Southern Oscillation Index Plunge Triggers “Big Winter Storm Signal,” Meteorologist Warns

Folks across the Mid-Atlantic, especially in the Washington, DC-Baltimore metro area, woke up to a rare December winter scene: a few inches of fresh snow on the ground. With a solid cold pattern locked in through the middle of the month, this early-season snow is something the region hasn’t seen this early in the cold season in many years.

#Snow falling on Friday at the Capitol in #DC! @capitalweather @KaitlynMcGrath pic.twitter.com/8vDSz7nSJ3

— Jeanne Stuart McVey (@RiverGirl707) December 5, 2025

With below-normal temperatures forecast across the Mid-Atlantic through at least mid-month – and potentially extending into the third week according to models, meteorologist Mark Margavage wrote on X that alarm bells are going off that signal a potential winter storm threat for the region could be a few weeks away. 

Margavage pointed to a sudden plunge in the Southern Oscillation Index (SOI), which measures the sea-level pressure difference between Tahiti and Darwin, Australia. Such a drop signals a shift from La Niña–like conditions (positive SOI values) toward neutral or even El Niño conditions (negative SOI values).

This transition weakens trade winds across the equatorial Pacific, warms sea surface temperatures in the central and eastern Pacific, and disrupts the Walker circulation, leading to widespread global weather anomalies.

One such anomaly that may occur at a 2 to 3 week lag is the increased risk of major winter storms or polar vortex disruptions in the Eastern U.S. 

Margavage explained more:

The Southern Oscillation Index (SOI) is a key indicator of the strength and phase of the El Niño–Southern Oscillation (ENSO). Sustained strongly positive SOI values (typically 30-day average > +7 or +8) are associated with La Niña conditions, and sustained strongly negative values with El Niño.

This graph shows a very sharp collapse of the SOI from strongly positive (La Niña-like) values around +18 in mid-to-late November 2025 down to slightly positive or near-neutral values by early December 2025.

According to @grok “This is one of the fastest SOI drops on record.”

After a sudden SOI plunge of this magnitude and speed, the most common lag time to the first big Eastern U.S. winter storm or polar vortex disruption-driven cold wave is about 2–3 weeks, with a broader 10–35-day window of elevated risk.

So for the drop shown in this image (finalized ~5 Dec 2025), the highest winter storm threat in the Eastern U.S. is roughly December 18 – January 10, peaking late December to very early January 2026. So Christmas could be extremely Wintery this year!

🚨Record Setting SOI Drop🚨
⚠️Big Winter Storm Signal⚠️

The Southern Oscillation Index (SOI) is a key indicator of the strength and phase of the El Niño–Southern Oscillation (ENSO). Sustained strongly positive SOI values (typically 30-day average > +7 or +8) are associated with… pic.twitter.com/qskr0SWRmP

— Mark Margavage (@MeteoMark) December 5, 2025

Cold is here. 

First colder than normal EIA week of new pattern – from Fri Nov 28 to Thu Dec 4 – was coldest of 2000s for matching dates. pic.twitter.com/KdRTefqDOm

— Commodity Wx Group (@commoditywx) December 5, 2025

Cold just keeps coming. I just don’t see a end in sight. pic.twitter.com/Ks7zvCnNzH

— Mitch West (@SCweather_wx) December 6, 2025

Energy markets have responded:

• Biggest NatGas Quarterly Spike In Decades As Deep Freeze Slams U.S. East

If Margavage is right about the downstream weather impacts in the Lower 48 from a plunging SOI, this could suggest rising odds of a white Christmas in portions of the Eastern U.S.