The jet stream explained

What is a jet stream?

A jet stream is a fast-flowing narrow band of wind, typically 100-250 mph, in the upper atmosphere.

• Discovered by pilots during WWII.
• Named by Carl-Gustaf Rossby.
• Occurs at the boundary between warm and cold air masses.
• Steers weather systems.
• Result of atmospheric temperature gradients + Earth's rotation.

The polar jet stream

• Between polar and mid-latitude air.
• Altitude: 20,000-40,000 ft.
• Wind speed: 80-200 mph.
• Meanders in Rossby waves.
• Position: north in summer, south in winter.
• Aligns with severe weather corridor when dipping south.
• Most active in Northern Hemisphere winter.

The subtropical jet stream

• Between subtropical and tropical air.
• Altitude: 30,000-45,000 ft.
• Weaker than polar jet: 40-100 mph.
• Farther south.
• Active in winter.
• Sometimes drives strong Southeast US storms.
• Weakens in summer.

The low-level jet

• Different beast entirely.
• 2,000-6,000 ft above ground.
• Nocturnal — strongest at night.
• Peak 40-60 mph.
• Great Plains phenomenon.
• Transports Gulf moisture northward.
• Critical for overnight severe weather.
• The reason many Plains tornadoes fire at 6 PM (as LLJ strengthens).

The polar night jet

• Stratospheric jet in polar regions.
• Only in winter.
• Related to polar vortex strength.
• Disruption can cause cold outbreaks.
• Sudden Stratospheric Warming (SSW) events break this jet.

How the jet drives weather

1. Warm and cold air masses have different temperatures.
2. Temperature gradient creates pressure gradient.
3. Pressure gradient drives geostrophic wind.
4. Coriolis rotates it into westerly flow.
5. Jet stream forms at the boundary.
6. Jet steers surface low-pressure systems.
7. Where jet dips south = trough = storm.
8. Where jet rises north = ridge = fair weather.

Zonal vs meridional flow

Storm-favorable jet configurations

• Left exit region of jet streak: divergence aloft, upward motion at surface.
• Right entrance region of jet streak: similarly favorable.
• These are severe weather sweet spots.
• When jet streak comes over an already-unstable environment: outbreak.
• 1974 Super Outbreak had jet streak position perfectly.
• 2011 Super Outbreak likewise.

The polar vortex connection

• Polar vortex is a broad area of low pressure and cold air over poles.
• Not a "storm" or a "tornado."
• Contained by polar night jet.
• When disturbed, cold air surges south (Arctic outbreak).
• Extreme cold events like 2021 Texas linked to polar vortex disruption.
• 2019 January polar vortex broke apart, dumped Chicago -30°F.

Climate change and the jet

• Arctic warming faster than mid-latitudes.
• Reduced temperature gradient = weaker polar jet.
• Weaker jet may develop more meandering patterns.
• Meandering = more extreme weather (blocking, cold outbreaks, heat waves).
• Research is contested but consensus growing.
• Weakening polar vortex may cause MORE frequent extreme winter cold outbreaks despite overall warming.

Reading a jet stream forecast

• Jet stream maps show winds at 300 hPa (about 30,000 ft).
• Look for the jet streak — enhanced wind band.
• Look for divergence (winds spreading apart).
• Look for jet position relative to your target.
• Ideal severe setup: left-exit / right-entrance region over surface low.
• HRRR shows lower-altitude wind fields.
• SPC mesoanalysis maps jet parameters.

Learn more

• Reading a hodograph
• Reading SPC mesoanalysis
• Atmospheric rivers
• El Niño and La Niña