OCTOBER FORECAST
Hey folks, I’m doing things slightly different this month. Due to a complex transition into Autumn, and for other reasons, I’m composing a more technical, extensive summary and polar vortex forecast, while adding some speculative thoughts about early Winter.
I will add more specific, short-term forecast thoughts in a few different places and highlight them for easier reference.
TECHNICAL AUTUMN SUMMARY
• (IOD INDEX)
The Indian Ocean Dipole, while still very weak, is just starting to rebound in response to La Niña conditions in the equatorial Pacific and delayed, but increasing, westerly winds in the west Indian Ocean tropics— a delay responsible for a fairly late retreat of India’s Summer Monsoon.
But, even as weak tropical convection (i.e. the Madden-Julian Oscillation, or MJO) is finally slowly moving east through the “Maritime Province” (waters around Indonesia and the Philippines), the MJO seems poised to accelerate across the Pacific in the next three weeks.
• (velocity potential anomaly)
Thus, velocity potential charts currently depict a few plumes of moisture advancing east in the next two weeks.
But, while VP charts show the current infusion of tropical moisture towards 120° W, the pool of warm water and convective conditions over the Maritime Province are still expected to mostly stay put for the next two or three weeks.
• (Earth-relative atmospheric angular momentum–AAM)
As for our recent windstorm, those westerly winds in the Indian Ocean combined forces with east Asian mountain torque from about 10 days ago and eastward progression of the MJO to add high atmospheric momentum to the north Pacific Jet Stream crossing the mid-latitudes.
(Please see x.com/Met4CastUK/status/1980694185553678832 for a deeper discussion.)
High winds and heavy rain are still possible through early Saturday evening, and moderately heavy rain is possible around mid-morning tomorrow.
Light showers are also possible late Tuesday night.
• (global relative AAM)
But, global winds are predicted to increase only slightly over the next 5–6 days, then tank again.
Steady movement towards La Niña and an easterly wind reversal in the tropical stratosphere (eQBO) have kept global winds mostly below average since early January.
POLAR VORTEX SYNOPSIS
• (72-hour SPV forecast)
Since AAM reached an unusually low point in early October, the stratospheric polar vortex (SPV) has slowly been gaining strength.
I do suspect that a mild SPV disruption around Oct. 18–19 will most likely have some noticeable effect here before and maybe a few days after Halloween, though lower global winds tend to weaken SPV disruptions. But I would note that a (relative) warm pool over the Canadian Arctic region is still deforming the SPV.
• (Arctic sea ice extent)
With Arctic sea ice on the rebound northeast of Siberia and Siberian snow cover decreasing some, I would expect these mild Canadian disruptions to generally continue off and on into early December.
High pressure domes over Siberia have consistently been channeling directly through east Siberia and across the Chukchi and Beaufort Seas, avoiding the thicker sea ice, on their way to Canada.
MAIN SHORT-TERM FORECAST
• (EPS 2-meter temperature anomaly)
Starting Tuesday or Wednesday we could see several days reach into the 60s, but mixed with a fair amount of rain early next weekend.
Today’s time-longitude Hovmöller also suggests a few days of showers starting next Monday, Nov. 3. It’s possible they could last more than 2 or 3 days, but I’m somewhat skeptical.
The stronger SPV, an upcoming Scandinavian ridge, and the subsequent downturn of global AAM could all maintain fairly warm weather into the first 10 days of November, maybe longer.
NORTHERN HEMISPHERE
• (2-meter temperature anomaly)
On a slight tangent, I should inform you all that in late August the Northern Hemisphere began to experience a highly unusual warming period– basically a heat wave that’s now covering most of it.
I suspect global wind stagnation, the negative IOD and sluggish MJO, easterly QBO, and consistently weak stratospheric polar vortex are all allowing the extreme north Pacific marine heatwave to steadily shed thermal energy into the atmosphere as sunlight decreases.
And sea surface temperatures have generally been decreasing since late September off the coastal Pacific Northwest.
• (daily temperature for 80+ °N)
Even the mean surface temperature in the high Arctic (above 80° N) has been unusually elevated since mid-August. We could even see polar air temperatures start to rise again towards December.
Combined with the easterly stratospheric QBO, La Niña, and depressed global winds, the polar vortex is not expected to gain normal strength anytime soon, making it vulnerable to more frequent disruptions into early Winter.
LA NIÑA–ENSO OUTLOOK
• (3-month ENSO probabilities)
La Niña conditions were declared on October 8, though a plume of warmer sub-surface sea temperatures subsequently began moving towards the central Pacific.
That plume and the north Pacific marine heatwave seem likely to limit how long La Niña can last, and other indicators lend support to a fairly clear transition to neutral ENSO conditions (ocean surface heat pooling towards the central Pacific) by mid-February, maybe sooner.
And again, the window of opportunity for El Niño to redevelop definitely opens up by late Summer 2026, at the latest. And, statistically we’re a little bit overdue for a Super El Niño, though it’s all very uncertain beyond Spring.