Major December Forecast Flip Signals Colder Jet-Stream Pattern for Indiana After Early Warm Outlook Collapses

INDIANA — A dramatic reversal in long-range weather modeling is reshaping expectations for December across Indiana and much of the northern United States. Updated CFSv2 data now shows a stronger Arctic influence, a sharper jet-stream dip, and colder temperature anomalies—a major shift from the warm, stable pattern originally forecast in early November.

A Flattened Jet Stream in November Led Models to Predict a Warm December

Early November projections suggested December would be dominated by zonal flow, meaning a flatter, west-to-east jet stream positioned across southern Canada.
That pattern typically produces:

• Mild Pacific air sweeping across the U.S.
• Limited Arctic intrusions
• Weak storm systems
• Lower snow potential for the Midwest

The CFSv2 run issued around November 11 showed broad warmth nationwide, including strong positive anomalies of +2°C to +4°C across Indiana, Illinois, and much of the central U.S.

In other words: December looked more like extended fall than winter.

A New Disruption Over the Pacific Northwest Triggered the Entire Flip

But late-November model updates reveal something the early forecasts missed:
A strong upper-level disturbance crashing into the Pacific Northwest has altered the jet-stream pattern across North America.

This disruption has created:

• A deeper trough over central Canada
• A sharper dip in the jet stream through the northern U.S.
• A pipeline for Arctic air to drain southward
• Slower east-coast flow, which helps cold air linger

This shift is exactly why the updated anomaly map looks nothing like the original one.

Updated Temperature Maps Now Show Widespread Cold Across the Northern U.S.

The newer CFSv2 run (initialized Nov 29 – Dec 8) replaces the warm December outlook with:

• Deep negative anomalies from Montana to the Great Lakes
• A cold belt running from the Pacific Northwest into the Upper Midwest
• Cooler trends extending toward northern Indiana and Michigan
• A stronger thermal boundary across the central U.S.

Meanwhile, warmth is now confined mostly to:

• The Southwest
• Southern Plains
• Gulf Coast
• Parts of the Southeast

This is a classic signature of an amplified jet stream, not the flat one models predicted in early November.

What This Means for Indiana: Colder Shots, More Active Weather, Higher Snow Potential

Indiana now sits much closer to the southern edge of the cold air mass, which means:

1. Colder Surges Are More Likely

Northern Indiana in particular could see more frequent dips into below-normal temperatures, especially when the Arctic trough deepens.

2. Storm Tracks Become More Favorable for Snow

A stronger temperature gradient across the Midwest increases the chance of:

• Clipper systems
• Mixed-precipitation events
• Early-season snow opportunities

This doesn’t guarantee big storms—but it opens the door, which the warm early-November outlook did not.

3. Central & Southern Indiana May See Sharp Temperature Swings

The amplified jet can bring:

• Sudden warm-ups
• Rapid cooldowns
• Quick-moving systems with rain or snow

This kind of volatility is typical during major pattern transitions.

What About Christmas Week? Warm or Cold?

The early-November outlook strongly favored a warm, snow-free Christmas for Indiana.

The updated outlook is much more uncertain—but definitely colder than before.

Right now, the signal suggests:

• Colder air may hold across the northern U.S. through Christmas week
• Indiana sits near the boundary between warm southern air and cold northern air
• Snow chances depend on whether the jet retracts westward or pushes southeast again

If the trough re-amplifies, Indiana could be positioned for:

• A colder Christmas
• Increased lake-effect influence in northern counties
• Better storm opportunities statewide

If the jet relaxes temporarily, a warm brief spell is still possible.

Does This Shift Help With Drought or Rainfall?

Yes — an amplified, active pattern typically increases:

• Rainfall chances in southern Indiana
• Snowfall and moisture in northern counties
• More frequent storm systems overall

This is beneficial for regions still dealing with fall precipitation deficits.

A Model Bust of Rare Scale

Meteorologists are calling this one of the largest long-range busts in recent years.

Early November:
Warm everywhere, low snow risk, quiet December

Late November:
Cold north, active jet, heightened snow potential

The difference highlights how sensitive winter forecasting is to upper-air changes over the Pacific—something global models often struggle with.

The Bottom Line for Indiana

• December will not be as warm as originally forecast
• The jet-stream pattern now supports colder outbreaks
• Snow chances have improved, especially late-month
• Christmas week is trending more uncertain but colder-leaning
• Expect a more “real winter” feel compared to early modeling

More updates will follow as the pattern continues evolving.