A split Polar Vortex in early November? 👀 The latest ECMWF 00z shows two vortex cores/lobes at 10mb, over northern Eurasia and Canada, with a wave-2 ridge cutting in-between. The same pattern reaches down to 50mb, so it’s not just an upper-level feature, but an upward reflective event. Both ECMWF and GFS push 60°N/10mb winds below the historical minimum (5–11 m/s by Nov 4-5). Even the GEFS extended mean, with its usual strong-SPV bias, is trending weaker in the latest available data. #PolarVortex #Winter2026

Thanks to a big jump in the latest IMS snow analysis, the latest SAI forecast and the final slope estimate got a bit of a boost in the latest run, showing a decent above average Snow Advance Index. NOTE: There are some changes in the IFS snow depth (sd) parameter packaging since the 12z run, along with the addition of 0.1° model output. Effectively, they slightly increased the minimum SWE (snow-water equivalent) threshold needed before a grid box is marked as snow-covered, though calibration against observations keeps the forecast curve in check. @judah47

Polar vortex outlook for winter 2026/27: El Niño analogs vs ECMWF SEAS5 🌀 Polar cap (60-90°N) temperature anomaly, Oct-Apr, from the surface to the stratosphere. 📊 Analogs (strong East-Pacific El Niño years 1997, 2015, 1982, 1957; NOAA CORe): A cold, strong polar vortex from November through most of January. Then a sharp stratospheric warming in late January / early February that works its way down towards the lower stratosphere by March. ECMWF SEAS5 (October run, 51 members): The same overall story: a cold vortex in November and December, then a strong warming signal that arrives earlier, from late December, peaking in January and reaching below 100mb by February. The takeaway: both point to a strong vortex in early winter, followed by a vortex disruption in mid-to-late winter. The main difference is timing: SEAS5 brings the warming about a month earlier than the analogs. A weaker vortex in late winter raises the odds of a negative AO / NAO pattern and cold outbreaks into mid-latitudes, but the timing is uncertain at this range. Also, the analogs are only 4 years, with high variance. @judah47 @webberweather @RyanWeather

Pattern repeat: how the 500 mb pattern evolves from November to December in strong East-Pacific El Niño analog years 🌍 ERA5 analog composite for 2026/27 (1997, 2015, 1982, 1957), weighted by how closely each year's Jul–Sep SST pattern matches 2026. 🇺🇸 United States: the November pattern repeats and amplifies (r = 0.61). Both months show lower heights over the West and South and a ridge over the East. In November it's a trough over the Southwest and a weak East Coast ridge. In December the trough slides towards Texas and the Gulf, and the ridge grows into a strong high over the Great Lakes and the Northeast. Upstream, a deep low develops over Alaska and the Bering Sea, the classic strong El Niño signal. That means a mild northern and eastern US, with the storm track favoring the southern tier. This is something that @webberweather already mentioned as "Merry Torchmas" 🇪🇺 Europe: the same pattern in both months (pattern correlation r = 0.91). High pressure is anchored over central Europe in November and simply strengthens into December, spreading across the Alps, the Balkans and the Mediterranean. The Atlantic trough stays in place but shifts north, from west of Ireland towards Iceland, keeping the storm track over the northwest. That points to a persistent mild and mostly dry regime for central and southern Europe, wetter for the British Isles and Norway. The bottom line for both regions: whatever sets up in November tends to lock in and strengthen into December. Keep in mind that analogs show tendencies, not forecasts.

The latest SAI forecast is starting to adjust into reality, while exhibiting a positive snow area bias over much of the month. The forecast still ends at an above average SAI. @judah47

The Polar Vortex is about to take a big hit 🌀📉 After a short boost to 22 m/s around Oct 24, both ECMWF and GFS show a sharp deceleration, with 10mb zonal winds dropping to 8–10 m/s by early November. That’s below the 1958–2025 record minimum for the date. At the same time, the GEFS extended range forecast is completely clueless about what's going on, and as usual, exhibits a positive bias. #PolarVortex #Stratosphere

The latest forecast update on the October SAI snow advance in Eurasia still shows an above-normal snow advance trend. @judah47

I was experimenting with SEAS5 seasonal data to see what it has for SAI, based on the great work by @judah47 . To keep it simple, a higher SAI favors a negative winter AO and/or a potentially more disrupted #winter circulation. Data shows a positive SAI upslope, but it's not ideal with a 1° data grid. And whether it's related or not, the October SEAS5 data also shows a significant deceleration of the Stratospheric Polar Vortex in mid-winter for a 3rd consecutive run now, which is remarkable in itself and likely a sign of being more Super El Niño-related. All strato plots use the model's own hindcast climo. Looks like a very dynamic stratospheric Polar Vortex season is ahead for Winter 2026/2027

The latest July 2026 ERA5 ranking for zonal winds at 850 mb is remarkable, ranking among the strongest or in the top 5 across a large region in the 1940-2026 dataset. This reveals the massive westerly wind bursts in the western tropical Pacific, which have enabled the development of a powerful subsurface warm anomaly, boosting the still-growing Super El Niño seen on the surface.