In short: The low-level jet stream is a band of fast-moving wind, often near 5,000 feet MSL, that forms over the Great Plains and southeastern United States. Rod Machado shows that it comes down to two ideas: uneven heating creates a pressure differential, and the Coriolis force bends the resulting wind into a southerly flow. One type forms at night; the other rides ahead of cold fronts.
Video: “Understanding the Low Level Jet Stream by Rod Machado” by Rod Machado, from his YouTube channel.
What you’ll learn
- How the low-level jet differs from the high-altitude jet stream
- Typical altitudes and speeds for the two types of low-level jet
- How evening cooling over the western plains tilts the 850 millibar pressure level
- Why Coriolis force turns the flow into a southerly wind
- Why a surface-based inversion can produce strong wind shear on approach
- How cyclone-induced jets carry Gulf moisture ahead of cold fronts
Two simple ingredients
This excerpt comes from Rod’s animated Understanding Weather eLearning course. He contrasts the familiar jet stream, a high-altitude band of west-to-east winds along the polar front, with lower bands of fast wind that develop over the Great Plains and the Southeast. These low-level jets are commonly found around 5,000 feet MSL, the 850 millibar level, with speeds of roughly 25 to 50 knots or 40 to 70 knots depending on the type.
All wind starts with a pressure gradient, flowing from high to low pressure, and in the Northern Hemisphere the Coriolis force curves it to the right. Combine those two principles and the low-level jet stops being mysterious.
The nocturnal low-level jet
The terrain slopes upward toward the Rockies, and the higher, drier western plains heat and cool faster than the lower, more humid eastern plains. After sunset, the west cools quickly while the east holds its warmth. Because cold air columns compress and warm columns expand, the 850 millibar pressure level slopes downward toward the west. At the same altitude, pressure is now higher in the east than in the west, so air begins flowing westward, and Coriolis bends it into a wind from the south that can run northward for hundreds of miles.
It is strongest in the early morning. A surface-based inversion often forms over the eastern plains, and the warm air above it flows with little friction. Rod describes the possibility of 3 to 4 knot surface winds beneath 40-plus knots at 1,000 feet AGL, a setup for significant wind shear on approach. As the day warms, the inversion breaks up and the jet usually fades.
The cyclone-induced jet
The second type forms in the warm air ahead of an advancing cold front. Here, lower pressure ahead of the front tilts the 850 millibar level, producing a low-level jet that runs parallel to and moves with the front, at speeds that can reach about 70 knots. These jets act like a conveyor belt for warm, moist Gulf of Mexico air, adding instability to air lifted by the front, which Rod connects to the severe weather of Tornado Alley.
Go deeper with Rod Machado
Rod’s pilot handbook builds weather understanding from the same simple principles, with plenty of illustrations. Learn more in Rod Machado’s Private/Commercial Pilot Handbook.
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