brace first, read later-

I'll be pretty suprised if we don't see at least one round of significant blocking.

Modern conceptualization of "colder" is probably near normal.

Region 1.2 has warmed again, speakaing to the volatility that Paul mentioned, but the subsurface remains supportive of an east-biased event.

37.2 this AM.

Now, maybe these trends reverse and guidance will be wrong, but that is not what we are seeing now. It's not becoming less east-based.

Also, if you look at guidance, this has been expected and well forecast.

Yes, I think Jerry is focusing on region 3...while 1.2 is volatile....this marked cooling in that zone isn't a function of volatility. If you have been paying attention, the subsurface cold pool emerged due to sustained trades.

No; it invalidates it. The eastern zone of 1.2 began warm and the subsurface cold pool emerged there.

I think that is a larger problem in December..not so much late season.

I think you have it backwards accordingly to the weeklies,,,eastern zones recently caugt up-this is supported by JAMESTEC EMI data. East-to-west 23JUL2025 22.2 0.6 25.5-0.1 27.0-0.2 28.7-0.1 30JUL2025 21.8 0.4 25.3-0.2 26.7-0.3 28.6-0.1 06AUG2025 21.9 0.8 25.2-0.1 26.7-0.3 28.7 0.0 13AUG2025 21.6 0.6 25.2 0.0 26.6-0.3 28.6-0.1 20AUG2025 20.8-0.1 24.7-0.3 26.4-0.4 28.6-0.1 27AUG2025 20.5-0.3 24.3-0.6 26.3-0.4 28.5-0.2 03SEP2025 20.5-0.2 24.5-0.4 26.4-0.3 28.5-0.2 10SEP2025 20.5-0.2 24.4-0.5 26.2-0.5 28.3-0.3

Maybe a hair stronger than last year-

We need the trades colocated with the subsurface cold pool...we had that big time througout much of August, when we had the burst of intensification of La Nina due to the upwelling of the cold pool over the eastern half of ENSO. However, trades died down and shifted west in Septmeber. They look to pick up out east again next week and into October, but not as strongly as August. SOI has also been inching upwards. I think at the end of the day, the pedestrian coupling of the trades with the cold pool, owed at least in part to the diffuse PAC pressure dipole (Low west/high east) is what will have this La Nina struggling to be acknowledged by CPC in the record books. Will be very close. The subsurface says game on...but the hemisphere isn't totally on board.

IRI plume def. more aggressive with La Nina. I think they have finally caught onto to an event that will will fall just shy of official designation. I called -0.5 to -0.7 ONI peak back in July and it looks like dynamical guidance now peaks at -0.71, statistical -0.53 and mean of all guidance -0.61. https://easternmassweather.blogspot.com/2025/07/la-neutral-enso-conditions-will.html Will probably go onto to overcorrect a bit next month, too before tickling back later in the fall. Subsurface is quite stout, but I don't think we will have the trade clout to surface as much of that in region 3.4 as we did in 3 and 1.2.

Yea, well don't forget the E QBO. This shouldn't be a super nova +NAO that is devoid of blocking.

I would go north of Tahoe this season.

My extra tropical Pacific composite actually looks just about identical to that BAM Atlantic composite. Primary Extratropical Pacific Analogs: 1971-1972, 2017-2018, 2021-2022, 2024-2025

Given the embryonic nature of this cool ENSO event, I'm not sure the -IOD is really going to couple with it enough to be a huge deal....bottom line is I would expect a weak La Nina response...this shouldn't be 2022-2023. However, I think the development of the -IOD does ensure that this will indeed be a cool ENSO season, and that the WPO is likely to be at least somewhat positive....two things that I had already been assuming.

Looks similar, but I'm a bit more aggressive with some episode(s) of blocking per the easterly QBO/solar max composite.